Monocrystalline silicon does not have grains nor domains; it is monolithic. However, it is not isotropic. Its Young modulus is different in different crystallographic directions. The strength is also different in different crystallographic directions. See below for more.
Lapped or Alkaline etched Silicon Wafer surfaces scatter light. Polished Silicon wafers are excellent reflectors of both visible and IR light. They are used for astronomic mirrors, for laser scanners and for controlled microscopic mirror arrays.
A scientist asked us the following:
I also need to know how good the surface polish is over a 25mm diameter section in the dead center of the wafer in static situ? I am interested in the fineness of the grind and the flatness? Is their any grain to a wafer?
Thermal conduction properties are needed also. The end product would be a flexible reflector.
If my idea is correct it will generate a new manufacturing stream for you.
Below are the mechanical properties of monocrystalline silicon. Please send us any question.
| PROPERTY | VALUE | UNITS | |
| Bulk modulus of elasticity | 9.8·1011 | dyn/cm2 | |
| Density | 2.329 | g/cm3 | |
| Hardness | 7 | on the Mohs scale | |
| Surface microhardness (using Knoop's pyramid test) | 1150 | kg/mm2 | |
| Elastic constants | C11 = 16.60·1011 | dyn/cm2 | |
| C12 = 6.40·1011 | dyn/cm2 | ||
| C44 = 7.96·1011 | dyn/cm2 | ||
| Young's Modulus (E) | [100] | 129.5 | GPa |
| [110] | 168 | GPa | |
| [111] | 186.5 | GPa | |
| Shear Modulus | 64.1 | GPa | |
| Poisson's Ratio | 0.22 to 0.28 | - | |
Compressive Strength 960 MPa
Tensile Strength 350 MPa
Shear Strength 240 MPa
Thermal Expansion Coefficient 2.6E-6 /ºC
Thermal Conductivity 149 W/m/ºC
Note that Silicon wafers are made from monocrystalline Silicon.
Such material does not have grains nor domains; it is monolithic.
However, it is not isotropic. Its Young modulus is different in different crystallographic directions.
The strength is also different in different crystallographic directions.
A silicon wafer when bent will break along cleavage planes.
A (100) orientation wafer will break into rectangular pieces, that is into pieces with 90º angles..
A (111) orientation wafer will break into pieces with 60º angles.
A (110) orientation wafer will break into pieces with 70.5 and 109,5º angles.
The compressive, tensile and shear strengths listed are bulk values.
In monocrystalline materials they are different in different directions
Standard 4"Ø wafers have total thickness variation of <10µm. Flatness, as measured by TIR is normally <7µm and central 25mm diameter area probably has TTV <2.5µm. Specially made 4"Ø wafers can have TTV<5 or even <1µm with TIR and TTV of central 25mm diameter, proportionally smaller.
Standard lapped or fine diamond ground 4"Ø wafers have TTV<4µm (which after polishing degrades to TTV<10µm) and Surface Roughness of about 50µm.
Standard polished 4"Ø wafers have TTV<10µm but Surface Roughnss <0.5nm as measured by an Atomic Force Balance over 2×2µm area {polishing a lapped wafer degrades its TTV some but improves Surface Roughness a lot}.