3.1 Synthesis and purification of bulk semiconductors  (Page 3/7)

The reasons for the predominant use of SiHCl ₃ in the synthesis of EGS are as follows:

1. SiHCl 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 SiCl 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, [link] .

Since the reaction is actually an equilibrium and the formation of SiHCl ₃ highly exothermic, efficient removal of generated heat is essential to assure a maximum yield of SiHCl 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 FeCl ₃ (b.p. = 316 °C), AlCl ₃ (m.p. = 190 °C subl.), and BCl ₃ (b.p. = 12.65 °C), respectively. Fractional distillation of SiHCl 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 SiHCl ₃ in a chemical vapor deposition (CVD) process similar to the epitaxial growth of Si. The high-purity SiHCl ₃ is vaporized, diluted with high-purity hydrogen, and introduced into the Seimens deposition reactor, shown schematically in [link] . 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 SiHCl ₃ to yield silicon, as described by the reverse reaction shown in Eq. 5.

The shift in the equilibrium from forming SiHCl ₃ from Si at low temperature, to forming Si from SiHCl ₃ at high temperature is as a consequence of the temperature dependence ( [link] ) of the equilibrium constant ( [link] , where ρ = partial pressure) for [link] . Since the formation of SiHCl ₃ is exothermic, i.e., ΔH<0, an increase in the temperature causes the partial pressure of SiHCl 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.

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.