Reflector for excimer laser and excimer laser apparatus using the reflector.

1. FIELD OF THE INVENTION

0001The present invention relates to a reflector which is applicable to an excimer laser, and more particularly to a reflector having high reflectance in a wide wavelength range and high damage resistance against the excimer laser light in ultraviolet range and an excimer laser apparatus using the reflector.

DESCRIPTION OF THE RELATED ART

0002An excimer laser radiates ultraviolet light, and is expected to be applicated to a semiconductor fabri­cating process, chemical industry, medicine and energy generating field.

0003In a discharge excitation excimer laser, excimer is generated by high speed pulse discharge which is uniform in spatial distribution in high pressure laser gas (2-6 atmospheric pressure).

0004Two kinds of structures are known as a total reflector for the excimer laser in the prior art. A first structure is that, as shown in FIG. 1, aluminum (Al) for a metal layer 12 having a high reflectance is depositted on a substrate 21 such as glass, quartz or fluorite (such as CaF₂). Then, the metal layer 12 is covered by a single layer (generally of optical thickness nd = λ/2) of magnesium fluoride (MgF₂) for a protection layer 23 for protecting the aluminum layer from the excimer laser gas which is a corrosive gas. Since the total reflector is free from wavelength dependency in reflectance, it is usable to excimer lasers of laser gas of such as ArF (193 nm), KrF (248 nm) or XeCl (308 nm). However, the reflector is inferior in reflectance, which is under 90 %, and further is inferior in the damage resistance, too. Moreover, in case that the total reflector is used in a condition which is directly exposed to the excimer laser gas, the corrosion resistance to the gas of the magnesium fluoride layer of λ/2 optical thickness is insufficient. On the other hand, if the optical thickness of the deposition layers of the magnesium fluoride is increased by integer times of λ/2 in order to improve corrosion resistance to gas, absorption of the excimer laser light in the deposited layers increases and causes both the damage resistance and reflectance to decrease. The reflectance with respect to the wavelength of the total reflector is shown by a curve 41 in FIG.2.

0005The second structure is that, as shown in FIG.3, dielectric multilayers 31 are formed on a substrate 21 of glass, quartz or fluorite. The dielectric multilayers 31 are composed of alternately piled layers of plural pairs of two kinds of layers of dielectric materials having low refractive index and high refractive index, both having λ/4 optical thickness (nd). The reflectance with respect to the wavelength of the above-mentioned total reflector is, as shown by curves 42, 43 and 44 of FIG.2, high in particular wavelengths of the excimer laser. In other words, the wavelength dependency is very narrow, and the reflectance is low in other wavelength of the excimer laser. Therefore there is an inconvenient that the reflector must be changed for other reflectors prepared exclusively therefor when the oscillation wavelength is changed by change of the laser gas in a laser cavity. In the dielectric multilayer, the more the number of layers increase, the higher the reflectance becomes in theoretical consideration, but improvement of the reflectance is limited on account of absorption and scattering in the layers and increase of a mechanical stress in the actual dielectric multilayers.

0006Besides, in general, unexpected impurities are liable to be included in boundaries of the layers on the layer forming process. It seems that a high electric field induced by light concentrates to the impurities and local heating is generated on the impurities and the layers are damaged. A distribution of standing wave electric field intensity which is obtained by calculation in the multilayers of each λ/4 optical thickness in the prior art is shown in FIG.4. Referring to FIG.4, a part "A" having high electric field intensity of the standing wave is positioned at a interface of the layers including much impurities, and it is thought to be one of causes of drop in the damage resistance.

0007The above-mentioned two kinds of the total reflector for the excimer laser in the prior art have both merits and demerits. As a common problem, they are inferior in the damage resistance, and usable power level is only 100 W in the most.

0008In case that the output of the laser is such high as 1 KW, which is 10 times the output in the prior art, a problem of heat which is generated on the surface of the total reflector must be solved. The amount of heat which is generated on the surface of the total reflector of 98 % in reflectance is estimated 20 W (1 KW x 2 % = 20 W). A temperature at a portion impinged by the laser beam rises in the substrate which is low in thermal conductivity such as conventional glass or quartz, and deformation of the total reflector and damage of the reflecting layer are inescapable. In improvement in the damage resistance, therefore, selection of material for substrate of the total reflector is considered.

0009Though the aluminum deposited layer has a merit which is wide in reflection wavelength range, it has a demerit of low in reflectance. On the other hand, the dielectric multilayer can provide higher reflectance than that of aluminum layer as an advantage, but it has a disadvantage of narrowness in reflection wavelength range. Moreover, in order to improve the reflectance, the number of the layers must be increased, however, there is a problem that an absorption increases and damage resistance decreases in proportion to the increase of the number of the layers. Furthermore, in a layer structure of the optical thickness of λ/4, since the interface of the layers is inevitably positioned at a portion where the electric field is high in intensity, the damage resistance is diminished. Then, in order to improve the damage resistance in the layer having a high reflectance, the reflection layer structure, dielectric material for multilayer structure and a thickness of the layer must be considered.

0010Corrosion by halogen gas contained in the laser gas is unavoidable in either case of aluminum reflector surface and dielectric multilayer surface. In order to solve the above-mentioned problem in the conventional method, a protection layer of magnesium fluoride (MgF₂) having a thickness of λ/2 is depositted on the reflecting surface, but making of a protection layer of the magnesium fluoride having no defects such as pinholes and having thickness of λ/2 optical thickness is impossible, (for example, when wavelength λ is 0.3µm and a refractive index n is 1.5, the layer is 0.1µm (1000 angstrom) in a geometrical thickness d.). Therefore, it is not complete means for solving the problem.

OBJECT AND SUMMARY OF THE INVENTION

0011An object of the present invention to provide a total reflector which is utilized for a high power excimer laser such as a power level of 1 KW, have a high reflectance in a wide range of the reflecting wavelength and having high damage resistance for excimer laser.

0012The reflector for excimer laser in accordance with the present invention comprises:     a substrate for forming reflecting materials on a surface thereof,     an aluminum layer formed on the substrate, and     dielectric multilayers formed on the aluminum layers.

BRIEF DESCRIPTION OF THE DRAWINGS

0013<ul id="ul0001" list-style="none"><li>FIG.1 is the cross-sectional view of the total reflector for excimer laser in the prior art;</li><li>FIG.2 is the graph showing the reflectance with respect to the wavelength of the total reflector in the prior art;</li><li>FIG.3 is the cross-sectional view of the other example of the total reflector for excimer laser in the prior art;</li><li>FIG.4 is the graph showing the distribution of electric field in the layers of the total reflector in the prior art;</li><li>FIG.5 is a cross-sectional view of a total reflector of a first embodiment in the present invention;</li><li>FIG.6 is a graph showing reflectance with respect to a number of pairs in the embodiment and the prior art:</li><li>FIG.7 is a graph showing a reflectance with respect to an wavelength of the total reflector in the embodiment;</li><li>FIG.8 is a graph showing distribution of an electric field intensity in the layers of the embodiment;</li><li>FIG.9 is a graph showing a reflectance with respect to the wavelength of the total reflector in a second embodiment of the present invention;</li><li>FIG.10 is a graph showing distribution of the electric field intensity in the layers of the second embodiment of the present invention;</li><li>FIG.11 is a perspective view of an excimer laser apparatus in accordance with the present invention.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0014FIG.5 is a cross-sectional view of a total reflector of a first embodiment in the present invention. Referring to FIG.5, a surface of a silicon substrate 11 is precisely polished. An aluminum layer 12 having a thickness of about 1000 Å is formed on the silicon substrate 11 by a vacuum evaporation method such as a vacuum evaporation by resistance heating, ion beam evapo­ration method or sputtering method. A plurality of dielectric multilayers 13 are formed on the aluminum layer 12. The dielectric multilayers are composed of alternative layers of magnesium fluoride (MgF₂) as a low refractive index material and scandium oxide (Sc₂O₃) as high refractive index material. In the embodiment, three pairs of both the materials are formed by the vacuum evaporation method. The first layer 13-1, third layer 13-­3 and fifth layer 13-5 of the dielectric multilayers are the magnesium fluoride layers, and the second layer 13-2, fourth layer 13-4 and sixth layer 13-6 are scandium oxide layers. An optical thickness (nd) of the first layer 13-1 and the third layer 13-3 are λ/4, and a geometrical thickness (d) is about 43.79nm.

0015Namely, in case that a central wavelength λ of the laser light is 250.5 nm and the refractive index (n) is 1.43, the geometical thickness (d) 43.79 nm is obtained by substituting the above-mentioned values of λ and n to a known equation d = 1/n (λ/4).

0016The optical thickness of the scandium oxide layers of the second layer 13-2 and fourth layer 13-4 are (λ/4), and the geometrical thickness (d) thereof is about 29.68 nm (λ = 250.5 nm, n = 2.11).

0017The optical thickness (nd) of the magnesium fluoride layer of the fifth layer 13-5 is 3λ/8, and the geometrical thickness (d) is about 65.69 nm. The optical thickness (nd) of scandium oxide layer of the sixth layer 13-6 is λ/8, and the geometric thickness (d) is about 14.84 nm.

0018Features of the layer structure are that the respective optical thicknesses of the magnesium fluoride layer of the fifth layer 13-5 and the scandium oxide layer of the sixth layer 13-6 are made to deviate from λ/4, and a total optical thickness of both the layers is made to be λ/2 (3λ/8 + λ/8 = λ/2).

0019A single crystal polished protection plate 14 of fluorite (CaF₂) is about 1 mm thickness and is transparent (having high purity and is negligible of absorption) in the wavelength range of the embodiment.

0020The reflecting layers 12 and 13 are heated by impingement of the laser light. The heat generated on the reflecting layers 12 and 13 causes deformation of the substrate 11 and damage of the reflecting layers. Therefore, it is desirable that material of the substrate 11 has a high thermal conductivity (K) and a low coefficient of linear expansion (α). Figure of merit K/α is defined in order to estimate characteristic of material of the substrate 11. Referring to the figure of merit K/α, material having high figure of merit K/α is feasible for the substrate 11.

0021Table 1 shows physical properties of several kinds of substrate materials. <tables id="tabl0001" num="0001"><img file="EP0280299A2_D0001.tif" /></tables>

0022Referring to Table 1, silicon (Si), silicon carbide (SiC) and tungsten (W) have serial hundred times of thermal conductivity of a kinds of glass which is used in the prior art, and are almost equal to metal in the thermal conductivity. Moreover, silicon, silicon carbide and tungsten are superior than other kinds of glasses in the figure of merit (K/α).

0023An effect of high reflectance dielectric multilayers 13 which are formed on the aluminum deposited layer 12 is elucidated hereinafter. A curve 46 in FIG.6 shows a reflectance dependency with respect to a number of pairs of the magnesium fluoride layer and scandium oxide layer which are formed on a quartz substrate and have λ/4 optical thickness, and a curve 47 shows a reflectance dependency with respect to a number of pairs of the magnesium fluoride layer and scandium oxide layer which are formed on the aluminum layer on the silicon substrate. At least seven pairs of the magnesium fluoride layer and scandium oxide layer are necessary to achieve over 99 % of the reflectance in case of using the conventional quartz substrate, while it can be effectuated by only three pairs of the magnesium fluoride layer and scandium oxide layer formed on the aluminum layer deposited on the silicon substrate in the present invention as shown on these curves 46 and 47. Absorption, dispersion and mechanical stress in the multilayers increase in proportion to the increase of the layers, and cause drop of the damage resistance by irradiation of the laser light. In the embodiment, since the number of layers is half of that of the conventional reflector, the deposition process of the layers is simplified and a high quality layers having high damage resistance are obtainable.

0024Effects of the multilayer structure in the embodiment are elucidated hereinafter. Calculated reflectance with respect to the wavelength is shown in FIG.7. As shown in the curve of FIG.7, a reflectance which is over 90 % is achieved in the respective wavelengths of laser lights of ArF laser (193 nm), KrF laser (248 nm) and XeCl laser (308 nm), and a reflector operable for a wide wavelength range is obtainable.

0025FIG.8 is a graph showing a distribution of the electric field intensity in the layers of the embodiment which is obtained through calculation. Referring to the graph, a interface 48 is positioned at a comparatively low electric field portion as shown by a circle A by deviating the optical thickness of the outer pair of layers from λ/4. Consequently the damage resistance is improved.

0026In case that the optical thickness of the respective layers of the outer pair are changed from λ/4, only making of the total optical thickness of the pair to be λ/2 does not always gives desirable result for in any combination of the optical thickness of the respective layers. Several examples of the combination of the optical thickness of the respective layers of the outer pair are shown in Table 2, in which values of calculated reflectance of the respective examples are listed. <tables id="tabl0002" num="0002"><img file="EP0280299A2_D0002.tif" /></tables>

0027In the example 1 of the table 2, the optical thickness of both the layers of the outer pair is λ/4, and the reflectance (R) is below 90 % in the wavelength of 193 nm. And desirable characteristic is not obtainable, because the reflectance of 90 % is used as criterion of the characteristic of the reflector in the embodiment. Therefore, in the column of "Judgment" of the table 2, a cross as being infeasible is marked. In the examples 2, 3 and 4, the optical thickness of the magnesium fluoride layers are thicker than that of scandium oxide layer, and the reflectance (R) exceeds 90 % in any wavelength. Therefore, a circle as being feasible or a double circle as being most feasible is marked in the column of Judgment. In the example 5, the optical thickness of the scandium oxide layer is thicker than that of the magnesium fluoride layer, and the reflectance is below 90 % for the wavelength of 193 nm. Consequently, the layer structure is infeasible in the embodiment as marked by a cross.

0028An effect of the single crystal polished plate of calcium fluoride (CaF₂) is elucidated hereinafter. In case that the total reflector is exposed to the excimer laser gas in the resonance chamber, the calcium fluoride plate 14 is effective to protect the aluminum layer 12 and dielectric multilayer 13 from corrosion by the laser gas.

0029According to the embodiment as mentioned above, since silicon which is superior in thermal characteristic is used as a substrate 11 and the reflecting layer is composed of combination of the aluminum layer 12 and the dielectric multilayers 13, the number of the layers, which is a cause to drop the damage resistance, can be reduced. Moreover, the range of wavelength with respect to a predetermined reflectance can be expanded by deviation of the optical thickness of the respective layers in the outer pair of the dielectric multilayers from λ/4. As a result, the interface of the dielectric layers in the outer pair is positioned in the portion of low electric field intensity, and the wide band total reflector having high reflectance and high damage resistance is realized. In case that further corrosion resistance to gas is required, the calcium fluoride plate 14, for example, is mounted on the total reflector, and thereby, a circumference resistance is improved.

0030In a second embodiment of the present invention, the substrate 11 is made of silicon carbide in stead of silicon of the first embodiment. The magnesium fluoride as low refractive index material in the first embodiment is replaced here, by lithium fluoride (LiF); and scandium oxide as high refractive index material is replaced by aluminum oxide (Al₂O₃). Further, magnesium fluoride plate is used instead of calcium fluoride plate for the protection plate 14. The layer structure is shown by the following representation:     Al//[LiF(λ/4)Al₂O₃(λ/4)]²LiF(3λ/8)/Al₂O₃(λ/8).

0031In case that a center wavelength is 250.5 nm, the refractive index (n) of lithium fluoride is 1.37 and the refractive index (n) of aluminum oxide is 1.72. Then, the geometrical thickness d of a lithium fluoride layer of λ/4 optical thickness is about 45.7 nm, and the geometrical thickness d of aluminum oxide layer of λ/4 optical thickness is about 36.4 nm, the geometrical thickness d of lithium fluoride layer of 3λ/8 optical thickness is about 68.6 nm and the optical thickness d of aluminum oxide of λ/8 optical thickness is about 18.2 nm.

0032Reflectance with respect to wavelength in the second embodiment is shown by a curve in FIG.9. Referring to the curve, reflectance exceeding 90 % is obtainable in the respective wavelength of the laser lights of ArF laser (193 nm), KrF laser (248 nm) and XeCl laser (308 nm), and a total reflector having wide band is realized.

0033In a similar manner to the first embodiment, calculated reflectances for the case where the optical thickness of the respective layers of the outer pair of the dielectric layers is deviated from λ/4 are shown in Table 3.

0034Referring to Table 3, the optical thickness of both the layers in the outer pair are all λ/4 in the example 1, and reflectance (R) is over 90 % in both the wavelength 193 nm and 248 nm. However, in the wavelength of 308 nm, the reflectance is under 90 %, and the example 1 of Table 3 can not pass the criterion. <tables id="tabl0003" num="0003"><img file="EP0280299A2_D0003.tif" /></tables>

0035In the examples 2, 3 and 4, the lithium fluoride layer is thicker than the aluminum oxide layer in the optical thickness, and the reflectance is slightly less than 90 % in the wavelength 308 nm of the example 2, but the judgment criterion is satisfied for the most part in the wavelength range. Therefore, a triangle mark is put in the judgement column. On the contrary in the example 5, the aluminum oxide layer is thicker than the lithium fluoride layer in the optical thickness, but the reflectance is lesser than 90 % in the wavelength of 193 nm and 308 nm, and the judgment criterion can not be satisfied.

0036FIG.10 is a graph showing a distribution of the optical electric field intensity in the layers with respect to the thickness of the multilayers in the second embodiment. Since the optical thickness of both the layers in the outer pair is deviated from λ/4, the electric field intensity in the interface of both the layers is comparatively low as shown by a circle A in FIG.10. Consequently, the damage resistance is improved.

0037As mentioned above, the similar effect to the first embodiment is realized by the embodiment, and a wide band total reflector having high reflectance and high damage resistance is obtainable.

0038Experiments show that results are obtainable by employing tungsten (W), molybdenum (Mo), tungsten-­ molybdenum alloy (W-Mo) or thermal resistance ceramics for the substrate. Moreover, a substrate which is coated with silicon carbide (SiC) on the above listed materials is usable in the embodiment. Materials for forming the high reflectance dielectric multilayer can be selected from oxides and fluorides which has little absorption for the laser light in a wavelength range of the excimer laser (for example, oxides such as yttrium oxide (Y₂O₃), hafnium oxide (HfO₂), magnesium oxide (MgO) and silicon oxide (SiO₂), and fluoride such as yttrium fluoride (YF₃), lanthanum fluoride (LaF₃), calcium fluoride (CaF₂) and thorium fluoride (ThF₄)) can be employed.

0039In the embodiments 1 and 2, the optical thickness of both the layers of the outer pair are made deviate from λ/4 in order to obtain a wide band characteristic, but in case that a high reflectance and high damage resistance are required in a specific wavelength of the excimer laser, if the optical thickness of both the layers is λ/4, the high damage resistance is realized by the layer structure in accordance with the present invention.

0040Though the total reflector of the present invention is shown as a plane mirror in the cross-­sectional view of FIG.5, the present invention is applicable to a spherical mirror and aspherical mirror.

0041As mentioned above in the total reflector for the excimer laser of the present invention, a heat which is generated on the reflecting plane by irradiation of the laser light is effectively conducted to a heatsink which is mounted on the reflector, since the substrate is made of silicon, silicon carbide or molybdenum tungsten alloy (Mo-W) which is superior in thermal conductivity. Consequently, local temperature rise in the substrate is prevented and deformation of the substrate by heat and temperature rise of the reflector is also prevented, and thereby the damage resistance is further improved. Moreover, the number of the dielectric multilayers can be reduced by half, and thereby, the reflecting layers are formed by aluminum and dielectric multilayers, and it is effective to improve the damage resistance.

0042An example of an excimer laser apparatus using the total reflector in accordance with the present invention is shown in FIG.11.

0043Referring to FIG.11, two bar-shaped main discharge electrodes 1 and 2 are arranged parallelly, and a total reflector 9 is positioned adjacent to one ends of the main discharge electrodes 1 and 2. An output coupling reflector 10 is placed adjacent to the other ends of the main discharge electrodes 1 and 2. Ten pairs of needle-­shaped predischarge electrodes 3 and 4 are arranged along the main discharge electrodes 1 and 2, and the predischarge electrodes 3 and 4 are held by two holding members 7 which are connected by an isolation member 8. The respective predischarge electrodes 3 of one of the holding members 7 oppose to the respective predischarge electrodes 4 of the other of the holding members 7 so as to maintain a predetermined gap therebetween. The main discharge electrodes 1 and 2, and the predischarge electrodes 3 and 4 are enclosed in an envelope with excimer laser gas. The total reflector 9 and output coupling reflector 10 are mounted on openings of the envelope.

0044Operation of the apparatus is elucidated hereinafter. Prior to the main discharge, arc discharge is generated between the predischarge electrodes 3 and 4, and ultraviolet light is emitted between the gap. Then the arc discharge is generated along the length of the main discharge electrodes 1 and 2, and a domain between the main discharge electrodes 1 and 2 is irradiated by a stable and uniform ultraviolet light of the predischarge, and a lot of uniform ionized electrons are produced by the pre-ionization effect by the predischarge.

0045In the above-mentioned state, when a glow discharge is generated between the main discharge electrodes 1 and 2, stable glow discharge is maintained because of existence of a lot of uniform ionized electrons. Then, laser beam 15 of ultraviolet light which is stable in output and is uniform in spatial distribution is output.

0046In order to generate the uniform glow discharge, it is important and essential to ionize the laser gas previously and slightly (pre-ionization) previous to the above-mentioned main discharge. Concerning output of the laser light, stability of the output and a diameter of the laser beam significantly depend on the number and spatial uniformity of the electrons which are produced in the pre-­ionization.

0047In the embodiment, since the total reflector 9 has the high reflectance, high output beam is obtained. Moreover, since the total reflector 9 has a wide range of wavelength in high reflectance, even if the wavelength of the laser beam is changed by change of the kind of the excimer laser gas, the apparatus is usable without exchange of the total reflector 9.