Projection exposure method and projection exposure device for microlithography
阅读说明：本技术 用于微光刻的投射曝光方法与投射曝光装置 (Projection exposure method and projection exposure device for microlithography ) 是由 A.沃尔夫 于 2018-05-28 设计创作，主要内容包括：在用以在投射曝光装置的操作控制系统的控制下以掩模的图案的至少一个像曝光辐射敏感基板的投射曝光方法中,在投射镜头的协助下将位于照明区域中的图案的一部分投射至在基板上的像场上,其中有助于像场中的像产生的投射辐射的所有射线形成投射光束路径。通过致动操纵器来影响投射辐射的波前,该操纵器具有配置在投射光束路径中的操纵器表面及用以可逆地改变操纵器表面的光学效果的致动装置。致动装置的操纵值变化由操作控制系统基于一控制程序来决定,其中控制程序具有优化目标函数的校正算法。用于至少一个操纵器的目标函数包含远心敏感度,其描述在操纵器的定义的操纵值变化与可因此实现的对像场中的投射辐射的远心度的影响之间的关系。(In a projection exposure method for exposing a radiation-sensitive substrate with at least one image of a pattern of a mask under the control of an operation control system of a projection exposure apparatus, a portion of the pattern located in an illuminated area is projected onto an image field on the substrate with the aid of a projection lens, wherein all rays of projection radiation contributing to the image generation in the image field form a projection beam path. The wavefront of the projection radiation is influenced by actuating a manipulator having a manipulator surface arranged in the path of the projection beam and actuating means for reversibly changing the optical effect of the manipulator surface. The change in the actuation value of the actuator is determined by the operating control system based on a control program having a correction algorithm that optimizes the objective function. The objective function for at least one manipulator comprises a telecentricity sensitivity, which describes the relationship between the defined change in the manipulator value and the influence that can be achieved thereby on the telecentricity of the projection radiation in the image field.)
1. A projection exposure method for exposing a radiation-sensitive substrate with at least one image of a pattern of a mask under the control of an operation control system of a projection exposure apparatus, comprising the steps of:
holding the mask between an illumination system of the projection exposure apparatus and a projection lens such that the pattern is arranged in the region of an object plane of the projection lens;
holding the substrate such that a radiation-sensitive surface of the substrate is arranged in the region of an image plane of the projection lens, the image plane being optically conjugate with the object plane;
illuminating an illumination area of the mask with illumination radiation provided by the illumination system;
projecting a portion of the pattern located in the illumination area onto an image field on the substrate with the aid of the projection lens, wherein all rays of projection radiation resulting from the image in the image field are caused to form a projection beam path;
influencing the wavefront of the projection radiation by actuating a manipulator having a manipulator surface arranged in the projection beam path and actuating means for reversibly changing the optical effect of the manipulator surface;
wherein the operating control system determines a change in a manipulated variable of the actuator based on a control program having a correction algorithm that optimizes an objective function,
the method is characterized in that:
the objective function for at least one manipulator comprises a telecentric sensitivity, wherein the telecentric sensitivity describes a relationship between a defined change in the manipulation value at the manipulator and a thus achievable effect on the telecentricity of the projection radiation in the image field.
2. Projection exposure method according to claim 1, characterized in that the Z1 sensitivity of at least one manipulator is determined, wherein the Z1 sensitivity describes the relationship between the defined change in the manipulator-value of the manipulator and the effect achieved thereby on the field distribution of the Zernike coefficients Z1 or mathematically equivalent variables thereof.
3. Projection exposure method according to claim 1 or 2, characterized in that the telecentricity sensitivity (S (Z1)) of the manipulator for changing the telecentricity is stored in a memory (SP) of the operation control system, wherein the operation of the projection exposure apparatus is controlled taking into account the telecentricity sensitivity.
4. Projection exposure method according to claim 3, characterized in that, taking into account the telecentric sensitivity, the change in the manipulation value of the manipulator is limited to a magnitude below a manipulation value limit value.
5. Projection exposure method according to one of the preceding claims, characterized in that an OPL surface conjugated to the object surface is calculated during the optimization of the objective function, which OPL surface is defined by the overall image point located at an optical distance of a constant Optical Path Length (OPL) from a conjugated object point.
6. A projection exposure method according to claim 5, characterized in that for the determination of the OPL surface a distribution of constant displacements of the wavefront of the projection radiation over the image field is calculated.
7. Projection exposure method according to any of the preceding claims, characterized in that the telecentric control of the projection lens incorporated into the projection exposure apparatus during operation of the projection exposure apparatus comprises the following steps:
determining a starting value of telecentricity at a starting time;
calculating a change in telecentricity caused by the adjustment of the manipulator using the change in the manipulator manipulation value and the assigned value of telecentricity sensitivity; and
determining a telecentricity value for the decision time from the start value and the change in telecentricity effected between the start time and the decision time.
8. Projection exposure method according to claim 7, characterized in that the starting value is determined by measuring the telecentricity at start-up or after readjustment.
9. Projection exposure method according to any of the preceding claims, characterized in that the telecentricity of the projection lens is changed by actuating at least one dedicated telecentricity manipulator.
10. A projection exposure apparatus for exposing a radiation-sensitive substrate using at least one image of a pattern of a mask, comprising:
an illumination system (ILL) for receiving primary radiation from the primary radiation source and for generating illumination radiation (ILR) directed to the mask (M) in the illumination area;
a projection lens (PO) to project a portion of the pattern located in the illumination area onto an image field at the substrate using projection radiation;
a mask holding device (RST) for holding the mask between the illumination system and the projection lens such that the pattern is arranged in the region of an object plane (OS) of the projection lens;
a substrate holding device (WST) for holding the substrate such that a radiation-sensitive surface of the substrate IS arranged in the region of an image plane (IS) of the projection lens, the image plane being optically conjugate to the object plane; and
an operation control system configured to control operation of the projection exposure apparatus;
a wavefront manipulation system (WFM) to dynamically influence a wavefront of projection radiation traveling from the object plane to the image plane, wherein:
the wavefront manipulation system comprises a Manipulator (MAN) actuatable by control signals of the operation control system and having a Manipulator Surface (MS) arranged in the projection beam path and actuating means (DR) for reversibly changing the optical effect of the manipulator surface,
the method is characterized in that:
the telecentricity sensitivity (S (Z1)) of the manipulator for changing the telecentricity, which telecentricity sensitivity describes the relationship between the defined change in the manipulator value and the effect achievable on the telecentricity of the projection lens, is stored in a memory (SP) of the operating control system, wherein the operating control system is configured such that the operation of the projection exposure apparatus is controllable taking into account the telecentricity sensitivity.
11. The projection exposure apparatus according to claim 10, wherein the operating control system is configured to control the projection exposure apparatus to carry out the projection exposure method according to any one of claims 1 to 9.
12. Projection exposure apparatus according to the introductory part of claim 10, in particular according to any of claims 10 and 11, characterized in that the projection lens comprises at least one dedicated telecentricity manipulator.
13. Projection exposure apparatus according to claim 12, characterized in that the dedicated telecentricity manipulator comprises a first manipulator element (ME1) and a second manipulator element (ME2) separate therefrom,
the first manipulator element (ME1) is arranged in the projection beam path in or optically close to a first field plane (FE 1);
the second manipulator element (ME2) is arranged in the projection beam path in or optically close to a second field plane (FE2) optically conjugate to the first field plane;
an imaging lens portion having an enlarged or reduced imaging ratio and disposed between the first field plane and the second field plane; and
actuation means (DR1) assigned to the manipulator elements and configured to cause relative changes of the manipulator elements with respect to each other such that one of the manipulator elements causes a change in telecentricity, distortion and defocus and the other manipulator element partially or completely compensates for the caused change in distortion and defocus.
14. Projection exposure apparatus according to any of claims 10 to 13, characterised in that the imaging lens section is enlarged or reduced by at least a factor of two.
15. Projection exposure apparatus according to any of claims 10 to 14, characterized in that the first field plane is the object plane and the second field plane is an intermediate image plane, or the first field plane is the object plane and the second field plane is the image plane, or the first field plane is a first intermediate image plane and the second field plane is a second intermediate image plane, or the first field plane is an intermediate image plane and the second field plane is the image plane.
16. Projection exposure apparatus according to any of claims 10 to 15, characterized in that the first manipulator element (ME1) and the second manipulator element (ME2) form an Alvarez manipulator or the first manipulator element (ME1) is a first Alvarez lens element and the second manipulator element (ME2) is a second Alvarez lens element.
17. A projection lens (PO) for imaging a pattern arranged in an object plane (OS) of the projection lens into an image plane (IS) of the projection lens by means of electromagnetic radiation, comprising:
a plurality of optical elements having optical surfaces, which are arranged in the projection beam path between the object plane (OS) and the image plane (IS), such that a pattern arranged in the object plane IS imageable in the image plane by the optical elements; and
a wavefront manipulation system (WFM) to dynamically influence a wavefront of the projection radiation traveling from the object plane to the image plane,
characterized in that the projection lens comprises at least one dedicated telecentricity manipulator.
18. Projection lens according to claim 17, characterized in that the dedicated telecentricity manipulator comprises a first manipulator element (ME1) and a second manipulator element (ME2) separate therefrom,
the first manipulator element (ME1) is arranged in the projection beam path in or optically close to a first field plane (FE 1);
the second manipulator element (ME2) is arranged in the projection beam path in or optically close to a second field plane (FE2) optically conjugate to the first field plane;
an imaging lens portion having an enlarged or reduced imaging ratio and disposed between the first field plane and the second field plane; and
actuation means (DR1) assigned to the manipulator elements and configured to cause relative changes of the manipulator elements with respect to each other such that one of the manipulator elements causes a change in telecentricity, distortion and defocus and the other manipulator element partially or completely compensates for the caused change in distortion and defocus.
19. Projection lens according to claim 17 or 18, characterized in that the imaging lens section is enlarged or reduced by at least a factor of two.
20. Projection lens according to one of claims 17 to 19, characterized in that the first field plane is the object plane and the second field plane is an intermediate image plane, or the first field plane is the object plane and the second field plane is the image plane, or the first field plane is a first intermediate image plane and the second field plane is a second intermediate image plane, or the first field plane is an intermediate image plane and the second field plane is the image plane.
21. Projection lens according to one of claims 17 to 20, characterized in that the first manipulator element (ME1) and the second manipulator element (ME2) form an Alvarez manipulator or in that the first manipulator element (ME1) is a first Alvarez lens element and the second manipulator element (ME2) is a second Alvarez lens element.
Technical Field
The invention relates to a projection exposure method as defined in the introductory part of claim 1, a projection exposure apparatus as defined in the introductory part of claim 10 which is suitable for carrying out a projection exposure method, and a projection lens as defined in the introductory part of claim 17.
Background
At present, microlithographic projection exposure methods are predominantly used for the production of semiconductor components and other finely structured components, such as, for example, photolithographic masks. In this case, a mask (reticle) or other pattern generating device is used which carries or forms the pattern of the structure to be imaged, for example a line pattern of a layer of a semiconductor component. The pattern is located in the region of the object plane of the projection lens between the illumination system and the projection lens in the projection exposure apparatus and is illuminated by the illumination radiation provided by the illumination system. The radiation altered by the pattern travels through the projection lens in the form of projection radiation, which images the pattern on a reduced scale onto the substrate to be exposed. The surface of the substrate is arranged in an image plane of the projection lens optically conjugate to the object plane. The substrate is typically coated with a radiation sensitive layer (resist, photoresist).
One of the goals of development of projection exposure apparatus is to produce structures of increasingly smaller dimensions on substrates by means of photolithography. In the case of, for example, semiconductor components, smaller structures lead to higher integration densities; this generally has a favorable effect on the properties of the resulting microstructured component.
The size of the structures that can be produced depends primarily on the resolving power of the projection lens employed and the latter can be increased firstly by lowering the wavelength of the projection radiation used for projection and secondly by increasing the image-side numerical aperture NA of the projection lens used in the process. Currently, projection exposure apparatuses comprising high-resolution projection lenses operate at wavelengths of less than 260nm in the Deep Ultraviolet (DUV) range or in the Extreme Ultraviolet (EUV) range.
Projection lenses typically have multiple optical elements to meet partially conflicting requirements in terms of correction of imaging aberrations, possibly even with large numerical apertures. Refractive and catadioptric projection lenses in the field of microlithography typically have ten or more transparent optical elements. In systems for EUV lithography, efforts are made to process with as few reflective elements as possible, for example using four to six mirrors.
In addition to the inherent imaging aberrations that the projection lens may have due to its optical design and production, imaging aberrations may also occur during use, in particular during operation of the projection exposure apparatus on the user side. Such imaging aberrations are typically caused by variations in the optical elements mounted in the projection lens due to the projection radiation used during use. This problem is usually dealt with by the keyword "lens heating". Other internal or external disturbances may also cause impairment of imaging performance. They include, among others, possible dimensional errors of the mask, variations in air pressure in the surrounding environment, differences in the strength of the gravitational field between the position of the original lens adjustment and the position used by the customer, changes in the refractive index and/or shape of the optical element due to material changes (e.g. compression) caused by high-energy radiation, deformations due to relaxation processes in the holding device, drifts of the optical element, etc.
Modern projection exposure apparatuses for microlithography comprise an operation control system which allows a near instantaneous fine optimization of the imaging-relevant properties of the projection exposure apparatus to be performed in response to environmental influences and other disturbances. For this purpose, at least one manipulator is actuated in a manner suitable for the current system state in order to counteract the adverse effect of the disturbance on the imaging performance. In this case, the system state may be estimated, for example, based on measurements, from simulations, and/or based on calibration results, or may be decided using other means.
The operation control system comprises a subsystem belonging to the projection lens in the form of a wavefront manipulation system for dynamically influencing the wavefront of the projection radiation travelling from the object plane to the image plane of the projection lens. In the course of the dynamic influencing, the effect of the components of the wavefront manipulation system arranged in the path of the projection beam can be adjusted in a variable manner in dependence on control signals for operating the control system, as a result of which the wavefront of the projection radiation can be modified in a targeted manner. The optical effect of the wavefront manipulation system can be modified, for example, in the case of a particular predefined situation, or in accordance with the situation before exposure, or during exposure.
The wavefront manipulation system comprises at least one manipulator having at least one manipulator surface arranged in the projection beam path. In this case, the term "manipulator" denotes a device which, on the basis of corresponding control signals of an operating control system of the projection exposure apparatus, is configured to actively influence individual optical elements or groups of optical elements in order to change their optical effect, in particular to change the optical effect in order to compensate at least partially for the occurring aberrations.
The manipulator contains one or more actuating members or actuators, the current manipulation value of which can be changed or adjusted as a result of a change in the manipulation value on the basis of control signals of the operating control system. Variations in the manipulation value may cause, for example, displacements or deformations of the optical element. If the change in the manipulated variable is a movement of an actuator, for example in order to displace or tilt an optical element, the change in the manipulated variable may also be referred to as a "manipulator stroke". The change in the actuation value can also occur, for example, as a temperature change or as a voltage change.
The change in the manipulation value causes a change in an imaging characteristic that can be influenced by the (at least one) manipulator. The efficacy of a manipulator with respect to a particular imaging aberration is often described by the so-called "sensitivity" of the manipulator to that imaging aberration. The term sensitivity describes the relationship between the defined change in the manipulation value at the manipulator and the effect achieved thereby on the imaging quality or on the lithographic aberration.
In known operation control systems, a change in a manipulated value at a manipulator or an actuator of the manipulator, which is required for performing a desired intervention on the system, is decided based on a control program of a correction algorithm (correction algorithm) having an optimization target function (optimization function). Thus, among other things, the goal is to minimize individual residual aberrations not at the expense of others, but to make a favorable, balanced reduction of all relevant influencing variables.
European patent EP 1251402B 1 describes an operation control system using an objective function. In this case, the objective function describes the quality of the exposure program as a weighted sum of a plurality of "lithographic aberrations". In this case, the term "lithographic aberration" is intended to encompass all defects associated with lithography during imaging. The lithography aberrations include, among others, aberrations such as distortion (distortion) (non-uniform displacement of an image point in an image field), deviation (displacement) of lateral image position (uniform displacement of an image point in an image field), image rotation, asymmetric imaging scale, distortion (deformation) of the focal position (non-uniform displacement of an image point perpendicular to the image plane), as well as variations of critical dimensions (CD variations) over an image field, variations of critical dimensions in mutually orthogonal directions (HV aberrations), etc. Typically, these aberrations are not uniform across the image field, but vary within the image field. Distortion and deformation of the focal plane may result in superimposed aberrations (e.g., superimposed aberrations between different patterns (or mask structures)) and focal aberrations. The lithographic aberration is influenced by various characteristics of the projection exposure apparatus or the projection exposure program, including the substrate, the radiation-sensitive layer on the substrate, the projection radiation provided by the light source, the mask and the projection system.
As the dimensions of structures to be fabricated become smaller and smaller, the lithographic aberrations that are still acceptable in the case of larger structures also become critical. There is therefore a need for an improved control and further reduction of the likelihood of lithographic aberrations in microlithographic projection exposure methods.
Disclosure of Invention
The invention solves the problem of providing a projection exposure method and a projection exposure apparatus for microlithography which allow different optical lithography processes to be performed with low level of lithographic aberrations under different operating conditions. In particular, the aim is to control and limit the possible overlay aberration well. Furthermore, it is an object of the present invention to provide a projection lens which can be used in particular in the context of a projection exposure method and a projection exposure apparatus.
This problem is solved by a projection exposure method comprising the features of claim 1 and by a projection exposure apparatus comprising the features of claim 10. A projection lens having the features of claim 17 provides a further solution. Advantageous developments are specified in the dependent claims. The wording of all the claims is incorporated by reference into the content of the description.
The present invention is based upon the following considerations and insights, among others.
Projection lenses for microlithography are nowadays usually designed as telecentric lenses (telecentric lenses). Telecentric lenses are unique in that the entrance pupil and/or the exit pupil are located at infinity. There are differences between object-side telecentricity, image-side telecentricity and telecentricity on both sides. A beam path telecentric at the object side is used to capture objects without perspective distortion. The entrance pupil is located at infinity so that the chief rays in object space all travel perpendicular to the object plane or parallel to the optical axis. In the case of axial object displacement, the imaging scale (imaging scale) does not change. Thus, the image size is always the same, independent of the object distance. In the case of a beam path that is telecentric on the image side, the exit pupil is located at infinity so that the cone of rays all impinges perpendicularly on the image plane. The beam path telecentric from both sides (double telecentricity) is a combination of a beam path telecentric on the object side and a beam path telecentric on the image side. Both entrance and exit pupils are at infinity; thus, the system is afocal. Theoretically, the image plane can be refocused without changing the imaging scale. A projection lens that is telecentric on both sides is not sensitive to defocus (defocus).
Projection lenses for microlithography, which operate at wavelengths in the DUV range, are nowadays usually designed to be doubly telecentric (telecentric on both sides) in order to meet the requirements on the imaging scale. If the projection lens is telecentric in object space (object-side, object-side telecentricity) and in image space (image-side, image-side telecentricity), the sensitivity of the imaging scale to height adjustments of both the reticle and the wafer is reduced.
In the case of projection lenses for microlithography, which have an operating wavelength in the EUV range (which are composed entirely of mirrors), reflective masks are used which require oblique illumination. At this point, telecentricity is only possible in image space (image-side telecentricity).
During operation of the projection exposure apparatus, individual lens elements and/or other optical elements (e.g. mirrors) of the projection lens are manipulated, for example moved out of their nominal position and/or deformed, for example to compensate for environmental disturbances (e.g. pressure variations and/or other disturbances). In the case of "manipulation" of the optical unit, telecentricity (telecentricity) is ignored in conventional operation control systems. This may cause the telecentricity to be adjusted significantly and, for example, cause a scale error.
Although it is possible to measure the telecentricity of a projection lens in isolation (see, for example, DE102005026628 a1), there is currently no measurement technique for telecentricity control of a projection lens incorporated into a projection exposure apparatus during operation of the projection exposure apparatus according to the knowledge of the inventors.
One known telecentricity calculation is based on the following method: the centroid ray (center in pupil coordinates) is determined by aiming the system stop. The deviation of the centroid ray direction from the desired 90 ° direction is called telecentricity error (telecentricity error). This error is usually expressed in millirads (mrad). Projection lenses used for microlithography typically have a nominal telecentricity error in the range of about 1mrad to 0.1 mrad.
In contrast, known measurement techniques in projection exposure apparatuses are based on wavefront data (e.g. interferometric measurement techniques to determine the phase) and are therefore not compatible with radiation data. In the case of an operation control system based on wavefront measurement, it is impossible to use the sensitivity of the direction of the centroid ray (centroid ray) in the software (lens model) used.
Furthermore, to the best of the inventors' knowledge, the problem of telecentricity has not previously been considered when calculating the change in the value of the manipulation value of the steerable optical element.
According to the claimed invention, the objective function optimized by the control program is modified with respect to a conventional objective function such that the objective function for the at least one manipulator contains a telecentricity sensitivity (telecentricity), wherein the telecentricity sensitivity describes a relationship between a defined change in the manipulation value (defined value change) at the manipulator and a thus achievable influence on the telecentricity of the projection radiation in the image field. Preferably, each manipulator takes into account telecentric sensitivity.
Optimization of the objective function typically involves simulating a plurality of manipulated value changes of the manipulator and calculating its effect on the objective function. If the procedure according to the claimed invention is employed, the influence of the change of the manipulation values at the manipulator on the telecentricity can be taken into account in the optimization of the objective function. What is achieved thereby is, for example, that without telecentricity measurement, the end customer (user of the projection exposure apparatus) is provided with the possibility of recording deviations of the telecentricity from the telecentricity of the transport state and subsequent adjustments made by the manipulator and ensures that the telecentricity remains within predefined specifications.
In the field of geometric optics, Zernike polynomials are commonly used to represent wavefronts, which in turn describe the imaging aberrations of the optical system. In this case, the respective imaging aberrations can be described by the coefficients of Zernike polynomials, i.e., Zernike coefficients or numerical values thereof (in [ nm ]). In a representation customary in the field of lithography, for example, Zernike coefficients Z2 and Z3 represent the tilt of the wavefront in the x-direction and y-direction, respectively, which results in distortion-like aberrations. The Zernike coefficients Z4 describe the curvature of the wavefront and thus the defocus aberration. The Zernike coefficients Z5 describe the saddle-shaped deformation of the wavefront and thus the astigmatic part of the wavefront deformation. The Zernike coefficients Z7 and Z8 represent coma aberration, the Zernike coefficient Z9 represents spherical aberration, and the Zernike coefficients Z10 and Z11 represent third-order aberration and the like.
Heretofore, the Zernike coefficient Z1, which describes a constant displacement of the wavefront, has not been considered in the adjustment of projection lenses for microlithography. This displacement results in a time delay, i.e. a change in the Optical Path Length (OPL) of the ray, rather than a distortion of the wavefront. The Z1 variation over the field only causes aberrations associated with defocus at the image or object plane, not at the normal position (no defocus). For this reason, assume that the field variation of Z1 is within a conventional optical design program (e.g., see
The inventors have recognized that the field profile (or a mathematically equivalent variable) of the Zernike coefficient Z1 can be used as a size map of telecentricity. The field distribution of the Zernike coefficients Z1 describes quantitatively how the Zernike coefficients Z1 vary over the effective image field, i.e. the dependence of Z1 on the position or field coordinates in the image field. The field distribution of the Zernike coefficients Z1 is a variable which has a well-defined calculable relationship with the telecentricity and in this respect defines the telecentricity or makes it calculable from the wavefront data.
It is possible to calculate the sensitivity of the field distribution of the Zernike coefficients Z1 to manipulation and implement it in existing operation control models. This makes it possible to quantitatively determine how the change in the manipulation value at the manipulator affects the field distribution (and thus the telecentricity) of the Zernike coefficients Z1.
The sensitivity of the field distribution of the Zernike coefficients Z1 to movement of the lens elements or to changes in other steering values at the manipulator may be described using a format corresponding to that of conventional manipulator software in a lithographic lens. Thus, implementation in existing systems is possible.
In a projection exposure apparatus comprising a manipulator, one possibility of taking into account changes or variations in the telecentricity under the control of the projection exposure apparatus also comprises storing telecentricity sensitivities, i.e. the sensitivities of the manipulator to changes in the telecentricity, in a correction algorithm of the operating control system, or determining such sensitivities and storing them in a memory of the operating control system and controlling the operation of the projection exposure apparatus under consideration of the telecentricity sensitivities.
If it occurs here that, for example, even a small change in the manipulation value of the manipulator results in a large change in the telecentricity to critical regions, it is possible to limit the change in the manipulation value of the manipulator in view of the telecentricity sensitivity for relatively small magnitudes, so that the lithographic aberrations caused by the telecentricity change remain sufficiently small. This can be achieved by limiting the permissible manipulation value variation of the manipulator by control engineering means to a magnitude below a manipulation value limit, taking into account telecentric sensitivity. This measure has the following effect: the range of permissible manipulation values (the so-called "range" of the manipulator) can be varied when telecentric sensitivity is taken into account, in comparison with projection exposure apparatus in which telecentric sensitivity is not taken into account.
Telecentricity determination of the field distribution equivalent to the Zernike coefficient Z1 is possible, for example if an OPL surface conjugate to the object surface is calculated during optimization of the objective function, which OPL surface is defined by all image points located at an optical distance of constant Optical Path Length (OPL) from the conjugate object point. For example, the distribution of constant displacements of the wavefront of the projection radiation over the image field can be calculated to determine the OPL surface.
The invention provides for the first time the possibility of controlling the telecentricity of a projection lens incorporated into a projection exposure apparatus during operation of the projection exposure apparatus. To this end, the following steps can be performed, for example: determining a starting value of telecentricity at a starting time; calculating a change in telecentricity caused by the adjustment of the manipulator using the value of the change in the operating value of the manipulator and the assigned telecentricity sensitivity; the telecentricity value at the decision time is determined from the start time and the telecentricity variation generated between the start time and the decision time. The starting value can be determined, for example, by measuring the telecentricity at start-up or after readjustment. Thus, a sensitivity-based telecentricity monitor can be realized.
For the case where telecentricity can be measured, it is possible to use the Z1 sensitivity to adjust telecentricity. This means in particular that the stroke of the manipulator can be determined such that the telecentricity has the desired distribution.
It is sufficient to decide with sufficient accuracy in which way and to what extent the telecentricity changes during operation, for example due to a change in the manipulation values on the manipulator. Some embodiments provide for a variation of the telecentricity of the projection lens by actuating at least one dedicated telecentricity manipulator. The term "dedicated telecentricity manipulator" here means a manipulator which can cause a targeted change in the telecentricity of the projection lens in response to control signals operating the control system, wherein the influence on the telecentricity is dominant compared to the influence which may also have on other aberrations, such as distortion and defocus.
The provision of a dedicated telecentricity manipulator for the projection lens or a projection lens with a dedicated telecentricity manipulator may also be a separate invention, independent of telecentricity control or consideration of telecentricity sensitivity.
The invention also relates to a projection lens for microlithography comprising a dedicated telecentricity manipulator, and to a projection exposure apparatus comprising such a projection lens. It is thus possible, if desired, during the adjustment operation or during operation of the projection exposure apparatus to change the telecentricity characteristic of the projection lens in a targeted manner without it being necessary at the same time to likewise change other aberrations (e.g. distortion and defocus) to a relevant extent.
Advantageous concepts of construction of dedicated telecentricity manipulators are explained in more detail below in connection with detailed exemplary embodiments.
Drawings
Further advantages and aspects of the invention are apparent from the claims and the following description of preferred exemplary embodiments of the invention, which is to be read in connection with the accompanying drawings.
Fig. 1 schematically shows how an ideal imaging system converts an input spherical wave to an output spherical wave, where fig. 1A depicts a double telecentric imaging system and fig. 1B depicts a non-double telecentric imaging system;
fig. 2 shows the distribution of surfaces of constant optical path length (OPL surfaces), with fig. 2A and 2B for a double telecentric imaging system with different ray directions and fig. 2C for a non-double telecentric imaging system;
fig. 3 shows a schematic drawing of a surface (OPL surface) showing a constant OPL for a selected field point and for a selected spatial direction (dashed line), wherein the OPL surface is planar in image space in the case of double telecentricity (fig. 3A) and curved in the image in the case of non-double telecentricity (fig. 3B);
FIG. 4 shows a schematic meridional lens element cross-sectional view of a reference system in the form of a catadioptric projection lens;
FIG. 5 schematically shows a distribution plot of Zernike coefficients Z1 as a function of field coordinates x and y of the reference system of FIG. 4;
FIG. 6 shows the average profile of Z1 as a function of the x-coordinate of the reference system of FIG. 4;
FIG. 7 shows the distribution of sensitivity for those optical surfaces of the reference system, where changes in surface shape result in particularly large changes in the Z1 distribution;
FIG. 8 shows in FIG. 8A a comparison of the sensitivity of Z1 versus varying wavelength compared to that in the case of the reference system (see FIG. 6), and the derivative of the function of FIG. 8A in FIG. 8B;
fig. 9 schematically shows the waveform distribution of the principal ray and the coma ray at the manipulator surface of the telecentric manipulator;
FIG. 10 shows an example of a projection lens comprising a dedicated telecentricity manipulator in the form of a modified Alvarez manipulator;
FIG. 11 shows an example of a projection lens comprising a dedicated telecentricity manipulator with two Alvarez lens elements; and
fig. 12 shows a schematic view of a microlithographic projection exposure apparatus according to an exemplary embodiment.
Detailed Description
To explain the background of aspects of the claimed invention in more detail, an explanation of the relationship between the telecentric properties of the optical imaging system and the alternative modes described, such as the field distribution of the Zernike coefficients Z1 or the OPL function, will first be given below.
The textbook "hamilton optics Introduction (An Introduction to hamiltonian optics) (cambridge university press (1970))" by buchdahahl shows that the optical system can be characterized entirely by a scalar function with the name "property". Mixing characteristic W1Particularly for use in imaging systems. This function corresponds to the sum of the optical path length OPL indicated on the direction cosine in the object plane and the spatial coordinates in the image plane with a weighted scalar product of the start position and the start direction:
W1(kx，ky；X，Y)：＝OPL(kx，ky；X，Y)+no(kxx+kyy)
where it is customary to choose to describe the coordinates in object space as lower case letters and the coordinates in image space as upper case letters.
The combination of features forming the ray data follows the fermat principle. In particular, W1Derivative generation with respect to the direction cosine in the object plane and refractive index noThe intersection of the proportional rays in this plane,
and derivative generation with respect to position in image plane with refractive index NBProportional direction cosines in the respective planes.
The ideal aberration-free behavior of an optical system with an imaging ratio β can be written as:
from differential equations if imaging conditions are taken into account
X＝βx and Y＝βy
the aberration of the optical system is described as W1Deviation of the characteristic from the ideal characteristic, wherein
WAberration＝W1-Wideal
This is in the optical design process (e.g. in the case of optical design programsOr
Telecentricity will now be considered. In the previous text on the study of wavefront aberrations, (W)ideal) The term R (X, Y) in the equation is controversial because it does not contribute to imaging quality. However, this item becomes important if statements about the telecentricity of the optical system are intended. In particular, if the ideal characteristics are inserted into the equation
these equations correspond to sinusoidal conditions. They indicate that the change in ray direction in image space is proportional to the change in ray direction in object space. The proportionality constant is the inverse of the imaging proportion β.
Ray (k) starting vertically in the object for a double telecentric optical unitx＝0，ky0) must arrive at the image (K) vertically at the same timex＝0，KY0). These conditions are satisfied if R (X, Y) is a constant. This shows that the telecentricity of the optical system is described by the item R (X, Y).
Furthermore, the ideal characteristics of a double telecentric lens must have the following form:
wherein c represents an arbitrary constant.
Projection lenses (EUV optical units) for microlithography with EUV radiation cannot be doubly telecentric because the masks are reflective. The light beam in such a system starts at the object plane with a fixed chief ray angle a, which is typically a few degrees (e.g. between 3 ° and 10 °, e.g. about 6 °). However, the EUV optical unit is telecentric in the image plane (telecentric on the image side only). For ideal properties, this meets the following requirements:
the solutions to these equations are as follows:
where c is an arbitrary constant. Using similar argument(s), the requirement for an ideal distribution of the function R (X, Y) can also be easily derived for systems that are not telecentric on both sides.
The combination of Zernike expansions can be understood as follows. Wavefront aberration (W)aberration) Usually expanded as Zernike polynomials Zn(kx，ky) Wherein
This expansion has a very attractive explanation since the Zernike polynomials can be interpreted as known image aberrations. For example, Z2And Z3Corresponding to distortion, and Z4Corresponding to the defocus of the optical unit.
Zernike polynomial Z1Corresponding to a constant and a Zernike coefficient c1The field distribution of (X, Y) thus corresponds to the distribution of telecentricity.
All optical design procedures tested by the inventors (a)
A visual representation of the results will be presented below with reference to fig. 1-3. Characteristic W1(kx，ky(ii) a X, Y) depends on four variables making it difficult to represent it. However, if the two variables are fixed, a simple physical explanation can be given.
The incident position (X, Y) is typically chosen to be a fixed representation. This corresponds to the procedure supported by all optical design procedures. In this case, a field point is defined (fixed incident position) and then wavefront aberration at that point is controlled. In this case, as schematically depicted in fig. 1A, the ideal characteristics (for (W)ideal) Equation (c) shows a direction (k) forx，ky) All rays at the beginning, the OPL, are constant. Thus, the surface of the constant OPL (also called OPL surface) is spherical. As a result, a requirement that the ideal optical unit converts the incident spherical wave into the output spherical wave is obtained.
For a fixed incident position (X, Y), the ideal characteristic is the cosine k in the directionxAnd kyDown to a linear function Wideal＝c1kx+c2ky+c0And represents a spherical wave. If it is only in a certain placeConsidering ideal characteristics, it is impossible to distinguish a double telecentric lens (fig. 1A) from a non-double telecentric lens (fig. 1B).
However, if special cases not considered so far are considered, where the direction (k) isx，ky) Fixed, then from the equation for the ideal characteristics of a double telecentric system ((W)ideal-doublyTel) Equation (b) the following requirements are obtained: a planar surface of constant OPL in object space is imaged onto a planar surface in image space. Fig. 2 schematically shows this relationship. For a fixed starting direction (k)x，ky) Ideal characteristics of a double telecentric system
The diagrams in FIGS. 2A and 2B show the diagram for (k)x＝0，ky0) (fig. 2A) and (k)x≠0，kyNot equal to 0) (fig. 2B) surface profile of constant OPL. For having a characteristic
In fig. 3, a representation is selected which depicts a surface of constant OPL (OPL surface) for a selected field point and for a selected spatial direction (dashed line). In this representation, a double telecentric system can be clearly distinguished from a system that is telecentric on the object side.
The dashed lines in the two figures indicate the presence of a pair (k)x＝0，ky0) surface profile of constant OPL. In the case of double telecentricity (fig. 3A), this surface is a plane in image space. For the non-doubly telecentric case (fig. 3B), this surface is curved in the image.
The quantitative aspects regarding the sensitivity of Z1 will be explained below for the design of operational controls and for the design of manipulators that affect telecentricity, according to an example.
Fig. 4 shows a schematic sectional view of a sub-meridian lens element of an embodiment of a catadioptric projection lens PO with a selected beam to illustrate the imaging beam path of the projection radiation through the projection lens during operation. The projection lens IS provided as an imaging system with a reduction effect for imaging the pattern of the mask arranged in its object plane OS onto its image plane IS aligned parallel to the object plane at a reduced scale (for example at a 4: 1 scale). Here, exactly two real intermediate images IMI1, IMI2 are produced between the object plane and the image plane. The first lens portion OP1, which consists only of transparent optical elements and is thus refractive, is designed such that the pattern of the object plane is imaged into the first intermediate image IMI1 substantially without any dimensional changes. The second folded reflective lens portion OP2 images the first intermediate image IMI1 into the second intermediate image IMI2 without substantially changing size. The third refractive lens portion OP3 IS designed to image the second intermediate image IMI2 greatly reduced into the image plane IS.
Pupil surfaces or pupil planes P1, P2, P3 of the imaging system are located between the object plane and the first intermediate image, between the first intermediate image and the second intermediate image, and between the second intermediate image and the image plane, respectively, wherein the principal ray CR of the optical imaging intersects the optical axis OA. The aperture stop AS of the system is attached in the area of the pupil surface P3 of the third lens portion OP 3. The pupil surface P2 in the catadioptric second lens portion OP2 is directly adjacent to the concave mirror CM.
With respect to the optical configuration thereof, the exemplary embodiment shown in fig. 4 is similar to the second exemplary embodiment in WO 2006/121008a1 (corresponding to US2009/092925 a1), modified by comparison therewith.
The catadioptric second lens portion OP2 comprises the only concave mirror CM of the projection lens. The negative group NG with two negative lens elements is located directly upstream of the concave mirror. In this configuration, sometimes referred to as Schupmann achromatization, Petzval correction (i.e., correction of the curvature of the image field) is achieved due to the curvature of the concave mirror and the negative lens element in its vicinity, and color correction is achieved due to the refractive index of the negative lens element upstream of the concave mirror and the position of the stop relative to the concave mirror.
The reflective deflection means serve to separate the beam or a corresponding partial beam path transmitted from the object plane OS to the concave mirror CM from the beam or partial beam path which, after reflection at the concave mirror, passes between the concave mirror and the image plane IS. For this purpose, the deflection device has a planar first deflection mirror FM1 and a planar second deflection mirror FM2, the first deflection mirror FM1 having a first mirror surface (surface 26) for reflecting radiation from the object plane onto the concave mirror CM, the planar second deflection mirror FM2 being aligned at right angles to the first deflection mirror FM1 and having a second mirror surface (surface 36), wherein the second deflection mirror deflects the radiation reflected from the concave mirror in the direction of the image plane IS. Since the optical axis is folded at the deflecting mirror, the deflecting mirror is also referred to as folding mirror in this application. The deflecting mirror is tilted, for example by 45 °, about a tilt axis extending perpendicular to the optical axis and parallel to the first direction (x-direction) with respect to the optical axis OA of the projection lens. For this purpose, the deflection means are realized by prisms, the outer reflection-coated right-angled surfaces of which are oriented perpendicularly to one another and act as deflection mirrors.
The intermediate images IMI1, IMI2 are optically adjacent to the deflecting mirrors FM1 and FM2, respectively, closest to the deflecting mirrors but still at a minimum optical distance therefrom, so that possible defects on the mirror surfaces are not imaged sharp in the image plane, and the planar deflecting mirrors (planar mirrors) FM1, FM2 are located in the region of medium radiation energy density.
The position of the (paraxial) intermediate image defines the field plane of the system optically conjugate to the object plane and to the image plane, respectively. The deflecting mirror is thus optically adjacent to the field plane of the system, which is also referred to as "near field" in the context of the present application. In this case, the first deflection mirror is arranged optically adjacent to a first field plane belonging to the first intermediate image IMI1, and the second deflection mirror is arranged optically adjacent to a second field plane optically conjugate to the first field plane and belonging to the second intermediate image IMI 2.
The optical proximity or optical distance of an optical surface from a reference plane (e.g. a field plane or a pupil plane) is described in this application by the so-called subaperture ratio SAR. For the purposes of the present application, the subaperture ratio SAR of an optical surface is defined as follows:
SAR＝sign h(|r|/(|h|+|r|))
where r denotes the edge ray height, h denotes the chief ray height, and the sign function sign x denotes the sign of x, in accordance with the convention sign 0 ═ 1. The chief ray height is understood to mean the ray height of the chief ray of the field point of the object field which has the greatest field height in number. Ray height is understood to be signed. The marginal ray height is understood to mean the ray height of the ray with the largest aperture starting from the intersection between the optical axis and the object plane. This field point need not facilitate transfer of the pattern disposed in the object plane, particularly in the case of off-axis image fields.
The sub-aperture ratio is a signed variable that is a measure of the field or pupil proximity of the plane in the beam path. By definition, the subaperture ratio is normalized to a value between-1 and +1, wherein the subaperture ratio is zero in each field plane, and wherein the subaperture ratio jumps from-1 to +1 in the pupil plane, and vice versa. Therefore, a subaperture ratio of 1 in absolute value determines the pupil plane.
Then, if the sub-aperture ratios of the two surfaces are comparable in numerical terms, the optical surface or plane is designated as being (optically) close to the "optical reference surface. In particular, an optical surface or plane is designated as "optically" near field "if it has a sub-aperture ratio close to 0. An optical surface or plane is designated as "(optically) near the pupil" if the absolute value of the sub-aperture ratio of the optical surface or plane is close to 1.
For both deflection mirrors, no optical element is indeed arranged between the deflection mirror and the closest intermediate image (in direct proximity), and the subaperture ratio SAR is less than 0.3, in particular less than 0.2, in absolute value.
The projection lens PO has an image-side numerical aperture NA of 1.35. The size of the effective image field is 26mm x22 mm. The telecentricity deviation from the perfect image side telecentricity is less than 1 mrad.
Figure 5 shows the distribution of Zernike coefficients Z1 as a function of the field coordinates x and y of this reference system. These lines are lines of the same value of Z1(X, Y) in microns. Since projection lenses with a slotted image field are typically used for scanning operations, the aberration distribution of the scan (averaged in the scan direction (y-direction)) is particularly important. Fig. 6 shows the average distribution of Z1 as a function of the x coordinate. These figures are primarily used to illustrate the magnitude of the allowed Z1 variation based on a system with well corrected telecentricity. In the case of projection lenses of this type with a telecentricity distribution of the order of 1mrad, an amplitude of Z1 of the order of about 5 μm is correspondingly permitted. It must be emphasized here that the key to the imaging is not the absolute value of the Z1 distribution, but rather its gradient.
To determine how much the changes at the various optical surfaces of the optical system affect telecentricity, i.e. to determine the sensitivity of the various optical surfaces to the Z1 distribution in the projection lens, a value of the form x is added to each optical surface2+y2Has a maximum amplitude of 2 μm, is located in the optically free region of the respective optical surface. The difference between the new Z1 distribution and the distribution of the reference design is then determined in each case. The diagram in fig. 7 shows the ten most sensitive distributions for those optical surfaces for which a change in surface shape and/or a change in positioning or position (e.g., due to displacement parallel or perpendicular to the optical axis or due to tilt) results in a particularly large change in the Z1 distribution. Obviously, the optical surfaces near the intermediate images IMI1, IMI2 at the deflection mirrors FM1, FM2 (see fig. 4) have the highest sensitivity to telecentricity (see fig. 7B). The two deflecting mirrors FM1, FM2 show the highest sensitivity because they are directly close to the intermediate image and because as mirrors they themselves have a higher optical sensitivity.
If one considers that typical steering value variations or the travel of the manipulator may be on the order of 1 μm or 2 μm, then it is immediately apparent from fig. 7 that in the worst case of steering value variations of the manipulator, the sum of the sensitivities may significantly exceed the nominal distribution of Z1. This clearly shows the problem addressed in the present application, namely that telecentricity is influenced by a change in the manipulation value at the manipulator of the projection lens.
In some wavefront manipulation systems, variations in the operating wavelength are also used as manipulators. For example, the wavelength and the resulting change in the refraction time in the case of large changes in the gas pressure in the vicinity of the projection exposure apparatus can be used as a manipulator. Experience has shown that typical values for wavelength variation can be in the range of ± 50 pm. Fig. 8A shows a comparison of the sensitivity of Z1 with respect to varying wavelengths compared to the sensitivity in the case of the reference system (see fig. 6). Fig. 8B shows the derivative of the function of fig. 8A. The derivative corresponds to the directional cosine of the ray in the image plane, denoted mrad. Obviously, during operation, by using a manipulator that varies the wavelength of the radiation used by a specified order of magnitude, the telecentricity may for example be more than doubled compared to a reference telecentricity.
The comprehensive analysis of the inventors, which is explained on the basis of only a few examples, leads in particular to the following insights: (i) typical variations in the manipulation values used to emphasize or compensate for external disturbances (e.g., due to pressure variations) in many systems may be sufficient to significantly interfere with telecentricity. (ii) Since the sensitivity of Z1 is particularly high here, the Z1 distribution can be controlled or set particularly effectively at the near-field optical surface. (iii) The greater the sensitivity of Z1, the smaller the numerical aperture at the optical surface considered respectively.
It is presently believed that it is highly possible to solve this problem by taking into account the Z1 sensitivity (or other relationship suitable as telecentric sensitivity) in the actuation of the manipulators of the operational control system of the projection exposure apparatus. It seems advantageous to limit Z1 to the smallest possible value, for example to the value 0, which would correspond to the ideal state. What is achieved thereby is that the telecentricity is not adjusted arbitrarily to a large extent during operation, but that possible variations in the telecentricity characteristic are limited to relatively non-critical values.
Quantitative analysis showed that Z1 sensitivity is typically on the order of microns. In contrast, many other Zernike coefficients have typical units (usually in the nm range) that are three orders of magnitude smaller. It is therefore advantageous to provide telecentricity or Z1 in the definition of the objective function of the control, which is weighted significantly less than the other Zernike coefficients.
Considerations regarding the design criteria of the dedicated telecentricity manipulator will be explained below. The term "dedicated telecentricity manipulator" here means that this is a manipulator which may cause a targeted change in the telecentricity of the projection lens in response to a control signal of the operational control system, wherein the influence on the telecentricity is dominant compared to the influence which may also have on other aberrations, such as distortion and defocus. In other words: a dedicated telecentricity manipulator allows a targeted variation of the telecentricity, wherein the level of other aberrations (in particular distortion and defocus) that may also be generated is low compared to the level of variation of the telecentricity.
In some applications, it may be advantageous for telecentricity (quantified by Z1) for the manipulator to have a sensitivity at least three orders of magnitude (at least 1000 times) greater than that of Z2/Z3. Still other applications intentionally do not use a projection lens in the best focus position, such as focus drilling. In these cases, among others, a difference in sensitivity of 10-fold or 100-fold may be sufficient.
To clarify these considerations, FIG. 9 schematically shows an excerpt of the projection beam path in the region of field plane FE1 of the projection lens. The field plane may, for example, be an intermediate image plane optically conjugate to the object plane. In the case of ideal optical imaging between the object plane and this field plane, the beam rays emanating from the object field point (field point in the object plane) will intersect at a single intersection point KP at the intermediate image plane FE 1. The projection beam path from left to right in fig. 9 is represented in fig. 9 by a main ray CR (which ideally runs parallel to the optical axis or at a small angle relative to the optical axis in the region of the intermediate image plane) and a coma ray (coma ray) COR, which forms an aperture angle α with the main ray at a point of intersection KP. In this case, the coma ray COR represents a ray transmitted from the field point of the object field to the opposite edge of the aperture stop with respect to the optical axis. The coma ray is an extreme ray of the light beam and may, together with the distribution of the principal ray CR, elucidate the numerical aperture of the projected radiation at the location of the field plane FE 1. The larger the aperture angle α, the larger the numerical aperture in field plane FE 1.
Reference character MS denotes the manipulator surface of the manipulator element of the dedicated telecentricity manipulator. The manipulator surface is first placed in a first operative position of the manipulator in field plane FE1Such that the principal ray and the coma ray intersect at the manipulator surface. Refractive index n of left-hand side (light incident side) of manipulator surface1And n of the right-hand side (light exit side) of the manipulator surface2Different. For example, there may be a gas or vacuum on the light incident side (left) (where n is11) and the manipulator surface MS is the optical surface of a transparent optical element, the material of which has a refractive index n2＞n1. However, n is also possible2＜n1。
If the manipulator surface MS is then displaced a displacement distance dx parallel to the optical axis or chief ray to a position MS' indicated by the dashed line, the chief ray CR will undergo a phase change or a change in optical path length according to:
OPDchief＝dx(n1-n2)＝dxδn
the abbreviation δ n stands for n1And n2The refractive index difference between. In contrast, coma ray COR undergoes a different phase change, which can be expressed as follows:
in this case, the parameter α represents the aperture angle of the coma ray COR with respect to the main ray. The optical path length difference OPD indicated abovechiefWhich can be described by the Zernike coefficient Z1, corresponding to the global phase of the beam.
The difference between the optical path length difference of the principal ray CR and the optical path length difference of the coma ray COR
These considerations also show that the manipulator surface in the region of the intermediate image with a relatively small aperture angle mainly affects the telecentricity, compared to which the influence on deformation and/or defocusing is smaller.
For the field of projection lenses for microlithography, in which the image-side numerical aperture should be relatively high in order to achieve high resolutions, it may be difficult to change the telecentricity to a desired extent primarily without simultaneously changing the defocus and the distortion significantly, by means of a single manipulator element in the field plane or in the optically adjacent field plane.
Conversely, according to the inventors' insight, it is possible to provide a dedicated telecentricity manipulator for a projection lens for microlithography if the projection lens comprises two field planes which are attachable manipulator elements and optically conjugate to each other and in which the coma ray has a different size with respect to the aperture angle (or numerical aperture) of the chief ray. Under these preconditions, the manipulator elements can ideally be changed relative to one another such that the sum of the phase changes of the coma rays disappears, so that the remaining induced aberrations only leave telecentricity or telecentricity changes. The condition under which the sum of the phases of the coma rays disappears can be expressed as follows:
or
In this case, the following relationship leads to a change in the Z1 coefficient or can thus describe a change in the telecentricity:
this shows that it is in principle possible to position the two manipulator elements in different field planes to achieve a pure telecentricity manipulator or a dedicated telecentricity manipulator with the desired effect.It is immediately apparent from the last equation that the aperture angle (aperturable) α if at field planes optically conjugate to each other1And alpha2The difference is large, the effect of the dedicated telecentricity manipulator becomes particularly large.
These insights can be used to configure a dedicated telecentricity manipulator for a truly existing projection lens. This will be explained based on the example of the projection lens PO in fig. 4. Wherein the object plane OS and the image plane IS are field planes optically conjugate to each other. The other field planes optically conjugate thereto are the intermediate image plane of the first intermediate image IMI1 and the intermediate image plane of the second intermediate image IM 12. The total image ratio (between the object plane OS and the image plane IS) IS 4: 1, i.e., 4 times less. The numerical aperture in the region of the object plane OS IS therefore 4 times smaller than the image-side numerical aperture in the region of the image plane IS.
The dedicated telecentricity manipulator may comprise a first manipulator element whose manipulator surface is as close as possible to the object plane OS, for example in a region with a sub-aperture ratio SAR of 0.1 or less. The second manipulator element adapted thereto may be configured to be in direct optical proximity to the image plane IS, for example in the form of a manipulator surface (plate PP) formed on the entry side or exit side of the last optical element on the image side.
A possible practical implementation of the concept in the projection lens PO will be described below with reference to fig. 10. The projection lens PO IS configured such that the pattern arranged in its object plane OS IS imaged into the image plane IS at a reduced imaging scale (e.g., 4: 1 or 5: 1). This is a three-dimensional system with three individual imaging lens sections, where the first lens section forms a first intermediate image IMI1, the first intermediate image IMI1 is imaged by the second imaging lens section as a second intermediate image IMI2, and the second intermediate image is imaged into the image plane with the aid of the third lens section.
The dedicated telecentric manipulator is implemented in the manner of an Alvarez manipulator, which comprises two transparent plate-type manipulator elements ME1, ME2, in which case one of the plate surfaces is planar and the other plate surface (the first manipulator surface) has an aspheric shape that is significantly different from the planar surface. The combination of two manipulator elements may also be referred to as an Alvarez manipulator. The first manipulator element ME1 is arranged directly downstream of the object plane OS optically adjacent to this field plane such that both the planar entrance surface and the aspherical exit surface are located in regions with a subaperture ratio SAR of less than 0.3 or less than 0.2 or even less than 0.1. With the aid of the first actuating device DR1, the first manipulator element ME1 is movable in a plane perpendicular to the optical axis OA.
The first manipulator element is assigned to a second manipulator element ME2, which is part of an Alvarez manipulator and has an aspherical surface similar to the first manipulator element ME 1.
The second manipulator element ME2 IS the last optical element of the projection lens closest to the image plane IS and IS located in a region where the sub-aperture ratio of the two surfaces IS smaller than 0.2 or smaller than 0.1. The second manipulator element ME2 is likewise steerable and actuated upon displacement of the first manipulator element such that its parasitic effects on distortion and defocus are partially or completely compensated or minimized such that essentially only the desired effect on telecentricity is produced.
The two manipulator surfaces MS1 and MS2 assigned to one another are designed with respect to their surface shape such that each surface corresponds to the form of the inverse derivative of the telecentricity error to be corrected. They are complementary in shape to each other (allowing for reduced imaging scale). In the first operating position shown (zero position), the first manipulator element ME1 is positioned relative to the second manipulator element ME2 such that the overall effect of the two manipulator elements on the radiation or wavefront traveling from the object plane to the image plane compensate each other such that the two manipulator elements as a whole do not produce any significant wavefront deformation.
In order to achieve a targeted variation of the telecentricity of the projection lens, the first manipulator element ME1 can be moved perpendicular to the optical axis, for example with the aid of a first actuator DR1, into the shown dashed second operating position ME 1'. For the main ray CR discernible in fig. 10, this has the following effect: the chief ray no longer passes through a relatively thick portion of the Alvarez plate ME1, but passes through a relatively thin portion. With regard to the first manipulator surface MS1, at the position of the chief ray CR, this corresponds to a displacement of the manipulator surface from the shown solid line position to the dashed line position, i.e. parallel to the optical axis OA (see fig. 9 and the associated explanation).
Then, in view of the reduced imaging scale applicable between the object plane and the image plane, the surface shapes of the manipulator surfaces are adapted to each other such that the influence of the displacement of the first manipulator element ME1 on the distortion (Z2/Z3) and on the defocus (Z4) IS compensated to the greatest possible extent by the second manipulator element ME2, while the change in telecentricity or global phase due to the displacement IS not substantially compensated, such that the change in telecentricity in the image plane IS still the resulting change.
Thus, in the case of the variant in fig. 10, the first field plane (which is optically close to the first manipulator element ME1) is the object plane and the second field plane (which is optically close to the second manipulator element ME2) is the image plane, wherein the demagnified imaging scale between these two planes corresponds to the overall imaging scale of the projection lens PO.
Many variations are possible. For example, the first manipulator element ME1 may also be positioned in the region of the first intermediate image plane at the first intermediate image IMI1 or in the second intermediate image plane at the second intermediate image IMI2, provided that the third lens portion imaging the second intermediate image IMI2 to the image plane IS has a sufficiently large demagnified imaging scale. If a reduction imaging is realized overall between the object plane and the intermediate image plane, the second manipulator element can also be arranged in the region of the respective intermediate image plane.
A variation of the embodiment shown in fig. 10 is schematically shown in fig. 11. In this variation, the first Alvarez lens element AL1 IS optically close to the object plane OS, while the second Alvarez lens element AL2 IS located in or near a plane conjugate to the object plane (i.e., the image plane IS). Each Alvarez lens element corresponds to a manipulator element ME1 and ME2, respectively, of the wavefront manipulation system. Optical imaging with dimensional changes (here demagnification) occurs between the Alvarez lens elements. An Alvarez lens element in this sense is an optical element consisting of two plates placed side by side (or in tandem in the beam path) with each other and each having a flat plate surface and an aspherical plate surface. The surfaces of the aspherical surfaces facing each other are complementary aspherical surfaces and form a manipulator surface. The two manipulator surfaces assigned to one another are designed with respect to their surface shape such that each surface corresponds to the form of the inverse derivative of the telecentricity error to be corrected. At the zero position, the result of the optical effect of the plate is global for this Alvarez lens element. Under relative displacement of the plates (e.g., by actuating drivers DR1 and DR2, respectively), an "air lens" is created between the aspheric plate surfaces that has the desired optical effect. An explanation of the basic principle is shown, for example, in the original patent specification US 3,305,294A.
Suitable manipulator elements are not limited to refractive elements that are transmissive or refractive elements that transmit radiation. It is also possible to design one manipulator element or both manipulator elements of a dedicated telecentricity manipulator as a steerable mirror, for example with a deformable mirror surface, which can function as a manipulator surface. A reflective manipulator element with a specularly reflective manipulator surface (e.g. a bendable mirror) may be advantageous for several reasons. First, for a mirror or reflective manipulator surface, the difference in refractive index is n1-n2So that in this respect a higher sensitivity is provided, even small deformations may have a large influence on the telecentricity. Secondly, in the case of the projection lens in fig. 4, the mirror surfaces of the folding mirrors FM1, FM2 are directly approached by, for example, the intermediate image IMI1, IMI2 or a designated intermediate image plane, so that the level of parasitically induced aberrations (which are caused by decentering of the manipulator surface relative to the nearest field plane) is still very low.
The following describes exemplary embodiments of projection exposure apparatuses which are realized in hardware and software.
Fig. 12 shows an example of a microlithographic projection exposure apparatus WSC which can be used for producing semiconductor components and other finely structured components and can be operated using light from the Deep Ultraviolet (DUV) range or electromagnetic radiation to achieve resolutions down to fractions of a micrometer. An ArF excimer laser having an operating wavelength λ of about 193nm is used as the primary radiation or light source LS. Other UV laser sources (e.g. F operating at 157nm2Lasers or ArF excimer lasers operating at 248 nm) are also possible.
In its exit surface ES, the illumination system ILL arranged downstream of the light source LS produces a large, clearly delimited and substantially uniformly illuminated illumination field which is suitable for the requirements of telecentricity of the projection lens PO arranged downstream thereof in the light path. The illumination system ILL has means to set different illumination modes (illumination settings) and it can be switched between conventional on-axis and off-axis illumination with different phase dryness σ, for example. For example, the off-axis illumination modes include annular illumination or dipole illumination or quadrupole illumination or any other multipole illumination. The design of suitable illumination systems is known per se and will therefore not be explained in detail here. Patent application US 2007/0165202 a1 (corresponding to WO 2005/026843 a2) shows an example of a lighting system that can be used within the scope of the various embodiments. In this respect, the disclosure of this patent application is incorporated by reference into the content of the present specification.
Those optical elements which receive light from the light source LS and from which the illumination radiation is shaped (which is directed to the illumination field lying in the exit plane ES or to the reticle M) are part of the illumination system ILL of the projection exposure apparatus
Arranged downstream of the illumination system is a device RS for holding and manipulating a mask M (reticle) such that the pattern PAT arranged at the reticle is located in the region of an object plane OS of the projection lens PO, wherein the object plane OS coincides with the exit plane ES of the illumination system and is also referred to herein as reticle plane OS. For the purpose of the scanning operation, the mask can be moved parallel to this plane in a scanning direction (y-direction) perpendicular to the optical axis OA (z-direction) with the aid of a scanner driver.
The means RS comprise an integrated lifting device for linearly moving the mask in the z-direction, i.e. perpendicular to the object plane, relative to the object plane, and an integrated tilting device for tilting the mask about a tilt axis extending in the x-direction.
Downstream of the reticle plane OS IS a projection lens PO which acts as a reduction lens and images an image of the pattern arranged at the mask M at a reduced scale (for example at a scale of 1: 4(| β | ═ 0.25) or 1: 5(| β | 0.20)) onto a substrate W coated with a photoresist layer, the photosensitive substrate surface SS of which IS located in the region of the image plane IS of the projection lens PO. The projection lens is nominally doubly telecentric, i.e., there is no or little deviation from perfect telecentricity at the object and image sides.
A substrate to be exposed (in the exemplary case a semiconductor wafer W) is held by a device WS comprising a scanner drive to move the wafer in synchronization with the reticle M perpendicularly to the optical axis OA in the scanning direction (y-direction). The device WS further comprises: a lifting device for linearly displacing the substrate in the z-direction relative to the phase line, and a tilting device for tilting the substrate about a tilt axis extending in the x-direction.
The device WS (also referred to as "wafer stage") and the device RS (also referred to as "reticle stage") are constituent parts of a scanner device, which is controlled via a scanning control device, which in an embodiment is integrated in a central control device CU of the projection exposure apparatus.
The illumination field generated by the illumination system ILL defines an effective object field OF used during the projection exposure. In an exemplary case, the effective object field is rectangular, having a height a measured parallel to the scan direction (y-direction) and having a width B > a measured perpendicular thereto (in the x-direction). Typically, the appearance ratio AR ═ B ═ a is between 2 and 10, in particular between 3 and 6. The effective object field is located at a distance in the y-direction adjacent to the optical axis (off-axis field). The effective image field in the image surface IS optically conjugated to the effective object field IS likewise an off-axis field and has the same form as the effective object field and the same aspect ratio between height B and width a, but the absolute field size reduces the imaging ratio β of the projection lens, i.e. a ═ β | a＊and B＝|β|B＊。
If the projection lens IS designed and operates as an immersion lens, the radiation IS transmitted through a thin layer of immersion liquid during operation of the projection lens, which thin layer IS located between the exit surface of the projection lens and the image plane IS. During immersion operation, an image-side numerical aperture NA > 1 is possible. Configurations of dry lenses are also possible; in this case, the image-side numerical aperture is limited to a value NA < 1.
The projection exposure apparatus WSC has an operational control system which is configured to perform a near instantaneous fine optimization of imaging-related properties of the projection exposure apparatus in response to environmental influences and other disturbances and/or on the basis of stored control data. For this purpose, the operating control system has a plurality of manipulators which allow targeted intervention in the projection behavior of the projection exposure installation. Actively actuatable manipulators comprise one or more actuation members (or one or more actuators), whose current manipulation values can be modified by ongoing defined manipulation value changes based on control signals of the operating control system.
The projection lens or projection exposure apparatus IS in particular equipped with a wavefront manipulation system WFM which IS configured to modify the wavefront of the projection radiation travelling from the object plane OS to the image plane IS in a controllable manner within the meaning that the optical effect of the wavefront manipulation system can be variably adjusted by control signals of the operation control system.
The wavefront manipulation system in the exemplary embodiment has a plurality of mutually independently drivable manipulators MAN1, MAN2, etc., each having at least one manipulator element ME1, ME2, etc., which are arranged in the projection beam path of the projection lens and have (at least one) manipulator surface MS1, MS2 which is arranged in the projection beam path and whose position (position) and/or orientation and/or surface shape and/or refractive index distribution can be reversibly changed with the aid of actuating devices DR1, DR2, etc. For example, the manipulator may be designed for decentration or displacement of the optical element parallel or perpendicular to a reference axis, tilting of the optical element, local or global heating or cooling of the optical element and/or deformation of the optical element. The term "manipulator" also encompasses devices which act on the mask or the substrate, for example to displace, tilt and/or deform the mask or the substrate, on the basis of corresponding control signals of the operating control system.
In the memory SP of the operation control system, the sensitivities S (Z1), S (Z2),. and S (zn) of the manipulator for a plurality of aberrations are stored, which are in each case represented by the associated Zernike coefficients Z1 (for telecentricity), Z2 (for the tilt of the wavefront in the x-direction), etc. A set of dedicated sensitivities is stored for each manipulator. Importantly, in addition to the sensitivities stored in some conventional systems, the telecentricity sensitivity S (Z1) of the manipulator for changing the telecentricity is also stored here. Telecentric sensitivity quantitatively describes the relationship between the defined change in the manipulation value at the manipulator and the effect achieved thereby on telecentricity in the image field. Thus, the operation of the projection exposure apparatus can also be controlled taking into account the telecentric sensitivity, for example in that the change in the manipulation value of the manipulator is limited to a magnitude below a manipulation value limit, taking into account the telecentric sensitivity.
When deciding that the manipulator value of the manipulator changes, the operation control system uses an objective function that describes the quality of the exposure process as a weighted sum of a plurality of "lithographic aberrations". In this case, the term "lithographic aberration" is intended to encompass all defects associated with lithography during imaging. The lithography aberration includes, among others, aberrations such as distortion (uneven displacement of an image point in an image field), deviation of a lateral image position (even displacement of an image point in an image field), image rotation, asymmetric imaging ratio, distortion of a focal position (uneven displacement of an image point perpendicular to an image plane), and the like, and variations in critical dimension (CD variations) on an image field, differences in critical dimension in mutually orthogonal directions (HV aberrations), and the like. In general, these aberrations are not uniform across the image field, but vary within the image field. Distortion and deformation of the focal plane may lead to overlay aberrations (e.g., overlay aberrations between different patterns (mask structures)) and focal aberrations. Lithographic aberrations are affected by various characteristics of the projection exposure apparatus or process, including the substrate, the radiation-sensitive layer on the substrate, the projection beam provided by the light source, the mask and the projection system.
In the case of the projection exposure apparatus WSC, the objective function optimized by the control program for each manipulator contains a telecentricity sensitivity S (Z1) which describes the relationship between the defined change in the manipulator value at the manipulator and the effect achievable thereby on the telecentricity of the projection radiation in the image field. Optimization of the objective function typically involves simulating many manipulated value changes of the manipulator and calculating its effect on the objective function. Since telecentricity sensitivity is also taken into account, the influence of the change in the manipulation value at the manipulator on the telecentricity can be taken into account in the optimization of the objective function. What is achieved thereby is, for example, that, without telecentricity measurement, the end customer (user of the projection exposure apparatus) is provided with the possibility of recording deviations of the telecentricity from the telecentricity of the transport state and subsequent adjustments by the manipulator and ensures that the telecentricity remains within predefined specifications. The predefined specification may be such that the image-side telecentricity is kept in the range of less than 20mrad, with preferred values of the image-side telecentricity being in the range of less than 10mrad, the latter limitation being particularly applicable to DUV systems.
The projection lens may be equipped with a dedicated telecentricity manipulator of the type described in the present application to intervene specifically on the telecentricity characteristics of the projection lens. However, this is not mandatory.
In principle, a lithography optical unit in the EUV range cannot be doubly telecentric, since no transmissive reticle is present in this wavelength range. Therefore, these optical systems are only telecentric (at the wafer) in their design. The insights and concepts described here can also be used in projection exposure apparatuses for microlithography using EUV radiation.
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