MEMS | ARTICLE
We therefore decided to re-evaluate the UV/LIGA technique for fabricating HARM structures. Some more investigation revealed various optical methods to improve the maximum aspect ratio to around 50:1, adequate for our needs. And what had seemed a formidable barrier — removing the cross-linked epoxy polymer after mould filling — could be overcome by some newly developed proprietary techniques, despite the polymer’s resistance to solvents. The two lithographic techniques were rather different but the next result was the same — a polymer mould used for electroplating the reed switch components. The processes are outlined in figure 3, using an escapement wheel from a watch as an example. Briefly, DXRL uses a positive photoresist, polymethyl methacrylate (PMMA), that has its molecular weight (MW) reduced by X-rays in non-masked regions. The lower MW polymer is then developed by dissolution in a selective solvent that only dissolves the reduced MW material, leaving a hollow mould. Gold, being a heavy metal, is relatively opaque to X-rays and is therefore used for the mask material. In contrast, the epoxy polymer is a negative photoresist. It cross-links and solidifies when exposed to UV radiation, so the mask used is opaque to UV except in regions that need to be solidified into a mould structure. After UV exposure, the non-crosslinked polymer is dissolved away. The subsequent polymer mould is used for electroforming the reed switch components, after which the surplus polymer is stripped, in the case of PMMA by solvent dissolution, and in the case of epoxy polymer, by a proprietary technique. After wafer capping and test probing, the wafer is diced into individual switches for final test, tape-and-reel packaging and customer shipment.
Subsequent testing of switches made by both the DXRL and UV methods showed no significant difference in electrical performance or contact life, and in fact somewhat less spread in magnetic closure sensitivity for the UV-fabricated switches, a sure sign that the mould dimensions were well under control. A new production line is being built including the UV exposure equipment, with the RedRock micro fabricated switch moving from DXRL pilot production to UV mass production in early 2014.
Summary
We have developed a new type of MEMS micro fabricated reed switch based on high aspect ratio micro fabrication using the UV LIGA technique. The switch is being commercialised under the trade name RedRock. Initial applications include medical devices such as hearing aid and capsule endoscopes, where the switch’s unique combination of very small size, zero power operation and hot-switching capability are enabling features. In a subsequent article, we will discuss potential alternatives for building micro fabricated reed switches, including subtractive techniques such as micro electro discharge machining, micro ultrasonic milling, micro abrasive waterjet milling and deep reactive ion etching, as well as providing an overview of some of the more innovative applications that the RedRock switch is currently addressing.
References
Roger Grace is President of Roger Grace Associates of Naples, Florida, a marketing consulting firm that he founded in 1982, specialising in the commercialisation of MEMS. His firm provides business development, custom market research, market strategy and integrated marketing communications services to high tech clients worldwide. He has published over 20 articles in industry publications, organised and chaired over 50 MEMS technical sessions and conferences and is frequently quoted in the technical and business press as a MEMS industry guru. He was a visiting lecturer in the School of Engineering at the University of California Berkeley from 1990 to 2003. He holds BSEE and MSEE (as a Raytheon Company Fellow) degrees from Northeastern University where he was awarded the “Engineering Alumni Engineer of the Year Award” in 2004.
Stephen Day is retained by Coto Technology Inc. as an engineering consultant, specialising in introducing MEMS devices to the company’s product portfolio. He holds the
B.Sc. and Ph.D. degrees in Physical Chemistry and Analytical Instrumentation from the University of Bradford (UK) and did post-doctoral work at Rensselaer Polytechnic Institute in Troy, NY. Most recently, he was the VP of Engineering and Technology for Coto Technology Inc., developing small signal switching solutions and IP relating to MEMS. Prior to joining Coto, he held senior engineering management positions at the Foxboro Company (Foxboro, Mass.) and Xerox Corporation (Milton Keynes, UK). Steve is the inventor or co-inventor of several patents in the fields of quantitative infrared spectroscopy, analytical instrumentation and high frequency electronic component design. He is a member of the Royal Institute of Chemistry and a Chartered Chemist in the UK.
[1] Vincent, M, et. al, “Electrical Contact Reliability in a Magnetic MEMS Switch,” Proceedings of the 54th Holm Conference on Electrical Contacts, pp. 145 – 150, 2008
[2] Gueissaz, F, “The MicroReed, an Ultra- small Passive MEMS magnetic Proximity Sensor Designed for Portable Applications,” in Proceedings of the 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2001)
[3]
http://www.memscap.com/products/ medical-and-biomedical/switches
[4] See for example Schonig, F., “LIGA and its Application to Electrical Interconnects,” presented at IEEE SW Test Workshop, June 10-13, 2012, San Diego, CA, USA
[5] Christenson, T., “High Aspect Ratio Microfabricated Structures”, in “X-Ray Based Fabrication,” Ch. 5, Vol. 2 in the MEMS Handbook, (2nd ed.), CRC Press, 2005
[6] Cohen, A.L., “Electrochemical Fabrication”, in the MEMS Handbook, (2nd ed.), Ch. II “Design and Fabrication,” 19-1, CRC Press, 2002
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