Preparation of Micro- and Nanostructures

Abstract

This paper focuses mainly on the development of dry etching for structures of high aspect ratios, which will offer the potential to manufacture micro-sensors, micro-engines, micro-turbines, micro-actuators, and electronic circuits onto a single IC silicon chip. This technology is based on the highly anisotropic and selective dry etching of Simonocrystals. 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 with the ICP (Inductive Coupled Plasma) Source and low substrate temperatures. For reactive ion etching based on Cl₂ or Cl₂/HBr plasma, a slightly “positive” (top wider than bottom) slope is achieved when structures are etched with a depth of several 10 pm, 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 Iow temperature (in range ≈-100–). Further optimization of plasma chemistries and reactive ion etching procedures should result in runout depths of the order of 0.1 µm/100 gm in Si as well as in organic materials. Etching processes are demonstrated in the realisation in Si microturbine. Axles or stators (nonmoving parts) are etched into the original Si-wafer. The movable parts (rotors, beams, etc.) are prepared from electrochemically etched Si-membranes with defined thicknesses. Then all movable parts are created lithographically on the SiN_xO_y surface. This is followed by dry etching the mono-crystalline Si-membrane down to the SiN_xO_y 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 axle and then positioned and centred. The SiN_xO_y sacrificial layer is then dissolved by a chemical wet or vapour etch process. Subsequent bonding with a Pyrex glass wafer seals the parts. The topic of lithography (masked ion beam lithography, MIBL), which delivers high resolution and large focus depths as well as e-beam lithography with tunnel-tips, is also addressed in this paper.