Laser apparatus

Abstract: A laser apparatus includes an optical resonator having a laser medium (1) and generating a laser light. A wavelength selection element (4) selects a given wavelength of the laser light. The selected wavelength resides within a gain bandwidth of the laser medium. A sealed container (5) houses the wavelength selection element (4). A sealed container may be filled with a gas. A density of the gas within the sealed container (5) may be changed to vary the selected wavelength.

Description: BACKGROUND OF THE INVENTION This invention generally relates to a laser apparatus. For example, this invention specifically relates to a laser apparatus usable in an exposure system. During the manufacture of large-scale integrated circuits, photolithography processes are used in printing patterns on semiconductor wafers. The photolithography processes are generally performed via exposure systems. Lasers are frequently used as light sources of the exposure systems. The manufacture of very large-scale integrated circuits requires shorter-wavelength light sources. Excimer lasers are applicable to such light sources. Generally, an excimer laser using an optical resonator generates an oscillation line having a half-width of about 0.5 nm. To use such an excimer laser in the manufacture of very large-scale integrated circuits, it is necessary to decrease the half-width of its oscillation line to about 0.005 nm or less. The decrease in the width of the excimer laser oscillation line is conventionally realized by wavelength selectors such as a prism, a diffraction grating, or a Fabry-Perot etalon. These wavelength selectors are sensitive to temperature and pressure, so that changes in the temperature or pressure cause considerable variations in the selected wavelength. It is important to stabilize the wavelength of the light of the exposure system during the manufacture of very large-scale integrated circuits. In Applied Optics, vol. 13, no. 7, July 1974, pp 1625-1628, Wallenstein et. al., a pressure-tunable dye laser system is disclosed having a first main pressure chamber and a second optional pressure chamber physically linked thereto, wherein a wavelength selection and first optical line width reduction is achieved by the use of an air-spaced Fabry-Perot interferometer disposed in the laser light path in combination with a diffraction grating, mounted in a common main pressure chamber. Moreover, Optics and Laser Technology, vol. 18, no.4, August 1986, pp 187-189, Henderson et. al. discloses an etalon based wavelength stabilization system, wherein the stabilization is performed by detecting a fringe pattern position and movement in a system using a reference laser beam, an analysing etalon, a reference fringe pattern, and an analyzing etalon compensation control loop for compensating a movement of the etalon mirrors. The laser beam to be stabilized in its wavelength passes through the system, thereby producing a second fringe pattern, which is used to stabilize the laser light wavelength. SUMMARY OF THE INVENTION It is an object of this invention to provide a stable laser apparatus. The above object is achieved by a laser apparatus comprising an optical resonator generating a laser light, a first sealed container filled with a gas, a wavelength selection element disposed within said first sealed container and selecting a wavelength of said laser light, means for detecting a central wavelength of said laser light output from a combination of said optical resonator and said wavelength selection element, and means for adjusting a density of said gas within said first sealed container, characterized in that said means for adjusting said density of said gas adjusts the density of said gas in accordance with said detected central wavelength of the output laser in a feedback manner, and said means for detecting a central wavelength comprises a second sealed container filled with a gas, a wavelength dispersion element disposed within said second sealed container and forming a spectrum pattern of said output laser light, means for sensing said spectrum pattern, and means for determining said central wavelength of said output laser light on the basis of said sensed spectrum pattern. In the following description of the drawings and the invention, basic explanations are given with reference to illustrating examples in 1 to 5 in combination with figures 1 to 7 for facilitating the understanding of the following preferred embodiments 1 to 7 and figs. 8 to 19 essential for the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of a laser apparatus according to a first illustrating example. Fig. 2(a) is a graph showing the relationship between intensity and wavelength of light generated by a general excimer laser. Fig. 2(b) is a graph showing the relationship between intensity and wavelength of light generated by the laser apparatus of Fig. 1. Fig. 3 is a diagram of a laser apparatus according to a second illustrating example. Fig. 4 is a diagram of a laser apparatus according to a third illustrating example. Fig. 5 is a diagram of part of a laser apparatus according to a fourth illustrating example. Fig. 6 is a diagram of a laser apparatus according to a fifth illustrating example. Fig. 7 is a diagram of the wavelength detector of Fig. 6. Fig. 8 is a diagram of a laser apparatus according to a first embodiment of this invention. Fig. 9 is a diagram of a laser apparatus according to a second embodiment of this invention. Fig. 10 is a diagram of a wavelength detector, a signal processor, and valves in a first modification of the embodiment of Fig. 9. Fig. 11(a) is a diagram showing the fringe, the linear image sensor, and a light intensity signal in the modification of Fig. 10. Fig. 11(b) is a graph showing the relationship between the degree of the non-symmetry of the light intensity signal and the distance beween the center of the fringe and the linear image sensor in the modification of Fig. 10. Fig. 12 is a diagram of a wavelength detector, a signal processor, and valves in a second modification of the embodiment of Fig. 9. Fig. 13 is a diagram of a laser apparatus according to a third embodiment of this invention. Fig. 14 is a diagram of the linear image sensor, the signal processor, and the valves of Fig. 13. Fig. 15 is a diagram of a linear image sensor, a signal processor, and valves in a modification of the embodiment of Fig. 13. Fig. 16 is a diagram of a laser apparatus according to a fourth embodiment of this invention. Fig. 17 is a diagram of a laser apparatus according to another example. Fig. 18 is a diagram of a laser apparatus according to another example. Fig. 19 is a diagram of a laser apparatus according to another example. DESCRIPTION OF THE FIRST illustrating example As shown in Fig. 1, a laser apparatus includes a discharge tube 1 extending between a total reflection mirror 2 and an output mirror 3. The discharge tube 1, the total reflection mirror 2, and the output mirror 3 form an optical resonator. The discharge tube 1 contains a known excimer laser medium including a mixture of a rare gas and a halogen gas. In this laser apparatus, laser oscillation occurs at a frequency within an ultraviolet range. An air space etalon 4 is disposed between the discharge tube 1 and the total reflection mirror 2. An optical axis of the optical resonator extends through the air space etalon 4. The air space etalon 4 is composed of one type of a Fabry-Perot etalon, including a pair of parallel flat-face quartz plates opposed to and spaced from each other by a small gap. The air space etalon 4 is disposed within a sealed container 5. The devices 1, 2, 3, and 5 are arranged on a common base (not shown). In addition, the air space etalon 4 is mounted on the wall of the sealed container 5 by use of suitable members (not shown). As shown in Fig. 2(a), a general excimer laser has a gain bandwidth of about 1 nm. The air space etalon 4 decreases the gain bandwidth of the excimer laser to a value of about 0.001 nm as shown in Fig. 2(b). Generally, in cases where the air space etalon 4 is exposed to atmosphere, the central wavelength of the laser light varies by about 0.01 nm or less during the operation of the laser apparatus. The inventors and others found that, in cases where the air space etalon 4 was disposed within a sealed container 5 as in this first illustrating example, the variation in the central wavelength of the laser light was suppressed. The found effect seems to be based on the following reasons. The wavelength selected by the air space etalon 4 depends on the refractive index related to the gap of the air space etalon 4. The refractive index is determined solely by the density of the gas in the gap of the air space etalon 4. In cases where the air space etalon 4 is exposed to atmosphere, as the pressure or temperature changes and thus the density of the gas in the gap of the air space etalon 4 varies, the refractive index changes and, thus, the selected wavelength shifts. In cases where the air space etalon 4 is disposed within a sealed container 5, the density of the gas in the gap of the air space etalon 4 is independent of external pressure and temperature so that the refractive index remains unchanged even when the pressure or temperature varies. Accordingly, in the latter cases, the wavelength selected by the air space etalon 4 remains essentially constant and independent of pressure and temperature. According to experiments, in the laser apparatus of this first illustrating example, the variation in the central wavelength of the laser light was held within a range of ±0.001 nm. DESCRIPTION OF THE SECOND illustrating example Fig. 3 shows a second illustrating example which is similar to the first illustrating example of Fig. 1 except for the following design changes. The second illustrating example of Fig. 3 uses a grating 6 in place of the air space etalon 4 (see Fig. 1). Light travels between mirrors 2 and 3 while being reflected by the grating 6. The grating 6 selects light having a predetermined wavelength. In general, the wavelength selected by a grating is determined by the density of the gas surrounding the grating. Accordingly, the grating 6 is disposed within a sealed container 5 so that the wavelength selected by the grating 6 is kept essentially constant. DESCRIPTION OF THE THIRD illustrating example Fig. 4 shows a third illustrating example which is similar to the first illustrating example of Fig. 1 except for the following design changes. In the third illustrating example of Fig. 4, an air space etalon 4 and a grating 6 are disposed within a common sealed container 5. Light travels between mirrors 2 and 3 while being reflected by the grating 6 and passing through the air space etalon 4. The air space etalon 4 and the grating 6 select lights having predetermined wavelengths, respectively. The wavelengths selected by the air space etalon 4 and the grating 6 are set equal so that the width of wavelengths of the laser light can be acceptably small. For example, the wavelength width is about 0.005 nm. As in the illustrating examples of Figs. 1 and 3, the wavelength selected by the air space etalon 4 and the grating 6 is essentially independent of the pressure and the temperature outside the sealed container 5. According to experiments, in the laser apparatus of this third illustrating example, the variation in the central wavelength of the laser light was kept within a range of ±0.001 nm. A high pressure air source 7 and a low pressure air source 8 are connected to the sealed container 5 via electrically-driven valves 9 and 10, respectively. When the valve 9 is opened with the valve 10 being closed, high pressure air enters the sealed container 5 and, thus, the density of the air residing in the gap of the air space etalon 4 and surrounding the grating 6 increases, so that the wavelength selected by the air space etalon 4 and the grating 6 varies in one direction. When the valve 9 is closed with the valve 10 being opened, low pressure air enters the sealed container 5 and, thus, the density of the air residing in the gap of the air space etalon 4 and surrounding the grating 6 decreases, so that the wavelength selected by the air space etalon 4 and the grating 6 varies in an opposite direction. A controller 11 outputs control signals to the valves 9 and 10. An exposure system including the laser apparatus has a device (not shown) detecting a shift of the position of the image formation. The control signals applied to the valves 9 and 10 are generated by the controller 11 in accordance with a signal from the detecting device, so that the valves 9 and 10 are opened and closed in accordance with the shift of the position of the image formation. In other words, the density of the air within the sealed container 5 is controlled in accordance with the shift of the position of the image formation. During the operation of the laser apparatus, the air space etalon 4 and the grating 6 absorb the laser light and, thereby, the temperature of the devices 4 and 6 increases. This temperature increase would cause a variation in the central wavelength of the laser light and, thus, cause a shift of the position of the image formation. The control of the density of the air within the sealed container 5 in response to the shift of the position of the image formation is designed so as to compensate for such a variation in the central wavelength of the laser light caused by the temperature increase. This compensation control enables a better stabilized central wavelength of the laser light. The central wavelength of the laser light can be positively varied by changing the characteristics of the controller 11 or the detecting device. Accordingly, the laser apparatus of this third illustrating example is of the tunable type. The variable range of the central wavelength of the laser light is determined by a gain bandwidth of a laser medium contained in the discharge tube 1. It should be noted that various modifications may be made in this third illustrating example. In a first modification, the sealed container 5 including the air space etalon 4 and the grating 6 is disposed between the discharge tube 1 and the output mirror 3. In a second modification, the grating 6 and the total reflection mirror 2 are formed by a common optical device. In a third modification, the grating 6 and the output mirror 3 are formed by a common optical device. In a fourth modification, the grating 6 is replaced with a prism or a solid etalon. In a fifth modification, either the air space etalon 4 or the grating 6 is disposed outside the sealed container 5. DESCRIPTION OF THE FOURTH illustrating example Fig. 5 shows a fourth illustrating example which is similar to the third illustrating example of Fig. 4 except for the following design changes. The fourth illustrating example of Fig. 5 uses a combination of bellows 12 and an actuator 13 in place of the combination of the high pressure air source 7, the low pressure air source 8, and the valves 9 and 10 (see Fig. 4). A sealed container 5 is connected to the bellows 12, so that the pressure or the density of the air within the sealed container 5 can be varied by expanding or contracting the bellows 12. The bellows 12 is driven by the actuator 13. It should be noted, that the air within the sealed container 5 may be replaced with other gases such as a nitrogen gas, a rare gas, or an inert gas. DESCRIPTION OF THE FIFTH illustrating example Fig. 6 shows a fifth illustrating example which is similar to the third illustrating example of Fig. 4 except for the following design changes. The fifth illustrating example of Fig. 6 includes a beam splitter or a semitransparent mirror 15 exposed to laser light emitted via an output mirror 3. A portion of the output laser light passes through the semitransparent mirror 15. Another portion of the output laser light is reflected by the semitransparent mirror 15 and entering a wavelength detector 16. The wavelength detector 16 outputs a signal which represents the central wavelength of the output laser light. The output signal from the wavelength detector 16 is applied to a comparator 17. A reference signal representative of a reference wavelength is also applied to the comparator 17. The device 17 compares the output signal from the wavelength detector 16 with the reference signal and generates control signals which depend on a difference between the detected central wavelength of the output laser light and the reference wavelength. Valves 9 and 10 are driven by the control signals from the comparator 17, so that the valves 9 and 10 are controlled in accordance with a difference between the detected wavelength and the reference wavelength so that the central wavelength of the output laser light can be kept essentially equal to the reference wavelength. In this way, the central wavelength of the output laser light is controlled in a feedback manner. The central wavelength of the output laser light can be positively varied by changing the reference signal applied to the comparator 17. The variable range of the central wavelength of the laser light is determined by a gain bandwidth of a laser medium contained in the discharge tube 1. As shown in Fig. 7, the wavelength detector 16 includes a combination of a lens 16A, a Fabry-Perot etalon 16B, and a lens 16C, which forms an image of a band pattern of the laser light on a linear image sensor 16D. An output signal from the linear image sensor 16D varies as a function of the central wavelength of the laser light. DESCRIPTION OF THE FIRST PREFERRED EMBODIMENT Fig. 8 shows a first embodiment of this invention, which is similar to the illustrating examples of Figs. 6 and 7 except for the following design changes. The embodiment of Fig. 8 uses an air space etalon 6A in place of the grating 6 (see Fig. 6). The air space etalon 6A and another air space etalon 4 have different free spectral ranges. The lens 16A of a wavelength detector 16 is omitted from the embodiment of Fig. 8. In the wavelength detector 16, the air space etalon or the Fabry-Perot etalon 16B is disposed within a sealed container 16E. Since the refractive index of gas is determined solely by the density of the gas as described previously, the refractive index related to a gap of the air space etalon 16B remains essentially constant. Accordingly, the characteristics of the wavelength detector 16 are essentially independent of the temperature the pressure outside the sealed container 16E. The sealed container 16E is preferably filled with a nitrogen gas, a rare gas, or an inert gas. The combination of the air space etalon 16B and the lens 16C forms an image of a spectrum of the laser light on the linear image sensor 16D. The image takes a pattern of concentrical fringes 16F. The positions of the fringes 16F are detected by the linear image sensor 16D. It should be noted, that the positions of the fringes 16F depend on the central wavelength of the laser light. An output signal from the linear image sensor 16D which represents the positions of the fringes 16F is applied to a signal processor 18. The signal processor 18 determines the position of a predetermined one of the fringes 16F by processing the output signal from the linear image sensor 16D, calculates the central wavelength of the laser light from the determined fringe position, and determines a difference between the calculated central wavelength and a setting wavelength. The signal processor 18 generates control signals in accordance with the difference between the central wavelength of the laser light and the setting wavelength, which are used for driving the Valves 9 and 10 in accordance with the difference between the two wavelengths. The control of the valves 9 and 10 is designed so that the central wavelength of the laser light can be kept essentially equal to the setting wavelength set in the signal processor 18. The central wavelength of the laser light can be positively varied by changing the setting wavelength. Since the wavelength detection via the device 16 is independent of variations in the temperature and the pressure as described previously, the control of the central wavelength of the laser light can be accurate. DESCRIPTION OF THE SECOND PREFERRED EMBODIMENT Fig. 9 shows a second embodiment of this invention, which is similar to the embodiment of Fig. 8 except for the following design changes. The embodiment of Fig. 9 includes a mirror 19 directing the laser light from the semitransparent mirror 15 toward the wavelength detector 16. The air space etalon 6A (see Fig. 8) is omitted from the embodiment of Fig. 9. The laser light enters the wavelength detector 16 via an optical front plate 16G. The wavelength detector 16 includes a combination of the lens 16A, the air space etalon 16B, and the lens 16C, which forms an image of a spectrum of the laser light on a plane where the linear image sensor 16D is located. The image takes a pattern of concentrical fringes 16F. The linear image sensor 16D is positioned relative to the fringes 16F so as to extend through the center of the fringes 16F. The positions of the fringes 16F are detected via the linear image sensor 16D. The signal processor 18 determines a diameter of a predetermined one of the fringes 16F by processing the output signal of the linear image sensor 16D, calculates the central wavelength of the laser light from the determined fringe diameter, determines a difference between the calculated central wavelength and a setting wavelength, and generates control signals in accordance with the difference between the central wavelength of the laser light and the setting wavelength. The valves 9 and 10 are driven by the control signals from the signal processor 18 so that the valves 9 and 10 are controlled in accordance with the difference between the two wavelengths. If the air space etalon 16B and the linear image sensor 16D move out of a normal positional relationship due to any factors, the diameters of the fringes 16F vary to a degree less than a degree of the movement of the devices 16B and 16D. Accordingly, the central wavelength of the laser light can be accurately detected via the wavelength detector 16 for a long time. The accurate wavelength detection enables accurate control of the central wavelength of the laser light. Fig. 10 shows a first modification of the embodiment of Fig. 9. In this modification, the linear image sensor 16D is offset from the diameters of the concentrical fringes 16F. The signal processor 18 determines the diameter of a predetermined one of the fringes 16F by processing an output signal from the linear image sensor 16D. As shown in Fig. 11(a), a light sensitive portion of the linear image sensor 16D has a uniformely distributed area. Accordingly, in cases where the linear image sensor 16D is offset from the diameters of the fringes 16F, a light intensity signal output from the linear image sensor 16 has a non-symmetrical shape. As shown in Fig. 11(b), a distance "z" between the center of the fringes 16F and the linear image sensor 16D varies in accordance with a given function "f" of "x", which equals a ratio between the integrals I1 and I2 of the signal intensity in both sides of a peak in the light intensity signal, and which represents the degree of non-symmetry of the light intensity signal. The diameter D of the predetermined fringe is calculated by the equation D = (L² + 4z²) 1/2 , where the letter L denotes the distance between the points at which the predetermined fringe crosses the linear image sensor 16D. Fig. 12 shows a second modification of the embodiment of Fig. 9. This modification uses a two dimensional image sensor 21 in place of the linear image sensor 16D (see Fig. 9). The signal processor 18 determines the diameter of a predetermined one of the fringes 16F by processing an output signal from the two dimensional image sensor 21. DESCRIPTION OF THE THIRD PREFERRED EMBODIMENT Figs. 13 and 14 show a third embodiment of this invention, which is similar to the embodiment of Fig. 9 except for the following design changes. In the embodiment of Figs. 13 and 14, the air space etalons 4 and 6A are disposed within a common sealed container 5. As best shown in Fig. 14, the linear image sensor 16D includes a linear array of photo sensitive cells forming respective sensor channels. The signal processor 18 processes output signals from the respective channels of the linear image sensor 16D and thereby calculates an integral I1 of the intensity of the laser light on an area of the linear image sensor 16D which extends inward of a boundary between reference cells of the sensor 16D. In addition, the signal processor 18 calculates an integral I2 of the intensity of the laser light on an area of the linear image sensor 16D which extends outward of the boundary between the reference cells of the sensor 16D. The signal processor 18 includes a subtracter 18A deriving a difference DIFF between the integrals I1 and I2, and a controller 18B outputting control signals to the valves 9 and 10. A reference signal representative of a reference value Vr is applied to the controller 18B. The controller 18B adjusts the valves 9 and 10 via the control signals so that the difference DIFF can be essentially equal to the reference value Vr. Since the difference DIFF depends on the position of a peak of the intensity of the laser light on the linear image sensor 16D, the adjustment of the valves 9 and 10 enables the position of the peak of the light intensity to be kept essentially at a reference position corresponding to the reference value Vr. In addition, since the position of the peak of the light intensity depends on the central wavelength of the laser light, the adjustment of the valves 9 and 10 enables the central wavelength of the laser light to be kept essentially equal to a reference wavelength determined by the reference value Vr. As the reference value Vr is varied continuously, the central wavelength of the laser light is changed continuously without any limitation related to spaces between the cells of the linear image sensor 16D. According to experiments, the accuracy in controlling and setting the central wavelength of the laser light was within a range below 0.5 pm. Reflecting film coatings on the surfaces of the air space etalons 4 and 6A tend to be deteriorated. This phenomenon seems to be caused in the following processes. Oxygen is activated by absorbing the laser light. The activated oxygen bonds to atoms of the film coats. This reaction deteriorates the film coats. According to experiments, in cases where the pressure in the sealed container 5 was adjusted within a low pressure range around 1 Pa, the life of the film coats of the air space etalons 4 and 6 was longer by a factor of about 100 than that obtained under conditions where the pressure in the sealed container 5 was around the atmospheric pressure. The longer life seems to result from the fact that the reduction of the pressure in the sealed container 5 lowers the density of oxygen in the sealed container 5. Accordingly, it is preferable that the pressure in the sealed container 5 is adjusted within a range below the atmospheric pressure. In addition, the sealed container 5 may be filled with a nitrogen gas, a rare gas, or an inert gas. Fig. 15 shows a modification of the embodiment of Figs. 13 and 14. In this modification, the linear image sensor 16D is of a scanning type, and incident light signals are sequentially read out from respective channels of the linear image sensor 16D in accordance with three-phase clocks fed from a clock generator 18L within the signal processor 18. The output signals from the linear image sensor 16D are transmitted to a switch 18E via the amplifier 18D within the signal processor 18. A counter 18M controls the switch 18E in response to pulses from the clock generator 18L, so that the output signals from the (x-m)-th channel to the x-th channel of the linear image sensor 16D are passed to an integrator or low pass filter 18F, and the output signals from the (x+1)-th channel to the (x+m+1)-th channel of the linear image sensor 16D are passed to an integrator or low pass filter 18G. The x-th channel is a reference adjacent to a predetermined fringe. The measurement width "m" is set around the bandwidth of the predetermined fringe. The low pass filter 18F generates a signal I1 representing an integral of the received signals. The low pass filter 18G generates a signal I2 representing an integral of the received signals. The subtracter 18A generates a signal DIFF representing a difference between the integrated intensity signals I1 and I2 which is applied to a differential amplifier 18H. A potentiometer 18N generates a reference signal Vr representing a reference value which is applied to the differential amplifier 18H. The differential amplifier 18H compares the difference signal DIFF and the reference signal Vr and generates a signal dependent on a difference between the signals DIFF and Vr. A driver 18J generates a control signal on the basis of the output signal from the differential amplifier 18H. A driver 18K generates a control signal on the basis of the output signal from the differential amplifier 18H. The control signals are transmitted from the drivers 18J and 18K to the valves 9 and 10, respectively, which are driven by the control signals so that the difference signal DIFF becomes equal to the reference signal Vr. As a result, the position of the predetermined fringe is kept essentially at the reference position determined by the reference signal Vr. In addition, the central wavelength of the laser light is kept essentially at the reference wavelength determined by the reference signal Vr. The central wavelength of the laser light can be varied by adjusting the potentiometer 18N to change the reference signal Vr. DESCRIPTION OF THE FOURTH PREFERRED EMBODIMENT Fig. 16 shows a fourth embodiment of this invention, which is similar to the embodiment of Fig. 9 except for the following design changes. In the embodiment of Fig. 16, laser light reflected by the semitransparent mirror or the beam splitter 15 is directly applied to the wavelength detector 16. The valve 9 connected between the high pressure gas source 7 and the sealed container 5 is controlled in response to the output signal from the wavelength detector 16. The valve 10 connected between the sealed container 5 and the low pressure source or pump 8 is also controlled in response to another output signal from the wavelength detector 16. Accordingly, the pressure or the density of gas within the sealed container is controlled in accordance with the detected wavelength of the laser light. The gas supplied from the high pressure gas source 7 to the sealed container is preferably a nitrogen gas, a rare gas, or an inert gas. The gas is most preferably a helium gas or an argon gas from the standpoint of the life of film coatings on surfaces of the air space etalon 4. DESCRIPTION OF ANOTHER EXAMPLE Fig. 17 shows an example which is similar to the embodiment of Fig. 16 except for the following design changes. In the example of Fig. 17, the discharge tube 1, the total reflection mirror 2, and the output mirror 3 are supported on a common base 31. The etalon 4 disposed between the discharge tube 1 and the total reflection mirror 2 is exposed. An angle of the etalon 4 is adjustable relative to an optical axis of an optical resonator composed of the devices 1-3. The wavelength selected by the etalon 4 depends on the angle of the etalon 4. The etalon 4 is driven by an actuator 33 arranged on the base 31. Laser light reflected by the beam splitter 15 is guided to the wavelength detector 16 via an optical fiber cable 32. The wavelength detector 16 is fixed to the base 31. The actuator 33 is driven in response to an output signal from the wavelength detector 16 so that the angle of the etalon 4 is controlled in accordance with the detected wavelength of the laser light. The control of the angle of the etalon 4 is designed so that the wavelength of the output laser light becomes essentially equal to a setting wavelength. The base 31 is preferably made of metal. The discharge tube 1 and the wavelength detector 16 are located at opposite sides of the base 31 respectively so that electromagnetic noises from the discharge tube 1 interfere with the wavelength detector 16 to a negligible degree. DESCRIPTION OF ANOTHER EXAMPLE Fig. 18 shows an example which is similar to the example of Fig. 17 except for the following design changes. In the example of Fig. 18, the wavelength detector 16 outputs an optical control signal to the etalon actuator 33 via an optical fiber cable 35. Accordingly, the output signal from the wavelength detector 16 is protected from electromagnetic noises generated by the discharge tube 1. DESCRIPTION OF ANOTHER EXAMPLE Fig. 19 shows an example which is similar to the example of Fig. 18 except for the following design changes. An exposure system 41 using laser light includes a wavelength sensor detecting a wavelength of the laser light. An output signal from the wavelength sensor is applied to a controller 42. The controller 42 generates an optical control signal in accordance with the output signal from the wavelength sensor. The optical control signal is guided from the controller 42 to the etalon actuator 33 by an optical fiber cable 43. Accordingly, the control signal to the etalon actuator 33 is protected from electromagnetic noises generated by the discharge tube 1.

Claims: A laser apparatus comprising a) an optical resonator (2, 3) generating a laser light; b) a first sealed container (5) filled with a gas; c) a wavelength selection element (4; 4, 6; 4, 6A) disposed within said first sealed container (5) and selecting a wavelength of said laser light; d) means (16) for detecting a central wavelength of said laser light output from a combination of said optical resonator (2,3) and said wavelength selection element (4; 4, 6; 4, 6A); and e) means (7, 8, 9, 10) for adjusting a density of said gas within said first sealed container (5), characterized in that f) said means (7, 8, 9, 10) for adjusting said density of said gas adjusts the density of said gas in accordance with said detected central wavelength of the output laser in a feedback manner; and g) said means (16) for detecting a central wavelength comprise a second sealed container (16E) filled with a gas, a wavelength dispersion element (16B) disposed within said second sealed container (16E) and forming a spectrum pattern (16F) of said output laser light, means (16D) for sensing said spectrum pattern (16F), and means (18) for determining said central wavelength of said output laser light on the basis of said sensed spectrum pattern. A laser apparatus according to claim 1, characterized in that said laser light is an ultra-violet laser light. A laser apparatus according to claim 1, characterized in that said gas within said first sealed container (5) includes an inert gas. A laser apparatus according to claim 3, characterized in that a pressure within said first sealed container (5) is lower than an atmospheric pressure. A laser apparatus according to claim 1, characterized in that said wavelength selected by said wavelength selection element (4; 4, 6; 4, 6A) resides within a gain bandwidth of said laser medium. A laser apparatus according to claim 5, characterized in that said selected wavelength is variable within said gain bandwidth in accordance with said density of said gas within said first sealed container (5); A laser apparatus according to claim 1, characterized in that said central wavelength is determined by said selected wavelength; A laser apparatus according to claim 1, characterized in that said gas within said second sealed container (16E) includes an inert gas. A laser apparatus according to claim 1, characterized by means for determining a target wavelength. A laser apparatus according to claim 9, characterized by comprising means (17) for deriving a difference between said detected central wavelength and said target wavelength. A laser apparatus according to claim 10, characterized by comprising means for varying said target wavelength to vary said central wavelength of said output laser light. A laser apparatus according to claim 1, characterized in that said means (18) for determining said central wavelength comprise means (18G) for calculating an integral of an intensity of light in said spectrum pattern (16F) located in a shorter wavelength side of a reference point. A laser apparatus according to claim 12, characterized in that said means (18) for determining said central wavelength comprise means (18F) for calculating an integral of an intensity of light in said spectrum pattern (16F) located in a longer wavelength side of said reference point. A laser apparatus according to claim 13, characterized in that said means (18) for determining said central wavelength comprise means (18A) for comparing said calculated integrals. A laser apparatus according to claim 14, characterized in that said means (18) for determining said central wavelength comprise means (18H) for calculating said central wavelength on the basis of the comparison between said integrals. A laser apparatus according to claim 1, characterized in that said wavelength detecting means (16) outputs a signal to controlling means (33) via an optical fiber cable (35).

Inventors: KAWAHARA HIDEHITO, MIKI TADAAKI, OGATA YOSHIRO, SHIMADA YASUHIRO, WANI KOICHI, KAWAHARA, HIDEHITO, MIKI, TADAAKI, OGATA, YOSHIRO, SHIMADA, YASUHIRO, WANI, KOICHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H01S3
IPC codes: H01S3