Abstract
Nonlinear optical phenomena in silicon such as self-focusing and multi-photon absorption are strongly dependent on the wavelength, energy, and duration of the exciting pulse, especially for wavelengths > 2 µ m . We investigate the sub-surface modification of silicon using ultra-short pulsed lasers at wavelengths in the range of 1950–2400 nm, at a pulse duration between 2 and 10 ps and pulse energy varying from 1 µJ to 1 mJ. We perform numerical simulations and experiments using fiber-based lasers built in-house that operate in this wavelength range for the surface and sub-surface processing of Si-wafers. The results are compared to the literature data at 1550 nm. Due to a dip in the nonlinear absorption spectrum and a peak in the spectrum of the third-order nonlinearity, the wavelengths between 2000 and 2200 nm prove to be more favorable for creating sub-surface modifications in silicon. This is the case even though those wavelengths do not allow as tight focusing as those at 1550 nm. This is compensated for by an increased self-focusing due to the nonlinear Kerr-effect around 2100 nm at high light intensities, characteristic for ultra-short pulses.
Highlights
AND STATE OF THE ARTSilicon wafers are currently largely separated from the bulk mono-crystalline Si-block using thin diamond saws, which introduce a loss of material of up to 50% [1]
We showed that the increased laser pulse energy does not necessarily result in the higher pulse energy reaching the focal spot inside the silicon, for pulse energies above 10 μJ
The model is based on the nonlinear Schrödinger equation and accounts for the wavelength dependence of the nonlinear refractive index and multi-photon absorption
Summary
Silicon wafers are currently largely separated from the bulk mono-crystalline Si-block using thin diamond saws, which introduce a loss of material of up to 50% [1]. Alternative methods for wafer separation have been in development, such as epitaxial Si lift-off, stress-induced spalling, and smart-cut [4,5]. This latter technique employs the fact that by the introduction of defects in a target layer below the Si wafer surface, this layer will be weakened, allowing the wafer itself to be removed.
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