Low-contamination impact tool for breaking silicon

Abstract: The present invention is a low-contamination impact tool especially useful for the breaking of semiconductor-grade silicon into pieces. The low-contamination impact tool comprises a core forming a handle portion (1) and a head portion (2), the head portion (2) contacting a tungsten carbide alloy striking element (4). The core is encapsulated in a synthetic resin material (3). The preferred synthetic resin is urethane.

Description: The present invention is a low-contamination impact tool especially useful for the breaking of semiconductor-grade silicon into pieces. The low-contamination impact tool comprises a core forming a handle portion and a head portion, the head portion contacting a tungsten carbide alloy striking element. The core is encapsulated in a synthetic resin. High density, integrated, electronic circuits require wafers of monocrystalline silicon of high purity. Of particular problem is transistional metal impurities including among others copper, gold, iron, cobalt, nickel, chromium, tantalum, zinc and tungsten and impurities such as carbon, boron, phosphorous, aluminum and arsenic. These impurities, even in small quantities, introduce defect sites in semiconductor grade silicon which can ultimately result in degraded device performance and limit circuit density. Typically, a polycrystalline silicon of high purity is formed by chemical vapor deposition of a high purity chlorosilane gas onto a heated silicon substrate. The resulting product is rods of polycrystalline silicon. The polycrystalline silicon rods must be further processed to produce a monocrystalline silicon from which silicon wafers can be cut. A significant portion of the monocrystalline silicon required by the semiconductor industry is produced by the well known Czochralski process. In a typical Czochralski type process, silicon pieces are melted in an appropriate vessel and a monocrystalline silicon seed crystal is used to draw a monocrystalline rod of semiconductor-grade silicon from the melt. Control of this crystal growth process requires that the silicon pieces added to the melt containing vessel be within a defined size range. Therefore, it is necessary that the polycrystalline silicon rods formed during the chemical vaporization deposition process be broken into pieces of suitable size. The low-contamination impact tool described as the present invention is especially useful for breaking silicon into pieces. The inventors have discovered that during the breaking process, the silicon can be significantly contaminated by contact with the surfaces of the breaking instrument, including the handle and striking surfaces. The present invention reduces the contamination associated with the breaking instrument by covering all surfaces, but the striking surface, with a low-contamination synthetic resin. The exposed striking surface is formed of a tungsten carbide alloy, which is also of a low-contamination nature. Maeda, U.S. Patent No. 4,697,481, issued October 6, 1987, describes a hammer including a head core and a handle core, where the head core and handle core are imbedded in a resin, with the exception of one end of the head core which serves as a striking surface. Maeda describes the striking surface as being made of a ferrous metal. Porter, U.S. Patent No. 3,640,324, issued February 8, 1972, describes a forged steel hammer head having a striking face provided with a layer of electrodeposited tungsten carbide. The tungsten carbide layer is reported to provide an antislip and wear-resistant surface on the striking face. FR-A-2337612 describes a hand hammer having exposed metallic impact surfaces made from steel with the remainder of the hammer covered by a unitary resilient encasement. The present invention is a low-contamination impact tool especially useful for the breaking of semiconductor-grade silicon into pieces. The low-contamination impact tool suitable for breaking silicon comprises: (a) a core forming a handle portion and a head portion, (b) a shell of synthetic resin encapsulating the core, and (c) a single tungsten carbide alloy striking element with a single exposed striking surface, wherein the striking element has an end in contact with the head portion of the core and the synthetic resin encapsulates a portion of the striking element to secure the head portion in contact with the tungsten carbide alloy striking element. The preferred synthetic resin is urethane. Figure 1 illustrates a cross-sectional view of an embodiment of the present invention. The present invention is a low-contamination impact tool. The tool is designed especially to break semiconductor grade silicon into pieces without imparting significant contamination to the pieces. The low-contamination impact tool comprises: (A) a core forming a handle portion and a head portion, (B) a tungsten carbide alloy striking element having an end contacted with the head portion of the core and (C) a shell of synthetic resin encapsulating the core. In order to further clarify the concept of the present invention, one exemplary embodiment of the invention will be specifically described referring to the drawing provided as Figure 1. The low-contamination impact tool comprises a core consisting of handle portion 1 and head portion 2 . The core can be formed from any metal, metal alloy, plastic or composite of sufficient rigidity and strength to deliver an impact to a surface. Preferred is when the core is formed from a metal or metal alloy, for example, carbon steel, stainless steel, inconel, monel or hasteloy. More preferred is when the core is formed from AISI 1018 cold rolled steel. The size of the core is not critical to the present invention. Those skilled in the art will recognize that the core must have sufficient cross-sectional area to prevent bending and breaking of the core during use of the low-contamination impact tool as a breaking instrument. The required cross-sectional area will depend upon the material from which the core is constructed as well as the length of handle portion 1 . When AISI 1018 cold rolled steel is used as the material of construction of the core material, and the low-comtamination impact tool is to be used for the breaking of silicon, a length of about 203,2mm 304,8mm (8 inches to 12 inches) for handle portion 1 and a cross-sectional diameter of about 10,2mm to 12,7mm (0.4 to 0.5 inches) for handle portion 1 is suitable. Head portion 2 can be constructed of the same or different material than handle portion 1 . Preferred is when head portion 2 is formed from the same material as handle portion 1 . Head portion 2 and handle portion 1 are connected. The connection can be achieved by forming the core as a single element by, for example, molding, casting, stamping, cutting or machining, depending upon the particular material of fabrication. Alternatively, head portion 2 and handle portion 1 can be formed separately and connected by, for example, wedging, welding, brazing, fusing, threading or other standard means for connecting two solid objects. When the core is formed from AISI 1018 cold rolled steel, it is preferred that head portion 2 and handle portion 1 be formed separately and connected by welding. The size of head portion 2 is determined by the material of fabrication, the size of handle portion 1 , the method of securing striking element 4 and the size of striking element 4 . Generally, when handle portion 1 and head portion 2 are formed from AISI 1018 cold rolled steel, it is preferred that head portion 2 has a length of about 25,4mm to 50,8mm (one inch to two inches) and a diameter of 12,7 to 25,4mm (0.5 to one inch) Head portion 2 is secured in contact with striking element 4 . According to the present invention striking element 4 is secured in contact with head portion 2 during the process of encapsulating the core with a synthetic resin. The synthetic resin maintains the position of striking element 4 , as illustrated in Figure 1. The advantage of this method of securing striking element 4 is that the striking element can be easily recovered and reused if the remainder of the low-contamination impact tool is damaged. Striking element 4 is formed from a tungsten carbide alloy, where cobalt is the alloying metal. It is preferred that the tungsten carbide alloy contain about 8 to 15 weight percent cobalt. More preferred is when the tungsten carbide alloy contains about 10 to 13 weight percent cobalt. In general, the shape of striking element 4 is not critical to the present invention. However, in a preferred embodiment of the present invention striking element 4 is formed in a generally cylindrical shape with a constricted central portion. The constricted central portion helps secure striking element 4 in contact with head portion 2 , when a synthetic resin is used as the securing means. The constriction can be about one to 30 percent of the diameter of striking element 4 . Preferred is when the constriction is about five to 20 percent of the diameter of striking element 4 . In a preferred embodiment of the present invention, the diameter of striking element 4 is within a range of about 12,7mm to 25,4mm (0.5 to one inch). Striking element 4 has striking face 5 . The radius of curvature of the edge of striking face 5 is important to minimize breaking of particles from striking element 4 during use. A radius of about 0,76mm to 6,35mm (0.03 to 0.25 inch) is considered useful. Preferred is a radius of about 1,8mm to 3,05mm (0.07 to 0.12 inch). The core is encapsulated in a synthetic resin to form cover 3 . The purpose of encapsulating the core in the synthetic resin is to prevent the core from contacting the material to be broken with the low-contamination impact tool. The synthetic resin is selected so as to impart minimal undesirable contamination to the material to be broken. By "synthetic resin" is meant highly cross-linked polymeric materials that are not naturally occurring. The synthetic resin can be for example, polyurethane, polypropylene, polyethylene or polycarbonate. Preferred is when the synthetic resin is polyurethane. Even more preferred is when the synthetic resin is a polyurethane having a Shore A Hardness of about 90 to 97. Cover 3 can be formed around the core and striking element 4 by injecting or casting the synthetic resin into a cavity of a mold which has the same shape as the external shape of cover 3 . In the preferred embodiment, the core is placed in the mold, striking element 4 positioned as illustrated in Figure 1 and the synthetic resin injected and cured, securing striking element 4 in contact with head portion 2 . Example 1 (Not within the scope of the present invention) Silicon samples were prepared by breaking a rod of polycrystalline silicon with an impact tool having a non-encapsulated handle and head formed from AISI 1018 cold rolled steel. A tungsten carbide alloy striking element was attached to the head of the impact tool. The tungsten carbide alloy contained about 12 weight percent cobalt. During the breaking process, care was taken to contact each piece of silicon with the handle of the impact tool. Samples of silicon pieces were analyzed for iron and phosphorus surface contamination by graphite furnace atomic absorption and photoluminesience techniques, respectively. The results are presented in Table 1. Contamination of Silicon Pieces by Contact With Impact Tool Uncoated Handle Sample No. Fe ppb P ppb 1 0.90 0.27 2 0.74 0.29 3 0.79 0.43 4 0.80 0.37 5 0.63 0.07 6 0.67 0.20 Mean 0.76 0.27 Example 2 Silicon samples were prepared by breaking a rod of polycrystalline silicon with an impact tool having a polyurethane encapsulated handle and head. The handle and head were formed from AISI 1018 cold rolled steel. The polyurethane coating was formed from a polyether based liquid, isocyanate-terminated prepolymer using (4,4'-methylene-bis(orthochloroaniline)) as catalyst to effect cure. The cured polyurethane had a Shore A durometer of about 95. A tungsten carbide alloy striking element was attached to the head of the impact tool by molding into the polyurethane. The tungsten carbide alloy was as described for Example 1. During the breaking process, care was taken to contact each sample of silicon with the urethane coated handle of the impact tool. The silicon samples were analyzed as described in Example 1 and the results are presented in Table 2. Contamination of Silicon Pieces by Contact With Impact Tool Polyurethane Coated Handle Sample No. Fe ppb P ppb 1 0.35 0.09 2 0.56 0.13 3 0.36 0.14 4 0.40 0.02 5 0.45 0.01 6 0.40 0.06 Mean 0.42 0.08 The data presented in Table 2, when contrasted with the data of Table 1, demonstrate the contamination that can occur to silicon pieces when they are contacted with the unencapsulated handle of the impact tool.

Claims: A low contamination impact tool suitable for breaking silicon, which comprises: (a) a core forming a handle portion (1) and a head portion (2), (b) a shell of synthetic resin (3) encapsulating the core, and (c) a single tungsten carbide alloy striking element (4) with a single exposed striking surface (5), wherein the striking element (4) has an end in contact with the head portion (2) of the core and the synthetic resin (3) encapsulates a portion of the striking element (4) to secure the head portion (2) in contact with the tungsten carbide alloy striking element (4).

Inventors: NEMETH JOHN DEE, RICE DANIEL PATRICK, RUHLIG ELDEN EMERY, SCHMIDT CHRIS TIM, NEMETH, JOHN DEE, RICE, DANIEL PATRICK, RUHLIG, ELDEN EMERY, SCHMIDT, CHRIS TIM
Applicants: HEMLOCK SEMICONDUCTOR CORP, HEMLOCK SEMICONDUCTOR CORPORATION
Application year: 1992
Main classification: B25D1
IPC codes: B25D1, H01L21