Summary Description

Si is a high refractive index material (3.4 @ 8 microns) with very broad spectral transmission in the mid-IR and far-IR regions of the spectrum. However, this broad spectral transmission is not contiguous and Si has very strong absorption in the mid-IR due to impurities; interstitial oxygen at 1107 cm^-1, and substitutional carbon at 605 cm^-1. These impurity bands can be reduced by using Si produced by the “float zone” technique but another absorption mechanism is also present in Si, namely the multi-phonon lattice bands, the “two phonon band” centered at 630 cm^-1 and the “three phonon band” centered at 1118 cm^-1. All of these absorption bands are fairly broad and thus overlap. As a result Si optical elements have never been popular in general mid-IR spectroscopic applications. Alternatives are ZnSe and Ge.

Advantages for all Spectral Regions and Application

1. Zero water solubility
2. Good chemical resistance
3. Excellent abrasion resistance

Advantages as a Far-IR Beamsplitter

1. A single Si pellicle beamsplitter can provide excellent spectral response from 600 cm-1 to below 10 cm-1.
2.  Si beamsplitters are not subject to acoustic noise like Mylar or coated Mylar beamsplitters.

Disadvantages as a Far-IR Beamsplitter

1. The reference laser in FT-IR systems requires a separate optical path

Advantages for General Spectroscopy Sampling, Window or ATR

1. High refractive index in ATR applications means low depth of penetration.
2. Excellent transmission when broad band anti-reflec tive (BBAR) coating is used.

Disadvantages for General Spectroscopy Sampling, Window or ATR

1. Spectral range limited: 6000 cm-1 to 1100 cm-1
2. Windows can produce large interference fringing.

Advantages for Use in Si Semiconductor Process Chemical Research

1. Large ATR elements can be produced for existing commercial spectrometer accessories which can acquire spectra with mono-layer sensitivity.
2. ATR and window elements can be produced with crystal orientation as that used in the semiconductor wafer production.

Physcial Data

Melting Point: 1420 °C

Density: 2.3 g/cm³

Solubility in H₂O: Insoluble

Hardness: 1150 kg/mm²

Appearance: Metallic

Reflective Index

Specific index listed; Generic: 3.42 @ 10 microns

Spectral Range

Short Wavelength Limit: 8900 cm^-1 (1 mm)

Long Wavelength Limit (mid-IR): 624 cm^-1 (1 mm), 969 cm^-1 (4 mm)

Coatings

The infrared throughput of Si can be significantly increased with application of specialized BBAR coatings to levels exceeding 95% transmission. It may also be coated to control depth of penetration in ATR applications.

Typical Uses

• IR transmission analysis
• ATR elements
• Si has been used as an FT-IR beamsplitter in the far-IR spectral region

Notes

Short and Long Wavelength Limits defined for which transmissivity is greater than 50% of stated crystal thickness.

1. Reference for above RI values: The Infrared Handbook, The Infrared Information Analysis Center, Environmental Institute of Michigan, 1985.