Researchers in the United States and Singapore have been pursuing the optimization of electric fields in trench-based gallium nitride (GaN) power electronic devices containing vertical architectures. Vertical structures are capable of deeply pushing peak fields within GaN material in order to avoid premature breakdowns from surface effects. Researchers from MIT in the Singapore-MIT Alliance for Research and Technology stated that trench-based structures achieved the best performance for advanced Schottky rectifiers and power transistors.
Having said that, premature breakdowns can occur when sharp corners are flooded by electric fields. One approach to stabilizing this condition is through dry etch techniques. Etches are used to create trenches although they tend to result in sharp corners and tough sidewalls, which promotes unwanted current leakage. If the structure is heated, the trenches can be rounded off, which can also smooth the sidewalls, however the material’s quality is degraded during the process.
A slower approach involves second etch using wet solutions, which also works when trying to smooth rough sidewalls. Etching n– GaN layers on two-inch silicon or n+ -GaN substrates into the wafers, the trench process was developed using samples on silicon, while free-standing GaN was formed using trench-based devices.
The dry etch part is comprised of boton trishloride and chlorine inductively couple plasma processing through nickel hard masks, which gives trenches of the order 2 μm wide and deep. In comparison to conventional oxide masks, using metal hard masks enables smoother etch sidewall since there’s less oxide edge erosion under high ion energies.
One of the ways researchers achieved corner rounding and etch damage repair was using tetramethylammonium hydroxide wet etching, along with piranha clean. In temperatures of 85 degrees Celsius, the anisotropic process (which preferred to eat the sidewalls), took 70 minutes to complete, during which scientists discovered how slight differences in smoothness for trenches aligned to different crystal orientations, were subjected to TMAH treatment. After just ten minutes into the process, piranha clean removed nickel and mask residues.
Having said that, dry etch is capable of adjusting to provide flat-bottom or tapered-angled-down trenches. A less anisotropic dry etching is capable of enhancing the lateral etching, which reduces the tapered angle along the dry etching sidewalls, and produces a tapered trench bottom after the TMAH wet etching. The less-anisotropic chlorine-boron trichloride-based ICP etching also can be achieved by reducing the bias power or increasing the ratio between boron trichloride and chlorine. Dry etch containing lower bias power and higher boron trichloride flow rate produced a pointed trench bottom, which rounds after having a wet etch treatment applied.
Trench metal-insulator-semiconductor barrier Schottky rectifiers were produced with several different trench profiles such as unrounded and rounded with tapered bottom, and flat-bottomed rounded. Following the etching, the structure was coated with 250 nm of plasma-enhance chemical vapor deposition silicon nitride. The top Schottky contacts were comprised of nickel and gold, while the backside contact contained titanium and aluminum.
Without the rounded trench corners, the device endured reverse-bias leakage and breakdown at -150V. Researchers are certain the leakage mainly attributed to this trap-assisted space charge limited current. The structure with flat-bottom rounded-corner trenches gave the lowest reverse-bias leakage current and highest breakdown at 500V. Leakage is attributed mainly to a combination of dominant factors affecting the unrounded and flat-bottom devices.
Researchers have predicted further current-blocking improvement could derive from enhancement of dielectric quality, insertion, or implanted field rings near the trench bottoms by using the optimize trench. In addition, the introduction of carbon-doped GaN/p-GaN hybrid blocking layers has also been considered a viable method.
Filed Under: M2M (machine to machine)