Radiant energy – Ion generation – Field ionization type
Patent
1989-06-26
1991-02-12
Anderson, Bruce C.
Radiant energy
Ion generation
Field ionization type
31511181, H01J 304
Patent
active
049926659
DESCRIPTION:
BRIEF SUMMARY
The known electrostatic Kaufman ion source (US PS 3 156 090) provides for ion generation by electrostatic fields. For this purpose an electron emitter (thermionic cathode) and metallic electrodes are required in the plasma chamber. Contamination of the plasma by sputter products from the metallic electrodes is, however, undesirable.
Further developments, for example those of Sakudo et al., resulted in ion sources that operate without thermionic cathodes. Such ion sources are fitted with a 2.4 GHz magnetron, a waveguide, a short circuit slider member, DC magnets and extraction electrodes (US PS 4 058 748). It is difficult, however, to provide the ionization chamber with a vacuum-tight seal with respect to the magnetron. For this purpose discs consisting of a dielectric such as boron nitride have hitherto been proposed (US PS 4 058 748, 4 316 090, 4 393 333, 4 409 520). There is furthermore the disadvantage that the contact of the gas with the metallic waveguide results in the production of sputter products. As a result of the contamination of the gas, undesired implants or dopants may appear on the substrate.
In another known design, the substrate is treated directly in the ionization chamber on a substrate platform (US PS 4 438 368).
Compared with these known plasma ion sources a novel device is now disclosed that overcomes the above mentioned disadvantages in an elegant and simple manner and at the same time ensures an especially good yield of ions.
In accordance with the invention a filamentless magnetron-ion source is provided that comprises the following elements:
The ions are generated in the magnetron-ion source of the invention by means of an electrode-free microwave gas discharge, which maintains itself, so that no electrode emitter (thermionic cathode) is required. Compared with an arrangement having a thermionic cathode and anode, the thermal load originating from the ion source and acting on the substrate or a material specimen is drastically reduced.
The ionization chamber is surrounded by a quartz dome, preferably in the shape of the cap of a sphere, which shields the ionizing gas from all of the components of the waveguide. The result of this is that a plasma completely free of sputter products is obtained in an elegant manner. This is particularly important since the gas obtained is preferably used for etching substrates. A plasma contaminated by impurities would result in the undesirable implantation of sputter products in the substrates.
Another advantageous feature of the ion source of the invention is the incorporation of a lambda/4-short circuit slider member. This provides an excellent sliding contact between the waveguide and the cylindrical cavity resonator and causes the resonator to oscillate in optimum manner. In this manner impedance and frequency are fixed and the desired degree of ionization is produced.
As regards the other components, the ski--ed person can retain those of the ion sources already described in the literature.
The microwave energy (2.4 GHz) produced in a magnetron (microwave generator) is coupled into a tuned cylindrical resonator either by way of a waveguide or a coaxial line, as desired, with the aid of a lambda/4-short circuit slider member. By varying its length in the axial direction, the cylindrical cavity resonator may have its resonant frequency matched to the magnetron frequency. A gas, for example argon, flows into the ionization chamber surrounded by the quartz dome and, for the ionization, the gas is acted upon by an electric alternating field. The gas can be introduced by way of an annular gas inlet system.
In one embodiment, the ionized gas is drawn off in the form of directed ion beams by way of a mu-ti-aperture electrode extraction system, and directed onto the material to be etched, which is arranged in an adjacent vacuum chamber. The extraction system may consist of two or three successive graphite electrodes. Graphite is the preferred material since it is inert and also, with the passage of time, causes no deposits on the surface of the quart
REFERENCES:
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patent: 4507588 (1985-03-01), Asmussen
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patent: 4788473 (1988-11-01), Mori et al.
patent: 4857809 (1989-08-01), Torri et al.
"The Experimental Test of a Microwave Ion Beam Source in Oxygen", Asmussen et al., J. Vac. Sci. Tech. B, vol. 5, No. 1, Jan. 1987, pp. 328-332.
Anderson Bruce C.
Technics Plasma GmbH
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