1. Field of the Invention
The present invention concerns an apparatus for the efficient excitation of a low pressure gas discharge (plasma). The apparatus generates a highly ionized and charge compensated plasma beam, which is extracted from a very dense low pressure plasma. The properties of the plasma beam (i.e. ion energy, ion current density, composition of the ion beam) can be well controlled and adjusted. A positive or negative particle current can be extracted alternatively in connection with a system extracting electrons or ions selectively.
2. Description of Prior Art
Plasma systems are of high significance for production, processing and treatment of solid state materials in modern technology. There is a special interest in plasma reactors, which produce quasi neutral plasma beams. These plasma reactors, also known as plasma sources, can be used in many applications in plasma processing. These include the growth of thin films, dispersion, etching and cleaning. A plasma beam contains positively charged ions and negatively charged electrons as well as neutral particles. The total positive charge of the ions equals the total negative charge of the electrons and hence the plasma beam is overall electrically neutral. The quasi neutrality of the plasma beam enables deposition and surface treatment of electrically insulating materials, so that no extra apparatus is required for the neutralization of the plasma beam.
Current applications in plasma processing often demand a high fraction of ions in the plasma beam with exactly adjustable ion energy for the formation of the desired chemical bonding. Thus the growth of hard films such as diamond-like carbon (DLC) or cubic boron nitride requires a highly ionized plasma beam with an ion energy of about 100 eV to maximize the fractions of sp3 bonds in the material. These high ion energies are required to overcome the nucleation threshold and to grow a continuous film. The demand for a very high ion fraction in the plasma means that plasmas typically need to be generated at pressures below 10−3 mbar. A plasma excitation supported by a magnetic field is necessary to generate plasmas at these low pressures, so recombination at the walls of the plasma chamber can be avoided. An efficient excitation of the plasma is the basic requirement for a high plasma density and a high growth and etch rate, so that short and cost efficient processing times can be guaranteed. For economic reason and for high efficiency it is also important that the plasma source can process large area substrates.
There are different systems for plasma supported treatment of solid state surfaces. A fraction of these systems is based on the use of high frequency electromagnetic fields for plasma generation. Most of these systems are based on two plates, cathode and anode; the high frequency power is usually coupled into the plasma capacitively through the cathode. In between the plasma and the two plates a voltage drop exhibits. The voltage of this drop is dependent on the area of the electrodes and the amplitude of the high frequency. To maximize ion bombardment, the substrate is placed onto the cathode. The disadvantage of capacitively coupled high frequency systems is the low plasma density, which is a consequence of the inefficient coupling of the high frequency power into the plasma. At typical pressures of 10−3 mbar the particle current onto the substrate consists of only 5% of energetic particles. For many applications which demand a high energy ion dominant process this particle current is not sufficient. Further disadvantages of conventional high frequency plasma sources are the broad ion energy distribution, low growth rates and the dependency of other process parameters on the specific environment.
U.S. Pat. No. 5,017,835 discloses a high frequency ion source for generation of large area ion beams. This high frequency source couples the high frequency power inductively into the plasma. The source uses the electron cyclotron wave resonance excitation of a plasma in a tube shaped plasma chamber, which is clamped between a mounting plate and a top plate. A tunable electrical circuit connects the high frequency generator with the load carrying coil. A weak direct current magnetic field is applied across the plasma. On the mounting plate there is an ion optical system, consisting of multiple electrodes, for the extraction of the ion beam.
U.S. Pat. No. 5,156,703 describes a process for the ablation and structuring of surfaces, production of surface doping and for the production of surface layers by particle bombardment from a plasma. The plasma is generated between two electrodes, one of which is connected to a high frequency power source. A quasi neutral plasma beam is extracted through the powered electrode. The energy of the extracted ions is determined by the amplitude of the high frequency voltage between the plasma and the extraction electrode.
M. Weiler et al described in Applied Physics Letters, Vol. 64 (1994), pages 2797-2799, and in Physical Review B, Vol. 53 (1996), pages 1594-1608 the deposition of tetrahedrally bonded amorphous carbon with a plasma beam source, which creates a capacitively coupled magnetic field supported plasma discharge. This source consists of a large, movable high frequency electrode as well as a smaller grid electrode, which is held on ground potential. Across the plasma a hyperbolic static magnetic field is applied. A positive voltage drop exhibits between electrode and plasma and the grounded extraction grid and the plasma. The electrode can be displaced vertically. This alters the effective area of the electrode and thus the positive voltage drop between plasma and electrode which leads to an adjustment of the ion energy. Hence the ion energy can be adjusted, without any change in gas pressure or high frequency power. The innovation of this source is that the ion energy is adjustable by the internal voltage drop, rather than by applying a DC bias to the substrate. Like other capacitively coupled plasma sources, the plasma density of this system below a pressure of 10−3 mbar is very low.
An improved version of the plasma source was described by M. Weiler in Applied Physics Letters Vol. 72 (1998) pages 1314-1316. The plasma beam is generated by a high frequency (13.56 MHz), inductively coupled plasma discharge with a superimposed transversal static magnetic field. The ion energy can be adjusted by applying a high frequency voltage to an electrode situated behind the plasma.
The U.S. Pat. No. 5,858,477 describes processes and apparatus for the production of protective overcoats on magnetic recording media and other industrial applications, by deposition of tetrahedrally bonded amorphous carbon. One of the systems describes a plasma source, which inductively ionizes a source material in a plasma chamber with an antenna disposed circumferentially about the plasma chamber so as to maintain a plasma in the plasma chamber, the plasma containing ions which comprise carbon. The ions are then energized by applying an alternating potential between a coupling electrode adjacent one end of the plasma chamber and an extraction electrode adjacent another end of the plasma chamber so as to form a stream of ions through the extraction electrode. Magnetic coils for the generation of a rotating transverse magnetic field which homogenizes the plasma beam are placed around the plasma chamber.
One problem of conventional plasma sources is that ion energy and ion current density are not adjustable independently. A further problem of conventional high frequency sources is that a separate high frequency matching network is required. The high frequency matching network feeds the power from the power supply through a cable to the excitation electrode, resulting in power losses. In the matching network of conventional plasma sources there is further the problem that the amplitude of the high frequency voltage and the amplitude of the high frequency current cannot be tuned independently. This means that resonance effects as the electron cyclotron wave resonance or Landau damping can not be used optimally.