Patent Grant
9419407
1. Field of the Disclosure
The present application relates to lasers suitable for generating radiation at deep UV (DUV) and vacuum UV (VUV) wavelengths, and to methods for generating laser light at DUV and VUV wavelengths. In particular it relates to systems and methods for reducing and controlling the spectral bandwidth of DUV and VUV lasers. The lasers are particularly suitable for use in inspection systems including those used to inspect photomasks, reticles, and semiconductor wafers.
2. Related Art
The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles whose sizes may be about 100 nm or smaller. Furthermore these inspection tools must operate at high speed in order to inspect a large fraction, or even 100%, of the area of photomask, reticle or wafer, in a short period of time, e.g. one hour or less.
Generally short wavelengths such as DUV and VUV wavelengths have higher sensitivity for detecting small defects compared with longer wavelengths. Inspection of a photomask or reticle is preferably done using the same wavelength as used the lithography used when printing from the photomask or reticle. Currently a wavelength of substantially 193.4 nm is used for the most critical lithography steps and a wavelength of substantially 248 nm for less critical lithography steps. Where a wavelength value is mentioned herein without qualification, it should be assumed that value refers to the vacuum wavelength of the light or radiation.
High-speed inspection requires high power lasers in order to illuminate the samples being inspected with high intensity in order to detect the small amount of light scattered from small particles or defects or allow detection of small changes in reflectivity due to defects in the pattern. The required laser power levels may range from approximately 100 mW for the inspection of photomasks and reticles up to more than 10 W for the detection of small particles and imperfections on a bare silicon wafer.
Typically inspection in the semiconductor industry requires lasers with very narrow bandwidth. Such inspection systems usually use an objective lens with a large field of view (typically from a few hundred microns to a few mm in dimensions) in order to allow imaging of a large area to achieve high inspection speeds. An objective lens with low distortions and a large field of view is expensive and complex. Requiring that objective lens to operate over a large bandwidth (such as more than a few tens of μm) significantly increases the cost and complexity. DUV lasers with bandwidths of approximately 20 μm or less are very desirable for inspection applications in the semiconductor industry.
DUV lasers are known in the art. U.S. Pat. No. 5,144,630 entitled “Multiwave Solid State Laser Using Frequency Conversion Techniques” that issued on Sep. 1, 1992 to Lin and U.S. Pat. No. 5,742,626, entitled “Ultraviolet Solid State Laser Method Of Using Same And Laser Surgery Apparatus”, issued on Apr. 21, 1998 to Mead et al. describe exemplary DUV lasers. Fourth and fifth harmonics are generated from a pulsed fundamental infra-red laser operating at a wavelength near 1064 nm, thereby resulting in wavelengths of approximately 266 nm and 213 nm. Lin and Mead also teach generating an infra-red wavelength longer than 1064 nm from the fundamental laser using an optical parametric oscillator (OPO).
The output bandwidth of a laser oscillator is determined by its intra-cavity dynamics. In prior-art pulsed lasers, to further reduce laser bandwidth, various bandwidth limiting devices, such as an etalon, a birefringent filter, or an optical grating, have been incorporated into a laser cavity. Because all of these approaches are invasive, they inevitably introduced detrimental effects to the lasers. These detrimental effects include extra power losses and greater complexity, which often led to lower laser efficiency, poor thermal stability, tighter misalignment sensitivity, and longer laser system warm-up time. Furthermore, because intra-cavity beam size is often small and predetermined by the laser cavity design, and intra-cavity laser power density is normally much higher than laser output power, these intra-cavity components are much more susceptible to damage.
In prior-art pulsed DUV lasers, the bandwidth of the DUV output depends directly on the bandwidth of the fundamental infra-red laser. That is, the broader the bandwidth of the fundamental laser, the broader the DUV broader the DUV output bandwidth. Reducing the bandwidth of a laser requires redesigning the laser oscillator cavity. Since the cavity may control many properties of the laser including bandwidth, repetition rate, as well as average and peak powers, redesigning the cavity to reduce the bandwidth while maintaining the other laser parameters may be a complex and time consuming task. Furthermore it may not be possible to achieve a specific DUV laser bandwidth specification using a readily available infra-red fundamental laser.
Reducing bandwidth by frequency doubling by combining two femtosecond pulses with opposite chirp is known in the art (see Raoult et al., Opt. Lett. 23, 1117-1119 (1998)). A femtosecond pulse was first chirped and stretched to about ins using a grating-pair stretcher and then, after amplification, split into two pulses. The two pulses were incompletely compressed into tens of picosecond pulses with opposite chirp by using two grating-pair dispersers. Sum frequency generation of these two pulses resulted in a much narrower bandwidth. However, this approach relies on grating-based stretchers and compressors, which are bulky and lack the mechanical stability needed for demanding commercial industrial applications. Furthermore femtosecond pulses are generally unsuited to use in semiconductor inspection applications as the wide bandwidth (multiple nm) greatly complicates the design of the system optics, and the high peak power can easily damage the article being inspected.
Therefore, a need arises for DUV laser overcoming some, or all, of the above disadvantages. In particular a need arises for a means of reducing or controlling the bandwidth of a DUV laser, including DUV lasers with pulse lengths of between a few picoseconds and a few hundred picoseconds.
The present invention is generally directed to bandwidth narrowing apparatus and methods that facilitate reducing and/or controlling the bandwidth of output laser light by way of dividing fundamental laser light pulses into two sub-pulses, stretching and adding opposite chirps to the two sub-pulses using one or more monolithic optical devices (e.g., one or more chirped volume Bragg gratings or chirped fiber Bragg gratings), and then recombining (mixing) the stretched/chirped sub-pulses to produce sum frequency (output) light made up of pulses having frequencies that are equal to two times the fundamental frequency (i.e., where the pulses have wavelengths that are equal to one-half of the fundamental laser light pulses' fundamental wavelength). According to an aspect of the invention, mixing of the two sub-pulses is performed using a sum frequency module configured such that the opposite (positive and negative) chirps of the two stretched/chirped sub-pulses are canceled during the sum frequency mixing process, thereby producing sum frequency output light having a much narrower bandwidth than is produced by direct second harmonic generation. An advantage of generating sum frequency output light in this manner is that this approach wastes very little of the fundamental laser power, as compared with methods that use a filter or etalon simply to reject unwanted wavelengths.
According to exemplary embodiments of the present invention, a laser assembly includes a fundamental laser and a bandwidth narrowing apparatus generally made up of a pulse dividing element, a single monolithic device (e.g., a monolithic chirped volume Bragg grating (monolithic CBG)), a frequency mixing module, and additional optical elements (e.g., mirrors, polarizing beam splitters, quarter-wave plates (QWPs) and fold mirrors) operably arranged to provide sub-pulse light paths between the pulse dividing element and the monolithic device and between the monolithic device and the frequency mixing module. The fundamental laser (e.g., a Nd:YAG or a Nd-doped vanadate laser including, in one embodiment, a second-harmonic conversion module) generates fundamental light made up of pulses having a frequency disposed within a fundamental frequency bandwidth. The bandwidth narrowing apparatus is disposed downstream of the fundamental laser (i.e., outside of the laser cavity) in order to avoid the detrimental effects of intra-cavity bandwidth-controlling devices, and also to facilitate maintaining other laser parameters (i.e., other than the bandwidth) without having to redesign the laser oscillator cavity. Specifically, the pulse dividing element (e.g., a partial reflector or beam splitter) is disposed to receive the fundamental laser light, and is configured to divide each fundamental laser light pulse into a pair of corresponding (first and second) sub-pulses having approximately equal energies. In a presently preferred embodiment, the two sub-pulses are respectively directed along separate light paths onto opposite surfaces of the monolithic device, whereby two oppositely chirped stretched sub-pulses are generated that are mirror images of each other (i.e., such that the two stretched sub-pulses have changes in frequency with time that are approximately equal in magnitude but opposite in sign). The use of a single monolithic device in this manner provides superior optical and mechanical stability, and occupies only a fraction of the space compared with grating-based stretcher and compressor approaches, and guarantees that the two stretched sub-pulses are chirped with substantially mirror-image pulse frequency patterns. Furthermore, a properly designed CBG has much higher dispersion than grating-based stretcher and compressor, can be used to stretch narrow-bandwidth picosecond pulses, which remains very challenging to do with gratings due to the lack of angular dispersion from gratings. The two stretched, oppositely chirped sub-pulses are then directed along separate light paths to the frequency mixing module. The two sub-pulse light paths between the pulse dividing element and the frequency mixing module are preferably arrange matched to within about 10% of the pulse length, and the optical elements (e.g., mirrors) forming the light paths can be easily repositioned to change the relative time delay between the two stretched sub-pulses arriving at the frequency mixing module, thereby facilitating fine-tuning of the center wavelength of the sum-frequency output pulses, which is beneficial for some applications requiring a precise specific wavelength, such as 193 nm light for photo-mask inspection. The sum frequency module (e.g., a BBO, LBO or CLBO crystal that is configured for either Type I or Type II frequency mixing, or a periodically poled non-linear crystal such as lithium niobate or stoichiometric lithium tantalate, SLT) is configured to mix the corresponding positively-chirped and negatively-chirped stretched sub-pulses such that the resulting sum frequency pulses have center frequencies equal to two times the fundamental frequency (e.g., such that the sum frequency pulses have center wavelengths equal to one of approximately 532 nm and approximately 266 nm).
According to alternative specific embodiments, the bandwidth narrowing apparatuses of various laser assemblies utilize different optical element arrangements to achieve different advantages. For example, in one approach, the optical elements forming the two sub-pulse light paths are configured such that the two stretched sub-pulses have substantially orthogonal polarizations and enter the frequency mixing module along collinear paths, and the frequency mixing module is configure to mix the two stretched sub-pulses using Type II frequency mixing techniques. This orthogonal-polarization-collinear-path approach simplifies the optical arrangement and causes the stretched sub-pulses to overlap while passing through the frequency mixing module, which results in efficient generation of sum frequency light as a second harmonic of the fundamental laser light. In an alternative approach, the optical elements forming the two sub-pulse light paths are configured such that the two stretched sub-pulses have substantially parallel polarizations and enter the frequency mixing module at an acute relative angle (e.g., less than about 4°). This parallel-polarization-non-collinear-path approach provides the advantage of facilitating Type I mixing, which is more efficient than Type II mixing, and thus facilitates either the use of shorter length crystals or the generation of more output power (i.e., for a given input power and crystal length). The parallel-polarization-non-collinear-path approach may perform frequency mixing in a periodically polled crystal, such as periodically poled lithium niobate (PPLN) or periodically poled SLT (PPSLT). Periodically poled crystals may have higher non-linear coefficients than materials such as LBO, BBO and CLBO, and may be used in longer crystals, allowing more efficient conversion of the first harmonic into the second harmonic.
According to additional alternative specific embodiments, laser assemblies of the present invention utilize the CGB-based bandwidth narrowing apparatuses mentioned above in combination with at least one of a harmonic conversion module, an optical bandwidth filtering device and an additional frequency mixing module to achieve laser output light exhibiting the reduced and/or controlled bandwidth described above, where the additional structures facilitate generating the laser output light at a higher (i.e., above 2×) harmonic of a fundamental light frequency (i.e., above a second harmonic of the fundamental frequency). In one exemplary embodiment, a DUV laser assembly utilizes one of the CGB-based bandwidth narrowing apparatuses described above to generate sum frequency light at the second harmonic of the fundamental light, and then passes the sum frequency light through a harmonic conversion module to generate laser output light at a higher (e.g., fourth) harmonic of the fundamental light. In another exemplary embodiment, a DUV laser assembly directs fundamental light into an optical bandwidth filtering device (e.g., an etalon) that reflects a first (rejected) portion of each fundamental light pulse having frequencies that are outside a narrow bandwidth, and passes a second portion of each fundamental light pulse including frequencies within the narrow bandwidth. The rejected first portion of each fundamental light pulse, which in conventional systems would be discarded and thus wasted, is passed through one of the CGB-based bandwidth narrowing apparatuses described above, whereby the previously unusable out-of-band frequencies of the first portion are converted into usable sum frequency light having frequencies within the narrow bandwidth. The sum frequency light is then passed through an optional harmonic conversion module, and then either the sum frequency light or the (first) harmonic light exiting the optional harmonic conversion module is passed to a second frequency mixing module. The second frequency mixing module is configured to mix the sum frequency light (or the optional first harmonic) with the second fundamental light portion (i.e., the narrow bandwidth portion passed by the optical bandwidth filtering device) or a harmonic thereof to produce laser output light having a higher harmonic and higher energy than could be produced without using the CGB-based bandwidth narrowing apparatus.
An exemplary inspection system is described. This inspection system includes an illumination source, optics, and a detector. The illumination source includes a DUV laser assembly that utilizes the CGB-based bandwidth narrowing apparatus mentioned above (i.e., a pulse dividing element, one or more monolithic devices, a frequency mixing module and associated optical elements) to generate DUV radiation of a desired wavelength and bandwidth. The optics are configured to direct and focus the DUV radiation from the illumination source onto a sample. The sample is supported by a stage, which moves relative to the optics during the inspection. The detector is configured to receive reflected or scattered light from the sample, wherein the optics are further configured to collect, direct, and focus the reflected or scattered light onto the detector. The detector includes one or more image sensors. At least one image sensor may be a time-delay integration (TDI) sensor.
The exemplary inspection system may include one or more illumination sources that illuminate the sample from different angles of incidence and/or different azimuth angles and/or with different wavelengths and/or polarization states, wherein one or more of the illumination sources incorporate the novel bandwidth control approach described above. The exemplary inspection system may include one or more collection paths that collect light reflected or scattered by the sample in different directions and/or are sensitive to different wavelengths and/or to different polarization states. The exemplary inspection system may include a TDI sensor with readout circuits on two sides that are used to read out two different signals simultaneously. The exemplary inspection system may include an electron-bombarded image sensor.
An exemplary method of inspecting a sample is described. The exemplary method includes directing and focusing radiation from a DUV laser illumination source onto the sample, wherein the DUV laser illumination source is configured to implement bandwidth control in the manner described above. The sample is supported by a stage, which moves relative to the optics during the inspection. The method further includes using optics to collect, direct, and focus light reflected or scattered by the sample onto a detector. The detector includes one or more image sensors. At least one image sensor is a time-delay integration (TDI) sensor. The method further includes controlling the bandwidth of the DUV laser with a chirped volume Bragg grating (CBG).
The present invention relates to an improvement in sensors for semiconductor inspection systems. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “underneath”, “upward”, “downward”, “vertical” and “horizontal” are intended to provide relative positions or orientations for purposes of description, and are not intended to designate an absolute frame of reference. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.
An illumination source 102 may comprise one or more lasers and/or a broad-band light source. Illumination source 102 may emit DUV radiation and/or VUV radiation (collectively referred to herein as “UV radiation”). Illumination source 102 includes at least one fundamental laser configured to generate the UV radiation, wherein the fundamental laser incorporates the bandwidth control described herein, and is positioned to direct the UV radiation as a beam that passes through an optical system (optics) 103 to sample 108. Optics 103 including an objective lens 105 configured to direct UV radiation towards, and focuses it on, sample 108. Optics 103 may also comprise mirrors, lenses, and/or beam splitters. A portion of the UV radiation (referred to below as “light”) that is redirected (i.e., reflected or scattered) from sample 108 is collected, directed, and focused by optics 103 onto a detector 106, which is disposed within a detector assembly 104.
Detector assembly 104 includes a detector 106. Detector 106 may include a two-dimensional array sensor or a one-dimensional line sensor. In one embodiment, the output of detector 106 is provided to a computing system 114, which analyzes the output. Computing system 114 is configured by program instructions 118, which can be stored on a carrier medium 116.
One embodiment of inspection system 100 illuminates a line on sample 108, and collects scattered and/or reflected light in one or more dark-field and/or bright-field collection channels. In this embodiment, the detector 106 may include a line sensor or an electron-bombarded line sensor.
Another embodiment of inspection system 100 illuminates multiple spots on sample 108, and collects scattered and/or reflected light in one or more dark-field and/or bright-field collection channels. In this embodiment, the detector 106 may include a two-dimensional array sensor or an electron-bombarded two-dimensional array sensor.
Additional details of various embodiments of inspection system 100 can be found in U.S. Published Patent Application 2013/0016346, entitled “WAFER INSPECTION”, by Romanovsky et al. which published on Jan. 17, 2013, U.S. Published Patent Application 2009/0180176, by Armstrong et al., which published on Jul. 16, 2009, U.S. Published Patent Application 2007/0002465 by Chuang et al., which published on Jan. 4, 2007, U.S. Pat. No. 5,999,310, by Shafer et al., which issued on Dec. 7, 1999, and U.S. Pat. No. 7,525,649 by Leong et al., which issued on Apr. 28, 2009. All of these patents and patent applications are incorporated by reference herein.
More details of inspection systems in accordance with the embodiments illustrated in
In the oblique illumination channel 312, the second polarized component is reflected by beam splitter 305 to a mirror 313 which reflects such beam through a half-wave plate 314 and focused by optics 315 to sample 309. Radiation originating from the oblique illumination beam in the oblique channel 312 and scattered by sample 309 is collected by paraboloidal mirror 310 and focused to sensor 311. The sensor and the illuminated area (from the normal and oblique illumination channels on surface 309) are preferably at the foci of the paraboloidal mirror 310.
The paraboloidal mirror 310 collimates the scattered radiation from sample 309 into a collimated beam 316. Collimated beam 316 is then focused by an objective 317 and through an analyzer 318 to the sensor 311. Note that curved mirrored surfaces having shapes other than paraboloidal shapes may also be used. An instrument 320 can provide relative motion between the beams and sample 309 so that spots are scanned across the surface of sample 309. U.S. Pat. No. 6,201,601, which issued on Mar. 13, 2001 and is incorporated by reference herein, describes inspection system 300 in further detail.
In a dark-field mode, adaptation optics 402 control the laser illumination beam size and profile on the surface being inspected. Mechanical housing 404 includes an aperture and window 403, and a prism 405 to redirect the laser along the optical axis at normal incidence to the surface of a sample 408. Prism 405 also directs the specular reflection from surface features of sample 408 out of objective 406. Objective 406 collects light scattered by sample 408 and focuses it on sensor 409. Lenses for objective 406 can be provided in the general form of a catadioptric objective 412, a focusing lens group 413, and a tube lens section 414, which may, optionally, include a zoom capability. Laser system 401 incorporates bandwidth control as described herein.
In a bright-field mode, broad-band illumination module 420 directs broad-band light to beam splitter 410, which reflects that light towards focusing lens group 413 and catadioptric objective 412. Catadioptric objective 412 illuminates the sample 408 with the broadband light. Light that is reflected or scattered from the sample is collected by objective 406 and focused on sensor 409. Broad-band illumination module 420 comprises, for example, a laser-pumped plasma light source or an arc lamp. Broad-band illumination module 420 may also include an auto-focus system to provide a signal to control the height of sample 408 relative to catadioptric objective 412.
Published US Patent Application 2007/0002465, which published on Jan. 4, 2007 and is incorporated by reference herein, describes system 400 in further detail.
The inspected object 530 may be a reticle, a photomask, a semiconductor wafer or other article to be inspected. Image relay optics 540 can direct the light that is reflected and/or transmitted by inspected object 530 to a channel one image mode relay 555 and to a channel two image mode relay 560. Channel one image mode relay 555 is tuned to detect the reflection or transmission corresponding to channel one illumination relay 515, whereas channel two image mode relay sensor 560 is tuned to detect the reflection or transmission corresponding to channel two illumination relay 520. Channel one image mode relay 555 and channel two image mode relay sensor 560 in turn direct their outputs to sensor 570. The data corresponding to the detected signals or images for the two channels is shown as data 590 and is transmitted to a computer (not shown) for processing.
Other details of reticle and photomask inspection systems and methods that may be configured to measure transmitted and reflected light from a reticle or photomask are described in U.S. Pat. No. 7,352,457, which issued to Kvamme et al. on Apr. 1, 2008, and in U.S. Pat. No. 5,563,702, which issued to Emery et al. on Oct. 8, 1996, both of which are incorporated by reference herein.
Additional details regarding exemplary embodiments of image sensor 570 are provided in U.S. Published Patent Application 2014/0158864, entitled “METHOD AND APPARATUS FOR HIGH SPEED ACQUISITION OF MOVING IMAGES USING PULSED ILLUMINATION”, filed by Brown et al. which published on Jun. 12, 2014, and in U.S. Pat. No. 7,528,943 entitled “METHOD AND APPARATUS FOR SIMULTANEOUS HIGH-SPEED ACQUISITION OF MULTIPLE IMAGES” by Brown et al., issued May 5, 2009. These patents and patent applications are incorporated by reference herein
Referring to the left side of
Pulse dividing element 602 is positioned to receive fundamental laser light L, and is configured to divide (split) each laser light pulse LP into two associated sub-pulses referred to below as first sub-pulse LSP1 and second sub-pulse LSP2. In the embodiment illustrated in
Sub-pulses LSP1 and LSP2 are directed along two different optical paths to monolithic device 607 by way of corresponding optical elements that are operably disposed between pulse dividing element 602 and monolithic device 607. As depicted by the single-dot-dashed line exiting to the right from pulse dividing element 602, first sub-pulse LSP1 passes from partial reflector 602 through a first polarizing beam splitter (PBS) 603, and is then converted into a first circularly polarized sub-pulse by way of passing through a first quarter-wave plate (QWP) 604 to a first end surface 607-1 of monolithic device 607. In contrast, as depicted by the double-dot-dashed line extending upward from pulse dividing element 602, second sub-pulse LSP2 is reflected by flat fold mirrors 611 and 612 to a second polarizing beam splitter (PBS) 605, and is then redirected horizontally to the left and converted into a second circularly polarized sub-pulse by way of passing through a second quarter-wave plate (QWP) 606 that is disposed adjacent to a second end surface 607-2 of monolithic device 607. The first and second circular-polarized sub-pulses are thus transmitted in opposite directions onto opposing surfaces of monolithic device 607.
According to an aspect of the invention, one or more monolithic devices are configured and positioned such that associated first and second circular-polarized sub-pulses are converted into stretched sub-pulses LSSP1 and LSSP2 respectively having opposing (i.e., positive and negative) chirps. In the single monolithic device embodiment shown in
Stretched sub-pulse LSSP1 and LSSP2 are then directed via corresponding optical paths from monolithic device 607 to mixing module 608A by way of corresponding optical elements. As depicted by the short-dash-long-dash line extending horizontally from first surface 607-1, first stretched sub-pulse LSSP1 passes from monolithic device 607 through QWP 604 to PBS 603, and is redirected downward from PBS 603 to fold mirror 613, from fold mirror 613 to fold mirror 614, and from fold mirror 614 upward to second polarizing beam splitter (PBS) 605, from which first stretched sub-pulse LSSP1 is redirected to the right toward frequency mixing module 608A. In contrast, as depicted by the double-short-dash-long-dash line extending horizontally to the right from second surface 607-1, second stretched sub-pulse LSSP2 passes from monolithic device 607 through QWP 606 to PBS 605, and is passed through PBS 605 to frequency mixing module 608A. Note that, after being reflected and chirped by monolithic device 607, each stretched sub-pulse LSSP1 and LSSP2 is converted back to linearly polarized light with polarization orthogonal to the incoming beam by corresponding QWPs 604 and 605. These two orthogonally polarized pulses are combined at PBS 605, and sent to a frequency mixing module 608A to generate sum frequency light FSF, which has a center frequency of 2νf. Note that the optical path lengths followed by the two sub-pulses from partial reflector 602 to where they recombine at PBS 605 are substantially equal, so that the two sub-pulses arrive at frequency mixing module 608A substantially overlapped. In a preferred embodiment the optical sub-pulse light path lengths traveled by the two sub-pulses are matched to within about 10% of the pulse length. In one embodiment the polarization of the fundamental light L may be oriented at about 45° to partial reflector 602A, which comprises a polarized beam splitter, so that substantially equal fractions of the fundamental light L are transmitted and reflected by partial reflector 602A. In this embodiment, PBS 603 should transmit the same polarization as partial reflector 602 and should efficiently reflect the orthogonal polarization. Similarly, PBS 605 reflects the polarization reflected by partial reflector 602A and directs it to QWP 606, which converts each pulse to circular polarization. After reflection from 607, QWP 606 converts the circular polarization to linear polarization, but rotated 90° with respect its initial polarization, so that it passes through PBS 605. The light reflected from PBS 603 also reflects from PBS 605, so that the two orthogonally polarized pulses travel together substantially collinearly into frequency mixing module 608A. Combining the two orthogonally polarized pulses at PBS 605A is described in additional detail below with reference to
Referring to the right side of
Utilizing bandwidth narrowing apparatus 610A in the manner described above and in the following examples, sum frequency light pulses LSFP are thus generated having a narrower bandwidth than could be produced without using the monolithic-device-based bandwidth narrowing apparatus 610 (i.e., by way of simply doubling the frequency of the fundamental). By way of example,
One advantage of the exemplary embodiment illustrated in
The approach implemented by the embodiment of
In an exemplary embodiment, fundamental laser 601 is a Nd:YAG or Nd-doped vanadate laser with a second-harmonic conversion module, and generates fundamental light L at fundamental wavelength λf of approximately 532 nm with a pulse length (duration) Df between a few picoseconds and a few tens of picoseconds, for example, a FWHM pulse length Df of about 20 ps. A monolithic device 607 comprising a CBG is configured to stretch the 532 nm fundamental pulses to a stretched FWHM pulse length (duration) Ds between about ten picoseconds and about one hundred picoseconds (e.g., a FWHM pulse length Ds of about 80 ps). In this case, frequency mixing module 608A is configured to generate sum frequency (output) light LSF having a wavelength λsf of approximately 266 nm, and having a narrower bandwidth and longer pulse length Dsf than would result from simply doubling the frequency of fundamental light L. In a specific exemplary embodiment, frequency mixing module 608A is implemented using a beta barium borate (BBO) crystal that is critically phase matched for Type II mixing of light at 532 nm at a phase-matching angle of about 82° at a temperature of about 100° C. Other temperatures can be used with an appropriate adjustment in phase matching angle. Other appropriate non-linear optical crystals could be substituted for the BBO crystal with an appropriate temperature and phase-matching angle. A periodically poled non-linear crystal with an appropriate poling period could also be substituted for the BBO crystal.
Referring to the central region of
Referring to the center of
One advantage of the exemplary embodiment illustrated in
Alternatively the exemplary embodiment illustrated in
In yet another exemplary embodiment (not shown), two pulses with substantially parallel polarizations are reflected twice from a chirped volume Bragg grating or a chirped fiber Bragg grating in a manner similar to that illustrated in
In an exemplary embodiment, the configuration of fundamental laser 601 and bandwidth narrowing apparatus 610 is similar to any one of the above described laser assemblies 600A, 600B and 600C. In one embodiment, fundamental laser 601 is a Nd:YAG or Nd-doped vanadate laser, generating fundamental light L at a wavelength of approximately 1064 nm with a pulse width of between a few picoseconds and a few tens of picoseconds, for example of pulse length of about 20 ps.
As in the above-described embodiment, monolithic device 607 functions to stretch the fundamental (e.g., 1064 nm) pulses to a pulse length between a few tens of picoseconds and a few hundred picoseconds, for example to a length of about 80 ps, and frequency mixing module 608 functions to generate sum frequency light LSF at a wavelength of approximately 532 nm. Frequency mixing module 608 can use Type II frequency mixing in lithium triborate (LBO) or cesium lithium borate (CLBO). For example, LBO can be used for Type II frequency mixing of light at a wavelength of about 1064 nm to generate light at a wavelength of about 532 nm using the YZ plane at a temperature of about 50° C. and phase matching angles of approximately θ=23° and φ=90°. Alternatively frequency mixing module 608 can use a periodically poled SLT crystal.
In one embodiment, harmonic conversion module 703 is configured to convert the sum frequency light LSF into laser output light Lout comprising pulses LoutP at the fourth harmonic of fundamental light L (e.g., having a wavelength of approximately 266 nm). Harmonic conversion module 703 may include a CLBO crystal which can be critically phase matched for Type I generation of the second harmonic of 532 nm at a phase-matching angle of about 61.8° at a temperature of about 100° C. Other temperatures can be used with an appropriate adjustment of the phase-matching angle. CLBO is particularly useful when a high power (such as 500 mW or more) of output light at 266 nm is needed as CLBO can have a higher damage threshold than other materials at DUV wavelengths. Annealed, deuterium-treated or hydrogen-treated CLBO crystals are preferred for power levels of about 1 W or higher at DUV wavelengths. More information on annealed, deuterium-treated and hydrogen-treated CLBO can be found in U.S. Published Patent Application 2015/0007765 entitled “CLBO Crystal Growth” by Dribinski and published on Jan. 8, 2015, U.S. Pat. No. 8,873,596 entitled “Laser With High Quality, Stable Output Beam, And Long Life High Conversion Efficiency Non-Linear Crystal” by Dribinski et al., issued Oct. 28, 2014, U.S. Published Patent Application 2013/0088706 entitled “Hydrogen Passivation of Nonlinear Optical Crystals” by Chuang et al. published on Apr. 11, 2013, and U.S. Published Patent Application 2014/0305367 entitled “Passivation of Nonlinear Optical Crystals” by Chuang et al. published on Oct. 16, 2014. All of these patent and/or patent applications are incorporated by reference herein.
In an exemplary embodiment, fundamental laser 601 generates fundamental light pulses LP having a fundamental wavelength of approximately 1064 nm using, for example, a Nd:YAG or Nd-doped vanadate laser. Sum frequency light LSF is generated with pulses LSFP having wavelengths of approximately 532 nm is generated using bandwidth narrowing apparatus 610. Harmonic conversion module 806 converts it into the harmonic light Lhar at a wavelength of approximately 266 nm. Frequency mixing module 808 produces laser output light Lout at a wavelength of approximately 213 nm by mixing the narrowed fundamental light LN at a wavelength of approximately 1064 nm, and the narrowed harmonic light Lhar at a wavelength of approximately 266 nm. In a preferred embodiment, one or both of harmonic conversion module 806 and frequency mixing module 808 include a CLBO crystal, an annealed CLBO crystal, a deuterium-treated CLBO crystal or a hydrogen-treated CLBO crystal.
Depending on the wavelength and power level of the fundamental light, suitable non-linear crystals for Type I frequency mixing may include BBO, LBO, CLBO and periodically poled materials such lithium niobate, stoichiometric lithium tantalate, and Mg-doped stoichiometric lithium tantalate. Note that with periodically poled crystals, the polarization of the sum frequency light LSF may be either perpendicular (as shown) or parallel to the polarization of the input pulses LSSP1 and LSSP2 depending on the material and quasi phase matching used.
The above exemplary embodiments describe lasers that generate an output wavelength corresponding to an integer harmonic of the fundamental. The bandwidth narrowing apparatuses and methods disclosed herein can be used in lasers that generate output frequencies that are not an integer harmonic of the fundamental. For example, a laser may generate an output wavelength by mixing a harmonic of the fundamental laser with another wavelength, such as one generated by an optical parametric oscillator, optical parametric amplifier or Raman laser pumped by a portion of the fundamental. In such a laser, the bandwidth of the harmonic may be narrowed using an apparatus or method disclosed herein, thus resulting in a narrower output bandwidth.
For example a laser can generate an output wavelength between about 180 nm and about 200 nm, such as a wavelength near 193 nm, by mixing the fifth harmonic of a fundamental near 1064 nm with an infra-red wavelength between about 1.1 μm and about 3.3 μm. More detailed descriptions of lasers generating wavelengths near 193 nm that can benefit from incorporating the bandwidth-controlling apparatus and methods described herein are described in U.S. Pat. No. 8,755,417 entitled “Coherent light generation below about 200 nm” to Dribinski, and in U.S. Published Patent Application 2013/0077086, entitled “Solid-State Laser and Inspection System Using 193 nm Laser” by Chuang et al. published on Mar. 28, 2013 (now abandoned), U.S. Published Patent Application 2013/0313440, entitled “Solid-state laser and inspection system using 193 nm laser” by Chuang et al. published on Nov. 28, 2013, U.S. Pat. No. 8,929,406 entitled “193 nm laser and inspection system” by Chuang et al. U.S. Published Patent Application 2014/0226140, entitled “193 nm Laser And Inspection System” by Chuang et al. published on Aug. 14, 2014, Ser. No. 14/210,355, entitled “A 193 nm Laser and an Inspection System Using a 193 nm Laser” and filed by Chuang et al. on Mar. 13, 2014. All of these patents and applications are incorporated by reference herein.
Note that the above described 193 nm lasers can be operated at other wavelengths shorter than about 200 nm by appropriate selection of the fundamental wavelength, the wavelength of the signal light, and appropriate changes to frequency mixing modules within the laser. In particular vacuum UV wavelengths shorter than 190 nm can be generated by such lasers. Lasers capable of generating wavelengths shorter than about 200 nm are also described in U.S. Provisional Patent Application 62/059,368 by Chuang et al., entitled “183 nm laser and inspection system” and filed on Oct. 3, 2014. This provisional application is incorporated by reference herein. The bandwidth reduction apparatus and methods described herein may be used in lasers described in this provisional application.
Exemplary embodiments of image sensors suitable for use in an inspection or imaging system incorporating any of the lasers described herein can be found in US Published Patent Application 2013/0264481 entitled “Back-Illuminated Sensor with Boron Layer” by Chern et al., which published on Oct. 10, 2013 and is incorporated by reference herein.
The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the particular embodiments described. For example, different harmonic conversion schemes and/or different non-linear crystals could be used. In another example, additional mirrors, prisms or other optical components may be used to direct laser pulses within a laser assembly and to adjust optical path lengths so as to be appropriately matched where needed.
The present application claims priority to U.S. Provisional Patent Application 62/055,605 entitled “Method for Reducing the Bandwidth of an Ultra-violet Laser and an Inspection System and Method Using an Ultra-violet Laser”, filed by Deng et al. on Sep. 25, 2014, and also claims priority to U.S. Provisional Patent Application 62/136,403 entitled “Method for Reducing the Bandwidth of an Ultra-violet Laser and an Inspection System and Method Using an Ultra-violet Laser”, filed by Deng et al. on Mar. 20, 2015. The present application is related to U.S. patent application Ser. No. 14/158,615, entitled “193 nm Laser and Inspection System”, filed by Chuang et al. on Jan. 17, 2014, U.S. patent application Ser. No. 13/797,939, entitled “Solid-State Laser and Inspection System Using 193 nm Laser”, filed by Chuang et al. on Mar. 12, 2013, U.S. patent application Ser. No. 14/170,384, entitled “193 nm Laser and Inspection System”, filed by Chuang et al. on Jan. 31, 2014, U.S. patent application Ser. No. 13/711,593, entitled “Semiconductor Inspection and Metrology System Using Laser Pulse Multiplier”, filed by Chuang et al. on Dec. 11, 2012, U.S. patent application Ser. No. 13/487,075, entitled “Semiconductor Inspection and Metrology System Using Laser Pulse Multiplier”, filed by Chuang et al. on Jun. 1, 2012, and U.S. patent application Ser. No. 14/300,227, entitled “A System and Method for Reducing the Bandwidth of a Laser and an Inspection System and Method Using a Laser”, filed by Deng et al. on Jun. 14, 2014. All these applications are incorporated by reference herein.
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