Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1993-04-02
2001-10-02
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192120, C204S298060, C204S298080, C204S298090, C204S298110, C204S298140
Reexamination Certificate
active
06296743
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the plasma vapor deposition of a layer of an electrically insulating material on a substrate by reactive sputtering. More particularly the inventive method and apparatus permits the sputter coating to be done using a DC plasma discharge.
2. Description of the Background Art
Sputter coating is a widely used technique for depositing a thin film of material on a substrate. In one form of this technique, known as DC sputtering, positive ions from a plasma discharge formed between an anode and a target cathode are attracted to and strike the target, dislodging the sputtering atoms from the target surface. Some of the dislodged atoms fall on the surface of the substrate and form a coating. In reactive sputtering a gaseous species is also present at the substrate surface and reacts with, and in some embodiments combines with, the atoms from the target surface to form the desired coating material. This material is also deposited on any other surface exposed to the sputtered atoms. It is recognized in the prior art that if the coating is an electrically insulating material, such as a metal oxide, the build up of the material on other parts of the sputtering apparatus can cause problems. In particular, the build up of an insulating coating on the anode interferes with the ability of the anode to remove electrons from the plasma, as required to maintain the plasma's charge balance. This destablizes the plasma and interferes with controlled deposition. As a result, it is common to use a different sputtering technique, RF sputtering, to deposit layers of insulating materials. However, RF sputtering is a less efficient, less controllable and more expensive process than DC sputtering.
Pinarbasi met this problem when trying to deposit layers of hydrogen-containing amorphous silicon. His work was published as a 1989 doctoral thesis for the University of Illinois at Urbana-Champaign entitled “Growth, Properties and Electrical Stability of DC Magnetron Reactive Sputtered Hydrogenated Amorphous Silicon Thin Films” and in
Thin Films,
171 (1989 ) Pp. 217-233 . To reduce this effect, Pinarbasi shielded the anode with a positively biased anode shield to carry the electron current from the plasma. This reduced the problem enough to permit deposition of the experimental films. (See especially Page 26 and
FIG. 7
) But a poorly understood transient effect, reported by Pinarbasi, indicates that the problem was not completely under control. This problem could seriously impact long term use of this process, such as in a production setting.
SUMMARY OF THE INVENTION
In the present invention a secondary anode electrode is included in the sputtering chamber and placed so as to be shielded from the stream of sputtered atoms. This secondary anode does not acquire a significant coating of electrically insulating material and retains its ability to attract enough electrons from the plasma to maintain the plasma's charge balance. This permits efficient use of DC sputtering for deposition of insulating materials and avoids the commonly considered need to use the less efficient and less controllable RF Sputtering technique.
In a DC Sputtering apparatus, an electric field is produced between a primary anode and a target holder, holding a target of the material to be sputtered. The target and target holder form the cathode. The field ionizes the sputtering gas, held at well below atmospheric pressure in the sputtering chamber and forms a plasma discharge. Positive ions from the plasma are attracted to the target, strike the target and dislodge atoms from the target surface. The atoms are emitted in all directions from the target surface. Since these atoms are almost entirely charge neutral, their paths are straight and not affected by electric or magnetic fields. They deposit on and coat any surface on which they fall. They fall on the substrate to be coated, on the walls of the sputtering chamber and on the primary anode. If the deposited material is electrically insulating, for example, if the sputtered atoms chemically combine with reactive gas species also present in the chamber and at the primary anode, an insulating coating on the primary anode interferes with its ability to absorb electrons from the plasma. The inventive placement of a secondary anode, shielded from the stream of sputtered atoms, prevents deposition of this coating or reduces deposition below the amount that represents a significant problem. The secondary anode can, for example, be placed behind a shield member, in a recess, or behind a shoulder, so long as it is out of geometric line of sight of the target surface. The effectiveness of this secondary anode in attracting electrons from the plasma can be enhanced by electrically isolating it from the primary anode and biasing it positively with respect to the primary anode.
REFERENCES:
patent: 4038171 (1977-07-01), Moss et al.
patent: 4619755 (1986-10-01), Hessberger et al.
patent: 4871434 (1989-10-01), Munz et al.
patent: 5126032 (1992-06-01), Szczyrbonski et al.
patent: 3612721 (1987-10-01), None
patent: 4042289 (1992-07-01), None
patent: 0534066 (1993-03-01), None
patent: 2568269 (1986-01-01), None
M. Pinarbasi, “Hydrogenated Amorphous Silicon Films Deposited by Reactive Sputtering”,Thin Solid Films(1989), pp. 217-233 at 220.
M. Pinarbasi, “Growth, Properties, and Electrical Stability of DC Magnetron Sputtered Hydrogenated Amorphous Silicon Thin Films” University of Illinois at Urban-Champaign Doctoral thesis, 1989, pp. 25-27 and Figure 7.
Applied Materials Inc.
McCarter & English
McDonald Rodney G.
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