Wietze Nijdam

Resistless patterning of sub-micron structures by evaporation through nanostencils

We describe a sub-micron shadow-mask evaporation or nanostencil technique for single-layer material patterning. The technique does not involve photoresist processing steps and is therefore applicable on arbitrary surfaces. It allows for rapid fabrication of sub-micron structures on a milimeter scale. The nanostencils used here are thin microfabricated silicon nitride membranes, 1 x 3 mm wide and 0.3-1.0 @mm thick. They are peforated by a regular two-dimensional array of sub-micron apertures of 1 @mm periode. Metal evaporation of 40 nm thick Cr/Au through the apertures directly onto the substrate yields the exact 1:1 replication of the aperture pattern. The smallest dot size on a flat substrate obtained is 120 nm, whereas 750 nm dots are reproduced, both on free-standing micromechanical beams and on a surface recessed by 5-10 @mm.

Deflection and maximum load of microfiltration membrane sieves made with silicon micromachining

With the use of silicon micromachining, an inorganic membrane sieve for microfiltration has been constructed having a silicon nitride membrane layer with thickness typically 1 /spl mu/m and perforations typically between 0.5 /spl mu/m and 10 /spl mu/m in diameter. As a support a -silicon wafer with openings of 1000 /spl mu/m in diameter has been used. The thin silicon nitride layer is deposited on an initially dense support by means of a suitable chemical vapor deposition method (LPCVD). Perforations in the membrane layer are obtained with use of standard photo lithography and reactive ion etching (RIE). The deflection and maximum load of the membrane sieves are calculated in a first approximation. Experiments to measure the maximum load of silicon-rich silicon nitride membranes have confirmed this approximation.

Large area photonic crystal slabs for visible light with waveguiding defect structures: Fabrication with focused ion beam assisted laser interference lithography

Extended photonic crystal slabs with light-guiding defects have been created by a combination of laser interference lithography (LIL) and local focused ion beam (FIB) assisted deposition. Large area, highly uniform photonic crystal slabs for visible light are thus made possible. The Figure shows a freestanding Si3N4-air photonic crystal with a light- guiding defect line running along the center of the slab (total length = 1 mm).

Microsieves made with laser interference lithography for micro-filtration applications

A microsieve with a very uniform pore size of 260 nm and a pore to pore spacing of 510 nm has been fabricated using multiple exposure interference lithography and (silicon) micro-machining technology. The sieve consists of a 0.1 µm thick silicon nitride membrane perforated with sub-micron diameter pores and a macro perforated silicon support. The calculated clean water flux is at least one to two orders higher than that of conventional inorganic membranes.

Filtration of lager beer with microsieves: flux, permeate haze and in-line microscope observations

Membrane fouling during filtration of lager beer with microsieves was studied through in-line microscope observations. It was observed that the main fouling was caused by micrometre-sized particles, presumably aggregated proteins. These particles formed flocks covering parts of the membrane surface. Most of the flocks could be removed by a strong temporary increase in crossflow. Underneath the flocks a permanent fouling layer was formed inside the pores. This made frequent removal of the flocks crucial in delaying the process of permanent in-pore fouling. Besides the fouling process the influence of pore size on permeate flux and turbidity was investigated. Centrifuged beer appeared to give a significantly clearer permeate than rough beer. For centrifuged beer and a microsieve with a pore diameter of 0.55 μm a haze of 0.23 EBC was obtained during 10.5 h of filtration at an average flux of 2.21×103 l/m2 h. For a sieve with slit-shaped perforations of 0.70 μm×3.0 μm a haze of 0.46 EBC was obtained during 9 h of filtration at an average flux of 1.43×104 l/m2 h. This flux is more than two-orders of magnitude higher than is commonly obtained with membrane-filtration of lager beer. Concentration of the beer by a factor of 12 hardly influenced the magnitude of the flux.

Fabrication of microsieves with sub-micron pore size by laser interference lithography

Laser interference lithography is a low-cost method for the exposure of large surfaces with regular patterns. Using this method, microsieves with a pore size of 65 nm and a pitch of 200 nm have been fabricated. The pores are formed by inverting a square array of photoresist posts with a chromium lift-off process and by subsequent reactive-ion etching using the chromium as an etch mask. The method has wider process latitude than direct formation of holes in the resist layer and the chromium mask allows for etching of pores with vertical sidewalls.

Microfabrication of palladium-silver alloy membranes for hydrogen separation

In this paper, a process for the microfabrication of a wafer-scale palladium-silver alloy membrane (Pd-Ag) is presented. Pd-Ag alloy films containing 23 wt% Ag were prepared by co-sputtering from pure Pd and Ag targets. The films were deposited on the unetched side of a -oriented silicon wafer in which deep grooves were etched in a concentrated KOH solution, leaving silicon membranes with a thickness of ca. 50 /spl mu/m. After alloy deposition, the silicon membranes were removed by etching, leaving Pd-Ag membranes. Anodic bonding of thick glass plates (containing powder blasted flow channels) to both sides of the silicon substrate was used to package the membranes and create a robust module. The hydrogen permeability of the Pd-Ag membranes was determined to be typically 0.5 mol H/sub 2//m/sup 2//spl middot/s with a minimal selectivity of 550 for H/sub 2/ with respect to He. The mechanical strength of the membrane was found to be adequate, pressures of up to 4 bars at room temperature did not break the membrane. The results indicate that the membranes are suitable for application in hydrogen purification or in dehydrogenation reactors. The presented fabrication method allows the development of a module for industrial applications that consists of a stack of a large number of glass/membrane plates.

Wet and dry etching techniques for the release of sub-micrometre perforated membranes

For the production of microsieves we studied the release of perforated silicon nitride membranes from a silicon substrate. During the release by KOH etching the pressure build-up due to hydrogen gas formation can be quite large and cause rupture of the membrane. We explored the use of anisotropic etching with an SF6/O2 plasma to replace KOH etching. For sub-micrometre pores excellent results were obtained.