# Patent

## Title
Optical apparatus, method of scanning, lithographic apparatus and device manufacturing method

## Abstract

An apparatus measures positions of marks on a lithographic substrate. A measurement optical system comprises illumination subsystem for illuminating the mark with a spot of radiation and a detecting subsystem for detecting radiation diffracted by the mark. A tilting mirror moves the spot of radiation relative to the reference frame of the measurement optical system synchronously with a scanning motion of the mark itself, to allow more time for accurate position measurements to be acquired. The mirror tilt axis is arranged along the intersection of the mirror plane with a pupil plane of the objective lens to minimize artifacts of the scanning. The same geometrical arrangement can be used for scanning in other types of apparatus, for example a confocal microscope.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to U.S. Provisional Patent Application No. 61/444,373, filed Feb. 18, 2011, and to U.S. Provisional Patent Application No. 61/468,208, filed Mar. 28, 2011, which are incorporated by reference herein in their entirety.
FIELD
The present invention relates to an optical apparatus, a method of scanning objects with beams of radiation, a lithographic apparatus, and a device manufacturing method. The present invention may be applied in measuring methods, including but not limited to measuring methods used in a lithographic apparatus.
BACKGROUND
The ability to scan objects optically is useful in many applications. By ‘scanning optically’ we mean to illuminate and/or detect radiation at/from a point on an object while scanning the point in one or more directions across the object.
A lithographic 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., 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. 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.
Typically, the position measurement is captured by moving the substrate and the alignment sensor relative to one another, without the substrate and sensor having to be brought to a stop. The position of the mark relative to the (known) position of the alignment sensor is measured optically, by scanning the marks with an optical spot. (It does not matter whether the sensor moves while the substrate is stationary, or only the substrate moves, or both move.) The alignment sensors should scan the marks as quickly as possible for optimum throughput, but this need for speed places limits on the accuracy of the position measurements that can be obtained. It is inherently more difficult to acquire a position accurately in a short time. In addition, a measurement acquired in a short time, even if the measurement itself is perfectly accurate, will be susceptible to disturbance by dynamic positioning errors (vibrations, servo errors etc.) in the positioning of the substrate and alignment sensor.
To increase the acquisition time, it is recognized that an optical scanning of the spot additional to the scanning of the sensor itself could be useful. However, conventional optical scanning mechanisms would add cost and bulk to the sensor, and/or lead to inaccuracies in the measurement, defeating the purpose of the optical scanning.
SUMMARY
It is an aim of the present invention in a first aspect to provide an optical scanning mechanism which avoids or reduces one or more of the drawbacks with known mechanisms.
The present invention in the first aspect provides an optical apparatus comprising an objective lens for transmitting rays of radiation between an optical system of the apparatus and an object. The optical apparatus further comprising least one moveable element comprising a mirror arranged between the optical system and the objective lens to tilt and so change the direction of the rays as they pass through the objective lens. The mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror. The reference to ‘mirror’ in this context includes a reflective element generally, whether plane or curved.
The present invention is not limited to optical apparatuses of any particular type or purpose. In embodiments described below, it may be for example an alignment sensor of the type described above, or it may be a confocal microscope.
The present invention further provides a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, the apparatus comprising a substrate table constructed to hold a substrate and an alignment sensor for measuring the positions of marks on the substrate relative to a reference frame of the lithographic apparatus. The alignment sensor comprises an optical apparatus according to the present invention as set forth above, and wherein the lithographic apparatus is arranged to control the transfer of a pattern onto the substrate by reference to the positions of marks on the substrate measured using the measuring apparatus.
The present invention yet further provides a method of scanning an object with rays of radiation via an objective lens. The method comprises arranging a mirror in a path of the rays prior to the objective lens and tilting the mirror so as to change the direction of the rays as they pass through the objective lens, and thereby to cause the rays to impinge on different parts of the object in accordance with a tilt angle of the mirror. The mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.
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(s) based on the teachings contained herein.



BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention.
FIG. 1 depicts an exemplary lithographic apparatus including an alignment sensor forming measuring apparatus according to an embodiment of the present invention.
FIGS. 2a and 2b illustrate various forms of an alignment mark that may be provided on a substrate in the apparatus of FIG. 1.
FIG. 3 is a schematic block diagram of a known alignment sensor scanning an alignment mark.
FIGS. 4a and 4b illustrate the operation of a novel alignment sensor scanning an alignment mark, in an embodiment of the present invention.
FIG. 5 is a more detailed exemplary schematic diagram showing the optical system of the novel alignment sensor in the lithographic apparatus of FIG. 1.
FIG. 6 illustrates a particular geometrical arrangement of a moving mirror in a scanning subsystem of the alignment sensor of FIG. 5.
FIG. 7 is a graph of mirror tilt angle against time in a scanning subsystem of the alignment sensor of FIGS. 5 to 6.
FIG. 8 is a schematic diagram of a tilt sensor associated with the moving mirror in the apparatus of FIG. 5.
FIG. 9 is a schematic diagram of a novel confocal microscope forming a measuring apparatus in an alternative embodiment of the present invention.
FIGS. 10a and 10b illustrate (a) a generalized geometric arrangement of a moving optical element usable in embodiments of the present invention and (b) properties of the optical path for different tilt angles.



The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.
DETAILED DESCRIPTION
This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.
The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
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 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, 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.
Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented.
The present invention concerns a mechanism whose principles can be applied to provide an optical scanning function in a range of apparatuses. Examples illustrated in the embodiments below include an alignment sensor in a lithographic apparatus and a confocal microscope, for example. The simple principle by which the mirror is arranged to tilt about an axis which is along the intersection between the plane of the mirror's surface and the pupil plane of the optical system can be applied to obtain the benefits of non-moving illumination in a range of other applications, not limited to these examples.
Application Example
Lithographic Apparatus
FIG. 1 schematically shows a lithographic apparatus LAP including a source collector module SO according to an embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; 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; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.
The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
The support structure supports, i.e., bears the weight of, the patterning device. 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.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.
The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
As 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 of a type as referred to above, or employing a reflective mask).
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. An example will be described below, with reference to FIG. 10. The present invention disclosed herein can be used in a stand-alone fashion, but in particular it can provide additional functions in the pre-exposure measurement stage of either single- or multi-stage apparatuses.
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 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.
Referring 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.
The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.
The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WTa/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 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 WTa/WTb 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 M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.
The depicted apparatus could be used in at least one of the following modes:
1. In step mode, the mask table MT and the substrate table WTa/WTb are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WTa/WTb is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the mask table MT and the substrate table WTa/WTb are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WTa/WTb relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WTa/WTb is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WTa/WTb or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
Lithographic apparatus LA is of a so-called dual stage type which has two substrate tables WTa and WTb and two stations—an exposure station and a measurement station—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 so that various preparatory steps may be carried out. The preparatory steps may include mapping the surface of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. This enables a substantial increase in the throughput of the apparatus. 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.
The apparatus further includes a lithographic apparatus control unit LACU which controls all the movements and measurements of the various actuators and sensors described. LACU also includes signal processing and data processing capacity to implement desired calculations relevant to the operation of the apparatus. In practice, control unit LACU will 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. For example, one processing subsystem may be dedicated to servo control of the substrate positioner PW. Separate units may even handle coarse and fine actuators, or different axes. Another unit might be dedicated to the readout of the position sensor IF. Overall control of the apparatus may be controlled by a central processing unit, communicating with these sub-systems processing units, with operators and with other apparatuses involved in the lithographic manufacturing process.
FIG. 2 shows examples of alignment marks 202, 204, provided on substrate W for the measurement of X-position and Y-position, respectively. Each mark in this example comprises a series of bars formed in a product layer or other layer applied to or etched in to the substrate. The bars on the X-direction mark 202 are parallel to the Y-axis, while the bars of the Y-direction mark 204 are parallel to the X-axis. The alignment sensor AS (shown in FIG. 1) scans each mark optically with a spot 206, 208 of radiation, to measure the position of substrate W relative to the apparatus. The scanning movement is indicated schematically by a broad arrow, with progressive positions of the spot 206 or 208 indicated in dotted outline. The pitch of the bars in the alignment pattern is typically much greater than the pitch of product features to be formed on the substrate, and the alignment sensor AS uses a wavelength of radiation (or usually plural wavelengths) much longer than the exposure radiation to be used for applying patterns to the substrate. Fine position information can be obtained, however, because the large number of bars allows the phase of the repeating pattern to be accurately measured.
An example application for optical scanning is in a novel alignment sensor for a lithographic apparatus, described in the related application U.S. Provisional Patent Application No. 61/444,373 (den Boef et al). Another example application is a confocal microscope. In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate must be measured accurately. The time taken for these measurements conflicts with the need to maximize throughput of the lithographic apparatus, otherwise device production will not be cost-effective. Various types of alignment sensors and marks are known, including for example the ones disclosed in U.S. Pat. No. 6,297,876 (Bornebroek), U.S. Pat. No. 6,961,116 (den Boef) and in U.S. published patent application No. 2009195768 A (Bijnen et al). All of these above noted U.S. Patents and applications are incorporated by reference herein in their entireties.
Coarse and fine marks may be provided, so that the alignment sensor can calculate which cycle of the pattern is at a given position, as well as the exact position (phase) within that cycle. Marks of different pitches can also be used for this purpose. These techniques are again known to the person skilled in the art, and will not be detailed herein. The design and operation of such sensors is known in the art, and each lithographic apparatus has its own design of sensor. For the purpose of the present description, it will be assumed that the alignment sensor AS comprises an alignment system of the form described in U.S. Pat. No. 6,961,116 (den Boef). FIG. 2(b) shows a modified mark for use with a similar alignment system, which X- and Y-positions can be obtained through a single scan. The mark 210 has bars arranged at 45 degrees to both the X- and Y-axes. This combined X- and Y-measurement can be performed using the techniques described in published patent application US 2009195768 A (Bijnen et al), the contents of which are incorporated herein by reference. It may be noted that US'768 discloses some embodiments in which the X-Y alignment marks have portions of different pitches, somewhat similar to the marks newly presented in the present application. However, the simpler mark shown in FIG. 2(b) is the form generally used in the commercial embodiment, and any moiré effect that may be observed between different pitches in the embodiments of US'768 is fixed and provides no measure of process performance.
FIG. 3 is a schematic block diagram of alignment sensor AS. Illumination source 220 provides a beam 222 of radiation of one of more wavelengths, which is diverted through an objective lens 224 onto mark such as mark 202, located on substrate W. As indicated schematically in FIG. 2, in the example of the present alignment sensor based on U.S. Pat. No. 6,961,116, mentioned above, the illumination spot 206 by which the mark 202 is illuminated may be slightly smaller in diameter then the width of the mark itself.
Radiation scattered by mark 202 is picked up by objective lens 224 and collimated into an information-carrying beam 226. A self-referencing interferometer 228 processes beam 226 and outputs separate beams onto a sensor array 230. Intensity signals 232 from individual sensors in sensor grid 230 are provided to a processing unit PU. By a combination of the optical processing in the block 228 and the computational processing in the unit PU, values for X- and Y-position on the substrate relative to the sensor are output. Processing unit PU may be separate from the control unit LACU shown in FIG. 1, or they may share the same processing hardware, as a matter of design choice and convenience. Where unit PU is separate, part of the signal processing may be performed in the unit PU and another part in unit LACU.
As mentioned already, the particular measurement illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of the sine wave is the one containing the marked position. The same process at coarse and/or fine level can be repeated at different wavelengths for increased accuracy, and for robust detection of the mark irrespective of the materials from which the mark is made, and on which it sits. The wavelengths can be multiplexed and demultiplexed optically so as to be processed simultaneously, and/or they may be multiplexed by time division. Examples in the present disclosure will refer to one measurement at one wavelength only, and the skilled reader can readily appreciate the modifications required to expand that teaching to provide a partial and robust measurement apparatus (alignment sensor).
Referring to the measurement process in more detail, an arrow labeled vW in FIG. 3 illustrates a scanning velocity with which spot 206 traverses the length L of mark 202. In this example, the alignment sensor AS and spot 206 in reality remain stationary, while it is the substrate W that moves with velocity vW. The alignment sensor can thus be mounted rigidly and accurately to the reference frame RF (FIG. 1), while effectively scanning the mark 202 in a direction opposite to the direction of movement of substrate W. The substrate is controlled in this movement by its mounting on the substrate table WT and the substrate positioning system PW. As discussed in the introduction, the high productivity requirements required of the expensive lithographic apparatus require the measurement of the alignment marks at numerous positions on the substrate be performed as quickly as possible, which implies that the scanning velocity vW is fast, and the time TACQ available for acquisition of each marked position is correspondingly short. In simplistic terms, the formula TACQ=L/vW applies.
Unfortunately, shortness of the acquisition time can lead to inaccuracy in the measured positions x and y. This is not only because of difficulties in the measurement process itself, but also because the acquisition time becomes too short to smooth out dynamic positioning errors in the movement of the substrate table WT and substrate W which is supported on it. These dynamic positioning errors, which may be thought of simplistically as vibrations, are referred to in the art as MSD error (MSD stands for moving standard deviation). To smooth out these MSD errors, one could repeat the measurement and use the average result, but this defeats the object of increasing throughput, because it increases the measurement time overall. In summary, there are competing factors that limit the accuracy that can be achieved in the known type of scanning alignment sensor. In a novel arrangement disclosed herein, spot 206 is caused to make scanning movements relative to the alignment sensor and reference frame, so that the acquisition time can be lengthened without slowing the operation overall. This technique will now be described in more detail.
In FIG. 4, we see (a) the start and (b) the end of a scanning operation of a modified alignment sensor AS on the mark 202, which is conveniently the same mark as described previously. The length of the mark is again L and the substrate W again moves under the alignment system with a velocity vW. In this modified operation, however, the illumination spot 406 or, more generally the point on the mark 202 which is being read by the alignment sensor at any given moment during the scanning operation, is controlled to move relative to the reference frame RF with a velocity vSPOT. This spot velocity vSPOT is somewhat less than the substrate velocity vW. The effect of this optical scanning movement of spot 406 is that the effective velocity vEFF of scanning, which is the difference between the substrate velocity vW and the spot velocity vSPOT, is much smaller than the substrate velocity itself. Consequently the acquisition time TACQ=L/vEFF is longer than would be implied by the overall scanning velocity. This allows the accuracy of position measurements on mark 202 to be increased, particularly to smooth out MSD error, without increasing the overall measurement time.
Naturally, there are limits to how great an effect of lengthening the acquisition time is achievable or desirable. These depend primarily on the optical properties of the alignment sensor optical system, which much have a field of view (FOV) wide enough to cover the whole scanning movement, and which must have a certain uniformity, stability and/or predictability of response across all spot positions. A practical arrangement will now be described, with reference to FIG. 5.
FIG. 5 illustrates an optical system 500 of a modified alignment sensor implementing the scanning spot whose principle of operation has been illustrated and described above with reference to FIG. 4. Optical system 500 is a modified version of the system described in the prior publication, U.S. Pat. No. 6,961,116, mentioned above. A first modification, particularly relevant for the present application, is the above mentioned scanning illumination spot. A second modification is the use of a particular segmented illumination mode which, among other things, allows a reduced pitch of alignment mark for greater accuracy, and which allows scatterometry type measurements to be performed with the alignment sensor, rather than a separate scatterometer instrument. A further modification can be made based on the disclosure of publication US 2009/195768, also mentioned above, so that simultaneous measurement of X and Y position can be made. For the purpose of the present description, it shall be assumed that measurement of position is made in one direction only.
For ease of comparison with the schematic diagram of FIG. 3, some parts of the optical system 500 are labeled with reference signs similar to those used in FIG. 3, but with prefix “5” instead of “2”. Thus, we see a light source 520, an illumination beam 522, an objective lens 524, an information carrying beam 526, an interferometer 528 and detectors 530a and 530b. Signals 532a and 532b from these detectors are processed by processing unit PU, which is modified as appropriate to implement the novel features described below. An optical axis O which has several branches is indicated by a broken line running throughout the optical system 500. Additional components illustrated in this more detailed schematic are as follows. In an illumination subsystem 540, radiation from source 520 is delivered via an optical fiber 542 to a point where it enters an optical system comprising lenses 544 and 546. Of course, where each lens is illustrated schematically by a single element, a practical embodiment may include groups of elements in practice or reflective optical elements may also be used. Between lenses 544 and 546, the beam is parallel, and passes through a plane P′, which is a back-projection of the pupil plane P of objective lens 524. A fixed or configurable illumination aperture 548 is provided in this plane, to allow specific modes of illumination, such as the symmetric, segmented illumination pattern illustrated in FIG. 5. Two diametrically opposite quadrants, labeled a and b, are bright in this aperture pattern (transparent), while the other two quadrants are dark (opaque). This type of aperture is known in scatterometry apparatus, from the patent application US 2010/201963. The merits of this modified illumination aperture will be described further below. After diffraction by a fixed mirror 550 and collimation by a lens 552, illumination beam 522 is delivered from the illumination subsystem 540 to a beam splitter 554, which separates the illumination beam 522 from the information-carrying beam 526, just as in the known instrument.
Unlike in the known alignment sensor, however, the illumination beam 522 and information-carrying beam 526 do not travel to and from the substrate W directly through objective 524 to the substrate W in a fixed manner. Rather, in this modified apparatus, a scanning subsystem 560 is interposed between objective 524 and the remainder of the optical system 500, for causing illumination spot 506 to implement a scanning movement additional to the scanning movement of the substrate W itself. This scanning movement is the one with the velocity vSPOT, indicated for spot 406 in the schematic diagrams of FIG. 4. Scanning subsystem 560 in this embodiment is based on a moving, specifically tilting mirror 562, which deflects the beams 522 and 526 through a small, variable angle. Mirror 562 is mounted on an arm 564 which is connected to a support 566 at a hinge point 568, so as to pivot to a desired angle among a range of angles. An actuator 570, which may be for example a magnetic actuator having no mechanical contact with mirror 562 or its support, causes movement of the mirror 562 to desired angles so as to move spot 506 to a desired position for the scanning movement. In the present embodiment, arm 564 and hinge point 568 implement with the mirror 562 a mass-and-spring vibration system, having a natural resonant frequency, as explained further below. In such an embodiment, actuator 570 can provide only a very slight energy input at an appropriate frequency, to cause a smooth oscillation of the mirror at a relatively well-defined frequency. A tilt sensor 572 is provided to monitor the tilt angle accurately, by optical measurement. Actuator 570 is under control of a controller (which may be part of unit PU), which receives also the signals from tilt sensor 572 and which is synchronized with the measurement operations of the alignment sensor generally, including the scanning movements of substrate table WT through positioning subsystem PW.
Further detail of the scanning subsystem 560 will be described further below, with reference to FIGS. 6-8. Referring now to the processing of information-carrying beam 526, this passes into interferometer 528 in the same manner as the known alignment sensor of U.S. Pat. No. 6,961,116. Interferometer 528 is shown in a simplified, two-dimensional arrangement, but in fact comprises a three-dimensional arrangement of prisms and other elements, as described in the prior patent. Similarly, polarizing elements that are part of the interferometer are omitted here, for clarity. The function of self-referencing interferometer 528, as in the known example, is to receive the information-carrying beam 526, to split it into two equal parts, to rotate these parts by 180° relative to one another, and to combine these parts again into an outgoing beam 582. Further lenses 584 and 586 together with a fixed deflecting mirror 588 deliver this beam to detectors 530a and 530b via polarizing beam splitter 590.
Beam splitter 590 produces two interference patterns that are in anti-phase with each other. Thus, if the interferometer 528 produces destructive interference at one point on detector 530a, there will be at the same time constructive interference at the corresponding point on the detector 530b. By subtracting the signals from the two detectors, one can reduce the influence of common-mode intensity noise, and so obtain a more accurate output from the alignment sensor as a whole.
Whereas, in the known alignment sensor of FIG. 3, detectors 230 are placed in an image plane corresponding to the plane of substrate W, the detectors 530a and 530b in the modified optical system 500 are positioned in a plane P′′, which is conjugated with the pupil plane P of objective 524. Each detector 530a and 530b in this example is an image sensor, for example a CCD camera sensor. In alternative embodiments, individual point detectors may be deployed instead of image sensors. In either case, the radiation field at plane P′′ is not an image of the substrate, but a diffraction spectrum of the mark 202, conjugated with the illumination aperture 548. In this type of detection system, the alignment signals required to obtain positional information of the mark 202 can still be obtained, but additionally the pupil plane images detected by detectors 530a and 530b can be used to analyze additional properties of the mark or other features on substrate W, to perform scatterometry. For example, asymmetry of alignment and/or overlay targets can be analyzed, which facilitates measurement of an overlay error between device layers, for example. A benefit of using the alignment sensor to measure target asymmetry, rather than a separate instrument, is that positioning errors between the alignment sensor and the overlay measurement apparatus are eliminated.
While the illumination pattern provided by aperture 548 has bright quadrants labeled a and b in FIG. 5, the diffraction pattern resulting from diffraction by the lines of the alignment mark 202 in a given direction is represented by the pattern at 548′. In this pattern, in addition to zero order reflections labeled a0 and b0 there are first order diffraction signals visible, labeled a−1, a+1, b−1 and b+1. Because other quadrants of the illumination aperture are dark, and more generally because the illumination pattern has 180° rotational symmetry, the diffraction orders a−1 and b+1 are “free” meaning that they do not overlap with the zero order or higher order signals from other parts of the illumination aperture. This property of the segmented illumination pattern can be exploited to obtain clear first order signals from a diffraction grating (alignment mark) having a pitch which is half the minimum pitch that could be imaged if a conventional, circularly-symmetric illumination aperture were used. This diffraction pattern 548′ and the manner in which it can be exploited for scatterometry, are described in the known application U.S. Published App. No. 2010/201963, which is incorporated by reference herein in its entirety. In the interferometer 528 of the alignment sensor optical system 500, a rotated copy of the diffraction pattern 548′, illustrated and labeled 548′′ in FIG. 5, is also formed and mixed with pattern 548′. These patterns will interfere with each other during the scanning of the alignment mark 202, so as to provide the position signal.
Concerning the scanning subsystem 560, the particular form of the scanning subsystem illustrated here has some beneficial features which will now be described.
As seen in FIG. 5 and FIG. 6, the geometry of the mounting of the tilting mirror 562 is such that the pivot point 568 is located in the same plane as the pupil plane P of objective lens 524. Tilting of mirror 562 back and forth is what causes a deviation of the direction of beam 522 as it enters objective lens 524, and consequently causes the back-and-forth scanning movement of spot 506, with the desired velocity vSPOT. More specifically, mirror 562 is mounted so as to tilt about an axis which is not within the body of the mirror, but is in or close to a line at which the plane of the mirror 562 and the pupil plane P of objective lens 524 intersect. As background, it is desirable that the direction of rays forming the illumination spot 506 should move to implement the scanning velocity vSPOT, but without movement of the illumination aperture in the back-projected plane P′. If mirror 562 were to be tilted about, say, its own central axis, as is common for tilting mirrors, then the illumination source would move back and forth in the back-projected pupil plane P′, and would result in a varying mean angle-of-incidence of the illumination spot on the substrate.
Better alignment accuracy is obtained by maintaining a normal angle-of-incidence on the substrate. To achieve this, a conventional solution would be to place a tilting mirror in a conjugate pupil plane of an optical system, to achieve a scanning motion of an illumination spot or field of view. In a practical instrument such as the alignment sensor illustrated here, however, a commercially available objective lens 524 is normally housed in a compact and self-contained sub-assembly, and there is no space at the pupil plane P where a tilting mirror or other moving element could be interposed. While a custom-designed assembly can be made which includes a tilting mirror in the pupil plane, or additional optical element can be provided to produce a conjugate pupil plane at a less crowded point in the optical path, the cost of the assembly would become much greater than before, and accuracy may be compromised.
Referring to FIG. 6, the geometry of the ray 522 and the mirror surface 562 is illustrated, firstly for a central or neutral position of the mirror (labeled 562(0)), and then for a deflected position 562(θ), in which the mirror has tilted through an angle θ away from the incoming wave 522, with rotation centered on the pivot point 568, which lies in the same plane as the pupil plane P of objective lens 524. The rest position of the mirror in this example is at 45° to the pupil plane and the incoming ray direction. While the angular movement θ is shown in one direction, the same movement may be imparted to the mirror in the reverse direction (nearer to the source of the ray 522), as mirror 562 oscillates back and forth through a range of angles θ. An enlarged detail within the broken circle shows the geometry in the region where the ray 522 is reflected by mirror 562 in the rest position and the deviated position. The range of angles θ through which the mirror deviate's in a practical embodiment may be on the order of millirads or tens of millirads, so that a “small angle approximation” can be applied in calculating the optical path changes caused by the tilting of the mirror. As can be seen in the diagram, pupil plane P and the path of the incoming ray 522 are separated by a distance D, while the default position of mirror 562 is at 45° so as to deflect ray 522 through a right angle. When the mirror tilts by angle θ, the deviated ray 522′ tilts by an angle 2θ, but its point of interception with the mirror also shifts to the left by a distance 2θD. Therefore the point 600 at which the deflected ray 522 intercepts the pupil plane P is, at least approximately, independent of the mirror tilt angle θ. From this we know that the effective position of the illumination aperture 548 is unchanged, though the direction of the rays, and hence the position of the illumination spot 506 moves back and forth in accordance with angle θ. (The linear velocity vSPOT is related to the angle 2θ by the magnification factor of objective lens 524.)
FIG. 7 shows a sinusoidal curve 700 which represents the change in tilt angle θ of mirror 562 over time t, in an exemplary embodiment. The trace 700 has a generally sinusoidal form, for the reason that the tilting mirror 562 and its mounting are driven as a resonant vibrator, as mentioned above. In other words, the mirror tilts with simple harmonic motion. The period of the mirror oscillation is labeled TOSC and the amplitude in terms of angle θ of the oscillation is θOSC. An advantage of this drive mode is that it requires minimal energy input, and therefore minimal heat dissipation within the actuator and the mirror mechanism. A disadvantage of this drive mode is that a scanning motion with the desired velocity and direction happens only at certain intervals, and only with limited linearity of motion. With careful design of the measurement process, however, including synchronizing this mirror oscillation with the way the substrate moves during scanning of a series of alignment marks 202 across substrate W, the frequency of the mirror oscillation can be selected so that measurements of alignment mark positions can be made during the intervals labeled M1, M2, M3 in FIG. 7. These are portions of the trace 700 in which the motion of the mirror and hence the movement of spot 506 along the alignment mark 202 are relatively linear and predictable. The range of angles over which scanning measurements take place is labeled θM and the time window within which a measurement or measurements can be made using the scanning motion of spot 506 is labeled TM. The apparatus is not limited to measuring one alignment mark during this interval, but could measure several marks, depending on their positions on the substrate W and the motion of the substrate on the substrate table WT.
FIG. 8 shows schematically the construction and operating principle of tilt sensor 572. As mentioned in relation to FIG. 5, the tilt sensor is provided to provide an accurate measurement of the tilt 562 at all times. This measurement can be used by unit PU or other controller to control and synchronize the scanning movement, the driving via actuator 570, corrections of non-linearity during the measurement periods M1, M2, M3, etc., and other measurements as desired. Tilt sensor 572 is based in this example on the phase-grating detector. A radiation beam 800 from a source 802 is diffracted by a first phase-grating having a pitch PIN. The phase-grating may be used, so that only first order rays, labeled +1 and −1 in the diagram, are transmitted with any strength. These rays are divergent as shown. A tilt sensor optical system 806 collimates the rays so that they become parallel, so that the +1 and −1 order rays impinge on the surface of the mirror 562 at distinct points, separated by a distance d. (As an example, this distance d may be on the order of millimeters.) The diffracted rays are reflected back through the optical system 806 and focused onto a detection grating 808. Detection grating 808 has a pitch Pdet which is half that of the input grating 804. A tilt angle in the mirror 562 causes different path lengths for the +1 and −1 diffraction orders, affecting their phase when they are recombined at grating 808. A detector 810 receives the diffracted radiation through the detection grating 808. As the mirror tilts one way and then the other, a rapid fluctuation in the intensity of radiation detected at 810 allows counting of very small deviations in the tilt angle. By counting these variations (fringe counting), a very high resolution measurement of tilt can be obtained over a relatively wide range of tilt angles. In order to obtain absolute knowledge of the tilt angle, a number of different grating systems 804, 808 and detectors 810 can be provided in parallel, with different pitches. This approach to disambiguation is well known to the skilled person, and these details will not be described further.
Alignment Sensor Variations
While the detectors 530a and 530b in the exemplary embodiment are described as image sensors having pixels across their area, alternative embodiments are possible in which detectors are provided at only certain points in the area of the pupil image plane P′′. In particular, alignment signals can be obtained by placing a few detectors at the peripheral positions where the first order diffracted illumination is expected. A set of detectors can be spaced all around the periphery of the pupil image, and signals from appropriate detectors can be selected for processing to generate the position measurements, according to whether the mark 202 is in the X or Y direction, or is an X-Y mark of the type shown in FIG. 2(b).
Numerous modifications and variations of the embodiments described above can be envisaged by the skilled reader, without departing from the scope of the present invention. As mentioned, for example, the alignment sensors shown can be adapted for use with the mark of FIG. 2(b) to obtain simultaneously positioned information in X and Y directions. If it is desired to use the segmented illumination aperture 548 in such an embodiment, the illumination aperture 548 should be rotated so that the bright segments a and b are at 45° to both X and Y axes, just as the grating lines in the mark 210 (FIG. 2(b)) are arranged at 45° also.
In this simple example, the oscillating mirror 562 only moves the spot in one dimension, and only one direction of marks can be scanned with the reduced effective velocity. If two-dimensional scanning is required, one can either use two separate mirrors with orthogonal pivot axes, or one mirror that can tilt around 2 orthogonal pivots. The skilled reader can extend the example to two dimensions in either way.
As mentioned in the introduction and elsewhere, certain features disclosed in the context of the alignment sensor with scanning illumination spot 406, 506 etc., can be used independently in other applications. The segmented aperture, combined with detection in the conjugate pupil plane which is a feature of the embodiment of FIG. 5 can be used in an alignment sensor without any scanning subsystem 560. As mentioned already, use of such an aperture allows denser grating lines to be used in the alignment mark, potentially improving positional accuracy and/or reducing the substrate area required by the mark.
It should be understood that the processing unit PU which controls alignment sensor, processes signals detected by it, and calculates from these signals position measurements suitable for use in controlling the lithographic patterning process, will typically involve a computer assembly of some kind, which will not be described in detail. The computer assembly may be a dedicated computer external to the apparatus, it may be a processing unit or units dedicated to the alignment sensor or, alternatively, it may be a central control unit LACU controlling the lithographic apparatus as a whole. The computer assembly may be arranged for loading a computer program product comprising computer executable code. This may enable the computer assembly, when the computer program product is downloaded, to control aforementioned use of a lithographic apparatus with the alignment sensor AS.
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.
Application Example
Confocal Microscope
The alignment sensor is an application of a tilting mirror whose pivot axis is displaced so as to lie in the same plane as the mirror and a pupil plane of the optical system in which it is used. This arrangement has the special ability to deflect an illumination spot in a substrate plane, without changing the average angle of incidence on the substrate. This principle may be applied used in applications other than the alignment sensor, and a confocal microscope is another example.
In FIG. 9, application of the tilting mirror is illustrated in the context of a confocal microscope 1500. Many parts of the system are common, at least in principle, to the alignment sensor example above, and parts such as the objective lens 1524 are numbered the same as the corresponding part 524 in FIG. 5, but with the addition of the prefix “1”. Some major differences will be described as follows. In the confocal microscope as illustrated, the illumination beam 1522 enters beam splitter 1554 from the top in the diagram, while the information carrying beam 1526 exits beam splitter 1554 towards the right hand side. Of course, the layout is a matter of practical implementation, and does not alter the principles of operation. The information carrying beam 1526 is focused firstly onto a narrow pinhole 1580, which in turn is imaged onto a detector 1582, which records the intensity of radiation passing through the pinhole. As is well known from the principles of confocal microscopy, the function of pinhole 1580 is to select both a position and a depth within the illumination spot 1506, from which illumination will reach detector 1582. The intensity of the detected radiation therefore depends on the composition of a target structure 1202 at a particular position and depth below the objective lens 1524. Scanning motions in two dimensions are implemented, so that the time-varying intensity signal detected by detector 1582 can be processed in a processing unit 1584 to obtain a two-dimensional image of the target 1202. In the diagram, the display 1586 is fed with the resulting information to provide the image 1588 showing, with very high resolution, features of the target 1202 at a desired depth.
In the illustrated example, scanning of the spot in at least one of the directions is implemented by the oscillating mirror 1562, which is mounted, driven and monitored in exactly the same manner as that of FIG. 3, described above. Typically in the operation of a confocal microscope, scanning in one direction, for example, the X-direction is implemented by some optical scanning of the spot 1506, while scanning in the Y-direction is implemented by a slow progression of the target 1202 beneath the objective lens 1524. If two-dimensional scanning by movable mirrors is desired, one can either use two separate mirrors with orthogonal pivot axes, or one mirror that can tilt around 2 orthogonal pivots. The skilled reader can extend the example to two dimensions in either way.
Other features of the implementation control and processing of the signals in the confocal microscope can be adapted from known examples, and need not be discussed in detail here.
Further Variations
In the examples described above, the oscillating mirror diverts the optical path through an angle of approximately 90°, and is mounted at an angle of 45° to the pupil plane of the objective 524 or 1524. The same principle of placing the pivot axis of mirror 562/1562 at the intersection of the mirror plane and pupil plane P of the objective lens 524/1524 can be extended to angles other than 45°.
FIG. 10(a) illustrates a generalized arrangement, similar to that of FIG. 6, but with the angle 45° changed to a generic angle α. The length along the mirror from the pivot axis 568 to the point of reflection of the illuminating ray 522 is labeled L, which was equal to the distance D times the square root of 2 in the example of FIG. 6, when α=45°. The relationships between various lengths and angles in this generalized example can be represented by the following equations relating the distance L, the distance D, and the angles α (rest position) and θ (deviation) under oscillation as follows.




  
    
      
        L
        =
        
          2
          ⁢
          D
          ⁢
          
            
              
                
                  cos
                  2
                
                ⁡
                
                  (
                  θ
                  )
                
              
              ⁢
              
                cos
                ⁡
                
                  (
                  α
                  )
                
              
            
            
              sin
              ⁡
              
                (
                
                  
                    2
                    ⁢
                    α
                  
                  -
                  
                    2
                    ⁢
                    θ
                  
                
                )
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  
  
    
      
        L
        =
        
          D
          
            sin
            ⁡
            
              (
              α
              )
            
          
        
      
    
    
      
        (
        2
        )
      
    
  
  
    
      
        
          
            d
            L
          
          
            d
            θ
          
        
        =
        
          4
          ⁢
          D
          ⁢
          
            
              
                cos
                ⁡
                
                  (
                  θ
                  )
                
              
              ⁢
              
                cos
                ⁡
                
                  (
                  α
                  )
                
              
              ⁢
              
                cos
                ⁡
                
                  (
                  
                    
                      2
                      ⁢
                      α
                    
                    -
                    θ
                  
                  )
                
              
            
            
              
                sin
                2
              
              ⁡
              
                (
                
                  
                    2
                    ⁢
                    α
                  
                  -
                  
                    2
                    ⁢
                    θ
                  
                
                )
              
            
          
        
      
    
    
      
        (
        3
        )
      
    
  
  
    
      
        
          
            
              d
              L
            
            
              d
              θ
            
          
          ⁢
          
            |
            
              θ
              =
              0
            
          
        
        =
        
          4
          ⁢
          D
          ⁢
          
            
              
                cos
                ⁡
                
                  (
                  α
                  )
                
              
              ⁢
              
                cos
                ⁡
                
                  (
                  
                    2
                    ⁢
                    α
                  
                  )
                
              
            
            
              
                sin
                2
              
              ⁡
              
                (
                
                  2
                  ⁢
                  α
                
                )
              
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




Equation (1) is a formula for the exact length L, which can be approximated by Equation (2), in the case where the deviation θ is much smaller than the rest angle α. In order to determine the best angle α for the design of the scanning mechanism, Equation (3) presents the derivative of length L in terms of θ. Equation (4) presents the special case of the derivative of L with respect to θ, when θ is zero.
FIG. 10(b) is a plot of Equation (4), normalized with respect to the distance D, for different values of rest angle α. As can be seen, the graph has an inflection point with value 0, where a has the value 45°. From this, we deduce that, for α=45°, the sensitivity of distance L to the oscillation θ will be minimized over a wider range of θ than for any other angle α. Consequently, since the invariance of the apparent position of the illumination source depends on the small angle approximation in the analysis presented with reference to FIG. 6, a rest angle α=45° is optimum for ensuring that these approximations hold substantially true over usable range of angles θ. Nevertheless, using an angle different from 45° might be of interest in order to accommodate particular layout constraints. It should be recalled that noted that oscillations of only a few millirad or a few tens of millirad will be ample for applications such as the ones described herein.
Although specific reference may have been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present 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. Confocal microscopy can be applied to inspect articles used in or produced by lithographic processes, but also for an unlimited range of other inspection tasks.
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.
The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.
While specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced otherwise than as described. For example, the present invention includes steps of signal processing and data processing which may be implemented by a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein.
The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set out below.

## Claims

1. An optical apparatus comprising:
an objective lens configured to transmit rays of radiation between an optical system of the apparatus and an object; and
a moveable element configured to oscillate the rays, wherein the movable element comprises a mirror arranged between the optical system and the objective lens, the mirror being configured to tilt to change the direction of the rays as they pass through the objective lens, and wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.

2. The apparatus of claim 1, wherein the movable element imparts a repetitive scanning movement to the rays.

3. The apparatus of claim 1, wherein the movable element is configured to oscillate at a resonant frequency.

4. The apparatus of claim 1, wherein the optical system comprises an illumination subsystem configured to illuminate a mark via the objective lens with a spot of radiation and a detecting subsystem for detecting via the objective lens radiation diffracted by the mark, the apparatus further comprising:
a first positioning subsystem configured to control movement of the substrate and measurement optical system relative to one another at a first velocity so as to scan the mark with the spot of radiation while detecting and processing signals representing the diffracted radiation to calculate a position of the mark relative to a reference frame of the measurement optical system,
wherein the movable optical element operable synchronously with the first positioning subsystem for moving the spot of radiation relative to the reference frame of the measurement optical system at a second velocity, the first and second velocities being related such that the spot scans the mark at a third velocity which is lower than the first velocity while the signals are detected.

5. The apparatus of claim 4, wherein in operation the first and second velocities are parallel, with the second velocity being lower than the first velocity, and the third velocity being equal to the first velocity minus the second velocity.

6. The apparatus of claim 4, wherein the objective lens is fixed in relation to the reference frame of the measurement optical system.

7. The apparatus of claim 4, wherein the optical system includes a self-referencing interferometer for rotating and recombining two portions of the diffracted radiation.

8. An apparatus comprising a confocal microscope for inspection of objects, the apparatus comprising:
an optical apparatus comprising:
an objective lens configured to transmit rays of radiation between an optical system of the apparatus and an object, the optical system comprising:
an illumination subsystem configured to illuminate an object via the objective lens with a spot of radiation; and
a detecting subsystem for detecting via the objective lens radiation reflected by the object at a specific point in the spot radiation; and

at least one moveable element comprising a mirror, the mirror being arranged between the optical system and the objective lens and configured to tilt and so change the direction of the rays as they pass through the objective lens, the movable optical element operable for scanning the spot of radiation and the specific point in at least a first scanning direction so as to record intensity of radiation detected from a line of points across the object,
wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.

9. The apparatus of claim 8, further comprising a positioning subsystem for moving the object and the objective lens relative to one another in a second scanning direction so as to record intensity of radiation from a series of lines of points across the object.

10. A method of scanning an object with rays of radiation via an objective lens, the method comprising:
arranging a mirror in a path of the rays prior to the objective lens;
oscillating the rays using the mirror, and
tilting the mirror so as to change the direction of the rays as they pass through the objective lens, and thereby to cause the rays to impinge on different parts of the object in accordance with a tilt angle of the mirror,
wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror.

11. The method of claim 10, wherein the oscillating comprises imparting a repetitive scanning movement to the rays.

12. The method of claim 10, wherein the oscillating comprises oscillating at a resonant frequency.

13. The method of claim 10, wherein the scanning is performed as part of a process of measuring the position of a mark on a substrate.

14. A method of manufacturing a device in which a lithographic process is used to transfer a pattern from a patterning device onto a substrate, and the transfer of a pattern onto the substrate is controlled by reference to positions of marks on the substrate measured using a method of scanning an object with rays of radiation via an objective lens, the method comprising:
arranging a mirror in a path of the rays prior to the objective lens; and
tilting the mirror so as to change the direction of the rays as they pass through the objective lens, and thereby to cause the rays to impinge on different parts of the object in accordance with a tilt angle of the mirror,
wherein the mirror is constrained to tilt about an axis which lies approximately along the intersection between a pupil plane of the objective lens and a plane of the mirror, and
wherein the scanning is performed as part of a process of measuring the position of a mark on a substrate.