Molecular fluorine laser with intracavity polarization enhancer

Details Molecular fluorine laser with intracavity polarization enhancer Molecular fluorine laser with intracavity polarization enhancer

2002-06-03

2004-12-21

Harvey, Minsun Oh (Department: 2828)

Coherent light generators

Particular active media

Gas

C372S058000

Reexamination Certificate

active

06834069

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a molecular fluorine (F
2
) laser, and particularly to an F
2
-laser with an improved resonator design and improved beam monitoring and line-selection for providing stable output beam parameters at high operating repetition rates.
2. Discussion of the Related Art
a. VUV Microlithography
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) will use the F
2
-laser operating around 157 nm.
The construction and electrical excitation of the F
2
-laser differs fundamentally from the rare gas-halide excimer lasers mentioned above. The laser gas of a rare gas-halide excimer laser, such as the KrF or ArF laser, includes a laser active molecular species that has no bound ground state, or at most a weakly bound ground state. The laser active gas molecule of the excimer laser dissociates into its constituent atomic components upon optical transition from an upper metastable state to a lower energy state. In contrast, the laser active gas constituent molecule (F
2
) of the F
2
-laser responsible for the emission around 157 nm is bound and stable in the ground state. In this case, the F
2
molecule does not dissociate after making its optical transition from the upper to the lower state.
The F
2
-laser has an advantageous output emission spectrum including one or more lines around 157 nm. This short wavelength is advantageous for photolithography applications because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. This permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package. The high photon energy (i.e., 7.9 eV) is also readily absorbed in high band gap materials like quartz, synthetic quartz (SiO
2
), Teflon (PTFE), and silicone, among others, such that the F
2
-laser has great potential in a wide variety of materials processing applications. It is desired to have an efficient F
2
laser for these and other industrial, commercial and scientific applications.
b. Line-Selection And Line-Narrowing
The emission of the F
2
-laser includes at least two characteristic lines around &lgr;
1
, =157.629 nm and &lgr;
2
=157.523 nm. Each line has a natural linewidth of less than 15 pm (0.015 nm), and in the usual pressure range between 24 bar, the natural linewidth can be less than 2 pm. The intensity ratio between the two lines is |(&lgr;
1
)/|(&lgr;
2
)=≈7. See V. N. Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5(1986).
FIGS. 1
a
and
1
b
illustrate the two above-described closely-spaced peaks of the F
2
-laser spontaneous emission spectrum.
FIG. 1
b
shows a third F
2
laser emission line around 157 nm that is observed when neon is used as a buffer gas, but that is not observed when the buffer gas used is strictly helium, as shown in
FIG. 1
a
(see U.S. Pat. No. 6,157,662, which is hereby incorporated by reference). Either way, the characteristic bandwidth of the 157 nm emission of the F
2
laser is effectively more than 100 pm due to the existence of the multiple lines.
Integrated circuit device technology has entered the sub-0.18 micron regime, thus necessitating very fine photolithographic techniques. Line narrowing and tuning is required in KrF- and ArF-excimer laser systems due to the breadth of their natural emission spectra (around 400 pm). Narrowing of the linewidth is achieved most commonly through the use of a line-narrowing unit consisting of one or more prisms and a diffraction grating known as a “Littrow configuration”. However, for an F
2
-laser operating at a wavelength of approximately 157 nm, use of a reflective diffraction grating may be unsatisfactory because a typical reflective grating exhibits low reflectivity and a laser employing such a grating has a high oscillation threshold at this wavelength (although an oscillator-amplifier configuration may be used to boost the power of an oscillator including a grating as described in U.S. patent application Ser. No. 09/599,130, which is assigned to the same assignee as the present application and is hereby incorporated by reference). The selection of a single line of the F
2
laser output emission around 157 nm has been advantageously achieved and described at U.S. patent application Ser. No. 09/317,695 and U.S. Pat. No. 6,154,470, which are assigned to the same assignee as the present application and are hereby incorporated by reference. It is desired to improve upon the line-selection techniques set forth in the '695 application and the '470 patent. Moreover, it is desired to have a way of monitoring the quality of the line selection being performed.
For an excimer laser, such as a KrF- or ArF-excimer laser, the characteristic emission spectrum may be as broad as 400 pm. To narrow the output bandwidth, one or more dispersive line-narrowing optics are inserted into the resonator. To increase the angular (and spectral) resolution commonly more than one optical dispersive element is introduced. A typical line-narrowing arrangement for a KrF- or ArF-excimer laser includes a multiple prism beam expander before a grating in Littrow configuration.
c. Absorption
The F
2
-laser has been known since around 1977 [see, e.g., Rice et al., VUV Emissions from Mixtures of F
2
and the Noble Gases-A Molecular F
2
laser at 1575 angstroms, Applied Physics Letters, Vol. 31, No. 1, 1 July 1977, which is hereby incorporated by reference]. However, previous F
2
-lasers have been known to exhibit relatively low gains and short gas lifetimes. Other parameters such as the pulse-to-pulse stabilities and laser tube lifetimes have been unsatisfactory. In addition, oxygen and water exhibit high absorption cross sections around the desired 157 nm emission line of the F
2
-laser, further reducing overall efficiency at the wafer when encountered by the laser beam anywhere along its path. To prevent this absorption, one can maintain a purged or evacuated beam path for the F
2
-laser free of oxygen, hydrocarbons and water (see U.S. Pat. No. 6,219,368, which is hereby incorporated by reference). In short, despite the desirability of using short emission wavelengths for photolithography, F
2
-lasers have seen very little practical industrial application to date. It is desired to have an F
2
-laser with enhanced gain, longer pulse lengths, enhanced energy stability, and increased lifetime.
F
2
-lasers are also characterized by relatively high intracavity losses, due to absorption and scattering in gases and optical elements within the laser resonator, particularly again in oxygen and water vapor which absorb strongly around 157 nm. The short wavelength (157 nm) is responsible for the high absorption and scattering losses of the F
2
-laser, whereas the KrF-excimer laser operating at 248 nm does not experience losses of such a comparably high degree. In addition, output beam characteristics are more sensitive to temperature induced variations effecting the production of smaller structures lithographically at 157 nm, than those for longer wavelength lithography such as at 248 nm and 193 nm.
d. Atomic Fluorine Visible Emission
The VUV laser radiation around 157 nm of the F
2
-molecule has been observed as being accompanied by further laser radiation output in the red region of the visible spectrum, i.e., from 630-780 nm. This visible light originates from the excited fluorine atom (atomic transition). It is desired to have an F
2
-laser wherein the output in the visible region is minimized and also to maximize the energy in the VUV region.
Although the active constituent in the gas mixture of the F
2
-laser is fluorine, the amount of pure fluorine amounts to no more than about 5 to 10 mbar of partial pressure within the gas mixture, and typically less than 5 mbar. A higher overall

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