Coherent light generators – Particular active media – Gas
Reexamination Certificate
1999-12-28
2002-04-30
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Gas
C372S060000, C372S108000
Reexamination Certificate
active
06381257
ABSTRACT:
BACKGROUND OF THE INVENTION
Prior Art Lithography Lasers
KrF excimer lasers are the state of the art light source for integrated circuit lithography. One such laser is described in U.S. Pat. No. 4,959,840 issued Sep. 25, 1990. The lasers operate at wavelengths of about 248 nm. With the KrF laser integrated circuits with dimensions as small as 180 nm can be produced. Finer dimensions can be provided with ArF lasers which operate at about 193 nm or F
2
lasers which operate at about 157 nm.
These lasers, the KrF laser, the ArF laser and the F
2
lasers, are very similar, in fact the same basic equipment used to make a KrF laser can be used to produce an ArF laser or an F
2
laser merely by changing the gas concentration and modifying the controls and instrumentation to accommodate the slightly different wavelength.
Control of lithography lasers and other lithography equipment require laser pulse energy monitors sensitive to the UV light produced by these lasers. The standard prior art detectors used for monitoring pulse energy in state of the art integrated circuit lithography equipment are silicon photo diodes.
A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIG.
1
and
FIG. 2. A
cross section of the laser chamber of this prior art laser is shown in FIG.
3
. As shown in
FIG. 2A
, pulse power system
2
powered by high voltage power supply
3
provides electrical pulses to electrodes
6
located in a discharge chamber
8
. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 inches per second. This is done with tangential blower
10
located in the laser discharge chamber. The laser gases are cooled with a heat exchanger
11
also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module
18
(sometimes referred to as a line narrowing package or LNP). Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
Pulse Power System with: high voltage power supply module,
commutator module and high voltage compression head module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module,
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes
6
consist of cathode
6
A and anode
6
B. Anode
6
B is supported in this prior art embodiment by anode support bar
44
which is shown in cross section in FIG.
3
. Flow is counter-clockwise in this view. One corner and one edge of anode support bar
44
serves as a guide vane to force air from blower
10
to flow between electrodes
6
A and
6
B. Other guide vanes in this prior art laser are shown at
46
,
48
and
50
. Perforated current return plate
52
helps ground anode
6
B to the metal structure of chamber
8
. The plate is perforated with large holes (not shown in
FIG. 3
) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor comprised of an array of individual capacitors
19
is charged prior to each pulse by pulse power system
2
. During the voltage buildup on the peaking capacitor, two preionizers
56
weakly ionize the lasing gas between electrodes
6
A and
6
B and as the charge on capacitors
19
reach about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 inch per millisecond, created by blower
10
, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of the subsequent pulse is close to the desired energy.
These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance. One problem experienced with these prior-art lasers has been excessive wear and occasional failure of blower bearings. A need exists in the integrated circuit industry for a modular, reliable, production line quality F
2
laser in order to permit integrated circuit resolution not available with KrF and ArF lasers.
Injection Seeding
A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band “seed” beam into a gain medium. In one such system, a laser producing the seed beam called a “master oscillator” is designed to provide a very narrow band beam and that beam is used as a seed beam in the second laser. If the second laser functions as a power amplifier, the system is referred to as a master oscillator, power amplifier (MOPA) system. If the second laser itself has a resonance cavity, the system is referred to as an injection seeded oscillator (ISO) and the seed laser is called the master oscillator and the downstream laser is called the power oscillator. These techniques reduced the heat load on the line narrowing optics.
Laser systems comprised of two separate lasers tend to be substantially more expensive, larger and more complicated than comparable single laser systems. Therefore, commercial applications of two laser systems has been limited. In most examples of prior art MOPA and ISO systems two separate laser chambers are utilized. However, systems have been proposed for using a single laser chamber to contain two sets of electrodes. For example,
FIG. 3A
shows a side-by-side arrangement described by Letardi in U.S. Pat. No. 5,070,513. Another arrangement shown in
FIG. 3B
described by Long in U.S. Pat. No. 4,534,035 in which the elongated electrode sets are positioned on opposite sides of the chamber. Gas flows from a common “in” plentum separately between the two sets of electrodes into a common “out” plenum. An arrangement proposed by McKee in U.S. Pat. No. 4,417,342 is shown in FIG.
3
C. This system has two elongated electrode sets mounted parallel to each other on one half of the chamber. A tangential fan and heat exchanger is located in the other half. Gas flows in parallel through between the two sets of electrodes. The system shown in
FIG. 3A
has not been considered suitable for high pulse rate laser because debris from the upstream discharge interferes with the downstream discharge. According to an article published in Applied Physics B Lasers and Optics 1998, this laser is operated at a pulse repetition rate of about 100 pulses per second. The authors indicate that an attempt to operate at 1000 Hz would lead to turbulent flow which is not desirable for generation of a high quality beam. The system shown in
FIG. 3C
has not been considered suitable for high pulse rate lasers because splitting of the flow reduces the velocity of the gas between the electrodes by about 50% as compared to a single set of electrodes on the system shown in FIG.
3
A. The system shown in
FIG. 3B
has not been considered satisfactory for high pulse rate lasers because the blower circulation is axial rather than tangential as shown in FIG.
3
.
F
2
Lasers Bandwidth
A typical KrF laser has a natural bandwidth of about 400 pm (FWHM) centered at about 248 nm and for lithography use it is line narrowed to about 0.6 pm. ArF lasers have a natural bandwidth of about 40 pm centered at about 193 nm and is typically line narrowed to about 0.5 pm. These lasers can be relatively easy tuned over a larger portion of thei
Das Palash P.
Ershov Alexander I.
Hofmann Thomas
Onkels Eckehard D.
Partlo William N.
Cymer Inc.
Davie James W.
Monbleau Davienne
Ross, Esq. John R.
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