LabAdviser/Technology Research/Smoothed advanced silicon NEMS devices

Smoothed advanced silicon NEMS devices

• Project type: Ph.d. project
• Project responsible: Vy Thi Hoang Nguyen
• Supervisors:Henri Jansen, Flemming Jensen, Jörg Hübner
• Partners involved: DTU Danchip

Project Description

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 (1414^oC). Based on this phenomenon, migrating atoms smooth out the surface roughness to minimize the total surface energy without losing volume.

Hydrogen-induced surface migration not only changes the surface morphology but also affects the global profile if the surface migration length is comparable to or larger than structural dimensions. This effect is similar to the reflow process in glasses or polymers, but unlike the reflow process, this mechanism only depends on surface-atom movement and the crystalline structure is preserved. Thermal annealing in hydrogen ambient has also been reported to produce round corners and various voids in bulk silicon.

This project aims to facilitate a reliable creation of smooth and slender nanostructures to demonstrate the ability in some state-of-the-art and novel NEMS applications include 1) resonators and sensors for physical and bio-chemical sensing down to the molecular level both because of the much reduced mass, 2) nanowires for novel transistors and photovoltaics increasingly exploring quantum effects starting at the sub-50nm level, 3) nanostructures for photonics (around the wavelength of the guided photons) and next generation storage (10nm and below), 4) black silicon for high area catalyzed reaction chambers and photovoltaics, 5) through wafer vias for packaging applications, and 6) nano imprint lithography (NIL) masters. The approach is to first create nanoscale silicon patterns using the modern high density plasma tool (SPTS DRIE-Pegasus). This structure is then annealed in pure hydrogen at high temperature (1100^oC) and high vacuum (10E-6 mbar) using an annealling tool (Annealsys RTP-150-HV). The quantitative analysis on the sidewall roughness reduction will be measured using scanning electron microscopes (SEM) and atomic force microscope (AFM).

Publications

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