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Note for VLSI Design - VLSI By ramji dr

  • VLSI Design - VLSI
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1 EC304 VLSI Module 1 Material Preparation- Purification, Crystal growth (CZ and FZ process), wafer preparation Thermal Oxidation- Growth mechanisms, Dry and Wet oxidation, Deal Grove model. Diffusion- Fick‘s Laws, Diffusion with constant surface concentration and from a constant source, diffusion techniques. Ion implantation-Technique, Range Theory, annealing. Material Preparation: Silicon is the most important semiconductor for the microelectronics industry. When compared to germanium, silicon excels for the following reasons: (1) Si has a larger bandgap (1.1 eV for Si versus 0.66 eV for Ge). (2) Si devices can operate at a higher temperature (150o C vs 100o C). (3) Intrinsic resistivity is higher (2.3 x 105 Ω-cm vs 47 Ω-cm). (4) SiO2 is more stable than GeO2 which is also water soluble. (5) Si is less costly. The first step in integrated circuit (IC) fabrication is preparing the high purity single crystal Si wafer. This is the starting input to the fab. Si wafer refers to a single crystal of Si with a specific orientation, dopant type, and resistivity. Typically, Si (100) or Si (111) wafers are used. The numbers (100) and (111) refers to the orientation of the plane parallel to the surface. Traditionally, bipolar devices are fabricated in oriented crystals whereas materials are preferred for MOS devices. Real crystals are imperfect and contain point defects, line defects or dislocations, area or plane defects, and volume defects. The wafer should have structural defects like point defects, line defects or dislocations, area or plane defects, and volume defects they are in the order of ppb (parts per billion). Defects in Si

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2 Point defects manifest themselves in several forms, as shown in Figure 1.1. Any non-silicon atoms incorporated into the lattice at either a substitutional or interstitial site are considered point defects Figure 1.1 Point defects in a simple lattice. Dislocations in a lattice are dynamic defects as shown in Figure 1.2. That is, they can diffuse under applied stress, dissociate into two or more dislocations, or combine with other dislocations. Figure 1.2 edge dislocation in a cubic lattice Purification (Converting MGS to EGS - Seimens process) EGS -Electronic grade Si MGS- Metallurgical grade Si Quartzite (SiO2) or sand

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3 The starting material for Si wafer manufacture is called Electronic grade Si (EGS). This is an ingot of Si. To get EGS, the starting material is called Metallurgical grade Si (MGS). The first step is the synthesis of MGS from the ore. Ore ( quartzite (SiO2) or sand ) MGS (Si (l))  EGS(Ingot) The starting material for Si manufacture is quartzite (SiO2) or sand. The ore is reduced to Si by mixing with coke and heating in a submerged electrode arc furnace. The SiO2 reacts with excess C to first form SiC. At high temperature, the SiC reduces SiO2 to form Si. The overall reaction is given by SiC (s) + SiO2 (s) → Si (l) + SiO (g) + CO (g) (1) Coke (fuel) - Wikipedia https://en.wikipedia.org/wiki/Coke_(fuel) Coke is a fuel with few impurities and a high carbon content, usually made from coal. It is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulphur bituminous coal. Cokes made from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is synthetic The Si(l) formed is removed from the bottom of the furnace. This is the MGS and is around 98% pure.

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4 One of the techniques for converting MGS to EGS is called the Seimens process. In this the Si is reacted with HCl gas to form tricholorosilane, which is in gaseous form. Si (s) + 3HCl (g)  SiHCl3 (g) + H2 (g) (2) o This process is carried out in a uidized bed reactor at 300 C, where the trichlorosilane gas is removed and then reduced using H2 gas. 2SiHCl3 (g) + 2H2 (g)  2Si (s) + 6HCl (g) (3)

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