Laser apparatus

Abstract: In a laser apparatus at least one capacitor (37) is disposed interiorly of a cavity (74) formed inside of at least one main discharge electrode (72), a pair of such main discharge electrodes (72) being provided defining a main discharge region therebetween, the discharge being initiated by pre-discharge means.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT 1. FIELD OF THE INVENTION The present invention relates generally to a laser apparatus, and more particularly to a laser apparatus which has an effective utilization of laser beam generated in a discharge chamber and is operable in low atmospheric pressure laser medium. 2. DESCRIPTION OF THE RELATED ART An excimer laser generates a laser light having high power and high repeat frequency in ultraviolet range by discharge excitation of a rare gas and a halogen gas. The laser apparatus is expected to be applied to various technical research and further to a semiconductor fabricating process, chemical industry, phermaceutical industry and energy generating field. Typical excimer lasers are that of ArF of 193nm wave length, KrF of 248nm wave length and XeCl of 308nm wave length, and the laser light is generated by discharge excitation of a laser medium of a mixture of a rare gas and a halogen gas enclosed in a discharge chamber. FIG.1(a) is a cross-sectional view on a sectional plane perpendicular to an optical axis 7 of a main portion of an excimer laser apparatus in the prior art. A known power source and other known electronic circuits for operating the excimer laser apparatus are omitted for simplicities in FIG. 1(a). Referring to FIG. 1(a), the discharge chamber 2 is filled with a laser medium. Two main electrodes 3 and 4 which are long in a direction of the optical axis 7 and are faced each other are provided in the discharge chamber 2. The main electrodes 3 and 4 are wired to the power source through a capacitor (not shown). Ultraviolet light is radiated from pre-discharge means 5 prior to application of a high voltage pulse across the main electrodes 3 and 4, and the laser medium between the main electrodes 3 and 4 are ionized. When a voltage of the high voltage pulse which is applied across the main electrodes 3 and 4 reaches a break down voltage of the laser medium, glow discharge 6 is generated between the main electrodes 3 and 4, and the laser medium is excited. Consequently, laser light is emitted between an output coupler 10 which is mounted perpendicularly to the optical axis 7 and a total reflector (not shown). It is well known that the higher an atmospheric pressure of the laser medium which is filled in the discharge chamber is, the higher a laser output energy per one pulse becomes. The atmospheric pressure of the laser medium is generally 2--4 Kg/cm². On the other hand, an optimum interval H of the main electrodes 3 and 4 (height of discharge region) depends on the atmospheric pressure of the laser medium, and is experimentally represented by the following relation: H(interval)×P(atmospheric pressure) = 6Kg/cm. Therefore, the higher the atmospheric pressure is, the narrower the interval H of the electrodes 3 and 4 becomes. Therefore, it is recommendable that a wide interval of the electrodes is employed in case of low atmospheric pressure of the laser medium. In chemical industry or energy generating field, increase of the output energy of laser per one pulse is required, thus operation in high atmospheric pressure of the laser medium which is over 3 Kg/cm² is feasible A cross section of the output laser beam in the above mentioned example is generally 20mm in height and 15mm in width. Increase of an average output by mean of increase of a repeat frequency of the laser oscillation is required rather than a higher output per one pulse for a light source of photolithography in the semiconductor fabricating process, in which the high output per one pulse is not required for exposing process of a photosensitive material such as photoresist. In a material of polymetamethyl alkylate (acrylate), for example, if a laser light having over 300 mJ/cm² per pulse is applied, a troublesome "ablation" arises. Therefore, a plurality of exposures by a low output light are preferable. A repeat frequency of at least 200 Hz is required for the light source of photolithography. In order to realize the laser oscillation of the high repeat frequency, a flow rate of the laser medium passing between the two main electrodes must be increased. For example, when a width of the discharge region of the main electrodes is designated by "W"(m) as shown in FIG.1(a) and the repeat frequency is designated by N(l/sec), a requisite flow rate U(m/sec) of the laser medium, in order that the laser medium existing in the discharge region after main discharge is perfectly replaced by new laser medium before the next main discharge, is defined by the following equation; U = C R ·W·N. The value of C R is 1 in an ideal condition, but the value of C R is valid by a structure of the discharge chamber and the shape of the electrodes, and is generally 2--3. For example, in a laser apparatus having a width W of 0.02m, a repeat frequency N of 500/sec and a value C R of 2, a flow rate of the laser medium of 20m/sec is required. In order to realize the above mentioned flow rate in the laser apparatus which is operated in a high atmospheric pressure laser medium, a fan which is driven by a high power electric motor must be provided in the discharge chamber because of the high atmospheric pressure of the laser medium, and further strong-build holding members and bearings for the fan are required. Consequently a complex mechanism is required and the cost thereof is expensive. Therefore, in order to realize high repeat frequency oscillation, a laser apparatus which is operated in low atmospheric pressure is advantageous. However, the wide interval of the two main electrodes, as mentioned above, is required in order to realize oscillation having high efficiency especially in the excimer laser which is operated in the low atmospheric pressure laser medium. Thus, as shown in FIG.1(b), a ratio of beam width(W′), which is almost equal to the width of the discharge region W, to the beam height (H′), which is almost equal to the interval of the electrodes 3 and 4 of the cross section of the output laser beam of the excimer laser, increases. In an operation of low atmospheric pressure of 2Kg/cm², an adequate interval H of the electrodes is about 3cm, and a cross section of the excimer laser radiated from the laser apparatus is rectangle of about 3cm in height(H′) and about 1.5cm in width (W′), and the ratio H′/W′ becomes about 2. When the atmospheric pressure of the laser medium is lower than the above mentioned case, and the ratio H′/W′ of the laser beam farther increases. The large ratio H′/W′ is not preferable for application of the excimer laser. For example, in application of the semiconductor fabricating process using photolithography, an expensive optical system of large diameter is required to utilize the output of the laser light effectively. On the other hand, in case that an optical system which is smaller than the beam height (H′) in an effective diameter is employed, the output beam of the laser is shielded by an aperture which is almost equal to the effective diameter the optical system. And thereby, effective utilization of the output beam cannot be attained. As mentioned above, in the prior art, cross-sectional shape of the laser beam which is output from the apparatus is equal to the shape of the discharge region 6, which is shown by the beam width (W′) and beam height (H′). Consequently, there is a defect that the ratio H′/W′ in low atmospheric pressure operation increases. The defect commonly exist not only in the excimer laser but also in gas laser such as CO₂ laser or He-Ne laser. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a laser apparatus which is operated in a low atmospheric pressure of a laser medium and are attainable effective utilization of an oscillated laser beam by a simple structure of the apparatus. A laser corresponding to the preamble of claim 1 is disclosed in EP-A-0 152 570. The laser apparatus in accordance with the present invention is defined by claim 1. Claims 2 to 7 describe preferred developments. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1(a) is the cross-sectional view of the laser apparatus in the prior art; FIG.1(b) is the cross-sectional view of the laser beam generated by the laser apparatus in the prior art; FIG.2(a) is a cross-sectional view which is perpendicular to an optical axis of a laser apparatus. FIG.2(b) is a cross-sectional view along the optical axis of the laser apparatus of FIG. 2(a); FIG.3 is an exciting circuit for operating the laser apparatus of FIG. 2; FIG.4 is a cross-sectional view of a laser apparatus; FIG.5 is a cross-sectional view of a laser apparatus; FIG.6(a) is a cross-sectional view along a axis of a laser apparatus; FIG.6(b) in a cross-sectional view which is perpendicular to the axis of the laser apparatus of FIG 5; FIG.7 is a cross-sectional view of a laser apparatus of an embodiment having an electrode containing a capacitor therein in accordance with the present invention; FIG.8 is a cross-sectional view of a laser apparatus of another embodiment having an electrode containing a capacitor in accordance with the present invention; DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.2(a) is a cross-sectional view on a plane of perpendicular to optical axes 7 end 7′ of an excimer laser apparatus, and FIG.(2b) is a cross-sectional view along the optical axes 7 and 7′ of the laser apparatus of FIG. 2(a). Referring to FIG.2(a) and FIG.2(b), a discharge chamber 2 is filled with a laser medium gas which is a mixture of a rare gas and a halogen gas of a predetermined atmospheric pressure under 3 Kg/cm² (2280torr.). The laser medium is circulated with a flow rate of 20--30m/sec in a direction perpendicular to the optical axes 7 and 7′ in a discharge region 6 defined between main electrodes 3 and 4. The main electrodes 3 and 4 are made to be a cross section of generally known Rogowsky or Chang type so that a uniform glow discharge is attainable. The main electrodes 3 and 4, as shown in FIG.3, are coupled to an exciting circuit of high voltage resonant charge transfer type. The uniform glow discharge is generated in the gap between the main electrodes 3 and 4 with a short time period by a charge and discharge energy of a peaking capacitor 37, and the discharge region 6 for generating the laser beam is formed. In FIG.3, a spark gap 36, a peaking capacitor 37 and the main electrodes 3 and 4 are mounted in the discharge chamber 2. In the exiting circuit as shown in FIG.3, a high voltage from a high voltage power source (not shown) is applied across terminals 31 and 32, and a storage capacitor 33 is charged through a charge coil 34. The electric charge in the storage capacitor 33 is transferred to a peaking capacitor 37 by closing a switch 35 such as spark gap or thyratron switch through the spark cap 36. Then, discharge occurs between the spark gap 36 during charge of the peaking capacitor 37. The laser medium between the spark gap 36 is ionized by the discharge, and ultraviolet light is emitted. The laser medium between the main electrodes 3 and 4 which are irradiated by the ultraviolet light are ionized. The ionization is called "pre-ionization". When the voltage which is applied across both the terminals of the peaking capacitor 37 reaches a break down voltage, a main discharge is generated across the main electrodes 3 and 4, and a laser light is emitted. In order to increase the output of the laser beam and improve a transfer efficiency to the laser light in the excitation operation, a large electric power must be inputted in a closed loop which is composed of the peaking capacitor 37, the spark gap 36, the main electrodes 3 and 4 and the stray inductance 38 during a short time period (several 10 nsec in excimer laser). In order to attain the purpose of reducing the size, the respective elements included in the closed loop must be miniaturized, and the stray inductance 38 due to wiring between the respective elements must be reduced to the minimum. In FIG.2(a) and FIG.2(b), the pre-ionizing devices 5 composed of the spark gap 36 and the peaking capacitor 37 ionize the laser medium prior to the main discharge of the discharge region 6, and makes a function for uniforming the flow discharge between the main electrodes 3 and 4. In the discharge operation, when the high voltage pulse is applied to the main electrodes 3 and 4, ultraviolet light is automatically radiated by the pre-ionizing devices 5. The discharge region 6 having height of H and width of W formed between the main electrodes 3 and 4 is divided into two half heights discharge regions 6a and 6b, each having H₁ = 1/2 H height, and the optical axes 7 and 7′ are set in the respective central portions of the two divided discharge regiones 6a and 6b. A total reflector 11 for the discharge region 6a and an output coupler 10 are provided on a left side wall 2a of the discharge chamber 2 perpendicularly to the respective optical axes 7 and 7′. A folding optical element 9 is provided on a right side wall 2d of the discharge chamber 2. An optical resonator in the example is composed of the output coupler 10, total reflector 11 and the folding optical element 9. The folding optical element 9 is consists of a total reflector 91 which in perpendicular to the axes 7 and 7′ and an optical member having two transparent plane 92 which is positioned in the respective optical axes 7 and 7′ with a Brewster angle, and mad to be minimum in a surface reflecting rate to P-polarization. In the above-mentioned laser apparatus, a path from the total reflector 11 to the output coupler 10 via the folding optical element 9 becomes an actual length of the discharge region, and the length of the discharge region becomes twice of the optical axis length of the discharge chamber 2. Consequently, the whole cross sectional area which is perpendicular to the axes 7 and 7′ of the discharge region 6 effectively serves to generate the laser light, and the laser beam is output from the output coupler 10 via the discharge region 6b which in an lower half of the discharge region 6. A ratio (H₁/W₁) of height H₁ to width W₁ of the discharge region as shown in FIG.2(a) can be made almost 1, in case that the discharge electrode has a structure of a large H/W ratio for the low atmospheric pressure gas. As a result, the diameter of the external optical system in sufficient to be a diagonal line of the cross section of the discharge pat 6b. Furthermore, the output laser light is made to be a vertical polarizing oscillation (P polarization) by an effect of the folding optical element 9. Therefore, when the output laser light is divided by an optical system which is incident with a predetermined angle, stable division is realized. FIG.4 is a cross-sectional view along the optical axis of a laser apparatus. In FIG.4, elements similar to the elements of FIG.2(b) are identified by like numerals. Only a folding optical element 12 is different from that of the laser apparatus of FIG. 2. The folding optical element 12 is composed of two reflectors 13 and 13′ which are mounted in the respective optical axes 7 and 7′ at 45° and a transparent optical member having a transparent plane 14 which is perpendicular to the axes 7 and 7′. Since the transparent plane 14 is perpendicular to the optical axes 7 and 7′, and the folding optical elements 12 are mounted in the respective optical axes 7 and 7′ at 45° (not Brewster angle), a laser light which is reflected by the folding optical element 12 does not deviate from the optical axes 7 and 7′ of the incident light in various wavelengthes of the laser. Thus, reflectance of the reflectors 13 and 13′ has a wide band covering the wavelength range of the excimer laser, for example, a laser light from XeCl laser (308 nm) to ArF (193 nm) can be oscillated by only change of the laser medium. Other effect of the second example are identical with that of the first example. FIG.5 is a cross-sectional view along the optical axis of a laser apparatus. The discharge region 6 is divided into four discharge regions 6a, 6b, 6c and 6d, and the respective optical axes 7a, 7b, 7c and 7d are set in the central portions of the respective discharge regions 6a, 6b, 6c and 6d, respectively. A total reflector 11 which is perpendicular to the optical axis 7a is mounted on a left end portion of the discharge region 6a, and a total reflector 13a is mounted in the optical axis 7a at 45° at a right end of the discharge region 6a. Total reflectors 13b are mounted in the optical axis 7b at 45° on the both end portions of the discharge region 6b, and total reflector 13c are mounted in the optical axis 7c at 45° on both the end portions of the discharge region 6c. A total reflector 13d is mounted in the optical axis 7d at 45° on the right end portion of the discharge region 6d, and an output coupler 10 which is perpendicular to the optical axis 7d is mounted on the left end portion of the discharge region 6d. In the discharge chamber 2, a length of the total discharge region becomes about four times of the length of the discharge chamber. According to the above-mentioned multi-division discharge region structure, the interval of the main electrodes 3 and 4 can be widened in order to improve efficiency of laser oscillation in low atmospheric pressure of the laser medium. The discharge region 6 which is enlarged in height direction can be divided by using a necessary number of the folding optical element. The folding optical element 9 which is shown in FIG.2(b) is applicable to this example. FIG.6(a) is a cross-sectional view of along an optical axis of an example which is applied to a CO₂ laser. A mixed gas of CO₂ , He , N₂ or the like is filled in the discharge chamber 2 as a laser medium. Two short tube-shaped discharge electrodes 22 and 23 are arranged with a predetermined distance in a common optical axis 8 in the discharge chamber 2. A discharge region 6 between the discharge electrodes 22 and 23 is divided into two discharge upper region 6a and lower region 6b, between which the optical axis 8 lies, and optical axes 7a and 7b are in the respective discharge regions 6a and 6b, respectively, as shown in FIG.6(b). Therein elements similar to the elements of FIG.2(b) are designated by like numerals. Since a length of an actual discharge region is doubled, a high output CO₂ laser is obtainable. In the high frequency pulse laser apparatus, the laser is oscillated in a low atmospheric pressure of the laser medium. In such apparatus, gap between two electrodes is made to wide in order to improve efficiency, and the cross section of the discharge region becomes inevitably rectangle. According to the present invention, the discharge region is divided into a predetermined number with the optical axis therebetween. Furthermore, the cross section of the output laser light is formed substantially in a square shape by mounting several reflectors at both end portions of the respective discharge regions, thereby to form permit one optical path to work as an optical resonator by means of the reflectors. Consequently, a diameter of the external optical system coupled thereto for receiving the output laser light can be reduced, and cost is saved. In the above-mentioned example, the output coupler is not essential to be formed integral with the total reflector. Furthermore, the folding optical element can be formed by combination of discrete reflectors. The above-mentioned example can be applied to a laser apparatus which is operated in high atmospheric pressure. FIG.7 is a cross-sectional view at a plane perpendicular to the optical axis,of the discharge chamber showing detailed constitution of the main electrodes and spark gaps serving as pre-discharge device of an embodiment of the present invention. Axially oblong electrodes 71 and 72 as main electrodes are provided in a discharge chamber 2, and they are disposed to face each other along an optical axis 7. The electrode 72 has an U-shaped cross section on a perpendicular plane to the optical axis, and a peaking capacitor 37 is mounted in a gutter of square section of the electrode 72. A lower surface of the peaking capacitor 37 is fixed on the gutter, and the upper surface 37a thereof is coupled with high voltage connecting plate 77 by screws 78 penetrating an insulating member 76a. A cooling chamber 74 is formed between side walls of the peaking capacitor 37 and the gutter, and a known coolant is filled therein. A plurality of spark pins 79 arrayed in the axial direction (perpendicular to the paper of FIG.7) are arranged along both sides of the main electrodes. The upper arrayed spark pins 79a are connected to the high voltage connecting plate 77, and the lower arrayed spark pins 79b are connected to the electrode 71. Each upper arrayed spark pin 79a faces to a corresponding lower arrayed spark pin (79b), each with predetermined gap. The coolant which is cooled by an external refrigerating apparatus (not shown) is circulated in the cooling chamber 74. In the above-mentioned constitution, a length of an electric current flowing path via the capacitor 37, the arrayed spark pin 79 and the electrodes 71 and 72 is reduced, and a stray inductance 38 as shown in FIG.3 is reduced. Consequently an electric power which is given to the laser medium between the electrodes 71 and 72 increases, and output of the laser light increases. Since the capacitor 33 is isolated from the laser medium, and is saved from corrosion. Moreover, deterioration of the laser medium is prevented, too. FIG.8 is a cross-sectional view at a plane perpendicular to the optical axis of another embodiment in the main electrodes portion. Referring to FIG.8, at least two flat board-shaped electrodes 81 and 82 are placed facing each other in a gutter 94 of an upper electrode 72 having an U-shaped perpendicular cross section of the optical axis 7, and thereby a capacitor 37a is formed. The electrode 81 of the capacitor 37a is connected to a high voltage connecting plate 32 by screw 78. Another electrode 82 is coupled to the outer electrode 72. The gutter 94 is covered by insulating member 76. non-conductive and high dielectric material such as pure water or oil which is chilled by an external refrigerating apparatus (not shown) is circulared in the gutter 94. In the embodiment, the dielectric material of the capacitor 37a serves as a coolant, and high power laser apparatus is miniaturized, and reliability of the apparatus is improved. The present invention is most suitable for the excimer laser, but is applicable to other laser apparatus.

Claims: A laser apparatus comprising: a discharge chamber (2) enclosing a laser medium, at least two main discharge electrodes (3,4) enclosed in said discharge chamber, said electrodes positioned so as to face each other with a discharge region therebetween, pre-discharge means (5) for preliminarily exciting said laser medium prior to initiation of a main discharge across said main discharge electrodes (3,4), and at least one capacitor (37), characterized in that said at least one capacitor (37) is disposed interiorly of a cavity (74) formed inside of at least one of said main discharge electrodes (72). A laser apparatus in accordance with claim 1, wherein said at least one capacitor (37) is enclosed in a cavity (74) which is formed along the length of said main discharge electrode (72). A laser apparatus in accordance with claim 1 or 2, wherein a coolant is circulated in said cavity (74) along the length of said main discharge electrode (72). A laser apparatus in accordance with claim 3, wherein said coolant is a dielectric material. A laser apparatus in accordance with one of the claims 1 to 4, characterized in that said main discharge electrodes are positioned so as to face each other with a main optical axis therebetween in which said discharge region of a predetermined height (H) and width (W) is formed, said height (H) being larger than said width (W). A laser apparatus in accordance with claim 5, characterized in that a total reflector (11) is located at one end of said discharge chamber (2) and has its reflection surface perpendicular to said main optical axis for reflecting a laser light beam by 180°, and an output coupler means (10) is located at a second end (2a) of said discharge chamber and allows laser light to pass therethrough, said output coupler means (10) being disposed so as to be perpendicular to said main optical axis. A laser apparatus in accordance with claims 5 and 6, characterized in that said discharge region is divided into plural laser regions in the direction of the height (H) of the discharge region by at least one light folding means (9;13) disposed at an end (2b) of said discharge chamber.

Inventors: HORIUCHI NAOYA, IWABUCHI TAKASHI, MIYATA TAKEO, ONO TAKUHIRO, SANO REIJI, HORIUCHI, NAOYA, IWABUCHI, TAKASHI, MIYATA, TAKEO, ONO, TAKUHIRO, SANO, REIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H01S3
IPC codes: H01S3