Abstract:
An F 2  laser having an etalon-based line narrowing output coupler and a technique for tuning the laser. The etalon based output coupler is adjusted to preferentially reflect a percentage of light at or near the spectral maximum of one of the primary F 2  spectral lines and to not reflect light at the other primary F 2  spectral line. Thus, a selected range of the selected line is preferentially amplified in the gain medium and the other line is transmitted out of the laser cavity and, therefore, receives no amplification and is suppressed. The result is substantial narrowing in the preferred embodiment of the 157.630 nm line and effective suppression of the 157.523 nm line. Substantial improvement in line narrowing of 157.630 nm line results from a wavelength selective properties of etalon based line-narrowing output coupler.

Description:
This application is a continuation-in-part application of U.S. Ser. No. 09/217,340, filed Dec. 21, 1998 which is a continuation-in-part of U.S. Ser. No. 08/869,239, filed Jun. 4, 1997, now U.S. Pat. No. 5,856,991; U.S. Ser. No. 08/886,715, filed Jul. 1, 1997; U.S. Ser. No. 08/926,721, filed Sep. 10, 1997, now U.S. Pat. No. 5,852,627; and U.S. Ser. No. 08/987,127, filed Dec. 8, 1997, now U.S. Pat. No. 5,901,163. This invention relates to line narrowed lasers and especially to etalon-based line narrowed lasers. 
    
    
     BACKGROUND OF THE INVENTION 
     TECHNIQUES FOR LINE NARROWING 
     Techniques for decreasing the bandwidth of the output of lasers are well known. Several such techniques used on excimer lasers are discussed by John F. Reintjes at pages 44-50 in  Laser Handbook , Vol. 5, North-Holland Physics Publishing, Elsevier Science Publishers B.V. These techniques include the utilization of gratings, including echelle gratings for wavelength selection. Use of beam expanding prisms ahead of the grating can increase the effectiveness of the grating. 
     A prior art narrow band KrF excimer laser is shown in FIG.  1 . The resonance cavity of excimer laser  2  is formed by output coupler  4  (which is a partially reflecting mirror) and echelle grating  16 . A portion of the laser output beam  20  (having a cross section of about 3 mm in the horizonal direction and 20 mm in the vertical direction) exits the rear of laser chamber  3 . This portion of the beam is expanded in the horizonal direction by prisms  8 ,  10  and  12  and reflected by mirror  14  onto echelle grating  16 . Mirror  14  is pivoted to select the narrow band output for laser  2 . Grating  16  is arranged in Littrow configuration so that the selected narrow band of wavelengths is reflected back off mirror  14  and back through prisms  12 ,  10  and  8  and into chamber  3  for amplification. Light at wavelengths outside the selected narrow band is disbursed so that this disbursed out-of-band light is not reflected back into the laser chamber. Total beam expansion for this laser is about 20X. The beam has a horizontal polarization (P-polarization for the prisms with the actual surface arranged vertically). Typical KrF lasers operating in a pulse mode may have a cavity length of about 1 m and produce pulses having a duration of about 15 to 25 ns. Thus, photons within the resonance cavity will make, on the average, about 3 to 5 round trips within the cavity. On each round trip, about 90 percent of the beam exits at the output coupler and about 10 percent is sent back for further amplification and line narrowing. The beam is repeatedly line narrowed as it passes through the line narrowing module. 
     With this prior art arrangement, the bandwidth of the KrF laser is reduced from its natural bandwidth of about 300 pm (full width half maximum or FWHM) to about 0.8 pm for KrF lasers and about 0.6 pm for ArF lasers. 
     Some applications of KrF lasers, however, require greater narrowing of the bandwidth. There is a need for smaller bandwidths such as FWHM values of 0.5 pm and 0.4 pm for KrF and ArF, respectively. 
     One prior art method is to add an etalon within the resonance cavity. In this case, the etalon is operated in a transmissive mode and the light is additionally line narrowed as it passes through the etalon. In such system one should use a relatively high finesse etalon, with a finesse value ƒ of about 1 or higher which produces sharp fringe patterns. The finesse value ƒ is determined by the equation: 
     
       
         ƒ=π r   ½ /(1− r ) 
       
     
     where r is the reflectivity of the etalon surfaces. The dependence of etalon transmission spectrum on r is shown graphically in FIG. 2 which is extracted from page 298 of  Fundamentals of Optics  by Jenkins and White, published by McGraw Hill. From FIG. 2, it is obvious why prior art transmissive etalons used for line narrowing have surfaces with reflectance of about 50% to 80% (see curves B and C of FIG.  2 ). FIG. 2 also shows that it would not be practical to use curve A-type low finesse etalon as it line-narrowing efficiency in this prior art arrangement would be very low. Prior art high finesse etalons used with diffraction gratings should enhance the line-narrowing capabilities provided by diffraction grating, and in general, improve the laser line-width. The major disadvantages of this technique are that the many reflections within the etalon tend to heat up the etalon producing distortions and that the tuning of the etalon synchronously with the grating does present a real technical challenge and is difficult to accomplish in practice. 
     F 2  gas molecular laser will likely be a successor to KrF and ArF excimer lasers as a production light source for next generation microlithography, enabling printing circuits with features as small as 50-70 nm. It is chosen for that purpose because of the very short wavelength of its output beam, which is about 157 nmn, as compared to currently used KrF and ArF excimer lasers, which produce beams with wavelengths of about 248 nm and 193 nm respectively. Narrowband versions of these lasers are currently used in semiconductor microlithography for printing circuits with smallest features in the range of 250 to 100 nm. The shorter wavelength of F 2  molecular laser permits much more tighter focusing and as a result, smaller features in the range of 50-70 nm can be printed. 
     However, all the available materials transparent at 157 nm have a significant dispersion in that region. Because of that, different portions of the laser beam, having slightly different wavelengths, will be focused by imaging lens onto different spots, so the image blurs. As a result, there is a tight requirement that the bandwidth of F 2  laser should be as small as possible. The estimations of possible lens designs show, that the bandwidth of F 2  laser should be about 0.2 to 0.5 pm at full-width half-maximum level (FWHM). 
     Unfortunately, the free running F 2  laser generates two lines, the stronger line at 157.630 nm and a weaker line at 157.523 nm. The separation between these two lines is about 107 nm which is much larger than the FWHM specification of 0.2 to 0.5 pm. Each line has a bandwidth of about 1.2 pm, which is also significantly higher than the required FWHM. Therefore, in order for F 2  laser to succeed in microlithography, its spectrum has to be line-narrowed. The weaker line should be suppressed as much as feasible, and the stronger line should be line-narrowed by at least ⅓ times. 
     One of the first choice for line-narrowing technique for F 2  laser might be diffraction grating-based technique, described above in FIG. 1, because this technique has been already proven in KrF and ArF excimer lasers to deliver line-narrowed pulses with pulse energy in the range of 5-10 mJ. 
     Unfortunately, when this technique is applied for F 2  laser line-narrowing, the pulse energy output becomes very small. There are a number of reasons for that. Because of line-narrowing requirements it is necessary to use high magnification prism beam expander in F 2  laser just like in KrF or ArF lasers. For example, the beam expansion coefficient for F 2  laser should typically be at least 20X. That requires at least three prisms to be used in beam expander as shown in FIG.  1 . Unfortunately, the absorption losses in prisms and losses from the surfaces of the prisms become very big as compared to KrF or ArF lasers. The limited number of materials which can be used for coatings at 157 nm wavelength greatly limits the quality of any anti-reflection coatings which can be applied to surfaces of the prisms in order to reduce the losses. Very high gain of F 2  laser produces a very strong amplified spontaneous radiation which increases divergence of the beam and broadens its spectrum. In addition to all that, the reflectivity of eschelle diffraction grating is also smaller at 157 nm as compared to 248 nm or 193 nm wavelengths of KrF or ArF lasers. 
     Because of all these factors, the typical pulse energy, which such a line-narrowed laser can produce is much less than 5 mJ and is not enough for microlithography applications. In addition to that the lifetime of anti-reflection coatings on optical components, such as prisms is too low, and correspondingly the cost of operation of such a system would be too high. 
     Therefore, there is a need to develop a more efficient line-narrowing scheme, which would have a higher lifetime of the components and preferably would provide higher pulse energy of line-narrowed light. 
     SUMMARY OF THE INVENTION 
     The present invention provides an F 2  laser having an etalon-based line narrowing output coupler and a technique for tuning the laser. The etalon based output coupler is adjusted to preferentially reflect a percentage of light at or near the spectral maximum of one of the primary F 2  spectral lines and to not reflect light at the other primary F 2  spectral line. Thus, a selected range of the selected line is preferentially amplified in the gain medium and the other line is transmitted out of the laser cavity and, therefore, receives no amplification and is suppressed. The result is substantial narrowing in the preferred embodiment of the 157.630 nm line and effective suppression of the 157.523 nm line. Substantial improvement in line narrowing of 157.630 nm line results from a wavelength selective properties of etalon based line-narrowing output coupler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 . shows a prior art line-narrowed laser. 
     FIG. 2 shows the percent of light transmitted by an etalon as a function of the reflection of the two inner etalon surfaces. 
     FIG. 3 shows a preferred embodiment of the present invention. 
     FIG. 4 shows details of a preferred etalon based output coupler. 
     FIG. 5 shows a technique for tuning the output coupler to reflect light a 157.6299 nm and to transmit light at 157.5233 nm. 
     FIG. 6 shows details of the preferred etalon based output coupler with gas pressure control. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention is shown in FIG.  3 . The resonator of the laser comprises high reflecting mirror  65  and an Fabri-Perot etalon  164 . The Fabri-Perot etalon consists of two prisms  164 A and  164 B made out of CaF 2 . Two sides  166 A and  166 B of the prisms are made parallel to each with very high precision and separated by a gap. The apex angle of each prism is about 33°. The laser beam  102  enters the prism  164 A and is refracted by this prism. The prism  164 A is aligned in such a way, that the portion of the beam  102 , which exits the prism  164 A, exits it at exactly normal incidence angle to the prism surface. The rest of the incoming beam is reflected from the surface  164 A straight back. Transmitted portion of the beam  102  is shown as a beam  103 . On the entrance surface  166 B of the second prism  164 B, part of the beam  103  is again reflected straight back and the rest transmits through the prism  164 B. Because the beam  103  is exactly perpendicular to both surfaces  166 A and  166 B, the reflected beam reflects straight back to the laser. Therefore, the Fabri-Perot etalon  164  serves as an output coupler for the laser  60 . Both prisms are enclosed in a nitrogen atmosphere at a pressure slightly above 1.0 atmosphere as shown in FIG.  4 . 
     Referring now to FIG. 4, the beam  102  enters the first prism  164 A at an angle of about 57 degrees which is close to Brewster&#39;s angle. Therefore, for the beam  102  polarized in the plane of incidence, there will be no reflection from the first surface  165 A of prism  164 A. The apex angle of prism  164 A is about 33 degrees. Therefore, because of refraction of the beam  102  on surface  165 A, the propagation direction of beam  102  is changed, so it will cross the second surface  166 A of prism  164 A at an angle of 90 degrees. The position of the prism is finely aligned so that this angle is exactly 90 degrees. The beam  102  enters the second prism  164 B at exactly the same 90 degree angle with surface  166 B as surfaces  166 A and  166 B are parallel to each other. Beam  102  will have another refraction at the second surface  165 B of prism  164 B, so that the existing beam will make a Brewster&#39;s angle of about 57 degrees with the surface  165 B. The apex angle of prism  164 B is again about 33 degrees. None of the surfaces of prisms  164 A and  164 B has any coatings. Windows  98  and  99  are positioned at an angle close to Brewster&#39;s angle if no reflection is desired from them. Or if a window is used to sample a portion of the beam  102 , the angle could be a little less than Brewster&#39;s angle. Therefore, the preferred embodiment has no coatings on any of its surfaces. 
     Each of the uncoated inside parallel surfaces of the prisms has a Fresnel reflectivity of about 4.7 percent for normal incident 157 nm light. These surfaces reflect a portion of beam 102 back to the laser. 
     More generally where the prism material has an index of refraction n m  and the adjacent gas has an index of refraction n a , the apex angle ∝ of the prism is such that                sin   ∝     =       1       1   +         (     n   m     )     2     /       (     n   a     )     2             .             (   1   )                                
     If the prism material is CaF 2  with n m =1.56 and the surrounding gas is nitrogen at about one atmosphere with an n a =1.0003, then ∝=32.67 degrees. 
     Tunable Etalon Output Coupler As Line-Selecting and Line-Narrowing Device 
     In the preferred embodiment, the reflectivity from each surface of  166 A and  166 B is about 4.7% as determined by Fresnel reflection from uncoated CaF 2  material with a refractive index of n m ≅1.56. Because of interference of two beams reflected from surfaces  166 A and  166 B correspondingly, the total reflectivity has a close to sinusoidal dependence on the laser wavelength as described by equation:              R   =       4   ·   r   ·       sin   2          [         2      π       λ   t       ·     n   a     ·   d     ]               (     1   -   r     )     2     +     4   ·   r   ·       sin   2          [         2      π       λ   l       ·     n   a     ·   d     ]                     (   2   )                                
     where r is reflection from surfaces  166 A and  166 B equal to 0.047, n a  is refractive index of nitrogen, d is the distance between surfaces  166 A and  166 B and λ 1  is the laser wavelength. This dependence is shown in FIG.  5 . 
     The maximum reflectivity is about 17% and the minimum reflectivity is about 0%. The distance between reflection peaks λ FSR  usually referred to as a free spectral range of the etalon is determined by its gap d, refractive index of nitrogen N a  and laser wavelength λ 1 :                λ   FSR     =       λ   l   2       2   ·   d   ·     n   a                 (   3   )                                
     The spectrum of F 2  laser is also shown in FIG.  5 . There are two peaks, each of them has FWHM of about 1.1 pm and they are separated by Δλ=106.6 nm. The effects of line-selection of the strong line and its simultaneous line-narrowing are achieved in the present embodiment by aligning the etalon output coupler such that one of its peaks lies right at the center of the strong line, and one of its minimums lies right at the center of the weak line. This can be done, if 
     
       
         Δλ=( N+l/ 2)· λ   FSR   (4) 
       
     
     where N is 1, 2, 3 . . . 
     For example, if λ FSR  =1.04 pm. (and Δλ=106.6 nm) N=102. Using formula (3) and N a =1.0004 for pressure about 30% above atmosphere, we obtain that d=11.941 mm. Fine tuning of EOC can be done by adjusting nitrogen pressure. 
     Tuning of EOC 
     Tuning of EOC can be done by changing pressure in chamber  92  (FIG. 4) using inject value  94  and release valve  95 . These valves are controlled by drivers  96  and  97  correspondingly. Signals to the drivers are sent by laser onboard computer (not shown). 
     In one embodiment, the He is used instead of nitrogen, and pressure in the chamber  92  is maintained to the required value. This value is determined at the factory as the most optimum for the output power and spectrum bandwidth. Preferably the percentage of light reflected at the chosen spectral line is between 10 and 17 percent and the light reflected at the other is about 0. The 17 percent reflectivity at the chosen line is obtained when no coatings are used on etalon surfaces. The lower reflectivity can be obtained by applying coatings to etalon surfaces  166 A and  166 B (FIG. 4) in order to reduce their reflectivity to below that of uncoated surfaces. 
     Control of the pressure for tuning the EOC can be accomplished using a feedback arrangement based on actual wavelength measurements in the manner described in U.S. Pat. No. 5,856,991 issued Jan. 5, 1999 entitled “Very Narrow Band Laser”. That patent is incorporated herein by reference. 
     A simpler alternate tuning method is to control the pressure of the etalon to a selected absolute pressure in order to keep the etalon adjusted to reflect at a selected wavelength range within the selected F 2  line and to transmit at the wavelength of the other line. This technique is possible because the F 2  laser of the present invention (unlike prior at KrF and ArF lasers like those described in the &#39;991 patent) can be operated at a narrow band without the use of grating based line narrowing unit which itself must be tuned. 
     Since both lines of the F 2  laser are absolute values, they do not change due to influences such as beam direction in the laser cavity or temperature of laser components. The reflectivity of the etalon (assuming a fixed spacing between the prisms and constant temperature of the etalon gas) are values which depend only on the etalon pressure. Therefore, the etalon can be calibrated in the factory to determine the proper etalon pressure to provide the desired reflectance at the two F 2  lines. Then a feedback control can be provided which utilizes an absolute pressure transducer reading etalon pressure in order to keep the etalon gas pressure at the desired pressure. If helium is the etalon gas the index of refraction for the etalon gap is:                n   He     =     1   +     4   ×     10     -   5              P   He       14.5                 psi                   (   5   )                                
     where P He  is the absolute helium pressure in the etalon. 
     According to equation 2, the peaks and minimums of the reflection curve shown in FIG. 5 shifts in proportion to n He ; thus the etalon is appropriately sensitive to pressure changes. For example, 1 psi change in absolute pressure will shift the reflectance curve by about 0.4 pm. Absolute pressure transducers are readily available for reading pressures to an accuracy of about 0.02 psi; thus a feedback control using such a transducer could easily control the etalon reflectance to an accuracy of better than 0.01 pm. These absolute pressure transducers are available from suppliers such as Druck, Inc. with offices in New Fairfield, Conn. An acceptable unit would be their Model No. PMP 4000. 
     Preferably the etalon temperature is regulated to an approximately constant temperature, but an alternative is to measure the temperature and to correct for temperature varation. This is done in accordance with the following formula.                n   He     =     1   +       (     4   ×     10     -   5              P   He       14.5                 psi         )          (         T   STD          (   K   )           T   He          (   K   )         )                 (   6   )                                
     where T STD (K) is a reference temperature in degrees Kelvin and T He (K) is the helium gas temperature in degrees Kelvin. In this case the constant distance between the surfaces  166 A and  166 B (FIG. 4) can be maintained by using spacers made out of low expansion material, such as fused silica or ULE glass. 
     Although this very narrow band laser has been described with a reference to a particular embodiment, it is to be appreciated that various adaptations and modifications may be made. For example, in addition, to pressure-tuned etalons and piezoelectric-tuned etalons, there are commercially available etalons which are compression-tuned using mechanical force to widen or narrow the gap between the plates of the etalon. Etalons with surface reflectances other than 4.7% could be used. Preferably, however, the reflectance of the reflecting surfaces should be between about 1 and about 20%. This corresponds to a range of 4% to 55% in the total etalon reflection according to equation 2. Also, a different apex angle of prisms  164 A and  164 B can be used. If this apex angle is different from 33 degrees, however, the outside surfaces  165 A and  165 B of prisms  164 A and  164 B should have an anti-reflection coating to reduce reflection losses from these surfaces. Particularly, the apex angle can be made as small as 0 degrees, at which point prisms  164 A and  164 B become flats. It is recommended, however, that the minimum apex angle to be at least 10 arcmin in order to reduce an effect of reflections from these surfaces on laser operation. Therefore, the invention is only to be limited by the appended claims.