Abstract:
A time delay is introduced in the optical path of the light pulse at fundamental wavelength relative to that for the fourth harmonic light pulse in a set up for generating the 5 th  harmonic, to compensate for at least a portion of the time delay of the fourth harmonic relative to the fundamental wavelength caused by 4HG generation. In one embodiment, this is achieved by introducing a time delay of the fundamental relative to the second harmonic wavelength, such as preferably by means of a timing compensator in the optical paths of the second harmonic and the fundamental wavelength. Preferably, any further delay of the fourth harmonic relative to the fundamental wavelength caused by other optical components can also be compensated for in this manner.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates in general to laser light generation, and in particular to efficient pulse laser light generation of higher harmonics from light at a fundamental wavelength. 
         [0002]    For many optical instruments, it is important to use light of the desired wavelengths, such as in telecommunication, and in semiconductor equipment. In recent years, the generation of light at smaller wavelengths, such as ultraviolet light, is desirable for different types of semiconductor equipment. For example, in order to reduce the size of transistors in semiconductors, it is desirable to use light of smaller wavelengths to improve resolution in photolithography. For discovering tiny defects in semiconductor devices during or after manufacture, it is desirable to use light of smaller wavelengths to improve resolution in anomaly detection. 
         [0003]    One common technique for generating light at smaller wavelengths is to pass light from a light source such as a laser through a non-linear crystal, which combines photons from the laser to form higher harmonics photons of higher energy, and hence smaller wavelengths. One such scheme generates light of the fifth harmonic (Fifth Harmonic Generation or 5HG). In this application, “HG” stands for “Harmonic Generation.” 
         [0004]    In a typical 5HG setup, 3 crystals are set up in line, with perhaps some focusing optics in between, as shown in  FIG. 1 . As shown in  FIG. 1 , a laser source (not shown) supplies a light pulse  12  at fundamental wavelength of 1064 nm to the Second Harmonic Generation crystal (SHG)  14 . The vertical double-sided arrow  12 ′ illustrates that the polarization of the pulse  12  supplied by the laser source is in the plane of the paper. SHG  14  passes light pulse  12  at fundamental wavelength of 1064 nm without changing its polarization and generates a second harmonic light pulse  16  at 532 nm with polarization  16 ′ orthogonal to the plane of the paper, as illustrated by the arrow pointing out of the paper. The relative temporal positions of the pulses  12  and  16  after SHG  14  are also illustrated by the positions of arrows  12 ′ and  16 ′ in their respective optical paths in  FIG. 1 . SHG  14  introduces only a small time delay to second harmonic light pulse  16  at 532 nm relative to the light pulse  12  at fundamental wavelength passed by crystal  14 , and the relative temporal positions of the two pulses outputted by crystal  14  are as illustrated by the points at the arrows  12 ′ and  16 ′ in  FIG. 1 . The same convention as noted above for pulses  12  and  16  is used in all of the figures of this application to illustrate polarization states and the relative temporal positions of the pulses in their respective optical paths. The Fourth Harmonic Generation or 4HG crystal  20 , however, introduces a significant time delay to fourth harmonic light pulse  22  at 266 nm relative to the light pulse  24  at fundamental wavelength passed by crystal  20 , and the polarizations and relative positions  22 ′ and  24 ′ of the two pulses outputted by crystal  20  are illustrated in  FIG. 1 . The second harmonic light pulse at 532 nm also passed by crystal  20  may be sent to a beam dump (not shown). Thus when the light pulses  22  and  24  reach the 5HG crystal  26 , they may overlap for only a short time period, or no longer overlap at all, so that the fifth harmonic pulse  28  at 213 nm is diminished in intensity or fails to be generated at all. 
         [0005]    In conventional schemes, the time delay to fourth harmonic light pulse  22  at 266 nm relative to the light pulse  24  at fundamental wavelength passed by crystal  20  is compensated by means of mirrors  32  to alter the relative optical path lengths experienced by the two pulses, as shown in  FIG. 2 , illustrating another conventional setup. However, the set up of  FIG. 2  is complicated and bulky. It is therefore desirable to provide an improved optical design whereby the above disadvantages of prior designs are avoided. 
       SUMMARY 
       [0006]    The problem above of the time delay of the fourth harmonic relative to the fundamental wavelength can be solved by introducing a time delay in the optical path of the light pulse at fundamental wavelength relative to that for the fourth harmonic light pulse, to compensate for at least a portion of the above explained time delay of the fourth harmonic relative to the fundamental wavelength. In one embodiment, this is achieved by introducing a time delay of the second harmonic relative to the fundamental wavelength, such as preferably by means of a timing compensator in the optical paths of the second harmonic and the fundamental wavelength. Preferably, any further delay of the fourth harmonic relative to the fundamental wavelength caused by other optical components can also be compensated for in this manner. 
         [0007]    In one implementation of the embodiment mentioned above, a laser light generating apparatus comprises a laser source emitting optical pulses at a fundamental wavelength λ 1 , and a first nonlinear crystal receiving the optical pulses at fundamental λ 1  and generates second harmonic optical pulses at wavelength λ 2 , where λ 2  is substantially equal to half of λ 1 . A second nonlinear crystal receives the optical pulses at wavelengths λ 1  and λ 2  and generates fourth harmonic optical pulses at wavelength λ 4  where λ 4  is substantially equal to half of λ 2 . The first and second nonlinear crystals cause a time delay of the optical pulses at wavelength λ 4  relative to the optical pulses at wavelength λ 1 . A third nonlinear crystal receives the optical pulses at wavelengths λ 1  and λ 4  and generates a fifth harmonic pulse λ 5  where frequency of the fifth harmonic pulse λ 5  is substantially equal to the sum of the frequencies of the optical pulses at wavelengths λ 1  and λ 4 . A birefringent crystal is placed between the first and second nonlinear crystals and receives the optical pulses at wavelengths λ 1  and λ 2 , wherein the optical pulses at wavelength λ 1  travel at a slower speed in the birefringent crystal than the optical pulses at wavelength λ 2 , to compensate for at least a portion of the time delay between the optical pulses at wavelength λ 4  relative to the optical pulses at wavelength λ 1 . 
         [0008]    In another implementation of the embodiment mentioned above, a method for higher harmonic light generation comprises supplying optical pulses at a fundamental wavelength λ 1  to a first nonlinear crystal so that the first nonlinear crystal generates second harmonic optical pulses at wavelength λ 2 , where λ 2  is substantially equal to half of λ 1 ; supplying the optical pulses at wavelengths λ 1  and λ 2  to a second nonlinear crystal so that the second nonlinear crystal generates fourth harmonic optical pulses at wavelength λ 4  where λ 4  is substantially equal to half of λ 2 . The first and second nonlinear crystals cause a first time delay of the optical pulses at wavelengths λ 4  relative to the optical pulses at wavelengths λ 1 . A second time delay of the optical pulses at wavelengths λ 1  relative to the optical pulses at wavelength λ 2  is caused before the optical pulses at wavelengths λ 1  and λ 2  reach the second nonlinear crystal, so that the second time delay compensates for at least a part of and reduces the first time delay. 
         [0009]    The above technique may be used for supplying light to a sample, such as in the case of photolithography or defect inspection in the semiconductor industry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic view of a conventional “in-line” 5 th  harmonic generation setup. 
           [0011]      FIG. 2  is a schematic view of a conventional “split-and-combine” 5 th  harmonic generation setup. 
           [0012]      FIG. 3  is a schematic view to illustrate the effect of the time delay of the higher order harmonics relative to the pulse at fundamental wavelength to illustrate the operation of a realistic implementation of the conventional “in-line” 5 th  harmonic generation setup of  FIG. 1 . 
           [0013]      FIG. 4  is a schematic view of an “In-line” 5HG configuration with timing slip-off compensation to illustrate the concept of an embodiment of the invention. 
           [0014]      FIG. 5  is a schematic view of an in-line 5 th  harmonic generation setup with timing compensation to illustrate an implementation of the configuration of  FIG. 4 . 
           [0015]      FIG. 6  is a schematic view of an in-line 5 th  harmonic generation setup with timing compensation to illustrate another implementation of the configuration of  FIG. 4 . 
           [0016]      FIG. 7  is a schematic view of an optical instrument for supplying light to a sample using an in-line 5 th  harmonic generation setup with timing compensation. 
       
    
    
       [0017]    For convenience in description, identical components are labeled by the same numbers in this application. 
       DETAILED DESCRIPTION 
       [0018]    A significant advantage of the “in-line” configuration of  FIG. 1  is the simplicity, as opposed to another conventional setup shown in  FIG. 2 . As explained above and depicted in  FIG. 1 , the fundamental and 4 th  harmonic pulses do not meet in the 5HG crystal  26 . The typical length of the crystals is of the order of centimeters, and the total thickness of the lenses used in the optical set up is also of the order of centimeters, and usually fused silica is used as the material. The group indices, which dictate the arrival time of the pulse at each wavelength, are listed in Table 1 below. As the group index is always smaller for the pulse at fundamental wavelength, the pulse at fundamental always advance with respect to other pulses. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Group indices of material used 
               
             
          
           
               
                   
                   
                   
                 Index 
               
               
                   
                 Group 
                   
                 difference from 
               
               
                   
                 Index 
                   
                 fundamental 
               
               
                   
                   
               
             
          
           
               
                 CLBO 4HG (9 = 62 deg.) 
                   
                   
                   
               
               
                 Fundamental (1064 nm) 
                 1.45568 
                 e~ray 
               
               
                 2nd harmonic (532 nm) 
                 1.47994 
                 o~ray 
                 Δn 2  = 0.06977 
               
               
                 4th harmonic (266 nm) 
                 1.62414 
                 e-ray 
                 Δn 4  = 0.16846 
               
               
                 CLBO 5HG (( ) = 68.4 deg.) 
               
               
                 Fundamental (1064 nm) 
                 1.49911 
                 o-ray 
               
               
                 4th harmonic (266 nm) 
                 1.69037 
                 o-ray 
                 Δn 5  = 0.19126 
               
               
                 fused silica 
               
               
                 Fundamental (1064 nm) 
                 1.4624 
                 o-ray 
               
               
                 4th harmonic (532 nm) 
                 1.48534 
                 o-ray 
                 Δns 2  = 0.0229 
               
               
                 4th harmonic (266 nm) 
                 1.61468 
                   
                 Δns 4  = 0.15228 
               
               
                   
               
             
          
         
       
     
         [0019]    CLBO in the table above stands for cesium lithium borate CsLiB 6 O 10 . We shall now estimate the difference in time of arrival of the light pulses at fundamental and 4 th  harmonic at the center of the 5HG crystal, in reference to  FIG. 3 . For the sake of argument, we shall ignore the dispersion of air, but can be taken into account later if necessary. 
         [0020]    As depicted in  FIG. 3 , L 1 , L 2  represents the total thickness of the fused silica (glass) between SHG&amp;4HG crystals  14  and  20  and that between 4HG&amp;5HG crystals  20  and  26 . They are the total sum of the thickness of the optics in the range, such as lenses or windows. L 4  and L 5  are the lengths of the 4HG crystal  20  and 5HG crystal  26 . 
         [0021]    We shall ignore the group velocity dispersion in the SHG crystal, as it is small, (Group velocity difference about 0.01.) Now, as the pulses enters the fused silica of length L 1 , the pulses at fundamental and 2 nd  harmonic are synchronous. Because of the difference in group velocity, at the exit of L 1 -long fused silica, and hence at the entrance of 4HG crystal, the time of arrival of the pulses are different by Δns 2 L 1 /c. (Where c is the speed of light in vacuum.) 
         [0022]    Likewise, at the center of 4HG crystal, they are different by Δn 2 L 4 /(2 c). 
         [0023]    From the center of 4HG crystal, we shall consider the difference in time of arrival between the fundamental and the 4th harmonic. From the center of 4HG crystal to the exit face of 4HG crystal, the difference is Δn 4 L 4 /(2 c), the delay caused by the L 2 -long fused silica is Δns 4 L 2 /c, and the 5HG center from the entrance to the center is Δn 5 L 5 /(2 c). 
         [0000]    
       
         
           
             
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         [0024]    If we take an example of typical values, L 1 =L 2 =10 mm, L 4 =15 mm, and L 5 =10 mm, the total delay is approximately 15 ps. If the pulsewidth is of the order of 10 ps, such delay would be more than sufficient to completely displace the fundamental pulses from the 4 th  harmonic, making the 5HG impractical. 
         [0025]    The present invention alleviates this problem, without having to split the beam paths between the fundamental and 4 th  harmonic in the configuration shown in  FIG. 4 , thus keeping the system simple. The inventors have identified a material in which the pulse at the fundamental travels at a slower speed than the second harmonic pulse. In one embodiment, this material includes barium borate BBO. As illustrated in  FIG. 4 , a BBO compensator  50  causes a delay of the pulse  12  from laser  11  at the fundamental (1064 nm) at position  24 ″ relative to the second harmonic pulse (532 nm) at position  22 ″, where the relative positions of the two pulses are as shown in  FIG. 4 . The 4HG CLBO  20  introduces a delay to the 4 th  harmonic (266 nm) pulse at position  36 ″ relative to the pulse at fundamental (1064 nm) at position  38 ″ upon exiting the 4HG CLBO  20  as shown in  FIG. 4 . However, due to the effect of the BBO compensator  50 , the 4 th  harmonic (266 nm) pulse still arrives at the 5HG CLBO  26  earlier than the pulse at fundamental (1064 nm). The pulse at fundamental (1064 nm) finally catches up with the 4 th  harmonic (266 nm) pulse upon reaching the center of the 5HG CLBO  26 , so that the fundamental and the 4th harmonic (266 nm) pulses overlap fully within the 5HG CLBO  26 , to generate the 5 th  harmonic pulse  52  at 213 nm. The frequency of the 5 th  harmonic pulse  52  is substantially the sum of the frequencies of the pulse at fundamental and of the second harmonic pulse. 
         [0026]    Barium borate, a negative uniaxial crystal, either α- or β-phase, has the property needed for the application of the present invention. Owing to large birefringence, barium borate has the group indices summarized in Table 2. Laser  11  may be a modelocked Nd:YAG or modelocked Nd:YVO 4  laser. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Group indices of BBO 
               
             
          
           
               
                   
                   
                 Group 
                   
                 Index difference from 
               
               
                   
                 BBO (⊖ = 90 deg.) 
                 index 
                   
                 fundamental 
               
               
                   
                   
               
               
                   
                 Fundamental 
                 1.67387 
                 o-ray 
                   
               
               
                   
                 2nd harmonic 
                 1.58883 
                 e--ray 
                 Δnc 2  = −0.08504 
               
               
                   
                 4th harmonic 
                 1.79684 
                 e--ray 
                 Δnc 4  = 0.12297 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    As evidently seen in Table 2, the only combination that allows delaying fundamental relative to harmonics is to have the fundamental wavelength in o-ray and the 2 nd  harmonic in e-ray to delay the fundamental with respect to the second harmonic. In order to compensate the 15-ps time difference, the thickness of the material needed is Δt c/Δnc 2 =52.9 mm. Thus, the above combination as an implementation of the scheme of  FIG. 4  will ensure that optical pulses with short durations (such as durations shorter than 100 ps) will overlap in the 4HG for generating the 5 th  harmonic. 
         [0028]      FIG. 5  is a schematic view of an in-line 5 th  harmonic generation setup  100  with timing compensation to illustrate an implementation of the configuration of  FIG. 4 , including the lenses that focus the rays to the elements  14 ,  50 ,  20  and  26 . 
         [0029]    The lenses that focus the rays to the elements  14 ,  50 ,  20  and  26  also cause the 4 th  harmonic pulses to be delayed relative to the pulses at fundamental wavelength. This time delay may be taken into account in choosing the design and thickness of the material in compensator  50 . Therefore the configuration as in  FIG. 5  would properly compensate for the difference in time of arrival of the pulses at fundamental and 4 th  harmonic, including that caused by the relative time delay between the fundamental and the 4 th  harmonic caused by the lenses, and hence allows efficient generation of 5 th  harmonic. 
         [0030]      FIG. 6  is a schematic view of an in-line 5 th  harmonic generation setup  200  with timing compensation to illustrate another implementation of the configuration of  FIG. 4 . The optical set up in  FIG. 6  includes a lens for focusing towards the 4HG  20 , an optical resonator for resonating the second harmonic together with an actuator for maintaining resonance of second harmonic and other mirrors for reflecting the pulses at fundamental and 4 th  harmonic wavelengths to the 5HG  26 . A technique for efficient frequency doubling of a modelocked laser using an optical resonator is described in the article “High-power second-harmonic generation with picosecond and hundreds-of-picosecond pulses of a cw mode-locked Ti:sapphire laser,” M. Watanabe, R. Ohmukai, K. Hayasaka, H. Imajo, and S. Urabe, Optics Letters 19, 637-639 (1994). This reference is incorporated herein by reference in its entirety so that the technique need not be described in detail here. The setup in this article is adapted for use in  FIG. 6  for 4HG generation, where the reflectors used in the setup have been modified where necessary as specified in  FIG. 6  for 4HG generation. 
         [0031]      FIG. 7  is a schematic view of an optical instrument for supplying light to a sample using an in-line 5 th  harmonic generation setup with timing compensation. The instrument  300  may be a piece of semiconductor equipment. For example, in order to reduce the size of transistors in semiconductors, it is desirable to use light of smaller wavelengths to improve resolution in photolithography, and instrument  300  may be an equipment used in photolithography, such as a stepper. For discovering tiny defects in semiconductor devices during or after manufacture, it is desirable to use light of smaller wavelengths to improve resolution in anomaly detection, and instrument  300  may be an equipment used in anomaly detection, such as for wafer or reticle inspection. The instrument  300  may also be an optical equipment for measuring properties of samples, such as their thickness, refractive index, critical dimension, height and profile of gratings, such as reflectometers and ellipsometers. Preferably, instrument  300  includes an apparatus such as setup  100  of  FIG. 5  or setup  200  of  FIG. 6  for generating the 5 th  harmonic. The instrument  300  may apply the 5 th  harmonic pulses in a direction normal to the surface of sample  400  along path  302 , or along an oblique path  304 , as shown in  FIG. 7 . 
         [0032]    While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their equivalent. All references referred to herein are incorporated by reference herein in their entireties.