Conventionally, as a semiconductor process becomes finer, an exposure wavelength of a laser used in a semiconductor exposure apparatus is made short. Excimer lasers such as a krypton fluoride (KrF: 248 nm) laser and an argon fluoride (ArF: 193 nm) laser have been put to practical use. In addition, an F2 laser (F2: 157 nm) oscillating in a vacuum ultraviolet ray region is anticipated as a light source of a next generation. Furthermore, in the F2 laser, photon energy is as large as approximately 7.9 eV. Even in a material having a large band gap, such as silica, an absorption coefficient is large. Therefore, new application to materials that have been said to be difficult in working is also anticipated.
Furthermore, gas used in the F2 laser is simple mixed gas composed of He and F2. The laser does not use expensive gas, such as Ne or Xe, and it does not contain a component, such as Ar, that tends to localize a discharge. Therefore, the gas used in the F2 laser has a merit in that stable discharge can be easily obtained.
In addition, in a study of the F2 laser, oscillation up to approximately 260 mJ/pulse is already obtained and its aspect suited to a higher output has been exhibited. If the F2 laser is put to practical use, it is anticipated to become a laser that can be applied to various fields.
As described above, the F2 laser is a short-wavelength laser having a great advantage. However, in existing circumstances in which a traditional F2 laser of a transverse direction excitation type, in which direction of laser light becomes perpendicular to a discharge current direction, and of an ultraviolet ray preliminary ionization type, there are a large number of problems. It has been considered that it is difficult to overcome these problems with a traditional way of thinking.
First, a high gas pressure is required for the F2 laser. At least 3 atm is required, and in a case where required gas pressure is high, a pressure as high as 10 atm is required. Therefore, it has been considered that it is necessary to use a large-sized chassis having a high strength. Therefore, an apparatus as a whole becomes large and expensive.
Second, for conducting high repetition operations, it is necessary to let gas flow at a high speed. For example, for causing operation at a repetition frequency exceeding 5 kHz, it has been considered that it is necessary to let gas flow at a speed exceeding several tens m/sec. This results in a problem of power required to drive a fan exceeding 20 kW.
In addition, there is a phenomenon called gas life. If the laser is activated, its output falls within a relatively short time. Unless laser gas is exchanged frequently, the output cannot be maintained. As regards the gas life, it has been coped with by exchanging the gas, while how a cause material decreases laser output is unknown.
In the F2 laser a self-excitation voltage of discharge is high and the discharge is apt to become unstable. In a transverse direction excitation type laser, therefore, an electrode interval is set to approximately 10 mm. However, understanding of homogeneous discharge generated in this narrow electrode gap of approximately 10 mm is insufficient. In a time of approximately 10 nanoseconds during which discharge is formed, even electrons cannot cross a gap between electrodes, and move little. Nevertheless, it is necessary to generate homogeneous discharge. An occurrence condition of homogeneous discharge has been looked for experimentally, that is, details of development that has been conducted heretofore.
In addition, in a lamp with F2 gas sealed therein at a low pressure, a luminous efficiency exceeding 50% is obtained in continuous lighting that extends over several thousands hours. Therefore, it is expected that a cause of a low coefficient and a short gas life exists in structure itself of the laser apparatus. Indeed, after the present inventors' study conducted for approximately 10 recent years, a conclusion that most of these are mere structural problems of the transverse direction excitation and an ultraviolet ray preliminary ionization type laser has been reached.
For example, as for high gas pressure required for the laser apparatus, it is considered that in a state in which an electrode gap in the transverse direction excitation type laser is narrow and preliminary ionization is sufficiently conducted, a discharge start voltage becomes a function of only pressure and an effective operating voltage does not rise unless a high gas pressure is used. A reason why the gas must be allowed to flow at high speed is also based on the narrow electrode gap in the transverse direction excitation type laser. If remaining electric charge exists, a discharge is localized. Therefore, it is considered that the gas is allowed to flow to remove a remaining electric charge.
As for gas life, this relates to a type in which discharge gas is subject to preliminary ionization using ultraviolet rays. Since impurities stored in the discharge gas absorbs preliminary ionization light, a quantity of light arriving at a discharge space decreases, resulting in a state of insufficient preliminary ionization. It is considered that as a result homogeneity in the discharge falls and output decreases.
Therefore, the present inventors have noticed a supposition that those problems might be solved by fundamentally altering structure of the laser apparatus, and decided to adopt an axial direction excitation type laser, in which an optical axis of laser light and a discharge current pass through the same path, instead of the conventional transverse direction excitation and ultraviolet ray preliminary ionization type laser. In this axial direction excitation type laser, excitation discharge occurs on the optical axis, and consequently an electrode gap becomes as wide as a range of 15 cm to 30 cm and it becomes possible to obtain a high discharge start voltage even at a low gas pressure. Therefore, the gas pressure can be made low, and consequently structure of the laser apparatus becomes very simple. As a result, not only remarkable size reduction of the apparatus becomes possible, but also a blower for preventing a remaining charge in a discharge space from affecting homogeneity of the discharge becomes unnecessary.
As regards an increase of impurities, which affect gas life, it also becomes possible to seal and use a discharge tube over a long period of time by hard-sealing components. In addition, in the axial direction excitation type laser, preliminary ionization can be conducted from outside of the discharge tube without using ultraviolet light. Therefore, there is a possibility that the gas life, which has posed a problem in the conventional transverse direction excitation type laser, will matter little. Indeed, for example, in a copper vapor laser, it has become clear that an output of kW class with repetition of several hundreds kHz is obtained with a low pressure gas without allowing flow of the gas, owing to use of the axial direction excitation type laser.
Therefore, the present inventors have adopted the axial direction excitation type laser as the F2 laser as well, and developed an axial direction excitation type F2 laser apparatus having a stable output and a high efficiency and allowing high repetition (Japanese Patent Application Publication No. 2000-265435).
In this developed axial direction excitation type F2 laser, however, the laser oscillates at a pressure near 1 atm. Therefore, the present inventors have continued studies in order to implement a highly efficient axial direction excitation type F2 laser that generates oscillation at a lower pressure.
The present invention has been achieved in view of the circumstances heretofore described. The present inventors aim at providing a small-sized, highly efficient, low-cost, low-pressure axial direction excitation type F2 laser apparatus oscillating at a pressure that is as low as 1/20 to 1/100 that of a conventional laser apparatus.