Non-critical phase matching in clbo to generate sub-213nm wavelenghts

0001This application claims the benefit of <patcit id="pcit0001" dnum="US70320905P" dnum-type="L"><text>U.S. Provisional Patent Application Serial No. 60/703,209, filed July 28, 2005</text></patcit>, which is a continuation in part of <patcit id="pcit0002" dnum="US05685505A" dnum-type="L"><text>U.S. Patent Application Serial No. 11/056,855</text></patcit>, entitled "Fiber Amplifier Based Light Source for Semiconductor Inspection", inventors Chuang et al., filed February 11, 2005, which claims the benefit of <patcit id="pcit0003" dnum="US60454004P" dnum-type="L"><text>U.S. Provisional Patent Application Serial No. 60/604,540</text></patcit>, entitled "Fiber Amplifier Based Light Source for Semiconductor Inspection", filed August 25, 2004, and <patcit id="pcit0004" dnum="US62081404P" dnum-type="L"><text>U.S. Provisional Patent Application Serial No. 60/620,814</text></patcit>, entitled "Coherent DUV Sources for Semiconductor Wafer Inspection", filed October 20, 2004.

BACKGROUND OF THE INVENTION

Field of the Invention

0002The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a fiber amplifier based light source for use with such inspection systems.

Description of the Related Art

0003The demands of the semiconductor industry for wafer and photomask inspection systems exhibiting high throughput and improvements in resolution are ongoing. Successive generations of such inspection systems tend to achieve higher resolution by illuminating the wafer or reticle using light energy having shorter wavelengths.

0004Certain practical advantages may be achieved when illuminating the wafer or reticle with light with wavelengths at or below 213nm. Providing suitable lasers for high quality wafer and photomask inspection systems presents a particular challenge. Exemplary light sources of that kind have been disclosed in the following publications: <nplcit id="ncit0001" npl-type="b"><text>WALL and ISYANOVA Y ET AL: "A quasi-continuous-wave deep ultraviolet laser source" IEEE JOURNAL OF QUANTUM ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, NJ, USA, vol. 39, no. 9, 1 September 2003 (2003-09-01), pages 1160-1169, ISSN: 0018-9197</text></nplcit>, describing the light source consisting of a 100-MHz, mode-locked, 15-W, 1047-nm master-ocsillateor-power-amplifier, a synchronously pumped optical parametric oscillator, and three nonlinear conversion stages; and <nplcit id="ncit0002" npl-type="b"><text>CHANG L B ET AL: "Efficient compact watt-level deep-ultraviolet laser generated from a multi-kHz Q-switched diode-pumped solid-state laser system" OPTICS COMMUNICATIONS, NORTH-HOLLAND PUBLISHING CO. AMSTERDAM, NL, vol. 209, no. 4-6, 15 August 2002 (2002-08-15), pages 397-401, ISSN: 0030-4018</text></nplcit>, where stable high-power operation in the deep ultraviolet is achieved from a multi-kHz diode-pumped multi-longitudinal-mode Q-switched Nd: YAG laser by fourth harmonic generation in an external resonant cavity and by fifth harmonic generation using sum-frequency mixing of the fourth harmonic and the residual fundamental in a β-BBO crystal. Conventional lasers generating DUV light energy are typically large, expensive devices with relatively short lifetimes and low average power. Wafer and photomask inspection systems generally require a laser generally having a high average power, low peak power, and relatively short wavelength for sufficient throughput and an adequate defect signal-to-noise ratio (SNR).

0005The primary method to provide adequate DUV power entails generating shorter wavelength light from a longer wavelength source. This wavelength alteration process is commonly called frequency conversion. Frequency conversion can be inefficient, and can require a high peak power laser to provide sufficient light to the frequency conversion crystal.

0006Generating light at wavelengths below 213nm, and especially below 200nm can be very challenging. Light sources used for semiconductor inspection require relatively high powers, long lifetimes, and stable performance. Light sources meeting these requirements for advanced inspection techniques are nonexistent. The lifetime and stability of current DUV frequency converted lasers is generally limited by the frequency conversion crystals, especially those exposed to DUV wavelengths like 266, 213, 200, and 195nm.

0007High efficiency is important for a DUV laser. High efficiency allows a lower power fundamental laser source that is more reliable, smaller, and produces less heat. A low power fundamental laser will produce less spectral broadening if a fiber laser is used. Higher efficiency also tends to lead to lower cost and better stability. For these reasons efficient frequency conversion to the DUV is relatively important.

0008High speed inspection requires a light source with a continuous in time temporal profile or a profile pulsed at a very high repetition rate. Such a light source is required because each imaged position on the sample desirably exhibits the same or similar intensity. A similar intensity is provided by, for example, a continuous light source. Similar intensity may also be provided if the light source is pulsed with a repetition rate high enough such that many pulses are averaged at each imaged position. In this case the pulsed nature of the laser generally will not effect the measurements.

0009The requirements for high efficiency frequency conversion and high speed inspection indicate the laser light source preferably operates in a pulsed mode at relatively high repetition rates. In this way, high conversion efficiency and high speed inspection can be obtained from the light source.

0010In this environment, a laser based illuminator or illuminating arrangement having wavelengths that include ultraviolet-deep ultraviolet (UV-DUV) wavelengths may provide benefits over previous types of illuminators. Such an illuminating arrangement operating at sufficient power levels, providing adequate SNRs, that are relatively inexpensive and have relatively long lifetimes may be preferable to other illuminators in certain advanced applications.

0011It would therefore be desirable to offer a light source that effectively and efficiently generates light energy at wavelengths at or below 213nm, using materials that have been processed specifically to provide increased lifetimes, stability, and damage thresholds over designs previously available.

SUMMARY OF THE INVENTION

0012According to one aspect of the present design, there is provided a method for producing light energy. The method comprises generating fundamental frequency laser energy at a wavelength of approximately 998 nm, converting a portion of the fundamental frequency laser energy to 2^nd harmonic frequency laser energy, further converting the 2^nd harmonic frequency laser energy to 4^th harmonic frequency laser energy, and mixing the 4^th harmonic frequency laser energy with a portion of the fundamental frequency laser energy to produce laser energy at a sum frequency. Mixing is accomplished by non-critical phase matching in a crystal of Cesium Lithium Borate (CLBO).

0013Other embodiment include<u>s</u> generating fundamental light energy at other fundamental frequencies and employing shifting a portion of the fundamental frequency laser energy to laser energy at a Raman line.

0014These and other advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

0015The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">FIG. 1</figref> illustrates a laser oscillator;</li><li><figref idref="f0001">FIG. 2</figref> shows an illuminator combining one or more laser oscillators with one or more fiber laser amplifiers;</li><li><figref idref="f0001">FIG. 3</figref> is a large diameter fiber having a tapered end;</li><li><figref idref="f0002">FIG. 4</figref> shows three examples of cross sections of photonic crystal fiber;</li><li><figref idref="f0002">FIG. 5</figref> illustrates an example of energy spreading using a single grating where the initial pulse leaving the laser occurs over a relatively short period of time and exhibits higher peak power;</li><li><figref idref="f0003">FIG. 6</figref> is a hybrid photonic crystal fiber into the fiber laser amplifier using pump cladding;</li><li><figref idref="f0003">FIG. 7</figref> shows an illuminator containing a plurality of fiber laser amplifiers each having similar output frequency values;</li><li><figref idref="f0004">FIG. 8</figref> illustrates a plurality of fiber laser amplifiers with substantially the same output frequency in a different configuration;</li><li><figref idref="f0004">FIG. 9</figref> shows an illuminator comprising multiple fiber laser amplifiers, with certain fiber laser amplifiers have different output frequencies;</li><li><figref idref="f0005">FIG. 10</figref> illustrates an alternate configuration of a plurality of fiber laser amplifiers having different output frequencies;</li><li><figref idref="f0005">FIG. 11</figref> shows an illuminator having a q-switched or mode-locked pulse train creation apparatus;</li><li><figref idref="f0006">FIG. 12</figref> shows a fiber laser oscillator;</li><li><figref idref="f0007">FIG. 13</figref> shows a fiber laser configured in a ring with an active intensity modulator;</li><li><figref idref="f0008">FIG. 14</figref> shows four cross sections of optical fiber incorporating pump cladding;</li><li><figref idref="f0008">FIG. 15</figref> is one example of end pumping;</li><li><figref idref="f0009">FIG. 16</figref> is a fiber laser oscillator where laser diodes pump the fiber from the end without the use of a pump combiner;</li><li><figref idref="f0010">FIG. 17</figref> illustrates laser oscillator pumping from the side;</li><li><figref idref="f0010">FIG. 18</figref> shows a grating with different grating frequencies, with multiple instances of four periods;</li><li><figref idref="f0011">FIG. 19</figref> is a chirped grating;</li><li><figref idref="f0011">FIG. 20</figref> shows one type of polarization-preserving fiber having a solid core;</li><li><figref idref="f0011">FIG. 21</figref> illustrates two cross sections of polarization-preserving pliotonic crystal fiber;</li><li><figref idref="f0012">FIG. 22</figref> shows two fiber laser oscillators and two fiber laser amplifiers all employing the same or similar wavelengths;</li><li><figref idref="f0012">FIG. 23</figref> is two fiber laser oscillators and two fiber laser amplifiers using different wavelengths;</li><li><figref idref="f0012">FIG. 24</figref> illustrates an illuminator comprising a laser oscillator, a fiber laser amplifier, and a frequency conversion medium;</li><li><figref idref="f0013">FIG. 25</figref> shows a birefringent crystal with input light entering the crystal;</li><li><figref idref="f0013">FIG. 26</figref> shows a periodically polled ferromagnetic material where the orientation of the magnetic domains alternates in successive regions;</li><li><figref idref="f0014">FIG. 27</figref> shows a single laser oscillator feeding two fiber laser amplifiers;</li><li><figref idref="f0014">FIG. 28</figref> is an illuminator including a laser oscillator feeding two fiber laser amplifiers, both amplifying the same or similar frequencies;</li><li><figref idref="f0014">FIG. 29</figref> shows an illuminator having a laser oscillator feeding two fiber laser amplifiers, each amplifying a different wavelength within the gain bandwidths of the laser oscillator;</li><li><figref idref="f0015">FIG. 30</figref> illustrates an illuminator having two laser oscillators, two fiber laser amplifiers, and two frequency conversion media;</li><li><figref idref="f0015">FIG. 31</figref> shows an illuminator having two laser oscillators, two fiber laser amplifiers, and two frequency conversion media, where the two laser oscillators may emit different wavelengths;</li><li><figref idref="f0015">FIG. 32</figref> represents a single laser oscillator feeding a single fiber laser amplifier, which then feeds two frequency conversion media;</li><li><figref idref="f0016">FIG. 33</figref> shows a beam exiting from one frequency conversion medium and undergoing a second stage of frequency conversion in another frequency conversion medium;</li><li><figref idref="f0016">FIG. 34</figref> illustrates light from a laser oscillator entering a fiber laser amplifier;</li><li><figref idref="f0016">FIG. 35A</figref> shows a laser oscillator, a fiber laser amplifier, a frequency conversion medium, and a temporal shifting medium;</li><li><figref idref="f0017">FIG. 35B</figref> shows a grating as a temporal shifting medium;</li><li><figref idref="f0018">FIG. 36</figref> illustrates beam splitters and one or more reflective surfaces serving as a temporal shifting medium;</li><li><figref idref="f0019">FIG. 37</figref> shows an etalon serving as a temporal shifting medium;</li><li><figref idref="f0019">FIG. 38</figref> shows a laser oscillator, a fiber laser amplifier, a frequency conversion medium, and a spatial smoothing medium;</li><li><figref idref="f0020">FIG. 39A</figref> illustrates a first spatial smoothing media;</li><li><figref idref="f0020">FIG. 39B</figref> shows a second spatial smoothing media;</li><li><figref idref="f0021">FIG. 40A</figref> shows a first additional example of spatial smoothing media;</li><li><figref idref="f0021">FIG. 40B</figref> shows a second additional example of spatial smoothing media;</li><li><figref idref="f0021">FIG. 40C</figref> shows a third additional example of spatial smoothing media;</li><li><figref idref="f0022">FIG. 41</figref> illustrates laser oscillator, a fiber laser amplifier, a frequency conversion medium, and a spectral shifting medium;</li><li><figref idref="f0022">FIG. 42A</figref> represents a photonic crystal filter;</li><li><figref idref="f0022">FIG. 42B</figref> shows cross sections of various spectral shifting media;</li><li><figref idref="f0023">FIG. 43</figref> illustrates a spectral shifting medium having a cell containing gas;</li><li><figref idref="f0023">FIG. 44</figref> shows an illumination subsystem including a laser oscillator, a fiber laser amplifier, a frequency conversion medium, and an image relay system;</li><li><figref idref="f0024">FIG. 45</figref> shows two examples of image relay systems that may lie downstream from the configurations shown in <figref idref="f0020">FIGs. 39</figref> and <figref idref="f0021">40</figref>;</li><li><figref idref="f0023">FIG. 46</figref> is an image relay system generally designed for dark field imaging.</li><li><figref idref="f0025">FIG. 47</figref> illustrates generating sub 213 nm laser energy by non-critical phase matching in Cesium Lithium Borate (CsLiB₆O₁₀ or CLBO) by mixing the fundamental and 4^th harmonic; and</li><li><figref idref="f0026">FIG. 48</figref> shows generating sub 213 nm laser energy by non-critical phase matching in CLBO by mixing the Raman shifted 4^th harmonic and the fundamental.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

0016According to the present design, an illuminator for a wafer inspection system or photomask inspection system is provided comprising at least one laser oscillator and at least one fiber laser amplifier. The laser oscillator may be a solid-state laser or semiconductor diode laser, and the laser oscillator may incorporate means for creating a q-switched or mode-locked pulse train. Various devices may be employed to enhance performance of the illuminator, including but not limited to peak power reducing hardware, frequency conversion media, spatial shifting media, spatial smoothing media, image relay hardware, and temporal shifting media.

0017Further, the present design may provide an advanced light source, wherein there is provided a novel method for producing light energy. The method comprises generating fundamental frequency laser energy, converting a portion of the fundamental frequency laser energy to 2^nd harmonic frequency laser energy, further converting the 2^nd harmonic frequency laser energy to 4^th harmonic frequency laser energy, and mixing the 4^th harmonic frequency laser energy with a portion of the fundamental frequency laser energy to produce laser energy at a sum frequency. Mixing is accomplished by non-critical phase matching in a crystal of Cesium Lithium Borate (CLBO).

0018Other embodiment include<u>s</u> generating fundamental light energy at other fundamental frequencies and employing shifting a portion of the fundamental frequency laser energy to laser energy at a Raman line.

Fiber Based Illuminator

0019<figref idref="f0001">FIGs. 1 and 2</figref> illustrate an illuminator for a wafer or photomask inspection system. <figref idref="f0001">FIG. 1</figref> shows a laser oscillator 101, while <figref idref="f0001">FIG. 2</figref> shows a slightly more complex illuminator combining one or more laser oscillators 201 with one or more fiber laser amplifiers 202. The present design is an illuminator for a wafer inspection system or a photomask inspection system wherein the gain media comprises optical fiber. Optical fiber technology, with the addition of optical crystals, allows for tuning light wavelengths by broadening narrowband light or shifting light from one narrowband wavelength to another.

0020The present design also addresses illuminating a wafer or photomask for inspection, including using one or more lasers to generate coherent light, and subsequently amplifying the coherent light with one or more fiber laser amplifiers. In certain embodiments, the design may also maintain single mode propagation within the fiber laser amplifier as discussed below.

0021Power-handling capability of the fiber laser amplifier 202 may be enhanced by employing a fiber with a relatively large diameter, but this can diminish wavefront quality and the fiber laser amplifier 202 may exhibit multi-mode behavior. In the present design, the fiber laser amplifier 202 may comprise a large diameter fiber having a tapered end as shown in <figref idref="f0001">FIG. 3</figref>. The photons enter the tapered area through the large area input 301. The size of the core 304 decreases in the tapered region 302, while the size of the cladding 303 may decrease or remain constant. The tapered end tends to suppress higher order modes in a fiber laser amplifier. The beam exits the fiber and propagates in a single mode to and through output 305.

0022An alternate design provides a high quality wavefront with a single mode (i.e., a Gaussian beam profile) by incorporating a photonic crystal fiber into the fiber laser amplifier 202 or an optical fiber with a modified index profile. The photonic crystal fiber may be a multi-core fiber. <figref idref="f0002">FIG. 4</figref> shows three examples of cross sections of photonic crystal fiber 401, 402, and 403. In all three examples, the gray regions represent either glass or doped glass and the white regions represent air. The first fiber 401 shows a doped fiber core 406, external cladding 404, and multiple channels of air cladding 405 arranged in a ring configuration. The second fiber 402 and third fiber 403 illustrate air cladding channels arranged in a honeycomb configuration. These configurations are generally spatially uniform along the fiber length. Air cladding channels 405 facilitate a high-quality wavefront having a single mode within a fiber having a diameter greater than approximately 50 microns.

0023The mode area may be increased, where the mode area is the area taken up by the fiber optic media, and a single mode output obtained within the fiber laser amplifier 202 by bending the fiber laser amplifier, by tapering the end of the fiber laser amplifier, or by modifying the refractive index of the fiber laser amplifier 202.

0024Within the bandwidth of the laser oscillator 201, the frequency for amplifying the fiber laser amplifier 202 may be assessed by doping the fiber laser amplifier 202 with an appropriate lanthanide element such as ytterbium, erbium, thulium, neodymium, praseodymium, or holmium.

0025Nonlinear effects can broaden the narrowband light which leaves the fiber laser amplifier and are generally undesirable. Broadband light can diminish the efficiency of the subsequent frequency conversion process. In the present design, nonlinear effects are addressed by either expanding the light energy pulse (in time) before the pulse enters the fiber laser amplifier 202, or using a fiber that can produce a relatively large single mode output. Relatively large single mode outputs can be achieved by incorporating a hybrid photonic crystal fiber with pump cladding into the fiber laser amplifier 202.

0026Spreading the energy of the pulse in time maintains the total energy of the pulse but can diminish peak power. <figref idref="f0002">FIG. 5</figref> illustrates an example of energy spreading using a single grating where the initial pulse 501 leaving the laser occurs over a relatively short period of time and exhibits higher peak power. The initial pulse 501 may pass through a grating 502. The subsequent pulse 503 occurs over a longer period of time and may have similar total energy but lower peak power. A focusing lens 504 may focus the pulsed laser beam into a fiber amplifier 505. More than one grating may be employed in addition to reflective surfaces such as mirrors in multiple pass or multiple pulse configurations.

0027To achieve energy spreading, the present design may illuminate the specimen, such as a wafer or photomask, by pumping a laser oscillator to generate coherent light, mode-locking or q-switching the laser oscillator to create a pulsed output beam, expanding the pulses of the pulsed output beam, and amplifying the coherent light with at least one fiber laser amplifiers. Note that in this arrangement multiple laser oscillators may be employed and pumped using the foregoing design. Mode locking and q-switching are laser techniques known to those skilled in the art, but in general, mode locking comprises modulating laser energy content for each mode within the laser to selectively produce energy bursts having desired properties. Q-switching may be employed in the optical resonator portion of a laser device to prevent lasing action until a relatively high level of inversion (optical gain and energy storage) may be achieved in the lasing medium.

0028The second technique used to avoid non-linear effects in the fiber laser amplifier is increasing the mode area. The present design may incorporate a hybrid photonic crystal fiber into the fiber laser amplifier 202 using pump cladding as shown in <figref idref="f0003">FIG. 6</figref>. For purposes of comparison, <figref idref="f0003">FIG. 6</figref> shows a traditional single-mode fiber 601 which allows a beam with a small single mode. The traditional single-mode fiber 601 has a relatively small doped fiber core 603 with external cladding 602. The hybrid photonic crystal fiber with pump cladding 604 allows a larger single-mode beam to propagate and may include a doped fiber core having a photonic crystal structure 607, pump cladding 606, and external cladding 605.

0029Multiple fiber amplifier illuminators can provide increased power or an increased number of available wavelengths. <figref idref="f0003">FIG. 7</figref> shows an illuminator containing a plurality of fiber laser amplifiers each having similar output frequency values. From <figref idref="f0003">FIG. 7</figref>, the illuminator may include two laser oscillators 701 and 703 and two fiber laser amplifiers 702 and 704. Both laser oscillators 701 and 703 generate similar central frequencies λ₁, where the subscript "1" indicates that the output power of the laser oscillators 701 and 703 is not amplified. Both fiber laser amplifiers 702 and 704 amplify light having substantially the same frequency. The letter "X" before λ₁ indicates the amplifier has multiplied the intensity of the laser's output power by a factor.

0030<figref idref="f0004">FIG. 8</figref> also illustrates a plurality of fiber laser amplifiers with substantially the same output frequency in a different configuration. From <figref idref="f0004">FIG. 8</figref>, the illuminator may include one laser oscillator 801 feeding two fiber laser amplifiers 802 and 803. Both laser amplifiers 802 and 803 amplify substantially the same frequency. The configuration of <figref idref="f0004">FIG. 8</figref> may require a beam splitter 804 or another device to deliver output light from the laser oscillator 801 to the two fiber laser amplifiers 802 and 803. The configurations in <figref idref="f0003">FIGs. 7</figref> and <figref idref="f0004">8</figref> can increase the available power or provide multiple channels of light with substantially the same frequency to allow bright-field imaging and dark-field imaging within the same inspection system.

0031<figref idref="f0004">FIG. 9</figref> shows an illuminator comprising multiple fiber laser amplifiers, with certain fiber laser amplifiers have different output frequencies. <figref idref="f0004">FIG. 9</figref> shows an illuminator with two laser oscillators 901 and 903 and two fiber laser amplifiers 902 and 904. Both laser oscillators 901 and 903 generate similar central frequencies (λ₁). The first fiber laser amplifier 902 may selectively amplify one wavelength, λ₁, within the gain bandwidth of the first laser oscillator 901, and the second fiber laser amplifier 904 selectively amplifies a different wavelength λ₂ within the gain bandwidth of the second laser oscillator 903.

0032<figref idref="f0005">FIG. 10</figref> illustrates an alternate configuration of a plurality of fiber laser amplifiers having different output frequencies. <figref idref="f0005">FIG. 10</figref> shows an illuminator with one laser oscillator 1001 and two fiber laser amplifiers 1002 and 1003. The first fiber laser amplifier 1002 selectively amplifies light having one wavelength λ₁ within the gain bandwidth of the laser oscillator 1001. The second fiber laser amplifier 1003 selectively amplifies light having a different wavelength λ₂ within the gain bandwidth of the same laser oscillator 1001. This alternate configuration can employ a beam splitter 1004 or another device for delivering the output light from the laser oscillator 1001 into two fiber laser amplifiers 1002 and 1003. The configurations in <figref idref="f0004">FIGs. 9</figref> and <figref idref="f0005">10</figref> can facilitate inspection of layers with different materials and different thicknesses.

0033The devices of <figref idref="f0004">FIGs. 9</figref> and <figref idref="f0005">10</figref> may be employed to illuminate a specimen, such as a wafer or photomask, by pumping the laser oscillator to generate a beam of coherent light, splitting said beam of coherent light into a plurality of sub-beams, and amplifying the sub-beams of coherent light using multiple fiber laser amplifiers. Alternately, the fiber laser amplifiers may amplify the same or similar wavelengths within the gain bandwidth of the laser oscillator. Another embodiment may entail each of the fiber laser amplifiers amplifying a different wavelength within the gain bandwidth of the laser oscillator.

0034In general, any number of laser oscillators and fiber laser amplifiers may be employed in configurations similar to those of <figref idref="f0003 f0004 f0005">FIGs. 7-10</figref>, and such designs may encounter certain tradeoffs in cost and performance.

0035The laser oscillator 201 may be virtually any type of laser, such as a solid-state laser or a semiconductor diode laser. Semiconductor lasers typically have output power of a few hundred milliwatts and a central wavelength of 600 to 900 nm.

0036High resolution inspection of small features on wafers and photomasks can be enhanced by light with relatively short wavelengths. Certain types of inspection can require an illuminator having hardware for converting light frequency output by the fiber laser amplifier. Efficient frequency conversion generally requires relatively high peak power, and generating relatively high peak power may employ a device or devices for creating a q-switched or mode-locked pulse train.

0037<figref idref="f0005">FIG. 11</figref> shows an illuminator for an inspection system having at least one laser oscillator 1101, at least one fiber laser amplifier 1102, and q-switched or mode-locked pulse train creation apparatus 1103. Illumination using the device of <figref idref="f0005">FIG. 11</figref> may comprise pumping the laser oscillator to generate coherent light, mode-locking or q-switching said laser oscillator or laser oscillators to create a pulsed output beam, and amplifying the coherent light with one or more fiber laser amplifiers.

0038High peak power is generally useful for efficient frequency conversion, but peak power in the beam is inversely proportional to the repetition rate. Thus a relatively low repetition rate generates a relatively high peak power. A high repetition rate can enhance the performance of a time domain integration (TDI) sensor, used to capture signals reflected from the specimen or through the specimen.

0039Creating a q-switched or mode-locked pulse train 1103 may employ a passive device or an active device. A passive device could be, for example, a saturable absorber fabricated from either a semiconducting material or carbon nanotubes. An active device could be, for example, an acousto-optic modulator, an electro-optic modulator, an active mirror with controlled reflectivity, or an electronic switch.

0040<figref idref="f0006">FIG. 12</figref> shows a fiber laser oscillator 1206, where the bracket indicates that all the components to the right of the bracket collectively make up the fiber laser oscillator. One mirror may be a saturable absorber 1201 producing a q-switched or mode-locked pulse. The saturable absorber 1201 may be a Bragg reflector made from semiconducting material, typically a gallium arsenide heterostructure with in the range of approximately 50 to 80 layers, alternately n-doped and p-doped, that change from absorptive to reflective over short time scales when receiving light from a continuous-wave laser. A collimating lens 1207 may be positioned between the saturable absorber 1201 and the doped fiber 1205 or the saturable absorber 1201 may be affixed directly to the doped fiber 1205.

0041The saturable absorber 1201 may generally deliver a repetition rate ranging from approximately 5 to over 100 MHz, depending on the length of the fiber. As long as the saturable absorber 1201 operates faster than the round trip time of the cavity, the length of cavity determines the repetition rate. In this arrangement, a fiber length of 1.875 meters generally yields a repetition rate of somewhere in the range of about 80 MHz.

0042The second mirror of the fiber laser oscillator 1206 is an output grating 1204. Multiple laser diodes 1202 may pump the gain medium, a doped fiber 1205 surrounded by pump cladding. A pump combiner 1203 may direct light from the laser diodes 1202 into the pump cladding surrounding the doped fiber 1205.

0043<figref idref="f0007">FIG. 13</figref> shows an example of a fiber laser oscillator 1301 configured in a ring with an active intensity modulator 1302. The fiber ring resonator 1303 may include optical fiber with pump cladding. Two laser diodes 1304 provide pump energy. Doped fiber 1305 may be provided as shown.

0044The present design may illuminate by pumping one or more fiber laser oscillators to generate coherent light. Fiber laser oscillators tend to be relatively compact and inexpensive. The illuminator may include one or more laser oscillators and one or more fiber laser amplifiers, where at least one laser oscillator is a fiber laser oscillator. For example, one or more of the laser oscillators 201 of <figref idref="f0001">FIG. 2</figref> could be a fiber laser oscillator. The present design may include a combined fiber laser oscillator and fiber laser amplifier. Such a design may pump at least one fiber laser oscillators to generate coherent light and amplify the coherent light using one or more fiber laser amplifiers. The combination of the fiber laser oscillator and the fiber amplifier tends to provide relatively high power and high beam quality.

0045An illuminator having at least one fiber laser oscillator and at least one fiber laser amplifier may include one or more laser diodes 1202 as the pumping system for the fiber laser oscillator. Current individual laser diodes 1202 typically generate energy in the range of 1 to 5 watts. A bar or arrangement of laser diodes 1202 can generate in the range of 10 to 40 watts. The laser diodes 1202 may produce light energy at a wavelength near the absorption peak of the optical fiber. The pump diodes may emit light having a shorter wavelength than the fiber laser oscillator.

0046A closely related design is an illuminator for an inspection system comprising a fiber laser oscillator and a fiber laser amplifier in which either the fiber laser oscillator or the fiber laser amplifier incorporates pump cladding. <figref idref="f0008">FIG. 14</figref> shows four cross sections 1401, 1402, 1403, 1404 of optical fiber incorporating pump cladding. In each cross section, an internal white circle represents the doped fiber core 1405. The dark or black area represents the external cladding 1407. The intermediate or gray area represents pump cladding 1406.

0047Pump cladding 1406 includes optical fiber without doping to allow pump light to propagate. The pump cladding 1406 guides the pump light down the length of the fiber. The pump light reflects through the doped fiber core 1405 multiple times, increasing the probability that the doped fiber core 1405 will absorb the pump light. The pump cladding 1406 facilitates uniform absorption throughout the length of the doped fiber core 1405. Absorption can be improved by positioning the doped fiber core 1405 away from the center of the fiber assembly as shown in cross section 1402. Asymmetrical optical fiber configurations 1403 and 1404 can provide increased absorption relative to over the symmetrical configuration of cross section 1401.

0048Using this arrangement, illumination may include pumping a fiber laser oscillator to generate coherent light and amplifying the coherent light using one or more fiber laser amplifiers. Pumping may include pumping the fiber laser oscillator using pump cladding. Pump light may be injected into the fiber laser oscillator by pumping light from the end of the oscillator. <figref idref="f0008">FIG. 15</figref> shows one example of end pumping. The input pump light 1501 enters the fiber laser oscillator at the left. The cross sectional diagram shows the highest-index doped fiber core 1502, the white region at the center, the middle-index pump cladding 1503, the intermediate or gray region surrounding the doped fiber core, and the low-index outside cladding 1504 represented by the dark or black region on the outside of the fiber. The pump cladding 1503 can enhance pump laser photon absorption and increase the spatial uniformity of absorption throughout the fiber laser oscillator. Note that <figref idref="f0006">FIG. 12</figref> also illustrates end pumping of a fiber laser oscillator using pump cladding. Several laser diodes 1202 contribute pump energy and pump combiners 1203 direct energy into the pump cladding. End pumping may apply to the fiber laser amplifier as well as the fiber laser oscillator.

0049<figref idref="f0009">FIG. 16</figref> shows a diagram of a fiber laser oscillator or amplifier 1601 where laser diodes 1602 pump the fiber from the end without the use of a pump combiner. Dichroic mirrors 1603 may be employed to separate the pump light from the light originating within the fiber.

0050Pump light may alternately be injected into the fiber laser oscillator from the side. <figref idref="f0010">FIG. 17</figref> illustrates one embodiment of pumping from the side. The pump diode input 1704 enters the fiber laser oscillator at the upper left through a prism 1705 and an index-matching fluid or a mechanical coupling 1706. The index of refraction of the prism 1705 may match or approximate the index of refraction of the pump cladding 1702. The index-matching fluid 1706 may replace air between the prism 1705 and the pump cladding 1702 to minimize reflections.

0051The design may employ a fiber Bragg grating as the grating 1204 in <figref idref="f0006">FIG. 12</figref>, serving as the output coupling mirror for the fiber laser oscillator. In such a design, at least one mirror in the fiber laser oscillator may include a grating. The design of the grating significantly influences the reflectivity of the mirror or reflective surface, as the greater the number of periods in the mirror, the greater the reflectivity. The grating can also filter the fiber laser oscillator's broadband light. The spacing between the periods determines those frequencies within the fiber's gain bandwidth experiencing gain. Such a design enables the fiber laser oscillator to have narrowband output.

0052The grating can also control the group velocity dispersion of light, thereby breaking up the pulses to lower the peak power of the light entering the amplifier. Lowering the peak power of the light entering the amplifier can keep the amplifier operating linearly. The grating may also operate so that the fiber laser oscillator emits light at several discrete narrowband wavelengths. In this way, a single fiber laser oscillator can provide numerous inspection wavelengths. The illuminator may therefore include a grating employing different grating frequencies.

0053<figref idref="f0010">FIG. 18</figref> shows a grating 1801 with different grating frequencies, with multiple instances of four periods 1802, 1803, 1804, and 1805. A grating of this type can cause the fiber laser oscillator to emit four discrete wavelengths of narrowband light within the oscillator gain bandwidth, each discrete wavelength corresponding to one of the four periods.

0054The grating may allow the fiber laser oscillator to emit broadband light. Such a construction includes a fiber laser oscillator having a chirped grating, such as the chirped grating 1901 of <figref idref="f0011">FIG. 19</figref>. Such a construction can provide a continuously varying period, causing the fiber laser oscillator to have broadband output.

0055In certain applications, such as those benefiting from short-wavelength light energy, light may be transmitted through a frequency conversion crystal. A frequency conversion crystal provide phase-matching for light energy exhibiting a single degree of polarization. In such a design, the illuminator may include a fiber laser oscillator having a polarization-preserving fiber. <figref idref="f0011">FIG. 20</figref> illustrates one type of polarization-preserving fiber having a solid core. The white region at the center is the doped fiber core 2004. The gray or intermediate region surrounding the doped fiber core 2004 is the pump cladding 2003. The black or dark region surrounding the pump cladding is the external cladding 2002. The structure also contains a different material 2001 having either asymmetrical structure or is constructed to reduce the symmetry of the cross section of the solid core. The different material 2001 creates internal stresses which render the solid core birefringent, resulting in the fiber favoring light of one polarization. The different material 2001 may be, for example, a glass or a polymer.

0056<figref idref="f0011">FIG. 21</figref> illustrates two cross sections of polarization-preserving photonic crystal fiber 2101 and 2102. Within the cross section of the first fiber 2101, <figref idref="f0011">FIG. 21</figref> depicts the doped fiber core 2106, air cladding 2105, and the external cladding 2104. The fiber may contain another material 2103 with an asymmetrical structure. Within the cross section of the second fiber 2102, the air channels may be arranged asymmetrically. Both these configurations 2101 and 2102 cause the photonic crystal fiber to favor light of a single polarization. Light energy polarization may therefore be controlled in the fiber laser oscillator or the fiber laser amplifier.

0057More than one fiber laser oscillator may be used in the design. Multiple fiber laser oscillators with the same wavelength may be used to increase the available power or provide separate channels for bright-field and dark-field imaging. Thus in the illuminator, the fiber laser oscillators may have the same or similar emission wavelengths.

0058<figref idref="f0012">FIG. 22</figref> illustrates two fiber laser oscillators 2201 and 2203 and two fiber laser amplifiers 2202 and 2204. Both fiber laser oscillators 2201 and 2203 have the same emission wavelength (1λ₁), and both fiber laser amplifiers have the same emission wavelength (Xλ₁). The "1" subscript indicates that the output power from the fiber laser oscillators 2201 and 2203 is unamplified. The "X" value indicates that output power from the fiber laser amplifiers 2202 and 2204 is amplified by a value of X.

0059<figref idref="f0012">FIG. 23</figref> shows two fiber laser oscillators 2301 and 2303 and two fiber laser amplifiers 2302 and 2304. Both fiber laser oscillators 2301 and 2303 have the same or similar emission wavelengths (1λ₁). The first fiber laser amplifier 2302 may amplify one wavelength (Xλ₁) within the gain bandwidth of the first fiber laser oscillator 2301, and the second fiber laser amplifier 2304 may amplify a different wavelength (Xλ₂) within the gain bandwidth of the second fiber laser oscillator 2303. Two fiber laser oscillators may provide the ability to mix output beams to generate coherent light at a third frequency.

0060High-resolution inspection of small features on state-of-the-art wafers and photomasks may require light with a wavelength near the wavelength of the wafer exposure tool, which can be 198 nm or 248 nm. Inspection systems employing fiber optic gain media can benefit from an illuminator with a frequency conversion medium. <figref idref="f0012">FIG. 24</figref> shows an illuminator comprising a laser oscillator 2401, a fiber laser amplifier 2402, and a frequency conversion medium 2403.

0061The frequency conversion medium could be a nonlinear crystal such as lithium niobate, beta barium borate, lithium barium borate, cesium lithium barium borate, or other birefringent crystals. <figref idref="f0013">FIG. 25</figref> shows a birefringent crystal 2501 with input light 2502 entering the crystal. Frequency-converted light 2504 and non-frequency-converted light 2503 exit from the crystal.

0062The frequency conversion medium could also be a periodically polled material such as periodically polled lithium niobate, magnesium strontium Lithium niobate, BaMgF₄, or a material from the KTP family (<i>M</i>TiO<i>X</i>O₄, where <i>M</i> is K, Rb, or Cs and X is P or As). <figref idref="f0013">FIG. 26</figref> shows a periodically polled ferromagnetic material where the orientation of the magnetic domains alternates in successive regions. At the top 2601 of the periodically polled material, the dimensions of the periods are consistent. In the periodically polled material at the bottom 2602, the dimensions of the period change along the length of the material, or the periodic poling is "chirped." A periodically polled crystal in which the domains are "fanned" (wedge-shaped) may be provided as an alternate to a crystal that is chirped. Other types of frequency conversion media may be provided depending on the application.

0063Locating the fiber laser amplifier and frequency conversion medium in different places can provide certain benefits, including the inability of waste heat from the fiber laser amplifier and associated pump diodes to compromise the optics thermal stability, and effective space utilization.

0064<figref idref="f0014 f0015">FIGs. 27 through 30</figref> illustrate various configurations of laser oscillators, fiber laser amplifiers, and frequency conversion media. <figref idref="f0014">FIG. 27</figref> shows a single laser oscillator 2701 feeding two fiber laser amplifiers 2702 and 2703. The amplified light from both fiber laser amplifiers 2702 and 2703 may enter the same frequency conversion medium 2704. The design of <figref idref="f0014">FIG. 27</figref> may provide extra amplification before the frequency conversion step. <figref idref="f0014">FIG. 28</figref> shows an illuminator including a laser oscillator 2801 feeding two fiber laser amplifiers 2802 and 2803, both amplifying the same or similar frequencies. Each fiber laser amplifier may feed a single frequency conversion medium. The first fiber laser amplifier 2802 may feed one frequency conversion medium 2804, while the second fiber laser amplifier 2803 may feed another frequency conversion medium 2805. Such a design can provide two frequency-doubled beams exhibiting the same or similar wavelengths, and may be employed in an inspection system allowing both dark-field and bright-field inspection with high resolution.

0065<figref idref="f0014">FIG. 29</figref> shows an illuminator having a laser oscillator 2901 feeding two fiber laser amplifiers 2902 and 2903, each amplifying a different wavelength within the gain bandwidths of the laser oscillator. Each fiber laser amplifier may feed its own frequency conversion medium. Fiber laser amplifier 2902 may feed frequency conversion medium 2904, while fiber laser amplifier 2903 may feed frequency conversion medium 2905, providing two frequency-doubled beams with different wavelengths.

0066<figref idref="f0015">FIG. 30</figref> shows an illuminator having two laser oscillators 3001 and 3002, two fiber laser amplifiers 3003 and 3004, and two frequency conversion media 3005 and 3006. Both laser oscillators emit the same or similar wavelengths, and both fiber laser amplifiers amplify the same or similar wavelengths. This configuration provides extra power for the inspection system or enables the inspection system to provide one frequency-doubled beam for a bright-field inspection and another frequency-doubled beam with the same frequency for dark-field inspection.

0067<figref idref="f0015">FIG. 31</figref> shows an illuminator having two laser oscillators 3101 and 3102, two fiber laser amplifiers 3103 and 3104, and two frequency conversion media 3105 and 3106. The two laser oscillators may emit different wavelengths. After the frequency conversion, the illuminator may provide two frequency-doubled beams with different wavelengths. This configuration may provide two wavelengths for high-resolution inspection.

0068Converting the frequency of coherent light can involve generating either a second harmonic, third harmonic, or fourth harmonic, or even higher harmonics. Multiple stages of frequency conversion may be beneficial. For example, the infrared light exiting the fiber laser amplifier might enter a crystal of lithium barium borate or a periodically polled material. The frequency-doubled light exiting this first frequency doubling medium might enter a second frequency doubling medium, such as a crystal of beta barium borate or cesium lithium barium borate. The light exiting the second frequency conversion stage may have a wavelength near that of the wafer stepper or scanner. Fiber laser oscillators offer relatively sufficient pointing stability and emit a beam with a Gaussian profile.

0069Illuminating the wafer may therefore entail mode-locking or q-switching a laser oscillator, amplifying the resultant coherent light with at least one fiber laser amplifiers, and converting the frequency of the coherent light. <figref idref="f0015">FIG. 32</figref> shows a single laser oscillator 3201 feeding a single fiber laser amplifier 3202, which then feeds two frequency conversion media 3203 and 3204. The design may illuminate by mode-locking or q-switching the laser fiber oscillator, amplifying the coherent light with a fiber laser amplifier, splitting the amplified beam into two or more sub-beams, and converting the frequency of each sub-beam.

0070The frequency of each sub-beam may be converted to the same frequency, as shown in <figref idref="f0015">FIG. 32</figref>, or to different frequencies, as shown in <figref idref="f0016">FIG. 33</figref>, where the beam exiting from one frequency conversion medium 3303 undergoes a second stage of frequency conversion in another frequency conversion medium 3305. In either case, the presence of multiple frequency-doubled beams offers unprecedented flexibility to the inspection system.

0071The frequency conversion medium converts only a portion of the incident light. The medium may emit some light with the doubled frequency and some unconverted light, called residual fundamental-frequency light. Amplifying this residual fundamental-frequency light may provide certain benefits, as may providing the amplified residual fundamental-frequency light to another frequency conversion crystal. <figref idref="f0016">FIG. 34</figref> illustrates light from a laser oscillator 3401 entering a fiber laser amplifier 3402. The amplified light (xλ₁) may enter a frequency conversion medium 3403. Frequency-doubled light (λ₂) may exit the frequency conversion medium, as may residual fundamental-frequency light (λ₁). The residual fundamental-frequency light may enter a fiber laser amplifier 3404. Alternatively, the light leaving the fiber laser amplifier 3404 can enter another frequency conversion medium 3405.

0072Thus the present design may include amplifying residual fundamental-frequency light from the frequency conversion process using an additional fiber laser amplifier beyond those provided for in-line illumination purposes. Such a design may be enhanced by converting the frequency of the amplified residual fundamental-frequency light.

0073Frequency of the beam may be tuned by sending the beam through a crystal such as cesium lithium sapphire. If a cesium lithium sapphire crystal receives input light with a wavelength of 266 nm, the crystal can emit light with a wavelength from 266 nm to 320 nm, depending on the angular aspects associated with the crystal. Such a design enables tuning the frequency of frequency-converted light in the design described above.

0074Birefringent crystals can convert the frequency of an incoming beam when the power of the beam exceeds a certain threshold. An amplifier may be provided to increase the power of the beam before frequency conversion. In some cases, hardware for lowering the peak power of the frequency-converted light may be provided to avoid harming the specimen. Such a design may therefore include a temporal shifting medium. A temporal shifting medium spreads out the illumination pulse in time, thereby diminishing its peak power. <figref idref="f0016">FIG. 35A</figref> shows a laser oscillator 3501, a fiber laser amplifier 3502, a frequency conversion medium 3503 and a temporal shifting medium 3504. The temporal shifting medium 3504 may take various forms, including but not limited to a grating, an etalon, one or more beam splitters, or a combination of beam splitters and reflective surfaces such as mirrors. From <figref idref="f0017">FIG. 35B</figref>, a grating 3506 can serve as a temporal shifting medium that effectively stretches the pulse in time. The input pulse 3507 with pulse width T may enter the grating 3506. The grating 3506 may delay one side of the outgoing pulse 3508 by the time required for light to travel the distance 2L, resulting in a lower peak power of the pulse.

0075<figref idref="f0018">FIG. 36</figref> shows how beam splitters 3601 and 3602 and one or more reflective surfaces 3603 can serve as a temporal shifting medium. The beam enters <figref idref="f0018">FIG. 36</figref> from a transmitting device 3604, such as a frequency conversion medium. The beam splitters 3601 and 3602 and reflective surface 3603 cause portions of each pulse to take different paths resulting in a 10-nanosecond delay, or a 20-nanosecond delay, or both (a 30-nanosecond delay). This arrangement divides and prolongs the pulse, thereby diminishing the peak power of the beam. If the delay exceeds the pulse width, the beam splitters can also divide a single pulse into a train of separate pulses. Such a design may be implemented using any number of beam splitters, with or without reflective surfaces.

0076<figref idref="f0019">FIG. 37</figref> shows an etalon 3701 serving as a temporal shifting medium. The etalon 3701 in this design is a block of glass having a relatively high-reflectivity surface 3702 and a relatively low-reflectivity surface 3703. The incident pulse 3704 may strike the low-reflectivity surface 3703, and a portion of the light may reflect from the low-reflectivity surface 3703. The remainder of the light may pass through the low-reflectivity surface 3703, strike the high-reflectivity surface 3702, and reflect outward. The photons bounce back and forth within the etalon 3701 many times, a portion of the photons escaping each time. In this way, the etalon 3701 may divide a single pulse 3704 into a series of reflected pulses 3705 and diminish the peak power of the incident pulse 3704.

0077To provide spatial uniformity in light brightness, the present design may further include a spatial smoothing medium. A spatial smoothing medium may improve the spatial uniformity of the brightness of the frequency-converted light. <figref idref="f0019">FIG. 38</figref> shows a laser oscillator 3801, a fiber laser amplifier 3802, a frequency conversion medium 3503, and a spatial smoothing medium 3804. Various devices may serve as the spatial smoothing medium, including various combinations of diffusers, diffractive optics, lens arrays, light pipes, gratings, etalons, apertures, gradient transmission elements, apodization devices, lenses, and other optical components.

0078<figref idref="f0020">FIGs. 39A and 39B</figref> show two spatial smoothing media, including a homogenizer rod 3901 configured to receive a spatially non-uniform input and transmit a spatially uniform output, and a lens array with a focusing lens. Nonuniform illumination may strike the lens array 3902 from the left in the orientation shown. The lens array 3902 may divide the beam into many sub-beams. The focusing lens 3903 can focus all the sub-beams at an overlap plane 3904 where the illumination is spatially uniform. For simplicity, <figref idref="f0020">FIG. 39B</figref> shows light focusing at only two points at the overlap plane 3904, but the configuration shown may be employed to provide uniform brightness throughout the overlap plane.

0079<figref idref="f0021">FIGs. 40A, 40B, and 40C</figref> show three more examples of spatial smoothing media. In <figref idref="f0021">FIG. 40A</figref>, a moving piece of ground glass or a diffractive optic 4002, downstream from a focusing lens 4001, removes speckle from a laser beam. The term speckle refers to patterns of light and dark over short distance scales caused by interference. In <figref idref="f0021">FIG. 40B</figref>, a diffractive optic 4003 followed by a lens 4004 can provide a substantially spatially uniform brightness on the pupil plane. The spatial smoothing medium of <figref idref="f0021">FIG. 40C</figref> may include a diffractive optic 4006 to shape pupil illumination, a lens 4007, a diffractive optic to shape field illumination 4008, and a focusing lens 4009. Such a design may improve the spatial uniformity of frequency-converted light.

0080Narrowband light may be transformed into broadband light. A narrowband beam of light passing through certain media may undergo Raman scattering and four-wave mixing and exit the medium as a broadband beam known as a supercontinuum. Such a medium is considered a "spectral shifting medium" due to shifting the spectrum of the incoming light. <figref idref="f0022">FIG. 41</figref> illustrates laser oscillator 4101, a fiber laser amplifier 4102, a frequency conversion medium 4103, and a spectral shifting medium 4104.

0081Different substances can serve as the spectral shifting medium. The spectral shifting medium may comprise a photonic crystal fiber, or a gas comprising one or more of the following elements: deuterium, xenon, krypton, argon, neon, helium, fluorine, and/or nitrogen. The spectral shifting medium could also be a plasma, crystal, polymer, or liquid, including but not limited to ethanol or ammonium hydroxide. <figref idref="f0022">FIG. 42</figref> illustrates various spectral shifting media. A photonic crystal fiber 4201 can serve as the spectral shifting medium. A cross section of photonic crystal fiber 4202 shows air channels 4203 within the fused silica matrix 4204. A photonic crystal fiber can serve as an even more effective spectral shifting medium if the air channels are filled with gas 4206, as shown in cross-section 4205. Another spectral shifting medium is a fused silica capillary 4207, wherein gas 4209 fills the hollow core surrounded by the fused silica 4208. <figref idref="f0023">FIG. 43</figref> presents another spectral shifting medium having a cell 4301 containing gas. Narrowband light 4302 enters on the left and broadband light 4303 exits on the right.

0082A supercontinuum may be created by pumping a laser to generate coherent light, amplifying the coherent light using one or more fiber laser amplifiers, and broadening the bandwidth of the coherent light to create a supercontinuum. Frequency converted light and non-frequency-converted light may also be broadened to create a supercontinuum, and multiple beams may be addressed. Mode looking may be employed in the single beam or multiple beam context.

0083The illumination subsystem of the present design may include both an illuminator and an image relay system for relaying the beam into the inspection system. As shown in <figref idref="f0023">FIG. 44</figref>, the illumination subsystem may include a laser oscillator 4401, a fiber laser amplifier 4402, a frequency conversion medium 4403, and an image relay system 4404.

0084<figref idref="f0024">FIG. 45</figref> shows two examples 4501 and 4506 of image relay systems that may lie downstream from the configurations shown in <figref idref="f0020">FIGs. 39</figref> and <figref idref="f0021">40</figref>. Both may deliver light to the pupil plane in the imaging optics 4505 and 4509. Configuration 4501 may employ three lenses 4502, 4503, and 4504, while configuration 4506 may employ two lenses 4507 and 4508. The illuminators disclosed herein may be combined with an image relay system to create an illumination subsystem.

0085The image relay systems shown in <figref idref="f0024">FIG. 45</figref> are generally designed for bright field imaging. The image relay system shown in <figref idref="f0023">FIG. 46</figref> is generally designed for dark field imaging. The three lenses 4601, 4602, and 4603 combine to illuminate a single point in the pupil plane.

Advanced Design Employing CLBO

0086Using CLBO for DUV frequency conversion can provide high frequency conversion efficiency, high damage thresholds, and relatively good stability.

0087High frequency conversion efficiency depends on the, peak power of the laser pulse, the nonlinear coefficient of the crystal, and the interaction length within the crystal. Higher peak powers, larger nonlinear coefficients, and longer interaction lengths produce higher efficiency conversion. High peak powers can be delivered by mode locked fiber lasers as discussed above. CLBO and BBO (Beta Barium Borate, or Beta-BaB₂O₄) crystals are generally suitable for DUV frequency conversion. Both have reasonable nonlinear coefficients, however, BBO is generally larger than CLBO. However, the large walkoff angle in BBO limits the interaction length within the crystal and thus the resulting frequency conversion efficiency. This large walkoff also can create a poor beam profile for high efficiency conversion in BBO.

0088Two frequency conversion crystals may be employed in series to compensate for walkoff effects and greatly improve efficiency where non-critical phase matching is difficult or impossible. However, use of two frequency conversion crystals can also add cost and complexity, especially to the DUV portion of the frequency conversion process. When using CLBO, it is possible to use non-critical phase matching rather than employing multiple frequency conversion crystals. Non-critical phase matching allows the longer interaction lengths because there no walkoff exists. In addition, the spatial beam profile for the frequency converted beam is much better for non-critical phase matching. For these reasons it is desirable to use non-critical phase matching for frequency conversion and especially the final stage of DUV frequency conversion. However, there are only specific wavelength combinations that can produce non-critical or near non-critical phase matching conditions in a particular crystal.

0089The phase matching requirements for a negative uniaxial crystal such as CLBO also require the following conditions be met. <maths id="math0001" num="(1)"><math display="block"><msub><mover><mi>n</mi><mo>‾</mo></mover><mi>e</mi></msub><mfenced><msub><mi>ω</mi><mn>3</mn></msub></mfenced><mo>*</mo><msub><mi>ω</mi><mn>3</mn></msub><mo>=</mo><msub><mi>n</mi><mi>o</mi></msub><mfenced><msub><mi>ω</mi><mn>1</mn></msub></mfenced><mo>*</mo><msub><mi>ω</mi><mn>1</mn></msub><mo>+</mo><msub><mi>n</mi><mi>o</mi></msub><mfenced><msub><mi>ω</mi><mn>2</mn></msub></mfenced><mo>*</mo><msub><mi>ω</mi><mn>2</mn></msub></math><img file="EP1908179B1_D0001.tif" /></maths><maths id="math0002" num="(2)"><math display="block"><mfrac><mn>1</mn><mrow><msub><mi>n</mi><mi>e</mi></msub><mo>⁢</mo><msup><mfenced><mi>θ</mi></mfenced><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfrac><mrow><msup><mi>sin</mi><mn>2</mn></msup><mo>⁢</mo><mi>θ</mi></mrow><msubsup><mover><mi>n</mi><mo>‾</mo></mover><mi>e</mi><mn>2</mn></msubsup></mfrac><mo>+</mo><mfrac><mrow><msup><mi>cos</mi><mn>2</mn></msup><mo>⁢</mo><mi>θ</mi></mrow><msubsup><mi>n</mi><mn>0</mn><mn>2</mn></msubsup></mfrac></math><img file="EP1908179B1_D0002.tif" /></maths> where <i>n̅_e</i> is the index along the extraordinary axis of the crystal and <i>n_o</i>, is the index along the ordinary axis of the crystal. θ is the angle between the propagation vector of the light to be converted and the optical axis of the crystal. If 0 is not 90 degrees, the phase matching condition is called critical phase matching. In this condition, the direction of the Poynting vector and the propagation vector diverge. This walkoff effect decreases spatial overlap as the light propagates through the crystal. Decreased spatial overlap limits the length of the crystal that can be used, and thus the frequency conversion efficiency. When 0 is 90 degrees, the phase matching is said to be non-critical and the Poynting vector and the propagation vector do not generally exhibit walkoff. A θ of 90 degrees allows long crystals to be used for increased frequency conversion efficiency. For non-critical phase matching, Equation (2) simplifies to <i>n_e</i>(90) = <i>n̅_e</i> . Using this result and examining Equation (1), specific mixing frequencies or wavelengths are needed to satisfy this equation and achieve non-critical phase matching. Thus, to determine the non-critical phase matching condition for a crystal, the index values versus wavelength values must be ascertained.

0090For CLBO the index values vs wavelengths are generally described in a paper in the <nplcit id="ncit0003" npl-type="s"><text>Japan Journal of Applied Physics, Vol. 34, pp. 296-298, "Nonlinear Optical Properties of Cesium Lithium Borate", by Yusuke Mori </text></nplcit>et al, which is hereby incorporated by reference. Using these values, the wavelength combinations that will produce non-critical phase matching can be determined.

0091In the case of CLBO with resultant wavelengths less than 213nm, the required mixing wavelengths are not available from standard laser sources. This makes using non-critical phase matching in CLBO at these short wavelengths difficult, especially if an efficient high power laser is desired. Using fiber based lasers to produce the mixing wavelengths enables several solutions to this problem.

0092The fundamental frequency laser generator and the frequency conversion crystals both benefit from having long lifetimes. The fiber laser based fundamental source described above can in most instances have an expected lifetime of more than 50,000 hours, a lifetime generally greater than frequency conversion crystals. Standard frequency conversion crystals used in high power frequency converted lasers have relatively short lifetimes, on the order of 200 hours per crystal site. For generating high powers in the DUV at wavelengths like 266nn, 213nm, 200nm, and 195nm, crystals having long lifetimes are highly beneficial. CLBO is a very good crystal for this application due to its good frequency conversion efficiency and high transparency. However, standard CLBO crystals are subject to long term damage and degradation.

0093Using appropriate crystal growth techniques, surface processing techniques, and handling techniques can improve the lifetime of CLBO crystals. CLBO damage mechanisms can be divided into bulk damage and surface damage. Bulk damage comes from two primary sources, crystallinity defects and impurities inside the crystal.

0094The number of crystallinity defects can be reduced by improving the uniformity of the solution in the crystal growth chamber. This can be challenging because the high viscosity limits mixing. However, adding solution stirring and crucible rotation can dramatically improve the solution uniformity. The primary impurities inside the crystal are residual carbonates and hydroxides. Carbonate impurities can be reduced by preparing the starting materials in water before sintering. The traditional approach seeks to remove water due to the hygroscopic nature of the crystal and the reduction in damage threshold in the final material due to hydroxides. This processing can completely decompose the carboxylates.

0095Hydroxide impurities may be removed by baking the grown crystal for an extended time. Baking at 150 deg C for 120 hours can significantly reduce the number of hydroxide impurities. Surface damage comes from residual optical polishing material embedded in the crystal. This material can be removed by ion milling or other similar technique. About 40-60nm of crystal material may be removed to excise the contaminated crystal material. Using one or more of these techniques in combination can increase the crystal damage threshold significantly.

0096In the field of, for example, semiconductor inspection, relatively high stability is particularly beneficial. Stability desirably occurs over both short time periods and long time periods. For a frequency converted DUV laser, stability over short time periods generally mandates the fundamental laser power and beam profile remain constant. Stability also generally mandates no change in the frequency conversion process, an advantage of using a fiber laser as explained above. For frequency conversion stability, non-critical phase matching can provide certain advantages over other illumination methods and schemes. For example, non-critical phase matching can make maintaining consistent DUV output power easier. Also, for stability over long time periods, a longer crystal lifetime is highly beneficial. Frequency conversion crystals have limited lifetimes when generating light, especially at wavelengths at or below 266nm. As power increases, lifetimes tend to rapidly decrease. Thus use of non-critical phase matching can provide longer crystal lifetimes.

0097Thermal walkoff can be a problem with crystals such as BBO due to absorption of the DUV light causing local heating in the crystal. Local heating changes the walkoff angle within the emanated laser beam. When non-critical phase matching is employed, the walkoff angle is essentially zero degrees and minimal changes occur due to absorption of DUV light in the crystal. Further, the temperature of the crystal can be tuned by using a heater or heating unit to maintain and essentially preserve the non-critical phase matching condition. Heating can be particularly useful when ambient temperatures around the laser fluctuate significantly.

0098The first embodiment of this aspect of the design is based on a fundamental laser source at a wavelength of 995.444nm. This laser is then frequency converted to the 2^nd harmonic and then subsequently to the 4^th harmonic. The 4^th harmonic is then mixed in CLBO using non critical phase matching to obtain a wavelength of 199.7nm. The fundamental laser can be frequency doubled to 499.222nm using standard frequency conversion crystals such as LBO, newly developed crystals such as BiBO (Bismuth Triborate), or periodically polled materials. The frequency conversion method yields best results when efficiency is high while generally maintaining a good beam profile. Use of two frequency conversion crystals in series to compensate for walkoff and thermal walkoff effects can greatly improve efficiency.

0099In operation, the 4^th harmonic at 249.611 can then be produced from the 2^nd using another doubling stage. Production of the 4^th harmonic can employ CLBO or BBO for this stage. The final process mixes the fundamental wavelength at 998.444 with the 4^th harmonic to produce a wavelength of 199.7nm. Mixing wavelengths is done using non-critical phase matching in CLBO. Slight variations from the identified wavelengths are possible by temperature tuning the crystal, where temperature tuning entails possibly monitoring and heating the crystal to maintain a relatively constant temperature over time.

0100Obtaining a high power fundamental laser at 998.444nm can be challenging. One method to produce this fundamental wavelength is to employ a direct diode laser. Currently direct diode lasers at this wavelength have somewhat limited pulsed output powers. Alternately, a fiber laser/amplifier approach may be employed. Three fiber based methods can be used to produce such a fundamental laser. The first approach is to use a fiber laser having a 998.444nm fundamental wavelength. Generation of a laser having this wavelength has recently been demonstrated in a paper by <nplcit id="ncit0004" npl-type="s"><text>Okhotnikov et al. ,"Mode-Locked ytterbium fiber laser tunable in the 980-1070nm spectral range" Optics letters, Vol. 28, No. 17</text></nplcit>, the entirety of which is incorporated herein by reference.

0101The second method uses a frequency doubled Tm (Thulium) fiber laser operating at a wavelength of 1996.88 nm. This laser can be frequency doubled using LBO, BiBO or periodically polled materials. The third method employs a fiber laser with a Raman shifted wavelength of 998.444nm.

0102An example of the first embodiment is illustrated in <figref idref="f0025">FIG. 47</figref> and conforms to the example in the Table below.

First embodiment example

0103<tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="3" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="16mm" /><colspec colnum="2" colname="col2" colwidth="17mm" /><colspec colnum="3" colname="col3" colwidth="25mm" /><tbody><row><entry>1ω</entry><entry align="char" char=".">998.444</entry><entry /></row><row><entry>2ω</entry><entry align="char" char=".">499.222</entry><entry>LBO or BIBO</entry></row><row><entry>4ω</entry><entry align="char" char=".">249.611</entry><entry>CLBO or BBO</entry></row><row><entry>4ω+1ω</entry><entry align="char" char=".">199.7</entry><entry>CLBO</entry></row></tbody></tgroup></table></tables>

0104The second embodiment is based on a fundamental laser source at a wavelength of 1064nm that is Raman shifted to 1193.33nm. The Raman shifted wavelength of the laser is then frequency doubled to the 2^nd harmonic frequency or 2^nd harmonic frequency laser energy level and then subsequently to the 4^th harmonic frequency. The 4^th harmonic may then be mixed in CLBO using non critical phase matching to obtain a wavelength of 194.9nm. The Raman shifted laser can be frequency doubled to 477.332nm using standard frequency conversion crystals such as LBO, BiBO, or periodically polled materials. A highly efficient frequency conversion method is desirable, particularly while maintaining a good laser beam profile. Using two frequency conversion crystals in series to compensate for walkoff and thermal walkoff effects can greatly improve overall efficiency.

0105The 4^th harmonic can be produced at 238.666nm from the 2^nd harmonic using another frequency doubling stage. This frequency doubling stage can employ CLBO or BBO. The final step is to mix the fundamental wavelength at 1064nm with the 4^th harmonic to produce a component having a wavelength of 194.9nm. Mixing occurs using non-critical phase matching in CLBO. Slight variations from these wavelengths are possible by temperature tuning the crystal, again typically getting the crystal to maintain a certain temperature by heating and/or monitoring the crystal. In addition, other fundamental wavelengths near 1064nm and Raman shifts may be employed to produce non-critical phase matching in CLBO below 213nm.

0106An example of the second embodiment is illustrated in <figref idref="f0026">FIG. 48</figref> and conforms to the example in the Table below.

<u>Second embodiment example</u>

0107<tables id="tabl0002" num="0002"><table frame="none"><tgroup cols="4" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="19mm" /><colspec colnum="2" colname="col2" colwidth="25mm" /><colspec colnum="3" colname="col3" colwidth="17mm" /><colspec colnum="4" colname="col4" colwidth="25mm" /><tbody><row><entry>1ω</entry><entry>1064</entry><entry align="char" char="." /><entry /></row><row><entry>1ω</entry><entry>Raman shift to</entry><entry>1193.33</entry><entry /></row><row><entry>2ω</entry><entry>Raman</entry><entry>477.332</entry><entry>LBO or BIBO</entry></row><row><entry>4ω</entry><entry>Raman</entry><entry>238.666</entry><entry>CLBO or BBO</entry></row><row><entry>4ω+1064</entry><entry /><entry>194.9</entry><entry>CLB</entry></row></tbody></tgroup></table></tables>

0108The frequency conversion embodiments disclosed herein may be employed in association with microscopes, photomask repair tools, telecommunication systems, medical diagnosis and treatment systems, and/or laboratory equipment for conducting experiments in the physical or biological sciences.

0109The design presented herein and the specific aspects illustrated are meant not to be limiting, but may include alternate components while still incorporating the teachings and benefits of the invention. While the invention has thus been described in connection with specific embodiments thereof, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within known and customary practice within the art to which the invention pertains.