Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-04-09
2001-05-29
Font, Frank G. (Department: 2877)
Coherent light generators
Particular beam control device
Nonlinear device
C372S020000, C372S022000, C372S028000, C372S032000, C372S034000, C372S035000, C372S070000
Reexamination Certificate
active
06240111
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to laser beam generating apparatus and method, and more particularly to laser beam generating apparatus and method for generating CW (continuous wave) or pulse laser beams of short wavelength by using a non-linear optical crystal device (BBO).
2. Description of the Related Art
Recently, the microstructure of semiconductor devices has been increasingly enhanced, and this enhancement of the microstructure of the semiconductor devices has also caused increasing requirements to shorten the wavelengths of light beams emitted from light sources in an exposure process for use in a semiconductor device manufacturing process. Of these light sources used in the exposure process, a YAG full solid-state laser for generating fifth higher harmonic wave of 213 nm in wavelength by using &bgr;-BaB
2
O
4
(hereinafter referred to as “BBO”) which is a non-linear optical crystal device has been expected as a light source for the semiconductor exposure process.
Besides, the market has required in an earlier stage that light sources of the next generation should have wavelengths of 200 nm or less. At the present situation, the requirement to shorten the wavelength of the light source to be less than 200 nm has not been satisfied for the full solid-state laser insofar as the BBO crystal device is used because the transmittance of the BBO crystal is sharply reduced in the wavelength range of 200 nm or less. For example, in the case of 3.5 mm-thickness BBO crystal, the transmittance at the same wavelength as an ArF excimer laser is sharply reduced from 70% to 10% which contains the surface reflectance of the end face of the crystal.
In order to avoid this problem, there has been proposed a method of producing 205 nm or 201 nm wavelength. However, this method has not yet been practically used (Japanese Unexamined Patent Applications No. Sho-62-162374, No. Sho-62-162377). Further, there has been proposed a method of producing 193 nm wavelength from the sum of 213 nm wavelength of YAG fifth higher harmonic and 2.0 &mgr;m of infrared ray by using an LBO (LiB
3
O
5
) crystal device at the final stage. However, a light source based on this method outputs only small output power, and it is difficult to obtain large output power.
The BBO crystal is one of crystals which are most suitable for practical use and highest reliability in lots of wavelength-converting non-linear optical crystals. If the full solid-state laser having the light source which is easier in maintenance, more compact in size and higher in efficiency and quality as compared with the ArF excimer laser, can produce the same wavelength (193 nm) as the ArF excimer laser with the same-level laser power as the ArF laser, its contribution to the industries would be extremely great.
SUMMARY OF THE INVENTION
The present invention has been implemented in view of the foregoing description, and has an object to provide laser beam generating apparatus and method which can achieve, at a practical level, a coherent ultraviolet light source in a wavelength range of 200 nm or less which is shorter in wavelength than that of a conventional BBO crystal device, by using a BBO crystal device which can be easily handled and highly practically used and give satisfactory results in the wavelength range above 200 nm.
In order to attain the above object, a laser beam generating apparatus according to a first aspect of the present invention has a BBO crystal device and means for controlling the temperature of the BBO crystal device at 273K or less, thereby generating a CW (continuous wave) or pulse laser beam of 200 nm or less in wavelength by utilizing a non-linear optical effect.
A laser beam generating apparatus according to a second aspect of the present invention is characterized by comprising: a first laser beam source which oscillates in a near infrared-rays region of any one of a solid-state laser such as Nd-ion doped Nd:YAG, Nd:YVO
4
, Nd:YLF, Nd:YAD, Nd:Glass or the like, a solid-state laser such as Cr-ion doped CR:LiSAF, Cr
4+
:Forsterite or the like; second higher harmonic wave generating means for generating, from the laser beam emitted from the first laser beam source, a second higher harmonic wave having a half wavelength of the first laser beam source; splitting means for splitting the second higher harmonic wave; a second laser beam source for inputting a part of the second higher harmonic wave thus split to a Ti:Sapphire laser or alexandrite laser to excite and oscillate the laser and generate a laser beam of substantially 700 nm in wavelength; fourth higher harmonic wave generating means for generating a fourth higher harmonic wave from the remaining part of the second higher harmonic wave; sum frequency mixing means comprising a BBO crystal device to which a laser beam of substantially 700 nm in previous wavelength and the fourth higher harmonic wave are input, and means for controlling the temperature of the BBO crystal device to substantially 100K or less, thereby obtaining a CW or pulse laser beam of substantially 193 nm in wavelength as an output of the sum frequency mixing means.
A laser beam generating method according to a third aspect of the present invention in which a BBO crystal device is used to utilize a non-linear optical effect, is characterized by comprising a step of controlling the temperature of the BBO crystal device to 273K or less, thereby generating a CW or pulse laser beam of 200 nm or less in wavelength.
A laser beam generating method according to a fourth aspect of the present invention is characterized by comprising: a second higher harmonic wave generating step of generating a second higher harmonic wave from a first laser beam source which oscillates in a near infrared-rays region of any one of a solid-state laser such as Nd-ion doped Nd:YAG, Nd:YVO
4
, Nd:YLF, Nd:YAD, Nd:Glass or the like, a solid-state laser such as Cr-ion doped Cr:LiSAF, Cr
4+
:Forsterite or the like, the second higher harmonic wave having a half wavelength of the first laser beam source; a splitting step of splitting the second higher harmonic wave; a laser beam generating step of inputting a part of the second higher harmonic wave to a second laser beam source such as a Ti:Sapphire laser to excite and oscillate the laser and generate a laser beam of substantially 700 nm in wavelength; a fourth higher harmonic wave generating step of generating a fourth higher harmonic wave from the remaining part of the second higher harmonic wave; sum frequency mixing step using a BBO crystal device to which a laser beam of substantially 700 nm in previous wavelength and the fourth higher harmonic wave are input; and a step of controlling the temperature of the BBO crystal device to substantially 100K or less, thereby obtaining a CW or pulse laser beam of substantially 193 nm in wavelength as an output of the sum frequency mixing step.
In the laser beam generating apparatus of the first aspect of the present invention and the laser beam generating method of the third aspect of the present invention, it is preferable to preserve the BBO crystal device in refrigerant through a cold finger which is thermally brought into contact with the BBO crystal device, the refrigerant being supplied and controlled to substantially 273K or less with ±0.1K by any one of an electrical cooling device such as Peltier element or the like, a liquid nitride cryostat, an He cryostat, a Stirling refrigerating machine and a closed cycle refrigerating machine. Further, it is preferable to preserve the BBO crystal device in an adiabatic vacuum container.
In the laser beam generating apparatus of the second aspect of the present invention and the laser beam generating method of the fourth aspect of the present invention, it is preferable to preserve the BBO crystal device in refrigerant through a cold finger which is thermally brought into contact with the BBO crystal device, the refrigerant being supplied and controlled to substantially 100K or less with ±0.1K by any one of a liquid
Fukui Tatsuo
Kubota Shigeo
Masuda Hisashi
Tatsuki Koichi
Umezu Nobuhiko
Font Frank G.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Rodriguez Armando
Sony Corporation
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