The road to SiC process control

Manufacturers of SiC power devices produce better transistors when they turn to metrology techniques involving Fourier transform infrared spectroscopy, optical critical dimension and picosecond ultrasonics.

BY NICK KELLER FROM ONTO INNOVATION

Efforts at curbing carbon dioxide emissions are stepping up, with more electric vehicles on our roads and the installation of renewable energy sources on the rise. Alongside these advances, the makers of these green technologies are increasing the electrical efficiency of their offerings, with silicon-based power devices being ditched in favour of superior alternatives based on the likes of SiC.

Supporting this move are the superior physical properties of these compounds. Compared with silicon, semiconductors such as SiC have wider-bandgaps, a higher electron saturation velocity, a higher critical electric field and a larger thermal conductivity. Drawing on all these strengths, power transistors offer higher operating frequencies, higher power ratings, elevated operating temperatures, better cooling capability and lower energy loss – just the traits that the market wants.

Today’s manufacturers of SiC power devices are tending to focus on trench-based devices, a design that reduces on-resistance and increases carrier mobility. But there is a trade-off, with these strengths coming at the expense of increased fabrication complexity.

Figure 1. SiC trench MOSFET process flow.

To address this issue, high-volume manufacturers of SiC power devices must adopt inline process control at several key steps, including optical metrology methods like Fourier transform infrared (FTIR) spectroscopy, optical critical dimension (OCD) and picosecond ultrasonics. When armed with these techniques, chipmakers are far better informed when undertaking critical processing steps, including epilayer growth, trench etch, gate poly-silicon etch back, and frontside/backside contact metallisation.

All of these three process control techniques that have just been mentioned can play a major role in streamlining SiC production. When FTIR is adopted alongside advanced algorithms, SiC manufacturers can extract epilayer thickness and carrier concentrations for two- and three-layer stacks. What’s more, FTIR can non-destructively characterise the depth and the dopants in the implant layer directly on SiC substrates before and after the anneal process step. That’s a significant benefit, as it removes the need for monitoring silicon wafers and secondary ion mass spectrometry when undertaking implant characterisation. Meanwhile, the introduction of a multi-channel OCD tool in a SiC fab can accurately and non-destructively determine trench depth, bottom and top widths, and bottom rounding at the trench etch step, when this technique draws on electromagnetic solvers that utilise advanced rigorous coupled wave analysis. Note that bottom rounding of the trench is critical to preventing a high electric field density, and ultimately premature device failure. Lastly, picosecond ultrasonics can improve efficiency in a SiC fab by measuring frontside and backside metal contact thickness. Together, these non-destructive, in-line process control methods empower chipmakers to solve many of the challenges posed by the increased fabrication complexity of SiC power devices.