Abstract:
A molecular fluorine (F 2 ) laser is provided wherein the gas mixture includes molecular fluorine for generating an emission spectrum including two or three closely spaced lines around 157 nm. An optical method and means are provided for selecting an emission line from among the plurality of closely spaced emission lines of the molecular fluorine laser gas volume and broadening the spectrum of said selected emission line. This approach of broadening the spectrum reduces the coherence length of the output beam. As a result, speckle may be reduced or avoided in microlithography applications.

Description:
PRIORITY 
     This Application claims the benefit of U.S. Provisional Application No. 60/121,350, which provisional application was filed Feb. 24, 1999 and is hereby incorporated by reference in its entirety. 
    
    
     RELATED U.S. APPLICATIONS 
     The subject matter of this Application is related in part to that of the following four U.S. patent applications Ser. No. 09/090,989, filed Jun. 4, 1998; Ser. No. 09/136,353, filed Aug. 18, 1998; Ser. No. 09/317,527, filed May 24, 1999; and Ser. No. 09/317,695, filed May 24, 1999. Said applications relate generally to molecular fluorine lasers with single line selection. U.S. application Ser. No. 09/317,527 also relates generally to line narrowing of the selected emission line and is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a means and method of broadening the spectrum of a single line output beam of a molecular fluorine laser and thereby decreasing the beam&#39;s temporal coherence length. 
     BACKGROUND OF THE INVENTION 
     Line-narrowed excimer lasers used for microlithography must provide output beams having certain spectral and temporal coherence properties according to the requirements of lithographic imaging systems. The design of the projection optics sets an upper limit to the spectral linewidth of the excimer laser. The design of the illumination optics determines the upper limit of the coherence length and thus the lower limit of the spectral bandwidth of the excimer laser. When the coherence length is longer than a specified minimal length (i.e., the spectral bandwidth is narrower than allowed by the design of the illumination optics), the image on the wafer shows random modulation due to the effect of speckle. 
     Narrowing of the linewidth of excimer lasers is achieved most commonly through the use of a wavelength selector consisting of prisms and diffraction gratings. In a molecular fluorine laser operating at the wavelength of approximately 157 nm, use of reflective diffraction gratings is limited due to their low reflectivity and the high oscillation threshold inherent with this type of laser. 
     However, F 2  excimer lasers emit light at two or three or more spectral lines, where each line is relatively narrow (on the order of picometers). The present invention provides a means of achieving upper and lower limits on spectral bandwidth output by first, selecting one of these lines and, second, increasing the spectral bandwidth of the selected single line by appropriate means. The increase in the spectral bandwidth is equivalent to a decrease of temporal coherence, thus reducing speckle effects. 
     BACKGROUND ART 
     U.S. Pat. No. 5,856,991 describes the use of an optical etalon as an outcoupler, which etalon serves at the same time to line-narrow KrF or ArF excimer lasers. U.S. Pat. 4,977,563 describes the use of a pressure-tuned optical etalon. V. N. Ischenko, S. A. Kochubei, A. M. Razhev, “High-power efficient vacuum ultraviolet laser excited by an electric discharge”, Soviet Journal of Quantum Electronics, v.16, pp.707-709, 1986 show molecular fluorescence spectra of fluorine. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to select an emission line of a molecular fluorine laser and to provide a means and method of broadening the spectrum of the single output beam line and thereby decrease the laser beam&#39;s temporal coherence length. 
     The detailed optical means and method for doing this are described below in relation to each of the various embodiments of the invention. 
     The spectral bandwidth of a selected emission line is dependent in part on the laser chamber pressure and the reflectivity of the outcoupling mirror, if present. Accordingly, the embodiments below function better when the laser chamber pressure is relatively high and the reflectivity of any outcoupling mirror is also relatively high. As our experience confirms, the spectral bandwidth of the selected laser line increases with increasing chamber pressure; therefore, for all embodiments of the present invention, the laser chamber pressure should preferably be above 2.5 bar. 
     From the fluorescence spectra of molecular fluorine F 2  published elsewhere (see e.g. V. N. Ischenko, S. A. Kochubei, A. M. Razhev, “High-power efficient vacuum ultraviolet laser excited by an electric discharge”, Soviet Journal of Quantum Electronics, v. 16, pp.707-709, 1986), it is known that the gain curve for a single line is rather broad in spectral width. Conventionally, excimer lasers employ low reflectivity outcoupling mirrors (reflectivity &lt;8%) because of the high gain of the excimer. This is also the case for the F 2  excimer laser. Laser oscillations can only occur in the part of the spectral gain curve in which the gain exceeds the losses. Therefore, increasing the reflectivity of the outcoupling mirror will lead to a broader spectral bandwidth for the single line F 2  laser output. Accordingly, for the embodiments of the present invention which have an outcoupling mirror, the reflectivity of the outcoupling mirror should preferably be above 8%. With proper selection of the outcoupler reflectivity and the laser chamber pressure, the spectral bandwidth and therefore the coherence length of the output can be adjusted to meet the requirements of microlithography. 
     In the first embodiment, line selection is achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using a high reflectivity outcoupling mirror (reflectivity R between 8% and 90%) and a gas pressure in the laser chamber above 2.5 bar. 
     In the second preferred embodiment, line selection is again achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using a high reflectivity mirror at the other side of the cavity (reflectivity R between 90% and 100%) and a gas pressure in the laser chamber above 2.5 bar. Outcoupling of the laser beam is achieved by reflection at one surface of the prism. The design of the prism, i.e., the angles of the prism as well as the coatings on its surface, can be used to vary the outcoupling ratio. 
     In the third embodiment, line selection is also achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using an etalon as an outcoupling mirror which is tuned antiresonant to the selected single laser line. Gas pressure in the laser chamber is again above 2.5 bar. 
     In the fourth preferred embodiment, line selection is again achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using an etalon as a reflecting mirror which is tuned antiresonant to the selected single laser line. Gas pressure in the laser chamber is again above 2.5 bar. Outcoupling of the laser beam is achieved by reflection at the one surface of the prism. 
     In the fifth embodiment, an etalon provides line selection and, therefore, the use of a prism is not necessary. In addition, the etalon acts as an antiresonant outcoupling mirror which suppresses the center of the selected single laser line. The advantage over the first four embodiments is simplicity and a reduction of elements in the resonator which leads to reduced optical losses and increased lifetime. However, the suppression of the second line may be less efficient. 
     In the sixth embodiment, line selection is achieved through the use of a high reflectivity etalon at one side of the resonator. Further line shaping, i.e., spectral broadening of the line, is achieved by using a second etalon as an outcoupling mirror which is tuned antiresonant to the selected single laser line. Gas pressure in the laser chamber is again above 2.5 bar. 
     In the seventh embodiment, line selection is achieved through the use of a high reflectivity etalon at one side of the resonator. Further line shaping, i.e., spectral broadening of the line, is achieved by using a higher reflectivity outcoupling mirror (reflectivity R between 8% and 90%) and a gas pressure in the laser chamber above 2.5 bar. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a first preferred embodiment of the present invention. 
     FIG. 2 shows a second preferred embodiment of the present invention. 
     FIG. 3 shows a third preferred embodiment of the present invention. 
     FIG. 4 shows a fourth preferred embodiment of the present invention. 
     FIG. 5 shows a fifth preferred embodiment of the present invention. 
     FIG. 6 shows a sixth preferred embodiment of the present invention. 
     FIG. 7 shows a seventh preferred embodiment of the present invention. 
     FIG. 8 a  shows a free-running output emission spectrum of an F 2 -laser with helium as a buffer gas. 
     FIG. 8 b  shows a free-running output emission spectrum of an F 2 -laser with neon as a buffer gas. 
     FIG. 9 a  shows a further preferred embodiment of the present invention. 
     FIG. 9 b  shows a still further preferred embodiment of the present invention. 
     FIG. 10 a  shows the gain/loss spectrum of a single selected line of an F-2 laser as a function of wavelength. The top dashed line indicates the gain spectrum in the case where a low reflectivity outcoupler is used and the bottom dashed line indicates the gain spectrum in the case where a high reflectivity outcoupler is used. 
     FIG. 10 b  shows the laser output intensity of an F2 laser as a function of wavelength given the use of two different outcouplers. The figure graphically illustrates that the use of a high reflectivity outcoupler has the effect of broadening the laser output spectrum as compared with the use of a low reflectivity outcoupler. 
     FIG. 11 shows in its top graph the reflectivity of an outcoupler etalon as a function of wavelength in the vicinity of an F 2 -laser emission line. In its bottom graph, FIG. 11 shows both a single selected line from the emission spectrum of a free-running F 2 -laser without any line broadening effect and also the same selected laser emission line with the line broadening effect caused by the outcoupler etalon. 
     FIG. 12 shows an etalon as an outcoupler for an F 2 -laser in accord with the present invention. 
     FIG. 13 shows in its top graph the wavelength dependence of the reflectivity of an outcoupler etalon as a function of wavelength in the vicinity of two F 2 -laser emission lines. In its bottom graph, FIG. 13 shows the emission spectrum of the free-running F 2 -laser without line selection or line broadening (solid line) and with the line selection and line broadening effects (dashed line) caused by the outcoupler etalon. 
     FIG. 14 illustrates the line selecting effect of a higher finesse etalon and the line broadening effect of a lower finesse etalon in accordance with the sixth preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The F 2 -laser of the present invention includes a discharge chamber  2  filled with a laser gas including molecular fluorine. The laser gas may be stimulated to emit radiation through the use of electrodes  3  coupled to a power supply circuit such that a voltage is applied across the electrodes to create a pulsed discharge. A UV-preionization of the electrical discharge may also provided and may be realized by means of an array of spark gaps or by another source of UV-radiation (surface, barrier or corona gas discharges), disposed in the vicinity of at least one of the solid electrodes of the main discharge of the laser. A preferred preionization unit is described in U.S. patent application Ser. No. 09/247,887 which is hereby incorporated by reference in its entirety. 
     The first preferred embodiment includes a dispersive prism  4  which has a highly reflective coating  6  at its back surface. A highly reflective mirror  10  serves as an output coupler of the beam. The prism  4  may also serve to seal one end of the discharge chamber, while the mirror  10  also serves to seal the other end of the discharge chamber. Also, one of the mirror  10  and the prism  4  may serve to seal one end of the discharge chamber while the other end is sealed by a window. An energy monitor  8  is included in the first embodiment and measures the energy of the output beam of the laser. Beam splitters  11  are also shown. 
     In the first preferred embodiment of the invention, selection of the appropriate spectral line is accomplished by using the dispersive prism  4  in the laser resonator. FIGS. 8 a  and  8   b  show the spectra of a free-running F 2  laser with helium and neon as buffer gases. Two or three well-defined, relatively narrow spectral lines are present. Due to the wavelength dependent nature of the refractive index of the material of the prism  4 , light entering the prism  4  is refracted according to wavelength at various angles. Only a line having a wavelength within a particular range of wavelengths that exit the prism  4  within the acceptance angle of the resonator of the laser will later be outcoupled as output laser beam. In other words, after retroreflecting from the highly reflecting surface  6  at the back of the prism  4 , lines of different wavelengths will enter the discharge chamber at different angles to the optical axis of the resonator. Lines having wavelengths within the range of wavelengths reflected within the acceptance angle of the resonator are selected, and all others are not selected or suppressed. The prism  4  may be adjusted so that a desired center wavelength may be aligned parallel to the optical axis so that it suffers the least optical losses and, therefore, dominates the output. This center wavelength is at or near the center of the F 2 -emission line it is desired to select. 
     Alternatively, in order to increase the wavelength separation effect, one may use multiple (two or more) prisms as shown in FIG. 9 a . Also, the use of a separate highly reflective mirror instead of a highly reflective coating on the prism side is shown in FIG. 9 b . Such a separate highly reflective mirror can be used in combination with multiple prisms as well. An advantage of the separate mirror is that it can be manufactured more readily than the prism. Therefore, it is less expensive to replace. At the same time, the use of a separate mirror increases the number of optical surfaces that the beam traverses, thus increasing both optical losses and wavelength dispersion. Consequently, a decision on the number of prisms and whether to use a separate mirror is dependent on the total magnitude of the dispersion required to achieve reliable selection of a single line. 
     All of the above considerations hold for the embodiments of FIGS. 1-4. 
     Conventionally, excimer lasers employ low reflectivity outcoupling mirrors (reflectivity &lt;8%) because of the high gain of the excimer. This is also the case for the F 2  excimer laser. Because laser oscillations only occur in the portion of the spectral gain curve in which there is a net gain, increasing the reflectivity of the outcoupling mirror leads to a broader spectral bandwidth for the single line F 2  laser output. Accordingly, the reflectivity of the outcoupling mirror should preferably be within the range of 8% to 90%. While the reflectivity of the outcoupling mirror should be no less than 8%, it may substantially higher and may be chosen to be above 30%, 50%, or 70%. 
     Further, the spectral bandwidth of a selected emission line is dependent in part on the laser chamber pressure and the reflectivity of the outcoupling mirror, if present. Accordingly, the embodiments of the present invention below function better when the laser chamber pressure is relatively high. As our experience confirms, the spectral bandwidth of the selected laser line increases with increasing chamber pressure; therefore, for all embodiments of the present invention, the laser chamber pressure should preferably be above 2.5 bar. 
     In the second preferred embodiment, higher reflectivities for the outcoupling mirror of FIG. 1 can be used (reflectivity of 90% to 100%). The mirror  16  acts as a high reflector. Outcoupling is achieved through Fresnel reflection or coating at one of the surfaces of the dispersive prism in FIG.  2 . The spectral width of the laser output is broader than in the first embodiment because of the reduced total losses in the cavity. 
     In the third preferred embodiment, after a single spectral line has been selected in a manner similar to that of the first embodiment, the spectral bandwidth of the single line is broadened by the use of an optical etalon  26  in place of a conventional outcoupler mirror, as explained below. 
     The reflectivity of the etalon R versus the optical frequency γ of the laser beam is shown in FIG.  11 . It is represented by the function 
     
       
         R(γ)˜F sin 2 (πγ/γ 0 )/(1+F sin 2 (πγ/γ 0 )), 
       
     
     where F=4R/(1−R) 2  is the finesse factor of the etalon, γ 0  is the free spectral range (FSR) of the etalon, γ 0 =1/(2nL) [cm −1 ], where n is the refractive index of the etalon gap material and L is the spacing of the etalon  26  in centimeters. The finesse factor F relates to the finesse F since F=πF/2. Spectral components having frequencies close to the minimum of the etalon&#39;s reflectivity R suffer the greatest losses in the resonator and, therefore, are suppressed. Spectral components having frequencies close to the maximum of the etalon&#39;s reflectivity R experience the lowest losses in the resonator and, therefore, are relatively enhanced. Therefore, if the FSR of the etalon is approximately equal to the linewidth of the free running laser, and the spectral minimum of the etalon&#39;s reflectivity is tuned to the spectral maximum of the selected F 2  laser line, the output spectral bandwidth is broadened. This is shown in the bottom portion of FIG.  11 . 
     It is clear from this consideration that the spectral reflectivity function of the etalon  26  must not be much wider than the free running linewidth of the single line, in order to enhance the sidebands in the spectrum and yet suppress the center of the line. 
     The following is an estimate of the etalon finesse and gap thickness needed to satisfy these conditions. The spectral bandwidth of the etalon reflectivity function is FSR/F. Since the linewidth of the free running F 2  laser is about 1 pm, the spectral bandwidth of the etalon should be approximately 1 pm. Since the FSR should be large enough for there to be only one reflectivity minimum in the spectrum, at a wavelength of approximately 157 nm, the FSR should be at least 0.4 cm −1 . This means that the etalon spacing, L, should be no more than about 8.3 mm if the etalon gap is filled with a material having a refractive index of 1.5 (such as MgF 2 , CaF 2 , LiF or crystalline quartz). Alternatively, the etalon gap may be filled with inert gas, in which case its thickness should be approximately 12.5 mm. Both of these spacings L are readily achievable. The selection of materials such as MgF 2 , CaF 2  or crystalline quartz is due to fact that those are among the few materials that are transparent at the wavelength of 157 nm. 
     When the finesse of the etalon is higher, the FSR of the etalon  26  may accordingly be wider. 
     Another important consideration is the stability of the etalon reflectivity with respect to the variations in ambient conditions, such as temperature. For example, MgF 2  has a linear expansion coefficient of 13.7 10 −6  K −1  along c-axis, and a temperature index coefficient of 1.47 10 −6  K −1  for an ordinary beam. This means that in order to maintain the centering of the spectral line with respect to the minimum of the reflection R(γ) within a margin of 10% of FSR, one needs to stabilize the temperature within 0.06 K. CaF 2  similarly requires stability within 0.05 K. 
     It is obvious from the above estimates that the better solution is to use an etalon  26  whose gap is filled with inert gas which is pressure controlled. For inert gases such as nitrogen, the refractive index changes by approximately 300 ppm per 1 bar of pressure. Therefore, where the spacing between reflecting surfaces, L, is 12.5 mm, achieving frequency control within 10% of FSR requires pressure control within 2 mbar of resolution. Preferably, one should use gases with a low refractive index such as helium, in order to relax the requirements for resolution as to pressure control. 
     Depending on the required maximum reflectivity of the etalon  26 , the internal surfaces of the etalon  26  can either be coated with partially reflective coatings, or can be uncoated. In the latter case, the reflectivity of each surface is approximately 4% to 6%, which results in a maximum reflectivity of the etalon of from 16% to 24%. Similar considerations apply to a solid etalon. 
     In a pressure-tuned etalon, one should preferably use an inert gas such as nitrogen, helium, argon and others, since air is not transparent at wavelengths near 157 nm, primarily due to the presence of oxygen, water vapor and carbon dioxide. 
     FIG. 12 shows that such an etalon should preferably act as a seal for the gas discharge chamber, in order to eliminate the need for an additional optical window in the chamber. 
     The fourth preferred embodiment acts like the third embodiment, but outcoupling is achieved by reflection at a highly reflecting surface  30  of the prism used for line selection in FIG.  4 . 
     In the fifth preferred embodiment, the etalon  40  is used for both bandwidth broadening, in a fashion similar to that described above, and, simultaneously, for selecting the single spectrum line. The latter is achieved by adjusting the FSR and the wavelength of the maximum reflectivity of the etalon  40  in such a way that at all lines in the free running spectrum the reflectivity is minimal. This can be done for two lines. If three lines are present, the following conditions should be satisfied: 
     
       
         ( m +½)γ 0 =γ 1 ; 
       
     
     
       
         ( k +½)γ 0 =γ 2 ; 
       
     
     
       
         ( j +½)γ 0 =γ 3 ; 
       
     
     where γ 1  m, j, k are integers, γ 0  is the free spectral range of the etalon in the optical frequency domain, γ 1  is the optical frequency of the line to be selected, and γ 2  and γ 3  are optical frequencies of the spectral lines to be suppressed. In the case where-only two lines are present, this set of equations reduces to two equations: 
     
       
         ( m +½)γ 0 =γ 1 ; 
       
     
     
       
         ( k +½)γ 0 =γ 2 ; 
       
     
     where γ 1  is the optical frequency of the line to be selected. Since the gain at the lines γ 2  and γ 3  is much smaller (by a factor of about {fraction (1/10)}) than the gain at γ 1 , the lines at γ 2  and γ 3  do not reach a laser oscillation threshold. Therefore, these lines are suppressed completely, while the intense line is only broadened. 
     FIG. 13 illustrates the above relations schematically. In order to satisfy the needed conditions, the etalon&#39;s FSR has to be adjusted either by changing its gap spacing, or by varying the pressure of the gas in the gap, as has been described above. The requirements respecting pressure resolution described above also apply to this case. 
     An advantage of this embodiment is simplicity, since no prisms are required. However, a possible disadvantage may be less efficient suppression of unwanted lines, leading to residual emission at those wavelengths. 
     In the sixth preferred embodiment, one etalon  44  is used for selecting the single line (left-hand side of FIG. 6) in a fashion similar to that described above. A second etalon outcoupler  48  (right-hand side, of FIG. 6) is employed for bandwidth broadening of the selected line, in a fashion similar to that described for the third embodiment. The advantage of this embodiment is the independence of line selection and line broadening. In FIG. 14 the reflectivity function of the line selecting etalon  44  is schematically shown. The contrast ratio is chosen so that minimum reflectivity occurs at the wavelength of the line which is to be suppressed. The line selecting etalon  44  possesses maximum reflectivity in the wavelength range of the line which is to be selected. On the other hand, the line broadening etalon  48  serves to broaden the selected emission line in a manner also shown in FIG.  14  and described above in conjunction with the third embodiment. The etalon  48  has a reflectivity minimum at the center of the selected emission line and is tuned in a manner described in conjunction with the third embodiment so as to enhance the side portions of the selected emission line relative to the center of the selected line. 
     In the seventh preferred embodiment, one etalon  32  is used for selecting the single line (left-hand side of FIG. 7) in a fashion similar to that described above. As in the first embodiment, the bandwidth broadening of the selected line is accomplished by the higher reflectivity of the outcoupling mirror  56  as well as the higher gas pressure. 
     The gas composition for the F 2  laser in the above embodiments includes Helium or Neon, or a mixture of Helium and Neon, as a buffer gas. The concentration of Fluorine in the gas ranges from 0.003% to 1.00%. Xenon and/or Argon may be added in order to increase the energy stability of the laser output. The concentration of Xenon or Argon in the mixture may range from 0.0001% to 0.1%. 
     In all seven embodiments described above, one can preferably include one or more apertures of a size approximately equal to the size of the generated beam. The purpose of such apertures is to reduce the amount of amplified spontaneous emission generated in the gas discharge chamber. Reducing this parasitic emission is advantageous because such emission does not generally possess the properties of the desired laser oscillations. Therefore, if not excluded by an aperture, such parasitic emission can deteriorate the quality of the output beam, including the beam&#39;s spectral purity and divergence. Generally, any of the embodiments described herein may be varied by adding one or more apertures for excluding this parasitic emission. 
     Wavefront curvature of the beam may also be compensated by using a cylindrical lens within the resonator (see U.S. patent application Ser. No. 09/073,070 filed Apr. 29, 1998). The etalons used in some embodiments of the present invention are generally sensitive to the wavefront curvature of the beam. Thus, one or more cylindrical lenses placed in the resonator can provide a more collimated beam at the etalon. Moreover, wavefront curvature compensation can be achieved through the use of one or more curved resonator mirrors. 
     The scope of the present invention is meant to be that set forth in the claims that follow, and equivalents thereof, and is not limited to any of the specific embodiments described above.