Wafer Manufacturing Process

Ingot Production

• Silicon
  • Metallurgical-grade silicon (MGS)
  • Upgraded MGS (UMGS)
  • Electronic-grade silicon (EGS)
• Crystal Growth
  • Czochralski growth (CZ)
  • Float-zone growth (FZ)
• Preparation
  • Ingot cropping
  • Ingot squaring
  • Rod grinding

Wafer Processing

• Slicing
  • Wire sawing
  • Internal diameter sawing (IDs)
• Profile control
  • Wafer edge profiling
  • Wafer lapping
  • Wafer etching
  • Wafer polishing
• Additional layers
  • Epitaxial deposition
    • Epitaxial wafer
  • Layer transfer (LT)
    • SOI wafer
• Preparation
  • Wafer inspection
  • Wafer packaging
  • Wafer shipping

Electronic-grade silicon (EGS or EG-Si) or semiconductor-grade silicon (SGS) is a highly-purified version of the metallurgical-grade silicon with extremely low impurities suitable for microelectronic device applications. Electronic-grade silicon is the raw material used for the growth of single-crystal silicon in the manufacturing of silicon wafers. From MGS to EGS, the impurities go from the order of parts per million to the low parts per billion.

Creating EGS is expensive and is thus only usually justified for integrated circuits. For other applications such as solar cells and liquid crystal displays, EGS is cost prohibitive, making upgraded metallurgical-grade silicon (UMG-Si) a much more attractive alternative.

Overview

While pure silicon (referred to as metallurgical-grade silicon) is of very good quality, it is still unsuitable for electronic device fabrication. Even impurities in the order of parts per million or less have significant impact on carrier mobility, reliability, and other aspects of the microelectronic device.

Impurity Concentrations in MG-Si (ppm)
Al	1000-5000		B	35-50
P	20-50		Ca	250-600
Cr	50-200		Cu	15-60
Fe	1600-6500		Mn	50-100
Mo	2-20		Ni	20-100
Ti	150-300		V	50-250
Zr	20-30

Metallurgical-grade silicon must be furthered purified to levels of usually less than a part per billion in order to be called electronic-grade silicon which can be used in subsequent fabrication steps.

Impurity Concentrations in EG-Si (ppb)
Element	Concentration		Element	Concentration
B	≤ 0.1		C	50-1000
Cr	≤ 0.01		Co	≤ 0.01
Cu	≤ 0.1		Fe	0.1 - 1
Ni	0.1 - 0.5		O	100-500
P	≤ 0.5		Zn	≤ 0.1

Purification

The most common way to further purify metallurgical-grade silicon into electronic-grade silicon is by converting it into a liquid volatile silicon such as monosilane (SiH₄), dichlorosilane (SiH₂Cl₂), and trichlorosilane (SiHCl₃).

Trichlorosilane

The most common method is using trichlorosilane (TCS), a volatile liquid at room temperature that can easily be purified through subsequent distillation. Trichlorosilane can also be stored in standard carbon steel tanks, making storage and handling very convenient. Conversion is done by reacting it with anhydrous hydrogen chloride at 300 °C in a fluidized bed reactor to form polychlorinated silanes.

MGS + 3HCl → SiHCl₃ + H₂

Note that in addition to TCS, Hydrogen is also produced. TCS boils at 31.8 °C and is liquid at room temperature meaning impurities can easily be removed through fractional distillation to produce extremely high purity liquid. The liquid TCS is then converted into solid polysilicon through the Siemens process which results in electronic-grade silicon with purity level as high as 9N or even 10N (99.99999999998%). The process results in TCS decomposing with the pure silicon being deposited on the highly-pure silicon slim rods located inside the reactor.

Monosilane

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