Coherent light generators – Particular pumping means – Gas dynamic
Reexamination Certificate
1999-11-30
2004-08-24
Wong, Don (Department: 2828)
Coherent light generators
Particular pumping means
Gas dynamic
C372S038020, C372S038040, C372S082000
Reexamination Certificate
active
06782031
ABSTRACT:
BACKGROUND OF THE INVENTION
Pulse Power Systems
Pulse power electrical systems are well known. One such system is described in U.S. Pat. No. 5,142,166 issued to Birx. That patent described a magnetic pulse compression circuit which shortens and amplifies an electrical pulse resulting from the discharge of a charge storing capacitor bank. The patent also describes a induction transformer for amplifying the pulse voltage.
Another pulse power circuit is described in U.S. Pat. No. 5,729,562 issued to Birx, et al. which describes a pulse power system for a gas discharge laser and includes an energy recovery circuit for recovering electrical energy reflected from the laser electrodes.
Gas Discharge Lithography Lasers
Gas discharge lasers are well known and many of these lasers utilize pulse power systems such as those described in the two above-referenced patents to provide short high-voltage electrical pulses across the electrodes of the lasers. One such gas discharge laser is described in U.S. Pat. No. 4,959,840.
Lasers similar to the one described in U.S. Pat. No. 4,959,840 utilizing pulse power systems like the one described in U.S. Pat. No. 5,729,562 are utilized as light sources for integrated circuit lithography. At the present time, most of these lasers are configured to operate as KrF lasers utilizing a laser gas comprised of about 0.1 percent fluorine, about 1.0 percent krypton and the rest neon. These lasers produce light at a wavelength of about 248 nm.
There is a need for lithography light sources at wavelengths shorter than 248 nm such as that produced when the lasers are configured to operate as ArF or F
2
gas discharge lasers which produce laser beams with wavelengths of about 193 nm and about 157 nm, respectively. In the case of the ArF laser the gas mixture is substantially argon, fluorine and neon and in the case of the F
2
laser the gas mixture is substantially F
2
and He or F
2
and neon.
Optical Damage
Fused silica is the primary refractive optical material used in integrated circuit lithography devices. At wavelengths in the range of 193 nm and 157 nm fused silica is damaged by high intensity ultraviolet radiation. The damage is caused primarily by double photon excitation so that for a given pulse energy, the extent of the damage is determined largely by the shape and duration of the pulse.
Modular Pulse Power System
An electrical drawing of a prior art modular pulse power system is shown in FIG.
1
. In a prior art system the components of the pulse power system are provided in a power supply module, a commutator module and a unit called the compression head which is mounted on the laser chamber.
High Voltage Power Supply Module
High voltage power supply module
20
comprises a 300-volt rectifier
22
for converting 208-volt three phase plant power from source 10 to 300-volt DC. Inverter
24
converts the output of rectifier
22
to high frequency 300 volt pulses in the range 1000 kHz to 2000 kHz. The frequency and the on period of inverter
24
are controlled by a HV power supply control board (not shown) in order to provide course regulation of the ultimate output pulse energy of the system. The output of inverter
24
is stepped up to about 1200 volts in step-up transformer
26
. The output of transformer
26
is converted to 1200 volts DC by rectifier
28
which includes a standard bridge rectifier circuit
30
and a filter capacitor
32
. DC electrical energy from circuit
30
charges 8.1 &mgr;F C
o
charging capacitor
42
in commutator module
40
as directed by the HV power supply control board which controls the operation of inverter
24
. Set points within HV power supply control board are set by a laser system control board in a feedback system in order to provide desired laser pulse energy and dose energy (i.e., the total energy in a burst of pulses) control.
The electrical circuits in commutator
40
and compression head
60
merely serve to utilize the electrical energy stored on charging capacitor
42
by power supply module
20
to form at the rate of (for example) 2,000 times per second electrical pulses, to amplify the pulse voltage and to compress in time the duration of each pulse. As an example of this control, the power supply may be directed to charge charging capacitor
42
to precisely 700 volts which during the charging cycle is isolated from the down stream circuits by solid state switch
46
. The electrical circuits in commutator
40
and compression head
60
will upon the closure of switch
46
very quickly and automatically convert the electrical energy stored on capacitor
42
into the precise electrical discharge pulse across electrodes
83
and
84
needed to provide the next laser pulse at the precise energy needed as determined by a computer processor in the laser system.
Commutator Module
Commutator module
40
comprises C
o
charging capacitor
42
, which in this embodiment is a bank of capacitors connected in parallel to provide a total capacitance of 8.1 &mgr;F. Voltage divider
44
provides a feedback voltage signal to the RV power supply control board
21
which is used by control board
21
to limit the charging of capacitor
42
to the voltage (called the “control voltage”) which when formed into an electrical pulse and compressed and amplified in commutator
40
and further compressed in compression head
60
will produce the desired discharge voltage on peaking capacitor
82
and across electrodes
83
and
84
.
In this embodiment (designed to provide electrical pulses in the range of about 3 Joules and 16,000 volts at a pulse rate of 1000 Hz to 2000 Hz, about 100 microseconds are required for power supply
20
to charge the charging capacitor
42
to 800 volts. Therefore, charging capacitor
42
is fully charged and stable at the desired voltage when a signal from commutator control board
41
closes solid state switch
44
which initiates the very fast step of converting the 3 Joules of electrical energy stored on charging capacitor C
o
into a 16,000 volt discharge across electrodes
83
and
84
. For this embodiment, solid state switch
46
is a IGBT switch, although other switch technologies such as SCRS, GTOs, MCTs, etc. could also be used. A 600 nH charging inductor
48
is in series with solid state switch
46
to temporarily limit the current through switch
46
while it closes to discharge the C
o
charging capacitor
42
.
The first stage of high voltage pulse power production is the pulse generation stage. To generate the pulse the charge on charging capacitor
42
is switched onto C
1
8.5 &mgr;F capacitor
52
in about 5 &mgr;s by closing IGBT switch
46
.
A saturable inductor
54
initially holds off the voltage stored on capacitor
52
and then becomes saturated allowing the transfer of charge from capacitor
52
through 1:23 step up pulse transformer
56
to C
p−1
capacitor
62
in a transfer time period of about 550 ns for a first stage of compression.
Pulse transformer
50
is similar to the pulse transformer described in U.S. Pat. Nos. 5,448,580 and 5,313,481; however, this prior art embodiment has only a single turn in the secondary and
23
separate primary windings to provide 1to 23 amplification. Pulse transformer
50
is extremely efficient transforming a 700 volt 17,500 ampere 550 ns pulse rate into a 16,100 volt, 760 ampere 550 ns pulse which is stored very temporarily on C
p−1
capacitor bank
62
in compression head module
60
.
Compression Head Module
Compression head module
60
further compresses the pulse.
An L
p−1
saturable inductor
64
(with about 125 nH saturated inductance) holds off the voltage on 16.5 nF C
p−1
capacitor bank
62
for approximately 550 ns then allows the charge on C
p−1
to flow (in about 100 ns) onto 16.5 nF Cp peaking capacitor
82
located on the top of laser chamber
80
and is electrically connected in parallel with electrodes
83
and
84
and preionizer
56
A. This transformation of a 550 ns long pulse into a 100 ns long pulse to charge Cp peaking capacitor
82
makes up the second and last stage of compression.
Las
Hofmann Thomas
Johanson Bruce D.
Cray William
Cymer Inc.
Jackson Cornelius H
Wong Don
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