1. Field of the Invention
The present invention is directed to a low pressure plasma generator with a localizable plasma combustion chamber.
2. Discussion of the Background
Treatment with low pressure plasmas is an important new method for modifying the surfaces of solid bodies. The surfaces can be, e.g., etched, i.e., partially removed, or activated, i.e., in an energy-rich state that is suitable for extensive modifications, or are coated by bonding gaseous substances. For all of these methods, the surface to be modified must be subjected to a plasma. As is well-known, a gas comprising excited molecules, radicals or ions is referred to as a plasma.
Plasmas can be generated at low gas pressures by means of microwave radiation. A prerequisite for the formation of a plasma is an adequately high field strength of the radiation. However, the field strength is the greatest in the immediate vicinity of the source of radiation and decreases with increasing distance therefrom. Therefore, the plasma may exist only in the vicinity of the source of radiation.
The uniform treatment of large surfaces or surfaces with complicated shapes with a plasma causes considerable difficulties. For reasons relating to their design and their energy supply, available sources of radiation cannot be disposed at any point and at any position in a low pressure chamber. Similarly, the surface to be treated cannot be moved to specified locations in the plasma combustion chamber. Therefore, the surfaces to be treated cannot be located near the plasma source.
The ability to ignite and maintain a plasma at a predetermined place, where it is supposed to unfold its technological effect, is called localization. The precise localization of plasma is of great importance primarily when a large surface is to be treated uniformly. This goal can be largely reached if one can succeed in localizing a plasma linearly and moving the plasma uniformly over the surface to be treated. For this purpose, either the plasma can be localized stationarily and the substrate can be moved relative thereto or the substrate can remain stationary and the plasma is moved at right angles to its longitudinal extension. However, just the linear localization of a uniform plasma causes considerable difficulties.
The literature reports on various possibilities for localizing microwave plasmas. These include, among others, the ignition of the plasma behind the inlet window for the microwave (Wertheimer et al., Thin Solid Films, 115 (1984), 109), the ignition of primary transmitting aerials (Alcatel DVM, 92240 Malakoff, France, machine GIR 820), the ignition by means of local pressure differences in a vacuum chamber (IKV reports, Mr. Ludwig) and the magnetic confinement with or without the utilization of an electron cyclotron resonance absorption (EP-A 279 895). Some of these possibilities were also used for localizing large area plasmas.
The use of surface waveguide structures, which are mounted outside the vacuum apparatus but which are in front of a microwave permeable window, allows a large area plasma to be ignited (Kieser et al., Thin Solid Films, 118 (1984), 203).
All of the described methods of localization have drawbacks that stand in the way of their practical application. The drawback of the arrangement described last is that the plasma burns only directly behind the window and cannot be moved within the vacuum to any arbitrary place therein by the operator. In the case of a coating plasma, the window is also coated, a feature that can lead to an absorption and reflection of the microwaves depending on the properties of the deposited layer. Long setting-up times then become necessary owing to the repeated cleaning or exchanging of the windows.
An ignition at a primary transmitting aerial yields a plasma whose intensity in most cases exhibits local inhomogeneities owing to the wavelength of the transmitting frequency (e.g., with a period of 12 cm at a frequency of 2.45 GHz). In this arrangement, compensating devices such as a mechanical movement of the aerial can hardly be used owing to the design of primary transmitting aerials
The ability to localize a plasma by means of local pressure differences is limited to the coating of largely closed bodies. This is a suitable method for coating bottles internally. However, in trying to process a flat substrate with such a system grave technological problems arise.
One successful method is magnetic confinement. This process is used, e.g, in the sputter technique. However, an effective magnetic confinement in achieved only if the gyration radius of the charged particles in the plasma with respect to the free path cannot be ignored. This is the case for conventional permanent magnets made of ferrite only below pressures of about 0.1 mbar.
With plasmas of higher pressures--of up to a few millibars--higher etching and deposition rates can be obtained during the etching and coating process. For this reason there is a need for a method for plasma confinement that also has a good localizability at higher pressures and thus allows homogeneous etching or formation of layers at simultaneously high etching and deposition rates.