Coating processes – Spray coating utilizing flame or plasma heat – Metal oxide containing coating
Reexamination Certificate
2001-08-31
2002-11-12
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Metal oxide containing coating
C427S446000
Reexamination Certificate
active
06479108
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to crucibles for use in the preparation of silicon crystals. The invention particularly relates to protective coating layers that are applied to the inner surface or both inner and outer surfaces of a silica crucible. The invention also relates to a process that forms the said protective coating.
BACKGROUND OF THE INVENTION
Crystal growth from melt that is contained in a crucible is a common way to prepare semiconductor materials. The materials for such a crucible are generally fused silica (also known as fused quartz) and graphite. Quartz crucibles are extensively used because of its availability in very high purity and comparatively lower reactivity with molten silicon. For instance, they are invariably used to grow single crystalline silicon in the Czo-chralski (CZ) process, where almost the entire melt is pulled out of the crucible during crystallization of silicon. Also, quartz crucibles are often used in directional solidification technique, the heat exchanger method (HEM), for example, to grow polycrystalline silicon, where the melt is solidified inside the crucible.
Besides the unique beneficial properties of quartz materials, there are a number of difficulties experienced in employing quartz crucibles when the temperature of the crucible is close to or exceeds the melting point of silicon. A major problem associated with the crucible that is composed of vitreous silica is its gradual devitrification, during which cristobalite islands are formed in the vitreous silica surface. The cristobalite islands can be released as particulate into silicon melt and cause dislocations in the silicon ingot. Another problem with quartz crucibles at high temperature is the deformation of the crucible because of the softness of vitreous silica at temperatures exceeding the melting point of silicon. For this reason, graphite susceptors are usually used to support the crucibles. However, the use of graphite susceptors may induce buckling of the quartz crucible during a prolonged holding period at high temperature. Moreover, graphite reacts with silica and forms carbon monoxide. The gaseous carbon monoxide diffuses onto the surface of silicon melt and forms the more stable silicon carbide which contaminates the melt. There is a particular problem related to the crucible used in directional solidification of polycrystalline silicon ingot where the melt is solidified inside the crucible in a controlled manner. At high temperature molten silicon reacts with quartz, and during solidification both silicon and quartz adhere with each other. Consequently, due to difference in the coefficient of thermal expansion, both crucible and ingot crack when they are cooled down.
A number of approaches has been suggested to overcome the above mentioned difficulties. The problems of gradual devitrification and loss of structural integrity of quartz crucibles in CZ process have been tackled by surface treatment of crucibles using several different processing methods. U.S. Pat. No. 4,429,009 describes a process for converting the vitreous silica surfaces of a crucible to cristobalite for passivating and enhancing the stability of the surface. In this process, the vitreous silica surfaces are converted to &bgr;-cristobalite at a temperature of 1200° C. to 1400° C. in an atmosphere containing atomic iodine for a period of about 24 hours. The &bgr;-cristobalite is transformed to &agr;-cristobalite when the surface is cooled back to a temperature of less than 260° C. When the crucible is thereafter reheated in a CZ furnace for crystal growth, the &agr;-cristobalite layer transforms back to &bgr;-cristobalite at temperatures around the melting point of silicon. This &bgr;-cristobalite serves as a protective layer between the crucible and the silicon melt as well as a reinforcing layer on the outer surface of the crucible. A problem with this process is the possibility of cracking the devitrified surface during the phase transformation of &agr;-cristobalite to &bgr;-cristobalite. Particulates formed by the cracking can be released into the silicon melt and cause the formation of dislocations in the silicon ingot.
U.S. Pat. No. 5,976,247 provides another way to form the preventive devitrified cristobalite layer during crystal growth process. By applying a thin layer of devitrification promoter on both sides of a crucible, which is preferably an oxide, hydroxide, carbonate or silicate of an alkaline-earth metal, a layer of substantially devitrified silica is formed on both sides when polysilicon material is melted in the crucible. The substantially devitrified silica layer on the inner surface significantly reduces the release of crystalline silica particulates into the silicon melt, while the layer on the outer surface reinforces the vitreous silica body.
U.S. Pat. No. 4,935,046 describes a quartz crucible having an opaque outer substrate of a quartz glass with a relatively high bubble content and an inner transparent glass layer that is substantially free from bubbles. The crucible is claimed to exhibit a stable dissolving rate of the crucible material into the melt and has a minimum possibility of causing cristobalite on the inner surface during the crystal growth process. The forming of both inner and outer glass surfaces is integrated in the fabrication of the crucible.
Other surface treatment methods have also been proposed to remove electrostatically adhering metallic contaminants from the inner surface of crucibles for the purpose of reducing the incidence of lattice defects and crucible deformation. Japanese Kokai No. 52/038873 suggests the use of a xenon lamp to irradiate the inner crucible surface to remove the metallic contaminants. Another method of subjecting a crucible to electrolysis is disclosed by Japanese Kokai No. 60/137892 to remove alkali metals. Both of these treatments do not address the control of the devitrification process in the presence of molten silicon.
Solutions to the adhesion problem encountered in directional solidification of polycrystalline silicon are mostly related to applying a protective coating layer on the inner walls of the quartz crucible. This thin layer acts essentially as a releasing agent. Among several materials for the coating layer, silicon nitride is the most widely used one. Saito et al. (A Reusable Mold in Directional Solidification for Silicon Solar Cells, in Solar Energy Materials, Vol. 9, (1983), p.337) reported the successful growth of polycrystalline silicon ingot by employing a coating layer of silicon nitride powder on the inner surface of a crucible.
Several different processes have been proposed for the preparation of the silicon nitride layer. A method of pyrolysis is proposed in U.S. Pat. No. 4,741,925, wherein reactive gases are fed through a tube into the crucible while the crucible walls are maintained at a temperature of at least about 1250° C. U.S. Pat. No. 4,565,711 discloses a method of using vapor generator in which granules of a low electrical conductivity and low thermal conductivity silicon materials are disposed between a pair of electrodes. The electric current passing through the granules causes the localized vaporization at the contact points between granules. The vaporized silicon reacts thereafter with the carrier gas nitrogen and forms silicon nitride on the crucible surface.
In an improved crucible coating system, a suspension is prepared by mixing the coating materials, for example silicon nitride, in powder form with a binder and defoamer in water. Such suspension is then spray-painted on the inner surface of the crucible. During spray painting, the inner surface is heated at a temperature of about 43°-65° C. in order to facilitate rapid drying of the coating. The dry release coating is then transferred to a kiln which will heat the crucible to remove the binder by thermal decomposition. The finished coating is intended to possess strength sufficient to maintain coating integrity during loading and manipulation of the crucible into a polysilicon growth furnace. Othe
Chandra Mohan
Costantini Michael
Hariharan Alleppey V.
Wan Yuepeng
Bareford Katherine A.
G.T. Equipment Technologies Inc.
Maine Vernon C.
Maine & Asmus
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