Coherent light generators – Particular component circuitry
Reexamination Certificate
2001-04-18
2004-12-21
Harvey, Minsun Oh (Department: 2828)
Coherent light generators
Particular component circuitry
C372S038020, C372S061000
Reexamination Certificate
active
06834066
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to laser systems. In particular, the present invention relates to implementing active loads in the discharge circuitry design of gas discharge lasers to provide stabilization of high repetition rate gas discharge laser systems.
2. Description of the Related Art
Pulse gas discharge lasers, emitting in the deep ultraviolet (DUV) and vacuum ultraviolet (VUV) region are widely used in various industrial applications such as microlithography, photoablation, and micro-machining, among others. For microlithographic applications, currently used systems include line narrowed excimer lasers, such as ArF (193 nm) lasers and KrF (248 nm) lasers, as well as molecular fluorine (F
2
) lasers emitting at 157 nm, which are efficient and exhibit high energy stability at high repetition rates, for example, at 1-2 KHz or more.
FIG. 1
illustrates a schematic arrangement of a pulsed gas discharge electrical circuit of a typical gas discharge laser system. As shown, a pair of discharge electrodes
101
,
102
is coupled to a discharge circuit which includes a peaking capacitance Cp and an inherent inductance L
d
between the peaking capacitance Cp and the discharge electrode
101
. Such discharge electrical circuitry may be found in current gas discharge lasers such as excimer lasers and molecular fluorine laser systems.
Referring to
FIG. 1
, the area between the discharge electrodes
101
,
102
defines a region referred to as a gas discharge region
103
. As can be seen, the pair of elongated discharge electrodes
101
,
102
of the gas discharge laser, one of which (in this case, the discharge electrode
102
) may be connected to a ground or reference potential, are separated by the gas discharge region
103
which is filled with a high pressure laser gas. Moreover, the discharge electrode
101
is connected to the output of the high voltage pulsed generator which is capable of providing fast and powerful charging of the peaking capacitor Cp up to the electrical breakdown voltage of the gas discharge gap. Powerful pulsed source(s) or preionization units for providing ultraviolet light in a spark or corona discharge provide an initial preionization of the gas mixture in the discharge region, and are typically positioned in the vicinity of the gas discharge region
103
between the discharge electrodes
101
,
102
. The preionizer(s) provide an initial ionization, or preionization, of the laser gas during the charging of the peaking capacitance Cp which receives an electrical pulse initially provided by the charging of a main storage capacitor by a high voltage (HV) power supply when the main storage capacitor is discharged through a switch such as a thyratron or a solid state switch.
Referring back to
FIG. 1
, in such gas laser systems the HV electrical circuitry which is used for the excitation of the gas discharge in the pulsed gas laser systems may be schematically sub-divided into two parts. The first part of the HV electrical circuitry may include the peaking capacitance Cp which is configured to store electrical energy, and used during the gas breakdown phase. The second part of the HV electrical circuitry may include the HV pulsed power generator which is used for the fast and efficient charging of the peaking capacitor Cp up to the breakdown voltage of the gas. In particular, the HV pulsed power generator may include a suitable HV pulsed device such as a gas filled thyratron, or a solid state switch such as a thyristor or an IGBT).
Additional information may be found in R. S. Taylor, K. E. Leopold, Applied Physics, B59 (1994) 479; U.S. Pat. Nos. 6,020,723, 6,005,880, 5,729,562, 5,914,974, 5,936,988, 6,198,761, 5,940,421, and 5,982,800, and pending U.S. patent applications Ser. Nos. 09/649,595, and 09/453,670, the disclosures of each of which are expressly incorporated herein by reference for all purposes.
A problem encountered with typical pulsed electrical gas discharges in strongly electronegative gas mixtures (i.e., containing a halogen component) at elevated pressures (for example, several bars) is a certain degree of instability. The short phase of the uniform glow discharge, usually less than 100 nanoseconds, corresponding to the pumping of the laser medium, is typically terminated by rapidly developing streamers. The streamers themselves are temporally inconsistent which leads to the discharge instabilities. In addition, the existence of streamers at the ends of discharges produces excessive wear on the electrodes. In view of these problems caused by streamers, it is desired to suppress them. It is therefore desired to have a gas discharge laser including a discharge circuit wherein the main input of the energy into the gas discharge is quickly realized, or that provides very short, intense electrical pulses to the main discharge load, and is terminated without extended and inconsistent streamers reducing the discharge stability from pulse to pulse and without excessively wearing the discharge electrodes.
SUMMARY OF THE INVENTION
In view of the foregoing, a discharge circuit for a pulsed gas laser system in accordance with one embodiment of the present invention includes a pair of spaced-apart electrodes defining a discharge region as a main load, a capacitance coupled to one of the pair of electrodes for providing electrical pulses to the electrodes, and an additional load electrically coupled between the capacitance Cp and one of the discharge electrodes.
The additional load may include one or more resistors, a resistor array, a resistor or resistor array coupled with a variable inductance and/or a saturable inductance, or another dissipative electrical component for dissipating electrically energy between the main load and the capacitance to facilitate termination of electrical discharges between the electrodes and in turn suppress the formation or influence of streamers. The additional load may be coupled in series or in parallel with the capacitance and the main load, and a portion of the additional load may be coupled in series and a portion may be coupled in parallel with the capacitance and the main load. Any series connected portion of the additional load may be coupled to a high voltage or grounded main electrode. The resistor or resistors may have a value comparable to a wave impedance of the discharge circuit. Alternatively, the resistor or resistors may have a value comparable to an active impedance of the gas discharge during a maximum discharge current phase. The additional load is preferably a passive resistance, and may alternatively have an active feature such as a voltage dependence or a temperature dependence.
The circuit may further include a cooling unit, wherein the additional load is provided in the cooling unit. The cooling unit may be provided within a pulsed power module of a laser system which contains electrical components of the discharge circuit particularly susceptible to heating effects. The cooling unit may include one of an air fan and an encapsulated volume with circulating isolating fluid.
One or more preionization units are preferably also provided for ionizing the laser gas within the discharge region between the pair of main electrodes during the charging of the capacitance just prior to discharging through the electrodes.
The capacitance may include a series of peaking capacitors, and may include a series of sustaining capacitors. The sustaining capacitors would be coupled to the electrodes by a different inductance than the peaking capacitors, and would be otherwise preferably coupled within the discharge circuit similarly to the peaking capacitors defining the capacitance coupled to the main electrodes. The pair of electrodes, the capacitance and the additional load may form a series configuration such as an electrical loop, or the additional load may be coupled to the electrodes in parallel with the capacitance. The circuit further includes a power generator configured to provide power to the capacitance for charging the capacitance. The power generato
Berger Vadim
Bragin Igor
Kleinschmidt Juergen
Lambda Physik AG
Nguyen Tuan N.
Stallman & Pollock LLP
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