阅读说明：本技术 光刻设备和方法 (Lithographic apparatus and method ) 是由 H·巴特勒 于 2018-11-29 设计创作，主要内容包括：一种光刻设备包括投影系统,所述投影系统包括位置传感器用于测量投影系统的光学元件的位置。位置传感器以传感器框架为参考。阻尼致动器阻尼传感器框架的振动。控制装置驱动致动器,并且配置成从加速度信号和传感器框架位置信号中的至少一种信号导出传感器框架阻尼力信号,从位置信号导出估计的视线误差,根据传感器框架阻尼力信号和估计的视线误差确定致动器驱动信号,使用致动器驱动信号驱动致动器,以阻尼传感器框架并至少部分地补偿估计的视线误差。(A lithographic apparatus includes a projection system including a position sensor to measure a position of an optical element of the projection system. The position sensor is referenced to the sensor frame. The damping actuator damps vibrations of the sensor frame. A control device drives the actuator and is configured to derive a sensor frame damping force signal from at least one of the acceleration signal and the sensor frame position signal, derive an estimated line of sight error from the position signal, determine an actuator drive signal from the sensor frame damping force signal and the estimated line of sight error, drive the actuator using the actuator drive signal to damp the sensor frame and at least partially compensate for the estimated line of sight error.)

1. A lithographic apparatus, comprising:

a projection system configured to project the patterned radiation beam onto a target portion of the substrate,

wherein the projection system comprises

A plurality of optical elements configured to optically interact with the patterned radiation beam to project the patterned radiation beam onto a target portion of the substrate,

a plurality of optical element position sensors for sensing the position of the optical element,

a sensor frame to which the optical element position sensor is mounted, the optical element position sensor configured to measure a position of an optical element relative to the sensor frame,

the force frame is provided with a force frame,

a plurality of vibration isolators connected between the force frame and the sensor frame, the force frame configured to support the sensor frame via the vibration isolators, the vibration isolators configured to isolate the sensor frame from vibrations in the force frame,

a plurality of sensor frame position sensors configured to measure a position of the sensor frame relative to the force frame,

a plurality of acceleration sensors configured to measure acceleration of the sensor frame, an

A plurality of actuators disposed between the sensor frame and the force frame and configured to apply a force between the sensor frame and the force frame, and

the lithographic apparatus further comprises a control device comprising:

an acceleration sensor input connected to the acceleration sensor input to provide an acceleration signal to the control device indicative of an acceleration of the sensor frame,

a position sensor input to which the sensor frame position sensor is connected to provide a position signal to the control device indicative of a position of the sensor frame relative to the force frame,

an actuator output connected to the actuator to enable the control device to drive the actuator, wherein the control device is configured to:

deriving a sensor frame damping force signal from at least one of the acceleration signal and the position signal,

an estimated line-of-sight error is derived from the position signal,

determining an actuator drive signal based on the sensor frame damping force signal and the estimated line of sight error,

driving the actuator using the actuator drive signal to dampen the sensor frame and at least partially compensate for the estimated line-of-sight error.

2. The lithographic apparatus of claim 1, wherein deriving the estimated line-of-sight error from the position signal comprises:

deriving an estimated vibration isolator induced sensor frame force from the position signal,

deriving the estimated line-of-sight error from the estimated vibration isolator induced sensor frame force.

3. The lithographic apparatus of claim 1 or 2,

wherein the sensor frame damping force signal is a sensor frame damping force signal of N degrees of freedom,

wherein the estimated line of sight error is an estimated line of sight error in M degrees of freedom,

wherein the actuator drive signals are N + M actuator drive signals,

wherein a plurality of the actuators are N + M actuators, and

wherein the actuator drive signal is determined from the sensor frame damping force signal and the estimated line of sight error using an N + M by N + M matrix.

4. The lithographic apparatus of claim 3, wherein N-6 and M-2, and wherein the estimated line-of-sight error is in 2 directions defining a plane substantially parallel to a target portion of the substrate.

5. The lithographic apparatus of claim 3 or 4, wherein the control device is further configured to:

frame stiffness matrix (K) by using N-by-N sensors_vis) Deriving an estimated sensor frame force due to the vibration isolator from the position signal, the N by N sensor frame stiffness matrix representing an N-dimensional sensor frame force as a function of an N-dimensional position signal.

6. The lithographic apparatus of any of claims 3-5, wherein the control device is further configured to:

an estimated line of sight error is derived from the estimated vibration isolator induced sensor frame force by using an M by N sensor frame force to line of sight error matrix (Q) representing an M dimensional estimated line of sight error as a function of an N dimensional sensor frame force.

7. The lithographic apparatus of any of claims 2-6, wherein the control device is further configured to: estimating a sensor frame force induced by the vibration isolator from the position signal by using a predetermined sensor frame displacement sensitivity function.

8. The lithographic apparatus of any of claims 2-7, wherein the control device is further configured to: deriving the estimated line of sight error from the estimated sensor frame force induced by the vibration isolator by using a line of sight sensitivity function representing line of sight error as a function of sensor frame force.

9. A method of reducing line-of-sight errors in a lithographic apparatus according to any of claims 3 to 6, comprising:

-determining the actuator drive signal from the sensor frame damping force signal and the estimated line of sight error by using an N + M by N + M matrix (G), the N + M by N + M matrix being determined by:

-providing an N by N + M matrix (V) representing forces of N degrees of freedom acting on the sensor frame resulting from the actuation of the N + M actuators,

-providing an M by N + M matrix (W) representing M dimensional line of sight errors due to the forces of the N + M actuators,

-combining the N by N + M matrix (V) and the M by N + M matrix (W) into an N + M by N + M intermediate matrix,

-determining the N + M by N + M matrix (G) as the inverse of the N + M by N + M intermediate matrix.

10. The method of claim 9, wherein an N + M by N matrix portion of the N + M by N + M matrix (G) contributes to the actuator drive signal to drive the N + M actuators without causing line-of-sight errors.

11. The method according to claim 9 or 10, comprising:

determining the N by N + M matrix (V) from the torques of N degrees of freedom acting on the sensor frame due to the actuation of each of the N + M actuators, respectively,

determining the M by N + M matrix (W) from the M-dimensional line of sight error due to forces of the N + M actuators.

12. The method of claim 11, wherein determining the M by N + M matrix (W) from the M dimensional line of sight error due to forces of the N + M actuators is performed above a resonant frequency of the vibration isolator.

13. The method of claim 11, wherein determining the M by N + M matrix (W) from the M-dimensional line of sight error due to forces of the N + M actuators comprises subtracting a contribution to the line of sight error due to the sensor frame forces caused by exciting the N + M actuators according to the N + M actuator drive signals.

Technical Field

The present invention relates to a lithographic apparatus and a method of reducing line of sight errors in such a lithographic apparatus.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). In such cases, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. The pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Typically, the transfer of the pattern is performed by imaging the pattern onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners; in a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

The lithographic apparatus includes a projection system configured to project a patterned beam of radiation onto a target portion of a substrate. Projection systems include optical elements such as reflective optical elements (e.g., mirrors) or transmissive optical elements (e.g., lenses). The optical element may be positioned by an optical element actuator: the optical element actuator may for example be driven to counteract vibrations, thermal effects, positioning inaccuracies etc. Thus, the position of the optical element is measured using a position sensor. In order to detect the position of the optical element, a position sensor may be provided to measure the position of the optical element. The optical element actuator is arranged between the force frame and the optical element, i.e. acts between the force frame and the optical element. In order to provide accurate position sensing of the position of the optical element, a sensor frame is provided, the position sensor sensing the position of the optical element relative to the sensor frame. The sensor frame is supported by the force frame through the use of vibration isolators. The vibration isolator is therefore intended to keep the vibrations acting in the force frame away from the sensor frame, thereby keeping the disturbance as far away as possible from the position sensor sensing the position of the optical element.

A problem associated with the sensor frame is that vibrations acting in the force frame and movements of the force frame can be converted into disturbing forces on the sensor frame via the vibration isolators. When the position sensor measures the position of the optical element relative to the sensor frame, disturbing forces on the sensor frame may be translated into position errors. The sensor frame actuator may be arranged to damp the sensor frame, e.g. to counteract an acceleration of the sensor frame and/or a displacement of the sensor frame. To this end, an acceleration sensor and/or a sensor frame position sensor may be provided to measure the acceleration response. The positions of the sensor frame and the sensor frame actuator may be driven to at least partially compensate for such acceleration and/or displacement, thereby attempting to compensate for such disturbances on the sensor frame.

However, the inventors have observed that attempts to compensate for such disturbances on the sensor frame appear to have the opposite effect and result in an overall reduction in projection accuracy rather than the expected increase in projection accuracy. In particular, the line of sight error appears to increase, rather than decrease as intended.

Disclosure of Invention

It is desirable to facilitate keeping line-of-sight errors low.

According to an aspect of the invention, there is provided a lithographic apparatus comprising:

a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the projection system comprises:

i. a plurality of optical elements configured to optically interact with the patterned radiation beam to project the patterned radiation beam onto a target portion of the substrate,

a plurality of optical element position sensors,

a sensor frame to which the optical element position sensor is mounted, the optical element position sensor configured to measure a position of an optical element relative to the sensor frame,

(iv) a force frame,

v. a plurality of vibration isolators connected between the force frame and the sensor frame, the force frame configured to support the sensor frame via the vibration isolators, the vibration isolators configured to isolate the sensor frame from vibrations in the force frame,

a plurality of sensor frame position sensors configured to measure a position of the sensor frame relative to the force frame,

a plurality of acceleration sensors configured to measure acceleration of the sensor frame, an

A plurality of actuators disposed between the sensor frame and the force frame and configured to apply a force between the sensor frame and the force frame, and

the lithographic apparatus further comprises a control device comprising:

an acceleration sensor input connected to the acceleration sensor input to provide an acceleration signal representative of an acceleration of the sensor frame to the control device,

a position sensor input, the sensor frame position sensor connected to the position sensor input to provide a position signal to the control device indicative of a position of the sensor frame relative to the force frame,

an actuator output connected to the actuator to enable the control device to drive the actuator,

wherein the control device is configured to:

deriving a sensor frame damping force signal from at least one of the acceleration signal and the position signal,

deriving an estimated line-of-sight error from the position signal,

determining an actuator drive signal from the sensor frame damping force signal and the estimated line of sight error,

xv. drive the actuator by using the actuator drive signal to damp the sensor frame and at least partially compensate for the estimated line of sight error.

According to another aspect of the invention, there is provided a method of reducing line of sight errors in a lithographic apparatus according to the invention, the lithographic apparatus comprising:

wherein the sensor frame damping force signal is a sensor frame damping force signal of N degrees of freedom,

wherein the estimated line of sight error is an estimated line of sight error in M degrees of freedom,

wherein the actuator drive signals are N + M actuator drive signals,

wherein a plurality of the actuators are N + M actuators, and

wherein the actuator drive signal is determined from the sensor frame damping force signal and the estimated line of sight error using an N + M by N + M matrix,

the method comprises the following steps:

-determining the actuator drive signal from the sensor frame damping force signal and the estimated line of sight error by using an N + M by N + M matrix (G), the N + M by N + M matrix being determined by:

-providing an N by N + M matrix (V) representing forces of N degrees of freedom acting on the sensor frame due to the actuation of the N + M actuators,

-providing an M by N + M matrix (W) representing M dimensional line of sight errors due to the forces of the N + M actuators,

-combining the N by N + M matrix (V) and the M by N + M matrix (W) into an N + M by N + M intermediate matrix,

-determining the N + M by N + M matrix (G) as the inverse of the N + M by N + M intermediate matrix.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 depicts a lithographic apparatus in which the invention may be embodied;

FIG. 2 depicts a schematic view of a projection system of a lithographic apparatus according to an embodiment of the invention;

FIG. 3 depicts a control scheme of the lithographic apparatus according to FIG. 2;

FIG. 4 depicts a control scheme on which the dimensioning of the lithographic apparatus according to an embodiment of the invention will be explained; and is

FIG. 5 depicts a frequency map on which possible effects of a lithographic apparatus according to an embodiment of the invention will be illustrated.

Detailed Description

FIG. 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or EUV radiation);

a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The support structure supports (i.e. bears) the weight of the patterning device. The support structure holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

The term "patterning device" used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the matrix of mirrors.

The term "projection system" used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

As depicted here, the apparatus is of a reflective type (e.g. employing a reflective mask). Alternatively, the apparatus may be of a transmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type: wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not mean that a structure such as a substrate must be submerged in a liquid; in contrast, "immersion" means only that liquid is located between the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. For example, when the source is an excimer laser, the source and the lithographic apparatus may be separate entities. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases, the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ -outer and σ -inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. IN addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

The depicted apparatus can be used in at least one of the following modes:

in step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C that is imaged in a single static exposure.

In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

FIG. 2 depicts a part of a projection system that may be employed in the lithographic apparatus according to FIG. 1. The projection system is configured to project a patterned beam of radiation onto a target portion of the substrate. The patterned radiation beam may be generated from any radiation source, such as an ultraviolet radiation source, a UV radiation source, a deep ultraviolet radiation source, a DUV or extreme ultraviolet radiation source, an EUV radiation source, i.e. a radiation source emitting UV radiation, DUV radiation or EUV radiation, respectively. The radiation beam may be patterned using any suitable patterning technique, such as a patterning device (e.g., a mask) or other patterning. Projection systems include optical elements such as reflective optical elements (e.g., mirrors) or transmissive optical elements (e.g., lenses, prisms, etc.), or a combination of transmissive and reflective optical elements. The optical transfer function of the projection system is determined, inter alia, by the optical properties of the optical elements and the positions of the optical elements.

The optical element may be positioned by an optical element actuator: the optical element actuator may for example be driven to counteract vibrations, thermal effects, positioning inaccuracies etc. to keep the optical element in its intended position. Thus, the position of the optical element is measured. In order to detect the position of the optical element, an optical element position sensor may be provided. The optical element actuator is arranged between, i.e. acts between, the force frame FFR of the projection system and the optical element. The force frame may be subject to disturbances, such as forces generated by the actuator. In order to enhance accurate position sensing of the position of the optical element, a sensor frame SFR is provided, the optical element position sensor sensing the position of the optical element relative to the sensor frame. The sensor frame is supported by the force frame by using a vibration isolator VIS. The vibration isolator is thus intended to distance the vibrations acting in the force frame away from the sensor frame, and thus the disturbance away from the position sensor.

The acceleration sensor ACC is provided on the sensor frame. The acceleration sensor senses an acceleration of the sensor frame. The acceleration sensor may also be constituted by a speed sensor, for example a time derivative thereof, if the speed signal of the speed sensor provides an acceleration signal. For example, a geophone may be applied. Alternatively or additionally, the sensor frame position sensor POS is arranged on the sensor frame. The sensor frame position sensor senses a position of the sensor frame. Based on the acceleration signal provided by the acceleration sensor and/or the position signal of the sensor frame position sensor, the control device CD of the lithographic apparatus calculates a damping force signal representing a damping force to be applied on the sensor frame to damp and/or position the sensor frame. In addition, an actuator ACT is provided between the sensor frame and the force frame. The control is performed by a control device CD comprising: a position sensor input PSI through which a position signal from the position sensor is input; an acceleration sensor input part ACI through which an acceleration signal from an acceleration sensor is input; and an actuator output AO via which the actuator ACT is driven. For example, the control device may derive the sensor frame damping force signal by using as an input the second derivative of the acceleration signal or the position signal from the acceleration sensor, which signal represents the force to be exerted on the sensor frame to damp the sensor frame. The acceleration sensor may for example comprise 6 sensors, each sensor exhibiting one degree of freedom, and may together provide an acceleration signal of the sensor frame exhibiting 6 degrees of freedom. The control device can also use the position signal to control the position of the sensor frame by providing a sensor frame damping force accordingly.

Since the projection system may be relatively large, the sensor frame may span a relatively large size. Thus, vibration isolation, measurement of acceleration of the sensor frame, and actuation of forces on the sensor frame for damping may be extended to different parts of the sensor frame. For example, the sensor frame may be substantially rectangular. Four vibration isolators may be provided to isolate the sensor frame, each at a corner of the sensor frame. Furthermore, the acceleration sensor may provide an acceleration signal of six degrees of freedom. In addition, eight actuators ACT may be provided to apply force to the sensor frame. For example, there are 2 actuators near each vibration isolator. For example, in the vicinity of each vibration isolator, an actuator that applies a force in a vertical direction and an actuator that applies a force in a direction in a horizontal plane are provided. Actuators that exert a force in a direction within the horizontal plane may actuate in different directions within the horizontal plane, e.g., two actuators exert a force in one direction and two other actuators exert a force in a direction perpendicular to that direction.

However, as mentioned above, the inventors have observed that attempts to compensate for such disturbances on the sensor frame by damping the sensor frame acceleration appear to have the opposite effect and result in an overall reduction in projection accuracy rather than the expected improvement in projection accuracy. In particular, the line of sight error appears to increase, rather than decrease as intended.

Fig. 3 depicts a schematic block diagram on the basis of which the operation of the control device will be explained. Fig. 3 depicts that the sensor frame damping controller SFDC determines the sensor frame damping force signal Fact from the acceleration signal ACC and/or the position signal POS. Any suitable damping function may be applied, for example, based on acceleration (or velocity) and/or position. A combined damping/positioning may be provided. The sensor frame damping force signal is provided to a matrix G, i.e., a matrix calculation as part of the calculation input. Other inputs to the matrix calculation are described below.

A position signal indicative of the position of the sensor frame is applied to estimate the force acting on the sensor frame caused by the vibration isolator. To this end, it may be assumed that the force depends linearly on the position (e.g., assuming a linear behavior of the vibration isolator). Note that any other relationship, such as a quadratic relationship or the like, may be used here. The force exerted by the vibration isolator on the sensor frame is estimated by using the matrix Kvis based on the stiffness of the vibration isolator. The estimated force acting on the sensor frame is input into a further determination, i.e. the line of sight error due to the force acting on the sensor frame is determined by using the matrix Q. Line-of-sight errors may be understood as position errors at the substrate level. An estimate of the line of sight error from the force may be derived from the modeling or measurements, as will be explained in more detail below. It is noted that instead of a two-step approach (in which the line-of-sight error is derived from the estimated sensor frame force, which in turn is derived from the position signal, i.e. a concatenation of matrices Kvis and Q), the line-of-sight error may be derived directly from the position signal, e.g. using a single sensitivity function or matrix.

Both the estimated line-of-sight error resulting from the (sensor frame) position signal and the calculated sensor frame force are now input into the matrix G to calculate an actuator drive signal that is representative of the force applied by the actuator to the sensor. Thus, the actuator force to be applied to the sensor frame is derived from the sensor frame damping force in combination with the estimated line of sight error due to the position of the sensor frame.

Thus, the matrix G that may be applied to calculate actuator forces from the 6 degrees of freedom sensor frame damping force (e.g., 8 actuators) may be expanded by two additional inputs to provide an 8 by 8 matrix. Two additional inputs are used to compensate for line-of-sight errors resulting from deformation of the sensor frame due to forces applied to the sensor frame by the vibration isolators. An expected line of sight error is calculated based on the position measurement of the sensor frame relative to the force frame. The expected line of sight error is input to two additional inputs of the matrix G. As a result, the actuator may cause deformation of the sensor frame that counteracts the deformation caused by the stiffness of the vibration isolator. For this purpose, two sensitivity matrices are required, on the one hand the relationship between the sensor frame forces caused by the vibration isolators and the line of sight errors, and on the other hand the relationship between the actuator forces and the line of sight errors. The described concept is based on the insight that: there is a large difference in the relationship between the vibration isolator force and the line of sight error (on the one hand) and the actuator force and the line of sight error (on the other hand). The relationship may be measured at the substrate stage using a sensor, such as a so-called transmission image sensor, to obtain an indication of line of sight error while causing relative frame displacement (e.g., using an air cushion shock absorber (airmount)) and while applying a force through an actuator.

The above-determined effect of the actuator drive signal may be twofold: on the one hand, the estimated line of sight error occurring due to forces on the sensor frame can be compensated for, since the estimated line of sight error is input into the calculation: thus, the force applied by the actuator to the sensor frame may compensate for the estimated line-of-sight error.

Second, the other input to the calculation, the sensor frame damping force signal, may no longer produce line of sight errors because the 6 degrees of freedom actuator force is within the null space of the line of sight error.

Thus, damping may be performed on the sensor frame while accounting for estimated line of sight errors due to sensor frame forces caused by the vibration isolators. The sensor frame damping force signal may be an N degree of freedom sensor frame damping force signal. The estimated line of sight error may be an estimated line of sight error for M degrees of freedom and the actuator drive signal may be N + M actuator drive signals that drive N + M actuators. As a result, the actuator drive signal may be determined from the sensor frame damping force signal and the estimated line of sight error by using an N + M times N + M matrix (matrix G). Thus, an additional degree of freedom can be utilized, since the number of actuators acting on the sensor frame exceeds the degree of freedom of the sensor frame to damp the force signal. In fact, the difference between the two enables to feed the estimated force signal to the matrix with as many degrees of freedom as the difference between the number of actuators acting on the sensor frame on the one hand and the degrees of freedom of the sensor frame to damp the force signal on the other hand.

For example, N may be 6 and M may be 2. Thus, the sensor frame damping force signal may be provided in 6 degrees of freedom, while applying two additional actuators (thus 8 actuators) and estimating the line of sight error in two degrees of freedom. The line-of-sight error may be estimated in a plane (e.g., a horizontal plane) that is substantially parallel to the surface of the target portion of the substrate. The largest line of sight error is expected to be in a plane parallel to the substrate, and therefore a corresponding reduction in line of sight error may be provided by using two additional inputs for line of sight estimation.

As depicted in fig. 3, the estimation of the sensor frame force is performed by using a stiffness Kvis representing the stiffness of the vibration isolator. Stiffness may be derived from measurements and/or from simulations, such as finite element model simulations. For example, the position of the sensor frame may be input as a 6 degree of freedom position from which 6 degrees of freedom force is derived. Thus, the matrix Kvis may be a 6 by 6 matrix. The estimated force applied to the sensor frame (e.g. in 6 degrees of freedom) is provided to a matrix Q from which an estimated line of sight error LOS in two dimensions is calculated. Thus, in the example of a sensor frame force of 6 degrees of freedom, the matrix Q would form a 2 by 6 matrix. Because 6 degrees of freedom of force may be considered and because there are 6 rigid body degrees of freedom, using 6 degrees of freedom of sensor frame force to calculate 2 degrees of freedom of line of sight error may yield an accurate model. The derivation of the values of the matrix Q will be described below with reference to fig. 4.

As depicted in fig. 3, both the sensor frame damping force signal and the estimated line of sight error are provided to a matrix G from which the actuator drive signals are calculated. The derivation of the values of matrix G from matrices V and W is described below with reference to fig. 4. In the example of a sensor frame damping force signal of 6 degrees of freedom, an estimated line of sight error of 2 degrees of freedom, and 8 sensor frame actuators, the matrix X would accordingly form an 8 by 6+2 matrix, i.e., an 8 by 8 matrix.

The dimensions of matrix Q and matrix G (according to V and W) will now be described with reference to fig. 4. FIG. 4 depicts how the position of the sensor frame translates into line of sight error, and the sensor frame damping force translates into line of sight error in the model described above, as follows:

FIG. 4 depicts position u_b，u_bIs the sensor frame position relative to the force frame, caused by the movement of the base frame, i.e. independent of the actuator force, but from an external source. Via transfer function H_ufAdding disturbances due to the actuator, resulting in a sensor frame u_SfrThe position of (a). Signal f_actsRepresenting the force of 8 actuators. By dynamic function H_ufThis results in a relative movement of the sensor frame which is added to the position u caused by the base frame_b. Position u of the sensor frame_SfrMeasured by the sensor frame position sensor POS in fig. 2. By K_visThe sensor frame position is converted into an estimated sensor frame force F_SFr. EstimatingVia the matrix Q into an estimated line of sight error LoS1₁. The matrix Q is also mentioned above in connection with fig. 3. Sensor frame force as a function of sensor frame position, i.e. K_visMay be derived from modeling and/or from measurements. Determine K_visThe matrix Q can now be derived by measuring the position of the sensor frame, while simultaneously measuring the line-of-sight error as a function of the position of the sensor frame. To position the sensor frame, the sensor frame may be actuated by an actuator.

FIG. 4 further depicts a sensor frame damping force F_actDamping force F of the sensor frame_actIs provided to a matrix G for determining actuator drive signals therefrom. Note that in the configuration according to fig. 4, the line-of-sight input of matrix G remains zero. As described above, matrix G is determined by matrix V and matrix W. The matrix V may be determined as follows: for each of the eight actuators, a 6 degree of freedom force acting on the sensor frame resulting from actuation by the actuator is determined. This provides a total of 8 by 6 numbers, i.e. 8 actuators and associated 6 degrees of freedom of force on the sensor frame. Said number provides a 6 by 8 matrix which can be derived from the geometry of the sensor frame, in particular the location of the actuators. Having determined the actuator force from the matrix V, the actuator force may result in a second line of sight error LoS₂As a result of the matrix W. Since the line of sight error is estimated as 2 degrees of freedom, and in the case of 8 actuators being driven to damp the sensor frame, the matrix W may form a 2 by 8 matrix. The matrix W may be determined in two ways. The movement of the sensor frame is also provided due to the actuation of the actuator, i.e. via H_ufThe transfer function, the actuation of the actuator, may cause further line-of-sight errors, i.e. due to sensor frame forces (due to movement of the sensor frame through the vibration isolator). A first possibility is to apply the actuator force above the cut-off frequency of the vibration isolator. For example, a vibration isolator may exhibit second order damping, resulting in a 16-fold reduction in response at 4 times the damping (resonant) frequency.

A second possibility is to correct line-of-sight errors caused by the vibration isolators. Since K has already been determined_visAnd Q, so this is possible. Thus, a line of sight error due to actuation of the sensor frame actuator may be determined, thus providing a line of sight error LoS that relates 8 actuator forces of 8 actuators to two degrees of freedom₂A correlation matrix. The matrix V and the matrix W are now known, and both the relationship between the sensor frame damping force and the actuator force (i.e., matrix V) and the relationship between the actuator force and the line of sight (matrix W) are known. It is now possible to obtain the relationship between the sensor frame damping force and the estimated line of sight error on the one hand and the actuator drive signal, i.e. the matrix G, on the other hand by combining the matrix V (6 by 8) and the matrix W (2 by 8) into an 8 by 8 intermediate matrix and inverting this 8 by 8 intermediate matrix.

In general, based on the damping forces of the sensor frame for N degrees of freedom, the estimated line of sight error for M degrees of freedom, and N + M actuators, matrix V may be M by N + M, matrix W may be N by N + M, matrix Q may be N + M by M, and matrix G may be N + M by N + M.

As described above, the matrix G is sized to be prescribed so as not to introduce a line-of-sight error. In other words, when converting the sensor frame damping force into an actuator drive signal, no additional forces are generated on the sensor frame that would cause line of sight errors, since the 6 degrees of freedom actuator forces are located within the null space of the line of sight error.

Any other dimensioning of the matrix V may also be applied. In this case, the additional line-of-sight error may be caused by a force acting on the sensor frame by the actuator. This line of sight error may be considered as such, i.e. included in the estimated line of sight error input to the matrix G, so as to be substantially compensated for. Fig. 5 depicts a frequency diagram on which the possible effects of the concept described above will be explained.

In fig. 5, the sensitivity Se in the horizontal direction (line of sight error as a function of VIS-induced force) is plotted along the vertical axis as a function of frequency f along the horizontal axis. The dashed top curve 510 represents an example of the sensitivity according to the prior art solution. The dashed top curve 520 represents an example of sensitivity using the G matrix described herein, giving 2 additional inputs a value of 0. The solid bottom curve 530 represents an example of sensitivity using the line-of-sight correction described herein. Thus, in various embodiments, low frequency sensitivity to line of sight errors may be reduced.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a coating and development system (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be imprinted into a layer of resist supplied to the substrate, the resist being cured by applying electromagnetic radiation, heat, pressure or a combination thereof. After the resist is cured, the patterning device is moved out of the resist, leaving a pattern therein.

The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including Ultraviolet (UV) radiation (e.g. having a wavelength of or about 365nm, 248nm, 193nm, 157nm or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5nm to 20 nm), as well as particle beams, such as ion beams or electron beams.

The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.