阅读说明：本技术 对准方法和相关联的量测装置 (Alignment method and associated metrology apparatus ) 是由 H·M·J·范德格罗斯 J·H·A·范德里德 M·P·J·彼得斯 曾倩虹 H·P·M·派勒曼 于 2020-02-14 设计创作，主要内容包括：披露了一种在设备内对准衬底的方法。所述方法包括基于多个目标的测量来确定衬底栅格,每个目标位于衬底上的不同部位处。所述确定步骤包括重复以下：在对目标的每次测量之后更新所述衬底栅格,以及使用经更新的栅格来对准后续目标的测量。(A method of aligning a substrate within an apparatus is disclosed. The method includes determining a grid of substrates based on measurements of a plurality of targets, each target located at a different location on the substrate. The determining step comprises repeating the following: the substrate grid is updated after each measurement of a target, and the updated grid is used to align measurements of subsequent targets.)

1. A method of aligning a substrate within an apparatus, comprising:

determining a grid of substrates based on measurements of a plurality of targets, each target located at a different location on the substrate; wherein the determining step comprises repeating:

updating the substrate grid after each measurement of the target, an

The updated grid is used to align measurements of subsequent targets.

2. The method of claim 1, comprising:

determining a target-to-position offset value for each target based on each measurement, the target-to-position offset value describing a difference between the measured target position and an expected target position for that measurement; and

determining the substrate grid from the target-to-location offset values.

3. The method according to claim 1 or 2, wherein the substrate grid is updated cumulatively at least in terms of coverage after each measurement, and wherein optionally the substrate grid is also updated cumulatively in terms of the number of fitting parameters used to describe the substrate grid.

4. The method of claim 3, wherein the substrate grid is described using at least one higher order term.

5. The method of claim 1 or 2, wherein the substrate grid is a moving local substrate grid determined from a fixed number of the targets within a moving window local to the subsequent target.

6. The method of claim 5, wherein the fixed number of targets is less than 6 in number.

7. The method of claim 5, wherein the fixed number of targets is 3 in number.

8. The method of claim 5, 6 or 7, wherein the substrate grid is described using only linear terms.

9. A method as claimed in any preceding claim, wherein the method comprises an initial coarse alignment step based on at least one alignment target, the coarse alignment step being sufficient to locate at least one target of the plurality of targets for use in one of the measurements.

10. The method of any preceding claim, wherein an updated grid to align a next measurement comprises an update based on an immediately previously measured target.

11. The method of any of claims 1-9, wherein the updated grid to align the next measurement does not include an update based on an immediately previously measured target, the update being delayed by at least one target.

12. The method of any preceding claim, wherein the apparatus comprises a metrology apparatus for determining a parameter of interest relating to a lithographic process, and wherein optionally the parameter of interest is one of an overlay or a focus.

13. The method of claim 12, wherein the target is a metrology target formed by the lithographic process for determining the parameter of interest, such that each of the measurements performed in the step of determining a substrate grid is used for the determination of the parameter of interest.

14. The method of claim 12 or 13, wherein each measurement is a scatterometer measurement, each metrology target comprises at least one grating, and wherein optionally each measurement is a dark field scatterometer measurement, and each of the targets is small and substantially comprised within a measurement spot during each of the measurements.

15. A metrology apparatus comprising a substrate holder for holding a substrate, a detector and a processor;

wherein the processor is operable to perform the method of any of claims 12 to 14 to align the substrate during the measurement of a parameter of interest.

Technical Field

The present invention relates to an alignment method for aligning a substrate, and in particular such an alignment method in relation to metrology applications in the manufacture of integrated circuits.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. For example, a lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). A lithographic apparatus may, for example, project a pattern (also commonly referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm and 13.5 nm. Lithographic apparatus using Extreme Ultraviolet (EUV) radiation having a wavelength in the range 4 to 20nm (e.g. 6.7nm or 13.5nm) may be used to form smaller features on a substrate than lithographic apparatus using radiation having a wavelength of, for example, about 193 nm.

Low k₁Lithography can be used for process features having dimensions smaller than the classical resolution limit of the lithographic apparatus. In this process, the resolution formula can be expressed as CD ═ k₁X λ/NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case half a pitch), and k is₁Is an empirical resolution factor. In general, k₁The smaller, the more difficult it becomes to reproduce patterns on the substrate that resemble the shapes and dimensions as planned by the circuit designer in order to achieve a particular electrical function and performance. To overcome thisWith some difficulties, complex fine tuning steps can be applied to the lithographic projection apparatus and/or the design layout. These steps include, for example but not limited to: optimization of NA, custom illumination schemes, use of phase-shifting patterning devices, various optimizations in the design layout, such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction"), or other methods commonly defined as "resolution enhancement techniques" (RET)₁Reproduction of the pattern in the case.

Metrology tools are used in many aspects of the IC manufacturing process. An alignment process may be initially performed to align a substrate within the metrology tool so that a target thereon may be positioned. Improvements in these adjustment procedures would be desirable.

Disclosure of Invention

In a first aspect of the invention, there is provided a method of aligning a substrate within an apparatus, comprising: determining a grid of substrates based on measurements of a plurality of targets, each target located at a different location on the substrate; wherein the determining step comprises repeating the steps of: the substrate grid is updated after each measurement of a target, and the updated grid is used to align the measurements of subsequent targets.

Other aspects of the invention include metrology apparatus operable to perform the method of the first aspect.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 depicts a schematic overview of an embodiment of a lithographic apparatus;

FIG. 2 depicts a schematic overview of an embodiment of a lithography unit;

fig. 3 depicts a schematic representation of global lithography, which represents the cooperation between three key technologies to optimize semiconductor manufacturing;

fig. 4 depicts a schematic overview of a scatterometry device used as a metrology apparatus, which may comprise a radiation source according to an embodiment of the present invention;

fig. 5 depicts a schematic overview of an alignment sensor device that may comprise a radiation source according to an embodiment of the invention;

fig. 6 shows steps for exposing a target portion (e.g. a die) on a substrate in an example of a dual-stage type lithographic apparatus;

figure 7 shows, in a conceptualized manner, an alignment method according to a first embodiment of the invention; and

fig. 8 shows, in a conceptualized manner, an alignment method according to a second embodiment of the invention.

Detailed Description

In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. having a wavelength of 365nm, 248nm, 193nm, 157nm or 126 nm) and extreme ultraviolet radiation (EUV, e.g. having a wavelength in the range of about 5-100 nm).

The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general purpose patterning device that can be used to impart an incident radiation beam with a patterned cross-section corresponding to a pattern to be created in a target portion of the substrate; the term "light valve" may also be used in this context. Examples of other such patterning devices, in addition to classical masks (transmissive or reflective; binary, phase-shift, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

FIG. 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. ultraviolet radiation, DUV radiation or EUV radiation); a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a substrate support (e.g. a mask table) MT constructed to accurately position the patterning device MA in accordance with certain parameters; a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate support 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.

In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to impart the radiation beam B with a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

The term "projection system" PS as used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or 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" PS.

The lithographic apparatus LA may 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 PS and the substrate W, which is also referred to as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such "multi-stage" machines the substrate supports WT may be used in parallel, and/or steps may be performed on a substrate W positioned on one of the substrate supports WT in preparation for subsequent exposure of the substrate W whilst another substrate W on the other substrate support WT is used to expose a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LA may include a metrology stage. The measuring platform is arranged to hold the sensor and/or the cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system providing the immersion liquid. The measurement platform may be moved under the projection system PS while the substrate support WT is away from the projection system PS.

In operation, the radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support MT, and is patterned by the pattern (design layout) present on the patterning device MA. After 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 measurement system IF, the substrate support WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B in focus and alignment. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in fig. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although substrate alignment marks P1, P2 are shown as occupying dedicated target portions, they may be located in spaces between target portions. When substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe-lane alignment marks.

As shown in fig. 2, the lithographic apparatus LA may form part of a lithographic cell LC (sometimes also referred to as a lithographic cell or (lithographic) cluster), which typically also includes equipment to perform pre-exposure and post-exposure processes on the substrate W. Conventionally, these apparatuses comprise a spin coater SC to deposit a resist layer, a developer DE to expose the resist, a chill plate CH and a bake plate BK, for example to regulate the temperature of the substrate W, for example to regulate the solvent in the resist layer. The substrate transport or robot RO picks up substrates W from the input port I/O1, the output port I/O2, moves them between different process tools, and transfers the substrates W to a load table LB of the lithographic apparatus LA. The devices in the lithography cell, also commonly referred to as coating and development systems or tracks (tracks), are typically under the control of a coating and development system control unit or track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, e.g. via a lithography control unit LACU.

In order to expose the substrate W exposed by the lithographic apparatus LA correctly and consistently, the substrate needs to be inspected to measure properties of the patterned structures, such as overlay error, line thickness, Critical Dimension (CD) and the like between subsequent layers. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, particularly if inspection is performed before other substrates W of the same batch or lot are still to be exposed or processed.

The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, in particular how properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W and may for example be part of the lithographic cell LC, or may be integrated into the lithographic apparatus LA, or may even be a separate device. The inspection device may measure the characteristics of a latent image (an image in the resist layer after exposure), a semi-latent image (an image in the resist layer after the post-exposure bake step PEB), or a developed resist image (in which the exposed or unexposed portions of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high accuracy in the determination of the size and placement of the structures on the substrate W. To ensure such high accuracy, the three systems may be combined in a so-called "global" control environment, as schematically depicted in fig. 3. One of the systems is a lithographic apparatus LA, which is (actually) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key to this "global" environment is to optimize the cooperation between the three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), typically allowing process parameters in a lithographic process or a patterning process to vary within the defined result.

The computer system CL may use (parts of) the design layout to be patterned to predict what resolution enhancement techniques are to be used and perform computational lithography simulations and calculations to determine which mask layouts and lithographic apparatus settings implement the largest overall process window of the patterning process (depicted in fig. 3 by the double arrow in the first scale SC 1). Typically, resolution enhancement techniques are arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g. using input from the metrology tool MT) to predict whether a defect (depicted in fig. 3 by the arrow pointing to "0" in the second scale SC 2) is present due to, for example, sub-optimal processing.

The metrology tool MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA to identify possible drifts, for example in a calibration or calibration state of the lithographic apparatus LA (depicted in fig. 3 by the plurality of arrows in the third scale SC 3).

In lithographic processes, it is desirable to frequently measure the resulting structures, for example for process control and verification. The tool that makes this measurement is commonly referred to as the metrology tool MT. Different types of metrology tools MT are known for making such measurements, including scanning electron microscopes or various forms of scatterometry metrology tools MT. A scatterometer is a multifunctional instrument that allows measurement of parameters of a lithographic process by placing a sensor in the pupil or conjugate plane to the pupil of the scatterometer objective (this measurement is usually referred to as a pupil-based measurement), or by placing a sensor in the image plane or conjugate plane to the image plane (in this case, the measurement is usually referred to as an image or field-based measurement). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753a1, US20120044470A, US20110249244, US20110026032 or ep1,628,164a, which are incorporated herein by reference in their entirety. The aforementioned scatterometers may use light ranging from soft X-rays, as well as visible to near IR wavelengths to measure gratings.

In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such scatterometers, reconstruction methods may be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction may for example be generated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the actual target.

In a second embodiment, the scatterometer MT is a spectral scatterometer MT. In such a spectral scatterometer MT, radiation emitted by a radiation source is directed onto the target, and radiation reflected or scattered from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e. a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target producing the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and non-linear regression or by comparison with a library of simulated spectra.

In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. Ellipsometry scatterometers allow for the determination of parameters of a lithographic process by measuring the scattered radiation for each polarization state. Such a metrology apparatus emits polarized light (such as linear, circular or elliptical) by using, for example, a suitable polarizing filter in the illumination section of the metrology apparatus. A source suitable for use in the metrology apparatus may also provide polarized radiation. Various embodiments of prior art ellipsometric scatterometers are described in U.S. patent applications 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968, 12/922,587, 13/000,229, 13/033,135, 13/533,110, and 13/891,410, which are incorporated by reference herein in their entirety.

A metrology device such as a scatterometer is depicted in fig. 4. It comprises a broadband (white light) radiation projector 2 projecting radiation 5 onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4, which spectrometer detector 4 measures the spectrum (i.e. the measurement of the intensity as a function of wavelength) of the specularly reflected radiation 10. From this data, the structure or profile 8 that produced the detected spectrum may be reconstructed by the processing unit PU, for example by tightly coupled wave analysis and non-linear regression, or by comparison with a library of simulated spectra as shown at the bottom of fig. 4. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from scatterometry data. Such scatterometers may be configured as normal incidence scatterometers or oblique incidence scatterometers.

The overall measurement quality for a lithographic parameter via measurement of the metrology target is determined, at least in part, by the measurement recipe used to measure the lithographic parameter. The term "substrate measurement recipe" may include measuring one or more parameters of itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation with respect to the substrate, the direction of the radiation with respect to the pattern on the substrate, and the like. One of the criteria for selecting a measurement recipe may be, for example, the sensitivity or sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application 2016-0161863 and published U.S. patent application US2016/0370717A1, which are incorporated herein by reference in their entirety.

Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place the applied pattern relative to features laid down in a previous layer (by the same apparatus or a different lithographic apparatus). To this end, the substrate is provided with one or more sets of marks. Each marker is a structure whose position can be later measured using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark".

The lithographic apparatus may comprise one or more (e.g. multiple) alignment sensors by which the position of an alignment mark provided on the substrate may be accurately measured. The one or more (e.g., multiple) alignment sensors may be part of a separate measurement, or alignment, system, or separate metrology tool. The alignment (or position) sensor may obtain position information from an alignment mark formed on the substrate using optical phenomena such as diffraction and interference. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer as described in US 69661116. Various enhancements and modifications of the position sensor have been developed, for example, as disclosed in US2015261097a 1. All of these disclosures are incorporated herein by reference.

Fig. 5 is a schematic block diagram of an embodiment of a known alignment sensor AS, such AS the alignment sensor AS described in US 69661116 (which is incorporated by reference). The radiation source RSO provides a beam RB of radiation having one or more wavelengths which is steered by the steering optics onto a mark, such as a mark AM located on the substrate W, as an illumination spot SP. In this example, the steering optics comprise a spot mirror SM and an objective lens OL. The diameter of the irradiation spot SP, by which the mark AM is irradiated, may be slightly smaller than the width of the mark itself.

The radiation diffracted by the marks AM is collimated (in this example via the objective lens OL) into an information carrying beam or an information carrying beam IB. The term "diffraction" is intended to include zeroth order diffraction (which may be referred to as reflection) from the mark. The self-referencing interferometer SRI (e.g. of the type disclosed in US 69661116 mentioned above) causes the beam IB to interfere with itself, after which it is received by the photodetector PD. In case more than one wavelength is generated by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element, or it may comprise a plurality of pixels, if desired. The light detector may comprise an array of sensors.

The steering optics, which in this example comprises the spot mirror SM, may also be used to block zero order radiation reflected from the marks, so that the information-carrying beam IB comprises only higher order diffracted radiation from the marks AM (which is not necessary for the measurement, but improves the signal-to-noise ratio).

The intensity signal SI is supplied to the processing unit PU. By a combination of the optical processing in the block SRI and the computational processing in the unit PU, values of the X-position and the Y-position on the substrate relative to a reference frame are output.

A single measurement of the type shown fixes the position of the marks only within a certain range corresponding to one pitch of the marks. A coarser measurement technique is used in conjunction with the single measurement to identify which period of the sine wave is the period containing the marked location. The same process at coarser and/or finer levels may be repeated at different wavelengths for increased accuracy and/or robust detection of the marks, regardless of the material from which the marks are made and on which materials the marks are disposed. The wavelengths may be multiplexed and demultiplexed optically to process the wavelengths simultaneously and/or the wavelengths may be multiplexed using time or frequency division.

In this example, the alignment sensor and spot SP remain fixed while the substrate W moves. Thus, the alignment sensor can be rigidly and accurately mounted to the reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in its movement by mounting the substrate W on a substrate support and a substrate positioning system that controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks AM are provided on the substrate support. Measurement of the position of the mark disposed on the substrate support allows calibration of the position of the substrate support as determined by the position sensor (e.g., relative to a frame to which the alignment system is coupled). The measurement of the position of the alignment mark disposed on the substrate allows the position of the substrate relative to the substrate support to be determined.

Fig. 6 illustrates the steps used to expose a target portion (e.g., a die) on a substrate or wafer in an example of a dual-stage type lithographic apparatus. Within the dashed box on the left-hand side of fig. 6, the process steps performed at the measurement station MEA are indicated; whereas the dashed box on the right-hand side of fig. 6 indicates the process steps performed at the exposure station EXP. Each of the metrology and exposure stations comprises a (separate) substrate support on which a substrate may be supported. Oftentimes, one of these substrate supports will be located at the exposure station EXP, while the other substrate supports are located at the measurement station MEA.

At step MEA1, a new substrate W' is loaded onto the substrate support at the measurement station MEA, while another substrate W has been loaded into the exposure station EXP. The substrate W, W' is processed in parallel (simultaneously) at the measurement station MEA and the exposure station EXP to increase the throughput of the lithographic apparatus.

At step MEA2, measurements are performed in the measurement station MEA to determine and record the position of the substrate W 'relative to the substrate support in the plane of the substrate support on which the substrate W' is disposed (i.e., the XY plane). In addition, a "wafer grid" may be measured (using, for example, an alignment sensor) that accurately describes the shape of the substrate W' and the position of a plurality of marks across the entire substrate, including any distortion (in the plane of the substrate, i.e., the XY plane) relative to a nominally rectangular grid of a plurality of marks.

At step MEA3, the topography of the substrate W' (normal to the XY plane of the substrate, i.e., along the Z axis) as a function of XY position is measured using, for example, a level sensor that generates a height map of the substrate (or wafer). Such a substrate height map is used, for example, to achieve accurate focusing of the exposed pattern at the exposure station EXP.

The measured wafer position, wafer grid and height map are added to recipe data RECI so that a set of exposure and measurement data MEADATA can be input to the exposure station EXP.

At step SWA, the substrate W and the substrate W 'are exchanged so that the measured substrate W' becomes the substrate in the exposure station EXP. In this example, the exchange is performed by exchanging the respective substrate supports such that the substrates remain accurately positioned on their respective supports to maintain relative alignment between the respective substrate supports and the substrates.

Before exposing the substrate W, the relative position between the projection system PS and the substrate support is determined so as to be able to utilize the data measured at steps MEA2, MEA3 for the position of the substrate on the substrate support when controlling the exposure step. At step EXP1, alignment between the reticle and the substrate support is performed using mask alignment marks M1, M2. In steps EXP2, EXP3 and EXP4, scanning movements and radiation pulses are applied at successive target sites across the entire substrate W to perform exposures of multiple patterns.

After exposing the substrate W, the substrate W is unloaded from the substrate support at step UNL. The substrate, currently labeled W ", will undergo photoresist processing, etching, and/or other semiconductor processing steps therein.

The skilled person will recognise that the above description is a simplified overview of many of the very detailed steps involved in one example of a real manufacturing scenario. For example, rather than measuring the position of the wafer in a single process, there will often be separate processes for coarse and fine measurements.

The alignment method described above is not only performed as a pre-exposure step within the scanner, but also prior to measurement within a metrology tool (such as illustrated in fig. 4). Described herein will be improved alignment strategies that focus on substrate (or wafer) alignment within a metrology tool, but it will be appreciated that the various concepts can be readily extended to pre-exposure alignment within a scanner.

In order to align the substrate during measurements (e.g., overlay measurements) in the metrology device, known substrate alignment processes may include two steps, a coarse alignment step and a fine alignment step, to achieve the final accuracy requirement. The alignment process may be performed using a dedicated alignment sensor, or using any sensor/optics for its primary metrology function other than that contained within the metrology device.

The alignment process may also include (immediately prior to the coarse alignment) a substrate pre-alignment step to initially adjust the substrate rotation to within, for example, +/-1mrad accuracy. During both the coarse and fine alignments, a substrate grid (or wafer grid) is constructed and updated by measuring the positions of a plurality of marks. The coarse alignment step is used to determine a coarse alignment grid to a sufficient accuracy so that it is used to calculate the desired position of the fine alignment marks. The fine substrate alignment grid is constructed in a fine alignment stage according to measurements of the final alignment marks at these desired locations.

Substrate alignment is a one-time operation per substrate and takes up to about 3 seconds; most of this time is consumed by the fine alignment step (coarse alignment may take less than 1 second). The final alignment accuracy may be desired to be, for example, less than +/-1.5 μm.

Two major drawbacks of current such two-step (coarse and fine) substrate alignment are its limited accuracy and impact throughput. Accuracy is limited by a number of factors. For example, the alignment accuracy depends on the exact moment the sensor captures the picture that is relied upon for determining the identification location. Furthermore, due to dynamic behavior (e.g., drift), there is an error of +/-3 μm (e.g., due to hysteresis in the alignment task). Drift may cause the desired target position to deviate from the true target position, thereby causing measurement error (MA). Differential vibration (dynamics) may also be a large error contribution factor that is not recorded by the encoder. The dynamics between the encoder and the sensor tip (point of interest, i.e. POI) are not taken into account when positioning the platform.

To solve these problems, an alignment method is proposed that replaces the fine alignment step with a step of determining the substrate grid based on overlay (or other parameter of interest) measurements to be performed in any case. Two embodiments of such an alignment method will be described.

The proposed method utilizes target-to-position offset (TPO) information obtained from each metrology target measurement (e.g., each measurement of a parameter of interest). TPO includes the error or residual between the desired target position (as calibrated) and the target position detected on the detector (e.g., camera) in the actual measurement. The camera measurement may be an mDBO dark field image, or for IDM case an image of the target captured by the YS alignment sensor.

The object measurement for determining the detected object position (and thus the object-to-position offset) may be obtained by dark-field imaging, e.g. in an overfill ADI (post-visualization inspection) measurement (e.g. of overlay or focus). Alternatively, the target measurements may each include additional measurements in the under-filled AEI (post etch inspection) measurements using alignment sensors of the metrology device (if any). In the latter case, the TPO information may result from confirmation of misalignment of the measurement patch with the target. This may have been measured as part of an existing control strategy so that this misalignment can be corrected for subsequent AEI measurements. More specifically, in an under-filled AEI measurement, the TPO offset can be determined by acquiring additional images using alignment optics at approximately the same time as or immediately after the actual AEI measurement. The image retrieved from the alignment optics may be used to determine the exact position of the target (by the size and/or shape of the target as seen by the optics) and thereby determine the TPO.

The target may be, for example, an overlay target or a focus target, depending on the parameter of interest being measured. In embodiments, the target may comprise a micro-diffraction based overlay (μ DBO) or a micro-diffraction based focus (μ DBF) target. Such targets may be measured using an "overfill" measurement method in which the measurement spot is larger than the target, such that the target is completely included within the measurement spot during measurement. Such a measurement method may determine the intensity values of different regions of interest (e.g. involving different grating structures than are contained within the object, which grating structures can all be measured in one image), and determine the parameter of interest from the intensity values of multiple diffraction orders. In particular, the difference in the corresponding higher diffraction orders may be used to determine the parameter of interest. Intensity values may be obtained using dark field metrology, examples of which may be found in international patent applications WO 2009/078708 and WO 2009/106279 (hereby incorporated by reference in their entirety). In addition, further developments of the technology have been described in various patent publications US20110027704A, US20110043791A and US 20120242940A. The contents of all of these applications are also incorporated herein by reference. Diffraction-based overlay or focusing using dark-field detection of the multiple diffraction orders enables overlay or focus measurements of smaller targets. Since these targets may be smaller than the illumination spot, they may be surrounded by product structures on the substrate. These methods are well known and will not be described in further detail herein.

Fig. 7 illustrates a first embodiment, where fig. 7(a) illustrates a high-level overall approach in a conceptualized way, and fig. 7(b) is a flow chart describing the proposed high-order cumulative grid adjustment CGA phase. The method includes coarse alignment CA by known methods (e.g., by performing known coarse alignment strategies such as coarse substrate alignment, COWA). Such a method may be performed by measuring alignment marks on two centrally located fields. The coarse alignment is accurate enough to target a parameter of interest (e.g., overlay or focus) within the sensor field of view.

After the coarse alignment CA, instead of the fine alignment step, a high order cumulative grid adjustment CGA stage is proposed. The CGA method is an accumulative method for calculating a substrate grid based on measurements of successive metrology targets and target-to-position offsets determined from each of these measurements. As such, the substrate grid is an ever-expanding local substrate grid that increases in at least coverage as more targets are measured at more and more locations. In an embodiment, it is also proposed that the cumulative substrate grid is also expanded in the number of fitting parameters, since more target positions are included in the calculation. By way of example, the substrate grid may start with a 4 parameter grid and increase to 10 or more parameters over time. As the number of fitting parameters increases, both intra-field and inter-field parameters may be included. In this definition, the term "local" is defined with respect to the next target to be measured (and/or the next field comprising said next target). For example, "local" may mean that the target being measured is in the same field as the closest measured target for which the measurement facilitates the determination of the accumulated substrate grid, in the adjacent field to the closest measured target, in at most two fields separate from the closest measured target, or in at most three fields separate from the closest measured target.

Fig. 7(b) illustrates several steps of the proposed method, starting from an intermediate stage immediately after the measurement of the target n-1. Based on this measurement, LG TPO is calculated as by the step₍₁₎To TPO_(n-1)"As indicated, a target-to-location offset TPO for this target n-1 is determined and the high order local grid is updated for all targets measured up to this time (e.g., including target 1 to target)n-1 inclusive). After that, the metrology device sensor is moved to a target n and a measurement meas (e.g., overlay or focus measurement) is performed on the target n. The moving step is based on the updated substrate grid such that this grid is used to locate the next target. MEA was based on this measurement_nThe TPO value of target n is determined and the cumulative underlay grid is again updated to a new value (step "calculate LG TPO)₍₁₎To TPO_(n)"). These steps are then repeated for subsequent measurements. The result is a cumulative (local) substrate grid with small coverage at the beginning of the substrate being measured, as represented by the CGA at the substrate_nIndicated by the three lighter shaded fields. The substrate coverage of the accumulated (local) substrate grid is increased for each measured object, as the CGA is represented by the later substrate after 8 further measurements_n+8Indicated by the lighter shaded field in (1).

A second embodiment is illustrated by fig. 8. This embodiment is similar to the embodiment illustrated by FIG. 7 in that both construct the substrate grid from TPO information obtained from overlay (or other parameter of interest) measurements. However, instead of constructing an increasingly larger cumulative high-order substrate grid, a local substrate grid of movement (e.g., a 6 parameter grid) is calculated as the measurements move to different regions of the substrate.

Fig. 8(a) is an equivalent conceptual view of fig. 7 (a). Again, the alignment includes an initial coarse alignment CA sufficient to position the metrology target in a sensor field of view. After that, the mobile local grid alignment MLGA process determines the local grid based on the same number of measurements obtained locally until the next measurement to be made. This moving coverage of the local grid is represented by a MLGA on the substrate_nAnd MLGA_n+aThree brighter fields.

Fig. 8(b) depicts several exemplary steps of the MLGA method. The TPO information is from the previous few (e.g., the previous 2 or more, here 3) target locations. In this figure, the flow starts at an intermediate stage immediately after the measurement of the target 3. Base ofCalculating LG TPO as by step₍₁₎To TPO₍₃₎"this measurement indicated, the target-to-position offset TPO of this target 3 is determined and the local substrate grid is calculated based on these three targets. After that, the measuring device sensor is moved to the target 4, and the measurement MEA is performed on the target 4₄(e.g., overlay or focus measurements). The moving step is based on the updated substrate grid such that this grid is used to locate the next target. MEA was based on this measurement₄The TPO value of target 4 is determined and a new local substrate grid is calculated based on targets 2 to 4 only (step "calculate LG TPO)₍₂₎To TPO₍₄₎"). These steps are then repeated for subsequent measurements.

The coverage of the grid moves with the path selection and the number of fitting parameters remains the same (e.g., 6 parameters). Alternatively, in this embodiment, the 6-parameter substrate grid may include only linear terms. Substrate overhead is reduced because fine alignment is skipped. Since the grid is always constructed very close to the next measurement, the grid reflects local printing errors and a higher position accuracy can be achieved.

As previously described, for this embodiment, the term "local" is defined with respect to the next target to be measured (and/or the next field comprising said next target). For example, "local" may mean that the object being measured is in the same field as the nearest measured object for which the measurement facilitates the determination of the moving local substrate grid, in the adjacent field to the nearest measured object, in at most two fields separate from the nearest measured object, or in at most three fields separate from the nearest measured object. As such, the mobile local grid may always involve (e.g., measured from objects within) a moving window of a fixed number of objects and/or fields local to the next object to be measured.

The example in fig. 8(b) describes an immediate grid update method, where the positioning for the nth target is made based on a grid of substrates determined from several (e.g., 3) immediately preceding targets; that is, it includes TPO information from the immediately preceding target n-1. However, delayed substrate grid updates may be more convenient; for example, the substrate grid is based on previous several measurements of at most only target n-2 (i.e., at most to targets that are two or three targets measured before the target being measured). Similarly, in an accumulated substrate grid embodiment, the update after each measurement may be delayed by one or more targets (e.g., such that the accumulated substrate grid for alignment to target n is based only on the measurements of targets 1-n-2).

The proposed alignment strategy should improve accuracy without yield loss and may result in yield gain. In addition, timely realignment (drift compensation) should not be required. As such, when the substrate grid is updated for each target, realignment is no longer necessary. The alignment drift compensation (more specifically for low frequency phenomena/components) is done on the fly.

Although the above description is described in terms of metrology tool alignment, the method is equally applicable to alignment measurements in scanners, and as such, to all exposure/scanners or metrology/inspection tools that use marks/targets at certain (known) locations on the substrate.

Further embodiments are disclosed in the subsequently numbered aspects:

1. a method of aligning a substrate within an apparatus, comprising:

determining a grid of substrates based on measurements of a plurality of targets, each target located at a different location on the substrate; wherein the determining step comprises repeating:

updating the substrate grid after each measurement of the target, an

The updated grid is used to align measurements of subsequent targets.

2. The method defined in aspect 1, comprising:

determining a target-to-position offset value for each target based on each measurement, the target-to-position offset value describing a difference between the measured target position and an expected target position for that measurement; and

determining the substrate grid from the target-to-location offset values.

3. The method defined in aspect 1 or 2, wherein the substrate grid is updated in an accumulated manner at least in terms of coverage after each measurement.

4. The method defined by aspect 3 wherein the substrate grid is also updated in an accumulated manner in terms of the number of fitting parameters used to describe the substrate grid.

5. The method defined in aspect 3 or 4 wherein the substrate grid is described using at least one higher order term.

6. The method defined by aspect 1 or 2 wherein the substrate grid is a moving local substrate grid determined from a fixed number of the targets within a moving window local to the subsequent target.

7. The method defined in aspect 6 wherein the fixed number of targets is less than 6 in number.

8. The method defined in aspect 6 wherein the fixed number of targets is 3 in number.

9. The method defined in aspect 6,7 or 8 wherein the substrate grid is described using only linear terms.

10. A method as defined in any preceding aspect, wherein the method comprises an initial coarse alignment step based on at least one alignment target, the coarse alignment step being sufficient to locate at least one target of the plurality of targets for use in one of the measurements.

11. A method as defined in any preceding aspect, wherein the updated grid to align the next measurement comprises an update based on an immediately previously measured target.

12. The method defined in any one of aspects 1 to 10 wherein the updated grid to align the next measurement does not include an update based on an immediately previously measured target, the update being delayed by at least one target.

13. A method as defined in any preceding aspect, wherein the apparatus comprises a metrology apparatus for determining a parameter of interest relating to the lithographic process.

14. The method defined in aspect 13, wherein the parameter of interest is one of an overlap or a focal length.

15. The method as defined in aspect 13 or 14, wherein the target is a metrology target formed by the lithographic process for determining the parameter of interest, such that each of the measurements performed in the step of determining a substrate grid is used for the determination of the parameter of interest.

16. The method defined in any one of aspects 13 to 15 wherein each measurement is a scatterometer measurement, each of the metrology targets comprising at least one grating.

17. The method defined in aspect 16 wherein each measurement is a dark-field scatterometer measurement and each of the targets is small and substantially included within a measurement spot during each of the measurements.

18. A metrology apparatus comprising a substrate holder for holding a substrate, a detector and a processor;

wherein the processor is operable to perform the method of any of aspects 13 to 17 to align the substrate during the measurement of a parameter of interest.

19. The metrology device of aspect 18, further operable to perform each of said measurements of said plurality of targets.

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. Possible other applications include 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.

Although specific reference may be made in this text to embodiments of the invention in the context of lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices may be generally referred to as lithographic tools. Such a lithography tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although the foregoing may make specific reference to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may not be limited to optical lithography, and may be used in other applications (e.g. imprint lithography), where the context allows.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. 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.