Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
  • H05G2/001—X-ray radiation generated from plasma
  • H05G2/003—X-ray radiation generated from plasma being produced from a liquid or gas
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
  • H05G2/001—X-ray radiation generated from plasma
  • H05G2/003—X-ray radiation generated from plasma being produced from a liquid or gas
  • H05G2/005—X-ray radiation generated from plasma being produced from a liquid or gas containing a metal as principal radiation generating component

Definitions

• the invention relates to a method and device for producing extremely short-wave UV and soft X-ray radiation from a dense hot plasma discharge of pinch type.
• the field of application includes lithography, particularly in the spectral range around 13.5 nm, lasers in the short-wave UV and X-ray ranges, and X-ray microscopy.
• a condition of effective operation is the addition of lithium vapor to the inert gas contained in the discharge chamber, and this substantially complicates the design of the source of radiation and contaminates the space outside the discharge.
• a method of producing short-wave radiation with the aid of a z-pinch involving RF pre-ionization is devoid of this disadvantage, but the dielectric wall of the discharge chamber at which the pinch-type discharge is initiated is subject both to exposure to powerful radiation flux and the substance that forms as a result of electrode erosion (see U.S. patent no. 5,504,795, hereby incorporated by reference). This limits the possibilities of achieving a long service life when this approacr i ⁇ implemented.
• a close technical achievement is a method of producing short-wave radiation from a gas-discharge plasma that consists in the pre-ionization of gas in the discharge region between coaxial electrodes achieved through an axial aperture in one of the electrodes, and in initiating a pinch-type discharge (see German patent DE 197 53 696 Al, hereby incorporated by reference).
• the device for implementing this method contains a discharge chamber having two axially symmetrical electrodes optically communicating through an aperture formed in one of the electrodes, with a source of pre-ionization disposed outside the discharge chamber (see the '696 published application).
• pre-ionization is achieved by a low-current discharge that is automatically formed in a cavity of the cathode when discharge voltage is applied and that then propagates into the discharge gap through the aperture in the hollow cathode.
• the internal dielectric wall of the discharge chamber may be disposed outside the zone irradiated by the discharge, and this enables a long service life to be achieved in a periodically pulsed operating mode.
• Disadvantages of this method and the device for implementing it are a low efficiency of conversion of the energy input into radiation in the short-wave range due to the low level of pre-ionization and its non-ideal spatial distribution in the gap between the electrodes of the discharge chamber. Since the pre-ionization is carried out substantially in the paraxial region of the discharge gap, increasing the cross-sectional area of a pinch-type discharge is made difficult at its initial stage, and this limits the possibility of increasing the energy and the average power of the short-wave radiation.
• the long time of formation (approximately 1 ms) of the automatic pre-ionization and of the initiation of a pinch-type discharge compared with the time interval between individual pulses and the low rate of growth (approximately 10 7 V/s) of the discharge voltage limit the possibility of achieving a high radiation energy stability from pulse to pulse.
• a method for producing short-wave radiation from a gas-discharge plasma including pre-ionization of the gas in the discharge region between coaxial electrodes achieved through an axial aperture formed in one of the electrodes and initiation of a pinch-type discharge.
• Pre-ionization is achieved simultaneously by a flux of radiation having wavelengths from the UV to X-ray range and by the flux of accelerated electrons from the plasma of the pulsed sliding discharge initiated in a region not optically communicating with the axis of the pinch-type discharge.
• a rate of growth of the discharge voltage across the region preferably and advantageously exceeds 10 11 V/s.
• Fluxes of radiation and electrons are preferably formed axially symmetrically and are directed into part of the discharge region outside the axis.
• the method can be implemented by a device containing a discharge chamber having two axially symmetrical electrodes optically communicating through an aperture formed in one of the electrodes, with a source of pre-ionization disposed outside the discharge chamber.
• the source of pre-ionization preferably derives from an axially symmetrical system of forming a sliding discharge comprising an elongated initiating electrode coated with a dielectric layer, on the surface of which there is disposed a trigger electrode, the initiating electrode being arranged coaxially with the electrodes of the discharge chamber and formed so that the dielectric layer is disposed in a region not optically communicating with the axis of the discharge chamber and one of the electrodes of the system for forming a sliding discharge being combined with one of the electrodes of the discharge chamber, a generator having a rate of growth of output voltage of more than 10 u V/s being introduced into the device, the output of positive polarity of which is connected to the initiating electrode, while the output of negative polarity of the pulsed generator is connected to the
• a dielectric insert in which an axial aperture is formed is preferably introduced into the discharge chamber, and the electrodes of the discharge chamber are disposed on the surface of the dielectric insert.
• a cylindrical plasma envelope having high conductivity forms in the discharge region as a result of pre-ionization. This establishes the initiation of a pinch-type discharge under ideal conditions and ensures an increase in the output of short-wave radiation from the hot plasma discharge.
• the cross-sectional size of the pinch-type discharge is advantageously increased according to the invention when it is initiated. This makes it possible to increase the kinetic energy of the plasma substantially at the stage when it is compressed by the magnetic field of the discharge, and this ensures a more effective heating of the plasma column and an increase in the energy of the short-wave radiation, and also in its average power in the periodically pulsed mode.
• Figure 1 shows diagrammatically a device for implementing the preferred method.
• Figure 2 shows a device into whose discharge chamber a dielectric insert has been introduced.
• the device comprises a supply source 1 that, in one case, comprises a storage capacitor with a commutator, charging induction coils, a pulse capacitor and a magnetic switch, and is connected to electrodes 2, 3 of the discharge chamber 4; a pulse generator 5, which is connected to the trigger electrode 6 and the initiating electrode 7 of the axially symmetrical system for forming a sliding discharge on the surface of the dielectric layer 8, and also a liquid coolant 9 and an insulator 10 of the discharge chamber.
• a dielectric insert 11 in which an axial aperture is formed and on the surface there are disposed electrodes 2, 3.
• the method of producing short-wave radiation from the gas-discharge plasma is preferably implemented as follows.
• the pulse generator 5 is switched on and the voltage pulse is applied to the electrodes 6, 7 of the pre-ionizer with a rate of growth greater than 10 u V/s, between which electrodes a sliding discharge is initiated on the surface of the dielectric layer 8.
• Alternative approaches to providing electrical pulses to preionization electrodes including wherein the preionization electrodes are coupled to the main electrodes, either directly or through capacitive, inductive and/or resistive elements for controlling the timing and/or magnitude of the preionization pulses with relative to main pulses, are understood to those skilled in the gas discharge arts.
• a beam of accelerated electrons is generated and, in the system for forming a sliding discharge, a homogeneous plasma layer that serves as a source of radiation having wavelengths from the UV to the X-ray range is formed on the surface of the thin dielectric layer.
• a high stability is achieved in initiating the sliding discharge from pulse to pulse and, in the energy balance of the pulsed sliding discharge at the stage when it is formed, the fraction of energy expended on the formation of the beam of escaping electrons and the generation of X-ray radiation becomes substantial.
• the negative polarity of the trigger electrode 6 with respect to the initiating electrode 7 decreases the voltage amplitude between the electrodes by a factor of several times compared with the case where the polarity is reversed.
• an axially symmetrical system for initiating a sliding discharge with an initiating electrode coaxial with the electrodes of the discharge chamber generated beams of accelerated electrons and irradiation are formed axially symmetrically.
• the beams of accelerated electrons and irradiation are emitted from a region not optically communicating with the discharge chamber and disposed outside it.
• the flux of accelerated electrons and the flux of radiation having wavelengths from the UV to X-ray range is introduced in a controlled manner into the discharge region.
• the radiation and electron beam propagates through the axial aperture in the electrode 3 into that part of the discharge region outside the axis that is optically communicating with the plasma layer of the sliding discharge and the gas in it is pre-ionized.
• a cylindrical plasma envelope is created between the electrodes 6, 7 of the discharge region.
• the magnetic switch opens and the pulse capacitor of the pulse source 1, which is fully charged at this instant, discharges through the electrodes 2, 3 onto the plasma envelope created as a result of the pre-ionization and the flow of the low-current discharge.
• the plasma envelope is compressed by the magnetic field of the current flowing over it and it is confined to the axis of the discharge region for a short time.
• the column of the dense hot plasma that forms on the axis of the discharge region emits short-wave radiation. The usable part of the radiation leaves the discharge region through the aperture in one of the electrodes.
• the surface of the dielectric layer 8 disposed in the region not optically communicating with the axis of the discharge region is not subjected to exposure to hard UV and X-ray radiation, beams of charged particles and plasma fluxes generated on the axis of the discharge chamber 4. This ensures the achievement of a long operational life of the system for forming the sliding discharge.
• the cycle of operation is repeated and, during the time between pulses, the device is cooled by a liquid coolant 9 circulating through the electrodes.
• the plasma envelope created as a result of the pre-ionization is formed on the internal surface of the cylindrical aperture of the dielectric insert, and this stabilizes the pinch-type discharge at stage when it is initiated. This results in an increase in the energy of the short-wave radiation at the final stage of the discharge and in an increase in its stability from pulse to pulse. Since the voltage between the electrodes on the surface of the dielectric insert is minimized as a result of the intense pre-ionization, the probability of its electrical breakdown is sharply reduced. Since the dielectric insert is not an element of the body of the discharge chamber, the mechanical loads in it are minimized. All this makes it possible to ensure a long operational service life of the device if a material is chosen for the dielectric insert that has a high thermal stability, for example silicon nitride Si 3 N 4 .
• the preferred method makes it possible to form a cylindrical plasma envelope that is optimum in shape, dimensions and conductivity stably from pulse to pulse as a result of the pre-ionization, and this results in an increase in the efficiency, average power and energy stability of the short-wave radiation of the gas-discharge plasma.

Applications Claiming Priority (5)

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• 2001
  • 2001-07-04 EP EP01965060A patent/EP1300056A2/de not_active Withdrawn
  • 2001-07-04 WO PCT/EP2001/007658 patent/WO2002007484A2/en not_active Application Discontinuation
  • 2001-07-04 TW TW90116371A patent/TW503669B/zh not_active IP Right Cessation
  • 2001-07-04 AU AU2001285797A patent/AU2001285797A1/en not_active Abandoned

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