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BUREAU OF MINES u , « 
 
 INFORMATION CIRCULAR/1989 
 
 Ultra-High-Purity Silicon for Infrared 
 Detectors: A Materials Perspective 
 
 By Clark R. Neuharth 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Mission: Asthe Nation's principal conservation 
 agency, the Department of the Interior has respon- 
 sibility for most of our nationally-owned public 
 lands and natural and cultural resources. This 
 includes fostering wise use of our land and water 
 resources, protecting our fish and wildlife, pre- 
 serving the environmental and cultural values of 
 our national parks and historical places, and pro- 
 viding for the enjoyment of life through outdoor 
 recreation. The Department assesses our energy 
 and mineral resources and works to assure that 
 their development is in the best interests of all 
 our people. The Department also promotes the 
 goals of the Take Pride in America campaign by 
 encouraging stewardship and citizen responsibil- 
 ity for the public lands and promoting citizen par- 
 ticipation in their care. The Department also has 
 a major responsibility for American Indian reser- 
 vation communities and for people who live in 
 Island Territories under U.S. Administration. 
 
Information Circular 9237 
 
 Ultra-High-Purity Silicon for Infrared 
 Detectors: A Materials Perspective 
 
 By Clark R. Neuharth 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Manuel Lujan, Jr., Secretary 
 
 BUREAU OF MINES 
 T S Ary, Director 
 

 ^ 
 
 Library of Congress Cataloging in Publication Data: 
 
 Neuharth, Clark R. 
 
 Ultra-high-purity silicon for infrared detectors : a materials perspective / by 
 Clark R. Neuharth. 
 
 p. cm. - (Bureau of Mines information circular; 9237) 
 
 Bibliography: p. 12 
 
 Supt. of Docs, no.: I 28.27:9237. 
 
 1. Silicon. 2. Infrared detectors-Materials. I. Title. II. Series: Information 
 circular (United States. Bureau of Mines); 9237 
 
 TN295.U4 [TN948.S6] 622 s-dc20 [355.2'4] 89-600256 
 
 CIP 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Semiconductors 2 
 
 Properties and grades 2 
 
 Semiconductivity 3 
 
 Semiconductor devices 4 
 
 Resources 5 
 
 Recovery technology 5 
 
 Silicon metal 6 
 
 Silanes 6 
 
 Polycrystalline silicon 7 
 
 Single-crystal silicon 7 
 
 Characterization 7 
 
 Supply and demand 9 
 
 Research and development 11 
 
 Government programs 11 
 
 Summary 12 
 
 References 12 
 
 Appendix-Major domestic and foreign firms involved in semiconductor silicon production 13 
 
 ILLUSTRATIONS 
 
 1. Face-centered cubic silicon lattice 3 
 
 2. Semiconductivity of silicon 4 
 
 3. Silicon junction devices 5 
 
 4. Photodetector device 5 
 
 5. Cross section of silicon metal furnace 6 
 
 6. TCS production flowsheet 6 
 
 7. Polysilicon reactor 8 
 
 8. Single-crystal silicon production methods 8 
 
 TABLES 
 
 1. Semiconductor demand, by major market 3 
 
 2. World silicon metal production 9 
 
 3. U.S. silicon metal statistics 10 
 
 4. Silicon supply-demand relationships and prices 10 
 
 5. World semiconductor silicon suppliers 11 
 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 
 
 in 
 
 inch 
 
 ppt part per trillion 
 
 kg 
 
 kilogram 
 
 st short ton 
 
 kg/yr 
 
 kilogram per year 
 
 yr year 
 
 ppm 
 
 part per million 
 
 
ULTRA-HIGH-PURITY SILICON FOR INFRARED 
 DETECTORS: A MATERIALS PERSPECTIVE 
 
 By Clark R. Neuharth 1 
 
 ABSTRACT 
 
 To assess the supply status of ultra-high-purity silicon for national defense needs, the U.S. Bureau 
 of Mines conducted a general study of the availability of materials and processing technology for silicon 
 used in the manufacture of infrared (IR) detectors from raw materials to the highly advanced device 
 state. The United States possesses all of the raw materials and processing technology for IR detector- 
 grade silicon production, but does not produce at all of the processing stages on a regular basis. 
 Consequently, the United States continues to rely on foreign sources for some of these critical materials. 
 
 1 Physical scientist, Division of Mineral Commodities, U.S. Bureau of Mines, Washington, DC. 
 
INTRODUCTION 
 
 Silicon is one of the most abundant elements in the 
 earth's crust. On a tonnage basis, it is used primarily as 
 ferrosilicon (an alloy containing iron and silicon) for deox- 
 idation and as an alloying agent in the production of iron 
 and steel. Metallurgical-grade silicon metal is also used on 
 a tonnage basis in aluminum alloys and by the chemical 
 industry as a feed material in the manufacture of silicones 
 and silanes. Subsequently, silanes are used to manufacture 
 high-purity silicon, which is the key material in today's 
 eletronics industry. The actual amount of silicon that is 
 ultimately processed to the point of usefulness in the man- 
 ufacture of semiconductor devices is less than a few per- 
 cent of total U.S. demand for silicon. The path that silicon 
 must follow from a raw material state to the manufacture 
 of certain semiconductor devices, such as infrared (IR) 
 detectors, is quite complex and involves a number of pro- 
 cessing stages. Currently, the United States is one of the 
 world's leaders in semiconductor device manufacture, but 
 
 does not produce on a regular basis all of the IR detector- 
 grade materials needed by the Department of Defense for 
 the manufacture of devices that guide highly advanced 
 weapons and serve numerous functions as part of satellite 
 surveillance systems. Capabilities to produce these ma- 
 terials do exist in the United States, but owing to limited 
 commercial applications, U.S. companies have been re- 
 luctant to enter respective phases of the market. Conse- 
 quently, the United States has relied almost entirely on 
 foreign sources for some of these ultra-high-purity 
 materials. 
 
 By presenting a digest of the available sources of silicon 
 materials and processing technology involved in the manu- 
 facture of IR detector-grade silicon, a perspective can be 
 drawn showing the overall importance of these materials 
 and whether or not there is a need for concern from a 
 national defense standpoint. 
 
 SEMICONDUCTORS 
 
 Although semiconductors serve thousands of functions, 
 the demand for these devices can be generally grouped 
 into three major categories: discrete semiconductors, 
 integrated circuits (IC's), and optoelectronic devices 
 (table 1). Discrete semiconductors include devices such as 
 diodes, high-power transistors, and thyristors. IC's, the 
 largest category based on total dollar value, include mem- 
 ories, processors, custom and semicustom IC's, linear IC's, 
 and logic devices. Imaging arrays, optically coupled iso- 
 lators, and photodetectors, make up the bulk of optoelec- 
 tronic devices. 
 
 Optoelectronic devices account for less than one-tenth 
 of one percent of U.S. semiconductor demand, but some 
 
 of these devices are extremely important in "high-tech" 
 military applications. IR detectors, a type of photodetector 
 capable of converting IR radiation into an electrical signal 
 (I), 2 are an essential part of a number of the military's 
 weapons systems (e.g., "smart" missile guidance and sur- 
 veillance satellites). The silicon starting material needed 
 to produce IR detectors must meet very stringent purity 
 requirements. Since most of these devices have almost no 
 commercial applications, semiconductor silicon producers 
 have been reluctant to enter this phase of the market (2). 
 
 PROPERTIES AND GRADES 
 
 Silicon never occurs free in nature, but is combined 
 with oxygen and other elements to form oxides and sili- 
 cates. Silicon dioxide (Si0 2 ), also referred to as silica, 
 quartz, or sometimes quartzite, is one of the most common 
 of these minerals. 
 
 In its metallic form, silicon is the result of chemically 
 reduced Si0 2 . The silica used to produce silicon metal is 
 typically >99.0% pure (Si0 2 content by weight), with alu- 
 minum, calcium, iron, and phosphorus constituting the 
 bulk of impurity elements. This grade of starting material 
 generally yields a silicon product that is 98% to 99% pure 
 (Si content by weight). 
 
 The crystal structure of silicon is diamond cubic, where 
 each atom in the lattice shares its four outermost electrons 
 (i.e., valence electrons) with the four nearest neighboring 
 atoms through covalent bonding (fig. 1). This crystal 
 structure, along with a number of other properties, gives 
 silicon its edge as a mainstay among semiconductor 
 materials. 
 
 2 Italic numbers in parentheses refer to items in the list of references 
 preceding the appendix. 
 
Table 1 .-Semiconductor demand by major market 
 
 (Millions of current dollars, January 1989) 
 
 Country 
 
 Discrete 
 
 Integrated 
 circuits 
 
 Opto- 
 electronic 
 
 Total 
 
 1989 1 
 
 $3,187 
 2,142 
 509 
 302 
 250 
 187 
 
 6,577 
 
 Japan 
 
 United States 
 
 Germany, Federal Republic of 
 
 United Kingdom 
 
 France 
 
 Italy 
 
 Total 
 
 Japan 
 
 United States 
 
 Germany, Federal Republic of 
 
 United Kingdom 
 
 France 
 
 Italy 
 
 Total 
 
 Japan 
 
 United States 
 
 Germany, Federal Republic of 
 
 United Kingdom 
 
 France 
 
 Italy 
 
 Total 
 
 Projected. 
 
 Source: Electronics. 
 
 Generally, the impurities contained in silicon for regular 
 semiconductor device manufacture must be held in the 
 low-ppm range, sometimes referred to as six 9's or 
 99.9999% pure. For IR-detector -grade material, these 
 
 $18,003 
 
 13,853 
 
 1,878 
 
 1,385 
 
 956 
 
 704 
 
 $2,585 
 
 377 
 
 134 
 
 120 
 
 81 
 
 36 
 
 $23,775 
 
 16,372 
 
 2,521 
 
 1,807 
 
 1,287 
 
 927 
 
 36,779 
 
 3,333 
 
 46,689 
 
 1988 
 
 $2,998 
 2,113 
 503 
 302 
 242 
 175 
 
 $16,214 
 
 12,599 
 
 1,783 
 
 1,385 
 
 870 
 
 651 
 
 $2,312 
 
 361 
 
 131 
 
 120 
 
 75 
 
 33 
 
 $21 ,524 
 
 15,073 
 
 2,417 
 
 1,807 
 
 1,187 
 
 859 
 
 6,333 
 
 33,502 
 
 3,032 
 
 42,867 
 
 1987 
 
 $2,815 
 1,822 
 480 
 277 
 223 
 154 
 
 $12,837 
 
 9,252 
 
 1,522 
 
 1,210 
 
 676 
 
 434 
 
 $1,996 
 
 332 
 
 124 
 
 108 
 
 66 
 
 31 
 
 $17,648 
 
 1 1 ,406 
 
 2,126 
 
 1,595 
 
 965 
 
 619 
 
 5,771 
 
 25,931 
 
 2,657 
 
 34,359 
 
 Figure 1 .-Face-centered cubic silicon crystal lattice. 
 
 impurity levels must be lowered to a range of 10 to 
 100 ppt, or eleven 9's. The primary impurities of concern 
 are the atoms of those elements that act as electron 
 donors and acceptors (i.e., dopants) in a silicon lattice, 
 such as boron and phosphorus (see Semiconductivity). As 
 few as 10 or 20 ppt impurity can significantly alter the 
 operation of a device such as an IR detector. Currently, 
 there are only a few characterization methods that can 
 detect impurities at these low levels, which is one of the 
 primary concerns in the production process. 
 
 SEMICONDUCTIVITY 
 
 The structure of silicon and how it relates to semicon- 
 ductivity can be represented by a two-dimensional diagram 
 of the crystal lattice. Figure 24 shows a theoretically 
 perfect, sometimes referred to as intrinsically pure, silicon 
 lattice. At room temperatures, valence electrons can break 
 free from the covalent bonds by acquiring energy from the 
 internal heat energy or vibrations of the crystal lattice 
 (fig. 2fl). These electrons hence become negative (n-type) 
 charge carriers, whereas the voids or holes they leave 
 behind become positive (p-type) charge carriers. The 
 energy required to bring about this condition is called the 
 
• • • 
 
 (7). .(T). .0 
 
 » > » 
 
 0. 0. 
 
 • • • 
 
 » > » 
 
 0.0.0 
 
 B 
 
 • • • 
 
 v Si y v Si y v si y * 
 
 » » » 
 
 v Si y v As y* *v Si y 
 
 • • • • 
 
 « • • 
 
 • Tsi J. '(sij. •( Si V 
 
 • • • 
 
 • 0..0..0 
 
 • 9 • 
 
 • (si \ •( Al V 'fsij 
 
 • o • 
 
 » » » 
 
 Figure 2.-Semiconductivity of silicon. A, No conductivity; 8, intrinsic; C, n- 
 doped; D, p-doped. 
 
 "gap" energy and varies with different materials. A mate- 
 rial is considered an insulator when the gap energy is too 
 great to be achieved. The conductivity exhibited in this 
 basic example is totally inherent (i.e., not brought about by 
 impurities) and is therefore known as intrinsic semicon- 
 ductivity. Extrinsic semiconductors, sometimes referred to 
 as doped semiconductors, contain intentionally introduced 
 impurities called dopants, which alter the semiconducting 
 characteristics of the material. Figure 2C shows how the 
 introduction of a pentavalent atom (i.e., containing five 
 valence electrons, such as phosphorus or arsenic) into the 
 silicon lattice creates an n-type semiconductive condition 
 similar to that shown in fig. 25. On the other hand, the 
 introduction of a trivalent atom, such as boron or alumi- 
 num, will create a p-type condition (fig. 2D) similar to that 
 of the hole left behind in the example in figure IB (3). 
 
 Whereas silicon exhibits semiconductive properties, Si0 2 
 acts as an insulator and forms naturally on a silicon sur- 
 face. This natural oxide-forming property allows con- 
 trolled formation of Si0 2 layers on silicon substrates. 
 These insulating layers serve a number of important func- 
 tions in the processing (e.g., planar technology) and 
 
 operation of semiconductor devices. Many other semicon- 
 ductor materials do not possess this natural oxide-forming 
 ability, making device manufacture and operation more 
 difficult and sometimes impossible (4). 
 
 SEMICONDUCTOR DEVICES 
 
 Junction devices, such as diodes, are the combination 
 of n-type and p-type semiconducting materials. In a simple 
 pn junction (fig. 3/1), the n-type portion of the semicon- 
 ductive material contains excess electrons, whereas the 
 p-type portion is electron deficient. Between these two 
 distinct regions lies a portion of material that possesses a 
 negligible amount of charge carriers called the depletion 
 layer. The depletion layer is void of charge carriers be- 
 cause the excess electrons from the n-type material have 
 crossed the junction to fill holes in the valence bands of 
 the nearest p-type region. This flow of electrons ceases 
 when the potential difference between the two portions 
 reaches a certain magnitude (i.e., the barrier potential). 
 The potential difference can be increased through the 
 application of an external source of electromotive force 
 
Electrons Depletion layer Ho|es 
 
 Light energy 
 
 Figure 4.-Photodetector device. 
 
 n-type 
 
 p-type 
 
 n-type 
 
 p-type 
 
 Negligible current 
 
 Electron flow 
 
 Figure 3.-Silicon junction devices. A, Diode and B, rectifier 
 (adopted from Jackson). 
 
 (emf). Current flow in the device is dependent on the 
 direction in which the external source is applied (fig. 3fi), 
 making the device function as a current rectifier (5). 
 
 Similarly, pn junctions are capable of generating emf 
 from electromagnetic waves, such as sunlight or IR radi- 
 ation (fig. 4). When sufficient energy is passed through a 
 transparent film of p-type silicon, excess electrons in the 
 adjacent n-type region acquire the energy and migrate 
 through the depletion layer into the p-type region, causing 
 a current to flow through the external load. This principle 
 forms the basis of operation for photodetectors. Probably 
 the most common example of photodetectors are solar 
 cells, which are used to convert sunlight into electrical 
 energy. 
 
 RESOURCES 
 
 The Earth's crust is made up almost entirely of silica 
 and silicates. These minerals constitute the bulk of most 
 common rocks, sands, soils, and clays. Based on present 
 requirements, domestic deposits of quartzite, sandstone, 
 and pegmatitic quartz could sustain the U.S. ferrosilicon 
 and silicon metal industries indefinitely. However, eco- 
 nomic factors such as accessibility to low-cost energy, 
 transportation costs, and a ready market for a product 
 
 determine resource development. Since metallurgical- 
 grade silicon is the primary raw material for the produc- 
 tion of silanes subsequently used to manufacture semicon- 
 ductor silicon, a shortage of raw materials is highly 
 unlikely. However, pitfalls in the production of 
 semiconductor silicon could occur if the capacity to 
 complete any phase of the silicon production chain was 
 interrupted. 
 
 RECOVERY TECHNOLOGY 
 
 There are a number of steps involved in the processing 
 of semiconductor silicon. Some companies involved in the 
 semiconductor industry are fully integrated, while others 
 specialize in just one or two phases of the processing 
 chain. The basic raw material for semiconductor silicon is 
 quartzite (Si0 2 ), which is abundant and for the most part 
 mined and processed domestically. The initial processing 
 step involves the reduction of quartzite to metallurgical- 
 grade silicon (sometimes referred to as chemical-grade 
 
 silicon in the chemical and electronics industries, or simply 
 as silicon metal). Quartzite used to produce silicon metal 
 is generally >99.0% pure. Following quartzite reduction, 
 the silicon metal is converted to a silicon-based chemical 
 that can be reduced to a purified silicon material in poly- 
 crystalline form. This polycrystalline silicon is then further 
 processed into a form that possesses the desired semicon- 
 ductive properties for device manufacture (i.e., single 
 crystal). 
 
SILICON METAL 
 
 SILANES 
 
 The principal raw material for silicon metal production 
 is beneficiated Si0 2 in the form of quartzite or certain 
 sandstones. The silica is reduced with carbon in a 
 submerged-arc electric furnace. The overall reaction is as 
 follows: 
 
 SiQ 2 + 2C 
 
 Si + 2CO 
 
 proceeding to the right above 1,164° C. However, in prac- 
 tice, temperatures vary in different furnace locations, and 
 a number of side and/or intermediate reactions also occur. 
 The products of these reactions subsequently migrate to 
 regions of the furnace where they react further or exit the 
 process (fig. 5). The silica starting material (i.e., quartzite) 
 is typically >99.0% pure, with aluminum, calcium, iron, and 
 phosphorus constituting the bulk of impurity elements. 
 Any iron contained in the silica will be reduced and report 
 to the metal. The amounts of aluminum, calcium, and 
 phosphorus present in the metal after reduction range 
 from 40% to 70% of their original content (6). 
 
 In the United States, silicon metal production of this 
 type ranges from 120,000 to 150,000 st annually. However, 
 demand is normally 30,000 to 40,000 st higher than produc- 
 tion, making the United States a net importer of silicon 
 metal. 
 
 Charge materials 
 SIO, + 2C 
 
 r~ 
 
 's 
 
 ♦ ♦ 
 Combustion 
 
 CO— CO, 
 
 SI0 — SI0, 
 
 H^4MNU 
 
 si -£"« 
 
 / 
 
 -\-l 1 
 
 L*-2C + SIO 
 
 Compounds containing hydrogen-silicon bonds are typ- 
 ically classified as silanes or sometimes as silicon hydrides. 
 Silane (SiH 4 ) is the simplest form of these compounds. 
 Other forms are named with the substituents prefixed, 
 such as disilane, H 3 SiSiH 3 ; dichlorosilane, H 2 SiCl 2 ; and 
 trichlorosilane (TCS), HSiCl 3 . TCS and silane are the pre- 
 ferred compounds for the production of polycrystalline 
 silicon. 
 
 TCS is produced by the reaction of powdered metal- 
 lurgical-grade silicon with anhydrous hydrogen chloride 
 (HC1) in a fluidized bed. The general equation for the 
 reaction is 
 
 Si + 3HC1 
 
 HSiCl 3 + H 2 . 
 
 However, a number of products other than TCS and H 2 
 are formed, including silicon tetrachloride (SiCl 4 ), other 
 chlorosilanes, unreacted HC1, and various metal chlorides 
 (A1C1 3 , BC1 3 , PC1 5 , etc.). The liquid TCS is separated from 
 the other products and purified by fractional distillation 
 (fig. 6) (7). 
 
 Silicon powder 
 HCI 
 
 Fluidized-bed 
 reactor 
 
 HCI H, 
 
 Recovery 
 
 \ 
 
 HCI 
 
 H 2 
 
 TCS 
 SiCI. 
 
 HCI 
 H 2 
 
 Separation 
 
 TCS 
 SiCI 4 
 
 Distillation 
 
 Figure 5.-Cross section of silicon metal furnace. 
 
 " 
 TCS SiCI 4 
 
 Figure 6.-TCS production flowsheet. 
 
Silane is produced by a number of methods involving 
 the reaction of metal silicides (e.g., those of aluminum, 
 lithium, and magnesium) with acids or ammonium salts. 
 Another silane process known to be used for commercial 
 production in the United States involves the conversion of 
 silicon metal to TCS, followed by the catalytic redistri- 
 bution and distillation of chlorosilanes (8). 
 
 POLYCRYSTALLINE SILICON 
 
 Polycrystalline silicon rods, often referred to as poly- 
 silicon or simply poly, are produced commercially by two 
 methods: (1) chemical vapor deposition from TCS in the 
 presence of hydrogen (i.e., the reverse of the fluidized-bed 
 reaction in TCS production), and (2) thermal decompo- 
 sition of silane. A simplified equation of the reaction of 
 TCS and hydrogen would be 
 
 HSiCl 3 + H 2 
 
 Si + 3HC1. 
 
 However, the actual process also yields SiC14 as a by- 
 product as well as unreacted TCS and H 2 . The ability to 
 recover and reuse the vent products plays an important 
 economic role in the production cycle (9). A typical de- 
 composition reactor containing a polysilicon filament (i.e., 
 starting rod) is shown in figure 7; thermal decomposition 
 of silane is carried out in a similar reactor. In this process, 
 reaction temperatures are much lower than those needed 
 for the TCS-hydrogen reaction, resulting in increased 
 polysilicon yields, lower impurity levels, and reduced pro- 
 duction costs through decreased power consumption (10). 
 A third method, also involving silane decomposition, is 
 used to produce polysilicon shot in a fluidized-bed reactor. 
 However, this material cannot be converted to single- 
 crystalline form using float-zoning (FZ) techniques. 
 
 SINGLE-CRYSTAL SILICON 
 
 Since polysilicon contains structural defects that would 
 affect electrical properties and interfere with semicon- 
 ductor device manufacturing processes, it must be con- 
 verted to a single-crystal form prior to device manufacture. 
 Single-crystal boules (rods) are grown by either the 
 Czochralski (CZ) or the FZ method. These boules are 
 later sliced into wafers (flat discs). In the CZ method 
 (fig. &4), a seed crystal is touched to the surface of a 
 molten silicon charge, and a solidified single crystal grows 
 as the seed is slowly pulled away from the melt. The 
 
 pulling rate controls the diameter of the boule. Diameters 
 generally range from 3 to 6 in, but significantly larger 
 diameters have been achieved. The CZ method accounts 
 for virtually all of the commercial production of single- 
 crystal silicon in the United States. The FZ method 
 (fig. 8B) starts with a solid polysilicon rod as grown in 
 the decomposition reactor. The rod is made to contact a 
 seed crystal after a small zone at the seed end is melted 
 with an induction coil. The coil is moved slowly along the 
 length of the rod, leaving the material behind the molten 
 zone solidified as an oriented single-crystal form. FZ 
 boules are typically smaller than those prepared by the CZ 
 method, ranging from 1 to 3 in. in diameter. Further puri- 
 fication can be achieved with both methods, since most 
 impurities tend to remain in the molten silicon rather than 
 solidify in the single crystal. However, a major advantage 
 of the FZ process is that the molten silicon zone is held 
 between the two solidified portions of the rod by surface 
 tension and is not in contact with any other material from 
 which it can pick up impurities. 
 
 CHARACTERIZATION 
 
 A primary concern in the preparation of ultra-high- 
 purity silicon is accurate analysis of the material for 
 impurity elements. Control of these impurities must be 
 maintained throughout the process. Methods of charac- 
 terization must be constantly improved and new methods 
 developed to keep pace with ever-increasing purity re- 
 quirements. Under current technology, characterization 
 is generally conducted on chlorosilane, silane, and single- 
 crystal silicon. Chlorosilane and silane gases can be mon- 
 itored by gas chromatography at certain stages of 
 production. To test silicon material, a small sample of 
 polysilicon must be FZ refined. Wafers cut from the FZ 
 material can be analyzed optically or tested for their elec- 
 trical transport properties. The two most common meth- 
 ods of optical analysis are (1) Fourier transform photo- 
 luminescence spectroscopy for donor/acceptor detection 
 and (2) Fourier transform infrared spectroscopy for mea- 
 surement of carbon and oxygen. Electrical transport mea- 
 surements based upon the Hall effect yield the most sensi- 
 tive data on donors and acceptors (e.g., <1 ppt) and are 
 used to calibrate the photoluminescence technique. Ac- 
 cording to Dr. Patrick M. Hemenger, U.S. Department of 
 the Air Force, these methods possess the necessary detec- 
 tion limits for present ultra-high-purity specifications. 
 
cJ^ 
 
 Vent 
 
 L 
 
 Heat shield 
 
 Quartz bell jar 
 
 Polycrystalllne 
 silicon U-rod 
 
 Graphite support 
 
 Power electrode 
 
 TCS 
 
 To recovery 
 
 Figure 7.-Polysilicon reactor. 
 
 Growing crystal 
 
 Fused silica liner 
 
 
 n 
 
 7 /\ Graphite crucible 
 
 # Zone heater 
 — Ingot 
 
 Single-crystal seed 
 
 Holder 
 
 B 
 
 Figure 8. -Single-crystal silicon production method. 
 A, Czochraiski and 8, Float-zoning. 
 
SUPPLY AND DEMAND 
 
 
 Since the supply of ultra-high-purity silicon for IR de- 
 tectors depends on the various aspects of a multistep pro- 
 cessing chain, the supply of materials involved in each step 
 should be examined if a quantitative materials perspective 
 is to be gained. However, very few official production or 
 production capacity data are available for silanes, poly- 
 silicon, or single crystals. The appendix lists the major 
 domestic merchant firms involved in the different phases 
 of the semiconductor silicon chain, along with some for- 
 eign firms that are involved in the more advanced pro- 
 cessing stages. 
 
 As discussed previously, raw silica is readily available 
 in most parts of the world, and a number of countries 
 produce silicon metal on a tonnage basis (table 2). 
 
 The United States produces 120,000 to 160,000 st of 
 silicon metal annually. However, demand is normally 
 30,000 to 40,000 st higher than production, making the 
 United States a net importer of silicon metal. The United 
 States does export silicon metal, but the amount is typically 
 less than 10% of domestic production (table 3). Both 
 production and imports of silicon metal account for rough- 
 ly one-third of production and imports of silicon materials 
 overall (table 4). 
 
 Production and production capacity for other phases of 
 the semiconductor silicon processing chain (i.e., silanes, 
 polysilicon, and single-crystal silicon) are adequate in the 
 United States. Annual production capacity for high-purity 
 polysilicon is about 4,000 st, and the major producers have 
 
 ample capacity to produce the necessary supplies of TCS 
 and silane starting materials. However, concerns do exist 
 at the latter end of the chain, since only one of the re- 
 maining major slice companies (i.e., producers of single- 
 crystal boules and wafers) is U.S.-owned, and the sale of 
 this company to a foreign buyer is currently under 
 consideration. 
 
 On a global basis, annual production capacity for high- 
 purity polysilicon is about 10,000 to 12,000 st, outpacing 
 demand by 2,000 to 3,000 st. As in the United States, 
 foreign producers generally possess adequate supplies of 
 TCS and silane starting materials. On the single-crystal 
 and wafer end of the spectrum, Japan is clearly the world's 
 leader (table 5). 
 
 U.S. military demands for ultra-high-purity polysilicon 
 and FZ single-crystal silicon used to make IR detectors 
 are approximately 500 and 200 kg, respectively. Currently, 
 the demand for both materials is met by foreign sources. 
 Foreign suppliers of ultra-high-purity material are listed 
 among the world polysilicon and single-crystal producers 
 in the appendix. Although ultra-high-purity polysilicon can 
 be produced domestically, and FZ equipment does exist, 
 neither material is produced on a regular basis. However, 
 current capabilities are being improved, and more regular 
 supplies of ultra-high-purity polysilicon as well as FZ single 
 crystals are being developed under Government-sponsored 
 programs. 
 
 Table 2. -World silicon metal production 
 
 (Thousand short tons) 
 
 1987 p 
 
 Country 
 
 1983 
 
 1984 
 
 1985 
 
 1986 
 
 -1988' 
 
 Brazil 
 
 Canada 
 
 China 
 
 France 
 
 Italy 
 
 Norway 
 
 South Africa, Republic of 
 
 Spain 
 
 Sweden 
 
 U.S.S.R 
 
 United States 
 
 Yugoslavia 
 
 Other 
 
 Total 
 
 p Preliminary. 
 Estimated. 
 
 23 
 28 
 24 
 72 
 15 
 85 
 30 
 19 
 22 
 70 
 122 
 29 
 43 
 
 582 
 
 30 
 28 
 24 
 78 
 
 15 
 100 
 38 
 66 
 22 
 70 
 141 
 31 
 43 
 
 32 
 28 
 30 
 77 
 15 
 
 112 
 39 
 68 
 22 
 66 
 
 121 
 36 
 45 
 
 686 
 
 691 
 
 41 
 29 
 45 
 77 
 13 
 
 110 
 39 
 68 
 22 
 72 
 
 124 
 35 
 42 
 
 717 
 
 44 
 33 
 70 
 77 
 13 
 
 110 
 37 
 77 
 22 
 72 
 
 147 
 35 
 40 
 
 777 
 
 87 
 33 
 75 
 77 
 13 
 
 110 
 37 
 77 
 22 
 72 
 
 164 
 55 
 36 
 
 858 
 
10 
 
 Table 3.-U.S. silicon metal statistics 
 
 (Thousand short tons, gross weight) 
 
 1983 
 
 1984 
 
 1985 
 
 1986 
 
 1987 
 
 1988 
 
 Production 
 Imports . . 
 Exports . . 
 
 123,602 
 
 28,173 
 
 2,767 
 
 144,005 
 
 25,221 
 
 4,420 
 
 122,787 
 
 51,801 
 
 2,120 
 
 125,966 
 
 40,851 
 
 5,378 
 
 150,080 
 
 36,930 
 
 9,247 
 
 164,348 
 62,030 
 10,304 
 
 Table 4. -Silicon supply-demand relationships and prices, including silicon metal 
 and silicon-containing ferroalloys 
 
 (Thousand short tons of Si) 
 
 1983 
 
 1984 
 
 1985 
 
 1986 
 
 1987 
 
 1988 
 
 PRODUCTION 
 
 United States 333 454 400 
 
 Rest of world 6 2,511 2,770 2,717 
 
 Total 6 2,844 3,224 3,117 
 
 COMPONENTS AND DISTRIBUTION OF U.S. SUPPLY 
 
 Components: 
 
 Primary production 333 454 400 
 
 Imports 133 121 154 
 
 Industry stocks-Jan. 1 109 86 90 
 
 Total 575 661 644 
 
 Distribution: 
 
 Industry stocks-Dec. 31 86 90 102 
 
 Exports 9 20 9 
 
 Industrial demand 480 551 533 
 
 U.S. DEMAND PATTERN 
 
 Chemicals 62 99 101 
 
 Construction 62 77 75 
 
 Machinery 101 94 91 
 
 Transportation 154 171 165 
 
 Other 101 VI0 101 
 
 Total 480 551 533 
 
 AVERAGE ANNUAL FREE-MARKET PRICES 
 
 Regular-grade 50% ferrosilicon, 
 
 actual (cents per pound Si) 36.1 40.7 36.6 
 
 Regular-grade 50% ferrosilicon, 
 
 based on constant 1988 dollars 
 
 (cents per pound Si) 42.3 46.0 40.2 
 
 Regular-grade 75% ferrosilicon, 
 
 actual (cents per pound Si) 36.0 41.3 35.0 
 
 Regular-grade 75% ferrosilicon, 
 
 based on constant 1988 dollars 
 
 (cents per pound Si) 42.2 46.7 38.4 
 
 Regular-grade silicon metal, 
 
 actual (cents per pound) 36.0 41.3 35.0 
 
 Regular-grade silicon metal, 
 
 based on constant 1988 dollars 
 
 (cents per pound) 42.2 46.7 38.4 
 
 Estimated. 
 
 NA Not available. 
 
 335 
 2,686 
 
 3,021 
 
 628 
 
 527 
 
 34.8 
 
 35.2 
 
 373 
 2,668 
 
 3,041 
 
 634 
 
 560 
 
 41.3 
 
 41.7 
 
 465 
 NA 
 
 NA 
 
 335 
 
 373 
 
 463 
 
 191 
 
 189 
 
 212 
 
 102 
 
 72 
 
 57 
 
 735 
 
 72 
 
 57 
 
 49 
 
 11 
 
 17 
 
 27 
 
 527 
 
 560 
 
 658 
 
 105 
 
 118 
 
 NA 
 
 69 
 
 73 
 
 NA 
 
 90 
 
 95 
 
 NA 
 
 169 
 
 179 
 
 NA 
 
 95 
 
 95 
 
 NA 
 
 NA 
 
 51.0 
 
 37.2 
 
 42.7 
 
 51.0 
 
 32.9 
 
 40.3 
 
 55.6 
 
 35.2 
 
 41.7 
 
 55.6 
 
 32.9 
 
 40.3 
 
 55.6 
 
 55.6 
 
 Source: Bureau of Mines. 
 
11 
 
 Table 5.-World semiconductor silicon suppliers 
 
 (Silicon and epitaxial wafer sales in million dollars) 
 
 Company 
 
 Shin-Etsu Handotai (Japan) 
 
 Mitsubishi Metal (Japan) 
 
 Osaka Titanium Co. (Japan) 
 
 Wacher (Germany, Federal Republic of) 
 Komatsu Electroonic Metals (Japan) . . . 
 
 Monsanto (United States) 
 
 Other 
 
 Total 
 
 NA Not available. 
 
 Source: Dataquest. 
 
 1985 
 
 1986 
 
 1987 
 
 1988 
 
 $310.0 
 
 $408.0 
 
 $452.1 
 
 128.0 
 
 195.0 
 
 241.3 
 
 160.0 
 
 197.6 
 
 235.5 
 
 205.0 
 
 194.6 
 
 166.5 
 
 116.0 
 
 168.5 
 
 197.3 
 
 137.0 
 
 154.0 
 
 185.0 
 
 210.5 
 
 233.8 
 
 249.3 
 
 1,266.5 
 
 1,551.5 
 
 1,727.0 
 
 NA 
 NA 
 NA 
 NA 
 NA 
 NA 
 NA 
 
 $2,172.0 
 
 RESEARCH AND DEVELOPMENT 
 
 Probably the most current domestic research aimed at 
 developing a more stable (i.e., production on a regular 
 basis and from a domestic source) supply situation for IR- 
 detector-grade silicon in the United States involves two 
 Government-sponsored programs. One is the Air Force's 
 
 recently completed "Boron-Free Silicon Detectors 
 Program", and the other is a title III contract under the 
 Defense Production Act (DPA) calling for the develop- 
 ment of a domestic ultra-high-purity silicon source. 
 
 GOVERNMENT PROGRAMS 
 
 In 1984, a project involving Hughes Aircraft Co. and 
 Union Carbide Corp. was initiated under Air Force 
 Contract F33615-84-C-5025 to produce starting materials, 
 including silane and polysilicon, to be used in the manu- 
 facture of extrinsic silicon IR detectors. This work, 
 known as the Boron-Free Silicon Detectors Program, was 
 completed in 1988 and concluded that, through Union 
 Carbide's modified silane process, ultra-high-purity silicon 
 could be produced on a regular basis. Also as a result of 
 the program, much improved characterization techniques 
 (i.e., impurity analysis) were developed (10). 
 
 Under title III of the DPA, President Reagan granted 
 the Defense Department $3 million in 1986 for 
 Government-sponsored development of an ultra- 
 high-purity silicon industry. Title III of DPA gives the 
 
 Administration the authority to expand capacity or develop 
 technological processes through the use of loans and guar- 
 anteed purchase contracts. Hemlock Semiconductor Corp. 
 in Hemlock, MI, recently completed a title III contract 
 (phase I) under which 150 kg of ultra-high-purity poly- 
 silicon was to be produced. Phase II of this title III 
 program is to be awarded to Hemlock sometime in 1989, 
 pending complete verification that phase I specifications 
 were met. The phase II contract calls for 1,000 kg/yr of 
 Phase I-grade polysilicon for each of the next 3 yr. At the 
 same time, title III contracts will be awarded to convert 
 100 kg of the phase I material by the FZ method. The 
 other 50 kg produced during phase I was used for ana- 
 lytical purposes. The FZ contracts will be limited to small 
 companies (i.e., less than 500 employees). 
 
12 
 
 SUMMARY 
 
 Both the raw materials necessary to produce silicon on 
 a tonnage basis and production capacity are abundant in 
 the United States and other parts of the world. Ample 
 production capacity for the silicon-based chemicals needed 
 for semiconductor silicon also exists. However, the indi- 
 vidual stages involved in the processing of raw silica into 
 the ultra-high-purity silicon needed for the manufacture of 
 today's IR detectors can be quite complex, and domestic 
 production in the latter stages is uncertain. The purity 
 requirements of such materials are very stringent at most 
 stages, and the control of impurity elements will become 
 even more crucial in the manufacture of more advanced 
 devices in the future. Currently, preparation of ultra-high- 
 purity silicon for IR detectors can be achieved domestically 
 
 through the polysilicon stage. However, there is no do- 
 mestic production on a regular basis. Many questions as 
 to the establishment of long-term domestic FZ capabilities 
 also exist, since there is currently no merchant FZ produc- 
 tion in the United States. Characterization of ultra-high- 
 purity silicon is a continuing concern. New and improved 
 characterization techniques must be developed to keep 
 pace with device requirements. Ongoing Government- 
 sponsored programs have closed some of the gaps in the 
 domestic ultra-high-purity silicon processing chain. How- 
 ever, the United States continues to depend on foreign 
 sources for a stable supply of this material for its defense 
 needs. 
 
 REFERENCES 
 
 1. Kirk-Othmer Encyclopedia of Chemical Technology. V. 17, 3d 
 ed., 1980, p. 560. 
 
 2. Business Week. No. 2970, Oct. 27, 1986, p. 46. 
 
 3. Van Vlack, L. H. Electron Transport in Solids. Ch. in Elements 
 of Materials Science and Engineering, ed. by M. Cohen. Addison- 
 Wesley, 4th ed., 1980, pp. 149-182. 
 
 4. Smith, P. C. Private communication. Data in brochure available 
 from Advanced Technology Div., Westinghouse Electric Corp., 
 Baltimore, MD. 
 
 5. Jackson, H. W. Insulators, Semiconductors, and Sources of EMF. 
 Ch. in Introduction to Electric Circuits. Prentice-Hall, 4th ed., 1986, 
 pp. 38-63. 
 
 6. Robiette, A. G. E. Manufacture of Silicon Alloys. Ch. in Electric 
 Smelting Processes. Halsted Press, 1973, pp. 102-125. 
 
 7. McCormick, J. R Polycrystalline Silicon. Paper in 
 Semiconductor Silicon 1986, ed. by H. R Huff, B. Kolbesen, and 
 T. Abe. Electrochem. Soc., 1986, pp. 46^8. 
 
 8. Taylor, P. A. Silane: Manufacture and Applications. Solid State 
 Technol., v. 30, No. 7, 1987, pp. 53-59. 
 
 9. Crossman, L. D., and J. A. Baker. Polysilicon Technology. Paper 
 in Semiconductor Silicon 1977, ed. by H. R Huff and E. Sirtl. 
 Electrochem. Soc., 1977. 
 
 10. Gittere, W. J. Private communication. Data in brochure 
 available from Market Development and Sales, Union Carbide Corp., 
 Tonowanda, NY. 
 
 11. Robertson, G. D., M. H. Young, J. P. Baukus, O. J. Marsh, and 
 R N. Flagella. High Purity Silicon for Detectors. IRIA IR Materials 
 Conf., 1988, Palo Alto, CA. 
 
13 
 
 APPENDIX.-MAJOR DOMESTIC AND FOREIGN FIRMS INVOLVED 
 IN SEMICONDUCTOR SILICON PRODUCTION 
 
 Silicon metal 
 
 Silanes 
 
 Domestic 
 
 Dow Corning 
 Elkem Metals 
 Globe Metallurgical 
 Silicon Metaltech 
 Simetco 
 SKW 
 
 Dow Corning 
 
 Ethyl 
 
 Union Carbide 
 
 Foreign 
 
 Numerous companies 
 worldwide produce 
 silicon metal. 
 See table 2 for 
 production by 
 country. 
 
 Komatsu 
 
 Shin-Etsu 
 
 Wacher 
 
 Polysilicon 
 
 Ethyl 
 
 Hemlock Semiconductor 
 (63% owned by Dow 
 Chemical, 37% by 
 Japanese firms) 
 
 Union Carbide 
 
 Dyamit-Nobel 
 
 Komatsu 
 
 Mitsubishi Metal 
 
 NKK 
 
 Rhone Poulenc 
 
 Shin-Etsu 
 
 Tokuyama 
 
 TopsU 
 
 Wacher 
 
 Single crystal 
 and wafers. 
 
 Crysteco 
 
 Monsanto 
 
 NBK (owned by Kawasaki 
 
 of Japan) 
 Siltec (owned by 
 
 Misubishi Metal of 
 
 Japan) 
 
 Kamatsu 
 
 Mitsubishi Metal 
 
 Monsanto 
 
 Osaka Titanium Co. 
 
 Shin-Etsu 
 
 Topsil 
 
 Wacher 
 
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