Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-06-08
2003-07-15
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029014, C372S057000, C372S055000, C372S020000
Reexamination Certificate
active
06594291
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultra narrow band laser apparatus for producing ultra narrow band laser light from a fluorine laser for use as a light source for a stepper or other fluorine exposure apparatus, and to a fluorine exposure apparatus.
2. Description of the Related Art
Qualities required of exposers for use in lithography include resolution, precision in alignment, processing capability, and apparatus reliability. Of these, resolution R, which is most intimately related to fine patterning ability, is expressed by R=k·&lgr;/NA (where k is a constant, &lgr; is the exposure wavelength, and NA is the numerical aperture of the projecting lens). Accordingly, lower exposure wavelengths &lgr; are more useful in terms achieving better resolution.
Conventional exposers utilize exposer light sources such as the i line (wavelength 365 nm) of a silver mercury lamp or a 248 nm-wavelength krypton-fluoride (KrF) excimer laser. These are respectively called i line exposers and KrF exposers. Reducing projection lens assemblies composed of a multitude of lenses comprising quartz glass are widely used as projection optical systems in these i line exposers and KrF exposers.
To enable processing of smaller features, exposers employing 193 nm-wavelength argon-fluoride (ArF) excimer lasers as light sources are coming into use as the next generation of lithography exposers. These are termed ArF exposers; these ArF exposers employ narrow band ArF excimer lasers having spectral width (bandwidth) of about 0.6 pm. For the reducing chromatic abberation, achromatic lenses comprising dual materials are used.
Narrow band elements known to provide ArF excimer laser bandwidths as small as about 0.6 pm include etalons, gratings, mode selectors, and other such elements. Of these elements, mode selectors are discussed in some detail in PROCEEDING OF THE EEE, VOL. 60, NO. 4, APRIL 1972, pp. 422-441.
ArF excimer laser apparatus include those employing two laser units. Specifically, a process termed injection seeding, wherein one of the laser units generates a seed light and this seed light is injected into an oscillator (the second laser unit), is implemented in an ArF excimer laser apparatus.
An injection seeding type ArF excimer laser apparatus is discussed, for example, in “Dai 59-kai Oyo Butsurigaku Kanko Rengo Koenkai Preprints”, p. 950, 17-a-P2-1, 1998.
A fluorine exposer employing as the light source an approximately 157 nm-wavelength fluorine laser is under study as a next-generation ArF exposer for lithography.
This fluorine laser produces two intense oscillation lines of different wavelengths and light intensities (also called output lines), the wavelengths of which are 157.6299 nm (wavelength &lgr;
1
) and 157.5233 nm (wavelength &lgr;
2
) respectively. Bandwidth of each of the two oscillation lines is from 1 to 2 pm.
When using this fluorine laser for exposure, it is typically advantageous to select the stronger wavelength line (&lgr;
1
=157.6299 nm) (hereinbelow termed the “strong line,” the other being termed the “weak line”) (this process is hereinbelow termed “single line mode”). Conventionally, one or two prisms were used for single line mode.
Experimental findings pertaining to single line mode in fluorine lasers for use in fluorine exposers are reported, for example, in “SPIE 24
th
International Symposium on Microlithography, February, 1999.
Double line mode for a fluorine laser is described, for example, in CAN. J. PHYS. VOL. 63, 1983, pp. 217-218.
In the fluorine exposers described above, it is a difficult matter to implement a refracting type reducing projection optical system using simply the lenses typically employed in exposers to date (i.e., exposers up through ArF exposers).
The reason is that with a 157 nm wavelength fluorine laser, transmittance through quartz glass is extremely low, imposing severe limitations on the materials that can be used to, for example, calcium fluoride. When a reducing projection optical system is constructed of monochromatic lenses of calcium fluoride only, when the fluorine laser is tuned to single line mode, the oscillation laser light from the fluorine laser will not have a sufficiently narrow band. The bandwidth resulting from band narrowing is about 1 pm, but in actual practice band narrowing to a bandwidth about ⅕ of that, namely, a bandwidth of about 0.2 pm, is thought necessary for single line mode.
Conventionally, since it has proven difficult to achieve band narrowing to bandwidths of 0.2 pm or smaller for single line mode for a fluorine laser, it has been thought necessary to implement the reducing projection optical system with a reflecting/refracting type reducing projection optical system (hereinbelow referred to as a catadioptric type) capable of being used over a bandwidth 10 times wider than a total reflection type optical system composed of lenses only.
The reason why it has been difficult in conventional practice to achieve band narrowing of fluorine lasers to bandwidths of 0.2 pm or smaller is that when one or two prisms are situated in the laser resonator for single line tuning, laser output drops down to about 40%. Installing an etalon or the like enabling greater band narrowing (i.e., one with reflectivity on the order of 80%) in order to achieve band narrowing of the bandwidth to 0.2 pm increases the insertion loss further by about 50%. This makes laser operation difficult or appreciably reduces laser output.
The reasons for the significant drop in laser output occurring with installation of an etalon in a fluorine are now discussed.
It is known that for an etalon having a high reflectivity reflective film, a low degree of planarity typically results in lower maximum transmittance. Where the etalon is fabricated with a calcium fluoride substrate, a typical optical system capable of being used at 157 nm wavelength, the etalon has a lower degree of hardness than quartz, and is moreover crystalline, which makes polishing difficult; for these and other reasons, planarity of no less than about {fraction (1/20)} the wavelength can be achieved. On the other hand, it is known that, with the use of quartz, etalons affording planarity on the order of {fraction (1/100)} the wavelength can be utilized.
Thus, where, for example, an etalon with finesse of 10 is used for band narrowing of 2 pm bandwidth laser light to a bandwidth of 0.2 pm, it is necessary for the etalon to have a coating with reflectivity of 80% or above. If the degree of planarity of the etalon is {fraction (1/20)} the wavelength, maximum transmittance on the order of only about 50% can be achieved in the etalon.
Accordingly, it is a first object of the present invention to provide an ultra narrow band fluorine laser apparatus capable of operation in single line mode, with the bandwidth of the line narrowed to about 0.2 pm, and additionally affording a reduction in the drop in laser output.
It is a second object of the invention to provide an ultra narrow band fluorine laser apparatus whereby oscillation laser light from a fluorine laser may be provided as an exposure light source to a fluorine exposer utilizing a lens-only total refraction type reducing projection optical system.
In systems where a single line of a fluorine laser (i.e., the line of wavelength &lgr;
1
=157.6299 nm) is used as-is, the line spectrum is determined absolutely spectrally, so wavelength stabilization is not needed. Where bandwidth is narrowed to about 0.2 pm, despite the need for the band narrowed wavelength to be stable within a 1 to 2 pm bandwidth single line spectrum, it is difficult to ascertain whether wavelength is in fact stable. This makes it difficult to correctly calibrate wavelength.
The reason is that in the vacuum ultraviolet region in proximity to the 157.6299 nm &lgr;
1
wavelength, it was difficult to use an absolute wavelength where the wavelength had been narrowed to about 0.1 pm (another stable light source (lamp) or absorption line).
It was also difficult to develop a fluorine exposer com
Komatsu Ltd.
Varndell & Varndell PLLC
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