Electric heating – Metal heating – By arc
Reexamination Certificate
1999-07-13
2001-01-09
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121480, C219S121540, C219S121520, C378S119000, C315S111310
Reexamination Certificate
active
06172324
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to plasma focused radiation sources, and more particularly to such a radiation source producing extreme ultraviolet (EUV) and/or soft x-ray radiation at a high pulse repetition frequency (PRF).
BACKGROUND OF THE INVENTION
The parent patent and the parent application both describe a plasma gun which may, among other things, be utilized to generate radiation in the EUV and soft x-ray bands with high reliability and at a PRF in excess of approximately 100 Hz, preferably in excess of 500 Hz, and preferably 1000 Hz or more for lithography and other applications requiring generation of such radiation. More specifically, the plasma gun of the parent application/patent involves a center electrode and an outer electrode substantially coaxial with the center electrode, a coaxial column being formed between the electrodes. A selected gas is introduced into the column through an inlet mechanism and a plasma initiator was provided at the base end of the column. A solid state high repetition rate pulse driver is provided which is operable on pulse initiation at the base of the column to deliver a high voltage pulse across the electrodes, the plasma expanding from the base of the column and off the end thereof. The pulse voltage and electrode lengths were selected such that the current for each voltage pulse is substantially at its maximum as the plasma exits the column. The outer electrode for this plasma gun embodiment is preferably the cathode electrode and may be solid or may be in the form of a plurality of substantially evenly spaced rods arranged in a circle. The inlet mechanism provides a substantially uniform gas fill in the column, resulting in the plasma being initially driven off the center electrode, the plasma being magnetically pinched as it exits the column to provide a very high temperature at the end of the center electrode. A selected gas/element fed to the pinch as part of the ionized gas, through the center electrode or otherwise, is ionized by the high temperature at the pinch to provide radiation at a desired wavelength. The wavelength is achieved by careful selection of various plasma gun parameters, including the selected gas/element fed to the pinch, current from the pulse driver, plasma temperature in the area of the pinch, and gas pressure at the column.
While radiation sources of the type indicated above, as described in far greater detail in the parent application and patent, can provide useful radiation at a desired wavelength, the high velocity of the plasma being driven down the column and off the center electrode can cause a problem which significantly limits the usefulness of such sources. In particular, temperatures at the pinch in the range of 100 eV (i.e., about 11,000° C.) to 1000 eV, depending on the desired frequency of radiation, require magnetic compression fields which are sufficient to drive the plasma to velocities of several centimeters per microsecond. Plasmas moving at these velocities down the center conductor and off the end forming the pinch tend to continue moving out into space away from the end of the center conductor, the plasma sheath eventually losing electrical connection to the pinch. This prematurely ends the pinch after as little as 100 nanoseconds and also results in a large voltage transient in the thousands of volts range, resulting in a restrike which can severely damage the electrodes.
Since a discharge can last for several microseconds, if premature loss of electrical connection between the plasma sheath and the electrode could be eliminated, the pinch lifetime could be extended dramatically and the potentially damaging restrike eliminated. This could result in significantly increased output efficiency for the plasma source and a greatly expanded electrode lifetime for the source, thus reducing source down time and maintenance, both of which can be expensive in for example a lithographic application. Significantly better performance at lower costs can thus be obtained.
Further, while materials to be fed to the pinch to achieve certain wavelengths of output were suggested in the parent application, a specific material for providing radiation at the desirable one nanometer wavelength was not specifically indicated.
SUMMARY OF THE INVENTION
In accordance with the above, this invention provides a high PRF radiation source at a selected wavelength which source includes a center electrode, an outer electrode substantially coaxial with the center electrode, a coaxial column being formed between the electrodes, which column has a closed base end and an open exit end; an inlet mechanism for introducing a selected gas into the column; a plasma initiator at the base end of the column; a solid state high repetition rate pulse driver operable on plasma initiation at the base of the column for delivering a voltage pulse across the electrodes, the plasma expanding from the base end of the column and off the exit end thereof; the pulse voltage and electrode lengths being such that the current for each pulse is at substantially its maximum as the plasma exits the column; the inlet mechanism providing a substantially uniform gas fill in the column, resulting in the plasma being initially driven off the center electrode, the plasma being magnetically pinched as it exits the column, raising the temperature at the end of the center electrode sufficient to cause an ionizable element appearing at the end of the center electrode to produce radiation at at least the selected wavelength; and a component for redirecting plasma driven of the center electrode back toward the center electrode without substantially affecting passage of the radiation. For preferred embodiments, the component which redirects is a shield of a high temperature, non-conductive material positioned a selected distance from the exit end of the center electrode and shaped to reflect plasma impinging thereon back toward the center electrode, the shield having an opening positioned to permit the radiation to pass therethrough. For preferred embodiments, the selected distance that the shield is spaced from the center electrode is no more than approximately 2R, where R is the radius of the center electrode, and is not less than approximately R. The shape of the shield may for example be generally spherical, generally conical, or generally parabolic. The opening for permitting passage of radiation is preferably substantially circular and located at substantially the center of the shield. More specifically, the opening is sized and positioned such that radiation exiting the center electrode at an angle of ±15° from the axis of the center electrode passes through the opening. The material for the shield is preferably at least one of a high temperature ceramic, glass, quartz and/or sapphire, the material for a preferred illustrative embodiment being Al
2
O
2
(aluminum oxide).
In accordance with another aspect of the invention, a high PRF source of radiation at approximately 1 nm is provided which includes a center electrode, an outer electrode substantially coaxial with the center electrode, a coaxial column being formed between the electrodes, which column has a closed base end and an open exit end; an inlet mechanism for introducing a selected gas into the column; a solid state high repetition rate pulsed driver operable on plasma initiation at the base of the column for delivering a high voltage pulse across the electrodes, the plasma expanding from the base end of the column and off the exit end thereof, the current for each voltage pulse initially increasing to a maximum and then decreasing to zero, the pulse voltage and electrode lengths being such that the current for each pulse is at substantially its maximum as the plasma exits the column, the inlet mechanism providing a substantially uniform fill in the column and ionizable sodium being applied to the pinch, the temperature of the pinch being sufficient to cause the sodium to emit radiation of at least said approximately 1 nm wavelength. A shield of the type previously descr
Paschall Mark
Science Research Laboratory, Inc.
Wolf Greenfield & Sacks P.C.
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