Silicon absorption coefficient and penetration depth calculator
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KEY POINTS
Silicon is an indirect band gap semiconductor, and its absorption strength changes by orders of magnitude with wavelength. Ultraviolet light is absorbed within about 10 nm, blue reaches about 0.4 µm, red about 3 µm and 850 nm about 20 µm, while silicon is nearly transparent beyond about 1100 nm. This is why excimer laser annealing can melt only the surface and stealth dicing can work inside the wafer.
Inputs
Values for lightly doped crystalline silicon at room temperature. At high temperature the band gap narrows and the absorption edge moves to longer wavelengths; heavily doped layers add free carrier absorption. Amorphous silicon absorbs more strongly than crystalline silicon in the visible range.
Wavelength and penetration depth
Penetration depth of common lasers and light sources
| Light source | Wavelength | Absorption coefficient | Penetration depth |
|---|---|---|---|
| KrF excimer (laser annealing) | 248 nm | 1.84×10⁶ cm⁻¹ | 5.43 nm |
| XeCl excimer (laser annealing, LTPS) | 308 nm | 1.59×10⁶ cm⁻¹ | 6.27 nm |
| Blue laser | 450 nm | 25,500 cm⁻¹ | 392 nm |
| Green laser (frequency-doubled Nd:YAG) | 532 nm | 7,800 cm⁻¹ | 1.28 µm |
| Red He-Ne (thickness gauges, alignment) | 633 nm | 3,210 cm⁻¹ | 3.12 µm |
| Diode laser (808 nm) | 808 nm | 789 cm⁻¹ | 12.7 µm |
| Near infrared (stealth dicing, etc.) | 1064 nm | 10.6 cm⁻¹ | 944 µm |
1064 nm is longer than silicon's band gap wavelength and is barely absorbed at low intensity. Stealth dicing focuses this transmitted light inside the wafer and uses nonlinear absorption to modify only the interior.
Absorption depth changes with wavelength
Silicon is an indirect band gap semiconductor, and its absorption strength varies by orders of magnitude with wavelength. Ultraviolet light is absorbed within about 10 nm of the surface, blue reaches about 0.4 µm, red about 3 µm and near-infrared 850 nm about 20 µm. Beyond about 1100 nm, matching the band gap, silicon is nearly transparent.
This property is put to different uses. Excimer laser annealing uses ultraviolet light to melt and activate only a very thin surface layer; in image sensors, the depth of the photodiode matters for each color separated by the color filters. Ranging sensors that need near-infrared sensitivity use thicker absorbing layers or surface microstructures to lengthen the optical path.
FAQ
How deep does 532 nm light penetrate silicon?
The 1/e penetration depth at 532 nm is about 1.3 µm. Enter any wavelength between 250 and 1150 nm to see the value.
Why can 1064 nm lasers process the inside of a wafer?
Silicon barely absorbs 1064 nm at low intensity, so the light passes through, and only at the tight focus does nonlinear absorption modify the material.
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Last updated: October 23, 2026