Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers

Ultra-short pulsed laser ablation and micromachining of n-type, 4H-SiC wafer was performed using a 1552 nm wavelength, 2 ps pulse, 5 μJ pulse energy erbium-doped fiber laser with an objective of rapid etching of diaphragms for pressure sensors. Ablation rate, studied as a function of energy fluence, reached a maximum of 20 nm per pulse at 10 mJ/cm², which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm². Scanning electron microscope images supported the Coulomb explosion (CE) mechanism by revealing very fine particulates, smooth surfaces and absence of thermal effects including melt layer formation. It is hypothesized that defect-activated absorption and multiphoton absorption mechanisms gave rise to a charge density in the surface layers required for CE and enabled material expulsion in the form of nanoparticles. Trenches and holes micromachined by the picosecond laser exhibited clean and smooth edges and non-thermal ablation mode for pulse repetition rates less than 250 kHz. However carbonaceous material and recast layer were noted in the machined region when the pulse repetition rate was increased 500 kHz that could be attributed to the interaction between air plasma and micro/nanoparticles. A comparison with femtosecond pulsed lasers shows the promise that picosecond lasers are more efficient and cost effective tools for creating sensor diaphragms and via holes in 4H-SiC.

Source:Applied Surface Science

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Defect mapping in 4H-SiC wafers

A new non destructive method has been applied for revealing micropipes in SiC wafers. Two 4H-polytype commercial wafers have been studied. The comparison made with the commonly used KOH etching shows some discrepancy in the absolute values, which can be attributed to the specificity of the two techniques. However the distribution course is identical. More micropipes are found close to the periphery of the wafers. In addition, the dislocation density and their distribution over the wafers are obtained and they are analysed in conjunction with microhardness measurements. It appears that these dislocations cannot be directly related to the micropipes.

Source:Materials Science and Engineering: B

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Single-crystalline 4H-SiC micro cantilevers with a high quality factor

Single-crystalline 4H-SiC micro cantilevers were fabricated by doping-type selective electrochemical etching of 4H-SiC. Using this method, n-type 4H-SiC cantilevers were fabricated on a p-type 4H-SiC substrate, and resonance characteristics of the fabricated 4H-SiC cantilevers were investigated under a vacuum condition. The resonant frequencies agreed very well with the results of numerical simulations. The maximum quality factor in first-mode resonance of the 4H-SiC cantilevers was 230,000. This is 10 times higher than the quality factor of conventional 3C-SiC cantilevers fabricated on an Si substrate.

Highlights

• We fabricated single-crystalline 4H-SiC cantilevers by electrochemical etching.

• Very sharp resonance was observed for the fabricated 4H-SiC cantilevers.

• The maximum quality factor of 4H-SiC cantilevers is 230,000.

• The quality factor is 10 times higher than that of 3C-SiC cantilevers.

Source:Applied Surface Science

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Variation of Etch Pit Size by Screw Dislocation Tilt in 4H-SiC Wafer

The wide size distribution of the hexagonal etch pit of screw dislocations (SD) in 4H-SiC wafer was found in spite of the narrow size distribution of the SD pit in epitaxial film. Calculation on the basis of the strain energy equation indicated that etch pit size depends on the Burgers vector and dislocation tilt. Size variation of SD etch pits in 4H-SiC wafer fabricated by sublimation method is explained to be caused by the dislocation tilt by observing the sizes and the positions of etch pits from the surface of the epitaxial film to the inside of 4H-SiC wafer. The SDs in 4H-SiC wafer fabricated by sublimation method propagate to c-axis direction in macroscopic but changing tilt in microscopic.

Source:Applied Surface Science

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