Crystal growth-bulk and epitaxial film-part 1

CRYSTAL GROWTH

• (1)Preparation of polycrystalline Electronic Grade Silicon material in Siemens Reactor .
• (2)Bulk Crystal preparation or Pure,Electronic Grade, Single Crystal Silicon ingot preparation by Czochralski method and by Float Zone Method .
• (3)Diamond cutting of Silicon Ingot into Silicon Wafers and lapping, polishing and chemical etching of the Wafers to obtain mirror finished Substrates for IC preparation.
• (4)Epitaxial Film Growth- Chemical Vapour Phase Deposition (CVD) or Liquid phase Epitaxy (LPE).
• (5)For Photonic Application , compound Semiconductor Epitaxial Film Growth is achieved by Molecular Beam Epitaxy (MBE).

PREPARATION OF POLYCRYSTALLINE ELECTRONIC GRADE SILICON MATERIAL.

Quartz Sand (SiO2) plus Coke are put in a container with submerged electrode Arc Furnace . Electric energy is consumed at the rate of 13 kWhr per kg and SiO2 is reduced to Silicon by Coke. This silicon is of 98% purity. The solid silicon is pulverized and kept in an oven where Hydrogen Chloride vapour is passed at 300ºC. Silicon is converted into SiHCl3 (liquid) which is called Trichloro Silane.

SiO2(Quartz Sand)+ 2C (coke) = (at high Temperature)Si +CO2

Si(pulverized form) +3HCl (vapour) =(At 300ºC) SiHCl3 (liquid) +H2↑

SiHCl3(liquid) has B.P 31.8ºC. Whereas most impurities are less volatile. Therefore by multiple distillation electronic grade TriChloroSilane is obtained which eventually is used in SIEMEN’S REACTOR.

H2 + SiHCL3 =(at 1000ºC) Si +3HCL

(Hydrogen reduction)

(a form of CVD of Silicon on slim rod of Silicon on Tantulum wire)

Slim rod contains Tantalum Wire surrounded by Silicon as shown in Figure 2. Slim rod can be formed either by CVD on hot Tantalum wires or can be pulled from the melt.

(When Si deposited by CVD on hot Tantalum wires then there can be thermal run away due to negative temperature coefficient of resistance(t.c.r) of surrounding silicon deposit dominating over positive t.c.r of the Tentalum wires. The thermal runaway has to be prevented by controlling the current.)

The resulting fattened slim rod has considerable metal contamination at the core of the rod.This can be removed by dissolving the central metallic core with acid. ANitric/Hydrofluoric acid mixture is then pumped through the hollow core. This cleaning method widens the hollow core by removing silicon and metal contamination(1mm/hr). First nitric acid is used to dissolve the metallic core . Next Hydrofluoric acid is passed through the hollowed core to dissolve the inner layer of silicon and in the process removing the metal contamination. The fattened, purified rod of silicon thus obtained will be used as the feedstock for crystal pulling. Hydrofluoric acid is very corrosive and it can effect our bones even . Therefore Teflon gloves , Teflon Aprons,Teflon tweezers and Teflon beakers have tobe used while working with HF acid.