Smooth advanced silicon NEMS devices

Plasma processes are important for micro electro-mechanical systems (MEMS) with critical dimensions of a few microns. The so-called Bosch etch process is probably the most popular technique in MEMS production facilities today. It uses a repeating sequence of plasma enhanced deposition to passivate silicon features, a physical etch for directional removal of this layer at the base of the features, and an isotropic etch for silicon removal at the cleared surfaces. However, it is not well suited to the nanoscale due to finite sidewall scallop size and undercut unless rate and selectivity are severely compromised. 

Anisotropic wet etching such as potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) can form smooth sidewalls, but the geometry is limited by crystal planes. Another solution is to develop post-dry etching processes to remove the sidewall scallops. Sacrificial thermal oxidation has been utilized to improve sidewall quality. However, the process consumes too much silicon and builds up residual stress. By employing hydrogen annealing, it was reported that sidewall scallops can be dramatically reduced. The surface mobility of silicon atoms is enhanced by heated hydrogen at temperatures much lower than the melting point (1414oC). Based on this phenomenon, migrating atoms smooth out the surface roughness to minimize the total surface energy without losing volume.

In order to facilitate a reliable creation of smooth and slender nanostructures, the approach is to first improve the Bosch process to minimize the scallops in the nanoscale etching. The etched nanostructure is then smoothened and reshaped by annealing at a high pressure in H2 ambient using a to-be-established RTP tool. The goal is to further reduce the scallops to ten times smoother. It is the quest to develop a fundamental understanding of the special challenges involved in surface smoothening at the nanoscale including the physics and chemistry involved. The aim is to establish a generic knowledge platform for nanoscale smoothening for future applications. The major part of this project will be related to experimental work in the cleanroom facility at DTU Danchip to demonstrate the ability to create and produce novel NEMS devices.

Contact

Henri Jansen
Professor
DTU Danchip
+45 45 25 64 33

Contact

Vy Thi Hoang Nguyen
PhD student
DTU Danchip
http://www.danchip.dtu.dk/english/research/phd-projects/smooth-advanced-silicon-nems-devices
18 JULY 2018