BACKGROUND OF THE INVENTION 0001 The present invention relates generally to a photolithographic molecular fluorine laser system, and more particularly to a two-stage laser mode of photolithographic molecular fluorine laser system in which the center wavelength of an oscillation-stage laser is matched to the center wavelength of an amplification-stage laser. 0002 For photolithographic technologies to achieve on semiconductors a semiconductor integrated circuit having a marking width of 70 nm or less, there are demanded exposure light sources of wavelengths of 160 nm or less. F2 (molecular fluorine) laser systems that give out ultraviolet rays of wavelengths of around 157 nm are now thought of as a promising candidate for those light sources. 0003 As shown in FIG. 16, a typical F2 laser system has two primary oscillation wavelengths (1157.6299 nm and 2157.5233 nm: Sov. J. Quantum Eelectron. 16(5), May 1986), with its spectral linewidth (FWHM: full width at half maximum) being of the order of about 1 pm. The intensity ratio of two such oscillation lines is I(1)/I(2)7. For exposure purposes, usually, the oscillation line of wavelength 1(157.6299 nm) having stronger intensity is used. 0004 By the way, the semiconductor photolithography technology is roughly broken down into the following two types that use: 0005 1) a dioptric system, and 0006 2) a catadioptric system. 0007 The use of the catadioptric system for photolithography ensures that chromatic aberrations are reduced. For this reason, an aligner using such a catadioptric system holds great promising in a wavelength range of the order of current 157 nm. However, the catadioptric systems are more troublesome than conventional dioptric systems in terms of the optical axis alignment of an aligner. 0008 On the other hand, the dioptric system is a projection optical system generally used on heretofore known semiconductor aligners. One grave problem with the semiconductor photolithographiy technology is how to correct an optical system for chromatic aberrations. In the dioptric system, correction of chromatic aberrations has been achieved by some combinations of optical elements such as lenses having varying refractive indices Because of considerable restrictions on the types of possible optical materials that are transparent to a wavelength range in the vicinity of 157 nm, however, there is still no option but to use CaF2. 0009 Thus, some line-narrowing means is needed for F2 laser systems used as a light source for the dioptric type aligners. For instance, the spectral full width at half maximum of a laser beam should be narrowed to 0.3 pm or less. 0010 On the other hand, the average output necessary for an F2 laser used as a photolithographic light source, for instance, is 20 W. To put it another way, when the repetitive frequency of the F2 laser is 2 kHz, a pulse energy per pulse is 10 mJ, and at the repetitive frequency of 4 kHz, a pulse energy per pulse is 5 mJ. 0011 However, when one wishes to obtain laser outputs of 5 to 10 mJ while, for instance, a coated etalon is located as line-narrowing means in a laser resonator, the coating of the etalon is damaged; there is no option but to use an etalon without any coating thereon. Accordingly, it is still impossible to make spectral linewidths narrow. In addition, only linewidths of 0.4 to 0.6 pm are merely obtained at best because of the presence of much ASE (amplified spontaneous emission) component. Thus, it is still difficult to achieve any narrow band at pulse energies of 5 to 10 mJ. 0012 Situations being like this, for instance, it is reasonalbe to rely on a two-stage laser system comprising an oscillation-stage laser and an amplification-stage laser for the purpose of obtaining laser beams having spectral linewidths of 0.3 pm or less and pulse energies of 5 mJ or greater. To be specific, the oscillation-stage laser gives out a laser beam that has low outputs yet spectral linewidths of 0.3 pm or less. Then, this laser beam is amplified at the amplification-stage laser to obtain a laser beam that has spectral linewidths of 0.3 pm or less and pulse energies of 5 mJ or greater. 0013 A typical two-stage laser system operates in two modes, the injection locking mode and the master oscillator power amplifier (MOPA) mode. In the injection locking mode, the amplification-stage laser is provided with a laser resonator for which an unstable resonator is used. In the MOPA mode, no laser resonator is used. In the MOPA mode where no laser resonator is provided to the amplification-stage laser, the amplification-stage laser functions as a one-pass amplifier for a laser beam coming from the oscillation-stage laser. 0014 For a line-narrowing element for the oscillation-stage laser, some combinations of one or more combined beam expansion and dispersion prism groups with gratings, a combination of an etalon with a total reflecting mirror, or the like are used. In what follows,-exemplary constructions of two-stage laser systems operating in the injection lock and the MOPA modes are explained. 0015 FIG. 17(a) is illustrative of the injection locking mode where a prism and a diffraction grating are used as line-narrowing element and FIG. 17(b) again of the injection locking mode where an etalon is used as a line-narrowing element. FIG. 18(a) is illustrative of the MOPA mode where a prism and a diffraction grating are used as line-narrowing element and FIG. 18(b) again of the MOPA mode where an etalon is used as a line-narrowing element. Throughout FIGS. 17(a) to 18(b), reference numeral 10 represents an oscillation-stage laser, 20 an amplification-stage laser, 1 a laser chamber in the oscillation-stage laser 10, 1 a laser chamber in the amplification-stage laser 20, 2 a line-narrowing module (line-narrowing element), 3 an output mirror in the oscillation-stage laser 10, 4 an aperture for limiting a laser beam in the oscillation-stage laser 10, 5 a diffraction grating that forms a part of the line-narrowing module 2, 6 a combined beam expansion and dispersion prism that forms a part of the line-narrowing module 2, 7 a concave mirror that forms a part of an unstable resonator in the amplification-stage laser 20, 8 a convex mirror that forms a part of the unstable resonator in the amplification-stage laser 20, 9 a reflecting mirror interposed between the oscillation-stage laser 10 and an amplification-stage laser 20, 11 an etalon that forms a part of the line-narrowing module 2, and 12 a total reflecting mirror that forms a part of the line-narrowing module 2. 0016 Specific reference is here made to the injection locking mode of FIG. 17(a), wherein the prism 6 and diffraction grating 5 are used as the line-narrowing element 2. The oscillation-stage laser 10 has a function of giving out a seed laser (seed laser light) for the laser system, and the amplification-stage laser 20 has a function of amplifying that seed laser. Namely, the overall spectral characteristics of the laser system are determined by the spectral characteristics of the oscillation-stage laser 10. Then, laser outputs (energy or power) from the laser system are determined by the amplification-stage laser 20. The oscillation-stage laser 10 comprises the line-narrowing module 2 including the expanding prism 6 and diffraction grating 5, so that laser beams having narrowed spectra linewidths are produced from the oscillation-stage laser 10. 0017 It is here noted that the line-narrowing module 2 may be made up of the etalon 11 and total reflecting mirror 12, as shown in FIGS. 17(b) and 18(b). 0018 The laser beam (seed laser beam) from the oscillation-stage laser 10 is guided to and poured into the amplification-stage laser 20 via a laser propagating system including the reflecting mirror 9 or the like. In the injection locking mode (FIG. 17), the amplification-stage laser is built up of the concave mirror 7 and convex mirror 8 so that the laser can be amplified even at a small input. For instance, an unstable resonator having a magnification of 3 or greater is used to this end. 0019 There is a hole in the concave mirror 7 that forms a part of the unstable resonator in the amplification-stage laser 10, so that the seed laser beam introduced through that hole is reflected and expanded at the convex mirror 8 as shown by an arrow. Then, the reflected and expanded laser beam passes effectively through a discharging section in the laser chamber 1, so that the power of the laser beam increases. Then, the laser emerges from the convex mirror 8. The concave mirror 7 is provided in its center with a spatial hole and on the rest with a mirror coating having high reflectivity. The convex mirror 8 is provided at its center with a high-reflectivity mirror coating and with an antireflection coating at a portion around that center, from which the laser beam is to emerge. For the concave mirror 7, it is acceptable to use a mirror substrate having a spatial hole which alone is provided with an antireflection coating. It is also acceptable to use an unstable resonator comprising a mirror free from a transmitting portion. 0020 A molecular fluorine laser uses helium and/or neon gases as buffer gases for both the oscillation-stage laser 10 and the amplification-stage laser 20. Whenever necessary, xenon may be used in combination. 0021 FIG. 19 is illustrative in schematic of the laser pulse waveform vs. change-with-time of spectral linewidth relationships in a two-stage laser mode of F2 laser system. In FIG. 19, t as abscissa is indicative of time, I as ordinate of intensity, and of a spectral linewidth. As can be seen from the pulse waveform of FIG. 19, the spectral linewidth becomes slender with the lapse of time (i.e., as the number of round trips in the resonator increases). In a conventional band-narrowing laser, the spectral linewidth takes the form of an integral value. In the two-stage laser mode, however, it is possible to extract the latter part of a pulse, whose linewidth becomes instantaneously narrow, and amplifying only the output while that linewidth is kept, thereby reducing the ASE component. 0022 By use of such a two-stage laser mode, it is thus possible to achieve high outputs simultaneously with very narrow bands. 0023 When F2 laser systems are used as photolithographic light sources, the spectral purity is critical. The term spectral purity is one index to the degree of concentration of spectral energy, referring to a linewidth that includes a certain area ratio in a spectral waveform. For instance, the 95% purity commonly used in the art refers to a linewidth that accounts for a 95% area of the total area of a spectral waveform, as measured from the center side as shown in FIG. 20. 0024 Referring specifically to an F2 laser system, the center wavelength differs between upon buffered with helium and upon buffered with neon; for instance, when an oscillation-stage laser using a neon buffer and an amplification-stage laser using a helium buffer are immediately oscillated, they will be synchronized with the center wavelength of the oscillation-stage laser in misalignment with that of the amplification-stage laser, as shown in FIG. 21(a). Because, by definition, the F2 laser is narrower in linewidth than ArF or KrF excimer lasers, this center wavelength misalignment will lead directly to deterioration in the spectral purity of a laser beam after amplification, as shown in FIG. 21(b).

SUMMARY OF THE INVENTION 0025 In view of such situations as mentioned above, an primary object of the invention is to provide a photolithographic molecular fluorine laser system operating in a two-stage laser mode, wherein the center wavelength of an oscillation-stage laser is matched to that of an amplification-stage laser, thereby oscillating a laser beam having a low spectral purity and a narrow linewidth. 0026 According to the present invention, this object is achievable by the provision of a photolithographic molecular fluorine laser system that comprises an oscillation-stage laser and an amplification-stage laser and operates in a two-stage mode, wherein: 0027 the center wavelength of a laser beam emitted out of the oscillation-stage laser is compared with and substantially matched to the center wavelength of a laser beam emitted out of the amplification-stage laser when said amplification-stage laser is oscillated by itself. 0028 According to the invention, the center wavelength of a laser beam emitted out of the oscillation-stage laser is compared with and substantially matched to that of a laser beam emitted out of the amplification-stage laser when the amplification-stage laser is oscillated by itself. It is thus possible to make the band of the photolithographic molecular fluorine laser system very narrow while the spectral purity is satisfactory. 0029 Referring here to the center wavelength of the laser beam emitted out of the amplification-stage laser when oscillated independently, this term means, in the case of the injection locking mode, literally the center wavelength of the laser beam emitted out the amplification-stage laser when oscillated independently and, in the case of the MOPA mode, the wavelength that is found in the vicinity of any one of primary wavelengths emitted out of the amplification-stage laser and has the highest gain (see FIG. 16). 0030 Still other objects and advantages of the invention will be in part be obvious and will in part be apparent from the specification. 0031 The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which sill be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS 0032 FIG. 1 is a system block diagram illustrative of one embodiment of the photolithographic molecular fluorine laser system according to the invention. 0033 FIG. 2 is a vector diagram for a molecular fluorine laser system, showing that there is a center wavelength misalignment between when helium is used as a buffer gas and when neon is used as a buffer gas. 0034 FIG. 3 is illustrative of a molecular fluorine laser system, showing a center wavelength misalignment due to a sealing gas pressure. 0035 FIG. 4 is illustrative of how to carry out tuning by controlling the gas pressure in the amplification-stage laser when the center wavelength of the amplification-stage laser is controlled while the center wavelength of the oscillation-stage laser remains fixed. 0036 FIG. 5 is a system block diagram for another embodiment of the photolithographic molecular fluorine laser system. 0037 FIG. 6 is illustrative of a tuning process wherein when the oscillation-stage spectral purity is compared with the amplification-stage spectral purity, the gas pressure for the amplification-stage laser is controlled. 0038 FIG. 7 is a diagram illustrative of a spectral 10% linewidth. 0039 FIG. 8 is illustrative of a tuning process wherein the gas pressure for the amplification-stage laser is controlled such that the spectral purity of a post-amplification laser beam emitted out of the amplification-stage laser is minimized. 0040 FIG. 9 is illustrative of a tuning process wherein the gas pressure for the amplification-stage laser is controlled such that the post-amplification spectral purity is minimized relative to the ideal spectral purity. 0041 FIG. 10 is illustrative of a spectral purity on a shorter wavelength side and a spectral purity on a longer wavelength side. 0042 FIG. 11(a) and 11(b) are illustrative of what spectral waveform a post-amplification laser beam emitted out of the amplification-stage laser has when there is a misalignment of the center wavelength of the laser beam emitted out of the amplification-stage laser with respect to that of the laser beam emitted out of the oscillation-stage laser. 0043 FIG. 12 is illustrative of a tuning process wherein the gas pressure for the amplification-stage laser is controlled such that the left-to-right ratio of the oscillation-stage spectral purity is compared with and matched to that of the post-amplification spectral purity. 0044 FIG. 13 is illustrative of a tuning process of controlling the gas pressure for the oscillation-stage laser so that the center wavelength of the oscillation-stage laser is controlled while the center wavelength of the amplification-stage laser remains fixed. 0045 FIG. 14 is illustrative of a tuning process of controlling the gas pressure for the oscillation-stage laser so that both the center wavelengths of the oscillation-stage laser and amplification-stage laser are controlled for turning in to the target wavelength. 0046 FIG. 15 is illustrative of a tuning process of controlling the gas pressure for the amplification-stage laser so that both the center wavelengths of the oscillation-stage laser and amplification-stage laser are controlled for turning in to the target wavelength. 0047 FIG. 16 is illustrative of primary oscillation wavelengths of a molecular fluorine laser. 0048 FIGS. 17(a) and 17(b) are illustrative of one exemplary arrangement of the injection locking mode of two-stage laser system. 0049 FIGS. 18(a) and 18(b) are illustrative of one exemplary arrangement of the MOPA mode of two-stage laser system. 0050 FIG. 19 is illustrative in schematic of the spectral waveform vs. spectral linewidth change-with-time relationships in a two-stage laser mode of molecular fluorine laser system. 0051 FIG. 20 is illustrative of a 95% purity. 0052 FIG. 21 is illustrative of the spectral waveform of a post-amplification laser beam emitted out of the amplification-stage laser in a state where there is a misalignment between the center wavelength of the oscillation-state laser and that of the amplification-stage laser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 0053 In what follows, the photolithographic molecular fluorine laser system of the present invention will be explained with reference to some embodiments. 0054 FIG. 1 is a system block diagram illustrative of the arrangement of one embodiment of the photolithographic molecular fluorine laser system to which the injection locking mode is applied. 0055 Before explaining the arrangement and action of FIG. 1, an account is given to the causes of why the center wavelength of an oscillation-stage laser 10 is in misalignment with that of an amplification-stage laser 20. 0056 The center wavelength misalignment is caused for the following reasons. 0057 The first reason is the difference in the buffer gas used. The difference in the buffer gas between the oscillation-stage laser 10 and the amplification-stage laser 20 (e.g., neon buffer for the oscillation-stage laser and helium buffer for the amplification-stage laser) is responsible for such a center wavelength misalignment as shown in FIG. 21(a). The band-narrowing laser that is the oscillation-stage laser 10 is capable of long pulsation for narrowing spectral linewidths, and the use of the neon buffer is favorable for that laser because it is possible to make much use of the line-narrowing element. To obtain high outputs, the helium buffer is preferable for the amplification-stage laser 20. When the helium buffer gas and neon buffer gas are used at the same sealing pressure, there is a center wavelength misalignment of the order of 0.32 to 0.36 pm as shown in FIG. 2. 0058 The second reason for the center wavelength misalignment is the difference in the gas-sealing pressure. Even when the same buffer gas is used for the oscillation-stage laser 10 and the amplification-stage laser 20, there is again a center wavelength misalignment due to a gas pressure difference if any. At a higher pressure, the center wavelength becomes long as shown in FIG. 3. 0059 The third reason for the center wavelength misalignment is a shift of wavelength selection in the line-narrowing unit. Referring as an example to the injection locking mode, a shift of wavelength selection, if any, in the line-narrowing unit will cause a misalignment of the center wavelength of the laser beam emitted out of the amplification-stage laser 20 with respect to that emitted out of the oscillation-stage laser 10 upon oscillated by itself. 0060 Often in this case, the center wavelength misalignment is caused by combinations of two or more of the aforesaid three reasons. 0061 Besides, other possible reasons for the center wavelength misalignment include (1) the partial pressure of fluorine in the laser gas, (2) discharge voltage (voltage applied to electrodes), and (3) gas temperature. However, the center wavelength misalignment due to (1) and (2) was negligibly small, and the center wavelength misalignment due to (3) the gas temperature is equivalent to that due to pressure because the laser chamber has a constant volume. 0062 Referring back to FIG. 1, the photolithographic molecular fluorine laser system to which the injection locking mode is applied comprises an oscillation-stage laser 10 and an amplification-stage laser 20. The oscillation-stage laser 10 is built up of a laser chamber 1, a line-narrowing unit (module) 2 and an output mirror 3 that are located at both ends of the laser chamber 1 to form a resonator and apertures 4, 4 for limiting a laser beam, and the amplification-stage laser 20 is made up of a laser chamber 1, and a reflecting mirror 13 and an output mirror 14 that are located at both ends of the laser chamber 1 to form a resonator. When an unstable resonator is used for the amplification-stage laser 20, a concave mirror is used for the reflecting mirror 13 while a convex mirror is used for the output mirror 14. The line-narrowing unit 2 in the oscillation-stage laser 10 may be constructed either using a prism 6 and a diffraction grating 5 as shown in FIGS. 17 and 18 or using an etalon 11 and a total reflecting mirror 12. 0063 In the arrangement of FIG. 1, a beam splitter 15 is interposed between the oscillation-stage laser 10 and the amplification-stage laser 20, a shutter 16 is interposed between the beam splitter 15 and the reflecting mirror 13, and another beam splitter 17 is located on the output side of the amplification-stage laser 20. 0064 A laser beam given out from the oscillation-stage laser 10 and singled out by the beam splitter 15 is entered into a monitor 21, and a laser beam given out of the amplification-stage laser 20 and singled out by the beam splitter 17 is entered into a monitor 22. Output signals from the monitors 21 and 22 are entered into a controller 23, and a control signal from the controller 23 is sent to the line-narrowing unit 2 and a gas unit 24 that controls the gas pressures within the laser chambers 1 and 1. 0065 Reference is now made to how to control central wavelength tuning in such an arrangement. 0066 There are the following three approaches to wavelength adjustment: 0067 (1) while 1 remains fixed, 2 is controlled; 0068 (2) 1 is controlled, while 2 remains fixed; and 0069 (3) 1 is controlled, and 2 is controlled. 0070 Here 1 is the center wavelength of the oscillation-stage laser 10, and 2 is the center wavelength of the amplification-stage laser 20. 0071 First of all, an account is given of how to control center wavelength tuning by controlling the wavelength of the amplification-stage laser 20 side while the wavelength 1 of the oscillation-stage laser 10 side remains fixed. To this end, wavelength tuning is performed by controlling the gas pressure, etc. for the amplification-stage laser 20. 0072 In the arrangement of FIG. 1, a part of the laser beam emitted out of the oscillation-stage laser 10 is singled out by the beam splitter 15, and then guided to the monitor 21. The monitor 21 detects the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10, and then sends data to the controller 23. 0073 While the shutter 16 between the oscillation-stage laser 10 and the amplification-stage laser 20 is placed in a closed state, a part of the laser beam emanating from the amplification-stage laser 20 per se and so not subjected to amplification is singled out by the beam splitter 17, and then guided to the monitor 22. The monitor 22 detects the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20, and then sends data to the controller 23. 0074 The monitors 21 and 22 used herein are capable of detecting the center wavelengths 1 and 2. On the basis of the received data, the controller 23 determines by calculation the center wavelengths 1 and 2 and compares both. 0075 The controller 23 controls the gas unit 24 to keep the pressure within the laser chamber 1 in the oscillation-stage laser 10 constant. The controller 23 also controls the line-narrowing unit 2 on the basis of center wavelength data coming from the monitor 21, so that the center wavelength of the laser beam emitted out of the oscillation-stage laser 10 is fixed at 1. 0076 Subsequently, the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20 itself is regulated such that 21. Regulation of the wavelength 2 is carried out by controlling the pressure of the gas within the laser chamber 1 in the amplification-stage laser 20 in the following manners.

2>1

0077 As already explained, when the pressure of the gas in the laser chamber 1 is high, the center wavelength is shifted to a longer wavelength side; the controller 23 controls the gas unit 24 so that, as shown in FIG. 4, the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20 is decreased until that gas pressure P2 coincides with the gas pressure Pa within the laser chamber 1, which corresponds to the wavelength 1.

2<1

0078 The controller 23 controls the gas unit 24 so that, as shown in FIG. 4, the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20 is increased until that gas pressure P2 coincides with the gas pressure Pa within the laser chamber 1, which corresponds to the wavelength 1. 0079 Suppose here that the same buffer gas species were used for the oscillation-stage laser 10 and the amplification-stage 20. As a result of the aforesaid control, the pressure within the laser chamber 1 in the oscillation-stage laser 10 will be substantially identical with that within the laser chamber 1 in the amplification-stage laser 20. Suppose on the other hand that different buffer gas species were used. As a result of the aforesaid control, the pressure within the laser chamber 1 in the oscillation-stage laser 10 will differ from that within the laser chamber 1 in the amplification-stage laser 20. 0080 In the aforesaid procedure, the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20 itself is detected using the shutter 16. In the MOPA mode, however, that wavelength 2 cannot be detected because there is no laser resonator in the amplification-stage laser 20. In other words, this control procedure can be applied to only the injection locking mode of two-stage laser. 0081 The construction of another embodiment of the photolithographic molecular fluorine laser system is now explained with reference to FIG. 5. Although this embodiment is directed to a photolithographic molecular fluorine laser system to which the injection locking mode is applied, it may also be applied to the MOPA mode where the reflecting mirror 13 and output mirror 14 that form the resonator for the amplification-stage laser 20 are dispensed with. 0082 The embodiment of FIG. 5 is different in construction from the embodiment of FIG. 1 in that there is no shutter 16 between the oscillation-stage laser 10 and the amplification-stage laser 20, i.e., in terms of the control process. More specifically, in this embodiment an oscillation-stage spectral purity p1 is compared with a post-amplification spectral purity p3. 0083 A part of the laser beam emitted out of the oscillation-stage laser 10 is singled out by the beam splitter 15, and then guided to the monitor 21. The monitor 21 detects the spectral waveform of the laser beam emitted out of the oscillation-stage laser 10, and then sends data to the controller 23. 0084 On the other hand, a part of the laser beam emitted out of the amplification-stage laser 20 where the seed laser beam from the oscillation-stage laser 10 has been amplified is singled out by the laser splitter 17, and then guided to the monitor 22. The monitor 22 detects the spectral waveform of a laser beam emitted out of the amplification-stage laser 20, and then sends data to a controller. 0085 The monitors 21 and 22 used herein are capable of detecting spectral waveforms. The monitor 21 can also detect the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10. 0086 On the basis of the received data, the controller 23 determines by calculation the spectral purity p1 of the laser beam emitted out of the oscillation-stage laser 10 and the spectral purity p3 of the post-amplification laser beam emitted out of the amplification-stage laser 20 and compares both. 0087 On the basis of the results, the gas pressure is regulated in the following two manners: 0088 (1) while the pressure on the oscillation-stage laser 10 side remains fixed, the pressure on the amplification-stage laser 20 side is controlled, and 0089 (2) the pressure on the oscillation-stage laser 10 side is controlled while the pressure on the amplification-stage laser 20 side remains fixed. 0090 Here how to control the pressure on the amplification-stage laser 20 side is explained. The controller 23 controls the gas unit 24 to keep the pressure within the laser chamber 1 in the oscillation-stage laser 10 constant, and controls the line-narrowing unit 2 on the basis of center wavelength data from the monitor 21, so that the center wavelength of the laser beam emitted out of the oscillation-stage laser 10 is fixed at 1. 0091 Subsequently, the controller 23 controls the gas pressure within the laser chamber l in the amplification-stage laser 20 in such a way that p3Ap1 where A is of the order 1 to 3. Usually, the laser beam emitted out of the amplification-stage laser 20 includes an ASE component occurring at the amplification-stage laser 20. Accordingly, the spectral purity p3 of the post-amplification laser beam emitted out of the amplification-stage laser 20 is often higher than the spectral purity p1 of the laser beam emitted out of the oscillation-stage laser 10. Experimentation by the inventor has taught that when the post-amplification laser beam emitted out of the amplification-stage laser 20 is narrowed to a spectral linewidth commonly available for photolithography, its purity p3 has a value between about 1 and about 3 times as high as the spectral purity p1 of the laser beam emitted out of the oscillation-stage laser 10. 0092 How to control the gas pressure in this case is now explained. As shown in FIG. 6, the controller 23 first controls the gas unit 24 to increase the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20. When the spectral purity p3 after the change in the gas pressure is lower than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 increases until p3Ap1. 0093 On the other hand, when the spectral purity p3after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 decreases until p3Ap1. 0094 This procedure may be modified as follows. That is, the controller 23 first controls the gas unit 24 to decrease the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20. When the spectral purity p3 after the change in the gas pressure is lower than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 decreases until p3Ap1. 0095 On-the other hand, when the spectral purity p3 after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 increases until p3Ap1. 0096 Because, as mentioned above, the spectral purity p3 of the post-amplification gas pressure emitted out of the amplification-stage laser 20 is monitored, there is no need of interposing any shutter between the oscillation-stage laser 10 and the amplification-stage laser 20, as shown in FIG. 5. 0097 Thus, the present control process is applicable just only to the injection locking mode but to the MOPA mode (that does not rely on the reflecting mirror 13 and output mirror 14) as well. 0098 Specifically but not exclusively, the spectral purity p1, p3 is 95% purity; for instance, however, it is acceptable to use spectral 10% linewidth p110%, p310%. The wording spectral 10% linewidth is understood to refer to a spectral linewidth at a position where the intensity peak of a spectral waveform becomes 10%, as shown in FIG. 7. In this conjunction, it is noted that instead of the spectral 10% linewidth, it is acceptable to use a spectral linewidth that accounts for 5%, 20%, . . . , of the spectral peak intensity. In this case, the aforesaid coefficient A is not always Aca. 1 to 3. 0099 In the embodiment explained with reference to FIG. 5, the spectral purity p1 (or 110%, etc.) of the laser beam emitted out of the oscillation-stage laser 10 and the spectral purity p3 (or 310%, etc.) of the post-amplification laser beam emitted out of the amplification-stage laser 20 are determined by calculation and compared with each other. However, it is acceptable to determine by calculation only the spectral purity p3 (or 310%, etc.) of the post-amplification laser beam emitted out of the amplification-stage laser 20, and then control the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20 in such a way that the purity is minimized as shown in FIG. 8. 0100 Next, one modification to the embodiment where the oscillation-stage spectral purity p1 is compared with the post-amplification spectral purity p3 is explained. In this modified embodiment, ideal spectral purity 0 is compared with the post-amplification spectral purity p3. 0101 To this end, the ideal purity for the post-amplification laser beam emitted out of the amplification-stage laser 20 should be predetermined by calculation, and stored in the controller 23. 0102 For the derivation of the ideal purity 0, the Gaussian function Iexp(x2/kg) or the Lorents function I1/(1x2)/k1) is used as a spectral waveform. Here I is the spectral intensity, x is the wavelength, and kg and k1 are each the constant. For instance, given a spectral waveform having an FWHM of 0.2 pm, kg1.4431026 and k111026. 0103 In the arrangement of FIG. 5, a part of the laser beam emitted out of the oscillation-stage laser 10 is singled out by the-beam splitter 15, and then guided to the monitor 21. The monitor 21 detects the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10, and then sends data to the controller 23. 0104 On the other hand, a part of the laser beam emitted out of the amplification-stage laser 20 where the seed laser beam from the oscillation-stage laser 10 has been amplified is singled out by the beam splitter 17, and then guided to the monitor 22. The monitor 22 detects the spectral purity p3 of the laser beam emitted out of the amplification-stage laser 20, and then sends data to the controller 23. 0105 The monitor 21 used herein is capable of detecting the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10, and the monitor 22 is capable of detecting the spectral purity p3. 0106 The controller 23 controls the gas unit 24 to keep the pressure within the chamber 1 in the oscillation-stage laser 10 constant. The controller 23 also controls the line-narrowing unit 2 on the basis of center wavelength data from the monitor 21, so that the center wavelength of the laser beam emitted out of the oscillation-stage laser 10 is fixed at 1. 0107 On the basis of the stored data and the received data, the controller 23 compares the ideal purity 0 with the post-amplification spectral purity p3. 0108 Subsequently, the controller 23 controls the gas pressure within the laser chamber in the amplification-stage laser 20 in such a way that p30. 0109 How to control the gas pressure in this case is now explained. As shown in FIG. 9, the controller 23 first controls the gas unit 24 to increase the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20. When the spectral purity p3 after the change in the gas pressure is lower than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 increases until p30. On the other hand, when the spectral purity p3 after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 decreases until p30. 0110 This procedure may be modified as follows. That is, the controller 23 first controls the gas unit 24 to decrease the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20. When the spectral purity p3 after the change in the gas pressure is lower than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 decreases until p30. On the other hand, when the spectral purity p3 after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P2 increases until p30. 0111 Because, as mentioned above, the spectral purity p3 of the post-amplification gas pressure emitted out of the amplification-stage laser 20 is monitored, there is no need of interposing any shutter between the oscillation-stage laser 10 and the amplification-stage laser 20, as shown in FIG. 1. Thus, the present control process is applicable just only to the injection locking mode but to the MOPA mode as well. Moreover, since it is not always required to provide the monitor 21, the arrangement can be simplified. 0112 An account is now given of a preferred embodiment of the invention wherein the left-to-right ratio of the oscillation-stage laser, p1short/p1long, is compared with the left-to-right ratio of the post-amplification spectral purity, p3short/p3long. 0113 In the arrangement of FIG. 5, a part of the laser beam emitted out of the oscillation-stage laser 10 is singled out by the beam splitter 15, and then guided to the monitor 21. The monitor 21 detects the spectral waveform of the laser beam emitted out of the oscillation-stage laser 10, and then sends data to the controller 23. 0114 On the other hand, a part of the laser beam emitted out of the amplification-stage laser 20 where the seed laser beam from the oscillation-stage laser 10 has been amplified is singled out by the beam splitter 17, and then guided to the monitor 22. The monitor 22 detects the spectral waveform of the laser beam emitted out of the amplification-stage laser 20, and then sends data to the controller 23. 0115 The monitors 21, 22 used herein are capable of detecting spectral waveforms, and the monitor 21 is capable of detecting the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10. 0116 On the basis of the received spectral data, the controller 23 determines by calculation the purities, p1short and p1long, on a shorter wavelength side (left) and a longer wavelength side (right) with respect to the center wavelength, which are included in the spectral purity p1 of the laser beam emitted out of the oscillation-stage beam 10, and the purities, p3short and p3long, on a shorter wavelength side (left) and a longer wavelength side (right) with respect to the center wavelength, which are included in the spectral purity p3 of the post-amplification laser beam emitted out of the amplification-stage laser 10. 0117 The term center wavelength used herein is understood to refer to the wavelength at which the intensity of a spectral wavelength peaks or the center value of a wavelength at half-value intensity. The purity pshort on the shorter wavelength (left) side is understood to mean a linewidth that accounts for 95% of the total area of a spectral waveform on the shorter wavelength (left) side with respect to the center wavelength, and the purity plong on the longer wavelength (right) side is understood to mean a linewidth that accounts for 95% of the total area of a spectral waveform on the longer wavelength (right) side with respect to the center wavelength, as shown in FIG. 10. 0118 The controller 23 determines by calculation P1short/p1long, and p3short/p3long. 0119 Then, the controller 23 controls the gas unit 24 to keep the pressure within the laser chamber 1 in the oscillation-stage laser 10 constant, and on the basis of center wavelength data from the monitor 21, the controller 23 controls the line-narrowing unit 2 in such a way that the center wavelength of the laser beam emitted out of the oscillation-stage laser 10 is fixed at 1. 0120 As the pressure is kept constant, the value of p1short/p1long in the oscillation-stage laser 10 is fixed. 0121 Subsequently, the controller 23 controls the gas pressure within the laser chamber 1 in the amplification-stage laser 20 in such a way that p3short/p3longp1short/p1long. 0122 How to control the gas pressure in this case is now explained.

p3short/p3long>p1short/p1long

0123 In this state, as shown in FIG. 11(a), the center wavelength of the laser beam emitted out of the amplification-stage laser 20 on the assumption that the laser 20 is oscillated by itself is found on the shorter wavelength side with respect to the center wavelength of the laser beam emitted out of the oscillation-stage laser 10. Accordingly, the controller 23 controls the gas unit 24 to increase the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20 as shown in FIG. 12, until that pressure coincides with the gas pressure Pa within the laser chamber 1, which corresponds to p1short/p1long.

p3short/p3long<p1short/p1long

0124 In this state, as shown in FIG. 11(b), the center wavelength of the laser beam emitted out of the amplification-stage laser 20 on the assumption that the laser is oscillated by itself is found on the longer wavelength side with respect to the center wavelength of the laser beam emitted out of the oscillation-stage laser 10. Accordingly, the controller 23 controls the gas unit 24 to decrease the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20 as shown in FIG. 12, until that pressure coincides with the gas pressure Pa within the laser chamber 1, which corresponds to p1short/p1long. 0125 According to this embodiment wherein, as mentioned above, the purity p3short on the shorter wavelength (left) and the purity p3long on the longer wavelength (right) side with respect to the center wavelength, which are included in the spectral purity p3 of the post-amplification laser beam emitted out of the amplification-stage laser 20, are compared with each other, there is no need of interposing the shutter 16 between the oscillation-stage laser 10 and the amplification-stage laser 20 as shown in FIG. 1. Accordingly, this control process is applicable just only to the injection locking mode but to the MOPA mode as well. Since the monitor 21 is not always necessary, the arrangement can be simplified. 0126 An account is now given of how to control the center wavelength 1 of the oscillation-stage laser 10 and how to control both the center wavelength 1 of the oscillation-stage laser 10 and the center wavelength 2 of the amplification-stage laser 20 while the shutter 16 is interposed between the oscillation-stage laser 10 and the amplification-stage laser 20 as shown in FIG. 1. 0127 In the arrangement of FIG. 1, a part of the laser beam emitted out of the oscillation-stage laser 10 is singled out by the beam splitter 15, and then guided to the monitor 21. The monitor 21 detects the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10, and then sends data to the controller 23. 0128 While the shutter 16 between the oscillation-stage laser 10 and the amplification-stage laser 20 is in a closed state, a part of the laser beam emitted out of the amplification-stage laser 20 per se is singled out by the beam splitter 17, and then guided to the monitor 22. The monitor 22 detects the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20, and then sends data to the controller 23. 0129 The monitors 21, 22 used herein are capable of detecting the center wavelengths 1, 2. 0130 On the basis of the received data, the controller 23 determines by calculation the center wavelengths 1 and 2, and compares both. 0131 The controller 23 controls one or both of the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10 and the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20 per se in such a way that 21. Wavelength regulation is carried out by controlling the corresponding gas pressure. 0132 For wavelength regulation, there are the following three approaches: 0133 (1) while 1 remains fixed, 2 is controlled, 0134 (2) 1 is controlled while 2 remains fixed, and 0135 (3) 1 is controlled, and 2 is controlled. 0136 Of (1), an account has already been given. Here, how to control the gas pressure in the case where (2) 1 is controlled while 2 remains fixed, and (3) both 1 and 2 are controlled is explained. 0137 To control 1 while 2 remains fixed, the controller 23 first controls the gas unit 24 to keep the pressure within the laser chamber 1 in the amplification-stage laser 20 constant, so that while the shutter 16 is in a closed state, the center wavelength of the laser beam emitted out of the amplification-stage laser 20 per se is fixed at 2. 0138 Subsequently, the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10 is regulated such that 12. Regulation of the center wavelength 1 is carried out by controlling the pressure of the gas in the laser chamber 1 in the oscillation-stage laser 10. Control of the gas pressure is performed as follows.

1>2

0139 The controller 23 controls the gas unit 24 to decrease the gas pressure P1 within the laser chamber 1 in the oscillation-stage laser 10 as shown in FIG. 13, until that pressure coincides with the gas pressure Pa within the laser chamber 1 in the oscillation-stage laser 10, which corresponds to the wavelength 2.

1<2

0140 The controller 23 controls the gas unit 24 to increase the gas pressure P1 within the laser chamber 1 in the oscillation-stage laser 10, until that pressure coincides with the gas pressure Pa within the laser chamber 1 in the oscillation-stage laser 10, which corresponds to the wavelength 2. 0141 Where (3) both 1 and 2 are controlled, the controller 23 should pre-store the target wavelength 10. 0142 Subsequently, both the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10 and the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20 per se are regulated such that 120. Regulation of the wavelengths 1, 2 are performed by control of the pressure of the gas within the laser chamber 1 in the oscillation-stage laser 10 and the pressure of the gas within the laser chamber 1 in the amplification-stage laser 20, respectively. It is here noted that for control of the center wavelength 2, the shutter 16 is put into a closed state. Control of the gas pressure is then performed as follows.

1>0

0143 The controller 23 controls the gas unit 24 to decrease the gas pressure P1 within the laser chamber 1 in the oscillation-stage laser 10 as shown in FIG. 14, until that pressure coincides with the gas pressure Pa within the laser chamber 1 in the oscillation-stage laser 10, which corresponds to the target wavelength 0.

1<0

0144 The controller 23 controls the gas unit 24 to increase the gas pressure P1 within the laser chamber 1 in the oscillation-stage laser 10, until that pressure coincides with the gas pressure Pa within the laser chamber 1 in the oscillation-stage laser 10, which corresponds to the target wavelength 0.

2>0

0145 The controller 23 controls the gas unit 24 to decrease the gas pressure P1 within the laser chamber 1 in the amplification-stage laser 20 as shown in FIG. 15, until that pressure coincides with the gas pressure Pa within the laser chamber 1 in the amplification-stage laser 20, which corresponds to the target wavelength 0.

2<0

0146 The controller 23 controls the gas unit 24 to increase the gas pressure P2 within the laser chamber 1 in the amplification-stage laser 20, until that pressure coincides with the gas pressure Pa within the laser chamber 1 in the amplification-stage laser 20, which corresponds to the target wavelength 0. 0147 With this control process wherein the center wavelength 2 of the laser beam emitted out of the amplification-stage laser 20 per se is detected using the shutter 16. In the MOPA mode, however, the wavelength 2 cannot be detected for lack of any laser resonator in the amplification-stage laser 20. In other words, the control process according to this embodiment is applicable only to the injection locking mode of two-stage laser. 0148 Generally, when the aforesaid control process is performed while the same buffer gas species is used for the oscillation-stage laser 10 and the amplification-stage laser 20, the pressure within the laser chamber 1 in the oscillation-stage laser 10 is substantially the same as that within the laser chamber 1 in the amplification-stage laser 20, and when the aforesaid control process is effected using different buffer gas species, the pressure within the laser chamber 1 in the oscillation-stage laser 10 differs from that within the laser chamber 1 in the amplification-stage laser 20. 0149 The present invention is now explained with reference to another embodiment wherein the oscillation-stage spectral purity p1 compared with the post-amplification spectral purity p3. 0150 In the arrangement of FIG. 5, a part of the laser beam emitted out of the oscillation-stage laser 10 is singled out by the beam splitter 15, and then guided to the monitor 21. The monitor 21 detects the spectral waveform of the laser beam emitted out of the oscillation-stage laser 10, and then sends data to a controller 23. 0151 On the other hand, a part of the laser beam emitted out of the amplification-stage laser 20 where the seed laser beam from the oscillation-stage laser 10 has been amplified is singled out by the laser splitter 17, and then guided to the monitor 22. The monitor 22 detects the spectral waveform of the laser beam emitted out of the amplification-stage laser 20, and then sends data to a controller. 0152 The monitors 21 and 22 used herein are capable of detecting spectral waveforms. The monitor 21 can also detect the center wavelength 1 of the laser beam emitted out of the oscillation-stage laser 10. 0153 On the basis of the received data, the controller 23 determines by calculation the spectral purity p1 of the laser beam emitted out of the oscillation-stage laser 10 and the spectral purity p3 of the post-amplification laser beam emitted out of the amplification-stage laser 20, and compares both. 0154 Specifically but not exclusively, the spectral purity 80 p1, p3 should be 95% purity as mentioned; for instance, however, it is acceptable to use spectral 10% linewidth p110%, p310%. Alternatively, it is acceptable to use a spectral linewidth that accounts for 5%, 20%, . . . of the spectral peak intensity. 0155 The controller 23 controls the gas unit 24 to keep the pressure within the laser chamber 1 in the oscillation-stage laser 10 constant, and controls the line-narrowing unit 2 on the basis of center wavelength data from the monitor 21 in such a way that the center wavelength of the laser beam emitted out of the oscillation-stage laser 10 is fixed at 1. 0156 Subsequently, the controller 23 controls the gas pressure within the laser chamber 1 in the-amplification-stage laser 20 in such a way that p3Ap1 (or 310%A110%, etc.) where A is of the order of 1 to 5. Here A is limited to the order to 1 to 3 for the same reasons as mentioned above. 0157 Regulation of the gas pressure is performed in the following two manners: 0158 (1) while the pressure on the oscillation-stage laser 10 stage remains fixed, the pressure on the amplification-stage laser 20 stage is controlled, and 0159 (2) the pressure on the oscillation-stage laser 10 side is controlled while the pressure on the amplification-stage laser 20 remains fixed. 0160 Here an account is given of the manner (2) because the manner (1) has been explained. 0161 The controller 23 controls 24 to keep the pressure within the laser chamber 1 in the amplification-stage laser 20 constant. 0162 Subsequently, the spectral purity p1 (or 110%, etc.) of the laser beam emitted out of the oscillation-stage laser 10 is regulated such that p3Ap1 (or 310%A110%, etc. ) where A is of the order of 1 to 5. Regulation of the spectral purity is performed by control of the pressure of the gas within the laser chamber 1 in the oscillation-stage laser 10. 0163 To control the gas pressure, the controller 23 first controls the gas unit 24 to increase the gas pressure P1 within the laser chamber 1 in the oscillation-stage laser 10. 0164 When the spectral purity p3 after the change in the gas pressure is lower than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P1 increases until p3Ap1. 0165 On the other hand, when the spectral purity p3 after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P1 decreases until p3Ap1. 0166 This procedure may be modified as follows. That is, the controller 23 first controls the gas unit 24 to decrease the gas pressure P1 within the laser chamber 1 in the oscillation-stage laser 10. 0167 When the spectral purity p3 after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P1 decreases until p3Ap1. 0168 On the other hand, when the spectral purity p3 after the change in the gas pressure is higher than the spectral purity p3 before the change in the gas pressure, the controller 23 then controls the gas unit 24 in such a direction that the pressure P1 increases until p3Ap1. 0169 Finally, reference is made to an embodiment of the invention wherein, when there is a shift of wavelength selection in the line-narrowing unit 2, tuning is carried out by control of the wavelength on the oscillation-stage laser 10 side and the wavelength on the amplification-stage laser 20 side. 0170 In the arrangement of FIG. 5, a part of the laser beam emitted out of the amplification-stage laser 20 where the seed laser beam from the oscillation-stage laser 10 has been amplified is singled out by the beam splitter 17, and then guided to the monitor 22. The monitor 22 detects the spectral waveform of the laser beam emitted out of the amplification-stage laser 20, and then sends data to the controller 23. 0171 The monitor 22 used herein is capable of detecting spectral waveforms. 0172 The controller 23 controls the gas unit 24 to keep the pressure within the laser chamber 1 in the amplification-stage laser 20 constant. 0173 The wavelength of the laser beam emitted out of the oscillation-stage laser 10 is regulated by wavelength tuning by a laser band-narrowing element (grating, etc.) in the line-narrowing unit 2 in the oscillation-stage laser 10 in such a way that the spectral purity 3 of the post-amplification laser beam emitted out of the amplification-stage laser 20 is minimized. This means that the wavelength on the oscillation-stage laser 10 side is matched to the wavelength on the amplification-stage laser 20 side. 0174 When a prism and a diffraction grating are used for wavelength tuning at the line-narrowing unit 2 (see FIG. 17(a) and FIG. 18(a)), the wavelength tuning is carried out by rotation of the grating. When a reflecting mirror is located as a turn-back mirror in the line-narrowing module comprising the prism and grating, the wavelength tuning is controlled by rotation of that mirror. Alternatively, the center wavelength may be controlled by use of both the turn-back mirror and the grating. 0175 When an etalon is used for the wavelength tuning at the line-narrowing unit 2 (see FIG. 17(b)and FIG. 18(b)), the wavelength tuning is carried out by rotation of the etalon, and when the etalon is an air gap etalon, the wavelength tuning is performed by rotation of the etalon or varying the pressure of an atmosphere in which the etalon is placed (i.e., varying the refractive index of the atmosphere). 0176 While the present invention has been described with reference to some preferable embodiments of the photolithographic molecular fluorine laser system, it is understood that the invention is in no sense limited thereto, and so could be modified in various manners. 0177 In accordance with the inventive photolithographic molecular fluorine laser system of the two-stage mode comprising an oscillation-stage laser and an amplification-stage laser, which is controlled such that the center wavelength of the oscillation-stage laser is matched to the center wavelength of the amplification-stage laser, the band of the photolithographic molecular fluorine laser system can be made very narrow with satisfactorily improved spectral purity.