Abstract:
A grating based line narrowing unit for gas discharge lasers with increased beam expansion to produce smaller bandwidths. The grating has a grating surface larger than 100 cm 2  and is a replica grating produced from a master grating produced with a lithography process on a single crystal substrate. In preferred embodiments, a beam from the chamber of the laser is expanded with four prism beam expanders. The large grating, much larger than gratings historically produced from diamond lined gratings, permit substantial reductions in bandwidth while maintaining laser efficiency. A narrow band of wavelengths in the expanded beam is reflected from a grating in a Littrow configuration back via the bi-directional beam expanders into the laser chamber for amplification.

Description:
[0001]    This invention relates to lasers and in particular to line narrowed excimer lasers. This invention is a continuation-in-part of Ser. No. 09/151,128, filed Sep. 10, 1998; Ser. No. 09/470,724, filed Dec. 22, 1999; Ser. No. 09/703,317, filed Oct. 31, 2000; Ser. No. 09/716,041, filed Nov. 17, 2000 and Ser. No. 09/943,343, filed Aug. 29, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
       Narrow Band Gas Discharge Lasers  
         [0002]    Gas discharge ultraviolet lasers used as light sources for integrated circuit lithography typically are line narrowed. A preferred line narrowing prior art technique is to use a diffraction grating based line narrowing unit along with an output coupler to form the laser resonant cavity. The gain medium within this cavity is produced by electrical discharges into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F 2  laser).  
         Prior Art Line-Narrowing Technique  
         [0003]    A sketch of such a prior art system is shown in FIG. 1 which is extracted from Japan Patent No. 2,696,285. The system shown includes output coupler (or front mirror)  4 , laser chamber  3 , chamber windows  11 , and a grating based line narrowing unit  7 . The line narrowing unit  7  is typically provided on a lithography laser system as an easily replaceable unit and is sometimes called a “line narrowing package” or “LNP” for short. This unit includes two beam expanding prisms  27  and  29  and a grating  16  disposed in a Litrow configuration so that diffracted beam propogates right back towards the incoming beam. The output of these excimer lasers are typically rectangular with the long dimension of for example 20 mm in the vertical direction and a short dimension of for example 3 mm in the horizontal direction. Therefore, in prior art designs, the beam is typically expanded in the horizontal direction so that the FIG. 1 drawing would represent a top view.  
         The Grating Formula  
         [0004]    Another prior art excimer laser system utilizing a diffraction grating for spectrum line selection is shown in FIG. 2. The cavity of the laser is created by an output coupler  4  and a grating  16 , which works as a reflector and a spectral selective element. Output coupler  4  reflects a portion of the light back to the laser and transmits the other portion  6  which is the output of the laser. Prisms  8 ,  10  and  12  form a beam expander, which expands the beam in the horizontal direction before it illuminates the grating. A mirror  14  is used to steer the beam as it propagates towards the grating, thus controlling the horizontal angle of incidence. The laser central wavelength is normally changed (tuned) by turning very slightly that mirror  14 . A gain generation is created in chamber  3 .  
           [0005]    Diffraction grating  16  provides the wavelength selection by reflecting light with different wavelengths at different angles. Because of that only those light rays which are reflected back into the laser will be amplified by the laser gain media, while all other light with different wavelengths will be lost. The diffraction grating in this prior art laser works in a Littrow configuration, when it reflects light back into the laser. For this configuration, the incident angle α and the wavelength λ are related through the formula: 
           2 dn  sin α= mλ   (1) 
           [0006]    where α is the incidence angle on the grating, m is the diffraction order, n is refractive index of the gas in the LNP, and d is the period of the grating.  
           [0007]    Because microlithography exposure lenses are very sensitive to chromatic abberations of the light source, it is required that the laser produce light with very narrow spectrum line width. For example, state of the art excimer lasers are now producing spectral linewidths on the order of 0.5 pm as measured at full width at half maximum values and with 95% of the light energy concentrated in the range of about 1.5 pm. New generations of microlithography exposure tools will require even tighter spectral requirements. In addition, it is very important that the laser central wavelength be maintained to very high accuracy as well. In practice, it is required that the central wavelength is maintained to better than 0.05-0.1 pm stability.  
         Making Gratings  
         [0008]    One traditional method of manufacturing diffraction gratings, and particularly echelle gratings, is to scribe or rule a series of grooves with a ruling engine on a good optical surface, such as a thin layer of aluminum or gold deposited on a suitable substrate. However, there are a number of difficulties associated with ruling gratings. Echelles are considered to be among the most difficult gratings to rule because high diffraction angles require exceptional ruling accuracy, yet this must be accomplished under high tool loads that usually accompany coarse groove spacing. The grooves must consistently have a uniform and correct shape to ensure high efficiency. Use at high diffraction orders requires blaze faces to be flat to nanometer tolerances if peak diffracted energy is to be concentrated in one blaze order. The grooves must also be ruled in a parallel and evenly spaced fashion because the density of grooves (e.g. grooves/mm) determines the dispersion and the accuracy in the position of the grooves determines the quality of the spectral image. Additionally, echelles typically have grooves that are deeper than other diffraction gratings (e.g. because of larger blazing angles) which in turn requires thicker metallic coatings consequently effecting the uniformity of the echelles flatness. Ruling engines used to fabricate echelles in this manner are complex mechanical devices that are slow and difficult to use, leading to gratings that are very expensive with long fabrication turnaround times. Large gratings are particularly difficult to make using the ruling techniques. Prior art gratings used for integrated circuit lithography have a lined surface about 24 cm×3.5 cm. Production of high quality gratings larger than this using ruling techniques would be difficult.  
           [0009]    Another technique produces so-called holographic gratings. An interference pattern created by two monochromatic, coherent laser beams is used to expose a photoresist film on a substrate. After exposure, the photoresist is developed and the substrate is etched. Although holographic gratings are relatively easy to manufacture, etching the desired blazing angle in such a grating is not, and fabricating high quality holographic gratings whose dimensions exceed 100 mm is very difficult.  
           [0010]    [0010]FIG. 8 shows a cross section of an echelle grating in the Littrow configuration. Grating  100  includes parallel grooves  110 , each with two facets and having a groove spacing d. Facet  120  is located at a blaze angle θ with respect to the plane of the grating. When the angle of incidence α is equal to the diffraction angle β and the blaze angle θ, incident light  130  is diffracted in a given diffracted order  140  (i.e., the m-th order) which propagates backward toward the source.  
           [0011]    A need exists for a better technique for making gratings especially large gratings needed to permit reduction in bandwidth for gas discharge lasers.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides for a grating based line narrowing unit for gas discharge lasers with increased beam expansion to produce smaller bandwidths. The grating has a grating surface larger than 100 cm 2  and is a replica grating produced from a master grating produced with a lithography process on a single crystal substrate. In preferred embodiments, a beam from the chamber of the laser is expanded with four prism beam expanders. The large grating, much larger than gratings historically produced from diamond lined gratings, permit substantial reductions in bandwidth while maintaining laser efficiency. A narrow band of wavelengths in the expanded beam is reflected from a grating in a Littrow configuration back via the bi-directional beam expanders into the laser chamber for amplification.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 shows a first prior art line narrowed laser system.  
         [0014]    [0014]FIG. 2 shows a second prior art line narrowed laser system.  
         [0015]    [0015]FIG. 3 shows the effect on wavelengths of vertical beam deviation.  
         [0016]    [0016]FIGS. 4A, 4B and  4 C show elements of a preferred embodiment of the present invention.  
         [0017]    [0017]FIG. 5 shows beam expansion coefficient possible with one prism.  
         [0018]    [0018]FIGS. 6 and 7 show techniques for controlling a tuning mirror.  
         [0019]    [0019]FIG. 8 shows a feature of a grating surface.  
         [0020]    [0020]FIG. 9 show features of a crystal.  
         [0021]    FIGS.  10 A-E,  11 A-E and  12 A-C illustrate a technique for making gratings. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]    Preferred embodiments of the present invention can be described by reference to the drawings.  
       Two Direction Beam Expansion  
       [0023]    In reality, formula (1) presented in the Background Section only works when all the beams incident on the grating have the same direction in the vertical axes, and this direction is normal to diffraction grating grooves. Diffraction grating grooves are placed vertically so formula (1) works for beams which lay in the horizontal plane.  
         [0024]    Real excimer laser beams, however, have some divergence in both horizontal and vertical directions. In this case, formula (1) is modified and becomes 
         2 dn  sin α·cos β= mλ   (2) 
         [0025]    In this formula, β is the beam angle in the vertical direction, the rest of the variables are the same as in (1). In the case of β=0; i.e., when the beam has no divergence in the vertical direction, cos β=1 and formula (2) becomes (1).  
         [0026]    It is important to note, that the grating does not have any dispersion properties in the vertical direction, that is, its reflection angle in the vertical direction does not depend on the light wavelength, but is rather equal to the incident angle. That means, in the vertical direction the reflecting facets of the grating face are behaving like ordinary mirrors.  
         [0027]    Beam divergence in the vertical direction has significant effect on line narrowing. According to formula (2), different vertical angles β would correspond to different Littrow wavelengths λ. FIG. 3 shows dependence of Littrow wavelength λ on the beam vertical deviation, β. Typical prior art excimer laser might have a beam divergence of up to ±1.0 mrad (i.e., a total beam divergence of about 2 mrad) in the vertical direction. FIG. 3 shows that a portion of a beam propogating with a 1 mrad vertical tilt (in either up or down direction) will have the Littrow wavelength shifted by 0.1 pm to the short wavelength direction for that portion of the beam. This wavelength shift leads to broadening of the whole beam spectrum. Prior art excimer lasers, having Δλ FWHM  bandwidth of about 0.6 pm does not substantially suffer from this effect. However, as the bandwidth is reduced, this 0.1 pm shift becomes more important. New excimer laser specifications for microlithography will require bandwidth of about 0.4 pm or less. In this case, it becomes important to reduce this broadening effect.  
         [0028]    A preferred line narrowing module of the present invention is shown in FIGS. 4A, B and C. It has three beam expanding prisms that expand the beam in the horizontal direction and one additional prism, which expands the beam in the vertical direction.  
         [0029]    [0029]FIG. 4A is a top view. FIG. 4B is a side view from the side indicated in FIG. 4A. (In FIG. 4B the prisms are depicted as rectangles representing the portion of the prisms through which the center of the beam passes.) FIG. 4C is a prospective view. Note that the grating  16  and mirror  14  are at a higher elevation than prisms  8 ,  10 , and  12 . Note that the expanded beam heads off in a direction out of the plane of the horizontal beam expansion. The beam then is redirected back into a second horizontal plane parallel to the plane of the horizontal expansion by mirror  14  onto the face of the grating  16  which is positioned in the Littrow configuration in the second horizontal plane. (Grating  16  is shown as a line in FIG. 4B representing the intersection of the horizontal center of the beam with the grating surface.)  
         [0030]    In the preferred embodiment, each of the three horizontally expanding prisms expands the beam by about 2.92 times. Therefore, total beam expansion in the horizontal direction is 2.92 3 =25 times. The beam expansion in the vertical direction is 1.5 times. (The degree of expansion is exaggerated in FIGS. 4B and C.) This vertical beam expansion does not directly affect the beam divergence in the laser cavity or the vertical beam divergence of the output laser beam, but it does reduce the vertical divergence of the beam as it illuminates the grating surface. After the beam is reflected from the grating, prism  60  contracts the beam in its vertical direction as it passes back through the prism thus increasing its divergence back to normal. This reduced divergence of the beam as it illuminates the grating results in a reduction in the wavelength shift effect thus producing better line-narrowing. A vertical tilt of 1 mrad of the beam before it goes through this prism is reduced to  
           1                 mrad     1.5     =     0.67                   mrad   .                             
 
         [0031]    According to FIG. 3, this will correspond to wavelength shift reduction from 0.1 pm to a mere 0.044 pm making this effect insignificant for line narrowing of the next generation of lasers.  
       Need for Large Grating  
       [0032]    The two direction beam expander requires a larger grating than prior art gratings used for integrated circuit light sources. In the case described above, the grating would need to be about 50 percent larger in the vertical direction.  
       Forty-Five X Horizontal Beam Expander  
       [0033]    [0033]FIG. 5 shows another technique for greatly reducing bandwidths of gas discharge lasers. Line narrowing is done by a line narrowing module  110 , which contains a four prism beam expander (112 a - 112   d ), a tuning mirror  114 , and a grating  10 C 3 . In order to achieve a very narrow spectrum, very high beam expansion is used in this line narrowing module. This beam expansion is 45× as compared to 20×-25×typically used in prior art microlithography excimer lasers. In addition, the horizontal size of front ( 116   a ) and back ( 116 B) apertures are made also smaller, i.e., 1.6 and 1.1 mm as compared to about 3 mm and 2 mm in the prior art. The height of the beam is limited to 7 mm. All these measures allow to reduce the bandwidth from about 0.5 pm (FWHM) to about 0.2 pm (FWHM). The laser output pulse energy is also reduced, from 5 mJ to about 1 mJ. This, however, does not present a problem, because this light will be amplified in a power amplifier  120  to produce a 10 mJ desired output per pulse. The reflectivity of the output coupler  118  is 30%, which is close to that of prior art lasers.  
         [0034]    [0034]FIG. 6 is a drawing showing detail features of a preferred embodiment of the present invention. Large changes in the position of mirror  14  are produced by stepper motor through a 26.5 to 1 lever arm  84 . In this case a diamond pad  81  at the end of piezoelectric drive  80  is provided to contact spherical tooling ball at the fulcrum of lever arm  84 . The contact between the top of lever arm  84  and mirror mount  86  is provided with a cylindrical dowel pin on the lever arm and four spherical ball bearings mounted (only two of which are shown) on the mirror mount as shown at  85 . Piezoelectric drive  80  is mounted on the LNP frame with piezoelectric mount  80 A and the stepper motor is mounted to the frame with stepper motor mount  82 A. Mirror  14  is mounted in mirror mount  86  with a three point mount using three aluminum spheres, only one of which are shown in FIG. 6. Three springs  14 A apply the compressive force to hold the mirror against the spheres.  
         [0035]    [0035]FIG. 7 is a second preferred embodiment slightly different from the one shown in FIG. 6. This embodiment includes a bellows  87  (which functions as a can) to isolate the piezoelectric drive from the environment inside the LNP. This isolation prevents UV damage to the piezoelectric element and avoid possible contamination caused by out-gassing from the piezoelectric materials.  
       Large Gratings Made Using Lithographic Techniques  
       [0036]    Applicants have developed techniques for making large gratings needed to provide bandwidth reductions for lithography laser light sources. These techniques utilize some of that same lithographic processes that the laser lithographic light sources support. This is a matter of bootstrap technology advancement.  
         [0037]    To fabricate a grating with a desired blaze angle using lithographic techniques, it is useful to etch silicon more rapidly along some crystal planes than others. This anisotropic etching allows the etch to significantly slow down or to etch specific shapes or structures in the silicon. In the diamond lattice of silicon, the (111) plane (or its equivalents generally designated as {111} planes) is more densely packed than the (100) plane (see FIG. 9). Consequently, etch rates of (111) oriented surfaces are expected to belower than those of with (100) orientations. One common anisotropic wet etchant for silicon is a mixture of potassium hydroxide (KOH) and isopropyl alcohol. The etch rate of this etchant is about 100 times faster along (100) planes than along (111) planes.  
         [0038]    In order to etch a diffraction grating with grooves whose facets are at a desired angle with respect to each other, a single crystal substrate must be carefully chosen keeping in mind both the relative angles of the crystallographic planes of the singlecrystal substrate, and the orientation of those planes with respect to the plane of the diffraction grating, for example the plane of the substrate. FIG. 9 shows a boule of single crystal silicon  200 . High purity, single crystal silicon is grown using a variety of techniques including the Czochralski method and the floating zone method. Additionally, single crystal silicon is grown in a variety of orientations depending on the desired application. Silicon boule  200  is grown with the (100) plane perpendicular to the length of the boule (i. e., the direction of growth), an orientation common in semiconductor manufacturing. Consequently, wafers sawn from the boule perpendicular to the growth axis has a surface with the (100) orientation. Silicon boule  200  includes flats  202  and  204  which are formed in the boule, by, for example, grinding, to help indicate the crystallographic axes of the silicon. In order to take advantage of the anisotropic etching of the {111} planes as noted above, a wafer to be etched should be cut from the boule at an angle φ with respect to the normal of the (100) plane, so that subsequent etching yields the desired angular grating groove facetfeatures. For example, in order to fabricate a grating groove facet at an angle of 78.81° with respect to the plane or surface of the substrate wafer (i.e. the grating&#39;s blaze angle) and using anisotropic etching, the substrate wafer should be cut from the boule so that the angle between the surface and one of the {111} planes is 78.81°. Thus, substrate  300  is cut from boule  200  at an angle φ=24.07° (because the (111) plane forms an angle of 54.74° with the (100) plane) with respect to the normal of the (100) plane and in the direction shown by arrow  220 . Substrate  300  then receives conventional wafer manufacturing processes including polishing both sides to provide thickness uniformity and flatness (e.g. a flatness of less than 5 μm).  
         [0039]    [0039]FIG. 10A shows a cross-section of substrate  300  including the location of a {100} plane and two {111} planes as shown by  302 ,  304 , and  306  respectively. Substrate  300  also includes an oxide layer  310 . Alignment marks (not shown) are etched into the substrate to determine precisely the crystallographic axes. Note that the alignment marks can be etched following the same general steps as outlined below for the etching of the grating grooves. Those having ordinary skill in the art will readily recognize that there are a variety of photolithographic and micromachining techniques suitable for use in fabricating the disclosed gratings including the alignment marks.  
         [0040]    [0040]FIG. 10B shows multiple photoresist mask features  320 . The photoresist mask features  320  are formed by coating the substrate with a layer of photoresist; selectively exposing the photoresist through a photomask, using, for example, a contact printing technique or direct writing; developing the photoresist; and curing the photoresist (e.g. baking) as necessary. The photomask can be generated, for example, by e-beam and have a plurality of parallel stripes. The width of the stripes defines the width of the etching mask, and the pitch of the stripes (i.e. the distance between the beginning edge of one stripe and the beginning edge of the next stripe) relates to the final groove spacing d. For example, the width of the stripes can be approximately 3 μm and the pitch can be approximately 12 μm.  
         [0041]    Next, oxide layer  310  is isotropicly etched, and photoresist mask features  320  are removed leaving a plurality of oxide hard mask features  330 , as seen in FIG. 10C. FIG. 10D shows the results of anisotropic etching of the substrate  300  such that a {100} plane is etched more rapidly than other crystallographic planes. Multiple grooves  340  are formed, each with facets  342  and  344 . In the example shown, both facets are {111 } planes, and the angle between the facets is defined by an inherent angle between {111 } planes in single crystal silicon. The oxide hard mask features  330  are removed, the substrate is cleaned, and a coating of reflective material  350 , for example vacuum deposited aluminum which has high reflectance for DUV light, is deposited on the surface of the etched substrate, as shown in FIG. 10E. Protective coatings such as SiO 2 , SiN 4 , and MgF 2  can be deposited prior to deposition of the reflective coating. Additionally, a variety of different metallic (e.g. chromium and nickel) and dielectric coatings (either single or multiple layers) can be deposited as indicated by the particular application for the diffraction grating. Protective coatings can even be deposited on top of the reflective coating or coatings. Once completed, the remaining portions of substrate  300  can serve as a substrate for mounting purposes. Alternatively, the grating can be attached to another substrate material. By attaching several gratings to the same substrate, a single, larger grating can be achieved.  
         [0042]    Flats  360  on the top edges between adjacent grooves  340  are caused by the mask used to etch the grooves. Flats  360  are generally undesirable because they prevent incident light from reflecting off a blazed facet such as facet  342 . Flats  360  can be reduced and even eliminated in some circumstances by over-etching the silicon and/or minimizing the width of the mask features. Alternatively, the flats can be eliminated by making a replica of the grating, as shown in FIGS.  11 A- 11 E.  
         [0043]    The fabrication of a replica grating begins with a master grating such as grating  400 . Grating  400  is similar to the grating of FIG. 10E, except that reflective coating  350  has not been deposited, and a thin film of a separating compound  410  has been deposited on the grating. Alternatively, separating compound  410  is deposited on top of reflective coating  350 , or in some circumstances, no separating compound is used. FIG. 11B shows that a reflective coating  420  is deposited over the thin film of separating compound. Reflective coating  420  will form the reflective surface of the replica grating. Alternatively, no reflective coating can be deposited at this point in the replication process, and instead a reflective coating can be added after the replica grating is separated from the master grating. Next, the coated master grating  400  is cemented to replica substrate  440  using a layer of resin  430 , allowing the resin to polymerize, as shown in FIG. 11C. Replica substrate  440  can be made from glass, such as standard optical glass, BK- 7 , Pyrex™, ZeroDur™, ULE®, or fused silica. Other materials, such as metal or light-weight composites can also be used. Additionally, a variety of different resins including both polyester and epoxy based resins are suitable for resin  430 . FIG. 11D illustrates the separation of the master grating from the replica once resin  430  is sufficiently set. Because of the separation layer and the resin, reflective coating  420  remains attached to the replica grating  450 . Because the facets meet at the bottom of each groove in the master grating, the top edge  460  between grooves in the replica grating is generally a sharp edge, and the flats  360  shown in FIG. 10E are eliminated.  
         [0044]    Another example of a technique for fabricating replica gratings makes use of compact disc (CD) manufacturing technology. With CDs, the mastering process typically begins with a polished, flat glass master. The master is coated with a layer of photoresist which is then exposed to light from a recording laser. If the photoresist is a positive photoresist, portions of the photoresist that are exposed to light are removed in a subsequent developing step. If the photoresist is a negative photoresist, non-exposed portions of the photoresist layer are removed in a subsequent developing step. Thus, a master is created with either pits or projections representing the binary data recorded on the disk. The master is then coated with a thin layer of metal (e.g. silver and/or nickel). The metalized master is then subjected to an electroforming process where additional metal is added to the thin layer of metal by, for example, electroplating, until a required thickness is achieved. This thick metal layer, often referred to as a “father,” is then separated from the master, and represents a negative image of the master. Because the father is a negative of the master, it can be used as a stamper to replicate CDs directly. Alternatively, the electroforming process can be performed using the father to replicate an additional master or “mother.” The mother, in turn, is used to electroform multiple copies (“sons”) of the stamper needed to produce CDs. Note that the electroforming process can be conducted using a variety of techniques and materials. Additional steps can be included, such as depositing a separation layer between either the master, the father, or the mother and a subsequent electroformed metal layer.  
         [0045]    Once a suitable stamper is produced, it is installed in a compression mold or injection mold. Molten plastic, such as polymethylacrylate or polycarbonate, is injected into the mold at high pressure against the stamper. The plastic is then cooled rapidly before the disc is removed. Next, a reflective layer such as aluminum is deposited on the data side of the disk. Finally, a protective layer is deposited over the deposited on the data side of the disk. Finally, a protective layer is deposited over the aluminum.  
         [0046]    In modifying this process for the fabrication of replica diffraction gratings, the CD glass master is replaced with a master diffraction grating such as grating  500  as shown in FIG. 12A. Grating  500  is similar to the grating of FIG. 10E, except that reflective coating  350  has not been deposited. Grating  500  can be used as the stamper in an injection or compression mold as shown in FIG. 12B. Mold  550  includes a cavity  552  within which grating  500  is placed to serve as the stamper. The remaining space of cavity  552  is filled by way of inlet  554  with plastic, such as polymethylacrylate or polycarbonate, to form replica grating  530 . After the plastic cools and hardens, grating  530  is removed from the mold as shown in FIG. 12C. The replica can then be coated with reflective and/or protective materials, and attached to another substrate if desired. Because the facets meet at the bottom of each groove in the master grating, top edge  565  between grooves in the replica grating is generally a sharp edge, and the flats  360  shown in FIG. 10E are eliminated.  
         [0047]    As in the case of CD replication, the stamper can be a father, mother, or son that has been electroformed based on the original master diffraction grating. Since one advantage of any replica created from the master diffraction grating described above is a sharp top edge between grooves, a preferred stamper would be an electroformed mother, that is a stamper with the same surface profile as the master grating and formed from a father which is, in turn, formed from the master diffraction grating. Using a mother stamper ensures that the flats  360  are located at the bottom of grating. Using a mother stamper ensures that the flats  360  are located at the bottom of grooves, and the edges between the grooves are sharp.  
         [0048]    Although the master diffraction grating of the present invention is shown fabricated from silicon, a number of different single crystal materials can be used, including, for example, gallium arsenide (GaAs). Additionally, a variety of different wet and dry etchants can be used to achieve the desired preferential etching leading to specific grating features given the material being etched, the orientation of the material&#39;s crystallographic planes, and the orientation of the surface of the grating substrate.  
         [0049]    Techniques for substantially real time control of several wavelength parameters are described in a U.S. patent application filed Sep. 3, 1999, Ser. No. 09/390,579 and in a U.S. patent application filed Oct. 31, 2000, Ser. No. 09/703,317 which are incorporated by reference herein. These techniques include fast feedback control of the position of the beam expanding prisms, grating curvature and tuning mirror position. Control of the position of the laser chamber is also provided.  
         [0050]    The description of the invention set forth herein is illustrative and is not intended to limit the scope of the invention as set forth in the following claims. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.