Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2002-06-04
2004-07-27
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S057000, C372S061000
Reexamination Certificate
active
06768765
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a high power laser, and particularly to a high power excimer or molecular fluorine laser having a window structure including multiple windows movable into position for transmitting the laser beam to/from the dicharge chabmer for increasing laser runtime.
2. Discussion of the Related Art
Excimer lasers are used in a wide variety of industrial applications. Among these applications are TFT annealing and photolithography. High power XeCl lasers are particularly used for TFT annealing and KrF, ArF and F
2
lasers are particularly used in or are being developed to be used in photolithographic applications. Gas discharges between electrodes in a discharge chamber may be used for energizing a halogen-rich gas mixture filling the discharge chamber, whereby a high power laser beam may be generated by disposing the discharge chamber within an optical resonator.
The discharge chamber has windows on either end so that light can enter and exit the chamber. These windows are exposed to the gas mixture. When the discharges occur between the electrodes, dust and/or other contaminants are produced. These contaminants can adhere to the windows and inhibit the ability of the beam to enter and exit the chamber. When the windows become significantly contaminated, then the windows are cleaned or replaced. The servicing of the laser windows may be performed while another component of the laser is being serviced, such as when a new gas fill is performed. It is desired to increase the lifetime of both the gas mixture and the laser windows to increase the uptime percentage of the laser and thereby increase the throughput of the industrial process for which the laser is being used.
A cryogenic gas circulation loop may be used for preventing contaminants from accessing the laser windows. The cryogenic loop would include an outlet port on the laser tube for removing a small flow of the laser gas mixture from the tube. The removed gas is then circulated through a cryogenic filter such as may be described at U.S. Pat. Nos. 5,136,605, 5,111,473 and/or 5,430,752, which are hereby incorporated by reference, to clean the gas by removing contaminants. The clean gas may then be flowed back into the laser tube at the windows. A gas circulation loop including an electrostatic precipitator filter may also be used preventing contaminant build-up on the laser tube windows, such as may be described at U.S. Pat. No. 4,534,034, which is hereby incorporated by reference, with or without the quiescent zones described in the '034 patent.
There are some disadvantages to using a cryogenic gas circulation loop in this manner. First, if, e.g., liquid helium is used, a helium compressor which converts the helium gas to liquid helium is used. The helium compressor runs at an operating frequency, e.g., 2.4 Hz, that is difficult to damp. It is desired to have a “quiet” gas circulation loop for keeping the laser tube windows clean. Second, when the laser is first initialized for operation, the cryogenic pump takes a significant amount of time to cool down, e.g., around one hour. This cool down period can prolong the downtime of the laser system. It is desired to have a gas circulation loop for cleaning the laser tube windows that does not have a significant cool down period. Third, a cryogenic filter has a limited volume, e.g., around one liter. This limits the gas flow volume through the gas circulation loop, and limits the degree to which the clean gas can be flowed past the laser tube windows. It is desired to have a gas circulation loop with a high clean gas flow volume past the laser tube windows.
It is therefore desired to provide a high power laser which includes a “quiet” gas circulation loop, such as not including a helium compressor, for keeping the laser tube windows clean.
It is further desired to provide a high power laser which includes a gas circulation loop for cleaning the laser tube windows that does not have a significant cool down period.
It is also desired to provide a high power laser which has a gas circulation loop with a high clean gas flow volume past the laser tube windows.
It is further desired to provide a high power laser having improved laser window lifetime and/or gas mixture lifetime.
SUMMARY OF THE INVENTION
An excimer or molecular fluorine laser system is also provided including a discharge tube filled with a gas mixture, multiple electrodes within the discharge tube and connected to a discharge circuit for energizing the gas mixture and a resonator for generating a laser beam. The system further includes at least one window structure including a first window and a second window. The first window initially seals the discharge tube and transmits the beam. The second window is initially unexposed to the gas mixture. The window structure is configured such that the second window is movable into position for sealing the discharge tube and transmitting the beam when the first window becomes contaminated.
The window structure may be further configured for replacing the first window with a third window when the second window is moved into position for sealing the discharge tube and transmitting the beam. The window structure preferably includes a valve for maintaining a seal on the discharge tube when the second window is moved into position for sealing the discharge tube and transmitting the beam. The window structure may be rotatable or translatable for moving the second window into position when the first window becomes contaminated.
The laser system may also include a gas handling unit for replenishing the gas mixture during laser operation and increasing the lifetime of the gas mixture. The laser system may also include an additional gas volume having fluid communication with the gas mixture for increasing a total gas volume and a lifetime of the gas mixture.
The laser system may also include components arranged in a gas circulation loop including the window structure and configured for drawing a flow of the gas mixture from the discharge tube and through the components, and for flowing filtered gas past the first window for preventing contaminants within the discharge tube from depositing on the window. One of the components may include a labyrinth filter and the window structure may include a baffle structure.
A high power excimer or molecular fluorine laser system for industrial processing is provided including a discharge tube filled with a gas mixture including a halogen component, multiple electrodes within the discharge tube and connected to a pulsed discharge circuit for energizing the gas mixture, a resonator for generating a laser beam, at least one window structure including a first window and a second window, wherein the first window initial seals the discharge tube and transmits the beam, and the second window is initially unexposed to the gas mixture, and wherein the window structure is configured such that the second window is movable into position for sealing the discharge tube and transmitting the beam when the first window becomes contaminated, and gas mixture contamination suppression means for suppressing contaminant build-up in the gas mixture, such that a lifetime of the gas mixture is increased, and wherein an interval between laser servicing procedures is increased due to the increased lifetime of the gas mixture and the movement into position of the second window, such that laser system downtime is reduced and industrial throughput is increased.
A method is also provided for generating an excimer or molecular fluorine laser beam such that the laser system generating the beam has an increased interval between laser servicing procedures such that laser system downtime is reduced and industrial throughput is increased. the method includes operating the laser for generating the laser beam which transmits a first window on a discharge tube of the laser, and wherein the first window is exposed to a gas mixture within the discharge tube. the method also includes moving the first window and replacing
Baumler Juergen
Osmanow Rustem
Schroeder Thomas
Ip Paul
Lambda Physik AG
Nguyen Tuan N.
Stallman & Pollock LLP
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