H. Wensink

Powder-blasting technology as an alternative tool for microfabrication of capillary electrophoresis chips with integrated conductivity sensors

The fabrication and characterization of a microfluidic device for capillary electrophoresis applications is presented. The device consists of a glass chip which contains a single separation channel as well as an integrated conductivity detection cell. In contrast to most microfluidic glass devices the channels are not wet etched in HF but machined by the newly developed micro powder-blasting technique which allows the creation of microstructures below 100 µm, and additionally makes parallel hole machining at very low costs outside the cleanroom environment possible [1, 2]. The integration of the conductivity detector was achieved by leading two thin-film metal electrodes inside the separation channel. For rapid sample injection the chip is mounted inside an autosampler-based capillary electrophoresis platform. The detection electrodes for conductivity detection are read out by lock-in amplifier electronics. First measurements show the successful separation of various ions in the sub-millimeter range.

High resolution powder blast micromachining

Powder blasting, or Abrasive Jet Machining (AJM), is a technique in which a particle jet is directed towards a target for mechanical material removal. It is a fast, cheap and accurate directional etch technique for brittle materials like glass, silicon and ceramics. By introducing electroplated copper as a new mask material, the feature size of this process was decreased. It was found that blasting with 9 /spl mu/m particles (compared with 30 /spl mu/m particles) result in a higher slope of the channel sidewall. The aspect ratio of powder blasted channels was increased by using the high resistance of the copper mask in combination with the use of 9 /spl mu/m particles. Furthermore, our measurements show how the blast lag (small channels etch slower compared to wider channels) is decreased by using smaller particles.

Mask materials for powder blasting

Powder blasting, or abrasive jet machining (AJM), is a technique in which a particle jet is directed towards a target for mechanical material removal. It is a fast, cheap and accurate directional etch technique for brittle materials such as glass, silicon and ceramics. The particle jet (which expands to about 1 cm in diameter) can be optimized for etching, while the mask defines the small and complex structures. The quality of the mask influences the performance of powder blasting. In this study we tested and compared several mask types and added a new one: electroplated copper. The latter combines a highly resistant mask material for powder blasting with the high-resolution capabilities of lithography, which makes it possible to obtain an accurate pattern transfer and small feature sizes (<50 µm).

The black silicon method VIII: a study of the performance of etching silicon using SF6/O2-based chemistry with cryogenical wafer cooling and a high density ICP source

This paper presents a study of the performance of current trends in high speed, highly controllable anisotropic plasma etching of silicon for its use in micro- and nano-engineering. Optimisation rules for tuning the equipment are formulated to enable maximum results with respect to etch rate, etch selectivity, and profile control (e.g. anisotropy). The optimisation process uses the black silicon method to allow for an easy to use design-of-experiment method. After optimisation, etch rates of silicon up to 15 μm/min were obtained at a relatively high pressure (4 Pa=30 m Torr and ICP power (2 kW), while keeping the anisotropy high. At the same time, the erosion rates of thermal silicon dioxide and ordinary photoresist (Shipley S1805) were around 7 nm/min (i.e. selectivities up to 2000). Increasing the pressure to 20 Pa, the selectivity increased to 10,000, although bottling became more pronounced. The high etch rate and high selectivity are especially important in the case of micro-engineering, where wafer through etching with the help of plasmas become a standard in the near future. In the case of nano-engineering, however, profile control is the main concern. To prevent undercut in such cases, in particular, bottling due to a broad ion angular distribution, the pressure should be sufficiently low. The best results were found at pressures below 0.2 Pa=1.5 mTorr and at a low ICP power of 350 W to prevent a too strong mask erosion caused by the low pressure. The silicon etch rate decreased to 1 μm/min and the erosion rate of the oxide and resist were both approximately 20 nm/min, giving a selectivity of 50. Reproducibility (wafer-to-wafer) and uniformity were also output factors of prime concern. Surprisingly enough, for two different equipment manufacturers the results were almost identical when using the same parameter setting. This indicates that the SF6/O2-based chemistry was optimised rather than the equipment itself.

Fine tuning the roughness of powder blasted surfaces

Powder blasting (abrasive jet machining) has recently been introduced as a bulk-micromachining technique for brittle materials. The surface roughness that is created with this technique is much higher (with a value of Ra between 1?2.5 ?m) compared to general micromachining techniques. In this paper we study the roughness of powder blasted glass surfaces, and show how it depends on the process parameters. The roughness can also be changed after blasting by HF etching or by using a high-temperature anneal step. Roughness measurements and scanning electron microscopy images show the quantitative and qualitative changes in roughness. These post-processes will allow us to investigate the influence of surface roughness on the microsystem performance in future research.

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.

Reduction of sidewall inclination and blast lag of powder blasted channels

Powder blasting (abrasive jet machining) is a fast directional machining technique for brittle materials like silicon and glass. The cross-section of a powder blasted channel has a rounded V-shape. These inclined sidewalls are caused by the typical impact angle dependent removal rate for brittle materials. It has a negative influence on the channel depth and aspect ratio, and results in the blast lag: wide channels become deeper compared to smaller channels. Two approaches are studied in this paper that can influence this effect: using smaller powder particles and decreasing the particle jet impact angle (oblique blasting). Calculations and measurements show that the blast lag can be reduced for these two cases. However, oblique blasting requires additional equipment and results in low mask/target selectivity while the reduction in blast lag is relatively small. Powder blasting with smaller particles is much easier to perform and leads to straighter sidewalls and a large blast lag reduction.

Multifractal properties of Pyrex and silicon surfaces blasted with sharp particles

The blasting of brittle materials with sharp particles is an important fabrication technology in many industrial processes. In particular, for microsystems, it allows the production of devices with feature sizes down to few tens of microns. An important parameter of this process is the surface roughness of post-blasted surfaces. In this work the scaling properties of Pyrex glass and silicon surfaces after bombardment with alumina particles are investigated. The targets were bombarded at normal incidence using alumina particles with two different average sizes, 29 μm and 9 μm. This investigation indicates that the resulting surfaces are multifractal. Applying multifractal detrended fluctuation analysis (MFDFA) allowed us to determine the singularity spectrum of the surfaces. This spectrum did not depend on the target material or on the size of the particles. Several parameters quantifying relevant quantities were determined. It was found that long range correlations are responsible for the observed multifractal behaviour.

Analytical model of micromachining of brittle materials with sharp particles

We present an analytical model for the powder blasting of brittle materials with sharp particles. We developed a continuum equation, which describes the surface evolution during the powder blasting, into which we introduced surface energetics as the major relaxation mechanism. The experimental and the calculated erosion rates match extremely well. Also, the developed model explains why the sidewalls of a powder blasted channel are not straight, e.g. have a blast lag, for which we also found a good agreement between experiments and theory.

New developments in bulk micromachining by powder blasting

Powder blasting is a fast, inexpensive and accurate directional etch technique for brittle materials like glass, silicon and ceramics. We have developed several improvements for a successful implementation of powder blasting as a micromachining technique.