Laser-assisted spalling of large-area semiconductor and solid state substrates
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Research Letter
Laser-assisted spalling of large-area semiconductor and solid state substrates Felix Kaule, Fraunhofer Center for Silicon Photovoltaics CSP, Otto-Eissfeldt-Str. 12, 06120 Halle, Germany Marko Swoboda, Christian Beyer, Ralf Rieske, Anas Ajaj, and Wolfram D. Drescher, Siltectra GmbH, Manfred-von-Ardenne-Ring 20, 01099 Dresden, Germany Stephan Schoenfelder, Fraunhofer Center for Silicon Photovoltaics CSP, Otto-Eissfeldt-Str. 12, 06120 Halle, Germany; Leipzig University for Applied Science, Karl-Liebknecht-Str. 132, 04277 Leipzig, Germany Jan Richter, Siltectra GmbH, Manfred-von-Ardenne-Ring 20, 01099 Dresden, Germany Address all correspondence to Jan Richter at [email protected] (Received 18 September 2017; accepted 19 December 2017)
Abstract Using kerf-free wafering technologies material losses in semiconductor manufacturing processes can be reduced drastically. By the use of externally applied stress, crystalline materials can be separated along crystal planes with clearly defined thickness. Nevertheless, during this process striations caused by the crack propagation occur. These crack growth features are river and Wallner lines. In this work, we demonstrate a process for spalling that scales favorably for large-area semiconductor substrates with a diameter up to 300 mm. To get rid of the crack growth features, a laser-conditioning process with a high numerical aperture at photon energies below the material bandgap energy, using multi-photon effects is utilized. The process affords a surface roughness Ra after spalling of 0.5) is required to confine the laser modification layer vertically. High NA optics lead to short Rayleigh ranges zR =
np w 0 l0 M 2
with n as the refractive index of the material, w0 as the beam diameter in the focal plane, λ0 as the used laser’s wavelength, and M2 as the beam quality parameter. Consequently, the highest laser intensity from focusing is also limited vertically to a thin sheet of few micron thickness. As the LAS process operates at wavelengths to which the material is transparent, high intensities are also needed to induce the required multiphoton processes at a significant rate. The probability of any multiphoton process scales with IN, where I is the laser intensity and N is the number of photons participating per event. According to this, the probability will scale most favorably for conditions with high instantaneous laser intensities, i.e. the use of pulsed lasers.
Pulsed lasers vary greatly in both average output power and pulse length, dependent on the technology they are based on. Typically, q-switched solid state lasers have nanosecond pulses, while pico- and femtosecond pulses (ultrashort pulses) require mode-locking. To achieve pulse energies suitable for material processing, ultrashort pulses require more elaborate amplification schemes in contrast to nanosecond pulses, so as to avoid damage to the amplification medium from exceedingly high instantaneous intensities. In our experiments, a range of pulsed lasers was used, with pulse durations and pu
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