Ž . Sensors and Actuators 77 1999 178–182 www.elsevier.nlrlocatersna High performance micromachined Sm Co polymer bonded magnets 2 17 B.M. Dutoit ) , P.-A. Besse, H. Blanchard, L. Guerin, R.S. Popovic ´ EPFL— Swiss Federal Institute of Technology, Institute of Microsystems, CH-1015 Lausanne, Switzerland Received 25 February 1999; accepted 8 March 1999 Abstract A new technique has been developed to fabricate thick permanent magnets in large batches with a remanence of up to 0.34 T and an energy product of over 22 kJrm 3 . To our knowledge, these are the best values reached in micromachined magnets. To illustrate the potential of this new technology, a new type of rotational speed microsensor has been fabricated. A 1 mVpp signal is obtained at 0.25 mm from a gearwheel. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polymer bonded magnets; Micromachining; Batch processing; Angular position sensor 1. Introduction There is a large potential for good magnets produced by micromachining. Such magnets can be used as a magnetic wx source either in sensors or in actuators 1 . Two approaches w x 2,3 were published on the integration of micromachined wx magnets. CoNiMnP has been electroplated 2 on silicon with a remanence between 0.2–0.3 T and an energy prod- uct of 14 kJrm 3 . The process needs a seed layer and a wx long deposition time. Another group 3 uses a ferrite powder that is embedded in polyimide and structured with photolithography. A remanence of 0.28 T and an energy product of 11.9 kJrm 3 has been obtained by this process. These magnets are easily and precisely shaped, but have the disadvantage of being very thin. Three depositions are necessary to reach a thickness of 10 to 20 mm. Sm Co magnets, on the contrary, have a high rema- 2 17 nence, a high coercivity, and a temperature dependence of Ž . only 0.03%r8C 0.2%r8C for Sr or Ba ferrite . The cost of Sm Co is high, but this is not so important for such 2 17 small quantities, considering the enhancement of the mag- wx netic properties 4 . Samarium has a very negative reduc- Ž . tion potential y2.68 V and is impossible to electroplate wx directly from aqueous media. One attempt 5 was made to electroplate rare earth with ferrous metals. The magnetic characteristics of these films were not measured, but the maximum rare earths density was only 17%. The maxi- ) Corresponding author. Tel.: q41-21-6936735; Fax: q41-21- 6936670; E-mail: bm.dutoit@epfl.ch mum remanence expected could only be 17% of the saturation value, that is to say 0.18 T for samarium–cobalt. For this reason, it is proposed here to use an alternative method by fabricating micromachined polymer-bonded Sm Co magnets. 2 17 2. Experimental A Sm Co powder is ball-milled in isopropanol at 100 2 17 rpm for 8 h down to particle diameters of about 10 mm Ž . Fig. 1 . During the ball-milling, the grains are treated with an epoxysilane to avoid oxidation and to improve the subsequent mixing with a polymer. The powder is then Ž . embedded in a photoepoxy SU-8 in concentrations from 18% to 60% vol. and mixed at 15,000 rpm during 10 min. The photoepoxy SU-8 allows the fabrication of ultrathick Ž . Ž . ) 1 mm and high aspect ratio structures ) 18 with a wx good geometrical precision 6 . Just before spin coating or tape casting, the mixture is ultrasonified for 5 min. A prebake takes place on a hot plate at 1158C. It is carried out under a flux density of 0.3 T to align the easy axis of the grains. After the photolithography, a postbake is car- ried out in an oven at 908C. The layer is developed in Ž . PGMEA propylene glycol methyl ether acetate and rinsed with isopropanol. 3. Results and discussion With one photolithographic step, magnets of 1250 = Ž . 1250 = 15 mm have been micromachined Fig. 2 . The 0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0924-4247 99 00193-4