Constricted glow discharge plasma source

Details Constricted glow discharge plasma source Constricted glow discharge plasma source

1999-02-19

2002-05-14

Bettendorf, Justin P. (Department: 2817)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

C315S111310, C118S7230DC, C219S121520

Reexamination Certificate

active

06388381

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to structures and methods for producing a linear array of streaming plasma with a very low level of contamination and low energy ions in a vacuum processing chamber, e.g., such as in those used to process compound films (e.g., oxide films), to synthesize thin films on the surface of a substrate.
BACKGROUND OF THE INVENTION
In a vacuum processing chamber ion sources are used to change the properties of substrate surfaces. Gas is fed through an electric field in a vacuum chamber to excite the gas to a plasma state. The energized ions or excited neutrals (such as excited atoms and disassociated molecules) of gas constituents bombard the surface of the substrate. The effect that the ions have on the surface is dependent on their atomic constituents and their energy (in one example to treat a surface by providing active oxygen).
Low-energy, ultra-clean flows of plasmas are required, to obtain a crystalline film with high crystal quality as required in the case of gallium nitride (GaN). To avoid ion damage the ion energy must not be too high, for instance, an ion with a kinetic energy of 20 or more eV for GaN must be considered a high energy ion. When an ion hits the surface during crystalline film growth and its kinetic energy is high enough to displace atoms which are already in place, a defect is created. So the materials grown with energetic plasma streams tend to have lots of defects. The defects create “carrier” densities for semiconductors such as GaN, leading to the creation of a material with n-type doped properties. In these cases it is difficult to obtain a p-type doped material. To overcome this problem a source of low energy ions is needed.
A nitrogen plasma flow or low-energy ion beam (ion energy of order 30-50 eV) is usually obtained by using a Kaufman ion source or an Electron-Cyclotron-Resonance (ECR) plasma source.
The drawback of a Kaufman source is that a hot tungsten filament is used as a cathode. The tungsten filament delivers large quantities of electrons by thermionic emission, which sustain the low-energy non-self-sustained arc discharge, but tungsten atoms evaporate from the filament and can be found in the stream of plasma and the growing films. This is not acceptable for instance in the growth of GaN because the stream is not clean and the film properties are altered by the impurities, and is not acceptable for growth of oxide films, because the tungsten filament will oxidize rapidly.
An ECR plasma source necessarily operates with a high magnetic field to fulfill the resonance condition of microwave frequency and electron cyclotron frequency. Typically, the standard microwave frequency of 2.45 GHz is used, leading to a required magnetic field of 875 Gauss. The gaseous microwave plasma is produced in the region of the resonance magnetic field. The ions gain kinetic energy when leaving the location of the high magnetic field and streaming towards the substrate. When a plasma is made this way there is a significant energetic component in the ion energy distribution, i.e., ions having 30-50 eV of kinetic energy are abundant. This energy is too high for the growth of a high quality crystalline films. Although ECR plasma sources are cleaner than Kaufman sources, ion damage is observed in growing films due to the relatively high ion energy. One way to overcome this energy problem is to bias the substrate electrically, to deflect the energetic ions but in doing so the low energy ions are also deflected and the growth rate decreases.
A better (cleaner) source of low-energy gaseous ions is needed to deposit high quality thin films on substrates in both research and commercial applications. This need includes not only MBE-type but also IBAD-type deposition of thin films (MBE=molecular beam epitaxy; that is film growth with reactive, activated gases; IBAD=ion beam assisted deposition, that is film growth assisted by the moderate kinetic energy of ions such as argon).
A plasma discharge chamber usable for an ion beam source, electron beam source, and a spectral light source was introduced by V. I. Miljevic and is described in several papers (Rev. Sci. Instrum. 55 (1984) 931; Rev. Sci. Instrum 63 (1992) 2619) (also see U.S. Pat. Nos. 4,871,918; 4,906,890). A preliminary explanation of the working principle of the discharge is given in a paper published in
Plasma Sources Science & Technology
, Vol. 4. (1995) p.571.
In one configuration as shown in
FIG. 1
, a gas flows through a discharge chamber
20
which consists of a metal cathode
22
(grounded) and a metal anode
24
(positively biased) separated by a Teflon insulator
26
. A flange
28
holds the anode
24
in place and seals it against the cathode flange using a series of O-rings
30
. By applying a sufficiently high voltage (500 V or more) to the electrodes, a glow discharge ignites in the flowing gas. The gas is introduced through an opening
40
in the cathode, and leaves the source through a small aperture
38
in the anode. A high positive voltage is applied to an extraction electrode
32
leading to acceleration of the ions from the source
20
in the direction shown by arrow
36
. A high negative voltage would accelerate electrons, turning the source into an electron beam source. An electromagnetic coil
34
produces a magnetic field around the anode
24
focusing the ions in an ion beam (whose direction is shown by the arrow
36
) departing from the discharge opening
38
in the anode
24
. Gas pressure supplied to the gas inlet
40
provides the motive force to discharge the ions from the discharge chamber
20
. The extraction electrode
32
and magnetic coil
34
assist in accelerating and focusing the ion discharge into a beam. The anode
24
is insulated from the grounded cathode
22
and grounded support flange
28
by a thin film of ceramic coating deposited on the respective mating surfaces of the anode
24
.
The feature which distinguishes this kind of discharge from an ordinary glow discharge is the actual exposure of a very small area of a large cross section anode facing the cathode, to the gas. In the Miljevic configuration this effect is obtained by blocking nearly all of the anode
24
by using an insulator
26
, except for a small discharge aperture. This discharge aperture forms a small hollow anode, and Miljevic named the discharge “hollow-anode discharge”. Related research has found (
Plasma Sources Science & Technology
, Vol.
4
.
(
1995
) p.
571
.
) that a voltage drop appears in front of the discharge opening, accelerating electrons which gain enough energy to ionize the working gas through inelastic collisions. A bright “anode plasma” forms in the anode channel, and this plasma is blown out by the gas flow in the channel due to the pressure gradient between the inside and the outside of the source. The “anode plasma” does not form when there is no blocking or covering such a large anode.
SUMMARY OF THE INVENTION
A structure and method according to the invention involves using a special type of glow discharge plasma source, namely the so-called “Constricted Glow Discharge Plasma Source”. The configuration of the prior art plasma source is adapted in a way that it delivers a downstream gaseous plasma of low contamination and very low kinetic energy, well-suited for the growth of high-quality thin films. The source can operate in a wide range of parameters, in particular it can also work at very low and very high gas pressures. It has been found that the anode does not necessarily need to form a small opening which is located next to the blocking insulator. The “hollow anode discharge” is just one possible configuration which makes use of a constriction element. A configuration according to the invention includes a special type of glow discharge characterized by a constriction between cathode and anode. The inventor(s) named this type “constricted glow discharge,” and the derived downstream plasma source “constricted glow discharge plasma source.”
The constricted glow discharge plasma source includes a discharge chamber wh

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