WO2021063663A1

WO2021063663A1 - Alignment sensor with modulated light source

Info

Publication number: WO2021063663A1
Application number: PCT/EP2020/075658
Authority: WO
Inventors: Ali ALSAQQA; Joshua Adams; Bin Li
Original assignee: Asml Holding N.V.
Priority date: 2019-09-30
Filing date: 2020-09-14
Publication date: 2021-04-08
Also published as: CN114450641A

Abstract

An apparatus for and a method of determining alignment of a substrate in which the intensity of a radiation source illuminating an alignment mark is varied in accordance with whether an element in the system for converting an analog signal into a digital signal is in a mode in which it is sampling the signal or in a mode in which it is converting the signal and so not sensitive changes in the signal, thus to reduce the amount of exposure of the substrate or system optical components to the illuminating radiation.

Description

ALIGNMENT SENSOR WITH MODULATED LIGHT SOURCE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Provisional Patent Application

Number 62/908,101, which was filed on September 30, 2019, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to the manufacture of devices using lithographic techniques. Specifically, the present disclosure relates to devices for sensing and analyzing alignment marks on reticles and wafers to characterize and control semiconductor photolithographic processes.

BACKGROUND

[0003] A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For that application, a patterning device, which is also 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. This 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). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so- called scanners, in which 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.

[0005] ICs are built up layer by layer, and modern ICs can have thirty or more layers.

On Product Overlay (OPO) is a measure of a system’s ability to print these layers accurately on top of each other. Successive layers or multiple processes on the same layer must be accurately aligned to the previous layer. Otherwise, electrical contact between structures will be poor and the resulting devices will not perform to specification. Good overlay improves device yield and enables smaller product patterns to be printed. The overlay error between successive layers formed in or on the patterned substrate is controlled by various parts of the exposure apparatus of the lithographic apparatus.

[0006] Process-induced wafer errors are a significant impediment to OPO performance.

Process-induced wafer errors are attributable to the complexity of printed patterns as well as an increase of the number of printed layers. This error is of relatively high spatial variation that is different from wafer to wafer, and within a given wafer. .

[0007] In order to control the lithographic process to place device features accurately on the substrate, one or more alignment marks are generally provided on, for example, the substrate, and the lithographic apparatus includes one or more alignment sensors by which the position of the mark may be measured accurately. The alignment sensor may be effectively a position measuring apparatus. Different types of marks and different types of alignment sensors are known from different times and different manufacturers. Measurement of the relative position of several alignment marks within the field can correct for process-induced wafer errors . Alignment error variation within the field can be used to fit a model to correct for OPO within the field

[0008] Lithographic apparatus are known to use multiple alignment systems to align the substrate with respect to the lithographic apparatus. The data can for example be obtained with any type of alignment sensor, for example a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116, issued November 1, 2005 and titled “Lithographic Apparatus, Device Manufacturing Method, and Device Manufactured Thereby,” which is hereby incorporated by reference herein in its entirety, that employs a self-referencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or ATHENA (Advanced Technology using High order ENhancement of Alignment), as described in U.S. Pat. No. 6,297,876, issued October 2, 2001 and titled “Lithographic Projection Apparatus with an Alignment System for Aligning Substrate on Mask,” which is hereby incorporated by reference in its entirety, which directs each of seven diffraction orders to a dedicated detector.

[0009] Reference is made in particular to the European application No. EP 1 372040

Al, granted March 5, 2008 and titled “Lithographic Apparatus and Device Manufacturing Method” which document is hereby incorporated by reference in its entirety. EP 1 372040 Al describes an alignment system using a self-referencing interferometer that produces two overlapping images of an alignment marker. These two images are rotated over 180° with respect to each other. EP 1 372040 Al further describes the detection of the intensity variation of the interfering Fourier transforms of these two images in a pupil plane. These intensity variations correspond to a phase difference between different diffraction orders of the two images, and from this phase difference positional information is derived, which is required for the alignment process. Reference is also made to U.S. Patent No. 8,610,898, “Self-Referencing Interferometer, Alignment System, and Lithographic Apparatus” issued December 17, 2013, the entire contents of which are hereby incorporated by reference in their entirety.

[0010] It is an object of the present invention to provide a sensor apparatus and method of determining a position of a target on the substrate which at least partially addresses one or more of the problems of the prior art, whether identified herein or elsewhere.

SUMMARY

[0011] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0012] According to one aspect of an embodiment there is disclosed an apparatus for measuring an alignment mark on a substrate, the apparatus comprising a source of radiation adapted to illuminate the alignment mark, the source of radiation being configured to have a first state in which the source of radiation produces radiation having a first intensity and a second state in which the source of radiation produces radiation having a second intensity less than the first intensity, a detector arranged to detect an intensity of radiation diffracted by the alignment mark and to produce a signal indicative of the intensity, and a converter arranged to receive the signal and to generate a digital signal based at least in part on the signal, the converter being configured to have a first mode in which the converter samples the signal and a second mode in which the converter does not sample the signal, the source of radiation being configured to produce radiation having the second intensity for at least part of a time when the when the converter is in the second mode. The apparatus may further comprise an attenuator wheel optically positioned to attenuate radiation from the radiation source. The converter may be configured to produce a signal indicating whether the converter is in the second mode and the further comprising a controller arranged to receive the signal and configured to control an intensity of the radiation source based at least in part on the signal. The first mode may be a sample mode and the second mode may be a hold mode. [0013] According to one aspect of an embodiment there is disclosed an apparatus for measuring a parameter of an alignment mark on a substrate, the apparatus comprising a source of radiation adapted to illuminate the alignment mark, a detector arranged to detect an intensity of radiation diffracted by the alignment mark and to produce a signal indicative of the intensity, and a converter arranged to receive the signal and to generate a digital signal based at least in part on the signal, the converter having a first mode in which the converter samples the signal and a second mode in which the converter does not sample the signal, the source of radiation being configured to reduce an intensity of radiation output by the source of radiation at least part of a time when the converter is in the second mode. The radiation source may comprise an attenuator wheel. The converter may be configured to produce a signal indicating whether the converter is in the second mode and the further comprising a controller arranged to receive the signal and configured to control an intensity of the radiation source based at least in part on the signal. The first mode may be a sample mode and the second mode may be a hold mode. [0014] According to another aspect of an embodiment there is disclosed photolithography method comprising the steps of using a source of radiation to illuminate at least one alignment mark, the source of radiation being configured to have a first state in which the source of radiation produces radiation having a first intensity and a second state in which the source of radiation produces radiation having a second intensity less than the first intensity, using a detector to detect an intensity of radiation diffracted by the at least one alignment mark and to produce a signal indicative of the intensity, and using a converter arranged to receive the signal to generate a digital signal based at least in part on the signal, the converter having a sample mode in which the converter samples the signal and a hold mode in which the converter does not sample the signal, the source of radiation producing radiation having the second intensity when the converter is in the hold mode.

[0015] According to another aspect of an embodiment there is disclosed a photolithography method comprising the steps of illuminating at least one alignment mark, the source of radiation being configured to have a first state in which the source of radiation produces radiation having a first intensity and a second state in which the source of radiation produces radiation having a second intensity less than the first intensity, detecting an intensity of radiation diffracted by the at least one alignment mark and producing a signal indicative of the intensity, generating a digital signal based at least in part on the signal, wherein the generating comprises a sampling phase in which the signal is sampled and a hold phase in which the signal is not sampled, and producing radiation having the second intensity when the generating step is in the hold phase. [0016] According to another aspect of an embodiment there is disclosed photolithography method comprising the steps of illuminating at least one alignment mark, detecting an intensity of radiation diffracted by the at least one alignment mark and producing a signal indicative of the intensity, generating a digital signal based at least in part on the signal, wherein the generating step comprises a sampling phase in which the signal is sampled and a hold phase in which the signal is not sampled, and reducing an intensity of radiation in the illuminating step when the generating step is in the hold phase.

[0017] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0018] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements.

[0019] FIG. 1 depicts selected parts of a photolithography system such as could be used to according to aspects of an embodiment disclosed herein.

[0020] FIG. 2 is a functional block diagram depicting selected parts of a known alignment system for explaining the principles of its operation.

[0021] FIG. 3 is a diagram describing some principles of operation of an analog-to- digital converter as might be used in systems according to aspects of embodiments.

[0022] FIG. 4 is a functional block diagram depicting selected parts of an alignment system according to aspects of an embodiment.

[0023] FIG. 5 is a flowchart showing steps of a method according to aspects of an embodiment.

[0024] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein. DETAILED DESCRIPTION

[0025] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments.

[0026] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include solid state memory, read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0027] FIG. 1 schematically depicts a lithographic apparatus. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or other suitable 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) PL 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. [0028] The illumination system may include various types of optical components, such as refractive, reflective, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0029] The support structure supports, i.e., bears the weight of, the patterning device.

It 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 can 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.” [0030] 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.

[0031] 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 mirror matrix. [0032] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, 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”.

[0033] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array or employing a reflective mask).

[0034] 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.

[0035] 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. An immersion liquid 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 liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0036] Referring again to FIG. 1 , the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. 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.

[0037] 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 s-outer and s-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. [0038] 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 PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D 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 (which is not explicitly depicted in FIG. 1) 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 PI, 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 wafer may also include additional marks such as, for example, marks that are sensitive to variations in a chemical mechanical planarization (CMP) process used as a step in wafer fabrication.

[0039] The target PI and/or P2 on substrate W may be, for example, (a) a resist layer grating, which is printed such that after development, the bars are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars may alternatively be etched into the substrate.

[0040] FIG. 2 shows a schematic overview of a known alignment system 10. A light source 12, also equivalently referred to herein as a radiation source, emits a spatially coherent beam of radiation which illuminates an alignment marker WM on a substrate (e.g., a wafer) e.g., TIS or PARIS plate, which reflects the radiation into positive and negative diffraction orders -i-n and -n. These diffraction orders are collimated by an objective lens 14 and enter an optical system such as a self-referencing interferometer 16. The self-referencing interferometer 16 outputs two images of the input with a relative rotation of 180° and which overlap and which can therefore be made to interfere. In a pupil plane the overlapping Fourier transforms of these images, with the different diffraction orders separated can be seen and be made to interfere. Detectors, e.g., photodetectors 18 in the pupil plane detect the interfered diffraction orders to provide positional information. The signal generated by the photodetectors 18 is generally a sinusoidally varying analog signal as shown. The analog signal is converted to a digital signal, for example, by an analog to digital converter 20. Based on this positional information a substrate can be aligned accurately with respect to a lithographic apparatus.

[0041] As described, The alignment sensor uses a light source for the purpose of scanning the mark. Different systems might have different light sources such as a laser or a white light source. These and similar light sources are referred to herein as radiation sources. The light source can be so strong that excessive exposure to the light source can damage the surfaces and components it hits. For example, the light source can damage to the fiducial, especially when combined with other effects. This can cause the fiducial plate to reach the end of its useful life prematurely. This contributes to downtime and a lengthy recovery process. [0042] The radiation source can also cause damage to the wafer. This damage may take the form of chemical changes in the resist which is yet-to-be exposed (for instance change in CD), or physical/chemical damage to layers on top of or below an alignment mark that ideally should be capable of being scanned repeatedly.

[0043] The radiation source can also cause damage to the significant optics. This includes a number of optical elements in optical modules which all have their own laser- induced damage thresholds which, if exceeded, can reduce the useful life of the elements. [0044] One method of avoiding or minimizing this damage is by reducing the radiation source power, for example, by using an attenuator, or by reducing the module power itself. This will reduce the damage induced accordingly. However, the disadvantage of reducing the radiation source power is that there is less light available to illuminate the alignment mark, and hence less light to be diffracted, which may lead to poor reproducibility. This in turn increases overlay error.

[0045] There are thus numerous potential damage mechanisms that can ultimately contribute to part failure. In particular, a high power light source can cause thermally induced damage because it has the potential to heat up and even vaporize/diffuse the material it hits. [0046] There is therefore a need to reduce the potential for damage caused by the alignment mark illumination system without compromising the ability to read the alignment mark. According to an aspect of an embodiment this need is met by reducing illumination of , for example, not illuminating the alignment mark at least some of the time when the system arranged to read the mark is not in a data acquisition mode.

[0047] As noted above, during an alignment determination the light source, e.g., laser shines light on an alignment mark. The alignment mark diffracts or refracts the light and the light then passes through optical elements, e.g., an optical module, to generate an optical alignment signal. The optical alignment signal is then supplied to a photodetector which generates an analog electrical signal based on the optical signal. The analog signal from the photodetector is then converted to a digital signal by an analog-to-digital converter (ADC). [0048] In accordance with an aspect of an embodiment, the ADC is of the type that alternates between a first mode in which it is sampling or acquiring the analog signal from the photodetector followed by a second mode in which it holds or converts into a digital signal the portion of the signal that it has just acquired. This is shown in FIG. 3. The broken line 30 is the analog signal from the photodetector. The solid line 32 represents the sample and hold output. During the hold period, one of which is indicated by P, the ADC is not measuring the photodetector output. Thus, it is not necessary to illuminate the alignment mark during these periods. According to an aspect of an embodiment, the light source is attenuated during these periods thus reducing the overall thermal load imposed by the light source.

[0049] According to an aspect of an embodiment, attenuation can be performed by synchronizing an attenuator wheel outside the laser module with the operation of the ADC. This is shown in FIG. 4. Attenuator wheel 40 is positioned optically between the light source 12 and the rest of the system. The ADC 20 generates a signal that indicates whether the ADC is reading the analog signal(s) from the photodetectors 18. A control system 42, which may be part of an overall control system, controls the attenuator wheel 40 based at least in part on the signal from the ADC 20. In this manner, the wafer and system optics are not exposed to radiation from the radiation source 12 for at least part of the time that the ADC is not measuring that radiation as diffracted. The light source may also be attenuated, for example, by closing a shutter between the light source and the rest of the system.

[0050] The operating of the attenuator wheel or the closing of the shutter may be synchronized with the operation of the ADC a priori rather than having the control system sense when the ADC is in a conversion mode and close the shutter when the ADC is in the conversion mode. Alternatively the control system could sense the onset of the conversion mode and close the shutter for a period of time known a priori to be the duration of the conversion mode. [0051] For a particular ADC the ratio of the conversion mode to the acquisition mode can be on the order of about 5:1, i.e., about 1.25 ps to .25 ps. Thus, if the light source is completely attenuated for the entire duration of the conversion mode then for such an ADC the amount of time the alignment mark is illuminated can be reduced by a factor of 6. Of course, it will be appreciated that while making the attenuation period completely coincident with the conversion period has the potential to yield the most benefit, benefits will still be achieved even if the attenuation period is of a shorter duration than the conversion period.

[0052] The invention may be equivalently conceptionalized as attenuating the illumination power during a period when analog data is not being converted and as increasing power during a sampling period, as long as the illumination source has a first mode in which it illuminates with a first power and a second mode in which it illuminates with a second power less than the first power, including the case where the second power is zero, i.e., complete attenuation, and the illumination source is in the second mode for at least part of a period in which the analog signal from the photodetectors is not being measured.

[0053] Reducing the laser power results in a reduction of temperature increase. Both continuous wave (CW) and pulsed lasers can be treated equally from the perspective of thermally-induced damage. A first order estimate of the temperature increase (and decrease) in a material due to changing laser power can be obtained using the relationship Q=mc \T, where Q is the power, m is the mass and c is the specific heat. Thus, there is a linear relationship between laser power and (average) temperature increase. Therefore, if the laser power by is reduced by a factor of 6 times, temperature increase can be expected to be reduced by a factor of 6.

[0054] In addition, other damage mechanisms may increase exponentially with temperature. One such mechanism is a thermally- activated diffusion process, where due to the excess heat diffusion starts to occur in the alignment mark plate. The diffusion has an exponential dependence on the temperature, and would be decreased by a much larger factor. [0055] The technique described may introduce a switching frequency into the alignment signal which might affect the position obtained. However, fundamentally, the switching occurs at a frequency f_SWitch that is equal to the sampling frequency f_s. Because an anti-aliasing filter provided in the system rejects all frequency components above f_s/2, f_SWitch will be filtered out as well and should not affect alignment signal. Thus there is no detriment to accurate determination of alignment.

[0056] According to another aspect of an embodiment a method of limiting damage to an alignment sensing system by modulating light source intensity is shown in FIG. 5. In a step S10 the ADC samples the analog signal from the photodetectors. In a step S20 the ADC converts the sample to a digital signal. As described above, while the ADC is so engaged it is not sampling the analog signal. Accordingly in a step S30 the radiation from the radiation source is partially or completely attenuated. In step S40 it is determined whether the ADC has completed conversion. This may be based on a signal from the ADC or, for example, on the passage of time or some other indicator. If conversion is not complete then the radiation remains attenuated. If conversion is complete then in a step S50 the attenuation is discontinued. [0057] While ADC-specific terms such as sample and hold are used herein the underlying concept is that there are modes or periods of time when the circuitry that converts the analog detector signal into a digital signal is insensitive to the analog signal so that it is possible to partially or completely attenuate the radiation illuminating the alignment mark during at least part of these periods to avoid unnecessary exposure of the wafer and/or system components to that radiation.

[0058] The embodiments may further be described using the following clauses:

1. Apparatus for measuring an alignment mark on a substrate, the apparatus comprising: a source of radiation adapted to illuminate the alignment mark, the source of radiation being configured to have a first state in which the source of radiation produces radiation having a first intensity and a second state in which the source of radiation produces radiation having a second intensity less than the first intensity; a detector arranged to detect an intensity of radiation diffracted by the alignment mark and to produce a signal indicative of the intensity; and a converter arranged to receive the signal and to generate a digital signal based at least in part on the signal, the converter being configured to have a first mode in which the converter samples the signal and a second mode in which the converter does not sample the signal, the source of radiation being configured to produce radiation having the second intensity for at least part of a time when the when the converter is in the second mode.

2. Apparatus for measuring an alignment mark on a substrate of clause 1 further comprising an attenuator wheel optically positioned to attenuate radiation from the radiation source.

3. Apparatus for measuring an alignment mark on a substrate of clause 1 wherein the converter is configured to produce a signal indicating whether the converter is in the second mode and the further comprising a controller arranged to receive the signal and configured to control an intensity of the radiation source based at least in part on the signal.

4. Apparatus for measuring an alignment mark on a substrate of clause 1 wherein the first mode is a sample mode and the second mode is a hold mode. 5. Apparatus for measuring a parameter of an alignment mark on a substrate, the apparatus comprising: a source of radiation adapted to illuminate the alignment mark; a detector arranged to detect an intensity of radiation diffracted by the alignment mark and to produce a signal indicative of the intensity; and a converter arranged to receive the signal and to generate a digital signal based at least in part on the signal, the converter having a first mode in which the converter samples the signal and a second mode in which the converter does not sample the signal, the source of radiation being configured to reduce an intensity of radiation output by the source of radiation at least part of a time when the converter is in the second mode.

6. Apparatus for measuring an alignment mark on a substrate of clause 5 wherein the radiation source comprises an attenuator wheel.

7. Apparatus for measuring an alignment mark on a substrate of clause 5 wherein the converter is configured to produce signal indicating whether the converter is in the second mode and the further comprising a controller arranged to receive the signal and configured to control an intensity of the radiation source based at least in part on the signal.

8. Apparatus for measuring an alignment mark on a substrate of clause 5 wherein the first mode is a sample mode and the second mode is a hold mode.

9. A photolithography method comprising the steps of: using a source of radiation to illuminate at least one alignment mark, the source of radiation being configured to have a first state in which the source of radiation produces radiation having a first intensity and a second state in which the source of radiation produces radiation having a second intensity less than the first intensity; using a detector to detect an intensity of radiation diffracted by the at least one alignment mark and to produce a signal indicative of the intensity; and using a converter arranged to receive the signal to generate a digital signal based at least in part on the signal, the converter having a sample mode in which the converter samples the signal and a hold mode in which the converter does not sample the signal, the source of radiation producing radiation having the second intensity when the converter is in the hold mode.

10. A photolithography method comprising the steps of: illuminating at least one alignment mark, the source of radiation being configured to have a first state in which the source of radiation produces radiation having a first intensity and a second state in which the source of radiation produces radiation having a second intensity less than the first intensity; detecting an intensity of radiation diffracted by the at least one alignment mark and producing a signal indicative of the intensity; generating a digital signal based at least in part on the signal, wherein the generating comprises a sampling phase in which the signal is sampled and a hold phase in which the signal is not sampled; and producing radiation having the second intensity when the generating step is in the hold phase.

11. A photolithography method comprising the steps of: illuminating at least one alignment mark; detecting an intensity of radiation diffracted by the at least one alignment mark and producing a signal indicative of the intensity; generating a digital signal based at least in part on the signal, wherein the generating step comprises a sampling phase in which the signal is sampled and a hold phase in which the signal is not sampled; and reducing an intensity of radiation in the illuminating step when the generating step is in the hold phase.

[0059] 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. The skilled artisan 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 track (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. Further, 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.

[0060] 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 may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured. [0061] 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 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0062] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, electromagnetic and electrostatic optical components.

[0063] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0064] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. [0065] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents..

Claims

2. Apparatus for measuring an alignment mark on a substrate as claimed in claim 1 further comprising an attenuator wheel optically positioned to attenuate radiation from the radiation source.

3. Apparatus for measuring an alignment mark on a substrate as claimed in claim 1 wherein the converter is configured to produce a signal indicating whether the converter is in the second mode and the further comprising a controller arranged to receive the signal and configured to control an intensity of the radiation source based at least in part on the signal.

4. Apparatus for measuring an alignment mark on a substrate as claimed in claim 1 wherein the first mode is a sample mode and the second mode is a hold mode.

5. Apparatus for measuring a parameter of an alignment mark on a substrate, the apparatus comprising: a source of radiation adapted to illuminate the alignment mark; a detector arranged to detect an intensity of radiation diffracted by the alignment mark and to produce a signal indicative of the intensity; and a converter arranged to receive the signal and to generate a digital signal based at least in part on the signal, the converter having a first mode in which the converter samples the signal and a second mode in which the converter does not sample the signal, the source of radiation being configured to reduce an intensity of radiation output by the source of radiation at least part of a time when the converter is in the second mode.

6. Apparatus for measuring an alignment mark on a substrate as claimed in claim 5 wherein the radiation source comprises an attenuator wheel.

7. Apparatus for measuring an alignment mark on a substrate as claimed in claim 5 wherein the converter is configured to produce signal indicating whether the converter is in the second mode and the further comprising a controller arranged to receive the signal and configured to control an intensity of the radiation source based at least in part on the signal.

8. Apparatus for measuring an alignment mark on a substrate as claimed in claim 5 wherein the first mode is a sample mode and the second mode is a hold mode.