Extreme Ultraviolet in Current Quenching
Current Quenching Phenomenon in Pseudospark and Capillary Discharges and Mechanism for Generation of an Extreme Ultraviolet Radiation from the Discharge Plasmas
Tech Area / Field
- PHY-PLS/Plasma Physics/Physics
8 Project completed
Senior Project Manager
Tyurin I A
Siberian Branch of RAS / Institute of High Current Electronics, Russia, Tomsk reg., Tomsk
- VNIITF, Russia, Chelyabinsk reg., Snezhinsk
- Philips Extreme UV GmbH, Germany, Aachen\nUniversity of Erlangen-Nurnberg, Germany, Erlangen\nFraunhofer Institut Lasertechnik, Germany, Aachen\nOld Dominion University/College of Engineering and Technology, USA, VA, Norfolk
Project summaryThe Project is addressed to the problem of research and development of the new methods for generation of extreme ultraviolet radiation (EUV, 50-10 nm) and soft X-ray radiation (10-1 nm) from the gas discharge plasmas. The main attention is concentrated on the investigations of a high current pseudospark discharge and on a capillary discharge.
The short wavelength radiation is widely used in a variety of technologies and scientific researches, such as:
– X-ray and UV spectroscopy, in particular, as applied to organic molecules and biological objects;
– high resolution microscopy of the biological objects;
– investigations in the field of solid state physics and physics of thing films;
– microlithography for semiconductor devices;
– technology of X-ray and EUV optics.
Currently considerable progress has been achieved in generation of EUV on the basic of synchrotron radiation. However, the synchrotrons are extremely expensive installations and only a limited number of the devices with the channels for output of radiation are available in the scientific laboratories and in industry. In this connection, the development of the alternative devices of a moderate cost is of a great importance.
Generation of EUV and soft X-ray radiation on the basis of magnetic compression of a plasma column are being studied intensively for the last 20 years. The problem is traditionally solved with a use of a plasma focus method, a capillary discharge or a gas puff Z pinch. Typical installations are powered by the pulsed generators with a current from hundred kiloamperes and more in a microsecond range of the pulse duration at energy stored in a capacitor bank larger than 1 kJ. Due to a high self-magnetic field, the plasma is compressed and accelerated to the axis of symmetry of the discharge system. At a stage of highest compression a high-temperature narrow plasma channel appears at the discharge axis and the plasma generates EUV or soft X-ray radiation in the lines of the multiple charged ions.
The current waveform for such systems has a specific feature. During the first quarter period, when the current approaches to its maximum, a current quenching phenomenon is observed. The current sharply decreases by 20 to 30 percents, and this stage of the reduced current lasts from 10 to 100 ns. The maximum output in EUV radiation coincides just with this stage, so it is conventionally accepted that a power input in the plasma reaches its maximum value in the stage of the current quenching.
In general case the sharp decrease in the current means that an effective resistance of the discharge gap is sharply enhanced. In view of the fact that an inductance of the compressing plasma channel increases with time, the effective resistance includes in itself both the term associated with increasing the inductance and the term related to a conductivity of the plasma-filled gap.
The overwhelming majority of investigators proceeds from the concept that the plasma column is compressed so fast and for such a small diameter that an increase in the inductance becomes the main reason for limiting the current and even for the current quenching. Correspondingly, the process of increasing the plasma temperature is treated as a transformation of the kinetic energy of the accelerated plasma into the inner energy of the plasma at the discharge axis. An alternative approach, which would be based on a supposition that the current quenching is mainly due to a sharp decrease in the gap conductivity, is not taken into account in most publications related to traditional Z-pinch with an extremely high stored energy.
On the other hand, considerable interest has recently been generated in the installations for a moderate energy in a single pulse that would be capable of working with a high pulse repetition rate. The prominent application of such systems is EUV lithography for semiconductor devices, and the most suitable wavelength for this application corresponds to about 13 nm. In this connection a development of new designs or a use of another kind of discharge, which would be intentionally directed to solving this problem, is of a great interest by now.
One of the new approaches is the application of the pseudospark discharge, or strictly to say of the pseudospark switch electrode system, for generation of EUV radiation. Such system schematically consists of two plane-parallel electrodes with the axial bore holes. A typical range of operating pressure (0.1 – 0.01 Torr) corresponds to the left branch of Paschen curve. The bore hole diameters, the main gap spacing and the thickness of the flat part of the cathode are comparable to each other. Therefore, from the viewpoint of the discharge operation mode, we can speak of a high-current hollow cathode discharge of a kind.
Proceeding from the specific features of the pseudospark discharge we can point out the following advantages of this discharge as compared to another discharge systems. In an initial stage of development the discharge column arises at the axis of symmetry of the electrode system and there is no need to use special methods (for instance surface discharge) to form a primary cylindrical discharge channel. Beside that, the pseudospark switches have been peculiarly developed to operate with high pulse repetition rate. Correspondingly, some technical solutions for this mode of operation are already available as applied to the pseudospark switches.
The first publications, which describe the use of the pseudospark electrode system in the source of EUV radiation are the following:
1. R. Lebert, K. Bergmann, G. Schriever, W. Neff, “A gas discharge based radiation source for EUV-lithography”, Microelectronic Engineering, 1999, v. 46, pp. 449 452;
2. K. Bergmann, G. Schriever, O. Rosier, M. Muller, W. Neff, R. Lebert, “Highly repetitive, extreme-ultraviolet radiation source based on a gas-discharge plasma”, Appl. Optics, 1999, v. 38, no. 25, pp. 5413 5417.
The papers demonstrate that the pseudospark discharge in oxygen and xenon at an extremely low stored energy (about 1 J) serves as a source of EUV radiation. A maximum current for the set-up was up to 25 kA with the first half-period duration of about 60 ns. The observed radiation in a range from 10 to 18 nm corresponds to beryllium and lithium like ions. For a wavelength in a vicinity of 13 nm an efficiency of 0.1% has been obtained at a pulse repetition rate up to 150 Hz.
This result has called attention of different scientific groups, including our groups, both from the viewpoint of a physical mechanism for generation of EUV radiation and of the problems of optimal technical solutions for installation. This Project proposal is mainly concentrated on the physical aspects of the discharge burning and generation of EUV radiation from this discharge.
The primary interpretation of the experimental data in the above papers was based on the traditional notion about magnetic compression of the gas discharge plasma. In our opinion this approach should be undergone to essential corrections. For a low stored energy and for short pulse duration the mechanism of the plasma heating and the current quenching due to magnetic compression of the plasma column does not look evident. Our preceding experience in investigation of the pseudospark discharge as applied to switches allows us to state another hypothesis for treating the discharge phenomena.
The current quenching phenomenon seems to be associated with the inner features of the pseudospark discharge and the mechanism of this phenomenon is not directly connected with the magnetic compression of the plasma column. Just during the development of the quenching the voltage on the gap sharply increases up to several tens of kilovolts. This voltage turns out to be applied to the near cathode region which provides for formation of a high-energy electron beam and the dissipation of the beam energy in the plasma. As a result of this process the power input in the plasma and the plasma temperature sharply increase which becomes the reason for generation of EUV radiation. The described approach resemble that for so-called plasma erosion opening switch where the current quenching is also associated with an increase in the gap resistance, but not with an increase in the inductance of the plasma channel.
It should be stressed that the process of magnetic compression of the plasma can go in parallel with the above-described phenomenon, however we do not consider the plasma constriction as the primary reason of the quenching. If the gas discharge column is already compressed to the instant of quenching then the conditions for power input from the electron beam to the plasma becomes more favorable. Nevertheless, the main reason of the sharp increase in the plasma temperature is the power input due to applying the high voltage directly to the gap, but not due to the transformation of kinetic energy of the compressed plasma into an inner energy of the discharge column. This is the working hypothesis on which this Project proposal is concentrated.
The principal objectives can be stated as follows:
– to investigate a mechanism of the current quenching phenomenon in the pseudospark and the capillary discharges, and to clarify a correlation between this phenomenon and generation of extreme ultraviolet radiation;
– to elucidate a mechanism for generation of the EUV radiation in the conditions of extremely low energy input in plasma of the low pressure gas discharges;
– to obtain the recommendations for the discharge operation conditions, which provide of an increase in efficiency of the EUV radiation sources based on the above discharges.
The group from Institute of High Current Electronics has a great experience in the experimental and theoretical investigations of the pulsed low-pressure gas discharges (in particular, the pseudospark discharge) and in development of the high pulse repetition rate pseudospark switches. During the last several years this group has been working in the close collaboration with the group of Prof. K. Frank (University of Erlangen, Germany). In turn, the group from Germany is recognized to be one of leaders in the investigations of the pseudospark discharge. Another foreign collaborator is the team from Fraunhofer Institute of Laser Technology (Germany, Dr. K. Bergman), which pioneered the application of the pseudospark discharge for EUV sources. The third collaborator is the Philips Extreme UV GmbH (Dr. J. Pankert). This department of the Philips Corporation has been intentionally established for development of the sources of EUV radiation and for different applications of the sources. The obtaining of EUV radiation on the basis of the pseudospark discharge is considered in this Corporation as one of the prominent methods.
The Russian group from VNIITF has the preliminary experience in the investigations of Z-pinch based on exploding wires, gas puff and capillary discharge. The specific feature of the installation SIGNAL, which is used in this group, is that the current pulse formes in the system with inductive energy storage where the pulse compression is provided with a plasma erosion opening switch. As compared to another Z-pinch installations with a microsecond powering, the output current pulse has a short duration (10 – 200 ns) and a current from 20 to 100 kA. Just this range of parameters corresponds to the tasks of the Project. First, we are going to use this installation for checking the main working hypothesis of the Project as applied to capillary discharge, and second, to investigate a pseudospark electrode system in this particular installation.
The approach and the methodology are based on experimental investigations and elaboration of the theoretical models. As a whole both groups have most experimental equipment and the corresponding experimental methods to carry out the work on the Project.
The new knowledge, which is going to be obtained, relates not only to the above-described particular cases. The quenching is characteristic of a variety of the low pressure pulsed discharges (in the plasma erosion opening switches, in the classical vacuum plasma-filled diodes, in the plasma- based electron and ion beam sources, in the thyratrons with a hot cathode and the like). We hope that the main ideas developed during this Project will be applicable to another discharges. So, one of the promising directions of the succeeding activity is to give interpretation of the current quenching phenomenon as applied to different kinds of gas discharge in the framework of unified concept.
In spite of the fact that the Project covers mainly the basic research, the results obtained will be definitely useful in designing the EUV sources. Close collaboration with the foreign scientific groups will promote the integration of the weapon scientists into international scientific community. Currently the research and development of the new methods for generation of EUV radiation from the gas discharge plasmas is of a great international interest and we hope that due to this Project we contribute to solution of this problem.
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