Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-04-18
2003-09-09
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S058000, C372S038060
Reexamination Certificate
active
06618421
ABSTRACT:
BACKGROUND OF THE INVENTION
Electric Discharge Gas Lasers
Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer Lasers
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of excimer lasers, useful for integrated circuit lithography, are described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact Excimer Laser” and U.S. Pat. No. 5,991,324 issued Nov. 23, 1999 entitled “Reliable, Modular, Production Quality Narrow-Band KrF Excimer Laser”. Both of these patents have been assigned to Applicants' employer, and these patents are hereby incorporated herein by reference. The excimer lasers described in the above patents are high repetition rate pulse lasers. In FIG. 1, the principal elements of the laser
10
are shown (FIG. 1 corresponds to FIG. 1 in Patent '884). The discharges
22
are between two long (about 23 inches) electrodes
18
and
20
spaced apart by about ⅝ inch. Repetition rates of prior art lasers, like the ones described, are typically within the range of about 100 to 2000 pulses per second. These high repetition rate lasers are usually provided with a gas circulation system. In the above referred to laser, this is done with a long squirrel-cage type fan
46
, having blades
48
as shown in FIG.
1
and in
FIG. 2
which is
FIG. 7
in Patent '884. The fan blade structure is slightly longer than the electrodes
18
and
20
and provides sufficient circulation so that at pulse rates between 100 to 2000 Hz, the discharge disturbed gas between the electrodes is cleared between pulses. The gas used in the laser contains fluorine which is extremely reactive. The fan rotor driving fan shaft
130
is sealed, within the same environmental system provided by housing structure members
12
and
14
, by sealing member
136
as explained at column 9, line 45 of Patent '884, and the motor stator
140
is outside sealing member
136
and thus protected from the corrosive action of the fluorine gas. Heat in the gas which is produced by the electric discharge and the rapid circulation of the gas is removed by finned, water-cooled heat exchanger
58
. An important use of these lasers is as a light source for integrated circuit lithography. The nominal output wavelength of these lasers is determined by the gas mixture. A KrF excimer laser operates at about 248 nm; an ArF excimer laser operates at about 193 nm and an F
2
excimer laser operates at about 157 nm.
Pulse Power
Electric discharge gas lasers of the type described in U.S. Pat. No. 5,023,884 utilize an electric pulse power system shown in
FIG. 3
to produce the electrical discharges, between the two electrodes. In such prior art systems, a direct current power supply
22
charges a capacitor bank called “the charging capacitor” or “C
0
”
42
to a predetermined and controlled voltage called the “charging voltage” for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts. After C
0
has been charged to the predetermined voltage, a solid state switch
46
is closed allowing the electrical energy stored on C
0
to ring very quickly through a series of magnetic compression circuits comprising capacitor banks
52
,
62
and
82
and inductors
48
,
54
and
64
and a voltage transformer
56
to produce high voltage electrical potential in the range of about 16,000 volts across the electrode which produces the discharge which lasts about 50 ns.
In prior art lithography laser systems, the time between the closing of the solid state switch and the discharge is in the range of about 5 microseconds; however, the charging of C
0
accurately to the pre-selected voltage has in the past required about 400 microseconds which was quick enough for pulse repetition rates of less than about 2,000 Hz. The reader should understand that accurate charging of C
0
is very important since the control of the voltage level on C
0
is in these systems the only practical control the laser operator has on the discharge voltage which in turn is the primary determiner of laser pulse energy. For laser light sources used for integrated circuit fabrication the precise timing of the pulses has not been critically important since for both stepper machines and scanning machines target areas on the wafer are illuminated with a number of pulses such as about 20 to 40 pulses during an interval of a few milliseconds.
Reticles
Reticles used for integrated circuit lithography contain the patterns to be applied to the silicon wafer as a part of the process to create the integrated circuit. The pattern on the reticle is typically 3 or 4 times larger than the corresponding image on the wafer. Nevertheless, the dimension on the reticle are still very small, i.e., a few hundreds of nanometers. These patterns on the reticles typically in the past have been created with electron beams, and both reticles and wafers typically have been inspected with visible light microscopes.
What is needed are excimer laser systems optimized for reticle creation and inspection of both reticles and wafers.
SUMMARY OF THE INVENTION
The present invention provides a high repetition rate, compact, modular gas discharge, ultraviolet laser. The laser is useful as a light source for very rapid inspections of wafers in an integrated circuit fabrication process. It is also useful for reticle writing at very rapid rates. A preferred embodiment operates at pulse repetition rates of 1000 to 4000 Hz and is designed for round-the-clock production line operation. This preferred embodiment comprises a pulse control unit which controls the timing of pulses to an accuracy of less than 4 nanoseconds. Preferred embodiments of this gas discharge laser can be configured to operate with a KrF gas mixture, an ArF gas mixture or an F
2
gas mixture, each with an approximate buffer gas, producing 248 nm, 197 nm or 157 nm ultraviolet light pulses.
REFERENCES:
patent: 4276516 (1981-06-01), Congdon
patent: 4553244 (1985-11-01), Benedict et al.
patent: 4660204 (1987-04-01), Dewhirst et al.
patent: 4847854 (1989-07-01), Van Dijk
patent: 5317589 (1994-05-01), Ogawa et al.
patent: 5658535 (1997-08-01), Thayer, III
patent: 6128323 (2000-10-01), Myers et al.
patent: 6240112 (2001-05-01), Partlo et al.
Anderson Stuart L.
Das Palash P.
Geiger Stephan
Hartmann Claudia A.
Partlo William N.
Cray William
Cymer Inc.
Ip Paul
Vy Hung
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