Page 58 - Sami Franssila Introduction to Microfabrication

Silicon 37

Table 4.1 Properties of silicon at 300 K
Structural and mechanical
Atomic weight 28.09
−3
Atoms, total (cm ) 4.995 × 10 22
Crystal structure Diamond (FCC)
Lattice constant ( ˚ A) 5.43
3
Density (g/cm ) 2.33
−2
Density of surface atoms (cm ) (100) 6.78 × 10 14
(110) 9.59 × 10 14
(111) 7.83 × 10 14
Young’s modulus (GPa) 190 (111) Crystal orientation
Yield strength (GPa) 7
Fracture strain 4%
Poisson ratio, ν 0.27
2
Knoop hardness (kg/mm ) 850
Electrical
Energy gap (eV) 1.12
−3
Intrinsic carrier concentration (cm ) 1.38 × 10 10
Intrinsic resistivity ( -cm) 2.3 × 10 5
Dielectric constant 11.8
Intrinsic Debye length (nm) 24
2
Mobility (drift) (cm /Vs) 1500 (electrons)
475 (holes)
−1
Temperature coeff. of resistivity (K ) 0.0017
Thermal
◦
−1
Coefficient of thermal expansion ( C ) 2.6 × 10 −6
◦
Melting point ( C) 1414
Specific heat (J/kg K) 700
Thermal conductivity (W/m K) 150
2
Thermal diffusivity 0.8 cm /s
Optical
Index of refraction 3.42 λ = 632 nm
3.48 λ = 1550 nm
Energy gap wavelength 1.1 µm (Transparent at larger wavelengths)
6
Absorption >10 cm −1 λ = 200–360 nm
5
10 cm −1 λ = 420 nm
4
10 cm −1 λ = 550 nm
3
10 cm −1 λ = 800 nm
<0.01 cm −1 λ = 1550 nm
Source: Data from Hull, R. (1999)
orientation is dipped into the silicon melt. The silicon away from the crystallization interface, and therefore
solidifies into a crystal structure determined by the seed large-diameter ingots have lower pulling rates. While a
crystal. A thin neck is quickly drawn to suppress the 100 mm diameter ingot can be pulled at 1.4 mm/min,
defects that develop because of a large temperature the 200 mm ingot pull rate is 0.8 mm/min. In order to
difference between the seed and the melt, and then the grow low vacancy concentration crystals, pulling rates
pulling rate is lowered. Both the ingot and the crucible as low as 0.35 mm/min are employed. Typical pulling
are rotated (in opposite directions); ingot rotation is ca. time is 30 h, not including heating and cooling, which
20 rpm and crucible rotation about 10 rpm. add another 30 h to the process, for 200 mm ingots.
The ingot diameter is determined by the ingot pull The ingot length is determined by the yield strength
rate. The pulling rate is limited by heat conduction of silicon neck and crucible size. The thin neck is not