Introduction

The synthesis and purification of bulk polycrystalline semiconductor material represents the first step towards the commercial fabrication of an electronic device. This polycrystalline material is then used as the raw material for the formation of single crystal material that is processed to semiconductor wafers. The strong influence on the electric characteristics of a semiconductors exhibited by small amounts of some impurities requires that the bulk raw material be of very high purity (> 99.9999%). Although some level of purification is possible during the crystallization process it is important to use as high a purity starting material as possible. While a wide range of substrate materials are available from commercial vendors, silicon and GaAs represent the only large-scale commercial semiconductor substrates, and thus the discussion will be limited to the synthesis and purification of these materials.

Silicon

Following oxygen (46%), silicon (L. silicis flint) is the most abundant element in the earth's crust (28%). However, silicon does not occur in its elemental form, but as its oxide ( SiO2SiO2 size 12{"SiO" size 8{2}} {} ) or as silicates. Sand, quartz, amethyst, agate, flint, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay and mica, etc. are a few of the numerous silicate minerals. With such boundless supplies of the raw material, the costs associated with the production of bulk silicon is not one of abstraction and conversion of the oxide(s), but of purification of the crude elemental silicon. While 98% elemental silicon, known as metallurgical-grade silicon (MGS), is readily produced on a large scale, the requirements of extreme purity for electronic device fabrication require additional purification steps in order to produce electronic-grade silicon (EGS). Electronic-grade silicon is also known as semiconductor-grade silicon (SGS). In order for the purity levels to be acceptable for subsequent crystal growth and device fabrication, EGS must have carbon and oxygen impurity levels less than a few parts per million (ppm), and metal impurities at the parts per billion (ppb) range or lower. Tables 1 and 2 give typical impurity concentrations in MGS and EGS, respectively. Besides the purity, the production cost and the specifications must meet the industry desires.

Table 1. Typical impurity concentrations found in metallurgical-grade silicon (MGS).

Table 2. Typical impurity concentrations found in electronic-grade silicon (EGS).

Metallurgical-Grade Silicon (MGS)

The typical source material for commercial production of elemental silicon is quartzite gravel; a relatively pure form of sand ( SiO2SiO2 size 12{"SiO" size 8{2}} {}). The first step in the synthesis of silicon is the melting and reduction of the silica in a submerged-electrode arc furnace. An example of which is shown schematically in Figure 1, along with the appropriate chemical reactions. A mixture of quartzite gravel and carbon are heated to high temperatures (ca. 1800 °C) in the furnace. The carbon bed consists of a mixture of coal, coke, and wood chips. The latter providing the necessary porosity such that the gases created during the reaction (SiO and CO) are able to flow through the bed.

Figure 1

Figure 1. Schematic of submerged-electrode arc furnace for the production of metallurgical-grade silicon (MGS).

The overall reduction reaction of SiO2SiO2 size 12{"SiO" size 8{2}} {} is expressed in Eq. 1, however, the reaction sequence is more complex than this overall reaction implies, and involves the formation of SiC and SiO intermediates. The initial reaction between molten SiO2SiO2 size 12{"SiO" size 8{2}} {} and C (Eq. 2) takes place in the arc between adjacent electrodes, where the local temperature can exceed 2000 °C. The SiO and CO thus generated flow to cooler zones in the furnace where SiC is formed (Eq. 3), or higher in the bed where they reform SiO2SiO2 size 12{"SiO" size 8{2}} {} and C (Eq. 2). The SiC reacts with molten SiO2SiO2 size 12{"SiO" size 8{2}} {} (Eq. 4) producing the desired silicon along with SiO and CO. The molten silicon formed is drawn-off from the furnace and solidified.

Figure 2

The as-produced MGS is approximately 98-99% pure, with the major impurities being aluminum and iron (Table 1), however, obtaining low levels of boron impurities is of particular importance, because it is difficult to remove and serves as a dopant for silicon. The drawbacks of the above process are that it is energy and raw material intensive. It is estimated that the production of one metric ton (1,000 kg) of MGS requires 2500-2700 kg quartzite, 600 kg charcoal, 600-700 kg coal or coke, 300-500 kg wood chips, and 500,000 kWh of electric power. Currently, approximately 500,000 metric tons of MGS are produced per year, worldwide. Most of the production (ca. 70%) is used for metallurgical applications (e.g., aluminum-silicon alloys are commonly used for automotive engine blocks) from whence its name is derived. Applications in a variety of chemical products such as silicone resins account for about 30%, and only 1% or less of the total production of MGS is used in the manufacturing of high-purity EGS for the electronics industry. The current worldwide consumption of EGS is approximately 5 x 106 kg per year.

Electronic-grade Silicon (EGS)

Electronic-grade silicon (EGS) is a polycrystalline material of exceptionally high purity and is the raw material for the growth of single-crystal silicon. EGS is one of the purest materials commonly available, see Table 2. The formation of EGS from MGS is accomplished through chemical purification processes. The basic concept of which involves the conversion of MGS to a volatile silicon compound, which is purified by distillation, and subsequently decomposed to re-form elemental silicon of higher purity (i.e., EGS). Irrespective of the purification route employed, the first step is physical pulverization of MGS followed by its conversion to the volatile silicon compounds.

A number of compounds, such as monosilane ( SiH4SiH4 size 12{"SiH" size 8{4}} {} ), dichlorosilane ( SiH2Cl2SiH2Cl2 size 12{"SiH" size 8{2}"Cl" size 8{2}} {} ), trichlorosilane ( SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} ), and silicon tetrachloride ( SiCl4SiCl4 size 12{"SiCl" size 8{4}} {} ), have been considered as chemical intermediates. Among these, SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} has been used predominantly as the intermediate compound for subsequent EGS formation, although SiH4SiH4 size 12{"SiH" size 8{4}} {} is used to a lesser extent. Silicon tetrachloride and its lower chlorinated derivatives are used for the chemical vapor deposition (CVD) growth of Si and SiO2SiO2 size 12{"SiO" size 8{2}} {}. The boiling points of silane and its chlorinated products (Table 3) are such that they are conveniently separated from each other by fractional distillation.

Table 3. Boiling points of silane and chlorosilanes at 760 mmHg (1 atmosphere).

The reasons for the predominant use of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} in the synthesis of EGS are as follows: (1) SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} can be easily formed by the reaction of anhydrous hydrogen chloride with MGS at reasonably low temperatures (200-400 °C); (2) it is liquid at room temperature so that purification can be accomplished using standard distillation techniques; (3) it is easily handled and if dry can be stored in carbon steel tanks; (4) its liquid is easily vaporized and, when mixed with hydrogen it can be transported in steel lines without corrosion; (5) it can be reduced at atmospheric pressure in the presence of hydrogen; (6) its deposition can take place on heated silicon, thus eliminating contact with any foreign surfaces that may contaminate the resulting silicon; and (7) it reacts at lower temperatures (1000-1200 °C) and at faster rates than does SiCl4SiCl4 size 12{"SiCl" size 8{4}} {}.

Chlorosilane (Seimens) Process

Trichlorosilane is synthesized by heating powdered MGS with anhydrous hydrogen chloride (HCl) at around 300 °C in a fluidized-bed reactor, Eq. 5.

Figure 3

Since the reaction is actually an equilibrium and the formation of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} highly exothermic, efficient removal of generated heat is essential to assure a maximum yield of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {}. While the stoichiometric reaction is that shown in Eq. 5, a mixture of chlorinated silanes is actually prepared which must be separated by fractional distillation, along with the chlorides of any impurities. In particular iron, aluminum, and boron are removed as FeCl3FeCl3 size 12{"FeCl" size 8{3}} {} (b.p. = 316 °C), AlCl3AlCl3 size 12{"AlCl" size 8{3}} {} (m.p. = 190 °C subl.), and BCl3BCl3 size 12{"BCl" size 8{3}} {} (b.p. = 12.65 °C), respectively. Fractional distillation of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} from these impurity halides result in greatly increased purity with a concentration of electrically active impurities of less than 1 ppb.

EGS is prepared from purified SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} in a chemical vapor deposition (CVD) process similar to the epitaxial growth of Si. The high-purity SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} is vaporized, diluted with high-purity hydrogen, and introduced into the Seimens deposition reactor, shown schematically in Figure 2. Within the reactor, thin silicon rods called slim rods (ca. 4 mm diameter) are supported by graphite electrodes. Resistance heating of the slim rods causes the decomposition of the SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} to yield silicon, as described by the reverse reaction shown in Eq. 5.

Figure 4

Figure 2. Schematic representation of a cross section through a Siemens reactor.

The shift in the equilibrium from forming SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} from Si at low temperature, to forming Si from SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} at high temperature is as a consequence of the temperature dependence (Eq. 6) of the equilibrium constant (Eq. 7, where ρ = partial pressure) for Eq. 5. Since the formation of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} is exothermic, i.e., ΔH < 0, an increase in the temperature causes the partial pressure of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} to decrease. Thus, the Siemens process is typically run at ca. 1100 °C, while the reverse fluidized bed process is carried out at 300 °C.

Figure 5

The slim rods act as a nucleation point for the deposition of silicon, and the resulting polycrystalline rod consists of columnar grains of silicon (polysilicon) grown perpendicular to the rod axis. Growth occurs at less than 1 mm per hour, and after deposition for 200 to 300 hours high-purity (EGS) polysilicon rods of 150-200 mm in diameter are produced. For subsequent float-zone refining the polysilicon EGS rods are cut into long cylindrical rods. Alternatively, the as-formed polysilicon rods are broken into chunks for single crystal growth processes, for example Czochralski melt growth.

In addition to the formation of silicon, the HCl coproduct reacts with the SiHCl3 reactant to form silicon tetrachloride ( SiCl4SiCl4 size 12{"SiCl" size 8{4}} {} ) and hydrogen as major byproducts of the process, Eq. 8. This reaction represents a major disadvantage with the Seimens process: poor efficiency of silicon and chlorine consumption. Typically, only 30% of the silicon introduced into CVD reactor is converted into high-purity polysilicon.

Figure 6

In order to improve efficiency the HCl, SiCl4SiCl4 size 12{"SiCl" size 8{4}} {}, H2H2 size 12{H size 8{2}} {} and unreacted SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} are separated and recovered for recycling. Figure 3 illustrates the entire chlorosilane process starting with MGS and including the recycling of the reaction byproducts to achieve high overall process efficiency. As a consequence, the production cost of high-purity EGS depends on the commercial usefulness of the byproduct, SiCl4SiCl4 size 12{"SiCl" size 8{4}} {}. Additional disadvantages of the Seimens process are derived from its relatively small batch size, slow growth rate, and high power consumption. These issues have lead to the investigation of alternative cost efficient routes to EGS.

Figure 7

Figure 3. Schematic diagram of the production of electronic-grade silicon (EGS) from the hydrogen reduction of trichlorosilane ( SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} ).

Silane Process

An alternative process for the production of EGS that has begun to receive commercial attention is the pyrolysis of silane ( SiH4SiH4 size 12{"SiH" size 8{4}} {} ). The advantages of producing EGS from SiH4SiH4 size 12{"SiH" size 8{4}} {} instead of SiHCl3SiHCl3 size 12{"SiHCl" size 6{3}} {} are potentially lower costs associated with lower reaction temperatures, and less harmful byproducts. Silane decomposes < 900 °C to give silicon and hydrogen, Eq. 9.

Figure 8

Silane may be prepared by a number of routes, each having advantages with respect to purity and production cost. The simplest process involves the direct reaction of MGS powders with magnesium at 500 °C in a hydrogen atmosphere, to form magnesium silicide ( Mg2SiMg2Si size 12{"Mg" size 8{2}"Si"} {}). The magnesium silicide is then reacted with ammonium chloride in liquid ammonia below 0 °C, Eq. 10.

Figure 9

This process is ideally suited to the removal of boron impurities (a p-type dopant in Si), because the diborane ( B2H6B2H6 size 12{B size 8{2}H size 8{6}} {}) produced during the reaction forms the Lewis acid-base complex, H3BH3B size 12{H size 6{3}B} {} ( NH3NH3 size 12{"NH" size 8{3}} {} ), whose volatility is sufficiently lower than SiH4SiH4 size 12{"SiH" size 8{4}} {}, allowing for the purification of the latter. It is possible to prepare EGS with a boron content of ≤ 20 ppt using SiH4SiH4 size 12{"SiH" size 8{4}} {} synthesized in this manner. However, phosphorus (another dopant) in the form of PH3 may be present as a contaminant requiring subsequent purification of the SiH4.

Alternative routes to SiH4SiH4 size 12{"SiH" size 8{4}} {} involve the chemical reduction of SiCl4SiCl4 size 12{"SiCl" size 8{4}} {} by either lithium hydride (Eq. 11), lithium aluminum hydride (Eq. 12), or via hydrogenation in the presence of elemental silicon (Eq. 13-16). The hydride reduction reactions may be carried-out on relatively large scales (ca. 50 kg), but only batch processes. In contrast, Union Carbide has adapted the hydrogenation to a continuous process, involving disproportionation reactions of chlorosilanes (Eq. 14-16) and the fractional distillation of silane (Table 3).

Figure 10

Pyrolysis of silane on resistively heated polysilicon filaments at 700-800 °C yields polycrystalline EGS. As noted above, the EGS formed has remarkably low boron impurities compared with material prepared from trichlorosilane. Moreover, the resulting EGS is less contaminated with transition metals from the reactor container because SiH4SiH4 size 12{"SiH" size 8{4}} {} decomposition does not cause as much of a corrosion problem as is observed with halide precursor compounds.

Zone Refining

The technique of zone refining is used to purify solid materials and is commonly employed in metallurgical refining. In the case of silicon may be used to obtain the desired ultimate purity of EGS, which has already been purified by chemical processes. Zone refining was invented by Pfann, and makes use of the fact that the equilibrium solubility of any impurity (e.g., Al) is different in the solid and liquid phases of a material (e.g., Si). For the dilute solutions, as is observed in EGS silicon, an equilibrium segregation coefficient ( k0k0 size 12{k size 8{0}} {} ) is defined by Eq. 17, where CsCs size 12{C size 8{s}} {} and Cl Cl size 12{C size 8{"l "}} {} are the equilibrium concentrations of the impurity in the solid and liquid near the interface, respectively.

Figure 11

If k0k0 size 12{k size 8{0}} {} is less than 1 then the impurities are left in the melt as the molten zone is moved along the material. In a practical sense a molten zone is established in a solid rod. The zone is then moved along the rod from left to right. If k < 1 then the frozen part left on the trailing edge of the moving molten zone will be purer than the material that melts in on the right-side leading edge of the moving molten zone. Consequently the solid to the left of the molten zone is purer than the solid on the right. At the completion of the first pass the impurities become concentrated to the right of the solid sample. Repetition of the process allows for purification to exceptionally high levels.

Table 4 lists the equilibrium segregation coefficients for common impurity and dopant elements in silicon; it should be noted that they are all less than 1.

Table 4. Segregation coefficients for common impurities in silicon.

Isolation and Purification of Gallium Metal

At 19 ppm gallium (L. Gallia, France) is about as abundant as nitrogen, lithium and lead; it is twice as abundant as boron (9 ppm), but is more difficult to extract due to the lack of any major gallium-containing ore. Gallium always occurs in association either with zinc or germanium, its neighbors in the periodic table, or with aluminum in the same group. Thus, the highest concentrations (0.1-1%) are in the rare mineral germanite (a complex sulfide of Zn, Cu, Ge, and As), while concentrations in sphalerite (ZnS), diaspore [AlO(OH)], bauxite, or coal, are a hundred-fold less. Industrially, gallium was originally recovered from the flue dust emitted during sulfide roasting or coal burning (up to 1.5% Ga), however, it is now obtained as side product of vast aluminum industry and in particular from the Bayer process for obtaining alumina from bauxite.

The Bayer process involves dissolution of bauxite, AlOx(OH)1-xAlOx(OH)1-x size 12{"AlO" size 8{x} \( "OH" \) size 8{"1-x"}} {} (0 < x < 1), in aqueous NaOH, separation of insoluble impurities, partial precipitation of the trihydrate, Al(OH)3Al(OH)3 size 12{"Al" \( "OH" \) size 8{3}} {}, and calcination at 1,200 °C. During processing the alkaline solution is gradually enriched in gallium from an initial weight ratio Ga/Al of about 1/5000 to about 1/300. Electrolysis of these extracts with a Hg cathode results in further concentration, and the solution of sodium gallate thus formed is then electrolyzed with a stainless steel cathode to give Ga metal. Since bauxite contains 0.003-0.01% gallium, complete recovery would yield some 500-1000 tons per annum, however present consumption is only 0.1% of this about 10 tons per annum.

A typical analysis of the 98-99% pure gallium obtained as a side product from the Bayer process is shown in Table 5. This material is further purified to 99.99% by chemical treatment with acids and O2O2 size 12{O size 8{2}} {} at high temperatures followed by crystallization. This chemical process results in the reduction of the majority of metal impurities at the ppm level, see Table 5. Purification to seven nines 99.9999% is possible through zone refining, however, since the equilibrium distribution coefficient of the residual impurities k0k0 size 12{k size 8{0}} {}≈ 1, multiple passes are required, typically > 500. The low melting point of gallium ensures that contamination from the container wall (which is significant in silicon zone refining) is minimized. In order to facilitate the multiple zone refining in a suitable time, a simple modification of zone refining is employed shown in Figure 4. The gallium is contained in a plastic tube wrapped around a rotating cylinder that is half immersed in a cooling bath. A heater is positioned above the gallium plastic coil. Thus, establishing a series of molten zones that pass upon rotation of the drum by one helical segment per revolution. In this manner, 500 passes may be made in relatively short time periods. The typical impurity levels of gallium zone refined in this manner are given in Table 5.

Table 5. Typical impurity concentrations found at the various stages in the purification of gallium.

Figure 12

Figure 4. Schematic representation of the zone melting apparatus for gallium. (a) gallium contained in a plastic tube, (b) rotating cylinder, (c) cooling bath, and (d) heater.

Isolation and Purification of Elemental Arsenic

Elemental arsenic (L. arsenicum, yellow orpiment) exists in two forms: yellow (cubic, As4As4 size 12{"As" size 8{4}} {} ) and gray or metallic (rhombohedral). At a natural abundance of 1.8 ppm arsenic is relatively rare, however, this is offset by its presence in a number of common minerals and the relative ease of isolation. Arsenic containing minerals are grouped into three main classes: the sulfides realgar ( As4S4As4S4 size 12{"As" size 8{4}S size 8{4}} {} ) and orpiment ( As2S3As2S3 size 12{"As" size 8{2}S size 8{3}} {} ), the oxide arsenolite ( As2O3As2O3 size 12{"As" size 8{2}O size 8{3}} {}), and the arsenides and sulfaresenides of the iron, cobalt, and nickel. Minerals in this latter class include: loellinginite ( FeAs2FeAs2 size 12{"FeAs" size