Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
2001-04-05
2002-08-06
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S7230ER, C118S724000, C118S716000, C118S7230AN, C156S345370, C156S345430, C156S345440, C156S345450, C427S593000, C427S592000
Reexamination Certificate
active
06427622
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of Hot Wire Chemical Vapor Deposition (HWCVD) deposition as used to produce semiconductor thin film(s) on a substrate member.
DESCRIPTION OF THE RELATED ART
Thin film semiconductors find utility in a variety of electronic devices, that are useful in applications such as active matrix liquid crystal displays, solar panels, and other related technologies.
Conventional HWCVD processes use a thin foil or a small diameter metal wire or filament, such as tungsten, tantalum, or molybdenum, as the hot element of the process. The process' hot element is heated to a high temperature, and a gas such as silane is caused to flow over, or into physical contact with, the hot element. The hot element operates to break down the gas into its constituents. These constituents then migrate to the location of a substrate member whereat a semiconductor film is formed.
Breakdown of a gas such as silane produces, as a by product, a large quantity of hydrogen. It is to be noted that hydrogen dilution may also be useful in the fabrication of certain semiconductor films.
There are several issues which are an impediment to the successful commercialization of the HWCVD technology. First, hot elements such as small diameter wire filaments are normally only about 1 mm in diameter. As a result, the manufacturing longevity of these small diameter filaments is generally limited to the deposition of about a 10-20 micron thick semiconductor film, and often less.
Also, it is believed that the surface of the prior art thin metal filaments are converted into silicides (in the presence of silane), which silicides eventually penetrate the entire depth of the filament. The presence of suicides is believed to promote wire brittleness, and this brittleness effect can be exacerbated by the hydrogen that is present during the deposition process. Therefore, the small diameter filament that is used in prior HWCVD processes usually must be replaced after a very short time period of manufacturing use.
The diameter of prior hot wires or filaments used in HWCVD processes is small, for example on the order of about 1 mm, and the filaments are physically supported at the opposite ends thereof, usually so that the filaments extend in a horizontal direction. These small diameter filaments tend to expand in a linear or horizontal direction during filament heating, and during semiconductor deposition. As a result, the hot and thin filaments tend to physically sag, thereby producing a bowed or arc-shaped filament. This bowed shape of the hot filament promotes manufacturing irreproducibility in the semiconductor film as the substrate to filament distance is thus altered.
A general tutorial on the subject of amorphous semiconductors can be found in the text THE PHYSICS AND APPLICATIONS OF AMORPHOUS SEMICONDUCTORS by Arum Madan and Melvin P. Shaw, Academic Press, Inc., 1988. As discussed therein, amorphous semiconductors can be roughly divided into hydrogenated amorphous silicon (a-Si:H) type alloys, and amorphous chalcogenides, wherein the classification of amorphous semiconductors is determined by the type of chemical bonding that is primarily responsible for the cohesive energy of the material. Also as stated therein, amorphous silicon films have been prepared using numerous deposition techniques such as: Glow Discharge (GD or Plasma Enhanced Chemical Vapor Deposition (PECVD)); CVD; reactive sputtering; and reactive evaporation, wherein glow discharge of a gas can be created by using either a DC or an RF electric field, with RF discharges being operable at lower pressures than DC discharges. This text also recognized that the opto-electric properties of a GD a-Si:H film depends upon many deposition parameters, such as the pressure of the gas, the flow rate, the substrate temperature, the power dissipation in the plasma, and the excitation frequency.
Very generally, a gas phase radical is incorporated into an amorphous silicon film by way of a surface dangling bond that is created via an H-abstraction reaction. This dangling bond diffuses to a lower energy site in a microscopic valley that is within the amorphous silicon film. The gas phase free radical then adsorbs to the surface of the amorphous silicon film, preferably at a high point on the film surface. Surface diffusion then brings the adsorbed radical into the vicinity of a dangling bond, where incorporation into the amorphous silicon film then occurs.
U.S. Pat. No. 4,237,150 to Wiesmann is of general interest for its description of the use of a vacuum chamber to produce hydrogenated amorphous silicon by thermally decomposing a gas containing silane and hydrogen, such as silane, dissilane, trisilane, tetrasilane and the like. In the Wiesmann process, the chamber is pumped down to a vacuum of about 10
−6
torr. The ambient pressure before evaporation is in the low 10
−6
torr range, and it rises into the low 10
−4
range during deposition. A stream of silane gas is directed from a copper tube toward a tungsten or graphoil sheet or foil that is resistive heated to 1400-1600 degrees C. Upon hitting the hot foil, a portion of the silane gas (SiH
4
) dissociates into a mixture of Si, H, SiH, SiH
2
and SiH
3
. A substrate, which may be sapphire, fused quartz or silicon, is located 1-12 inches above the heated sheet/foil, and a flux of silicon and hydrogen is deposited on the substrate. The substrate can be heated, if desired, to a temperature below 500-degrees C., and preferably to 225-325-degrees C., with 325-degrees C. being optimum. At 1600-degrees C., appreciable hydrogen is generated, which hydrogen reacts with the silicon condensing on the substrate, to thereby yield an amorphous silicon hydrogen alloy. An amorphous silicon film of 2500 angstroms was obtained in about 30 minutes.
What is needed in the art of hot wire chemical vapor deposition is a method/apparatus whose improved operation results in a commercially practical production process wherein the lifetime of the hot element, and semiconductor manufacturing reproducibility, are both greatly increased.
SUMMARY OF THE INVENTION
The present invention provides a HWCVD process wherein the process' hot or heated element comprises a plurality of generally parallel, physically spaced, coplanar, and relatively large diameter rods that are formed of an electrically conductive and a non-metallic material that has a high melting temperature (in excess of 2000 degrees centigrade) and that is inert to the constituents such as silicon and germanium, examples being carbon, graphite (the crystalline allotropic form of carbon), electrically conductive silicon carbide (SiC), and high temperature and electrically conductive ceramic.
Advantages achieved by the invention operate to convert prior HWCVD processes to a production-compatible semiconductor deposition technology. Materials other than graphite or graphite-like materials can be employed in accordance with the invention; for example, high temperature electrically conducting ceramic.
In accordance with the present invention, a HWCVD process includes a heated element in the form of a plurality of physically spaced and relatively thick graphite or graphite-like rods. Advantages of the present invention include, but are not limited to:
a) Non-metallic rod material such as graphite, has a melting temperature that is much higher than metallic tungsten; therefore, a wider range of temperature can be used during the deposition process.
b) Because the resistivity of a non-metallic graphite rod is much higher than that of small diameter metal wire, graphite rod heating elements in according with this invention can be constructed in the form of a plurality of relatively thick rods, and not thin filaments as was used in the prior art. This thick rod construction results in a graphite rod heating assembly that is more stable and more durable than prior metal filament heating assemblies.
c) Graphite is chemically more inert than metal; hence, the hot graphite rods of the present invention do not react with silico
Madan Arun
Morrison Scott
Xi Jianping
Hancock E. C.
Hassanzadeh P.
Holland & Hart LLP
Mills Gregory
MV Systems, Inc.
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