Coherent light generators – Particular component circuitry – Power supply
Reexamination Certificate
1999-11-12
2001-12-11
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular component circuitry
Power supply
C372S055000, C372S057000, C372S058000, C372S059000
Reexamination Certificate
active
06330260
ABSTRACT:
BACKGROUND OF THE INVENTION
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. 4,959,340 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. A
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
. 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 clockwise in this view. One comer 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.
SUMMARY OF THE INVENTION
The present invention provides a reliable, modular, production quality F
2
excimer laser capable of producing, at repetition rates in the range of 1,000 to 2,000 Hz or greater, laser pulses with pulse energies in the range of a few mJ with a full width half, maximum bandwidth of about 1 pm or less at wavelengths in the range of 157 nm. Laser gas mixtures are disclosed for maximizing laser efficiency while reducing unwanted infrared and visible emissions from the laser. Also disclosed are UV energy detectors which are substantially insensitive to infrared and visible light. Preferred embodiments of the present invention can be operated in the range of 1000 to 4000 Hz with pulse energies in the range of 1.0 to 10 mJ with average power outputs in the range of about 10 to 40 watts. Using this laser as an illumination source, stepper or scanner equipment can produce integrated circuit resolution of 0.1 &mgr;m or less. Replaceable modules include a laser chamber and a modular pulse power system.
In a preferred embodiment the laser was tuned to the F
2
157.6 nm line using a set of two external prisms. In a second preferred embodiment the laser is operated broad band and the 157.6 nm line is selected external to the resonance cavity. In a third preferred embodiment a line width of 0.2 pm is provided using injection seeding. In a fourth embodiment one of the two F
2
lines is selected with an etalon output coupler. Another embodiment utilizes a grating for line selection and increases the tuning range by operating the laser at a pressure in excess of 4 atmospheres.
REFERENCES:
patent: 4959840 (1990-09-01), Akins et al.
patent: 5313481 (1994-05-01), Cook et al.
patent: 5315611 (1994-05-01), Ball et al.
patent: 5448580 (1995-09-01), Birx et al.
patent: 5719896 (1998-02-01), Watson
patent: 6128323 (2000-10-01), Myers et al.
patent: 6154470 (2000-11-01), Basting et al.
Siegman,AnthonyE., “Lasers”, University Science Books, Mill Valley, California, Copyright 1986, pp. 279-283.
Duffey Thomas P.
Onkels Eckehard D.
Sandstrom Richard L.
Arroyo Teresa M.
Cymer Inc.
Monbleau Davienne
Ross, Esq. John R.
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