X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2002-08-09
2004-09-07
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S034000
Reexamination Certificate
active
06788763
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of EP Patent Document 99112403, filed Jun. 29, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for generating extreme ultraviolet and soft x-rays from a gas discharge. Preferred fields of application are those requiring extreme ultraviolet (EUV) radiation or soft x-rays at a wavelength ranging from approximately 1 to 20 nm, and in particular around 13 nm, such as in EUV lithography.
The German Reference DE 28 04 393 C2 discloses an electrode arrangement for generating and accelerating charged particles, wherein at least one plasma channel is formed in a space between the aligned openings of the electrodes. The charged particles exit the plasma channel and bombard a solid, while emitting electromagnetic radiation. Produced is visible light or x-rays. The drawback with this solution is that the particle accelerator, disclosed in the German Patent DE 28 04 393 C2, requires a decelerating solid, which is subject to wear and which decreases the lifespan of the device. Furthermore, a photon is generated only at approximately every 10
−4
colliding particles so that the efficiency of the radiation generation is low.
The U.S. Pat. No. 4,771,447 teaches the use of an electrode geometry, which is similar to that of the German Patent DE 28 04 393 C2 and wherein the space between the electrodes is evacuated and gas is injected in batches. In this gas puff operation the plasma is generated everywhere between the two electrodes and contracts then into a single radiation-emitting pinched plasma channel. During this gas discharge work is done on the right-hand branch of the Paschen curve. In contrast, an operation on the left-hand branch of the Paschen curve has the advantage that sparking in the gas volume is possible, thus exhibiting especially low wear. Furthermore, operating on the left-hand branch of the Paschen curve permits work without a switching element between the radiation generator and the voltage supply, a feature that enables low inductive and thus very effective coupling of energy into the plasma. The latter enables in turn for time averaged simultaneous energy input smaller pulse energies at higher repetition rates, a feature that also decreases the wear of the electrode arrangement. Depending on the requirements, an operation on the left-hand branch of the Paschen curve also permits work at relatively low gas pressures, a feature that results in low radiation absorption in the gas discharge system. For these reasons the gas puff is less suitable as the source of radiation.
German Patent DE 197 53 696 A1 discloses a generic device.
FIG. 1
from German Patent DE 197 53 696 A1 depicts an electrode arrangement with the geometry of a single channel pseudo spark switch and exhibits a cathode (
1
) and an anode (
2
) with a gas-filled space (
7
). Each of the two electrodes (
1
,
2
) exhibits one opening (
3
,
4
), by means of which an axis of symmetry (
5
) is defined. In the case of this electrode geometry the gas discharge cannot spread on the smallest path between the electrodes, because in this case the average free path length of the charge carrier is greater than the distance between the electrodes. The gas discharge seeks then a longer path, because only in the case of a sufficient discharge distance is an adequately large number of ionizing bursts to maintain the discharge possible. In this case this longer path can be specified by means of the openings (
3
,
4
), by means of which the axis of symmetry (
5
) is defined. The result is that only a single plasma channel is formed that has the above defined axis of symmetry (
5
) and whose lateral expansion is determined by the borehole limits. Thus, during this gas discharge the plasma ignites inside the cylinder, defined by the diameter of the openings, on the axis of symmetry (
5
). Then the plasma contracts onto a cylinder of smaller diameter. Thus, the plasma in DE 197 53 696 A1 is a pinched channel plasma; that is, both in the sparking phase and later after the plasma has contracted there is always plasma whose exterior dimensions represent a channel. This plasma itself is the radiating medium. The radiation is decoupled axially along the axis of symmetry (
5
) through the openings (
3
,
4
) of the main electrodes.
For commercial reasons, in particular for EUV lithography, an even greater radiation intensity is necessary than is achieved in the solutions known from the prior art. Thus, for example, the plasma channel and thus the electrode spacing can be typically only a few millimeters, thus limiting the intensity of the EUV radiation source. Moreover, in the electrode space the absolutely necessary working gas absorbs a portion of the generated radiation (that cannot be ignored) before it leaves the plasma chamber.
The invention is based on the technical problem of providing a device with a radiation-emitting plasma. With this device it is possible to obtain a particularly high radiation intensity in the EUV range (wavelength approx. &lgr;=10 to 20 nm) and in the soft x-ray wavelength range (wavelength approx. &lgr;=1 to 10 nm).
It was recognized, according to the invention, that the aforementioned problems associated with devices for generating extreme ultraviolet and soft x-rays from a gas discharge can be solved by providing two main electrodes, between which there is a gas-filled space; each main electrode exhibits an opening, by means of which an axis of symmetry is defined, and said main electrodes have additionally means to increase the conversion efficiency.
For applications requiring very high emitted output, thus for example EUV lithography, the input electrical power is limited. Therefore, it is also important, how efficiently this input energy is converted into radiant energy, that is, how high the conversion efficiency is. The radiant energy or radiation, which is of interest here, is supposed to be only that that is actually available to the user, thus that that finally leaves the apparatus.
With the means that are described below to increase the conversion efficiency, the output potential of the gas discharge can be better exhausted and higher radiant power can be obtained, as desired. This is primarily guaranteed by an increase in the plasma particle density. With all of these means the radiation intensity is increased, as desired.
According to an advantageous design of the inventive device, at least one auxiliary electrode, which can be positioned in arbitrary ways in the electrode arrangement, can be provided as the means to increase the conversion efficiency.
There is the possibility of positioning the auxiliary electrode(s) behind the openings of the main electrodes, that is on the side of the main electrode openings facing away from the space. Thus, it has proved to be advantageous to position an auxiliary electrode, located at a positive potential, behind the negatively charged main cathode. This wiring causes, first of all, an increase in the sparking field strength or the sparking voltage of the gas discharge. Since it would simultaneously result in more energy coupled into the gas discharge, the sparking voltage is held constant to compensate for this effect. According to Paschen's law, the sparking voltage is a function of the electrode spacing and the gas pressure. Therefore, the sparking voltage is held constant by maintaining a higher gas pressure when the device of the invention is operated on the left branch of the Paschen curve, because usually a higher gas pressure decreases the sparking voltage. However, a higher gas pressure leads to a higher density of plasma particles or there are in total more particles that contribute to the emission of radiation. The increased emission of radiation occurs at the same energy fed into the gas discharge and thus at greater conversion efficiency.
Furthermore, it is possible to position the auxiliary electrodes between the main electrodes. Then the auxiliary electrodes must exhibit a
Bergmann Klaus
Lebert Rainer
Neff Willi
Rosier Oliver
Bruce David V.
Crowell & Moring LLP
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Thomas Courtney
LandOfFree
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