ALE? What’s that I hear you say? And no, it’s not a type of beer brewed using a warm fermentation method, before you all rush off to Wikipedia. ALE is far more exciting. It’s another one of those semiconductor process acronyms that describes Atomic Layer Etching. ALE is the etch equivalent to Atomic Layer Deposition (ALD). It uses a cycled two-step approach to etching, made up of initial surface modification and subsequent removal, repeated multiple times.
So what’s it good for? Atomic layer etching allows much greater control when it comes to defining nano-features, maintaining critical dimensions, reducing aspect ratio dependent etching and providing smooth surfaces. These properties make the technique highly applicable to the sharp end of silicon transistor manufacturing. However, there may be another application that could potentially make use of ALE. The world of wide band gap power semiconductors is also looking for a solution. Compared with silicon, GaN offers higher temperature capability, much higher breakdown voltage, and lower switching losses leading to greater power conversion efficiency. The device of interest is the high electron mobility transistor. The specific device of interest is the enhancement mode HEMT or, E-mode HEMT.
There are a variety of device designs including recessed Schottky gate, p-GaN gate that have a number of disadvantages when it comes to properties such as low or unstable threshold voltage and / or high leakage current. The inversion type device shown in fig.1 has demonstrated low gate leakage current, higher and stable threshold voltage but, has also proven to be tricky to make. What it needs is a way of precisely controlling the etch stop for the gate recess etch and minimising fixed charges and interface traps. Oxford Instruments Plasma Technology is using both in house developed ALE and ALD to determine whether these techniques are suitable for application to mass production. So if you’re into making power devices using GaN, let’s see if ALE has found a new home. Failing that, it’s off to the pub for some warm beer. Watch this space.
Author – Knut Beekman, Senior Product Manager (Etch)