Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article
Reexamination Certificate
1999-08-30
2002-05-28
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
C264S039000, C264S129000
Reexamination Certificate
active
06395203
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to furniture such as a fixture or boat used in the manufacture of semiconductor wafers.
Semiconductor wafers are used as diodes, transistors and integrated circuits. The wafer manufacturing process involves a large number of process steps to form a multitude of extremely small and complex electrical circuits on the semiconductor wafer. As technology has progressed, the wafers have been made larger and the feature size of the elements of the circuitry on the wafer's surface have been greatly decreased. As the size of the circuitry has decreased, contamination has become a serious yield problem. Semiconductor standards require extremely low levels of impurities in the semiconductor processing system. Substantially no impurities should be incorporated into the semiconductor material, as even trace amounts can alter electronic semiconductor properties. Even very small sized particle contaminants can cause a short circuit or cause a spacing in the circuitry. A contaminating particle can block processing chemicals from reaching portions of the circuitry. Contaminants can cause incomplete etching in spaces between lines thus leading to an unwanted electrical bridge. Some contaminants can cause electrical failure through induced ionization or by creating trapping centers in gate dielectrics or the like.
Diffusion annealing is one of the critical steps in semiconductor wafer processing. In this step, the wafer surface is covered with a layer of oxide which has been selectively etched in areas where doping is needed. The wafer is then exposed to an atmosphere that transports various dopants onto the wafer surface. It is important that only intended dopants are transported to the wafer, so the cleanliness of the furnace tube and structure supporting the wafer (wafer carrier) is of paramount importance. Additional important properties of the wafer carrier are low thermal mass and resistance to thermal shock, so that the wafer can be heated and cooled rapidly to minimize processing time.
Fused silica, known in the trade as fused quartz is the most commonly used material for handling of semiconductor wafers because it can be produced with relatively low impurity content. However, silica components lose mechanical strength and progressively devitrify with time at the processing temperatures within the annealing furnace. The quartz components distort from the frequent heating and cooling within the furnace and they do not withstand hydrofluoric acid that is commonly used to clean semiconductor processing furniture. Increasingly, silicon carbide-based materials (SiC) are used for construction of the semiconductor-handling furniture. Silicon carbide has technological advantages over quartz. The silicon carbide has better deformation resistance and higher resistance to the acids that are used for furniture cleaning.
Alliegro et al., U.S. Pat. No. 3,951,587 discloses a diffusion furnace having a liner, process tube, paddle or boat that are composed of high purity sintered or recrystallized silicon carbide that has been impregnated with 5 to 30% by weight of high purity silicon metal. The silicon metal renders the components impervious to gases. Alliegro et al. teaches that both the silicon carbide and silicon metal should be as high purity as possible to avoid wafer contamination. The materials should be at least 99% pure and more desirably at least 99.9% pure.
Japanese Patent Publication No. 52-145419 teaches diffusion furnace components of the same general type. The Japanese Patent Publication teaches depositing an impervious high purity coating of silicon carbide onto the surfaces of the components. The coating is vapor deposited applied under vacuum at high temperature to form an impervious coating of silicon carbide.
Foster U.S. Pat. No. 4,761,134 teaches diffusion furnace components such as liners, process tubes, paddles, and boats made up of a pure sintered silicon carbide impregnated with silicon metal. The components are further protected by applying a coating of a high purity refractory material such as silicon carbide, silicon nitride or silicon dioxide. In the Foster patent, the term “high purity” is defined to mean at least 99% pure and preferably at least 99.9% pure.
Foster et al., U.S. Pat. No. 4,998,879 discloses furnace furniture constructed from a matrix of silicon carbide or silicon nitride, with an internal fibrous reinforcement of silicon carbide fabric or carbon or graphite fabric, including carbon/carbon composite converted to SiC. The fabric is of an open structure to allow infiltration and build up of the matrix by chemical vapor infiltration and deposition. The fabric is first formed into a desired final shape by using a temporary binder such as a liquid phenolic heat setting polymer or by use of a thermoplastic softened by a solvent. After shaping and burnout of the binder, the fiber preform is infiltrated and coated in a high temperature reaction chamber. Foster et al. teaches Infiltration and coating by silicon carbide by hydrogen reduction of methyltrichlorosilane and infiltration and coating by silicon nitride by the hydrogen reduction of silicon tetrachloride and ammonia.
These patents stress the need for high purity furniture for an annealing furnace. The patents provide materials that are stated to be at least 99% pure, preferably at least 99.9% pure. Foster et al., U.S. Pat. No. 4,998,879 at column 4, shows materials that include Fe impurities of 22 ppm and 72 ppm. Other impurities are present in amounts on the same scale. Unfortunately, annealing furnace furniture with even these low amounts of impurities substantially contributes to semiconductor impurity. Thus, there is a need to provide a composite with a lower impurity level for use as semiconductor wafer processing furniture.
SUMMARY OF THE INVENTION
The present invention provides a process to produce a low impurity level ceramic composite. The composite produced by the process is characterized by the presence of detrimental elements in concentrations lower than 0.5 ppm. In the process, an annealing furnace is treated with a halide-containing gas at elevated temperature to scavenge metal impurities, and a porous carbon body is infiltrated with silicon within the annealing furnace.
In another aspect, the invention relates to a process wherein a porous carbon body is formed from a carbon felt with a low level of metal impurity and the porous carbon body is infiltrated with silicon.
In still another aspect, the invention relates to semiconductor processing furniture produced by the process. The furniture comprises a high purity silicon melt infiltrated SiC matrix having a metal impurity content of less than about 0.5 ppm.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, semiconductor processing furniture is provided that comprises a high purity silicon melt infiltrated SiC matrix (Si/SiC) having a metal impurity content of less than about 0.5 ppm. The Si/SiC is produced by reacting a porous carbonaceous preform with liquid silicon. Liquid silicon reacts with carbon to form SiC and then fills the remaining porosity (Si/SiC). The carbonaceous preform generally consists of some form of carbon (powder or fiber) with the possible addition of other powders and a binder.
The preferred carbon source for Si/SiC is crushed carbon felt. However, other carbon sources can be used. Carbon felt is produced by companies such as Electrical Carbon Co. and is mainly used as furnace insulation. The carbon felt consists of carbon fibers with relatively low density, which is important for producing a preform with a desired porosity level. In accordance with the present invention, the silicon melt infiltrated SiC matrix purity is enhanced by the utilization of a high purity carbon matrix precursor. The carbon precursor is a carbon felt that has an impurity level of less than about 2 ppm of metal contaminant. The carbon precursor is produced by weaving a fibrous polymer into a felt. The felt is then heat treated in two steps. In a first step, it is heat
Cabou Christian G
Fiorilla Christopher A.
Johnson Noreen C.
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