Level sensor

Abstract

Disclosed is a sensor, comprising: at least one phase modulating projection grating element being configured to scatter measurement radiation and impose a phase modulation onto the resultant scattered radiation; at least one detector being configured to receive radiation derived from reflected radiation comprising said scattered radiation subsequent to reflection from a surface to be measured; and at least one processor unit being operable to determine a measurement signal comprising a parameter of said radiation received at the detector, and determine a height of said surface from a phase of said measurement signal.

Description

LEVEL SENSORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24197963.2 which was filed on 2 September 2024, and which is incorporated herein in its entirety by reference.FIELD OF THE INVENTION

[0002] The present invention relates to methods and apparatus usable, for example, in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques. In particular the invention relates to a level sensor which may used in such apparatuses.BACKGROUND ART

[0003] A lithographic apparatus or exposure apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of a die, one die, 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. These target portions are commonly referred to as “fields”.

[0004] In lithography, a flatness of the substrate (e.g. the wafer) may be measured and e.g. stored as a height map. The height map may be used to position a relevant target portion of the substrate at an appropriate height, in order to provide that, when projecting a pattern onto the target portion of the substrate, that target portion is positioned within a focal range (depth of focus) of a projection system (e.g., a projection lens) of the lithographic apparatus. Compiling the height map is also referred to as "level sensing" . The level sensing may be performed by a level sensor. The level sensor may be integral to a lithographic apparatus or may be a separate measurement arrangement. The level sensor may make use of an optical measurement, by means of projecting a measurement beam onto the substrate and detecting a reflection thereof. In certain detection schemes, use may be made of gratings in an optical path of the measurement beam, e.g. a projection grating upstream of the substrate and a detection grating downstream of the substrate.

[0005] Consider a level sensor having a projection grating, a detection grating and a detector. The projection grating is imaged on the substrate surface at an angle relative to the (ideal) surface normal. The image is reflected by the wafer surface and re-imaged on the detection grating. Due to the oblique incidence, a variation in the substrate's height will shift the image of the projection grating on the detection grating over a certain distance. The shifted image of the projection grating is partiallytransmited by the detection grating. The detector detects an intensity of the transmited image. The intensity is indicative of the substrate's local height. In other words, a height variation of the surface of the substrate results in a variation in the image transmited by the detection grating, allowing deriving height information from the detector signal. In a measurement principle as may be applied, the optical path of the image of the projection grating should be accurately set, so as to provide that the projection grating is correctly imaged onto the detection grating.

[0006] Present level sensors have a limited height range over which height can be determined unambiguously. Additionally, they require a large detector arrangement within the sensor critical volume.

[0007] Consequently, a solution is desirable which overcomes at least some of the above-mentioned disadvantages.SUMMARY OF THE INVENTION

[0008] The invention in a first aspect provides a sensor, comprising: at least one phase modulating projection grating element being configured to scater measurement radiation and impose a phase modulation onto the resultant scatered radiation; at least one detector being configured to receive radiation derived from reflected radiation comprising said scatered radiation subsequent to reflection from a surface to be measured; and at least one processor unit being operable to determine a measurement signal comprising a parameter of said radiation received at the detector, and determine a height of said surface from a phase of said measurement signal.

[0009] The invention in a second aspect provides a method of measuring a height of a surface, the method comprising: scatering measurement radiation; imposing a phase modulation onto the resultant scatered radiation; reflecting said scatered radiation off said surface to generate reflected radiation; detecting said reflected radiation; determining a measurement signal comprising a parameter of said detected radiation over time, and determining a height of said surface from a phase of said measurement signal.

[0010] The above and other aspects of the invention will be understood from a consideration of the examples described below.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 depicts a lithographic apparatus; and- Figure 2 depicts a schematic of a measurement and exposure processes in the apparatus of Figure 1; and- Figure 3(a) depicts a schematic view of a level sensor arrangement;- Figure 3(b) depicts a schematic view of a detection arrangement of the level sensor arrangement of Figure 3(a);- Figure 4 depicts a schematic view of a first example level sensor arrangement according to concepts disclosed herein;- Figure 5 depicts a plot of intensity against time as may be detected on a photodetector of the level sensor arrangement of Figure 4 during a measurement;- Figure 6 depicts schematically a dual-period grating and its constituent grating period component usable in an extended measurement range example of the level sensor arrangement of Figure 4;- Figure 7 depicts a schematic view of a second example level sensor arrangement according to concepts disclosed herein;- Figure 8 depicts a schematic view of a third example level sensor arrangement according to concepts disclosed herein.DETAILED DESCRIPTION OF EMBODIMENTS

[0012] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0013] Figure 1 schematically depicts an exposure apparatus or lithographic apparatus LA. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a patterning device support or 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; two substrate tables (e.g., a wafer table) WTa and WTb each constructed to hold a substrate (e.g., a resist coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. A reference frame RF connects the various components, and serves as a reference for setting and measuring positions of the patterning device and substrate and of features on them.

[0014] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0015] The patterning device support MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the exposure apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support MT may be a frame or a table, for example, whichmay be fixed or movable as required. The paterning device support may ensure that the paterning device is at a desired position, for example with respect to the projection system.

[0016] The term “paterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a patern in its cross-section such as to create a patern in a target portion of the substrate. It should be noted that the patern imparted to the radiation beam may not exactly correspond to the desired patern in the target portion of the substrate, for example if the patern includes phase-shifting features or so called assist features. Generally, the patern 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.

[0017] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive paterning device). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask). Examples of paterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “paterning device.” The term “paterning device” can also be interpreted as referring to a device storing in digital form patern information for use in controlling such a programmable paterning device.

[0018] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0019] The exposure 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 exposure 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.

[0020] In operation, the illuminator IL receives a radiation beam from a radiation source SO. The source and the exposure 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 exposure apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases the source may be an integral part of the exposure 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.

[0021] 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 aso-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross section.

[0022] The radiation beam B is incident on the patterning device MA, which is held on the patterning device support MT, and is patterned by the patterning device. Having traversed the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WTa or WTb 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 Figure 1) can be used to accurately position the patterning device (e.g., 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.

[0023] The exposure apparatus may comprise an aberration sensor for the verification of an aberration fingerprint of the proj ection system PS . In an embodiment such an aberration fingerprint, i .e . aberrations per field point of the projection system PS, may be determined using a such wavefront aberration sensor. A wavefront aberration sensor of a known type, for instance such as described in US2002 / 0001088, incorporated herein by reference, may be used. Such a wavefront aberration sensor may be based on the principle of shearing interferometry and comprises a source module and a sensor module. The source module may comprise a patterned layer of chromium that is placed in the object plane (i.e. where during production the pattern of the patterning means is) of the projection system PS and has additional optics provided above the chromium layer. The combination provides a wavefront of radiation to the entire pupil of the projection system PS. The sensor module may comprise a patterned layer of chromium that is placed in the image plane of the projection system (i.e. where during production the substrate W is) and a camera that is placed some distance behind said layer of chromium. The patterned layer of chromium on the sensor module diffracts radiation into several diffraction orders that interfere with each other giving rise to an interferogram. The interferogram is measured by the camera. The aberrations in the projection lens can be determined by software based upon the measured interferogram. The wavefront aberration sensor may be configured to transfer information with respect to the aberration fingerprint towards the control unit.

[0024] Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks may be located between the dies. Small alignment marks may also be included within dies, in amongst the device features, in which caseit is desirable that the markers be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment markers is described further below.

[0025] The depicted apparatus could be used in a variety of modes. In a scan mode, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. Other types of exposure apparatus and modes of operation are possible, as is well-known in the art. For example, a step mode is known. In so-called “maskless” lithography, a programmable patterning device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned.

[0026] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0027] Exposure apparatus LA is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations - an exposure station EXP and a measurement station MEA - between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. This enables a substantial increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height contours of the substrate using a height sensor or height measurement arrangement such as a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. If the position sensor IF is not capable of measuring the position of the substrate table while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations, relative to reference frame RF. Other arrangements are known and usable instead of the dual-stage arrangement shown. For example, other exposure apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate table undergoes exposure.

[0028] The exposure apparatus control unit LACU which controls all the movements and measurements of various actuators and sensors to receive substrates W and reticles MA and to implement the patterning operations. Exposure apparatus control unit LACU also includes signal processing and data processing capacity to implement desired calculations relevant to the operation of the apparatus, e.g., based on inter alia level sensor LS data, alignment sensor AS data and feedback metrology data (e.g., one or more of inter alia overlay, focus, dose, critical dimension data). In practice,control unit LACU may be realized as a system of many sub-units, each handling the real-time data acquisition, processing and control of a subsystem or component within the apparatus.

[0029] Figure 2 illustrates the steps to expose target portions (e.g. dies) on a substrate W in the dual stage apparatus of Figure 1. On the left hand side within a dotted box are steps performed at a measurement station MEA, while the right hand side shows steps performed at the exposure station EXP. From time to time, one of the substrate tables WTa, WTb will be at the exposure station, while the other is at the measurement station, as described above. For the purposes of this description, it is assumed that a substrate W has already been loaded into the exposure station. At step 200, a new substrate W’ is loaded to the apparatus by a mechanism not shown. These two substrates are processed in parallel in order to increase the throughput of the exposure apparatus.

[0030] Referring initially to the newly-loaded substrate W’, this may be a previously unprocessed substrate, prepared with a new photo resist for first time exposure in the apparatus. In general, however, the lithography process described will be merely one step in a series of exposure and processing steps, so that substrate W’ has been through this apparatus and / or other lithography apparatuses, several times already, and may have subsequent processes to undergo as well. Particularly for the problem of improving overlay performance, the task is to ensure that new patterns are applied in exactly the correct position on a substrate that has already been subjected to one or more cycles of patterning and processing. These processing steps progressively introduce distortions in the substrate that must be measured and corrected for, to achieve satisfactory overlay performance.

[0031] The previous and / or subsequent patterning step may be performed in other lithography apparatuses, as just mentioned, and may even be performed in different types of lithography apparatus. For example, some layers in the device manufacturing process which are very demanding in parameters such as resolution and overlay may be performed in a more advanced lithography tool than other layers that are less demanding. Therefore some layers may be exposed in an immersion type lithography tool, while others are exposed in a ‘dry’ tool. Some layers may be exposed in a tool working at DUV wavelengths, while others are exposed using EUV wavelength radiation.

[0032] At 202, alignment measurements using the substrate marks Pl etc. and image sensors (not shown) are used to measure and record alignment of the substrate relative to substrate table WTa / WTb. In addition, several alignment marks across the substrate W’ will be measured using alignment sensor AS. These measurements are used in one embodiment to establish a “wafer grid”, which maps very accurately the distribution of marks across the substrate, including any distortion relative to a nominal rectangular grid.

[0033] At step 204, a map of wafer height (Z) against X-Y position is measured also using the level sensor LS. Conventionally, the height map is used only to achieve accurate focusing of the exposed pattern. It may be used for other purposes in addition.

[0034] When substrate W’ was loaded, recipe data 206 were received, defining the exposures to be performed, and also properties of the wafer and the patterns previously made and to be made upon it.To these recipe data are added the measurements of wafer position, wafer grid and height map that were made at 202, 204, so that a complete set of recipe and measurement data 208 can be passed to the exposure station EXP. The measurements of alignment data for example comprise X and Y positions of alignment targets formed in a fixed or nominally fixed relationship to the product patterns that are the product of the exposure process. These alignment data, taken just before exposure, are used to generate an alignment model with parameters that fit the alignment model to the data. These parameters and the alignment model will be used during the exposure operation to correct positions of patterns applied in the current lithographic step. The model in use interpolates positional deviations between the measured positions. A conventional alignment model might comprise four, five or six parameters, together defining translation, rotation and scaling of the ‘ideal’ grid, in different dimensions. Advanced models are known that use more parameters.

[0035] At 210, wafers W’ and W are swapped, so that the measured substrate W’ becomes the substrate W entering the exposure station EXP. In the example apparatus of Figure 1, this swapping is performed by exchanging the supports WTa and WTb within the apparatus, so that the substrates W, W’ remain accurately clamped and positioned on those supports, to preserve relative alignment between the substrate tables and substrates themselves. Accordingly, once the tables have been swapped, determining the relative position between projection system PS and substrate table WTb (formerly WTa) is all that is necessary to make use of the measurement information 202, 204 for the substrate W (formerly W’) in control of the exposure steps. At step 212, reticle alignment is performed using the mask alignment marks Ml, M2. In steps 214, 216, 218, scanning motions and radiation pulses are applied at successive target locations across the substrate W, in order to complete the exposure of a number of patterns.

[0036] By using the alignment data and height map obtained at the measuring station in the performance of the exposure steps, these patterns are accurately aligned with respect to the desired locations, and, in particular, with respect to features previously laid down on the same substrate. The exposed substrate, now labeled W” is unloaded from the apparatus at step 220, to undergo etching or other processes, in accordance with the exposed pattern.

[0037] The skilled person will know that the above description is a simplified overview of a number of very detailed steps involved in one example of a real manufacturing situation. For example rather than measuring alignment in a single pass, often there will be separate phases of coarse and fine measurement, using the same or different marks. The coarse and / or fine alignment measurement steps can be performed before or after the height measurement, or interleaved.

[0038] In an exposure apparatus, it is known to control machine settings, e.g. lens settings and (reticle and substrate) stage settings, during exposures onto a substrate to optimize the projected image in terms of one or more placement parameters of interest, e.g., parameters related to placement of structures on the substrate. Such placement parameters of interest may comprise parameters on which device functionality and yield are dependent. In particular, the machine settings typically controlled includeinter alia stage settings, lens control settings (e.g., to correct for lens aberration) and dose control settings. Stage settings may describe a planned or set stage movement such as described by various stage setpoints determined by the control hardware and software (e.g., as controlled by lithographic apparatus control unit or exposure apparatus control unit LACU). These may relate to stage positioning, speed, and / or acceleration in each of the substrate plane (e.g., for placement / overlay control) and perpendicular to the substrate plane (e.g., for focus control).

[0039] In this manner, parameters of interest such as one or more of overlay, focus, and imaging (e.g., critical dimension (CD)) may be optimized. Such an optimization may comprise some trade-off between some of these parameters, but which ensure that each of these parameters of interest remain within- specification (e.g., within a tolerance indicative of a yielding or functional device). As such, “optimize” in the context of this disclosure may simply mean improve and / or ensure that the parameter is within- specification. Alternatively, or in addition, “optimize” may mean optimizing towards a target projection (e.g., an optimized or ideal projection) onto the substrate (e.g., where there is a single ideal setting rather than a specification range. Similarly “minimize” in the context of this disclosure may simply mean reduce, e.g., minimizing variation may be understood to reduce variation and / or to reduce said variation as much as is practically possible.

[0040] A sensor such as a level sensor can determine a height (of e.g., a substrate) by measuring an intensity imbalance between scattered radiation (e.g., between a pair of complementary diffraction orders) from a grating. A first grating (projection grating) projects a patterned beam onto a wafer, and a second grating (detection grating) forms part of a detection arrangement which measures the intensity of the transmitted light. A projection of the fringes generated by the projection grating onto the detection grating is used to determine asymmetry of the scattered radiation: if the detection grating is not symmetrically illuminated, caused by a change of wafer height, an intensity imbalance is measured. The reference grating may comprise wedges to enhance asymmetry detection.

[0041] Figure 3(a) schematically depicts a known level sensor arrangement 10 according to an embodiment of the present invention, which may be used in a lithographic apparatus as described earlier (e.g., level sensor LS described in relation to Figure 1). More detail on this level sensor arrangement 10 may be found in patent publication US2010 / 0233600, which is hereby incorporated by reference. A radiation source 16 is used (e.g., having a specific radiation spectrum), which emits radiation towards the substrate W. The radiation from the radiation source 16 is directed at a projection grating 17 positioned between the radiation source 16 and substrate W. Using a reflective projection system comprising mirrors 14 and 15 in the example shown, the image formed by the projection grating 17 is projected on the substrate W. The reflected radiation from the substrate W is again optically focused using further reflective elements 14 and 15 to a detector unit 18. The detector unit 18 is e.g. provided with a UV sensor to measure the radiation reflected from the substrate W and to determine the substrate height (profile) from the reflected radiation in operation.

[0042] Figure 3(b) schematically depicts a known type of detector unit 18 or detection arrangement which may be used with the level sensor arrangement 10 of Figure 3(a). The detector unit 18 may comprise a (relatively shallow ruled) detection grating 31 with a triangular grating profde (e.g., comprising wedges). The ruled grating 31 may function as a phase grating. The pitch P of the ruled grating 31 is larger than the wavelength of the radiation emitted by the radiation source 16. The pitch P is e.g., typically equal to the period of the image of the projection grating 17 (e.g., the projection grating and detection gratings are matched gratings). In Figure 3(b) a transmission grating 31 is shown, but as an alternative a ruled reflection grating maybe used.

[0043] The triangular grating profde of the ruled grating 31 acts as a series of wedges 3 la, 3 lb, which redirects the radiation beam 20 according to the well-known Snell's law. The image that is on a positive wedge 31a is redirected in an upward direction (‘u’) and the image that is on a negative wedge 31b is redirected in a downward direction (‘d’). If the image is exactly centered on the ruled grating 31 then the ‘u’ and ‘d’ images have equal intensity and the output of a processing unit 37 connected to two detectors 35, 36 will be zero. If the substrate height changes then the image of the projection grating 31 will also shift and this will result in an unbalance between the ‘u’ and ‘d’ signals.

[0044] Methods disclosed herein aim to improve on known level sensor arrangements such as illustrated in Figure 3(a). For example, in the present arrangement, the detection optics (i.e., the complete detector unit 18) are comprised within the critical volume. It is desirable to reduce the component volume within the critical volume.

[0045] Additionally, developments in IC manufacture means there are increasingly large height differences on wafer, e.g., trenches may be 20pm or more in depth, which may be outside of the capture range of present level sensors (e.g., more than one fringe period at a single frequency). It would be desirable to increase e.g., the height capture range of the sensor.

[0046] The basic working principle of the level sensor, as described, shares some commonality with some alignment sensors presently used for measuring alignment in a lithographic apparatus. Typically, alignment sensors measure substrate distortion in the substrate plane while a level sensor measures substrate distortion perpendicular to the substrate plane. It is proposed herein that some of these alignment sensor principles are implemented within the level sensor arrangement.

[0047] In alignment, instead of measuring an intensity difference between diffracted orders, it is the phase between diffracted orders which is used to determine position. To achieve this, the alignment sensor scans the phase over an alignment mark on the wafer. This is typically achieved by scanning the measurement radiation (measurement beam) with respect to the alignment mark (grating).

[0048] A level sensor arrangement is proposed herein which determines the substrate height from the phase of reflected scattered radiation, having been scattered by a projection grating (or illumination grating), reflected from the substrate, and imaged onto a detection grating. To achieve this, it is proposed that the projection grating is configured to impose a (e.g., linear) phase modulation onto the scattered radiation (e.g., onto the diffracted orders including at least one pair of complementary diffraction orders)from the projection grating. This modulation may be performed in an image plane, e.g., by moving the projection grating.

[0049] In an embodiment, the projection grating may comprise an acousto-optical grating, e.g., implemented using an acousto-optical modulator such as an acousto-optical tunable fdter (AOTF). Acousto-optical modulators are well known and function by inducing a change in refractive index of a medium by introducing acoustic waves in that medium. The acoustic waves produce a refractive index grating in the material, and it is this grating that is "seen" by the radiation wave. The acoustic waves may be induced in the medium using a piezoelectric transducer, for example. The piezoelectric transducer may be controlled to vary frequency (i.e., the effective grating pitch) and / or can be simultaneously driven at multiple frequencies (to impose an effective grating with multiple pitches simultaneously).

[0050] It can be appreciated that a linear phase scanning of the projection grating may be implemented in other ways, e.g., by physically moving a physical projection grating. For example, the projection grating may be carried on a rotating disk (e.g., an optical chopper).

[0051] The detection arrangement may comprise a detection grating, onto which the reflected scattered radiation from the projection grating and substrate surface is imaged. The detection grating may be static detection grating. The detection grating may be a simpler, conventional grating, rather than the wedge ruled grating such as illustrated in Figure 3(b), although the latter may still be used. As an alternative, the detection grating may also comprise an acousto-optical grating, e.g., implemented using an acousto-optical modulator such as an acousto-optical tunable fdter. Advantages of the latter alternative will be described below.

[0052] The detector may comprise an intensity detector, e.g., a photodiode, which measures the intensity of the radiation scattered from the detection grating. For example, the intensity detector may measure the intensity of the zeroth order scattered radiation from the detection grating. The intensity may be recorded over time to obtain an interferogram signal. The frequency of the interferogram is dictated by the scan speed of the projection grating with respect to the detection grating, and the grating pitch. The height of the substrate surface is determined from the phase of the interferogram.

[0053] Additionally, intensity data describing the intensity magnitude may also be used to improve measurement accuracy. For example, the intensity magnitude may be determined per-color in a similar matter to a spectral ellipsometer. By considering the reflection spectrum and color-to-color intensities (spectrally resolved intensity), it is possible to reconstruct intensity information. The intensity magnitude measured may comprise one or more of the AC (modulation) component, the DC component and the sum of these components. This intensity data may be used to mitigate and / or correct for height process dependency (HPD), which is a phenomenon whereby reflected light from underlying layers can create errors in the height measurement.

[0054] The fact that the interferogram signal can be measured using a single intensity measurement (e.g., using a photodiode) over a time period means that the detector and associated detector optics maybe removed from the critical volume of the level sensor. For example, a suitable light carrier (e.g., a multi-mode fiber) can be positioned to collect the (e.g., zeroth order) reflected scattered radiation from the detection grating and transport this reflected scattered radiation to a detection arrangement (e.g., detector / photodiode and associated detection optics) located remotely from the critical volume.

[0055] Figure 4 is a highly simplified schematic illustration of a level sensor arrangement 400 according to an embodiment. Measurement radiation from a radiation source (not shown) is directed at a linear phase modulating first grating element or linear phase modulating projection grating element 417. Such a linear phase modulating projection grating element 417 may be configured to impose a continuous linear phase modulation on the phase difference between diffraction orders of a complementary pair of diffraction orders (e.g., between the +1 and -1 diffraction orders, +1 and -2 orders etc.). The linear phase modulating projection grating element 417 may comprise an acousto- optical modulator (e.g., an AOTF) as described, e.g., driven to linearly scan the acoustic waves (acoustic grating) along the direction of periodicity (as symbolized by the arrow, with the waves moving in either one of the two directions illustrated), although other alternatives are possible (e.g., actuating a physical grating as has been described). In this way, the projection grating is scanned linearly in a direction substantially perpendicularly to the propagation direction of the radiation from the radiation source.

[0056] The scattered radiation from linear phase modulating projection grating element 417 may be directed to (e.g., focused on) the surface to be measured, e.g., a substrate W, for example, using a projection optical arrangement represented here by lens arrangement 422a, 422b. The angle of incidence onto the substrate may be an oblique angle (e.g., not normal to the substrate surface). Due to the sweeping of the projection diffraction grating, the measurement spot 426 on substrate W comprises scanning fringes.

[0057] A detection optical arrangement represented by lens arrangement 424a, 424b reimages the reflected radiation from the substrate onto the detection grating element 431. The illumination and detection optical arrangements may each comprise a 4f optical system.

[0058] The detection grating element 431 may comprise a static grating with a pitch matched to that of the linear phase modulating projection grating element 417. Alternatively, detection grating element 431 may comprise an acousto-optical modulator (e.g., an AOTF), again matched with the projection grating element 417. Radiation scattered by the detection grating (e.g., the zeroth order radiation) is detected by a detector 435 (e.g., a photodiode).

[0059] Optionally, the detection may be performed via a color filter or color filter module for demultiplexing of the level sensor signal, e.g., located between the detection grating element 431 and detector 435. In such an arrangement, the measurement radiation may comprise multiple wavelengths or multiple wavelength bands.

[0060] It can be appreciated that each of the optical elements shown, e.g., lens arrangements 422a, 422b, 424a, 424b, linear phase modulating proj ection grating element 417 and detection grating element431 may be transmissive or reflective. The actual projection optics and / or detection optics may be more complex and comprise more individual elements than shown.

[0061] Figure 5 is a plot of intensity over time illustrating an exemplary detected measurement signal 428 as may be determined from the detected radiation on detector 436 using the apparatus of Figure 4, whereby the projection grating is scanned during the measurement as has been described. The intensity varies substantially sinusoidally over time. The measured phase of this signal 428 correlates directly with the surface height (parameter of interest) of the substrate W. As such, a processing unit 436 is configured to determine this measurement signal 428 and the substrate surface height from the phase of the measurement signal 428. By way of a specific example, the fringe pattern is projected on the substrate with a known angle of incidence (e.g., an oblique, i.e., non-normal and non-parallel angle with respect to the substrate surface) When the substrate changes in height (e.g., has a thicker portion), the projected fringes will undergo a lateral displacement which corresponds to a phase change. As such, by measuring the phase change, it is possible to deduce the height change. To determine the absolute height, a calibration step may be performed, e.g., by performing measurements on an object of known height (e.g., measure a substrate with a known thickness). A calibration factor can then be determined and later used to convert the measured phase to a substrate height.

[0062] The signal magnitude may be used for ellipsometry-like measurements, as has been described.

[0063] It was mentioned that the detection grating element 431 may comprise an acousto-optical modulator. Where this is the case, the projection acousto-optical modulator and detection acousto- optical modulator should be locked and stable with respect to each other (each driven at one or more stable frequencies). However, they should not be driven at the same frequency or frequencies while also set to impose the same phase offset, otherwise the phase modulation cancels; the effect of two (e.g., acoustic) gratings which move in the same direction with the same frequency / pitch cancel each other out.

[0064] Using an acousto-optical modulator for detection provides enables the flexibility of matched programmable gratings to be used within the level sensor. The switch time to switch between different pitches is on the order of a millisecond or less. This can enable the level sensor to switch between a coarse measurement mode and a fine measurement mode, in a manner synonymous with present coarse and fine alignment capture presently performed. This may be useful in particular when measuring a substrate surface with large height differences, such as has been described. For example, in substrate regions where these differences may be larger than a typical capture range (limited in this example by a phase cycle of the measured signal), the gratings may be switched into a coarse measurement mode (e.g., driven at a first frequency corresponding to a larger pitch), extending the measurable height range of the level sensor. Once the height is determined coarsely, the gratings may be switched into a fine measurement mode (e.g., driven at a first frequency corresponding to a smaller pitch) to measure the height more precisely.

[0065] Rather than switching between frequencies, the projection acousto-optical modulator may be driven at two (or more) different frequencies simultaneously. The detection grating may be implemented as an acousto-optical modulator driven at the same two frequencies, or as a fixed grating comprising the two corresponding (matched) pitches. Figure 6 illustrates a detection grating 600 (whether implemented as a fixed grating or within an acousto-optical modulator medium) comprising a structure having a first period 605 and second period 610. In the resulting zeroth order (i.e., detected radiation comprising the signal detected by detector 435), the two frequency components can be disentangled and the Nonius principle (e.g., as used on a Vernier scale) applied to extend the height range with good precision.

[0066] Figure 7 illustrates an alternative method for implementing two-frequency level sensing, e.g., to increase the measurable height range. A first linear phase modulating projection grating element 417a has a first pitch or is driven at a first frequency depending on how it is implemented (e.g., a moving grating or an acousto-optical modulator), and a second linear phase modulating projection grating element 417b has a second pitch or is being driven at a second frequency. The first linear phase modulating projection grating element 417a projects first radiation of a first wavelength or first waveband towards a first dichromic mirror 441 and the second linear phase modulating projection grating element 417b projects second radiation of a second wavelength or second waveband towards the first dichromic mirror 441. The first dichromic mirror 441 combines the first radiation and second radiation, and the combined radiation is projected onto substrate W via projection optics 422a, 422b. The reflected radiation is captured by detection optics 424a, 424b and split into its component wavelengths / wavebands by a second dichromic mirror 443. The first radiation is detected via a first detection grating element 43 la and first detector 435a and the second radiation is detected via a second detection grating element 43 lb and second detector 435b. The first detection grating element 43 la and second detection grating element 431b may each comprise a fixed grating or an acousto-optical modulator as before, in each case matched to its corresponding projection grating.

[0067] A further application using an acousto-optical modulator for both projection and detection is that Doppler shifting may be employed, e.g., to reduce electronic noise (e.g., with respect to the signal of interest). Such an approach may comprise using two very close frequencies (e.g., a difference in driving frequencies of less than 1 MHz) respectively for each of the projection grating and detection grating. By way of a specific example, the projection modulator may be driven at 100.1 MHz and the detection modulator driven at 100.0 MHz. This results in an effective difference frequency to be detected at 0.1 MHz.

[0068] Figure 8 illustrates a variation on the level sensor arrangement examples provided above, wherein a self-referencing interferometer (SRI) 845 is used instead of a detection grating element. The concept of an SRI is known in alignment, and can be found in many alignment sensors. The SRI 845 produces two overlapping and relatively rotated images of the projection grating element 417. Respective detectors 835a, 835b detect intensities in a pupil plane where Fourier transforms of theimages are caused to interfere. The height is derived from the phase difference between diffraction orders of the two images, which manifests as intensity variations in the interfered orders. Respective SRIs may be used instead of detection grating elements 43 la, 43 lb in the example of Figure 7.

[0069] The concept of an SRI is described in more detail in US Patent No. 6,961,116, which is disclosed herein by reference. The advantage of using an SRI is that detection is independent of the projection pitch and orientation, such that there is no need for a matched detection grating.

[0070] In any of the examples described herein, the modulation frequency may be between 10kHz and 500kHz or between 20kHz to 200kHz in the acousto-optical modulation case (or correspondingly have a pitch of between 1 pm and 50 pm or 5 pm and 50 pm for a fixed grating). The smaller range, for example, assumes a 1mm measurement spot and 0.5ms detector integration time during which 10 to 100 fringes should be detected to obtain a reliable phase measurement.

[0071] In an embodiment, a form of lock-in detection may be employed, e.g., based on the modulation imposed by the projection grating. This lock-in detector may have a high bandwidth. The phase can be down sampled to match the lithographic apparatus sync. Present sync within the lithographic apparatus on x,y, positions has a lower clock frequency, e.g., in the region of 20 kHz. It may be advantageous to be able to measure and analyze fringes that have been cycled with e.g., 1000 KHz, but without increase the x,y sync in the lithographic apparatus. Therefore, using lock-in detection, e.g., using a fast diode and a lock in-amplifier with a suitable bandpass (e.g., 20 kHz), the output can be matched to sync of the scanner. In other words, the detector and analyzer may be fast, while the recorded output is matched to the slow sync of lithographic apparatus.

[0072] Another advantage of the proposed level sensor arrangement is that many of the detection elements may be the same as those of the alignment sensor. As such, these modules may be common with the equivalent alignment modules. Such modules may comprise, where provided, a multi-mode fiber to transport the radiation scattered from the detection grating element and any fiber coupling lens, a demultiplexing color filter and the detector photodiode.

[0073] In the above-described embodiments, a linear phase modulating projection grating element being configured to impose a linear phase modulation is described. It will be appreciated that such the phase modulating projection grating is not limited to a linear phase modulating projection grating, and the imposed phase modulation is not limited to a linear phase modulation. A non-linear phase modulation may also be utilized in the disclosed embodiments.

[0074] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0075] 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 resistsupplied 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.

[0076] 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 1-100 nm), as well as particle beams, such as ion beams or electron beams.

[0077] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are likely to be used in an apparatus operating in the UV and / or EUV ranges.

[0078] The breadth and scope of the present invention should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0079] Various embodiments of the present systems and methods are disclosed in the subsequent list of numbered clauses. In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A sensor, comprising: at least one phase modulating projection grating element being configured to scatter measurement radiation and impose a phase modulation onto the resultant scattered radiation; at least one detector being configured to receive radiation derived from reflected radiation comprising said scattered radiation subsequent to reflection from a surface to be measured; and at least one processor unit being operable to determine a measurement signal comprising a parameter of said radiation received at the detector, and determine a height of said surface from a phase of said measurement signal.2. A sensor as claimed in clause 1, further comprising at least one detection grating element, wherein said radiation received at the detector comprises at least a portion of said reflected radiation subsequent to scattering by said detection grating element.3. A sensor as claimed in clause 2, wherein said at least a portion of said reflected radiation comprises the zeroth order scattered radiation from the detection grating element.4. A sensor as claimed in clause 2 or 3, wherein each said at least one detection grating element comprises a detection acousto-optical modulator.5. A sensor as claimed in clause 2, 3 or 4, comprising a detection optical arrangement configured to image said reflected radiation onto said at least one detection grating element.6. A sensor as claimed in any preceding clause, comprising:at least a first phase modulating projection grating element being configured to receive first radiation of a first wavelength or first wavelength band and generate first scattered radiation and a second phase modulating projection grating element being configured to receive second radiation of a second wavelength or second wavelength band and generate second scattered radiation; a first dichroic mirror configured to combine said first scattered radiation and second scattered radiation prior to reflection from said surface to be measured; and a second dichroic mirror configured to split said reflected radiation into first reflected radiation comprising said first wavelength or first wavelength band and second reflected radiation comprising said second wavelength or second wavelength band subsequently to reflection from said surface to be measured.7. A sensor as claimed in clause 6, further comprising at least a first detection projection grating element being configured to receive said first reflected radiation and a second detection projection grating element being configured to receive said second reflected radiation.8. A sensor as claimed in clause 1, comprising a self-referencing interferometer being configured to receive said reflected radiation and derive said radiation received at the detector for detection by said at least one detector.9. A sensor as claimed in any preceding clause, wherein said at least one phase modulating projection grating element comprises a projection acousto-optical modulator.10. A sensor as claimed in clause 9, wherein said processor unit is operable to drive said projection acousto-optical modulator at two driving frequencies.11. A sensor as claimed in clause 10, wherein said processor unit is operable to drive said acousto- optical modulator at said two driving frequencies sequentially.12. A sensor as claimed in clause 10, wherein said processor unit is operable to drive said acousto- optical modulator at said two driving frequencies simultaneously.13. A sensor as claimed in clause 11, wherein a detected frequency comprises a Doppler shifted difference frequency.14. A sensor as claimed in clause 13, wherein the two driving frequencies are less than 1MHz apart.15. A sensor as claimed in any of clauses 10 to 14, wherein said processor unit is operable to determine said height in a coarse measurement mode and a fine measurement mode.16. A sensor as claimed in any preceding clause, wherein said at least one processor unit is operable to determine a measurement signal comprising a parameter of said radiation received at the detector over a time period.17. A sensor as claimed in any preceding clause, wherein said at least one phase modulating projection grating element is configured to impose a phase modulation onto the resultant scattered radiation.18. A sensor as claimed in any preceding clause, wherein at least one phase modulating projection grating element is located at an image plane of the height measurement arrangement.19. A sensor as claimed in any preceding clause, wherein each said at least one detector comprises a photodiode.20. A sensor as claimed in any preceding clause, comprising a light carrier configured to carry said radiation received at the detector to said detector.21. A sensor as claimed in any preceding clause, wherein said detector is comprised outside of a critical volume of the height measurement arrangement.22. A sensor as claimed in any preceding clause, wherein the processor is further operable to determine a magnitude of said measurement signal; and determine a correction for the determined height from said magnitude.23. A sensor as claimed in any preceding clause, wherein said parameter of said radiation received at the detector comprises intensity.24. A sensor as claimed in any preceding clause, comprising a color filter module for demultiplexing said radiation received at the detector prior to detection on said at least one detector.25. A sensor as claimed in any preceding clause, further comprising: a projection optical arrangement configured to direct said scattered radiation onto a surface to be measured.26. A sensor as claimed in any preceding clause, wherein said at least one phase modulating projection grating element is configured to impose the continuous phase modulation on a phase difference between diffraction orders of a complementary pair of diffraction orders comprised within said scattered radiation.27. A sensor as claimed in any preceding clause, being configured for measuring a height of a surface of a substrate.28. A sensor as claimed in any preceding clause, comprising a level sensor arrangement.29. A sensor as claimed in any preceding clause, wherein said at least one phase modulating projection grating element is a linear phase modulating projection grating element being configured to scatter measurement radiation and impose a linear phase modulation onto the resultant scattered radiation.30. A lithographic apparatus comprising the sensor of any preceding clause.31. A lithographic apparatus as claimed in clause 30, comprising: a substrate support for supporting a substrate; and wherein said sensor is configured for measuring height data describing the height of the surface of said substrate.32. A method of measuring a height of a surface, the method comprising: scattering measurement radiation; imposing a phase modulation onto the resultant scattered radiation; reflecting said scattered radiation off said surface to generate reflected radiation; detecting said reflected radiation;determining a measurement signal comprising a parameter of said radiation received at the detector over time, and determining a height of said surface from a phase of said measurement signal.33. A method as claimed in clause 32, wherein said imposing step comprises imposing the continuous phase modulation on a phase difference between diffraction orders of a complementary pair of diffraction orders comprised within said scattered radiation.34. A method as claimed in clause 32 or 33, comprising imaging said reflected radiation onto a detection grating element; and wherein said detecting step comprises detecting resultant scattered radiation from said detection grating element.35. A method as claimed in clause 34, wherein the resultant scattered radiation comprises a zeroth order from the detection grating element.36. A method as claimed in any of clauses 32 to 35, wherein said surface comprises a substrate.37. A method as claimed in any of clauses 32 to 36, wherein the step of imposing a phase modulation onto the resultant scattered radiation comprises imposing a linear phase modulation onto the resultant scattered radiation.

Claims

CLAIMS1. A sensor, comprising: at least one phase modulating projection grating element being configured to scatter measurement radiation and impose a phase modulation onto the resultant scattered radiation; at least one detector being configured to receive radiation derived from reflected radiation comprising said scattered radiation subsequent to reflection from a surface to be measured; and at least one processor unit being operable to determine a measurement signal comprising a parameter of said radiation received at the detector, and determine a height of said surface from a phase of said measurement signal.

2. A sensor as claimed in claim 1, further comprising at least one detection grating element, wherein said radiation received at the detector comprises at least a portion of said reflected radiation subsequent to scattering by said detection grating element.

3. A sensor as claimed in claim 2, wherein said at least a portion of said reflected radiation comprises the zeroth order scattered radiation from the detection grating element.

4. A sensor as claimed in claim 2 or 3, wherein each said at least one detection grating element comprises a detection acousto-optical modulator.

5. A sensor as claimed in any preceding claim, comprising: at least a first phase modulating projection grating element being configured to receive first radiation of a first wavelength or first wavelength band and generate first scattered radiation and a second phase modulating projection grating element being configured to receive second radiation of a second wavelength or second wavelength band and generate second scattered radiation; a first dichroic mirror configured to combine said first scattered radiation and second scattered radiation prior to reflection from said surface to be measured; and a second dichroic mirror configured to split said reflected radiation into first reflected radiation comprising said first wavelength or first wavelength band and second reflected radiation comprising said second wavelength or second wavelength band subsequently to reflection from said surface to be measured.

6. A sensor as claimed in claim 5, further comprising at least a first detection projection grating element being configured to receive said first reflected radiation and a second detection projection grating element being configured to receive said second reflected radiation.

7. A sensor as claimed in any preceding claim, wherein said at least one phase modulating projection grating element comprises a projection acousto-optical modulator.

8. A sensor as claimed in claim 7, wherein said processor unit is operable to drive said projection acousto-optical modulator at two driving frequencies, and wherein said processor unit is operable to drive said acousto-optical modulator at said two driving frequencies sequentially or simultaneously.

9. A sensor as claimed in claim 8, wherein a detected frequency comprises a Doppler shifted difference frequency, in case that said processor unit is operable to drive said acousto-optical modulator at said two driving frequencies sequentially.

10. A sensor as claimed in any of claims 8 and 9, wherein said processor unit is operable to determine said height in a coarse measurement mode and a fine measurement mode.

11. A sensor as claimed in any preceding claim, wherein said at least one processor unit is operable to determine a measurement signal comprising a parameter of said radiation received at the detector over a time period.

12. A sensor as claimed in any preceding claim, wherein said at least one phase modulating projection grating element is configured to impose a phase modulation onto the resultant scattered radiation.

13. A lithographic apparatus comprising the sensor of any preceding claim.

14. A lithographic apparatus as claimed in claim 13, comprising: a substrate support for supporting a substrate; and wherein said sensor is configured for measuring height data describing the height of the surface of said substrate.

15. A method of measuring a height of a surface by using the sensor of any of claims 1 to 12, the method comprising: scattering measurement radiation; imposing a phase modulation onto the resultant scattered radiation; reflecting said scattered radiation off said surface to generate reflected radiation; detecting said reflected radiation;determining a measurement signal comprising a parameter of said radiation received at the detector over time, and determining a height of said surface from a phase of said measurement signal.

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