MEMS | ARTICLE
<< Figure 2: Critical dimensions for the micro fabricated reed switch. >>
of the UV/LIGA process). In addition, DXRL is capable of producing features with aspect ratios exceeding 100:1 [5], an ideal capability for producing the wide, flat blades structures needed in a reed switch.
(4) EFAB (Electrochemical fabrication) is a MEMS technology developed the University of California under a DARPA contract, and is commercialised by Microfabrica Inc. [6]. EFAB involves electrochemically plating stacked multiple layers of photolithographically defined metal. It is an additive technique potentially capable of building reed switch structures. However, a serious drawback is the limited set of metals available, currently limited to 80% nickel / 20% cobalt, to rhodium or to palladium. Nickel-cobalt, though ferromagnetic, has poor electromagnetic properties, and rhodium or palladium are of course non-magnetic and also prohibitively expensive as bulk materials. We rejected EFAB and went back to the drawing board.
Comparing the DXRL and UV LIGA Processes
Modelling and prototyping by HT Microanalytical suggested that the structure shown in Figure 2 would provide an effective reed switch. The critical dimensions based on our market survey work included a
maximum length goal 2 mm, with a maximum height and width of 1 mm. From these dimensions, electromagnetic modelling allowed calculation of the optimum cantilever length and thickness, the required contact gap, and the contact overlap in order to provide a closure sensitivity of around 10 millitesla (mT). Simple mechanical calculations also showed that a single cantilever as shown provided a higher contact closure force for a given magnetic field strength and given total beam length compared to the dual cantilevers generally used in conventional reed switches. From our past experience building conventional reed switches, we knew that high contact forces were the key to long contact life when switching ‘hot’ electrical circuits.
The manufacturing question that remained was how to build the structure shown in Figure 2? The rectangular gray blocks shown in the figure are flux concentrators that pick up magnetic flux and transfer it down the flexible cantilever into the contact gap — the higher the field focused here, the higher the contact force. But since the bending moment of a flexible beam depends on the third power of its thickness but only the first power of its width, we needed very high precision for the beam thickness and the size of the contact gap in order to maintain fine control over the switch’s magnetic sensitivity. It appeared from these considerations that HARM fabrication would be ideal, and we initially selected DXRL based on its known precision. We would take a ceramic wafer and form a plastic mould on it, with
38 | commercial micro manufacturing international Vol 7 No.1
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