METROLOGY SYSTEM, AND LITHOGRAPHIC APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 62/883,889 which was filed on

August 7, 2019 and which is incorporated herein in its entirety by reference.

FILED

[0002] The present disclosure relates to a laser module assembly used as an alignment source in a metrology systems that may be used, for example, in a lithographic apparatus.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion ( e.g ., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the 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. Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of substrate through the use of a reflection system. The interference causes lines to be formed on at the target portion of the substrate. [0004] 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 alignment marks on a substrate can be measured with a high degree of accuracy. These alignment sensors in a metrology system for detecting positions of the alignment marks (e.g., X and Y position) and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask.

[0005] Alignment systems typically have their own illumination system. The signal detected from the illuminated alignment marks may be dependent on how well the wavelengths of the illumination system are matched to the physical or optical characteristics of the alignment marks, or physical or optical characteristics of materials in contact with or adj acent to the alignment marks. The aforementioned characteristics may vary depending on the processing steps used. Alignment systems may offer a narrow band radiation beam having a set of discrete, relatively narrow passbands in order to maximize the quality and intensity of alignment mark signals detected by the alignment system. [0006] Generally, the alignment sensors use a laser module assembly comprising more than one color as laser sources. The light from these laser sources is aligned in predetermined polarization state and thus may suffer complete or near-complete extinction due to certain stack variations. Since many stacks and/or alignment marks are polarizing and hence collected light from the alignment marks may be of very low magnitude thus compromising accuracy of calculated alignment position. Detected signals due to asymmetry of the alignment marks and associated position error also cannot be corrected without polarization diversity signals.

SUMMARY

[0007] Accordingly, there is a need for a new laser module assembly as an alignment source in a metrology system to include a polarization controller to dynamically change the polarization of the launch light to derive healthy alignment signal, even under harsh conditions of polarization fading. [0008] One aspect of the present disclosure provides a metrology system, comprising: a radiation source configured to generate a plurality of light beams, each centered at a different wavelength; a dynamic polarization controller configured to dynamically alternate polarization orientation of each of the plurality of light beams; a polarization multiplexer configured to combine the plurality of light beams with dynamically alternated polarization orientations into a combined light beam; a reflector configured to direct the combined light beam towards a substrate; an interferometer configured to receive light that has been diffracted from a pattern on the substrate and to produce output light from interference between the diffracted light; and a detector configured to detect optical signals based on the output light from the interferometer and to output a time-varying intensity signal. [0009] In some embodiments, the radiation source comprises at least a green laser, a red laser, a near-infrared laser, and a far-infrared laser.

[0010] In some embodiments, the dynamic polarization controller includes a plurality of polarization controllers each being positioned in a path of a corresponding one of the plurality of light beams such that each light beam is linearly polarized and dynamically alternated between orthogonal polarization orientations.

[0011] In some embodiments, the dynamic polarization controller is further configured to control a first set of light beams simultaneously in a first polarization orientation during a first time period and in a second polarization orientation during a second time period, and control a second set of light beams simultaneously in the second polarization orientation during the first time period and in the first polarization orientation during the second time period. In some embodiments, the first polarization orientation is orthogonal to the second polarization orientation. [0012] In some embodiments, the first set of light beams includes green light and near- infrared light, and the second set of light beams includes red light and far-infrared light.

[0013] In some embodiments, the dynamic polarization controller is further configured to control a time gap between alternating polarization orientation of each of the plurality of light beams less than 200 ps.

[0014] In some embodiments, the polarization multiplexer is further configured to maintain polarization orientations of components of the combined light beam.

[0015] In some embodiments, the metrology system further comprises a demultiplexer configured to separate the output light of the interferometer into multiple optical signals based on wavelength.

[0016] In some embodiments, the detector comprises at least four detector elements configured to detect time-varying intensity signals of the multiple optical signals respectively based on wavelengths.

[0017] Another aspect of the present disclosure provides a lithographic apparatus, comprising the disclosed metrology system.

[0018] Another aspect of the present disclosure provides a method for inspecting a target on a substrate, comprising: generating a plurality of light beams, each centered at a different wavelength; dynamically alternating polarization orientation of each of the plurality of light beams; combining the plurality of light beams with dynamically alternated polarization orientations into a combined light beam; directing the combined light beam towards a substrate; receiving light that has been diffracted from a pattern on the substrate and producing output light from interference between the diffracted light; detecting optical signals based on the output light from the interferometer; and outputting a time-varying intensity signal.

[0019] In some embodiments, generating the plurality of light beams comprises generating at least a green laser beam, a red laser beam, a near-infrared laser beam, and a far-infrared laser beam. [0020] In some embodiments, the method further comprises individually controlling each of the plurality of light beams to dynamically alternate between orthogonal linear polarization orientations.

[0021] In some embodiments, the method further comprises: controlling a first set of light beams simultaneously in a first polarization orientation during a first time period and in a second polarization orientation during a second time period; and controlling a second set of light beams simultaneously in the second polarization orientation during the first time period and in the first polarization orientation during the second time period. The first polarization orientation is orthogonal to the second polarization orientation.

[0022] In some embodiments, the method further comprises: controlling a time gap between alternating polarization orientation of each of the plurality of light beams less than 200 ps. [0023] In some embodiments, the method further comprises: maintaining polarization orientations of components of the combined light beam during combining the plurality of light beams. [0024] In some embodiments, the method further comprises separating the output light from interference into multiple optical signals based on wavelength.

[0025] In some embodiments, the method further comprises detecting time-varying intensity signals of the multiple optical signals respectively based on wavelengths.

[0026] In some embodiments, directing the combined light beam towards a substrate comprises scanning the pattern on the substrate by using the combined light beam with dynamically alternated polarization orientations.

[0027] 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

[0028] 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.

[0029] FIG. 1A is a schematic illustration of a reflective lithographic apparatus in accordance some embodiments.

[0030] FIG. IB is a schematic illustration of a transmissive lithographic apparatus in accordance some embodiments.

[0031] FIGs. 2 A and 2B illustrate various forms of an alignment mark that may be provided on a substrate in the apparatus of FIG.1.

[0032] FIG. 3 is a schematic block diagram of alignment sensor AS in accordance some embodiments.

[0033] FIG. 4 is a schematic illustration of an exemplary metrology system in accordance some embodiments.

[0034] FIG. 5 is a schematic illustration of switching polarization states of multiple light beams with different colors in accordance some embodiments.

[0035] FIG. 6 illustrates a flowchart for using a metrology system to inspect a target on a substrate in accordance some embodiments.

[0036] 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. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTION

[0037] 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.

[0038] 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.

[0039] 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. [0040] Example Reflective and Transmissive Lithographic Systems

[0041] FIGs. 1A and IB are schematic illustrations of a lithographic apparatus 100 and lithographic apparatus 100', respectively, in which embodiments of the present invention may be implemented. Lithographic apparatus 100 and lithographic apparatus 100' each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100' also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100', the patterning device MA and the projection system PS are transmissive. [0042] The illumination system IL may include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B. [0043] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100', and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA.

The support structure MT can be a frame or a table, for example, which can be fixed or movable, as required. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0044] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0045] The patterning device MA may be transmissive (as in lithographic apparatus 100' of

FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, 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 the radiation beam B which is reflected by a matrix of small mirrors.

[0046] The term “projection system” PS can encompass 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 on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0047] 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).

[0048] Lithographic apparatus 100 and/or lithographic apparatus 100' can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT. The two substrate tables WTa and WTb in the example of FIG. IB are an illustration of this. The 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.

[0049] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0050] Referring to FIGs. 1 A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100' can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100', and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100' — for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if required, can be referred to as a radiation system.

[0051] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as “s-outer” and “s-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0052] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask)

MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0053] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, 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 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WTa/WTb can be moved accurately (for example, 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 (not shown in FIG. IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0054] In general, movement of the mask table MT can 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 can 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 can be connected to a short-stroke actuator only or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks PI, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (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 can be located between the dies.

[0055] The lithographic apparatus 100 and 100' can be used in at least one of the following modes:

[0056] 1. In step mode, the support structure (for example, mask table) MT and the substrate table WTa/WTb are kept essentially stationary, while an entire pattern imparted to the radiation beam B 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.

[0057] 2. In scan mode, the support structure (for example, mask table) MT and the substrate table WTa/WTb are scanned synchronously while a pattern imparted to the radiation beam B 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 support structure (for example, mask table) MT can 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.

[0058] 3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WTa/WTb is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source SO can be 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 a programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0059] Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

[0060] 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.

[0061] 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 posi tion 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. [0062] Example Alignment Sensor

[0063] FIG. 2A shows examples of alignment marks 202, 204, provided on substrate W for the measurement of X-position and Y -position, respectively. Each alignment mark in this example comprises a series of bars formed in a product layer or other layer applied to or etched into the substrate. The bars are regularly spaced and act as grating lines so that the mark can be regarded as a diffraction grating with a sufficiently well-known spatial period (pitch). The bars on the X-direction mark 202 are parallel to the Y-axis to provide periodicity in the X direction, while the bars of the Y- direction mark 204 are parallel to the X-axis to provide periodicity in the Y direction.

[0064] The alignment sensor AS (shown in FIG. 1) scans each mark optically with a spot

206 (X direction), 208 (Y direction) of radiation, to obtain a periodically-varying signal, such as a sine wave. The phase of this signal is analyzed, to measure the position of the mark, and hence of substrate W, relative to the alignment sensor, which in turn is fixed relative to the reference frame RF of 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 (grating lines) 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 a repeating signal to be accurately measured.

[0065] Coarse and fine marks may be provided, so that the alignment sensor can distinguish between different cycles of the periodic signal, as well as the exact position (phase) within a cycle. Marks of different pitches can also be used for this purpose. These techniques are again well known to the person skilled in the art, and will not be detailed herein. The design and operation of such sensors is well known in the art, and each lithographic apparatus may have its own design of sensor. For the purpose of the present description, it will be assumed that the alignment sensor AS is generally of the form described in U.S. Pat. No. 6,961,116 (den Boef et at).

[0066] FIG. 2B shows a modified mark for use with a similar alignment system, which X- and Y-positions can be obtained through a single optical scan with the illumination spot 206 or 208. 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 U.S. Pat. No. 8,593,464 (Bijnen et al.), the contents of which are incorporated herein by reference. It may be noted that U.S. Pat. No. 8,593,464 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. 2B is the form generally used in the commercial embodiment, and any more effect that may be observed between different pitches in the embodiments of U.S. Pat. No. 8,593,464 is fixed and provides no measure of process performance. [0067] 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 di verted through an objective lens 224 onto mark such as mark 202, located on substrate W. As indicated schematically in FIGs. 2A and 2B, 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. In some embodiments, illumination source 220 can be a multi-color laser module assembly (LMA) described in detail below in connection with FIGs. 4 and 5. [0068] Radiation scattered by mark 202 is picked up by objecti ve lens 224 and collimated into an information-carrying beam 226. A self-referencing interferometer 228 is of the type disclosed in U.S. Pat. No. 6,961,116 mentioned above, and processes beam 226 and outputs separate beams (for each wavelength) onto a sensor array 230. Spot mirror 223 serves conveniently as a zero order stop at this point, so that the information carrying beam 226 comprises only higher order diffracted radiation from the mark 202 (this is not essential to the measurement, but improves signal to noise ratios). 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.

[0069] 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 exploit measurement at several wavelengths to provide a practical and robust measurement apparatus (alignment sensor) with reduced sensitivity to mark asymmetry.

[0070] Referring to the measurement process in more detail, an arrow labelled 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. All movements shown are parallel to the X axis. Similar actions apply for scanning the mark 204 with spot 208 in the Y direction. This will not be described further.

[0071] As discussed in U.S. Pat. No. 8,593,464, the high productivity requirements required of the lithographic apparatus require the measurement of the alignment marks at numerous positions on the substrate to be performed as quickly as possible, which implies that the scanning velocity vW is fast, and the time TACQ available for acquisition of each mark position is correspondingly short. In simplistic terms, the formula TACQ = L/vW applies. U.S. Pat. No. 8,593,464 describes a technique to impart an opposite scanning motion of the spot, so as to lengthen the acquisition time. The same scanning spot techniques can be applied in sensors and methods of the type newly disclosed herein, if desired.

[0072] There is interest in aligning on marks with smaller grating pitches. The measured overlay in real production is generally significantly larger than under controlled test conditions. Investigations suggest that this is due to the alignment marks on product wafers becoming asymmetric to varying degrees during processing. Reducing the pitch of the alignment marks decreases the effect of some types of asymmetry on the measured alignment position.

[0073] The skilled person knows that some options to allow reduction of the pitch of an alignment grating are (i) shortening the wavelength of radiation used, (ii) increasing the NA of the alignment sensor optics and (iii) using off-axis illumination. A shorter wavelength is not always possible since alignment gratings are often located underneath an absorbing film (for example an amorphous carbon hard mask). Increasing the NA is in general possible but is not preferred since there is a need for a compact objective with a safe distance from the wafer. Therefore using off-axis illumination is attractive.

[0074] Position Measurement with Multiple Wavelengths and Dynamic Polarizations

[0075] FIG. 4 illustrates an exemplary optical system 440 of an alignment sensor that is a modified version of one described in U.S. Pat. No. 6,961,116 and U.S. Pat. No. 8,593,464 mentioned above. An optical axis that has several branches is indicated by a broken line running throughout the optical system 440.

[0076] In some embodiments, illumination source 410 can be a multi-color laser module assembly (LMA) including multiple laser sources 413, multiple dynamic polarization controller (DPC) 415, a fiber connection plate (FCP) 417, and a laser power supply assembly (not shown in FIG. 4).

[0077] In some embodiments as shown in FIG. 4, the multiple laser sources 413 comprises four individual sources to produce radiation with of four wavelengths, such as a green laser centered around 532 nm, a red laser centered around 633 nm, a near-infrared (NIR) laser centered around 780 nm, and a far-infrared (FIR) laser centered around 850 nm. For convenience in the following discussion, the radiation at these four different wavelengths will be called “four colors of light,” which can be in the visible or non-visible bands of the electromagnetic spectrum. The four colors of light generated by the multiple laser sources 413 is unpolarized (e.g., includes both horizontal and vertical polarization states). In some embodiments, the power of the light emitted by the different laser sources can be controlled by a laser power supply assembly (not shown in FIG. 4.) In some other embodiments, the multiple laser sources 413 can include more individual sources to produce radiation with of more colors (e.g., 12 colors) with wider wavelength diversity.

[0078] The unpolarised multi-color laser light can be delivered via dynamic polarization controllers (DPC) 415 to dynamically modulate the polarizations of the multi-color light. In some embodiments, a dynamic polarization controller 415 is used in each path of the multiple laser sources 413 such that each color of light is linearly polarized and the polarization of each color of light can be individually changed dynamically. As shown in FIG. 5, each of the four-color light including green light 51, red light 52, near infrared light (NIR) light 53 and far infrared (FIR) light 54, can switch between a first polarization state (e.g., vertical polarization state 51-1, 52-1, 53-1 and 54-1) and a second polarization state that is orthogonal to the first polarization state (e.g., horizontal polarization states 51-2, 52-2, 53-2 and 54-2).

[0079] In some embodiments, each color of light is not simultaneously oriented in both polarization states, and the beams of all colors are not simultaneously oriented in a single polarization state. For example, as shown in the solid circuits in FIG. 5, the green light 51 and the NIR light 53 can be simultaneously oriented in the vertical polarization states 51-1 and 53-1, while the red light 52 and the FIR light 54 can be simultaneously oriented in the horizontal polarization states 52-2 and 54-2 in a same time. After the switch, as shown in the dashed circuits in FIG. 5, the green light 51 and the NIR light 53 can be simultaneously oriented in the horizontal polarization states 51-2 and 53-2, while the red light 52 and the FIR light 54 can be simultaneously oriented in the vertical polarization states 52-1 and 54-1 in a same time. In doing so, the detected alignment signal can have dynamically changed between both polarization states for all colors. Such polarization and wavelength diversity can ensure a high fidelity alignment signal presenting in all stack scenarios that may cause signal extinction at particular color and/or polarization.

[0080] The dynamic polarization controllers 415 can alternate the polarization states of each beam of the multi-color light in a designed frequency. It is noted that, the time for alternating the polarization states of alignment light 406 should be limited in a short time period, such as about 100 ps. This 100 ps time period for alternating the polarization states of alignment light may cause missing about 0.5-1.5 pm of a segment of the alignment mark 202 during the scanning, which is within the tolerate limit of missing 5.5-15 pm of a segment of the alignment mark 202 during the scanning.

[0081] It is noted that, any suitable polarization controllers, such as fiber squeezer technology polarization controllers, liquid crystal polarization controllers, thermally activated polarization controllers, LiNbOs polarization controllers, or the like, can be used as the dynamic polarization controllers 415. Specifically, examples of dynamic polarization controllers 415 or modification or combination thereof include: General Photonics’ PCD-M02 Polarization Controller, Lightwaves2020’s High Speed Polarization Controller, New Ridge Technologies’ NRT-2500 Versatile Polarization Control Platform, Phoenix Photonics’ Fiber Polarization Controllers, and the like. Some of the exemplary polarization controller devices work at typical wavelengths in an infrared band between about 1310 nrn to about 1650 nm, but these wavelengths can be modified to work at lower wavelengths (e.g., around 532 nm, 633 nm, 780 nm, and/or 850 nm, etc.).

[0082] Referring back to FIG. 4, the four color light with dynamically changing polarizations can be then transported via polarization maintaining optical fibers mounted in an fiber connection plate (FCP) 417 to a polarization multiplexer (PMUX) 502, where the four color light are combined into a single four-color beam. The polarization multiplexer 502 does not perform polarization to thereby maintain linear polarizations of components of the combined radiation. That is, the combined four-color beam can include: 1) vertically polarized green component and NIR component, and horizontally polarized red component and FIR at a first instance, and 2) horizontally polarized green component and NIR component, and vertically polarized red component and FIR at a second instance. [0083] The combined four-color beam can be delivered through an illumination profiling optics 446, and then traverses a first delivery optics 506 into beam splitter 454. Input beam 422 reflects from a partially- or fully reflecting surface (e.g. a 0.5 mm diameter spot mirror), which can be inside the beam splitter. An objective lens 424 can have a pupil plane P that focuses the reflected beam to a narrow beam thereby forming a spot (i.e. alignment light 406) on alignment mark 202 (or 204/210) of the wafer W. This alignment light 406 is reflected and diffracted by the grating formed by alignment mark 202 on the wafer. Light is collected by the objective lens 424, with for example numerical aperture NA = 0.6. This NA value allows at least ten orders of diffraction to be collected from a grating with 16 pm pitch, for each of the colors.

[0084] The reflected and diffracted light forming information carrying beam 426, passes through beam splitter 454, is then transported to the self-referencing interferometer 428. In some embodiments, before entering the self-referencing interferometer 428, the information carrying beam 426 is split by beam splitter 462 to supply a portion 464 of the information carrying beam 426 to the asymmetry measuring arrangement 460, when provided. Signals 466 conveying asymmetry measurement information are passed from arrangement 460 to the processing unit PU.

[0085] In some embodiments, before entering the self-referencing interferometer 428, the polarization of each color of light is rotated by 45° by a half wave plate 510. The self-referencing interferometer 428 can include a polarizing beam splitter, wherein half of each color of light is transmitted, and half of each color of light is reflected. Each half can be reflected (e.g., three times) inside the interferometer, thereby rotating each color’ s radiation field by +90° and -90°, giving a relative rotation of 180° between each corresponding color half. The two radiation fields are then superimposed on top of each other and allowed to interfere. A phase compensator 512 can be used to compensate for path differences of the -90° and +90° fields. That is, the self-referencing interferometer 428 splits the radiation field into two parts with orthogonal polarizations, rotates these parts about the optical axis by 180° relative to one another, and then combines all components of colors into an outgoing beam 482. The polarization of each color of light is then rotated again by 45° using another half wave plate 514 (having its major axis set at 22.5° to the X or Y axis). The half wave plates 510, 514 are wavelength insensitive, so that polarizations of all four color wavelengths are rotated by 45°.

[0086] After traversing a second delivery optics 516, the light is collected by a collector lens assembly 484 configured to focus the entire field onto each element of the detector 430. Aperture 518 is used to eliminate most of the light from outside the spot on the substrate. A multimode fiber 520 is used to transport the collected light to a demultiplexer 522. The demultiplexer 522 splits the light in the original four colors, so that four optical signals are delivered to detector 430. In one practical embodiment, fibers are arranged between the demultiplexer 522 to four detector elements respectively (e.g., located on one or more detector circuit boards).

[0087] The detector 430 including the four detector elements is configured to output time- varying intensity signal I for each color, as the apparatus scans the alignment mark 202 on substrate W, for example. The resulting intensity optical signals are position-dependent signals, and are received as time-varying signals (waveforms) synchronized with the physical scanning movement between the apparatus and the alignment mark, as discussed above in connection with FIG. 3. In some embodiments, the detector 430 includes effectively single photodiodes and does not provide any spatial information except by the scanning motion described already. A detector having spatial resolution in a conjugate pupil plane can be added, if desired. This may allow for angle-resolved scatterometry methods to be performed using the alignment sensor hardware.

[0088] Processing unit PU receives the intensity waveforms from the detector 430 and processes them to provide a position measurement signal (POS). Because there are four signals to choose from in the exemplary embodiment, based on different wavelengths and dynamically alternating incident polarizations, the apparatus can obtain useable measurements in a wide variety of situations. As described above, to increase diversity without impacting throughput, one can envisage an implementation similar to the four-color scheme presented here, but using more colors, for example eight or sixteen, with mixed polarizations.

[0089] In this regard it should be remembered that the alignment mark 202 may be buried under a number of layers of different materials and structures. Some wavelengths will penetrate different materials and structures better than others. PU processes the waveforms to provide a position measurement based on the one which is providing the strongest position signal, in a manner that would become apparent to persons skilled in the art. The remaining waveforms may be disregarded.

In a simple implementation, the ‘recipe’ for each measurement task may specify which signal to use, based on advanced knowledge of the target structure/ alignment mark, and experimental investigations. In more advanced systems, for example as described in the paper by Huijbregtse et ai, mentioned above, an automatic selection can be made, using “Color Dynamic” or “Smooth Color Dynamic” algorithms to identify the best signals without prior knowledge.

[0090] By dynamically alternating polarizations in all colors and being able to temporally separate the polarized states at detector 428, the metrology system need not use two self-referencing interferometers (one for each polarization state). And the alignment mark (e.g., XY mark) does not need to be scanned twice by using two different polarizations, and different portions of the XY mark can be scanned and measured without switching illumination mode. In some embodiments, the polarization of each color of light can be modulated with a characteristic frequency, selected to be much higher than the frequency of the time-varying signal that carries the position information.

[0091] In some embodiments, if it is desired to illuminate an alignment mark 202 with circular polarization, whether for position sensing or some other form of metrology, a quarter wave plate (not shown) can be inserted between beam splitter 454 and objective 424. This has the effect of turning a linear polarization into a circular one (and changing it back again after diffraction by the mark). The spot positions are chosen as before according to the mark direction. The direction of circular polarization (clockwise/counter-clockwise) can be changed by illumination profiling optics 446, for example.

[0092] Example Methods of Inspecting a Wafer Using a Metrology System

[0093] FIG. 6 illustrates a flowchart 600 for using a metrology system to inspect a target on a substrate according to some embodiments. Solely for illustrative purposes, the steps illustrated in FIG. 6 will be described with reference to example operating environments described above in connection with FIG. 4. However, flowchart 600 is not limited to these embodiments. It is to be appreciated that steps can be performed in a different order or not performed depending on specific applications.

[0094] In operation 602, multiple light beams certered at different wavelengths are generated from multiple laser sources. Each light beam may be pulsed or continuous. The multiple light beams may have four or more colors, and can be unpolarised. That is, each color of light beam can include both orthogonal polarization components (e.g., horizontal and vertical polarization components). [0095] In operation 604, each of the multiple light beams with different color is modulated to dynamically change the polarization orientation. The light beam of each color may be modulated by a dynamic polarization controller. In some embodiments, the multiple light beams of different colors are not simultaneously oriented in both polarization orthogonal states, and multiple light beams of all colors are not simultaneously oriented in a single polarization state.

[0096] In operation 606, the multiple light beams with different colors and dynamically changing polarizations are combined. In some embodiments, the multiple light beams can be combined by a polarization multiplexer into a single multi-color light beam. The polarization multiplexer (e.g., PMUX 502 as shown in FIG. 4) can have fixed polarization to maintain the linear polarizations of different components (i.e., different wavelength bands) of the combined light beam. The combined light beam (e.g., alignment beam 406 as shown in FIG. 4) has both color and polarization diversity, which can result in high fidelity alignment signal in all stack scenarios allowing improved signal extinction including accurate color and/or polarization.

[0097] In operation 608, the combined light beam is directed towards a substrate. In some embodiments, the combined light beam is directed towards a target (e.g., alignment mark 202 as shown in FIG. 4) on the substrate and is diffracted by the target. In some embodiments, the target on the substrate is scanned while a pattern imparted to the combined light beam is projected onto a portion of the target.

[0098] In operation 610, the light received from the substrate is interfered. In some embodiments, diffraction orders of each polarization mode of each color of light diffracted from the target on the substrate are interfered. The interference can be achieved by a self-referencing interferometer (SRI), for example.

[0099] In operation 612, the output light from the interferometer is detected. In some embodiments, different polarization modes of the detected light can be differentiated from one another at the detector, and the different wavelengths of light can also be differentiated from one another at the detector. The detected signal with both polarization and wavelength diversity can derive robust alignment position determination.

[0100] Other aspects of the invention are set out in the following numbered clauses.

1. A metrology system, comprising: a radiation source configured to generate a plurality of light beams, each centered at a different wavelength; a dynamic polarization controller configured to dynamically alternate polarization orientation of each of the plurality of light beams; a polarization multiplexer configured to combine the plurality of light beams with dynamically alternated polarization orientations into a combined light beam; a reflector configured to direct the combined light beam towards a substrate; an interferometer configured to receive light that has been diffracted from a pattern on the substrate and to produce output light from interference between the diffracted light; and a detector configured to detect optical signals based on the output light from the interferometer and to output a time-varying intensity signal.

2. The metrology system of clause 1, wherein the radiation source comprises at least a green laser, a red laser, a near-infrared laser, and a far-infrared laser.

3. The metrology system of clause 1, wherein the dynamic polarization controller includes a plurality of polarization controllers each being positioned in a path of a corresponding one of the plurality of light beams such that each light beam is linearly polarized and dynamically alternated between orthogonal polarization orientations.

4. The metrology system of clause 1, wherein the dynamic polarization controller is further configured to: control a first set of light beams simultaneously in a first polarization orientation during a first time period and in a second polarization orientation during a second time period; and control a second set of light beams simultaneously in the second polarization orientation during the first time period and in the first polarization orientation during the second time period; wherein the first polarization orientation is orthogonal to the second polarization orientation.

5. The metrology system of clause 4, wherein: the first set of light beams includes green light and near-infrared light; and the second set of light beams includes red light and far-infrared light.

6. The metrology system of clause 1, wherein the dynamic polarization controller is further configured to: control a time gap between alternating polarization orientation of each of the plurality of light beams less than 200 ps.

7. The metrology system of clause 1, wherein the polarization multiplexer is further configured to maintain polarization orientations of components of the combined light beam.

8. The metrology system of clause 1, further comprising: a demultiplexer configured to separate the output light of the interferometer into multiple optical signals based on wavelength.

9. The metrology system of clause 8, wherein the detector comprises: at least four detector elements configured to detect time-varying intensity signals of the multiple optical signals respectively based on wavelengths.

10. A lithographic apparatus, comprising the metrology system of clause 1.

11. A method for inspecting a target on a substrate, comprising: generating a plurality of light beams, each centered at a different wavelength; dynamically alternating polarization orientation of each of the plurality of light beams; combining the plurality of light beams with dynamically alternated polarization orientations into a combined light beam; directing the combined light beam towards a substrate; recei ving light that has been diffracted from a pattern on the substrate and producing output light from interference between the diffracted light; detecting optical signals based on the output light from the interferometer; and outputting a time-varying intensity signal.

12. The method of clause 11 , wherein generating the plurality of light beams comprises generating at least a green laser beam, a red laser beam, a near-infrared laser beam, and a far-infrared laser beam.

13. The method of clause 11, further comprising: individually controlling each of the plurality of light beams to dynamically alternate between orthogonal linear polarization orientations. 14. The method of clause 11, further comprising: controlling a first set of light beams simultaneously in a first polarization orientation during a first time period and in a second polarization orientation during a second time period; and controlling a second set of light beams simultaneously in the second polarization orientation during the first time period and in the first polarization orientation during the second time period; wherein the first polarization orientation is orthogonal to the second polarization orientation.

15. The method of clause 14, wherein: the first set of light beams includes green light and near-infrared light; and the second set of light beams includes red light and far-infrared light.

16. The method of clause 11, further comprising: controlling a time gap between alternating polarization or ientation of each of the plurality of light beams less than 200 ps.

17. The method of clause 11, further comprising: maintaining polarization orientations of components of the combi ned light beam during combining the plurality of light beams.

18. The method of clause 11, further comprising: separating the output light from interference into multiple optical signals based on wavelength.

19. The method of clause 18, further comprising: detecting time-varying intensity signals of the multiple optical signals respectively based on wavelengths.

20. The method of clause 11, wherein directing the combined light beam towards a substrate comprises scanning the pattern on the substrate by using the combined light beam with dynamically alternated polarization orientations.

[0101] Final Remarks

[0102] 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. [0103] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0104] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0105] In the embodiments described herein, the terms “lens” and “lens element,” where the context allows, can refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0106] Further, the terms “radiation,” “beam,” and “light” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (for example, having a wavelength l of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-20 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in an embodiment, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0107] The term “substrate” as used herein generally describes a material onto which subsequent material layers are added. In embodiments, the substrate itself may be patterned and materials added on top of it may also be patterned, or may remain without patterning.

[0108] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.

[0109] It is to be appreciated that the Detailed Description section, and not the Summary and

Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.

[0110] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0111] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. [0112] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.