Analysis of materials modifications caused by UV laser micro drilling of via holes in AlGaN/GaN transistors on SiC
T. Wernickea, O. Krügera, M. Hermsa, J. Würfla, H. Kirmseb, W. Neumannb, T. Behmc, G. Irmerc, G. Tränklea
Published in:
Appl. Surf. Sci., vol. 253, no. 19, pp. 8008-8014 (2007).
Abstract:
Pulsed UV laser drilling can be applied to fabricate vertical electrical interconnects (vias) for AlGaN/GaN high electron mobility transistor devices on single-crystalline silicon carbide (SiC) substrate. Through-wafer micro holes with a diameter of 50-100 µm were formed in 400 µm thick bulk 4H-SiC by a frequency-tripled solid-state laser (355 nm) with a pulse width of <=30 ns and a focal spot size of ~15 µm. The impact of laser machining on the material system in the vicinity of micro holes was investigated by means of micro-Raman spectroscopy and transmission electron microscopy. After removing the loosely deposited debris by etching in buffered hydrofluoric acid, a layer of <4 µm resolidified material remains at the side walls of the holes. The thickness of the resolidified layer depends on the vertical distance to the hole entry as observed by scanning electron microscopy. Micro-Raman spectra indicate a change of internal strain due to laser drilling and evidence the formation of nanocrystalline silicon (Si). Microstructure analysis of the vias´ side walls using cross sectional TEM reveals altered degree of crystallinity in SiC. Layers of heavily disturbed SiC, and nanocrystalline Si are formed by laser irradiation. The layers are separated by 50-100 nm thick interface regions. No evidence of extended defects, micro cracking or crystal damage was found beneath the resolidified layer. The precision of UV laser micro ablation of SiC using nanosecond pulses is not limited by laser-induced extended crystal defects.
a Ferdinand-Braun-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Straße 4, 12489 Berlin, Germany
b Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie, Newtonstr. 15, 12489 Berlin, Germany
c Technische Universität Bergakademie Freiberg, Institut für Theoretische Physik, Bernhard-von-Cotta-Str. 4, 09596 Freiberg, Germany
Keywords:
Pulsed UV laser; Laser micro processing; Laser-induced material modifications; AlGaN/GaN; High-electron mobility transistors; Silicon carbide; Via holes; Micro-Raman spectroscopy; Transmission electron microscopy.
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