Spectra in the near and mid-infrared ranges of the infrared spectrum are well-transmitted by silicon materials. They also have exceptional mechanical strength and are quite chemically inert. Infrared windows, infrared lens substrates, and filter substrates are frequently made of silicon. When used with laser mirrors with weight-sensitive designs, their excellent heat conductivity and light weight are also advantageous. Silicon can be used by Alfa Chemistry to create optical components that are made to order and can be almost any size and shape. Please get in touch with us if you require assistance.
Advantages and Uses of Silicon Materials
Silicon materials are excellent for IR LED lenses and can be used for windows and lenses in the 2 ~ 6 μm range. The strong heat conductivity and low weight of silicon, which has a refractive index near 3.4 over the whole range, make it a desirable material for IR or FTIR applications. For lasers and IR silicon viewports, silicon is also employed as a high-reflectivity mirror substrate.
Fig 1. Transmission spectrum of crystalline silicon from the visible to the near-IR. (Serpenguzel A, et al. 2010)
Optical grade silicon typically has a resistivity of 10 to 40 Ohm*cm, higher than most semiconductor applications, for optimal transmission over 10 nm. On special order, silicon materials with extremely high resistivity (>1000 Ohm*cm) are available, particularly for terahertz applications. It is typical for silicon materials to contain some oxygen when they are developed using the Czochralski pulling method (CZ), which produces an absorption band of 9 microns. This silicon material is primarily utilized as a substrate for the manufacture of filters and as an optical window in the 3 to 5 nm range.
Alfa Chemistry also offers floating zone (FZ) materials that lack this absorption potential if needed. Silicon also has a passband of 30 to 100 microns, which is only effective in very high resistivity uncompensated materials. The doping is usually boron (p-type) and phosphorus (n-type).
Fig 2. Transmittance of float zone silicon compared to float zone silicon with AR coating and filter. (Hilton A, et al. 2016)
Different types of silicon materials are available:
- Czochralski standard low-cost n-type (resistivity >10 Ohm*cm) or p-type (>20 Ohm*cm)
- High resistivity floating zone silicon with resistivity >1000 Ohm*cm
Properties of Silicon Materials
| Density | 2.33 g/cc |
| Molecular Weight | 28.09 |
| Solubility | Insoluble in Water |
| Class/Structure | Cubic diamond, Fd3m |
| Melting Point | 1420 ℃ |
| Hardness | Knoop 1150 |
| Refractive Index | 3.4223 @ 5 μm |
| Transmission Range | 1.2 ~ 15 μm |
| Reflection Loss | 46.2% at 5 μm |
| Absorption Coefficient | 0.01 cm-1 at 3 μm |
| dn/dT | 160 x 10-6/℃ |
| Youngs Modulus (E) | 131 GPa |
| Shear Modulus (G) | 79.9 GPa |
| Bulk Modulus (K) | 102 GPa |
| Apparent Elastic Limit | 124.1MPa (18000 psi) |
| Thermal Expansion | 2.6x10-6 / at 20℃ |
| Thermal Conductivity | 163.3 W/m/K at 273 K |
| Dielectric Constant | 13 at 10 GHz |
| Specific Heat Capacity | 703 J Kg-1 K-1 |
| Poisson Ratio | 0.266 |
| Elastic Coefficients | C11=167; C12=65; C44=80 |
About refractive index parameters.
"No" means ordinary light.
| µm | No | µm | No | µm | No |
|---|---|---|---|---|---|
| 1.357 | 3.4975 | 1.367 | 3.4962 | 1.395 | 3.4929 |
| 1.530 | 3.4795 | 1.660 | 3.4696 | 1.709 | 3.4664 |
| 1.813 | 3.4608 | 1.970 | 3.4537 | 2.153 | 3.4476 |
| 2.325 | 3.4430 | 2.714 | 3.4358 | 3.000 | 3.4320 |
| 3.303 | 3.4300 | 3.500 | 3.4284 | 4.000 | 3.4257 |
| 4.258 | 3.4245 | 4.500 | 3.4236 | 5.000 | 3.4223 |
| 5.500 | 3.4213 | 6.000 | 3.4202 | 6.500 | 3.4195 |
| 7.000 | 3.4189 | 7.500 | 3.4186 | 8.000 | 3.4184 |
| 8.500 | 3.4182 | 10.00 | 3.4179 | 10.50 | 3.4178 |
| 11.04 | 3.4176 |
References
- Serpenguzel A, et al. (2010). "Silicone Microspheres for VLSI Photonics." VLSI Micro- and Nanophotonics: Science, Technology and Applications. pp.3.1-3.12.
- Hilton A, et al. (2016). "WAFER-LEVEL VACUUM PACKAGING OF MICROBOLOMETER-BASED INFRARED IMAGERS." International Wafer-Level Packaging Conference.