MEMS | ARTICLE
Given these requirements, it appeared that planar or HARM micro fabrication technologies were the most promising candidates for building a magnetically operated reed switch. When we looked at the array of available techniques we produced the following short list, which we’ll describe in some detail, comparing the advantages and disadvantages of each technique.
Potential techniques for building reed switches by batch micro fabrication:
1. Silicon bulk or surface micro machining, adapted to build ferromagnetic structures.
2. UV/LIGA (Acronym for Lithographie, Galvanoformung and Abformung [Ger.], in English Lithography, Electrodeposition and Moulding, using UV photolithography)
3. DXRL (Deep X-ray lithography) 4. EFAB (Electrochemical microFABrication)
Description of the techniques
(1) Reed switches based on surface micro machining techniques adapted for ferromagnetic structure fabrication have been attempted in the past. Publicised devices include those from Schneider Electric [1] using a silicon substrate, Asulab S.A. [2] using
<< Figure 1: Reduction in reed switch length over time. >>
a glass substrate, and MEMSCAP using their MetalMUMPS [3] process. However, the switches were either not commercially available or their production has ceased. We had doubts about the utility of this technology in any case, since the planar fabrication imposed by conventional semiconductor fabrication techniques leads to reed switch blade structures too thin or stress- distorted to develop adequate contact forces in the presence of a magnetic field. The weak contact forces (in the low tens of micronewtons) in turn lead to low contact switching reliability. We therefore rejected this technique from further consideration.
(2) UV/LIGAlooked a lot more promising. LIGA is a photolithographic, additive, batch HARM technique capable of developing ferromagnetic structures such as reed switch blades [4], and since UV/LIGA is known to be capable of generating features with aspect ratios of at least 20:1, with about 1o
runout
from feature top to feature bottom, it looked like a promising candidate for building a MEMS reed switch. We felt we needed at least 50:1 and a runout of better than 0.1o
contacts closed almost perfectly face-to-face, and so we initially rejected the technique.
(3) DXRL (Deep X-ray Lithography)/LIGAis a batch HARM technique closely related to UV/LIGA, except that it uses parallel X-rays instead of UV radiation in a photolithographic process producing component moulds with exceptionally parallel walls (runouts of thousands to one compared to the twenty-to-one
to ensure the switch
37 | commercial micro manufacturing international Vol 7 No.1
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