Patent Publication Number: US-RE38372-E

Title: Narrow band excimer laser and wavelength detecting apparatus

Description:
This application is a continuation of application Ser. No. 07/793,339, filed Jan. 13, 1992, now abandoned. 
    
    
     TECHNOLOGICAL FIELD 
     This invention relates to a narrow band excimer laser and a wavelength detection device, and more particularly to a narrow band excimer laser suitable for a light source of a reduction projection aligner for use in manufacturing semiconductors. 
     BACKGROUND ART 
     Attention has been paid to the use of an excimer laser as a light source of reduction projection aligner (hereinafter called a stepper) for manufacturing semiconductor devices. This is because the excimer laser may possibly extend the light exposure limit to be less than (0.5 μm since the wavelength of the excimer laser is short (for example the wavelength of KrF laser is about 248.4 nm), because with the same resolution, the focal depth is greater than a g line or an i line of a mercury lamp conventionally used, because the numerical aperture (NA) of a lens can be made small so that the exposure region can be enlarged and large power can be obtained, and because many other advantages can be expected. 
     An excimer laser utilized as a light source of the stepper is required to have a narrow bandwidth with a beam width of less than 3 pm as well as a large output power. 
     A technique of narrowing the bandwidth of the excimer laser beam is known as the injection lock method. In the injection lock method, wavelength selecting elements (etalon, diffraction grating, prism, etc.) are disposed in a cavity of an oscillation stage so as to generate a single mode oscillation by limiting the space mode by using a pin hole and to injection synchronize the laser beam in an amplification stage. With this method, however, although a relatively large output power can be obtained, there are such defects that misshots occur, that it is difficult to obtain 100% locking efficiency, and the spectrum purity degrades. Furthermore, in this method, the output light beam has a high degree of coherency so that when the output light beam is used as a light source of the reduction type projection aligner, a speckle pattern generates. Generally it is considered that the generation of speckle pattern depends upon the number of space transverse modes. When the number of space transverse modes contained in the laser light is small, the speckle pattern becomes easy to generate. Conversely, when the number of the space transverse modes increases, the speckle pattern becomes difficult to generate art. The injection lock method described above is a technique for narrowing the bandwidth by greatly decreasing the number of space transverse modes. Since generation of speckle pattern causes a serious problem, this technique can not be adopted in the reduction projection aligner. 
     Another projection technique for narrowing the bandwidth of the excimer layer beam is a technique utilizing a air gap etalon acting as a wavelength selective element. A prior art technique utilizing the air gap etalon was developed by AT &amp; T Bell Laboratory wherein an air gap etalon is disposed between the front mirror and a laser chamber of an excimer laser device so as to narrow the bandwidth of the excimer laser. This system, however, cannot obtain a very narrow spectral bandwidth. In addition there are problems in that the power loss is large due to the insertion of the air gap etalon. Further, it is impossible to greatly increase the number of the space transverse modes. Furthermore, the air gap etalon has a problem of poor durability. 
     Accordingly, an excimer laser device has been proposed wherein a relatively high durable diffraction grating is used as the wavelength selective element. 
     In the excimer laser having a diffractive grating which acts as a wavelength selective element, a pin hole is provided in a resonator (laser cavity) to reduce the spread angle of the beam in the grating. Alternatively, a beam expander is provided to expand the laser beam incident to the grating. For this beam expander, a prism expander utilizing a prism is typically been used. 
     The narrow band excimer laser used as the stepper must not only have a narrow bandwidth with a line width of less than 3 pm, but also must produce a large output. 
     In the construction in which a pin hole is disposed in a resonator, however, the output becomes greatly reduced and the number of space transverse modes necessary for preventing generation of a speckle pattern decreases, so that such construction can not be used. 
     In the construction in which a prism beam expander is used, the expander must have a large magnifying power in order to narrow the line width. 
     However, when the magnifying power of the prism beam expander becomes large, the incident angle of the laser beam to a prism of the prism expander becomes large or it becomes necessary to increase the number of prisms, thereby increasing the loss. As a result, a large output cannot be produced. 
     Furthermore, where a narrow band excimer laser is used as the light source of a stepper, it is necessary to narrow the bandwidth of the output laser beam and then to control the wavelength of the output laser beam whose bandwidth has been narrowed to a stable condition at a high accuracy. 
     A monitor etalon has been used for measuring the line width of the output beam and for detecting the wavelength. The monitor etalon is constituted by an air gap etalon wherein a pair of partial reflective mirrors are disposed to confront each other with a predetermined air gap there-between. The transmissive wavelength of this air gap etalon is expressed by the following equation 
     
       
         mλ=2nd·cos θ 
       
     
     where m represents the order, d the partial mirrors spacing, n the refractive index of the medium the partial reflective mirrors, and θ an angle between the normal of the etalon and the axis of the incident light. 
     This equation shows that where n, d and m are constant, as the wavelength varies, the angle θ changes. The monitor etalon detects the wavelength of the beam by utilizing those characteristics. In the monitor etalon described above, when the pressure in the air gap and the ambient temperature vary, the angle θ varies even when the wavelength is constant. Accordingly, where the monitor etalon is used for detecting the wavelength, the pressure in the air gap and the ambient temperature are controlled to be constant. 
     However, it is difficult to precisely control the pressure in the air gap and the ambient temperature. Therefore, it is impossible to detect the absolute wavelength at a high accuracy. 
     For this reason, apparatus has been proposed wherein the beam to be detected is inputted to the monitor etalon together with a reference beam having a known wavelength, and the wavelength of the beam is detected by detecting a wavelength of the beam relative to the reference beam. 
     In this apparatus, a light beam transmitting through the monitor etalon is directly inputted to a beam detector such as CCD image sensor. 
     However, in this apparatus, since the output of the monitor etalon is directly inputted to the beam detector, the beam to be detected and the reference beam cannot be inputted to the beam detector with a sufficient beam quantity, and an interference fringe cannot be formed on the beam detector. 
     Accordingly, it is an object of this invention to provide a novel narrow band excimer laser of the type using a prism beam expander and a diffraction grating as a band-narrowing element capable of preventing increase of loss even when the magnifying power of the prism expander is increased. 
     Another object of this invention is to provide a novel wavelength detecting apparatus of a narrow band excimer laser capable of inputting a reference beam and a beam to be detected into a beam detector with a sufficient quantity and capable of detecting the interference fringes of both beams at a high accuracy. 
     DISCLOSURE OF THE INVENTION 
     According to one aspect of this invention, there is provided a narrow band oscillation excimer laser comprising a prism beam expander and a diffractive grating which are used as a bandwidth narrowing element, the ruling direction of the grating and the beam spreading direction of the prism beam expander being substantially coincided with each other; and selective oscillation means for selectively oscillating a linearly polarized wave which is substantially parallel with the beam expanding direction of the prism beam expander. The selective oscillation means may be constructed by a polarizing element disposed in the laser cavity. 
     The selective oscillation means may be constructed by a window provided on a rear side or a front side of the laser chamber in such a manner that the window is inclined with a Brewster&#39;s angle with respect to the optical axis of the laser beam in a plane containing the beam expanding direction of the prism expander and the optical axis of the laser beam. 
     The selective oscillation means may include a prism beam expander in which one surface of a prism is coated with a reflection preventive film for selectively preventing a reflection of a polarized component which is parallel with the beam expanding direction of the prism beam expander. 
     By selectively oscillating a linearly polarized wave which is substantially parallel with the beam expanding direction of the prism beam expander, the loss can be reduced even though the incident angle to the prism is large or the number of the prisms is increased. This is because a linearly polarized wave parallel with the beam expanding direction of the prism beam expander has a large transmissivity even the incident angle to the prism is large. With this construction, the narrow band laser device can generate a large output with a small spectrum line width. 
     According to the other aspect of this invention there is provided a wavelength detecting apparatus for use in a narrow band oscillation excimer laser comprising an etalon, light incidence means for projecting a reference light generated by a source of reference light and a laser beam to be detected upon the etalon; light condensing means for condensing light passed through the etalon; light detecting means disposed in a rear side focal plane of the light condensing means for detecting an interference fringe of light condensed by the light condensing means. 
     The reference light and the light to be detected are passed through the light condensing means, and then focused on the detection surface of the light detecting means disposed on the focal plane of the light condensing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG.  1 (a) is a plan view showing one embodiment of a narrow band excimer laser according to this invention; 
     FIG.  1 (b) is a side view of the embodiment shown in FIG.  1 (a); 
     FIG.  2 (a) is a plan view showing another embodiment of the narrow band excimer laser according to this invention; 
     FIG.  2 (b) is a side view showing the another embodiment shown in FIG.  2 (a); 
     FIG.  3 (a) is a plan view showing a still another embodiment of the narrow band excimer laser; 
     FIG.  3 (b) is a side view of the embodiment shown in FIG.  3 (a); 
     FIG. 4 is a diagrammatic representation showing one embodiment of the wavelength detecting apparatus for the narrow band excimer laser of this invention; 
     FIG. 5 is a diagrammatic representation showing another embodiment of the wavelength detecting apparatus for the narrow band excimer laser using a beam condensing mirror; 
     FIG. 6 is a diagrammatic representation showing still, another embodiment of the wavelength detecting apparatus for narrow band excimer laser using a converging type light filter; 
     FIG. 7 is a diagrammatic representation showing a further embodiment of the wavelength detecting apparatus for the narrow band excimer laser using a lamp as a source of reference light; 
     FIG. 8 is a diagrammatic representation showing a still of the wavelength detecting apparatus using a lamp and an optical fiber; 
     FIG. 9 is a diagrammatic representation showing another wavelength detecting apparatus for the narrow band excimer laser in which a shutter and a filter are inserted; 
     FIG. 10 is a flow chart showing one example of the main routine for detecting a wavelength when the wavelength detecting apparatus shown in FIG. 9 is used; 
     FIG. 11 is a flow chart showing and example of a reference light detecting subroutine; and 
     FIG. 12 is a flow chart showing one example of a light detecting subroutine. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The embodiments-of this invention will now be described with reference to the accompanying drawings. The narrow band excimer laser shown in FIGS.  1 (a) and  1 (b) comprises a front mirror  1 , and a grating  6  acting as a rear mirror and a wavelength selective element. Between the front mirror  1  and the grating  6 , a laser chamber  3 , a polarizing element  4 , and two prisms  5 a and  5 b acting as a beam expander (prism beam expander) are provided. Thus, a laser chamber or cavity is constructed between the front mirror  1  and the grating  6 . 
     The laser chamber  3  is filled with a laser gap containing KrF, etc. which can circulate in the chamber  3 . For the purpose of exciting the laser gas, discharge electrodes (not shown) are provided in the laser chamber  3 . Windows  2 a and  2 b are provided at both ends of the laser chamber  3  at predetermined angles. 
     The purpose of the grating  6  is to select a beam having a specific wavelength by utilizing diffraction of light beam. The grating  6  is provided with a number of grooves directed in the same direction. In this specification, the direction perpendicular to these grooves is termed a ruling direction. By changing the angle of the grating  6  with respect to an incident beam within a plane containing the ruling direction of the grating  6 , a beam having a specific wavelength can be selected. In other words, the grating  6  reflects only a specific diffracted light in a predetermined direction (in this case, the direction of incident beam), the specific diffracted light corresponding to the angle of the grating with respect to the incident beam. As a consequence, a beam having a specific wavelength can be selected. 
     The prism beam expander including prisms  5 a and  5 b is disposed such that its beam expanding direction substantially coincides with the ruling direction of the grating  6 . The grating  6  is irradiated by the laser beam expanded by the prism beam expander. 
     The polarizing element  4  selectively transmits only the polarized beam which is substantially parallel with the beam expanding direction of prism beam expander made up of prisms  5 a and  5 b. The polarizing element  4  may be constructed, for example, by a polarizing prism utilizing a birefrigence material (crystal, calcite, etc.), A Brewster&#39;s dispersion prism, a glass substrate (quartz, CaF 2  or MgF 2 ), which is arranged at a Brewster&#39;s angle or a glass substrate coated such that it transmits a certain polarized light component. 
     With such construction, the apparatus shown in FIGS.  1 (a) and  1 (b) selectively oscillates a linearly polarized light wave parallel to the beam spreading direction by the prism beam expander made up of prisms  5 a and  5 b. The linearly polarized light wave parallel with the beam expanding direction of the prism beam expander has a large transmissivity even when the incident angle of the beam to the prism is large. Therefore, even when the magnifying power of the beam expander is made larger for line-narrowing, the loss does not increase greatly. In other words, according to this invention, it is possible to construct a narrow band excimer laser with a small loss. 
     The modified embodiment of this invention shown in FIGS.  2 (a) and  2 (b) is constructed such that a specific linearly polarized light wave can be selected by a rear side window of the laser chamber  3 . In this embodiment, the rear side window  2 b of the laser chamber  3  makes substantially the Brewster&#39;s angle θ with respect to the beam expanding direction of the prism beam expander constructed by prisms  5 a and  5 b, in a plane including the beam expanding direction of the beam expander and the optical axis of the laser beam. 
     In the modification shown in FIGS.  2 (a) and  2 (b), only the rear side window  2 b is set to make the Brewster&#39;s angle. However, it is possible to arrange both the rear side window  2 b and the front side window  2 a to make the Brewster&#39;s angle. It is also possible to set only the front side window  2 a to make the Brewster&#39;s angle. 
     A still another embodiment shown in FIGS.  3 (a) and  3 (b) is constructed by coating the prisms  5 a and  5 b, which constitute the prism beam expander, with an anti-reflection coating film that selectively transmits only the polarized light component parallel to the beam expanding direction of the prism beam expander. In FIG.  3 (b), portions  5 c and  5 b shown by dotted lines indicate these coated portions. 
     The coating can be applied to one beam transmitting surface of at least one prism. With this construction, even when the incident angle increases, the transmissivity is higher than 99%. 
     FIG. 4 shows one embodiment of a wavelength detecting apparatus for the narrow band excimer laser according to this invention. In this embodiment, as the beam to be detected, the output beam La of the narrow band excimer laser  10  is used, and as a reference light source  20 , a He—Ne laser or an Ar laser or other types of laser is used. The reference laser beam generated by the reference light source  20  and the excimer laser beam have different wavelengths. 
     A part of the laser beam La outputted from the narrow band excimer laser  10  is sampled by a beam splitter  30 , and this sampled beam is inputted to a beam splitter  40 . The reference beam Lb generated by the reference light source  20  is inputted to the other surface of the beam splitter  40 . 
     The beam splitter  40  transmits a part of the sampled beam La outputted from the beam splitter  30  and reflects a part of the reference beam Lb outputted by the reference beam source  20 , thus combining the sampled beam with the reference beam. The combined beam outputted from the beam splitter  40  is spreaded by a concave lens  50 , and the spread beam is inputted to an etalon  60 . 
     The etalon  60  is constituted by two transparent plates  60 a and  60 b whose inner surfaces are made to act as partial reflecting mirrors. The wavelength of the beam transmitted through the etalon  60  varies corresponding to the angle of incident light to the etalon. Thus, the reflecting films are coated on the etalon plates in 2-wavelength coating so as to partially reflect both the reference beam Lb and the excimer laser beam La having different wavelengths from the reference beam Lb. 
     The beams transmitted through the etalon  60  are inputted to a condenser lens  70 . The condenser lens  70  may be an achromatic lens corrected for color aberration. When the laser beam transmits through the achromatic condenser lens  70 , the color aberration compensated. 
     The beam detector  80  is disposed at the focal point of the condenser lens  70  so that the light beam passed through the condenser lens  70  is focused on the beam detector  80 . On the detecting surface of the beam detector  80  are formed a first interference fringe corresponding to the reference beam and a second interference fringe corresponding to the beam to be detected. Base on these first and second interference fringes, the beam detector  80  detects the relative wavelength of the beam to be detected with respect to the wavelength of the reference beam. Thus, the beam detector  80  can detect the absolute wavelength of a beam to be detected when the wavelength of the reference beam is known. 
     The beam detector  80  may be constituted by a one-dimensional or two-dimensional image sensor, a diode array or a position sensitive detector (PSD). 
     As above described, the beams are inputted to the etalon  60  after being spread with the concave lens  50 , and the beams transmitted through the etalon  60  are focused on the beam detector  80  with the condenser lens  70 . Therefore, a sufficient quantity of beams are inputted to the beam detector  80  and the interference fringes of both beams can be clearly formed. 
     FIG. 5 illustrates another embodiment of the wavelength detecting apparatus of the narrow band excimer laser according to this invention. For convenience of description, elements doing the same performance are designated by the same reference numerals and characters in FIG.  5  and subsequent drawings. 
     In the embodiment shown in FIG. 5, instead of the achromatic condenser lens  70 , such condenser mirror  90  as a concave mirror or an eccentric parabolic mirror is used. More particularly, a reference beam Lb and an excimer laser beam La are inputted to an etalon  60  through a concave lens  50 , and the beam transmitted through the etalon  60  is reflected by the condenser mirror  90 . The reflected light beam is inputted to the detecting surface of a beam detector  80  disposed at the focus of the condenser mirror  90 . Since the condenser mirror  90  having a reflecting surface is used, there is no achromatic aberration so that it is possible to focus the interference fringes of the excimer laser beam La and the reference laser beam Lb at the same position, that is, on the beam detector  80  at the focal point of the mirror  90 . 
     In this embodiment shown in FIG. 5, it is possible to detect at a high accuracy the interference fringes of a sufficient light quantity by using the concave lens  50  and the condenser mirror  90  as in the previous embodiment. 
     FIG. 6 shows a still another embodiment of the wavelength detecting apparatus of the narrow band excimer laser according to this invention. 
     In this embodiment, the excimer laser beam La and the reference laser beam Lb are synthesized by using a synthesizer type optical fiber  40 a. More particularly, the excimer laser beam La sampled by a beam splitter  30  is applied to a beam synthesizer  14  through a condenser lens  11 , an optical fiber sleeve  12  and an optical fiber  13 , while the reference laser beam Lb generated by the reference light source  20  is applied to the beam synthesizer  14  via condenser lens  15 , an optical fiber sleeve  16  and an optical fiber  17 . The beam synthesizer  14  synthesizes these two beams La and Lb, and the beam thus synthesized is inputted to an optical fiber  18 , and the beam spread by a sleeve  19  is inputted to a monitor etalon  60 . The beam transmitted through the monitor etalon  60  is focused on the beam detector  80  through an achromatic condenser lens  70 . 
     In the embodiment shown in FIG. 6, the position of the interference fringe is not influenced by the position of the synthesizing type optical fiber  40 a, but is solely determined by the positional relation among the etalon  60 , the condenser lens  70  and the beam detector  80 . Therefore, there is an advantage that the optical system can be adjusted easily, in addition to the advantages of the previous embodiments. 
     Although in the embodiment shown in FIG. 6, correction of the color aberration is performed by using the achromatic lens  70 , this lens can be substituted by the concave mirror or the eccentric parabolic mirror  90  shown in FIG.  5 . 
     FIG. 7 shows further embodiment showing the wavelength detecting apparatus of the narrow band excimer laser in which a plane light source, that is a lamp  20 a is used as the reference light source. This lamp  20 a may be a mercury lamp generating a reference light having a wavelength of 253.7 nm, for example. More particularly, the excimer laser beam La sampled by a beam splitter  30  is spread by a concave lens  21  and then applied to a beam splitter  40  which synthesizes the spread beam with the reference beam Lc from the mercury lamp  20 . The synthesized beam is inputted to an etalon  60 . The beam transmitted through the etalon  60  is focused on the beam detector  80  through an achromatic condenser lens  70 . 
     In the embodiment shown in FIG. 7, the color aberration is corrected by using the achromatic condenser lens  70 . However, this lens can be substituted by the concave mirror or the eccentric parabolic mirror  90  shown in FIG.  5 . 
     FIG. 8 illustrates yet another embodiment of the wavelength detecting apparatus of the narrow band excimer laser of this invention in which a mercury lamp  20 a is used as the reference light source. In this embodiment, the excimer laser beam La is guided to a beam splitter  40  by using an optical fiber  23 . More particularly, the excimer laser beam sampled by a beam splitter  30  is inputted to a sleeve  22  through a condenser lens  11 . Thereafter, the beam is outputted from the sleeve  22  to a sleeve  24  through an optical fiber  23 . The beam which is spread by passing through the sleeve  24  is applied to a beam splitter  40  where the beam is synthesized with the reference light beam Lc from the mercury lamp  20  and is then inputted to an etalon  60 . The light beam transmitted through the etalon  60  is focused on a beam detector  80  via an achromatic condenser lens  70 . 
     In the embodiment shown in FIG. 8, the achromatic condenser lens  70  may be substituted by a concave mirror or an eccentric parabolic mirror  90  shown in FIG.  5 . 
     FIG. 9 shows another embodiment of the wavelength detecting apparatus of the narrow band excimer laser. In this embodiment, the achromatic condenser lens  70  in FIG. 8 is replaced with a condenser mirror  90  such as a concave mirror or an eccentric parabolic mirror. In addition, a filter  41 , and shutters  42  and  43  are added. 
     More particularly, a filter  41  which selects only a light beam having a predetermined wavelength among the reference light beams Lc generated by the mercury lamp  20 a is provided between a shutter  42  and a beam splitter  40 . Thus, only the reference light beam having the predetermined wavelength is inputted to beam splitter  40 . For example, where the apparatus is used as the wavelength detector of the beam (having a wavelength of 248.4 nm) generated by a KrF narrow band excimer laser, a mercury lamp is used as the lamp  20 a, and an interference filter for a beam having a wavelength of 253.7 nm, which is close to that of the excimer laser, is used as the filter  16 . Shutters  42  and  43  are provided for the purpose of independently detect the reference light beam and the light beam to be detected (the laser beam of excimer laser). For detecting the reference light beam (FIG.  11 ), the shutter  42  is opened and the shutter  43  is closed. For detecting the light beam to be detected (FIG.  12 ), the shutter  43  is opened and the shutter  42  is closed. 
     In this embodiment, the filter  41  is disposed between the beam splitter  40  and the shutter  42 . However, the filter  41  may be disposed between the lamp  20 a and the shutter  42 . Furthermore, where the wavelengths of the reference light beam and of the beam to be detected are close to each other, and where the beam to be detected transmits through the filter  41 , the filter  41  may be placed at a suitable position in the light path between the beam splitter  40  and the light beam detector  80 . 
     The detecting operation of the reference light beam and the beam to be detected of this embodiment shown in FIG. 9 will now be described with reference to the flow charts shown in FIGS. 10-12. 
     FIG. 10 shows the main routine for the wavelength detection. First, the reference light beam detection subroutine is executed at step  100 . As shown in FIG. 11, in this subroutine, the shutter  43  on the side of the beam to be detected is closed while the shutter  42  on the side of the reference light beam is opened so as to input only the reference light beam Lc to the beam detector  80  via the etalon  60  at steps  200  and  210 . Then, the radius Rs of the interference fringe of the reference beam formed on the beam detector  80  is detected and stored in a memory, not shown, at step  220 . 
     Upon completion of this reference beam detection subroutine, the count value T of a timer, not shown, is cleared to zero at step  110 . Then, at step  120 , a judgement is made as to whether the count value T is larger than a predetermined preset time K (for example, several minutes). Where T&lt;K, the detection subroutine for detecting the beam to be detected is executed at step  140 . In this detection, subroutine, as shown in FIG. 12, by closing the shutter  42  on the side of the reference light beam and opening the shutter  43  on the side of the beam to be detected, only the beam La to be detected is applied to the beam detector  80  at steps  300  and  310 . Then, the radius Re of the interference fringe of the beam to be detected formed on the surface of the beam detector  80  is detected at step  320 . 
     Upon completion of the detection subroutine for detecting the beam to be detected, the radius Re determined by this subroutine is compared with the radius Rs which has been determined and stored in the previous reference beam subroutine so as to detect the absolute wavelength of the beam to be detected at step  150 . Thereafter, the executions of steps  140  and  150  are repeated until a condition T&gt;K is obtained. 
     When T&gt;K at step  120 , the reference beam detection subroutine shown in FIG. 11 is executed again and the stored data Rs is renewed by the radius Rs of the interference fringe of the reference beam determined by the subroutine. Then, after the count value T of the timer is cleared to zero, the subroutine for detecting the beam to be detected is executed again. 
     All processings described above are executed automatically. 
     More particularly, in the wavelength detection processing described above, where the wavelengths of the reference beam and the beam to be detected are close to each other, it is difficult to simultaneously detect the interference fringes of both beams. Therefore, the interference fringes are independently detected by using the shutters  42  and  43 . Since the reference beam is relatively stable, it is advantageous to detect the interference fringe in a relatively long preset period K. In a case other than detecting the reference beam, it is designed that the beam to be detected is always detected. In other words, the detection period for the reference beam is set to be sufficiently longer than the detection period for the beam to be detected. 
     In this embodiment, the radius of an interference fringe is detected. However, the absolute wavelength of the beam to be detected can also be obtained by detecting the diameter or position of the interference fringe. 
     In the embodiments shown in FIGS. 4-9, when the light quantities of the reference beam and the beam to be detected are small so that the detection of the interference fringes is difficult, a collimator lens may be disposed in front of the etalon  60  so as to apply parallel light to the etalon, thus increasing the light quantity. 
     In the embodiments shown in FIGS. 4-9, the beam splitter  40  is disposed such that the beam to be detected is applied to the beam transmissive side while the reference beam is applied to the opposite side. However, their positional relationship may be reversed. Where the wavelengths of the reference beam and of the beam to be detected are close to each other, a partial reflection mirror may be used, whereas where the difference between wavelengths are large, a dichroic mirror may be used. 
     The embodiments described above are constituted by using an air gap etalon. However, it may be substituted by a solid etalon. 
     In the above described embodiments, the focused positions of the reference beam and of the beam to be detected are made to coincide with each other by correcting the color aberration of the condenser lens  70 , or by using the condenser mirror  90 . However, it may be so constructed that the condensing lens  70  or the beam detector  80  is disposed movable in the direction of the optical axis so as to correct the focused position. 
     Where the wavelengths of the reference beam and the beam to be detected are close to each other in the embodiments shown in FIGS. 4,  6 ,  7  and  8 , a condenser lens whose color aberration is not corrected may be inserted between the etalon  60  and the beam detector  80 . 
     In the embodiments shown in FIGS. 4,  5 , and  7 , the laser beam La is spread by the concave lens  50  or  21  and the spread laser beam is inputted to the etalon  60 . However, the concave lens may be substituted with a convex lens. According to this modification the laser beam is condensed, at first by the convex lens and then spread. The spread beam is inputted to the etalon  60 . Furthermore, instead of using the concave lens  50 , a diffusion plate (e.g., a frosted glass) may be used. 
     FIELD OF APPLICATION IN INDUSTRY 
     As above described, in the narrow band excimer laser according to this invention, by substantially coinciding the ruling direction of the grating with the beam expanding direction of the prism beam expander and by providing selective oscillation means for selectively oscillating a linearly polarized light wave substantially parallel with the beam spreading direction of a prism beam expander, it becomes possible to greatly decrease the loss in the prism beam expander. For this reason, it becomes possible to narrow the spectrum line width and to obtain a large output. 
     Further, according to the wavelength detecting apparatus of the narrow band excimer laser of this invention, with a simple construction of providing a light condensing device on the rear side of an etalon, a sufficient quantity of light can be applied to a light detector. As a result, it becomes possible to form clear interference fringes on the light detector, thus enabling to detect the absolute wavelength of the beam to be detected at a high accuracy. Especially, by using an achromatic lens or a light condensing mirror as light condensing means, even when the wavelengths of the reference beam and that of the detected beam are not equal, the absolute wavelength can be accurately detected without constructing the optical system to be movable. 
     The narrow band excimer laser and the wavelength detecting apparatus of the excimer laser are especially suitable to use as a light source of a reduction projection aligner for use in manufacturing semiconductor devices.