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Title:
LASER MODULE AS ALIGNMENT SOURCE, METROLOGY SYSTEM, AND LITHOGRAPHIC APPARATUS
Document Type and Number:
WIPO Patent Application WO/2021/052790
Kind Code:
A1
Abstract:
An improved light laser module is disclosed. The laser module can include a laser source (310) configured to generate laser light, and an infinite impulse response filter configured to reduce coherence effect of the laser light by decorrelating phase of components of the laser light. The infinite impulse response filter can include a plurality of optic couplers (410,420,430,440,450) to form a plurality of optical propagation loops with different optical path lengths (L1,L2,L3), respectively. The laser module can further include an acoustic-optic modulator arranged in an optical propagation loop, and configured to shift optical carrier frequency, such that an output of the laser module has a widen spectral to further reduce coherence effect.

Inventors:
AJGAONKAR MAHESH (US)
Application Number:
PCT/EP2020/074863
Publication Date:
March 25, 2021
Filing Date:
September 04, 2020
Export Citation:
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Assignee:
ASML HOLDING NV (NL)
International Classes:
G02B6/24; G02B27/48; G03F7/20; G03F9/00; H04B10/25
Foreign References:
US20170277046A12017-09-28
US20050185683A12005-08-25
US20050053305A12005-03-10
US20030175032A12003-09-18
EP0165773A21985-12-27
US6961116B22005-11-01
US8593464B22013-11-26
Other References:
LEE JUN ET AL: "Speckle reduction in optical coherence tomography images via dynamic infinite-impulse-response filtering", PROGRESS IN BIOMEDICAL OPTICS AND IMAGING, SPIE - INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING, BELLINGHAM, WA, US, vol. 8934, 4 March 2014 (2014-03-04), pages 89343I - 89343I, XP060033571, ISSN: 1605-7422, ISBN: 978-1-5106-0027-0, DOI: 10.1117/12.2041635
ERWIN H W CHAN: "High-Order Infinite Impulse Response Microwave Photonic Filters", JOURNAL OF LIGHTWAVE TECHNOLOGY, IEEE, USA, vol. 29, no. 12, 1 June 2011 (2011-06-01), pages 1775 - 1782, XP011325824, ISSN: 0733-8724, DOI: 10.1109/JLT.2011.2141116
Attorney, Agent or Firm:
ASML NETHERLANDS B.V. (NL)
Download PDF:
Claims:
CLAIMS

1. A laser module, comprising: a laser source configured to generate laser light; and an infinite impulse response filter configured to decorrelate phase of components of the laser light to thereby reduce coherence effect of the laser light.

2. The laser module of claim 1, wherein the laser source is configured to generate green laser light.

3. The laser module of claim 1, wherein the infinite impulse response filter comprises a plurality of optic couplers to form a plurality of optical propagation loops with different optical path lengths.

4. The laser module of claim 3, wherein the plurality of optical propagation loops comprises: a first optical propagation loop having a first fiber with a first fiber length; a second optical propagation loop having a second fiber with a second fiber length; and a third optical propagation loop having a third fiber with a third fiber length; wherein the first fiber length, the second fiber length, and the third fiber length are larger than a coherence length of the laser light.

5. The laser module of claim 4, wherein: an absolute value of a difference between the first fiber length and the second fiber length is larger than the coherence length of the laser light.

6. The laser module of claim 4, wherein: an absolute value of a difference between the third fiber length and a summation of the first fiber length and the second fiber length is larger than the coherence length of the laser light.

7. The laser module of claim 4, wherein: a summation of arbitrary integer multiples of any two of the three fiber lengths is not an integer multiple of the other one of the three fiber lengths.

8. The laser module of claim 4, wherein a combination of the first fiber length, the second fiber length, and the third fiber length is one of the following: the first fiber length is 1.17m, the second fiber length is 2.63m, and the third fiber length is 4.47m; the first fiber length is 1.31m, the second fiber length is 2.57m, and the third fiber length is 4.49m; the first fiber length is 1.67m, the second fiber length is 2.77m, and the third fiber length is 4.57m; or the first fiber length is 1.79m, the second fiber length is 3.73m, and the third fiber length is 5.93m.

9. The laser module of claim 3, further comprising an acoustic-optic modulator arranged in an optical propagation loop, and configured to shift an optical carrier frequency, such that an output of the laser module has a widen spectral to further reduce coherence effect.

10. The laser module of claim 9, further comprising a fiber phase modulator driven by a randomized phase signal to scramble a phase relationship among different spectral components of the output of the laser module.

11. The laser module of claim 1 , further comprising a variable optical attenuator configured as an optical switch.

12. The laser module of claim 4, wherein the plurality of optic couplers comprise: a first optic coupler including a first input port connected to the laser source; a second optic coupler including a first input port connected to a first output port of the first optic coupler; a third optic coupler including a first input port connected to a second output port of the first optic coupler, and a second input port connected to a second output port of the second optic coupler; and a fourth optic coupler including a first input port connected to a first output port of the third optic coupler, a second input port connected to a second output port of the third optic coupler, a first output port connected to a second input port of the first optic coupler, and a second output port connected to a second input port of the second optic coupler.

13. The laser module of claim 12, wherein: the first fiber is arranged between the first output port of the third optic coupler and the first input port of the fourth optic coupler; and the second fiber is arranged between the second output port of the third optic coupler and the second input port of the fourth optic coupler.

14. The laser module of claim 12, wherein: the first fiber is arranged between the second output port of the first optic coupler and the first input port of the third optic coupler; and the second fiber is arranged between the second output port of the second optic coupler and the second input port of the third optic coupler.

15. The laser module of claim 12, wherein: the third fiber is arranged between the second output port of the fourth optic coupler and the second input port of the first optic coupler; or the third fiber is arranged between the first output port of the fourth optic coupler and the second input port of the second optic coupler.

Description:
LASER MODULE AS ALIGNMENT SOURCE, METROLOGY SYSTEM, AND

LITHOGRAPHIC APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

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

62/901,369, which was filed on September 17, 2019, and which is incorporated herein in its entirety by reference.

FIFED

[0002] The present disclosure relates to a laser module 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 are in a metrology system and used for detecting positions of the alignment marks (e.g., X and Y position) for aligning the substrate using the alignment marks to ensure accurate exposure of the beam patterned by the mask. The metrology system may be used to determine a height of a wafer surface in the Z direction.

[0005] Alignment systems typically include an 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 adjacent 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 can detect more than one color produced by one or more laser sources. Typically these lasers are centered around 532nm, 633nm, 780 and 850nm. However, existing green laser modules that are used to product wavelengths at about 532nm are prone to have low longevity and are highly coherent, which results in alignment position uncertainty due to wafer induced coherence effects (WICO). Mode hopping of conventional green laser sources may also contribute to alignment problems.

SUMMARY

[0007] Accordingly, there is a need for a new laser module as an alignment source in a metrology system to achieve not only better working life but also reduce coherence effects.

[0008] One aspect of the present disclosure provides a laser module, comprising: a laser source configured to generate laser light; and an infinite impulse response filter configured to decorrelate phase of components of the laser light to thereby reduce coherence effect of the laser light .

[0009] In some embodiments, the laser source is configured to generate green laser light. [0010] In some embodiments, the infinite impulse response filter comprises a plurality of optic couplers to form a plurality of optical propagation loops with different optical path lengths. [0011] In some embodiments, the plurality of optical propagation loops comprises: a first optical propagation loop having a first fiber with a first fiber length; a second optical propagation loop having a second fiber with a second fiber length; and a third optical propagation loop having a third fiber with a third fiber length; wherein the first fiber length, the second fiber length, and the third fiber length are larger than a coherence length of the laser light.

[0012] In some embodiments, an absolute value of a difference between the first fiber length and the second fiber length is larger than the coherence length of the laser light.

[0013] In some embodiments, an absolute value of a difference between the third fiber length and a summation of the first fiber length and the second fiber length is larger than the coherence length of the laser light.

[0014] In some embodiments, a summation of arbitrary integer multiples of any two of the three fiber lengths is not an integer multiple of the other one of the three fiber lengths.

[0015] In some embodiments, a combination of the first fiber length, the second fiber length, and the third fiber length is one of the following: the first fiber length is 1.17m, the second fiber length is 2.63m, and the third fiber length is 4.47m; the first fiber length is 1.31m, the second fiber length is 2.57m, and the third fiber length is 4.49m; the first fiber length is 1.67m, the second fiber length is 2.77m, and the third fiber length is 4.57m; or the first fiber length is 1.79m, the second fiber length is 3.73m, and the third fiber length is 5.93m.

[0016] In some embodiments, the laser module further comprises an acoustic-optic modulator arranged in an optical propagation loop, and configured to shift an optical carrier frequency, such that an output of the laser module has a widen spectral to further reduce coherence effect.

[0017] In some embodiments, the laser module further comprises a fiber phase modulator driven by a randomized phase signal to scramble a phase relationship among different spectral components of the output of the laser module.

[0018] In some embodiments, the laser module further comprises a variable optical attenuator configured as an optical switch.

[0019] In some embodiments, the plurality of optic couplers comprise a first optic coupler including a first input port connected to the laser source; a second optic coupler including a first input port connected to a first output port of the first optic coupler; a third optic coupler including a first input port connected to a second output port of the first optic coupler, and a second input port connected to a second output port of the second optic coupler; and a fourth optic coupler including a first input port connected to a first output port of the third optic coupler, a second input port connected to a second output port of the third optic coupler, a first output port connected to a second input port of the first optic coupler, and a second output port connected to a second input port of the second optic coupler.

[0020] In some embodiments, the first fiber is arranged between the first output port of the third optic coupler and the first input port of the fourth optic coupler; and the second fiber is arranged between the second output port of the third optic coupler and the second input port of the fourth optic coupler.

[0021] In some embodiments, the first fiber is arranged between the second output port of the first optic coupler and the first input port of the third optic coupler; and the second fiber is arranged between the second output port of the second optic coupler and the second input port of the third optic coupler.

[0022] In some embodiments, the third fiber is arranged between the second output port of the fourth optic coupler and the second input port of the first optic coupler; or the third fiber is arranged between the first output port of the fourth optic coupler and the second input port of the second optic coupler.

[0023] In some embodiments, the laser module further comprises the first optic coupler has a splitting ratio of 10:90; and the second optic coupler, the third optic coupler, the fourth optic each has a splitting ratio of 50:50.

[0024] In some embodiments, the laser module further comprises a first acoustic-optic modulator arranged between the second output port of the first optic coupler and the first input port of the third optic coupler; and a second acoustic-optic modulator arranged between the second output port of the second optic coupler and the second input port of the third optic coupler.

[0025] In some embodiments, the laser module further comprises a first acoustic-optic modulator arranged between the first output port of the third optic coupler and the first input port of the fourth optic coupler; and a second acoustic-optic modulator arranged between the second output port of the third optic coupler and the second input port of the fourth optic coupler. [0026] Another aspect of the present disclosure provides a metrology system, comprising a multi-color radiation source including the disclose laser module, and configured to generate alignment light.

[0027] Another aspect of the present disclosure provides a lithographic apparatus, comprising the disclosed metrology system.

[0028] 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

[0029] 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.

[0030] FIG. 1A is a schematic illustration of a reflective lithographic apparatus according to an embodiment;

[0031] FIG. IB is a schematic illustration of a transmissive lithographic apparatus according to an embodiment;

[0032] FIG. 2 is a schematic block diagram of an alignment sensor scanning an alignment mark in accordance with some embodiments;

[0033] FIGs. 3-8 illustrate schematic diagrams of exemplary green laser modules in accordance with some embodiments; and

[0034] FIG. 9 includes schematic diagrams showing spectral broadening of output green laser according to some embodiments.

[0035] 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

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Example Reflective and Transmissive Lithographic Systems

[0040] 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.

[0041] 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 .

[0042] 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 RF, 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.

[0043] 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.

[0044] 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. [0045] 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”.

[0046] 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).

[0047] 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.

[0048] 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.

[0049] Referring to FIGs. 1A 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.

[0050] 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.

[0051] Referring to FIG. 1 A, 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.

[0052] 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).

[0053] 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. [0054] The lithographic apparatus 100 and 100' can be used in at least one of the following modes:

[0055] 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.

[0056] 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. [0057] 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.

[0058] Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

[0059] 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.

[0060] 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.

[0061] Example Alignment Sensor

[0062] FIG. 2 is a schematic block diagram of alignment sensor AS. Illumination source

220 provides an alignment beam 222 of radiation of one of more wavelengths and/or polarizations, which is diverted through an objective lens 224 onto mark such as mark 202, located on substrate W.

[0063] In some embodiments, illumination source 220 can include a multi-color laser module assembly (LMA) configured to generate multiple laser beams centered at different wavelengths. For example, the multi-color LMA can include four individual laser 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. In some embodiments, the multi-color LMA can further modulate the polarizations of the multiple laser beams, and then combine the multiple laser beams as the alignment beam 222.

[0064] 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 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.

[0065] 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.

[0066] Referring to the measurement process in more detail, an arrow labelled vw in FIG.

2 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 as shown in FIG. IB, 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.

[0067] As discussed in U.S. Pat. No. 8,593,464, the high productivity throughput demands of the lithographic apparatus requires 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 = F/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.

[0068] 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.

[0069] 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.

[0070] Improved Green Laser Module as Alignment Laser

[0071] As described above, the existing green laser module (e.g., with wavelength centered at 532 nm) is prone to have a low longevity much smaller than expectation. For example, the green laser module currently used in the existing multi-color laser module assembly (LMA) of alignment sensor AS employs diode-pumped solid-state (DPSS) laser, which is plagued by a low B10 life time of less than six months and a low B20 life time of about only one year. The skilled person knows that, a B10 life time is defined as a measurement of the time by which ten percent of a population of a product will have failed, while a B20 life time is defined as a measurement of the time by which twenty percent of a population of a product will have failed.

[0072] Further, wafer induced coherence effects (WICO), which are calibrated and tested for each laser, contribute to position uncertainty. The existing green laser module is generally a highly coherent source thus gives rise to alignment position uncertainty due to the WICO. In addition, mode hopping of the existing green laser module is generally also a contributor of shift between order (SBO) jumps, which are prominent in green color when used for alignment. And the laser beam switch (LBS) of the existing green laser module is also prone to malfunction and failure. Therefore, the present disclosure provide an improved green laser module to resolve these and other problems.

[0073] In some embodiments, the disclosed green laser module can ensure that, at any time of the measurement, the photon packets emitted from the disclosed green laser module include multiple copies of the green laser light, which was emitted several “coherence time” intervals ago. Stated in other words, light output of the disclosed green laser module contain theoretically infinite replicas of light delayed by more than coherence length of the laser itself hence breaking coherence relationship in photon packets.

[0074] In some embodiments of the present disclosure, the disclosed green laser module can include an Infinite Impulse Response (HR) filter constructed by using multiple optic couplers (also referred as “fiber optic splitters”) and patch cords. In some embodiments, light input of the IIR filter can be split into three loops with three different fiber lengths namely Li, L2 and L3 respectively. By specifically choosing the values of the three fiber lengths Li, L2 and L3 following a certain rule as described below, integer numbers of three loopings of signals from these fiber decorrelators cannot be in phase relationship with each other either, so as to eliminate the WICO. [0075] FIGs. 3-8 illustrate schematic diagrams of green laser modules including exemplary

IIR filters in accordance with various embodiments of the present disclosure.

[0076] Referring to FIG. 3, green laser module G1 can include a green light laser source

310. In some embodiments, green light laser source 310 can be any suitable laser source emitting continuous or pulsed laser radiation having a wavelength band centered around 532nm. In some embodiments, green light laser source 310 can have a Mean Time To Failure (MTTF) larger than 400KHrs, which can result on a much better B10 life time. However, green light laser source 310 can have a coherence length (L c ) about 10mm, and thus may have a worse WICO performance. [0077] In order to obtain a better WICO performance, green light laser source 310 can further include an Infinite Impulse Response (IIR) filter to reduced coherence effects. In some embodiments, the IIR filter can comprise multiple optic couplers (also referred as “fiber optic splitters”), such as first optic coupler 410, second optic coupler 420, third optic coupler 430, fourth optic coupler 440, and fifth optic coupler 450.

[0078] As shown in FIG. 3, the green light output from green light laser source 310 can be delivered to a first input port 410a of the first optic coupler 410. It is noted that, in FIGs. 3-8, each solid line with an arrow represents a fiber, and the arrow represents a propagation direction of the light within the fiber. The first optic coupler 410 can have a splitting ratio of 10:90. That is, the output light of the first output port 410c contains 10% power of the input light from the first input port 410a and 90% power of the input light from the second input port 410b, while the output light of the second output port 410d contains 90% power of the input light from the first input port 410a and 10% power of the input light from the second input port 410b. The output light from the first output port 410c of the first optic coupler 410 can be delivered to a first input port 420a of the second optic coupler 420. The output light from the second output port 410d of the first optic coupler 410 can be delivered to a first input port 430a of the third optic coupler 430.

[0079] In some embodiments, the fibers arranged between two devices can be connected by a polarization-maintaining (PM) splice 510. For example, a PM splice 510 can be used to connect the fibers arranged between the first output port 410c of the first optic coupler 410 and the first input port 420a of the second optic coupler 420, and another PM splice 510 can be used to connect the fibers arranged between the second output port 410d of the first optic coupler 410 and the first input port 430a of the third optic coupler 430. It is noted that, a solid oval shape is used to illustrate a PM splices 510 in FIGs. 3-12. The skilled person knows that, a PM splice 510 can be used to connect fibers to build an optical waveguide between two optical device. Thus, the PM splices 510 are not described in the following in connection with FIGs. 3-12.

[0080] In some embodiments, the second optic coupler 420 can be a 3dB coupler, and have a splitting ratio of 50:50. Stated in other words, each of the output light of the first output port 420c and the second output port 420d contains 50% power of the input light from the first input port 420a and 50% power of the input light from the second input port 420b. The output light from the first output port 420c of the second optic coupler 420 can be delivered to an input port 450a of a fifth optic coupler 450 via a variable optical attenuator (VOA) 520. The VOA 520 can be used as a shutter but is more reliable than the currently used laser beam switch (LBS) wheel. The output light from the second output port 420d of the second optic coupler 420 can be delivered to a second input port 430b of the third optic coupler 430.

[0081] In some embodiments, the fifth optic coupler 450 can have a splitting ratio of 99: 1.

The output light of the first output port 450c contains 99% power of the input light from the first input port 450a. The first output port 450c of the fifth optic coupler 450 is the main output of the green laser module Gl, and can be delivered to a fiber channel protocol (FCP, not shown in FIG. 3). The output light from the second output port 450d of the fifth optic coupler 450 contains 1% power of the input light from the first input port 450a, and can be used for a testing purpose, and can be delivered to a diagnostic output (not shown in FIG. 3).

[0082] In some embodiments, the third optic coupler 430 can be a 3dB coupler, and have a splitting ratio of 50:50. Stated in other words, each of the output light of the first output port 430c and the second output port 430d contains 50% power of the input light from the first input port 430a and 50% power of the input light from the second input port 430b. As shown in FIG. 3, the output light from the first output port 430c of the third optic coupler 430 can be delivered to a first input port 440a of the fourth optic coupler 440 via a first fiber having a first length Li . The output light from the second output port 430d of the third optic coupler 430 can be delivered to a second input port 440b of the fourth optic coupler 440 via a second fiber having a second length L2. [0083] In some embodiments, the fourth optic coupler 440 can be a 3dB coupler, and have a splitting ratio of 50:50. Stated in other words, each of the output light of the first output port 440c and the second output port 440d contains 50% power of the input light from the first input port 440a and 50% power of the input light from the second input port 440b. As shown in FIG. 3, the output light from the first output port 440c of the fourth optic coupler 440 can be delivered to the first input port 420a of the second optic coupler 420 to form a loop. The output light from the second output port 440d of the fourth optic coupler 440 can be delivered to the second input port 410b of the first optic coupler 410 to form a loop via a third fiber having a third length L3.

[0084] Each of the three fiber lengths Li, L2 and L3 is larger than the coherence length (L c ) of the green laser. The values of the three fiber lengths Li, L2 and L3 can be determined following certain rules. In some embodiments, an absolute value of the difference between the first fiber length Li and the second fiber length L2 is larger than the coherence length (L c ) of the green laser. In addition, an absolute value of the difference between the third fiber length L3 and a summation of the first fiber length Li and the second fiber length L2 is larger than the coherence length (L c ) of the green laser. Further, a summation of arbitrary integer multiples of any two of the three fiber lengths cannot be represented as an integer multiple of the other one of the three fiber lengths. That is, AiL x + A2L y ¹ A3L Z , wherein Ai, A2 and A3 are arbitrary integer numbers larger than or equal to 1, and the set {x, y, z} = { 1, 2, 3}. By specifically choosing the values of the three fiber lengths Li, L2 and L3 in such as a way, integer numbers of loopings of signals from these fiber decorrelators cannot be in phase relationship with each other either. That is, multiple photon packets generated by multiple optical propagation loopings are tuned out of phase so as to eliminate the WICO. [0085] Various combinations of the three fiber lengths Li, L2 and L3 can satisfy the rules described above for determining the values of the three fiber lengths Li, L2 and L3. For example, the first fiber length Li can be equal to 1.17m, the second fiber length L2 can be equal to 2.63m, and the third fiber length L3 can be equal to 4.47m. As another example, the first fiber length Li can be equal to 1.31m, the second fiber length L2 can be equal to 2.57m, and the third fiber length L3 can be equal to 4.49m. As yet another example, the first fiber length Li can be equal to 1.67m, the second fiber length L2 can be equal to 2.77m, and the third fiber length L3 can be equal to 4.57m. As still another example, the first fiber length Li can be equal to 1.79m, the second fiber length L2 can be equal to 3.73m, and the third fiber length L3 can be equal to 5.93m. It is noted that, the three fiber lengths Li, L2 and L3 are not limited by the above disclosed combinations but can have any other suitable values that satisfy the disclose rules.

[0086] Referring to FIGs. 4-5, schematic diagrams of other exemplary green laser module

G11 and G12 are shown in accordance with some other embodiments of the present disclosure. It is noted that, the same components that have been described above in connection with FIG. 3, such as green light laser source 310, optic couplers 410-450, PM splices 510, etc., are not repeated herein.

[0087] Comparing the green laser module Gil as shown in FIG. 4 with the green laser module G1 as shown in FIG. 3, acoustic-optic modulators (AOMs) are used in the HR filter to systematically upshift or downshift optical carrier frequency by 200MHz for every pass through. As shown in FIG. 4, a first AOM 530-1 can be connected between the second output port 410d of the first coupler 410 and the first input port 430a of the third optic coupler 430. The first AOM 530-1 can be driven by a first radio-frequency (RF) driver 540-1. A second AOM 530-2 can be connected between the second output port 420d of the second coupler 410 and the second input port 430b of the third optic coupler 430. The second AOM 530-2 can be driven by a second radio frequency (RF) driver 540-2. Both AOMs can also blue-shift the carrier frequency thus widening the spectral output of the green laser, which results in reducing WICO effect alignment position uncertainty.

[0088] Referring to FIG. 9, schematic diagrams showing spectral broadening of output green laser after one or more loopings, according to some embodiments of the present disclosure. As shown in the left figure, the initial green laser at 532nm wavelength has its power centered at a fixed l/f, wherein l is the nominal center wavelength of laser 310 (e.g., 532nm), and f is the RF frequency applied to the two AOMs 530-1 and 530-2. In some embodiments, the two AOMs can positively and negatively shift optical carrier frequency by 200MHz after one loop, as shown in the middle figure of FIG. 9. That is, the output of the up-converted AOM can be at a frequency corresponding to (532nm + 200MHz), and the output of the down-converted AOM can be at a frequency corresponding to (532nm -200MHz). Thus, the power can be apart at positive and negative 200MHz in spectral. After a large number of loops, the two AOMs can iteratively shift green laser frequency by 200MHz positively and negatively. That is, the spectral components can be broadly distributed at multiple values of wavelength/frequency at 200MHz apart. Thus, the output green laser at any time can have a large number of copies of the green light emitted by green light laser source 310 and multiple positive or negative 200MHz frequency shifted spectral components. As such, the coherent phase relationship existing in photon packets can be broken at any time, so as to reduce WICO effect.

[0089] It is noted that, the AOMs can be arranged at different positions of the optical circuit of the HR filter. For example, in the green laser module G12 as shown in FIG. 5, the first AOM 530-1 driven by the first radio-frequency (RF) driver 540-1 can be connected between the first output port 410d of the third coupler 430 and the first input port 440a of the fourth optic coupler 440, while the second AOM 530-2 driven by the second radio-frequency (RF) driver 540-2 can be connected between the second output port 430d of the third coupler 430 and the second input port 440b of the fourth optic coupler 430.

[0090] In some embodiments, a fiber phase modulator 550 can be connect after the first output port 450c of the fifth optic coupler 450. The fiber phase modulator 550 can be driven by a randomized phase sinusoid signal to further scramble the phase relationship among different spectral components of the output of the green laser, thereby further reducing WICO effect alignment position uncertainty.

[0091] Referring to FIGs. 6-8, schematic diagrams of other exemplary green laser module

G2, G3 and G4 are shown in accordance with some other embodiments of the present disclosure. It is noted that, the same components that have been described above in connection with FIG. 3, such as green light laser source 310, optic couplers 410-450, PM splices 510, etc., are not repeated herein. In some embodiments, the locations of the three fibers with different lengths Li, L2 and L3 can be arranged at different locations of the optical circuit of the HR filter.

[0092] In some embodiments, comparing the green laser module G2 as shown in FIG. 6 with the green laser module G1 as shown in FIG. 3, the first fiber with the first fiber length Li can be arranged between the second output port 410d of the first optic coupler 410 and the first input port 430a of the third optic coupler 430, while the second fiber with the second fiber length L2 can be arranged between the second output port 420d of the second optic coupler 420 and the second input port 430b of the third optic coupler 430. It is noted that, although not shown in the figures, two AOMs can be added at different locations in the HR filter of green laser module G2, as described above in connection with FIGs. 4 and 5.

[0093] In some other embodiments, comparing the green laser module G3 as shown in

FIG. 7 with the green laser module G1 as shown in FIG. 3, the third fiber with the third fiber length L3 can be arranged between the first output port 440c of the fourth optic coupler 440 and the first input port 420a of the second optic coupler 430. Similarly, although not shown in the figures, two AOMs can be added at different locations in the HR filter of green laser module G3, as described above in connection with FIGs. 4 and 5.

[0094] In some other embodiments, comparing the green laser module G4 as shown in

FIG. 8 with the green laser module G1 as shown in FIG. 3, the first fiber with the first fiber length Li can be arranged between the second output port 410d of the first optic coupler 410 and the first input port 430a of the third optic coupler 430, the second fiber with the second fiber length L2 can be arranged between the second output port 420d of the second optic coupler 420 and the second input port 430b of the third optic coupler 430, and the third fiber with the third fiber length L3 can be arranged between the first output port 440c of the fourth optic coupler 440 and the first input port 420a of the second optic coupler 430. Similarly, although not shown in the figures, two AOMs can be added at different locations in the HR filter of green laser module G4, as described above in connection with FIGs. 4 and 5.

[0095] It is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of embodiment of the disclosure can be made without departing from the spirit and scope of the disclosure. Features of the disclosed embodiments can be combined and rearranged in various ways. Without departing from the spirit and scope of the disclosure, modifications, equivalents, or improvements to the disclosure are understandable to those skilled in the art and are intended to be encompassed within the scope of the present disclosure. For example, the splitting ratios of the five optic couplers 410-450 described above are merely exemplary, and should not be limited. For example, any suitable optic couplers with other splitting ratios, such as 95:5, 90:10, 80:20, 75:25, 60:40, 40:60, 25:75, 20:80, 10:90, 5:95, etc., can be used in the HR filter of the disclosed green laser module. As another example, the number of the optic couplers is also not limited herein. In some embodiments not shown in the figures, an optic couplers array with a number M rows and a number N columns of optic couplers can be connected to constmct the HR filter.

[0096] Accordingly, the present disclosure provides an improved green laser module to achieve better working life, reduced coherence effects, and faster switching speed. By decorrelating phase of the green laser using the disclosed HR filter, and by adding spectral components using AOMS, the disclosed green laser module can avoid having same phase relationship in the photon packets under the spectral envelope when light is emitted from the output of the green laser module. The different green components can occupy same spectral range but all the photons at any time are configured to be out of phase (e.g., they do not have have same phase relationship) among the wave packets, since the phase relationship among different photons is broken when combining light after travelling coherence length (L c ) or for duration longer than coherence time (T c ). Thus, the generated green light undergos a large number of reflections within integrating sphere and light combined after such reflections can lose coherency.

[0097] Additionally, mode hopping can be less of lower magnitude than inherent laser noise, which can make SbO drift due to mode jumps in green laser less of an issue. Further, variable optical attenuator (VO A) is implemented in the disclosed green laser module as an optical switch, which can dramatically reduce laser beam switch (LBS) related malfunctions and failures. It is noted that, the present disclosure is good not only for lithographic systems but for any laser in any application (e.g., biomedical, sensor, telecom, etc.) to break the coherence of the laser source. It is also noted that, in the present disclosure uses green laser as an example. However, the present disclosure can work for any laser source with different wavelength (e.g., red laser, any other visible light laser, UV laser, or infrared laser, etc.) by choosing appropriate fiber lengths and other components in the disclosed scheme.

[0098] The embodiments may further be described using the following clauses:

1. A laser module, comprising: a laser source configured to generate laser light; and an infinite impulse response filter configured to decorrelate phase of components of the laser light to thereby reduce coherence effect of the laser light.

2. The laser module of clause 1, wherein the laser source is configured to generate green laser light.

3. The laser module of clause 1, wherein the infinite impulse response filter comprises a plurality of optic couplers to form a plurality of optical propagation loops with different optical path lengths.

4. The laser module of clause 3, wherein the plurality of optical propagation loops comprises: a first optical propagation loop having a first fiber with a first fiber length; a second optical propagation loop having a second fiber with a second fiber length; and a third optical propagation loop having a third fiber with a third fiber length; wherein the first fiber length, the second fiber length, and the third fiber length are larger than a coherence length of the laser light.

5. The laser module of clause 4, wherein: an absolute value of a difference between the first fiber length and the second fiber length is larger than the coherence length of the laser light.

6. The laser module of clause 4, wherein: an absolute value of a difference between the third fiber length and a summation of the first fiber length and the second fiber length is larger than the coherence length of the laser light.

7. The laser module of clause 4, wherein: a summation of arbitrary integer multiples of any two of the three fiber lengths is not an integer multiple of the other one of the three fiber lengths.

8. The laser module of clause 4, wherein a combination of the first fiber length, the second fiber length, and the third fiber length is one of the following: the first fiber length is 1.17m, the second fiber length is 2.63m, and the third fiber length is 4.47m; the first fiber length is 1.31m, the second fiber length is 2.57m, and the third fiber length is 4.49m; the first fiber length is 1.67m, the second fiber length is 2.77m, and the third fiber length is 4.57m; or the first fiber length is 1.79m, the second fiber length is 3.73m, and the third fiber length is 5.93m.

9. The laser module of clause 3, further comprising an acoustic-optic modulator arranged in an optical propagation loop, and configured to shift an optical carrier frequency, such that an output of the laser module has a widen spectral to further reduce coherence effect.

10. The laser module of clause 9, further comprising a fiber phase modulator driven by a randomized phase signal to scramble a phase relationship among different spectral components of the output of the laser module.

11. The laser module of clause 1, further comprising a variable optical attenuator configured as an optical switch.

12. The laser module of clause 4, wherein the plurality of optic couplers comprise: a first optic coupler including a first input port connected to the laser source; a second optic coupler including a first input port connected to a first output port of the first optic coupler; a third optic coupler including a first input port connected to a second output port of the first optic coupler, and a second input port connected to a second output port of the second optic coupler; and a fourth optic coupler including a first input port connected to a first output port of the third optic coupler, a second input port connected to a second output port of the third optic coupler, a first output port connected to a second input port of the first optic coupler, and a second output port connected to a second input port of the second optic coupler.

13. The laser module of clause 12, wherein: the first fiber is arranged between the first output port of the third optic coupler and the first input port of the fourth optic coupler; and the second fiber is arranged between the second output port of the third optic coupler and the second input port of the fourth optic coupler.

14. The laser module of clause 12, wherein: the first fiber is arranged between the second output port of the first optic coupler and the first input port of the third optic coupler; and the second fiber is arranged between the second output port of the second optic coupler and the second input port of the third optic coupler.

15. The laser module of clause 12, wherein: the third fiber is arranged between the second output port of the fourth optic coupler and the second input port of the first optic coupler; or the third fiber is arranged between the first output port of the fourth optic coupler and the second input port of the second optic coupler.

16. The laser module of clause 12, wherein: the first optic coupler has a splitting ratio of 10:90; and the second optic coupler, the third optic coupler, the fourth optic each has a splitting ratio of 50:50.

17. The laser module of clause 12, further comprising: a first acoustic-optic modulator arranged between the second output port of the first optic coupler and the first input port of the third optic coupler; and a second acoustic-optic modulator arranged between the second output port of the second optic coupler and the second input port of the third optic coupler.

18. The laser module of clause 12, further comprising: a first acoustic-optic modulator arranged between the first output port of the third optic coupler and the first input port of the fourth optic coupler; and a second acoustic-optic modulator arranged between the second output port of the third optic coupler and the second input port of the fourth optic coupler.

19. A metrology system, comprising: a multi-color radiation source including the laser module of clause 1, and configured to generate alignment light.

20. A lithographic apparatus, comprising the metrology system of clause 19.

[0099] Final Remarks

[00100] 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.

[00101] 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.

[00102] 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. [00103] 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. [00104] 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.

[00105] 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.

[00106] 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. [00107] 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.

[00108] 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.

[00109] 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.

[00110] 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.