As noted in an above referenced application(s) or patent(s), a fluoride layer generated on the anode in rare gas fluoride excimer lasers (a so-called reef due to its resemblance to a marine coral reef albeit on a much smaller scale) can greatly reduce the erosion rate, thereby increasing anode operating lifetime by at least a factor of 5×, and more likely a factor of 10×. However, unless the fluoride layer is distributed evenly across the surface of the discharge region, e.g., in the gas flow direction, and, e.g., along the entire length of the discharge, it can cause local, severe erosion of the opposing cathode surface, which can leads to pulse energy and other instabilities. Applicants propose and apparatus and method of operation to alleviate such problems.
Coaxial cable has traditionally been used for high voltage power connections between modules in pulsed powered lasers, e.g., for integrated circuit photolithography processing uses, e.g., in applicant's assignee's below noted laser systems, e.g., in connections from a high voltage power supply (“HVPS”) to a commutator section of a magnetic switch pulsed power system and from the commutator to a compression head portion of the pulsed power system, a so-called solid state pulse power system. The HVPS may be connected to the commutator section through, e.g., a resonant charger circuit. However, in many cases, the power delivered is not continuously applied. That is, the power is delivered in the form of pulses. The fundamental (and harmonic) frequency of these pulses can often result in a skin effect applied to the coaxial cable conductors (both the center conductor and outer return braid). This can have unwanted effects when the voltage and current delivered by the HVPS and inter-section connections is high as is the case with solid state magnetic switched pulsed power systems, e.g., for ArF, KrF, XeF, XeCl, F2 and like excimer/molecular fluorine laser systems, e.g., used in integrated circuit photolithography. The higher the pulse repetition rate, e.g., as these systems require upwards of 6 kHz and greater pulse repetition rates, the worse the problem. Applicants propose according to aspects of an embodiment of the present invention to alleviate such problems.
As noted in the above referenced co-pending patent application Ser. No. 10/607,407 a pulse power circuit known in the art, e.g., for use in supplying high pulse repetition rate (4 kHz and above) electrical pulses between electrodes in a gas discharge laser system may include, e.g., a high voltage resonant power supply, a commutator module, a compression head module and a laser chamber module. High voltage power supply module can comprise, e.g., for a 4 kHz pulse repetition rate laser a 600 volt rectifier for, e.g., converting the 480 volt three phase normal plant power from an electrical power AC source to about 600 volt DC. An inverter, e.g., converts the output of the rectifier to, e.g., high frequency 600 volt pulses in the range of 10 kHz to 100 kHz. The frequency and the on period of the inverter can be controlled, e.g., by a HV power supply control board (not shown) in order to provide course regulation of the ultimate output pulse energy of the system, e.g., based upon the output of a voltage monitor comprising, e.g., a voltage divider.
The output of the inverter can be stepped up to about 800 volts in a step-up transformer. The output of transformer is converted to 800 volts DC by a rectifier, which can include, e.g., a standard bridge rectifier circuit and a filter capacitor. A Resonant charger module can be used to take the DC output of circuit, e.g., to resonantly pulse charge, e.g., an 5.1 μF charging capacitor C0 in the commutator module as directed by a control board, which can, e.g., control the operation of the Resonant charger module to set this voltage. Set points, e.g., within the HV or Resonant charger control board can be provided by a laser system control board (not shown). In the discussed embodiment, e.g., pulse energy control for the laser system can be provided by a set of power supply and resonant charger modules.
The electrical circuits in commutator module and compression head module may, e.g., serve to amplify the voltage and compress the electrical energy stored on charging capacitor C0 by the power supply and resonant charger modules, e.g., to provide 800-1200 volts to charging capacitor C0, which during the charging cycle can be isolated from the down stream circuits, e.g., by a solid state switch.
The commutator module, which can comprise, e.g., the charging capacitor C0, which can be, e.g., a bank of capacitors connected in parallel to provide a total capacitance of, e.g., 5.1 μ.F, along with the voltage divider, in order to, e.g., provide a feedback voltage signal to the HV power supply or Resonant charger control board which can be used by control board to limit the charging of charging capacitor C0 to a voltage (so-called “control voltage”), which, e.g., when formed into an electrical pulse and compressed and amplified in the commutator and compression head, can, e.g., produce the desired discharge voltage on a peaking capacitor Cp and across electrodes in the lasing cavity chamber.
As is known in the art, such a circuit may be utilized to provide pulses in the range of 3 or more Joules and greater than 14,000 volts at pulse rates of 2,000-4,000 or more pulses per second. In such a circuit, e.g., at 4 kHz about 160 microseconds may be required for DC power supply and Resonant charger modules to charge the charging capacitor C0 to, e.g., 800-1200 volts, and at 6 kHz the charging time is reduced to about 100 microseconds, and so forth as pulse repetition rate increases. Charging capacitor C0, therefore, can, e.g., be fully charged and stable at the desired voltage provided the voltage and current applied to the charging capacitor C0 in the amount of time allowed by the pulse repetition rate can be accomplished. For example, when a signal from a commutator control board is provided, e.g., to close the solid state switch, which, e.g., initiates a very fast step of converting the 3 Joules of electrical energy stored on charging capacitor C0 into, e.g., a 14,000 volt or more charge on peaking capacitor Cp for creating a discharge across the electrodes, provided the charging capacitor has been adequately charged within the time allotted by the pulse repetition rate of the laser system. The solid state switch may be, e.g., an IGBT switch, or other suitable fast operating high power solid state switch, e.g., an SCR, GTO, MCT, high power MOSFET, etc. A 600 nH charging inductor L0 can be placed in series with the solid state switch and employed, e.g., to temporarily limit the current through the solid state switch while it closes to discharge the charge stored on charging capacitor C0 onto a first stage capacitor C1 in the commutator module, e.g., forming a first stage of pulse compression.
For the first stage of pulse generation and compression, the charge on charging capacitor C0 can be switched onto a capacitor, e.g., a 5.3 μF capacitor C1, e.g., in about 4 μs. A saturable inductor can hold off the voltage on capacitor C1 until it saturates, and then presents essentially zero impedance to the current flow from capacitor C1, e.g., allowing the transfer of charge from capacitor C1 through, e.g., a step up transformer, e.g., a 1:25 step up pulse transformer to charge a capacitor Cp-1 in the compression head module, with, e.g., a transfer time period of about 400 ns, comprising a second stage of compression. The design of pulse transformer is described in a number of prior patents assigned to the common assignee of this application, including, e.g., U.S. Pat. No. 5,936,988. For example, such a transformer is an extremely efficient pulse transformer, transforming, e.g., a 800 volt 5000 ampere, 400 ns pulse to, e.g., a 20,000 volt, 200 ampere 400 ns pulse, which, e.g., is stored very temporarily on compression head module capacitor Cp-1, which may also be, e.g., a bank of capacitors. The compression head module may, e.g., further compress the pulse. A saturable reactor inductor Lp-1, which may be, e.g., about a 125 nH saturated inductance, can, e.g., hold off the voltage on capacitor Cp-1 for approximately 400 ns, in order to, e.g., allow the charge on Cp-1 to flow, e.g., in about 100 ns, onto a peaking capacitor Cp, which may be, e.g., a 10.0 nF capacitor located, e.g., on the top of a laser chamber and which the peaking capacitor Cp is electrically connected in parallel with the laser system electrodes. This transformation of a, e.g., 400 ns long pulse into a, e.g., 100 ns long pulse to charge peaking capacitor Cp can make up, e.g., the second and last stage of compression. About 100 ns after the charge begins flowing onto peaking capacitor Cp mounted on top of and as a part of the laser chamber in the laser chamber module, the voltage on peaking capacitor Cp will have reached, e.g., about 20,000 volts and a discharge between the electrodes begins. The discharge may last, e.g., about 50 ns, during which time, e.g., lasing occurs within the resonance chamber of the, e.g., excimer laser.
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