1. Field of the Invention
The present invention relates to a continuous emission excimer laser oscillation apparatus and method that allow continuous light emission, and an excimer laser apparatus and a laser tube.
2. Related Background Art
An excimer laser has received a lot of attention as one and only high-power laser that can oscillate in the ultraviolet region, and its applications are expected in the electronics, chemical, and energy industries.
More specifically, the excimer laser is used in working, chemical reactions, and the like of metals, resins, glass, ceramics, semiconductors, and the like.
An apparatus for generating an excimer laser beam is known as an excimer laser oscillation apparatus. A laser gas mixture containing, e.g., Ar, Kr, Ne, F.sub.2, and the like filled in a manifold is excited by electron beam radiation, discharge, or the like. The excited F atoms bind to inactive KrF*.multidot.ArF* atoms in the ground state to form molecules that can exist in only an excited state. Such molecules are called excimers. Since the excimers are unstable, they immediately emit ultraviolet rays and drop to the ground state. Such phenomenon is called bond-free transition or spontaneous emission, and an excimer laser oscillation apparatus utilizes the excited molecules and amplifies them as in-phase light in an optical resonator made up of a pair of reflection mirrors to output a laser beam.
Conventionally, it is impossible for an excimer laser oscillation apparatus to attain continuous excitation since the lifetime of excimers as a laser medium is very short, and pulse excitation that intermittently supplies fast-rise-time current pulses (about 10 nsec) is normally performed.
For this reason, the service life of electrodes in the conventional excimer laser oscillation apparatus is as short as about half a year.
On the other hand, when, for example, a chemical sensitization type resist is exposed using a pulse oscillation type continuous emission excimer laser oscillation apparatus having a repeating frequency of 100 Hz to 1 kHz in a semiconductor working process, the service life of a lens material and a reflection-free multilayered film on the surface of the lens material is very short.
Such problem will be explained in detail below.
The sensitivity of the chemical sensitization type resist is about 20 mJ/cm.sup.2. Hence, light of 0.1 W/cm.sup.2 requires an exposure time of 0.2 sec. On the other hand, light of 1 W/cm.sup.2 requires an exposure time of 0.02 sec. In consideration of considerable losses in the optical system, an optical output of about 10 W suffices.
In pulse emission (1 kHz) used today, pulse light of about 10 nsec is generated about 1,000 times per sec. If the exposure time is 0.2 sec, 200 pulses and 20 mJ/cm.sup.2 are required. Assuming that energy drops to 1/100 due to losses of the optical system, the emission intensity I.sub.0 of each pulse is described as follows in consideration of the pulse duty shown in FIG. 31: EQU I.sub.0 (watt).times.10(nsec).times.2.times.10.sup.2 (pulses).times.10.sup.-2 (efficiency)=2.times.10.sup.-2 (Joul) EQU I.sub.0 =2.times.10.sup.-2 /10.sup.-8.times.2 EQU =1.times.10.sup.6 (watt)
If a constant optical output can be obtained for 10 nsec, pulse light of 1 MW is obtained. In practice, pulse light shown in FIG. 32 is obtained.
In practice, since the pulse light has a pulse waveform shown in FIG. 32, the intensity of light pulses has a peak power ranging from 2 to 3 MW. Since short-wavelength light of several MW intermittently are incident, the durability requirement of the lens material and the reflection-free multilayered film on its surface becomes very severe.
In the era of excimer laser lithography, step-and-repeat exposure is not simultaneously performed, but scanning exposure by scanning a mirror or lens is performed. When about 1,000 light pulses are generated per sec, and the exposure time is 0.2 sec, only about 200 pulses can be used per exposure. If, for example, a 25.times.35 mm.sup.2 area is to be exposed uniformly, the relative relationship between the scanning mechanism of the mirror or lens and the light pulses must be very strictly controlled, and a very complicated control system is required in optical elements. In addition, at present the light pulse outputs fluctuate by about 10%. For this reason, the mirror or lens scanning control system must inevitably be made very complicated, resulting in a sophisticated, expensive excimer laser exposure apparatus.
Furthermore, the conventional excimer laser oscillation apparatus also has the following problem. That is, since a KrF laser and ArF laser of excimer lasers use highly reactive fluorine gas as a laser gas, the concentration of fluorine in the laser chamber that stores the laser gas and gives discharge energy to the gas is low. In consideration of this, the voltage supplied to the laser chamber is raised so as to obtain a predetermined output. When the predetermined output becomes hard to obtain even by such control, oscillation is interrupted, and fluorine gas is refilled. When oscillation further continues, finally the predetermined laser output cannot be obtained even by refilling fluorine, and the laser chamber must be exchanged in such state.
In the case of an excimer laser emission apparatus that emits light for about several 10 ns by discharge using voltage pulses, since the emission time is too short, the wavelength half width of the light emission spectrum of outgoing light is as wide as about 300 pm. For this reason, a wavelength half width of 1 pm or less can be obtained by monochromating using a narrow-band module such as a grating or the like.
In the existing techniques, fluorine gas must be refilled at predetermined intervals, and oscillation must be done by raising the applied voltage. In other words, fluorine gas decreases in amount due to reaction with, e.g., the chamber inner surface as time elapses. Therefore, the service life of the laser chamber is not satisfactory, and in particular, when a laser is used for a long period of time upon working articles, the service life of the chamber is an important factor upon improving the manufacturing throughput of worked articles.
A wavelength half width of 1 pm or less can be currently obtained by monochromating using a narrow-band module such as a grating or the like. However, the emission intensity of outgoing light decreases due to a narrow bandwidth using a grating or the like, and such decrease in intensity seriously disturbs improvement of the manufacturing throughput of worked articles.