Etching processes for High Aspect Ratio Micro Systems Technology (HARMST)

Abstract

This paper reports on the development of a dry etching based HARMS-Technology which will offer the potential to manufacture micro-engines, micro-turbines, micro-sensors, micro-actuators, and electronic circuits onto a single silicon IC chip. This technology is based on the highly anisotropic and selective dry etching of Si-monocrystals. The suitability of reactive ion etching for the fabrication of micro electro mechanical systems (MEMS) has been evaluated by characterising the change of lateral dimensions vs. depth in etching deep structures in silicon. Fluorine, chlorine and bromine containing gases have provided the basis for this investigation. A conventional planar RIE (Reactive Ion Etching) reactor has been used, in some cases with magnetic field enhancement or ICP (Inductive Coupled Plasma) Source and low substrate temperature. For reactive ion etching based on Cl2 or Cl2/HBr plasma a slightly “positive” (top wider than bottom) slope is achieved when etching structures with a depth of several 10 μm, whereas a “negative” slope is obtained when etching with an SF6/CCl2F2 based plasma. Pattern transfer with vertical walls is obtained for reactive ion etching based on SF6 (with O2 added) when maintaining the substrate at low temperature (in range ≈−100 °C). Further optimisation of plasma chemistries and reactive ion etching procedures should result in runouts in the order or 0.1 μm/100 μm depth in Si as well as in organic materials. Etching processes for HARMST is demonstrated in the realisation in Si microturbine. Axes or stators (nonmoving parts) are etched into the initial Si-wafer. The movable parts (rotors, beams, etc.) are prepared from electro-chemically etched Si-membranes with defined thicknesses that, all movable parts are created lithographically on the SiNxOy surface. This is followed by dry etching the mono-crystalline Si-membrane down to the SiNxOy sacrificial layer on the back side of the membrane by an RIE-process. The wafer with the movable parts is flipped onto the wafer with the already etched axis and then positioned and centred. The SiNxOy-sacrificial layer is then dissolved by a chemical wet or vapour etch process. Subsequent bonding with a Pyrex glass wafer seals the parts.