Method and device for generating a laser beam

Details Method and device for generating a laser beam Method and device for generating a laser beam

1996-09-23

1998-10-06

Scott, Jr., Leon

Coherent light generators

Particular pumping means

Electrical

372 9, 372 25, 372 37, 372 38, 372103, H01S 3097

Patent

active

058188649

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a method for generating a laser beam, and a laser device for practicing the method.
More particularly, the invention relates to a pulsed, transversely excited gas discharge laser. An example of such a laser is an excimer laser, such as an XeCl laser.
Such a laser device generally has the following structure. A gas chamber is provided with two oppositely arranged electrodes, to which a voltage can be applied for generating an electric field in the gas chamber, the main direction of which is designated as y-direction. A plasma to be generated between those electrodes amplifies light by stimulated emission. A laser beam can be generated by means of a resonator consisting of mirrors which may also close off the gas chamber. At least one of these mirrors should be partly transparent to the laser light so that the laser beam can leave the gas chamber. The direction of egress of the laser beam is perpendicular to the y-direction and is designated as z-direction. The dimension of the laser beam in the y-direction is sometimes referred to as the height. The transverse dimension of the laser beam in the direction perpendicular to the y-direction, to be referred to as x-direction, is sometimes referred to as the width. An example of such a laser device is described in the publication "A New Mode to Excite a Gas-Discharge XeCl Laser" by J. C. M. Timmermans, F. A. van Goor and W. J. Witteman in Applied Physics B, vol. 57 (1993), pp. 441-445. With such a device, the generation of the light-inducing plasma proceeds substantially in three steps. In a first step, the gas is ionized by means of radiation, typically X-rays. In a second step, the electron density is increased through a pre-discharge (breakdown) induced by a relatively short and high voltage pulse. In a third step, the main discharge takes place, whereby a relatively large current flows through the plasma for a relatively long time.
For certain applications of the laser beam produced, it is desired that the laser beam has a profile as uniform as possible in a largest possible area of its cross section. This is to say that the intensities Int(x,y) at different points in the beam are equal to each other to the highest possible extent and so are dependent to the least possible extent on the distances x and y of those points with respect to the main axis of the laser beam, measured perpendicularly to that main axis; and that the beam is defined as sharply as possible at its edge. An example of an application where a uniform laser beam profile is desired, is the machining of a surface through a shadow mask, for instance in IC technology. It is then desired that an equal exposure strength occurs in all points of the surface to be machined, in order that an equal light exposure time results in an equal machining result (such as for instance the burn-off depth), in particular when the beam is used to machine several products simultaneously via a plurality of juxtaposed identical masks.
In general, the laser beam profile in the y-direction satisfies the above-mentioned uniformity desire to a sufficient extent. This is due to the sharp boundary provided by the electrodes. By contrast, in the x-direction there is in conventional lasers a gradual decrease in intensity from the beam centre (x=0) to the edge; such a conventional profile can be designated as a bell-shaped profile.
In the art, proposals have previously been made to adapt the laser device in such a manner that the laser beam profile has an improved uniformity in the x-direction. These proposals are based on two mutually different principles.
A first principle is a mechanical principle, and involves the use of specially designed electrodes with a suitable shape (profiling). It has been found, however, that electrode shapes that could yield a reasonable uniformity of the laser beam profile give rise to instability of the gas discharge. According to this principle, therefore, at best a compromise can be achieved between good uniformity on the one hand and sta

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patent: 5313487 (1994-05-01), Fujikawa et al.
patent: 5748635 (1988-05-01), McLellan

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