Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-01
2004-12-07
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S055000, C372S057000, C372S092000
Reexamination Certificate
active
06829279
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser oscillating apparatus for generating a laser beam by introducing an electromagnetic wave from a waveguide into a laser tube through a plurality of fine gaps formed in the waveguide wall and, more specifically, to a laser oscillating apparatus using a microwave as an electromagnetic wave for exciting a laser gas, an exposure apparatus using the same, and a device fabrication method.
2. Description of the Related Art
Recently, a so-called excimer laser has attracted attention as the only high-output laser which oscillates in the ultraviolet region. This excimer laser is expected to be applied to the electronic, chemical, and energy industries, particularly processing and chemical reactions of metals, resins, glass, ceramics, and semiconductors.
The principle of function of an excimer laser oscillator will be described below. First, laser gases such as Ar, Kr, Ne, F
2
, He, Xe, and Cl
2
contained in a laser tube are excited by electron beam irradiation or discharge. Excited F atoms bond to inert Kr and Ar atoms in the ground state to generate KrF and ArF as molecules existing only in an excited state. These molecules are called excimers. Since excimers are unstable, they immediately emit ultraviolet rays and fall to the ground state. This phenomenon is called spontaneous emission. An excimer laser oscillator uses this to amplify as an in-phase beam in an optical resonator constructed of a pair of reflecting mirrors and extract as a laser beam.
In the case of excimer laser emission, an excitation source using microwaves is known as a laser gas exciting source as described above. Microwaves are electromagnetic waves having an oscillation frequency of a few hundred MHz to several tens of GHz. As a laser gas exciting method using this microwave, a method has been proposed by which a microwave is introduced from a waveguide into a laser tube through a gap (slot) formed in the waveguide wall, thereby exciting a laser gas in the laser tube into a plasma.
In this excitation method, even if the intensity distribution of microwaves emitted through the slots is uniform, a slot array structure in which a plurality of slots are arrayed in the long-axis direction of a resonator must be formed in order to supply a microwave to a long space meeting the resonator length of a laser beam. This structure is shown in FIG.
55
. Referring to
FIG. 55
, a plurality of fine gaps (slots)
9202
are formed at equal intervals in a waveguide wall
9201
. For convenience, the interior of a laser tube is schematically shown as an emission space.
When this slot array structure is used, regions (hatched elliptic regions in
FIG. 55
) between adjacent slots
9202
are necessarily microwave non-irradiation regions. Accordingly, when a laser gas existing in the emission space is to be excited by a microwave, the existence of these non-irradiation regions produces variations in the microwave intensity. This generates plasma discharge having a nonuniform distribution as a whole.
As described above, it is difficult to uniformize the radiation characteristic of an electromagnetic wave from a slot formed in a waveguide wall in an entire region over the slot. Usually, the distribution is a sinusoidal distribution in the slot long-axis direction or a similar distribution. That is, as shown in
FIG. 56A
, an electric field intensity distribution in the center along the slot long-axis direction is largest, and the field intensity distribution at the ends in the slot long-axis direction is smallest.
Additionally, as shown in
FIG. 56B
, an excited plasma has a property of concentrating to the center in the slot long-axis direction with respect to the microwave field intensity distribution. This promotes the nonuniform distribution of the field intensity in the slot long-axis direction. This is a great cause of preventing a uniform distribution of a plasma excited in the slot longitudinal direction.
This phenomenon is caused by the property that a plasma is easily excited in a central position along the slot longitudinal direction because the intensity of an electromagnetic wave as an excitation source is a maximum in this central position, and by the property that the excited plasma readily concentrates into the form of a sphere having the smallest surface area. This plasma excited in the central position forms a region having a low spatial impedance in the center of the slot. This portion preferentially consumes energy. Also, the plasma functions as a shield to reduce the slot length, which is designed to be the one by which a microwaves is emitted, to half that required by a microwave. This makes it difficult to emit a microwave outside the slot. By these two factors, a plasma is readily formed only in the center of a slot, and it is very difficult to excite a uniform plasma over the slot.
Furthermore, a plasma is generated immediately close to the microwave emission surface over a slot. However, in a plasma sheath formed over the slot a microwave can propagate. As a consequence, a microwave extends in the slot short-axis direction via this sheath region and disperses the input power. This makes it impossible to satisfy energy density necessary to excite an excimer laser. The reason for this is that when a plasma is diffused in a wide space, energy used to generate the plasma disperses, and this makes it difficult to realize enough energy density to excite an excimer.
FIG. 57
is a schematic view showing the way a microwave propagates via a plasma sheath. That is,
FIG. 57
is a sectional view taken along a direction perpendicular to the longitudinal direction of a slot.
Although a plasma is not electrically grounded over the slot, the outside of the plasma is basically grounded directly to a waveguide
1
. This results in a sheath potential difference and a sheath width difference between them. Therefore, if the plasma density is insufficient, the thickness of the sheath increases, and this results in easy nonuniform outward leakage of a microwave. Consequently, the plasma is thin immediately above the open end of the slot and thick on the outside.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide a laser oscillating apparatus which uses a slot array structure and yet realizes electromagnetic wave radiation uniform as a whole over the length of a laser tube and allows uniform laser emission with minimum energy loss, a high-performance exposure apparatus including this laser oscillating apparatus, and a high-quality device fabrication method using this exposure apparatus.
It is another object of the present invention to provide a laser oscillating apparatus which uses a slot array structure and yet suppresses diffusion of a plasma generated over a slot and allows uniform laser emission with minimum energy loss, a high-performance exposure apparatus including this laser oscillating apparatus, and a high-quality device fabrication method using this exposure apparatus.
A laser oscillating apparatus for achieving the above objects according to the present invention is a laser oscillating apparatus for exciting a laser gas in a laser tube by introducing an electromagnetic wave from a waveguide into the laser tube through a plurality of fine gaps formed in a waveguide wall, and generating a laser beam by resonating light emitted from the laser gas, wherein the fine gaps and a wall of the laser tube are spaced apart by a predetermined distance to form an electromagnetic wave passage.
According to one aspect of the laser oscillating apparatus of the present invention, the distance from the fine gaps to the laser tube wall is an integral multiple of the half-wave length of an electromagnetic wave introduced from the waveguide.
According to another aspect of the laser oscillating apparatus of the present invention, an electromagnetic wave introduced from the waveguide is a microwave.
According to still another aspect of the laser oscillating apparatus o
Hirayama Masaki
Ohmi Tadahiro
Ohsawa Hiroshi
Shinohara Toshikuni
Suzuki Nobumasa
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Flores-Ruiz Delma R.
Wong Don
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