Energy stabilized gas discharge laser

Details Energy stabilized gas discharge laser Energy stabilized gas discharge laser

2000-02-25

2004-03-30

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular active media

Gas

C372S057000

Reexamination Certificate

active

06714577

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to gas discharge lasers, particularly to excimer and molecular fluorine lasers having gas mixtures with optimal concentrations of specific component gases, such as halogen containing species, active rare gases, buffer gases, and a xenon additive for improving pulse-to-pulse and peak-to-peak energy stabilities, energy dose stability and burst energy overshoot control, and increasing the lifetimes of laser system components.
2. Discussion of the Related Art
The term “excimer laser” describes gas lasers in which the lasing medium contains excimers (e.g. Ar
2
*), exciplexes (e.g. ArF*) or trimers (e.g. Kr
2
F*). The feature common to all is a gas discharge in which highly excited molecules that have no stable ground state are created. The following invention primarily concerns excimer lasers in which the lasing medium is composed of halogen-containing, particularly fluorine-containing exciplexes (e.g. ArF* and KrF*). In addition, the present invention relates to molecular fluorine (F
2
) lasers.
In a number of scientific, medical and industrial applications for excimer and molecular gas lasers, it is important that the radiation pulses emitted have a stable (constant) energy. In gas lasers, the fact that gas discharge conditions and characteristics can change has an impact on the achievement of a constant energy from pulse to pulse of the emitted radiation. Characteristics and conditions of the gas discharge are dependent upon a number of parameters that with adequate control can allow significant improvements toward exact reproducibility. The result is that the energy of the emitted laser radiation pulses is not maintained exactly constant from pulse to pulse. It is desired to have an excimer or molecular fluorine laser that demonstrates greater pulse-to-pulse stability.
Energy stability is described by various characteristics of the laser beam depending on the application. One of these characteristics is the standard deviation sigma of a distribution of energies of a large number of laser pulses. As many applications use laser output not continuously but in bursts of light pulses, other parameters are also used for stability (see U.S. Pat. No. 5,463,650, which is hereby incorporated by reference into the present application, and particularly the background discussion therein). Specific application of the excimer or molecular fluorine laser beam in optical lithography as an illumination source for wafer scanners, the energy dose stability is significant (see U.S. Pat. No. 5,140,600, which is assigned to the same assignee as the present application, and The Source™ (Cymer, Inc.), Vol. 1, Issue 2 (Summer 1999), each of which is hereby incorporated by reference into the present application).
Another significant characteristic is peak-to-peak stability. For measuring the peak-to-peak energy stability values, laser pulse energies are accumulated over some interval. The absolute difference between the maximum and minimum energies related to the average laser pulse energy is defined as the peak-to-peak stability.
Of particular interest in burst mode applications, the energy overshoot, as illustrated in
FIG. 1
, is a significant characteristic. Energy overshoot, or spiking, is observed when the laser isoperated with constant high voltage at the discharge chamber in burst mode and the first few pulses have higher energies than pulses later in the burst (see U.S. Pat. Nos. 5,710,787 and 5,463,650, hereby incorporated by reference). The energy overshoot (designated “ovs” in
FIG. 1
) is defined as the difference between the energy of the first pulse in a burst and the steady state energy in the entire burst.
The quality of the gas discharge and also the pulse energy of the emitted laser radiation pulses are dependent upon and are sensitive to variations in gas discharge conditions such as characteristics of the external electrical circuit, the composition and shape of the gas discharge electrodes, the type and quality of pre-ionization, etc. The purity of the gas mixture in the laser gas discharge chamber and the composition of the gas are also very important. Even tiny impurities of certain kinds are known to be very detrimental to the energy of the emitted radiation pulses, the stability of their energy (the consistency of energy per laser pulse from one firing to the next), the intensity distribution in the laser beam profile, the life of the laser gas and the life of individual optical and other laser components. Such impurities in the gas can be present in the gas mixture from the very beginning or they may form during operation of the laser, e.g. through interactions between reactive components of the laser gas mixture (e.g. of the halogen) and the laser chamber material or through diffusion from the materials or chemical reactions in the gas mixture. For example, during operation of a KrF-excimer laser, such contaminants as HF, CF
4
, COF
2
, SiF
4
have been observed to increase in concentration rapidly (see G. M. Jurisch et al.,
Gas Contaminant Effects in Discharge
-
Excited KrF Lasers,
Applied Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)). For a static KrF laser gas mixture, i.e., with no discharge running, increases in the concentrations of HF, O
2
, CO
2
and SiF
4
have been observed (see Jurisch et al., above).
It is known that the addition of certain substances to the gas mixture can improve particular characteristics of the emitted radiation. For example, U.S. Pat. Nos. 5,307,364 and 5,982,800 (hereby incorporated by reference) suggest that small amounts of oxygen be added to the gas mixture to achieve greater reproducibility of emitted radiation during laser operation. Oxygen, however, is not an inert gas, and its effects on other parameters of the excimer laser, such as the uniformity of the emission intensity curve and the life of the gas mixture are not yet fully understood and may be in fact detrimental. Oxygen, especially atomic oxygen and ozone which can form in the gas discharge, are extremely chemically reactive, and their effects on the laser gas mixture can be quite detrimental, especially during long periods of operation. Due to the presence of oxygen, other stable impurities such as OF
2
and FONO form in the excimer laser gas mixture. These can have a considerable absorption effect on the laser irradiation or the pre-ionization radiation. Tests recommended by the current state of technological developments in which the energy of excimer laser radiation impulses is stabilized through the addition of gases to the gas mixture have shown disadvantageous effects on other characteristics of the laser and the emitted radiation.
Filling an excimer or molecular fluorine laser with a gas mixture of precise composition and maintaining that composition is known to be advantageous for determining significant output beam parameters. For example, KrF-excimer laser gas mixtures typically comprise around 1% Kr, 0.1% F
2
and a 98.9% Ne buffer. For the ArF-excimer laser, the composition is around 1% Ar, 0.1% F
2
and 98.9% buffer. The molecular fluorine laser typically has around 0.1% F
2
and 99.9% buffer gas.
The introduction of very small quantities (≧0.1 Torr) of xenon in excimer and molecular fluorine laser gas mixtures has been proposed as increasing the photopreionization yield. See R. S. Taylor and K. E. Leopold,
Transmission Properties of Spark Preionization Radiation in Rare
-
Gas Halide Laser Gas Mixes,
IEEE Journal of Quantum Electronics, pp. 2195-2207, Vol. 31, No. 12 (December 1995). Taylor et al. demonstrate an enhancement of spark pre-ionization intensity by the action of a Xenon additive to the gas mixture. An advantageous result of this enhancement of the preionization density is an improvement of the homogeneity of the excimer laser discharge. Taylor et al. describe qualitatively, however, that if the xenon concentration is too high, then absorption of laser radiation would occur and degrade the output laser beam. The conclusion of Taylor et al. then is that only a s

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