Patent Publication Number: US-2003231687-A1

Title: Ultraviolet laser device

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an ultraviolet laser device.  
       [0003] 2. Description of the Related Art  
       [0004] Conventionally known ultraviolet laser devices include an excimer laser device with a wavelength narrow-banded and a fluorine molecular laser device. FIG. 10 shows a top view of the ultraviolet laser device according to a related art, which will be described below with reference to FIG. 10.  
       [0005] In FIG. 10, an ultraviolet laser device  11  is provided with a laser chamber  12  in which laser gas is sealed. In the fluorine molecular laser system, for example, the laser gas contains fluorine and inert gas such as helium or neon and sealed in under, for example, an absolute pressure of about 3 to 4 atmospheres higher than atmospheric pressure.  
       [0006] The laser chamber  12  has therein a pair of main electrodes  14 ,  15  which are disposed to perpendicularly oppose the drawing of FIG. 10. An unshown fan for feeding the laser gas to between the main electrodes  14 ,  15  and an unshown heat exchanger for cooling the laser gas are also disposed in the laser chamber  12 .  
       [0007] Windows  17 ,  19  allowing the passage of ultraviolet laser light  21  are respectively disposed before and after the laser chamber  12 . The windows  17 ,  19  each are fixed to the laser chamber  12  with an unshown window holder (not shown in FIG. 10). The windows  17 ,  19  are respectively disposed at Brewster angle θB with respect to an optical axis of the ultraviolet laser light  21 .  
       [0008]FIG. 11 shows a cross-sectional view of a window holder  37  for holding the rear window  19 . The front window  17  is also held in the same manner. As shown in FIG. 11, the window holder  37  has the rear window  19  held between a base  37 B and a lid  37 A, which are fixed by clamping with bolts  42 . And, O-rings  41 ,  41  are respectively held between the rear window  19  and the window holder  37  and between the window holder  37  and the laser chamber  12  to seal the laser gas. Thus, the windows  17 ,  19  are pressed against the O-ring via the lid  37 A by tightening the bolts  42  to receive a force.  
       [0009] The surface (hereinafter called as the inside surface  19 A) of the rear window  19  directly next to the laser chamber is in contact with the laser gas having a pressure equal to or higher than atmosphere pressure. Therefore, the rear window  19  is exposed to the pressure as indicated by arrows  36  from the inside surface  19 A to the surface  19 B (hereinafter called as the outside surface) of the rear window  19  away from the laser chamber.  
       [0010] In the description below, linearly polarized light passing through the windows  17 ,  19  disposed at the Brewster angle θB to the optical axis is called P polarized light, which is indicated by P in FIG. 11. Linearly polarized light perpendicular to the P polarized light and obstructed by the windows  17 ,  19  is called as the S polarized light and indicated by S in FIG. 11.  
       [0011] Then, the ultraviolet laser light  21  will be described.  
       [0012] In FIG. 10, a high voltage is applied from a high-voltage power supply  23  to between the main electrodes  14 ,  15  according to instructions from a laser controller  29  to cause main discharge. Thus, the laser gas is excited to produce the ultraviolet laser light  21 .  
       [0013] The produced ultraviolet laser light  21  passes through, for example, the rear window  19  to enter a line-narrowing module  31 . The line-narrowing module  31  has therein, for example, two prisms  32 ,  32  and a grating  33 . The ultraviolet laser light  21  enters the inside surfaces of the prisms  32 ,  32  at an angle close to the Brewster angle θB and goes out substantially perpendicularly from an outside surface  32 B. At this time, the ultraviolet laser light  21  is made to have an increased beam width by the prisms  32 ,  32 .  
       [0014] The ultraviolet laser light  21  having entered the grating  33  has a wavelength close to a desired center wavelength diffracted on a diffracting plane and is reflected in the incident direction. The ultraviolet laser light  21  is repeatedly reflected between the grating  33  and a front mirror  16  which is disposed in front of the laser chamber  12  so to be amplified by the main discharge and partly passes through the front mirror  16  to go out. The output ultraviolet laser light  21  enters an exposure  25  such as a stepper to become exposure light.  
       [0015] At the time, the windows  17 ,  19  are disposed at the Brewster angle θB to the optical axis, so that the ultraviolet laser light  21  passing through the windows  17 ,  19  becomes almost P polarized light. Therefore, the P polarized light is amplified in the laser chamber  12 , and the output ultraviolet laser light  21  also becomes almost P polarized light.  
       [0016] The above-described related art, however, has the following drawbacks.  
       [0017] Specifically, optical elements, such as the windows  17 ,  19  and the prisms  32 ,  32 , through which the laser light  21  passes are formed of, for example, a crystal of fluoride such as calcium fluoride. Such a crystal of fluoride might cause birefringence when the laser light  21  passes through it.  
       [0018] The birefringence is intrinsically held by the crystal of fluoride or produced when a force is applied to the optical elements.  
       [0019] In the latter, the force includes a residual stress such as a thermal stress at the time of production of crystal and a force which is applied when the optical elements are held by a holder or the like. As shown in FIG. 11, the windows  17 ,  19  are fixed to the window holder  37  to receive the force. They also receive the force due to a pressure difference from atmosphere pressure in order to seal the high-pressure laser gas as indicated by the arrows  36  in FIG. 11.  
       [0020] The other optical elements such as the prisms  32 ,  32  are also exposed to a fixing force applied by an unshown holder, and the birefringence is caused as a result.  
       [0021] The passage of the laser light  21  through the optical elements having the birefringence mixes an S polarized light with a phase delayed in addition to the P polarized light to the laser light  21 . The P polarized light passes at a high transmittance through the inside surfaces  32 A,  32 A of the prisms  32 ,  32 , which are disposed at approximately the Brewster angle θB to the laser light  21 , while the S polarized light is reflected from the inside surfaces  32 A,  32 A of the prisms  32 ,  32  and inside surfaces  17 A,  19 A of the windows  17 ,  19  as indicated by arrows  40 . Part of the S polarized light impinges on the inside surface of the line-narrowing module  31 , the prisms  32 ,  32  and the grating  33  to change to heat. As a result, the optical elements have an increased temperature, and a refractive index is partly changed. And, the laser light  21  may have a change in beam cross-sectional shape or intensity distribution. Besides, pulse energy may become low.  
       [0022] Another part of the S polarized light may reenter the light path of the laser light  21  and emerge from the front mirror  16 . The S polarized light includes light which is not narrow-banded by the grating  33 , so that the output laser light  21  may have its wavelength characteristics such as a spectral line width or center wavelength degraded. The degradation of the wavelength characteristics is caused when a stress applied to the optical element material is changed by the thermal expansion resulting from the temperature increase of the optical elements.  
       [0023] The present invention was achieved in view of the above-described drawbacks, and it is aimed to provide a laser device which stably emits laser light with the birefringence of the optical elements reduced.  
       SUMMARY OF THE INVENTION  
       [0024] The present invention has been made in view of the above circumstances and provides an ultraviolet laser device, wherein at least one of optical elements, which configure a laser resonator and through which laser light passes, is formed by annealing a crystal.  
       [0025] Thus, a birefringence amount of the optical element is reduced and birefringence becomes hard to occur when the laser light passes through the optical element interior, so that the polarized light state of the laser light does not change much.  
       [0026] The ultraviolet laser device of the invention is featured in that at least one of the optical elements is disposed so that the laser light passes therethrough substantially perpendicularly to its cleave plane.  
       [0027] When the laser light passes through the cleave plane perpendicularly, birefringence hardly occurs, so that the polarized light state of the laser light does not change much.  
       [0028] The ultraviolet laser device of the invention is featured in that the optical element is formed so that a cleave plane becomes substantially parallel to at least one of planes through which the laser light enters and leaves.  
       [0029] Thus, when the planes of incidence and outgoing are configured so that the laser light passes substantially perpendicularly, the configuration to reduce the birefringence can be realized with ease.  
       [0030] And, at the time when the planes of incidence and outgoing are polished, they can be polished with high precision.  
       [0031] The ultraviolet laser device of the invention is featured in that the cleave plane is a &lt;111&gt; plane or a &lt;100&gt; plane of the crystal.  
       [0032] When the laser light passes substantially perpendicularly to the &lt;111&gt; plane of the cleave plane, the birefringence becomes particularly small. And, when the laser light passes substantially perpendicularly to the &lt;100&gt; plane, the birefringence becomes relatively small.  
       [0033] The ultraviolet laser device of the invention is featured in that at least one of the optical elements is disposed so that the cleave plane becomes substantially perpendicular to a force applied to the optical element.  
       [0034] When the optical element has a force applied substantially perpendicularly to the cleave plane, particularly the &lt;111&gt; plane, the birefringence hardly occurs.  
       [0035] The ultraviolet laser device of the invention is featured in that the crystal is fluoride.  
       [0036] Specifically, the fluoride allows the passage of the ultraviolet laser light at high transmittance, so that it is suitable as the optical element of the resonator.  
       [0037] The ultraviolet laser device of the invention is featured in that the fluoride is calcium fluoride.  
       [0038] Specifically, the calcium fluoride has the smallest birefringence which is originally possessed by the material among the substances which allow the passage of the ultraviolet laser light.  
       [0039] The ultraviolet laser device of the invention is featured in that at least one of the optical elements configuring the ultraviolet laser device has selectivity of polarized light in a given direction of the ultraviolet laser light.  
       [0040] Thus, when the laser light of linearly polarized light is to be emitted, the birefringence is small, so that the laser light seldom becomes oval polarized light, and laser light of desired polarized light can be obtained.  
       [0041] The present invention further provides an ultraviolet laser device, wherein at least one of optical elements, which configure a laser resonator and through which laser light passes, is formed by annealing glass.  
       [0042] For example, when the ultraviolet laser device is a KrF excimer laser device, synthetic quartz may be used as the optical element of the resonator, and the synthetic quartz is also made to have the birefringence become small by annealing and the strength increased. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0043]FIG. 1 is a top view of a fluorine molecular laser device according to a first embodiment of the invention;  
     [0044]FIG. 2 is a flow chart showing a procedure for production of an optical element;  
     [0045]FIG. 3 is a top view of an optical element of the fluorine molecular laser device according to a second embodiment of the invention;  
     [0046]FIG. 4 is an explanatory diagram showing a birefringence amount of a calcium fluoride crystal;  
     [0047]FIG. 5 is a top view showing another configuration of the optical element of the fluorine molecular laser device according to the second embodiment of the invention;  
     [0048]FIG. 6 is a top view of the fluorine molecular laser device according to a third embodiment of the invention;  
     [0049]FIG. 7 is a sectional view of an etalon holder;  
     [0050]FIG. 8 is a top view of the fluorine molecular laser device according to a fourth embodiment of the invention;  
     [0051]FIG. 9 is a top view showing another configuration of the fluorine molecular laser device according to the fourth embodiment of the invention;  
     [0052]FIG. 10 is a top view of the fluorine molecular laser device according to prior art; and  
     [0053]FIG. 11 is a sectional view of a window holder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0054] Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
     [0055] First, a first embodiment will be described. FIG. 1 shows a top view of a fluorine molecular laser device  11  according to this embodiment. In FIG. 1, the fluorine molecular laser device  11  is provided with a laser chamber  12  in which laser gas is sealed.  
     [0056] The laser chamber  12  has therein a pair of main electrodes  14 ,  15  which are disposed to perpendicularly oppose the drawing of FIG. 10. An unshown fan for feeding the laser gas to between the main electrodes  14 ,  15  and an unshown heat exchanger for cooling the laser gas are also disposed in the laser chamber  12 .  
     [0057] Windows  17 ,  19  allowing the passage of ultraviolet laser light  21  are respectively disposed before and after the laser chamber  12  by a window holder  37  (not shown in FIG. 1) as shown in FIG. 11.  
     [0058] In FIG. 1, a high voltage is applied from a high-voltage power supply  23  to between the main electrodes  14 ,  15  according to instructions from a laser controller  29  to produce a main discharge. Thus, the laser gas is excited to produce the fluorine molecular laser light  21  having a wavelength of approximately 157 nm.  
     [0059] For example, the produced laser gas passes through the rear window  19  to enter a line-narrowing module  31 . The line-narrowing module  31  has therein, for example, two prisms  32 ,  32  and a grating  33 . The fluorine molecular laser light  21  enters the prisms  32 ,  32  at an entry angle θP approximate to the Brewster angle θB to inside surfaces  32 A,  32 A of the prisms  32 ,  32  and emerges substantially perpendicularly from an outside surface  32 B. At that time, the fluorine molecular laser light  21  has its beam width increased by the prisms  32 ,  32 .  
     [0060] The fluorine molecular laser light  21  having entered the grating  33  has only a wavelength close to a desired center wavelength diffracted on a diffracting plane and is reflected in the incident direction. The fluorine molecular laser light  21  is repeatedly reflected between the grating  33  and the front mirror  16  which is disposed in front of the laser chamber  12  so to be amplified by the main discharge and partly passes through the front mirror  16  to go out.  
     [0061] The output fluorine molecular laser light  21  enters an exposure  25  such as a stepper as exposure light.  
     [0062] As materials for the optical elements such as the prisms  32 ,  32 , the grating  33 , the windows  17 ,  19  and the front mirror  16 , calcium fluoride (CaF 2 ) is used. When calcium fluoride is used, the fluorine molecular laser light  21  has the Brewster angle θB of approximately 57.3 degrees.  
     [0063]FIG. 2 shows a flow chart of a procedure for production of an optical element, e.g., the windows  17 ,  19 .  
     [0064] First, calcium fluoride to be a material for the optical element is melted, and a monocrystalline ingot is produced from the molten material (step S 11 ). The method for production of the ingot includes, for example, a method of growing a monocrystal by, for example, a pull method, a method of gradually cooling the molten material from an end, and the like.  
     [0065] Then, the produced ingot is cooled, placed in a furnace, raised its temperature up to a prescribed temperature at which the ingot does not melt (step S 12 ), and slowly cooled over a prescribed time (step S 13 ). Steps S 12 , S 13  are annealing, thereby removing a residual stress from the interior of calcium fluoride and reducing a birefringence amount.  
     [0066] The birefringence amount indicates a delay amount of polarized light having a slow speed when the polarized light of a high speed propagates within the optical element by 1 cm and indicated in nm/cm.  
     [0067] After the annealing is completed, the ingot is machined into substrates having the shape of the windows  17 ,  19  (step S 14 ). The machined substrates are placed in the furnace again and raised their temperatures up to a prescribed temperature (step S 15 ) and gradually cooled over a prescribed time (step S 16 ).  
     [0068] Specifically, additional annealing is performed in the steps S 15 , S 16 , so that the birefringence amount is further reduced. The annealing temperature and time in the steps S 15 , S 16  are not limited to be the same as in the steps S 12 , S 13 .  
     [0069] The inside surfaces  17 A,  19 A and the outside surfaces  17 B,  19 B of the annealed windows  17 ,  19  are polished (step S 17 ). Thus, the production of the windows  17 ,  19  is completed.  
     [0070] The windows  17 ,  19  were described above as an example, but the other optical elements such as the prisms  32 ,  32  and the front mirror  16  are also processed in the same way. Prescribed coating may be applied to the surface depending on a type of optical element after the step S 17 .  
     [0071] As described above, annealing is conducted in the process of producing the optical elements according to the first embodiment. Thus, the crystalline structure of each optical element is stabilized, and the birefringence originally possessed by the optical element and the birefringence produced when the optical element is produced become very small.  
     [0072] It is also known that the birefringence, which is produced by the force received from the holder when the optical element is attached to the holder or the force that the windows  17 ,  19  receive from a difference in pressure between the laser gas and the atmosphere (arrows  36  in FIG. 10), can be reduced by annealing.  
     [0073] Thus, S polarized light is seldom mixed into the fluorine molecular laser light  21  having passed through the optical element, and P polarized light has high purity.  
     [0074] As a result, the fluorine molecular laser light  21  reflected from the inside surfaces  32 A,  32 A of the prisms  32 ,  32  and the inside surfaces  17 A,  19 A of the windows  17 ,  19  is reduced. Therefore, it is seldom that pulse energy of the fluorine molecular laser light  21  is lowered or the fluorine molecular laser light  21  not narrow-banded is mixed into the laser chamber  12 .  
     [0075] Besides, the line-narrowing module  31  is seldom heated its interior by the reflected fluorine molecular laser light  21 , and the optical element or a light path space (if not vacuum) through which the laser light  21  passes seldom has a change in refractive index. Therefore, it is possible to obtain the fluorine molecular laser light  21  having a stable beam cross-sectional shape, wavelength characteristics and pulse energy.  
     [0076] Referring to the flow chart of FIG. 2, it was described to perform two times of annealing in the steps S 12 , S 13  and steps S 15 , S 16 , but the two times of annealing is not essential, and only one of them may be performed. Besides, additional annealing may be performed after the step S 17 .  
     [0077] It was also described above that the temperature is raised in the steps S 12 , S 15  and gradually lowered in the steps S 13  and S 16  to perform only one set of temperature increase and decrease. But, it is not a limited procedure. For example, the steps S 12 , S 13  may be repeated a plurality of times just after the annealing is performed in the steps S 12 , S 13 .  
     [0078] Then, a second embodiment will be described.  
     [0079]FIG. 3 shows a top view of the structure of the optical elements in the fluorine molecular laser device  11  according to the second embodiment. Each optical element is previously annealed as described in the first embodiment. In FIG. 3, a cleave plane  35  of a calcium fluoride crystal of each optical element is indicated by a broken line. As the cleave plane  35 , a &lt;111&gt; plane which has a minimum birefringence amount is optimum. And, a &lt;100&gt; plane which does not match the &lt;111&gt; plane but has a relatively small birefringence amount in the cleave plane  35  may be used.  
     [0080] First, the prisms  32 ,  32  are configured to have their outside surfaces  32 B,  32 B substantially parallel to the cleave planes  35 ,  35 . And, the prisms  32 ,  32  are disposed so that the fluorine molecular laser light  21  passes through the prisms  32 ,  32  substantially perpendicularly to their cleave planes  35 ,  35 .  
     [0081] The windows  17 ,  19  are disposed at the Brewster angle θB to the optical axis of the fluorine molecular laser light  21  as described above. In that state, the cleave plane  35  is configured to make the fluorine molecular laser light  21  pass through the windows  17 ,  19  substantially perpendicularly to the cleave plane  35 .  
     [0082] At that time, the fluorine molecular laser light  21  enters the surfaces  17 A,  17 B,  19 A,  19 B of the windows  17 ,  19  at an entry angle of the Brewster angle θB and refracts there. When it is assumed that calcium fluoride has a refractive index of 1.56 and the Brewster angle θB is 57.3 degrees, the fluorine molecular laser light  21  which outgoes the surfaces  17 A,  17 B,  19 A,  19 B of the windows  17 ,  19  has an outgoing angle θC of approximately 32.6 degrees. In other words, the cleave planes of the windows  17 ,  19  are formed to be substantially perpendicular to the fluorine molecular laser light  21  having the outgoing angle θC to the surfaces  17 A,  17 B,  19 A,  19 B.  
     [0083] Besides, the inside surface  16 A of the front mirror  16  is disposed to be substantially perpendicular to the optical axis of the fluorine molecular laser light  21 , and the cleave plane  35  is configured to be substantially parallel to the inside surface  16 A. The outside surface  16 B of the front mirror  16  is slanted so to be not parallel to the inside surface  16 A.  
     [0084] According to the second embodiment as described above, the optical element made of calcium fluoride is annealed and disposed so that the fluorine molecular laser light  21  passes through the optical element substantially perpendicularly to the cleave plane  35  of the crystal. Thus, the birefringence amount of the optical element which has become small by annealing can further be made smaller. Besides, a much smaller birefringence amount can be obtained by using the &lt;111&gt; plane as the cleave plane  35 .  
     [0085]FIG. 4 shows a birefringence amount under application of a prescribed stress with and without annealing of the &lt;111&gt; and &lt;100&gt; planes of a calcium fluoride crystal. As shown in FIG. 4, when the annealing is not performed, the &lt;100&gt; plane has a birefringence amount of 42.4 nm/cm by the stress, but when the annealing is performed, it has birefringence amount of 2.4 nm/cm. When the annealing is not performed, the &lt;111&gt; plane originally has a very small birefringence amount of 4.1 nm/cm by the stress, but when the annealing is performed, it has a birefringence amount of only 0.8 nm/cm.  
     [0086] In other words, a very small birefringence amount is obtained by annealing so to produce and dispose the optical element which allows the laser light to pass through the &lt;111&gt; plane perpendicularly.  
     [0087] When the optical element is configured so that the plane into which the fluorine molecular laser light  21  enters becomes parallel to the &lt;111&gt; plane, the plane of incidence can be polished with high accuracy. Therefore, fluctuations in a wave front on the plane of incidence are reduced, and the fluorine molecular laser light  21  with high quality can be obtained.  
     [0088]FIG. 5 shows a top view of another example of configuration of the optical elements in the fluorine molecular laser device  11  according to the second embodiment. In FIG. 5, the windows  17 ,  19  are configured to have the cleave plane  35  (especially, the &lt;111&gt; plane) substantially parallel to the inside surfaces  17 A,  19 A and the outside surfaces  17 B,  19 B and are annealed when produced.  
     [0089] The inside surfaces  17 A,  19 A of the windows  17 ,  19  are in contact with the laser gas, and the outside surfaces  17 B,  19 B are in contact with the atmosphere. As described above, because the laser gas is sealed in the laser chamber (not shown) under a pressure considerably higher than the atmosphere pressure, the pressure of the laser gas is applied to the windows  17 ,  19  substantially perpendicularly to the inside surface  19 A (arrows  36 ).  
     [0090] At that time, the windows  17 ,  19  have the highest strength to a force perpendicularly applied to the cleave plane  35  (especially, the &lt;111&gt; plane). Therefore, when the cleave plane  35  is disposed to be parallel to the inside surface  19 A, the strength of the windows  17 ,  19  is increased, and the increase in birefringence caused by distortion of the windows  17 ,  19  becomes very small. Durability of the windows  17 ,  19  to the pressure of laser gas is improved. And, the windows  17 ,  19  are produced with ease.  
     [0091] Then, a third embodiment will be described.  
     [0092]FIG. 6 shows a top view of the fluorine molecular laser device  11  according to the third embodiment. In FIG. 6, the fluorine molecular laser device  11  is provided with the laser chamber  12 , which has the windows  17 ,  19  at either end, the front mirror  16  for emitting the laser light  21 , an etalon  38  for narrow-banding the laser light  21 , and the rear mirror  18  for total reflection of the laser light  21 .  
     [0093] The laser light  21  produced in the laser chamber  12  has its wavelength narrow-banded by the etalon  38 . And, the laser light  21  is amplified while being reciprocated between the rear mirror  18  and the front mirror  16  and partly passes through the front mirror  16  to go out.  
     [0094] The etalon  38  configured with a spacer  45  made of low-expansion glass held between two disc type parallel flat boards  44 ,  44 . A partial reflective coating is applied to surfaces  47 ,  47  of the parallel flat boards  44 ,  44  on the side of the spacer  45 , and a nonreflective coating is applied to the surfaces  46 ,  46  opposite to the surfaces  47 ,  47 . The cleave planes  35 ,  35  are produced to be parallel to the parallel flat boards  44 ,  44 .  
     [0095] For the fluorine molecular laser device  11  described above, the optical element is previously annealed and then produced and disposed to direct the cleave plane  35 , particularly the &lt;111&gt; plane, substantially perpendicularly to the optical axis of the laser light  21 .  
     [0096]FIG. 7 shows a cross-sectional view of an etalon holder  39  for holding the etalon  38 . The etalon holder  39  is configured by fixing, for example, an inner tube  39 A and an outer tube  39 B, and fixes the etalon by tightening in screws  43  from both sides of the etalon holder  39 . Thus, the screws  43  apply a force to the etalon  38  to produce birefringence, but the birefringence can be minimized by producing and disposing the cleave plane  35 , especially the &lt;111&gt; plane, to be substantially perpendicular to the optical axis of the laser light  21 .  
     [0097] It is not illustrated but the cleave plane  35  may be configured to be substantially perpendicular to the optical axis in the same way when the prisms  32 ,  32  and the grating  33  described in the first embodiment are used instead of the rear mirror  18  to make the wavelength more narrow-banded.  
     [0098] When the etalon  38  is used as an output coupler, the cleave plane  35  of the etalon  38  is desired to be substantially perpendicular to the optical axis.  
     [0099] The rear mirror  18  may also be produced and disposed to have cleave plane  35  substantially parallel to the reflecting plane  18 A of the laser light  21 . The rear mirror  18  does not allow the passage of the laser light  21 , so that no birefringence is produced in it, but when the cleave plane  35  is disposed to be substantially parallel to the reflecting plane  18 A, the reflecting plane  18 A can be polished with higher precision.  
     [0100] To hold the rear mirror  18 , the reflecting plane  18 A is held by an unshown holder, so that distortion of the reflecting plane  18 A due to holding is reduced by disposing the cleave plane  35  to be substantially parallel to the reflecting plane  18 A.  
     [0101] Then, a fourth embodiment will be described. FIG. 8 shows a top view of the fluorine molecular laser device  11  according to the fourth embodiment. In FIG. 8, the fluorine molecular laser device  11  is provided with, for example, two dispersion prisms  28 ,  28  and the rear mirror  18  disposed behind the laser chamber  12 . And, slits  26 ,  27  are disposed before and after the laser chamber  12 .  
     [0102] The fluorine molecular laser light  21  includes intense line light (center wavelength of approximately 157.63 nm) with a long wavelength and weak line light (center wavelength of approximately 157.52 nm) with a short wavelength together. The intense line light and the weak line light are different in wavelength, so that they have a different refraction angle when they enter and leave the dispersion prisms  28 ,  28 . Therefore, the intense line light and the weak line light have their light path gradually deviated from each other while passing through the dispersion prisms  28 ,  28 .  
     [0103] Specifically, the intense line light passes through the dispersion prisms  28 ,  28 , reflects from the rear mirror  18 , passes through the windows  17 ,  19 , passes through the dispersion prisms  28 ,  28  again, passes through the slits  26 ,  27 , and partly passes through the front mirror  16  to go out.  
     [0104] Meanwhile, the weak line light is caused to deviate its light path while reciprocating between the dispersion prisms  28 ,  28 , passes through the front window  17 , and is obstructed by the slits  26 ,  27  to stop oscillating. It is called single lining. Here, the single lining is assumed to be a kind of narrow banding.  
     [0105] The dispersion prisms  28 ,  28  are also made of calcium fluoride and configured so that the laser light  21  passes through the cleave plane  35  substantially perpendicularly to it as shown in FIG. 8. For example, when the dispersion prisms  28 ,  28  are formed to have the shape of an isosceles triangle as viewed from above as shown in FIG. 8, the dispersion prisms  28 ,  28  are produced in such a way that the cleave plane  35  becomes parallel to a bisector of the vertical angle of the isosceles triangle. And, the incident angle of the laser light  21  to the dispersion prisms  28 ,  28  is assumed to be the Brewster angle θB.  
     [0106] Thus, birefringence hardly occurs, and generation of S polarized light is reduced even when the laser light  21  passes through the dispersion prisms  28 ,  28 .  
     [0107]FIG. 9 shows another example of configuration of the fluorine molecular laser device  11  according to the fourth embodiment. As shown in FIG. 9, the windows  17 ,  19  of the fluorine molecular laser device  11  are disposed not to have the Brewster angle θB but substantially right angles to the optical axis as described in the above individual embodiments.  
     [0108] Specifically, the windows  17 ,  19  disposed, so that the cleave planes  35 ,  35  have the Brewster angle θB to the inside surfaces  17 A,  19 A as shown in FIG. 1 and have an advantage that a loss is less, but their production might be hard. As described above, when the cleave planes  35 ,  35  are exposed to the pressure of the laser gas (arrows  36 ), the inside surfaces  17 A,  19 A or the outside surfaces  17 B,  19 B may be deformed because the cleave planes  35 ,  35  are not perpendicular to the pressure.  
     [0109] According to the fluorine molecular laser device  11  as shown in FIG. 9, the fluorine molecular laser light  21  becomes substantially the P polarized light when it enters the dispersion prisms  28 ,  28  at the Brewster angle θB. And, the fluorine molecular laser light  21  passes through the dispersion prisms  28 ,  28  substantially perpendicularly to the cleave plane  35 , so that birefringence hardly occurs, and the S polarized light is seldom mixed.  
     [0110] The fluorine molecular laser light  21  suffers from a slight loss because it enters substantially perpendicularly the windows  17 ,  19  and passes through the windows  17 ,  19  substantially perpendicularly to the cleave plane  35 . Thus, birefringence hardly occurs, and the S polarized light is seldom mixed.  
     [0111] And, the windows  17 ,  19  are highly durable to the pressure and have low occurrence of birefringence by the pressure because the cleave planes  35  are substantially perpendicular to the pressure of the laser gas.  
     [0112] The above description was made on the fluorine molecular laser device as an example, but the present invention can be applied to an ultraviolet laser device such as KrF, ArF excimer laser with a wavelength of approximately 193 nm, or the like.  
     [0113] The laser device having a wavelength narrow-banded was described as an example, but the windows  17 ,  19  and the front mirror  16  are effective even when the wavelength is not narrow-banded. Specifically, the optical element is lowered its birefringence amount by annealing as described in the above respective embodiments. As a result, the fluorine molecular laser light  21  passing through the optical element does not cause birefringence. And the fluorine molecular laser light  21  of linearly polarized light can be obtained at all times.  
     [0114] Besides, when it is configured to have the cleave plane  35  to be substantially parallel to the plane in or from which the fluorine molecular laser light  21  enters or exits, the plane polishing precision is improved. Besides, the windows  17 ,  19  are improved their durability to the pressure of the laser gas when the cleave planes  35 ,  35  are configured to be substantially parallel to the inside surfaces  17 A,  19 A.  
     [0115] Furthermore, the material for the optical element in the above description was calcium fluoride. It was because the calcium fluoride had the smallest birefringence amount among the materials through which the ultraviolet laser light passed and was suitable as the optical element.  
     [0116] However, the present invention is also effective on another fluoride, which allows the passage of the ultraviolet laser light, such as magnesium fluoride (MgF 2 ) or lithium fluoride (LiF 2 ). When the laser device oscillates laser light having a relatively long wavelength (248 nm) such as the KrF excimer laser device, glass such as synthetic quartz, which allows the passage of light having the above wavelength, is used as the optical element. Glass does not have a cleave plane but has a small birefringence when it is annealed. Therefore, the present invention is also effective on glass.  
     [0117] The inside surfaces  17 A,  19 A and the outside surfaces  17 B,  19 B of the windows  17 ,  19  described were disposed to be parallel to each other. They may not be disposed to be parallel, and the cleave plane  35  may be arranged to be parallel to the inside surfaces  17 A,  19 A.