Excimer laser with pulse and beam multiplier

Details Excimer laser with pulse and beam multiplier Excimer laser with pulse and beam multiplier

1998-10-30

2001-11-06

Arroyo, Teresa M. (Department: 2881)

Coherent light generators

Particular active media

Gas

C372S025000

Reexamination Certificate

active

06314119

ABSTRACT:

FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
Prior Art Excimer Lasers
Krypton-Fluoride (KrF) excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. The KrF laser produces a laser beam having a narrow-band wavelength of about 248 nm and can be used to produce integrated circuits with dimensions as small as about 180 nm. Such a KrF laser is described in U.S. Pat. No. 5,023,844 which is incorporated herein by reference. A complete description of a state-of-the art production quality KrF laser is described in U.S. patent application Ser. No. 09/041,474 which is also incorporated herein by reference. The Argon Fluoride (ArF) excimer laser is very similar to the KrF laser. The primary difference is the laser gas mixture and a shorter wavelength of the output beam. Basically, Argon replaces Krypton and the resulting wavelength of the output beam is about 193 nm. This permits the integrated circuit dimensions to be further reduced to about 140 nm. A typical prior art excimer laser used in the production of integrated circuits is depicted in
FIG. 1. A
cross-section of the laser chamber of this prior art laser is shown in
FIG. 2. A
pulse power system comprised of a commutator module and a compression module and powered by a high voltage power supply module provides electrical pulses to electrodes
6
located in a discharge chamber
8
. Typical state-of-the-art 1 ArF lasers are operated at a pulse rate of about 1000 Hz with pulse energies of about 10 mJ per pulse if narrow band. Typical 1000 Hz broad bond ArF lasers may produce about 50 mJ per pulse. The laser gas for an ArF laser, about 0.1% fluorine, 3% argon and the rest neon) at about 3 to 3.5 atmospheres is circulated through the space between the electrodes at velocities of about 25 meters 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 a line narrowing module as shown in FIG.
1
. 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,
commutation module and high voltage compression head module,
Output Coupler Module
Line Narrowing Module
Wavelength Stabilization Module
Control Module
Gas Control Module
These and additional modules shown in
FIG. 1
are designed for quick replacement as individual units to minimize down time to the laser when maintenance is performed.
Electrodes
6
consists of a cathode and a grounded anode. The anode is supported in this prior art embodiment near the center of the chamber. Flow is counter-clockwise in this view. Peaking capacitor
54
is charged prior to each pulse by pulse power system. During the voltage buildup on peaking capacitor
54
a high electric field is created by two preionizers
56
which produce an ion field between the electrodes and as the charge on the peaking capacitor reaches about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse and discharging peaking capacitor
54
. Following each pulse, the gas flow between the electrodes of about 2.5 cm per millisecond, created by blower
10
, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring 1.0 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 the control module to adjust the power supply voltage so that the energy of subsequent pulses are close to a 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.
Pulse Multiplication To Avoid Speckle From Coherent Laser Beams
A major advantage of the excimer laser over many other lasers for use as a lithography light source is that the excimer laser beam is naturally very spatially incoherent compared to most other laser sources. Laser beams from other potential lithography laser sources such as a quintupled Nd-YAG is highly coherent and as a result would produce speckle if used for a lithography source. Techniques have been proposed to minimize the speckle produced by pulse beams from these solid state lasers. For example, see U.S. Pat. No. 5,233,460 which is incorporated herein by reference.
FIG. 3
shows a pulse delay technique from U.S. Pat. No. 5,233,460. In this case, the output pulses of a coherent laser beam are split into multiple beams which are each subjected to a different delay and are recombined to greatly reduce the coherence of the beam.
The background section of U.S. Pat. No. 5,233,460 recognizes that excimer lasers have multiple spatial mode characteristics and high average power which make the excimer laser well suited for use in microlithography. The multiple spatial mode characteristic is the feature of the excimer laser which is responsible for the naturally, comparatively incoherent output beam of the excimer laser.
Another system designed for speckle reduction is described in a patent by Scully (“Laser Target Speckle Eliminator”, U.S. Pat. No. 4,511,220, April 1985). Scully's technique is summarized in FIG.
4
.
Pulse Multiplication for Communication
Optical arrangements for multiplying pulses have been proposed for optical commnunication. The system similar to that shown in
FIG. 4
was proposed by Rubenstein in 1969 (C. B. Rubenstein, “Optical Pulse Generator”, U.S. Pat. No. 3,430,048) for increasing data transmission rates. Another pulse multiplying system designed for use in commnunication was proposed by De Lange (E. O. DeLange et al., “Optical Pulse Multiplexer”, (U.S. Pat. No. 3,447,856, June 1969) and an example of one of his techniques for multiplying the number of pulses by 32 is shown in FIG.
5
.
Another example of optical pulse multiplexer systems designed for optical is communication is described in a patent by Herriott and Schulte (U.S. Pat. No. 3,501,222).
What is needed is a reliable production quality excimer laser capable of producing high energy with low intensity pulses to reduce damage to optics in beam delivery and imaging systems such as those used in optical lithography steppers.
SUMMARY OF THE INVENTION
The present invention provides an excimer laser with optical pulse multiplication produced by separating a laser beam into a plurality of beams and multiplying the pulse rate in each beam. A pulse multiplier optical system receives the laser output beam and produces multiple output beams, each beam having a larger number of pulses with substantially reduced intensity values as compared to the laser output beam. In a preferred embodiment, CaF
2
flats are used along with maximum reflection mirrors to split the laser beam into four beams each with a 4× increased pulse rate.
The present invention is particularly important as an improvement to the ArF excimer laser industry to reduce two-photon absorption damage to optical equipment in lithography machines. For damage mechanisms involving two-photon processes, such as the compaction and solarization of fused silica in the DUV spectral region, a factor of four reduction in peak power decreases the quantity of two photon absorption damage done by the synthesized four-pulse burst by a factor of about 16 compared to delivering all of the energy in the single pulse emitted by the laser. This is a useful method of prolonging the lifetime of very expensive beam delivery systems such as those used in photolithography stepper systems without reducing the total dose available at the wafer. In preferred embodiments, the pulse multiplier system is contained in a module which can be pre-aligne

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