Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-08-06
2003-06-10
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S029021, C372S059000, C372S029020, C372S019000, C372S058000
Reexamination Certificate
active
06577663
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to molecular fluorine lasers emitting around 157 nm, and particularly to a very narrow characteristic bandwidth and low spectral purity emitting molecular fluorine laser oscillator followed by an amplifier for increasing the energy of the emitted laser beam to a desired power for applications processing.
2. Description of the Related Art
Ultra-narrow bandwidth emission is required for the F
2
-laser to make refractive optics projection lenses usable for 157 nm lithography. Recent investigations have shown that the natural bandwidth of emission may be around 0.60±0.10 pm for an F
2
-laser under normal operation conditions. Also, line narrowing is possible down to about ≦0.15 pm with special resonator designs containing sensitive optical elements. However, using such optical line-narrowing resonators causes the output energy to drop remarkably down to ≦1 mJ. To get a desired output energy, e.g., around 10 mJ, one solution is to use an oscillator-amplifier design (see U.S. patent application Ser. No. 09/599,130 which is assigned to the same assignee as the present application and is hereby incorporated by reference).
The advantageous oscillator-amplifier design described in the '130 application provides a very narrow bandwidth laser beam at desired energies for application processing such as photolithography. Use of the design set forth in the '130 application also involves synchronization of two laser systems, i.e., the oscillator and the amplifier, in parallel, which can be expensive, can tend to occupy a lot of space and may be unreliable under certain circumstances. Preliminary experiments have shown that the oscillator-amplifier stage described in the '130 application can provide an advantageous 157 nm laser beam for application processing. However, the design may also present, in some circumstances, a competitively disadvantageous economic solution relative to systems which would use the natural bandwidth of emission of the F
2
laser for use with an imaging system having a catadioptric lens design.
It is therefore an object of the present invention to provide a more compact, economic molecular fluorine laser system which emits a beam around 157 nm having a narrow bandwidth and sufficient power for application processing.
SUMMARY OF THE INVENTION
In view of the above object, a molecular fluorine (F
2
) laser system is provided including a seed oscillator and a power amplifier. The seed oscillator includes a laser tube including multiple electrodes therein which are connected to a discharge circuit. Seed radiation may alternatively be provided by an excimer lamp preferably maintained at low pressure. The laser tube is part of an optical resonator for generating a laser beam including a first line of multiple characteristic emission lines around 157 nm. The laser tube is filled with a gas mixture including molecular fluorine and a buffer gas. The gas mixture is at a pressure below that which results in the generation of a laser emission including the first line around 157 nm having a natural linewidth of less than 0.5 pm without an additional line-narrowing optical component for narrowing the first line. The power amplifier increases the power of the beam emitted by the seed oscillator to a desired power for applications processing.
A molecular fluorine laser system is also provided including a discharge tube filled with a gas mixture including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, a resonator for generating a laser beam including a first line of multiple characteristic emission lines around 157 nm, and a power amplifier for increasing the power of the beam generated by the resonator to a desired power for applications processing. The gas mixture has a total pressure which is sufficiently low such that the laser beam includes the first line around 157 nm having a linewidth of less than 0.5 pm while the resonator does not include an additional line-narrowing optical component for further narrowing the linewidth of the first line.
A molecular fluorine laser system is further provided including a discharge tube filled with a gas mixture including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, a resonator for generating a laser beam including a first line of multiple characteristic emission lines around 157 nm, and a power amplifier for increasing the power of the beam generated by the resonator to a desired power for applications processing. The resonator includes at least one line-narrowing optical component for narrowing a linewidth of the first line around 157 nm. The gas mixture has a total pressure which is sufficiently low such that the line-narrowed first line has a linewidth of less than 0.2 pm.
A molecular fluorine laser system is also provided including a discharge tube filled with a gas mixture including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, a resonator for generating a laser beam including a first line of multiple characteristic emission lines around 157 nm, a gas handling unit coupled with the discharge tube for flowing gas between the discharge tube and the gas handling unit, a processor for controlling the flow of gases between the gas handling unit and the discharge tube to control one or more parameters associated with the gas mixture, and a power amplifier for increasing the power of the beam generated by the resonator to a desired power for applications processing. The gas mixture has a total pressure which is sufficiently low such that the laser beam includes the first line around 157 nm having a linewidth of less than 0.5 pm.
A molecular fluorine laser system is also provided including a discharge tube filled with a gas mixture including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, a resonator for generating a laser beam including a first line of multiple characteristic emission lines around 157 nm, a gas handling unit coupled with the discharge tube for flowing gas between the discharge tube and the gas handling unit, and a processor for controlling the flow of gases between the gas handling unit and the discharge tube to control one or more parameters associated with the gas mixture. The gas mixture has a total pressure which is sufficiently low such that the laser beam includes the first line around 157 nm having a linewidth of less than 0.5 pm.
A method for controlling a bandwidth of an excimer or molecular fluorine laser system is also provided, including the steps of operating the laser system, monitoring the bandwidth of an output beam of the laser system, controlling a gas mixture pressure within a laser tube of the laser system at a predetermined pressure that is sufficiently low such that the output beam includes a first line of multiple characteristic emission lines around 157 nm having a linewidth of less than 0.5 pm, and amplifying the output beam for increasing the power of the beam generated by the resonator to a desired power for applications processing.
A method for controlling a bandwidth of an excimer or molecular fluorine laser system is further provided including operating the laser system, monitoring the bandwidth of an output beam of the laser system, and controlling a gas mixture pressure within a laser tube of the laser system at a predetermined pressure that is sufficiently low such that the output beam includes a first line of multiple characteristic emission lines around 157 nm having a linewidth of less than 0.5 pm.
A method for controlling a bandwidth of an excimer or molecular fluorine laser system is also provided including operating the laser system, and controlling a gas mixture pressure within a laser tube of
Jr. Leon Scott
Lambda Physik AG
Smith Andrew V.
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