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Design and fabrication of a magnetic bi-stable electromagnetic MEMS relay

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Design and fabrication of a magnetic bi-stable electromagnetic MEMS relay
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  Microelectronics Journal 38 (2007) 556–563 Design and fabrication of a magnetic bi-stable electromagneticMEMS relay Shi Fu  , Guifu Ding, Hong Wang, Zhuoqing Yang, Jianzhi Feng Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, National Key Laboratory of Nano/Micro Fabrication Technology,Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai, 200030, PR China Received 8 January 2007; accepted 3 March 2007Available online 23 April 2007 Abstract This paper describes the principles, design, fabrication and performance of a new type of electromagnetic bistable MEMS relay. Themicrodevice is operated by a wiggling microactuator, which is symmetrically assembled with two integrated planar windings and onepermanent rotor in the form of sandwich with coaxial sustained gaps between each other. The micromachined rotor moves to-and-froaround the axis and operates the joined brush to open or close external circuit. This hybrid MEMS relay has been provided with fastresponse and latching function owing to the special design. The response time is about 0.3ms and the maximum load current is 2A. r 2007 Elsevier Ltd. All rights reserved. Keywords:  MEMS relay; Bistable; Wiggling microactuator; Electrical contact 1. Introduction Miniaturization and corresponding lower power con-sumption were invariable aspiration especially for electro-mechanical relays. With the development of MEMStechnology, there has been an opportunity to achieve thisgoal. With the aid of MEMS technology, the electro-mechanical relay can be designed and manufactured assolid-state relay, thus, small size and low power consump-tion are realizable. Many works have been done to developmicrorelays, and varied MEMS relays have been reportedin recent years, such as electrostatic [1–9], electromagnetic[10–14], electrothermal [15–21] microswitches and micro- relays, in which microactuators play an essential role.Electrostatic microactuators could work with low powerconsumption. But because of small electrostatic force [1–3],big size is necessary and the driving voltage is too high tobe compatible with IC application. Because it is easy tofabricate a latching mechanism with surface micromachin-ing, electrothermal microactuators often integrate somespecial components to achieve latching operation mode.However, ascribed to the heat dissipation, it is difficult toimprove the response properties.For the capability of low driving voltage, fast response,high breakdown voltage as well as suitability in harshenvironment, electromagnetic MEMS relays have attractedconsiderable attention in recent years. Several componentsof magnetic MEMS relays [10–13,22,24] were designed andfabricated. Parts of them possess bi-stable mechanism toreduce power consumption.Generally, the magnetic bistable function is achieved bytwo approaches: out-plane running of permalloy cantileverin a external permanent magnetic field [10–13] and in-planerunning of permalloy cantilever in gap of magnetic circuit[14]. They both used the coupling of the elastic potentialenergy and the magnetic potential energy to carry out thebistable function. But the complicated linked structurerestricts the response, and the bistable structures tend tofatigue, so the life of the device was reduced drastically.In this paper, a hybrid electromagnetic microrelay with abistable wiggling microactuator and a electrical brush witha curved contactor is fabricated. The principle of linearmotor [23–27] was introduced into the electromagneticmicroactuator, whereas, the end-effect of the common ARTICLE IN PRESS www.elsevier.com/locate/mejo0026-2692/$-see front matter r 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.mejo.2007.03.015  Corresponding author. Tel.: +862162932517-828;fax: +862162823631. E-mail address:  realheartor@sjtu.edu.cn (S. Fu).  linear motor was reduced by the close annular drivemechanism. The friction induced by magnetic force in theaxis direction was counteracted by the symmetric sandwichmechanism of the microactuator. So the dynamic perfor-mance of the device was improved. The design, optimiza-tion, microfabrication, assembly and test results aredescribed in detail as follows. 2. Device concept As shown in Fig. 1(a), the microrelay contains twointegrated planar winding and one micromachined perma-nent magnet rotor in sandwich structure. The actuationpart is designed based on the linear motor. While the end-effect existing in common linear motors is reduced by theannular structure which makes the rotor rotate in twodirections more efficiently.Fig. 1(b) shows the detailed structure of integratedstator. There are six planar windings in each stator and sixpermalloy cores are placed in the center of every winding,respectively. Every winding contains two layers of planarcoil to increase the drive force and every magnetic core isdivided into six pieces for reducing the eddy current loss.The current direction in the adjacent windings is differentto produce the same direction of the current in the coilsbetween two adjacent permalloy cores. As the substrate of permalloy cores and planar windings, single crystal softmagnetic ferrite, a kind of high magnetic conductivitymaterial [28], was used to produce the uniform magneticfield for driving.The diameter of the round rotor is 4mm, and itsthickness is 100 m m as shown in Fig. 1(c). The magneticpoles of adjacent fan-shaped permanent magnetic chips areopposite. Seven tapers are fabricated on the axis for theconvenience of fine assembly. The acuracy can becontrolled at about 10 m m and the width of the gap isabout 50 m m.The electrical contact part on the cover board consists of an electrical brush and two pads for the outer circuit. Acurved contactor is fabricated at the end of the forkedbrush; thereby the line-to-face contact improves thecontact stability and insulation protection of the micro-relay. Stator is stuck onto the punched copper encapsula-tion. After the electrical brush being mounted on thearmature, as shown in Fig. 1, all the components areassembled to the whole device in order. 3. Design and analysis The basic structure of the microrelay can be analyzedand optimized by the FEM method. The FEM analysissoftware was used for the analysis of the irregular magneticstructure. The finite element modeling of the staticmagnetic field and force was done via the ANSYS \ E-MAG module. For the purpose of simplifying the process,2D magnetic scalar tetragonal elements were used to modelthe permanent magnet, gap, permalloy and ferrimagnet.Only half of the structure is considered as analytic objectdue to the symmetric structure. In order to reduce theinfluence of the end-effect, the length of the substrate wasextended properly; three permalloy cores that related tofour permanent magnetic chip on the rotor were designed.The material properties and parameters of the model arelisted in Table 1. ARTICLE IN PRESS Fig. 1. Sketch of the hybrid electromagnetic microrelay.Table 1Modeling parametersMaterials Parameter ValuePermanent magnet Relative permeability 4Coercive force 1.5e5Permalloy Relative permeability 1e4Ferro magnet Relative permeability 0.9e4Air Relative permeability 1Fig. 2. The magnetic field distribution in the gap. S. Fu et al. / Microelectronics Journal 38 (2007) 556–563  557  3.1. Analysis of the magnetic system Fig. 2 illustrates the effect of the permalloy core on themagnetic field in the gap. The magnetic flux always prefersto pass through higher magnetic permeability materials.The magnetic flux distribution in the gap is changed by apermalloy core in the center of every winding. Fig. 2 showsthe non-stable state of the magnetic system. The lateralmagnetic force depends on the interaction between thestator and rotor. But, at this time, there is no lateral forcebecause magnetic force in two opposite directions aresymmetric and counteracted absolutely. From the magneticlines, the permanent magnetic chips and soft magneticrotor are firmly combined together. 3.2. Analysis of stable state When there is a deviation( d  ) existing between the centerof the permalloy core and the edge of the adjacentpermanent magnet chip due to some disturbance in thehorizontal direction, the magnetic field distribution aroundthe permalloy core is changed as shown in the ‘‘ellipsecircle’’ of  Fig. 3. The magnetic intensity between twolateral surfaces of the permalloy core is different and thedifference of the magnetic intensity leads to a lateral forcetoward left direction when the rotor rotates to the left.Furthermore, the direction of resultant magnetic force inthe horizontal direction tends to the left when the rotorturns the left direction.In the simulation, as shown in Fig. 4, the conclusions areobtained as follows:(1) The magnetic force increases with the increasing of thedeviation( d  ) until the rotor runs in the distance of   L /2and then, it reduces rapidly.(2) The maximum value of the magnetic force( F  max )increases with the increasing of the  L  of permalloycore. The  F  max  increases rapidly when the  L  is shorterthan 400 m m, while the  F  max  increases slowly once the  L exceeds 400 m m.(3) The direction of the rotor’s rotating is the same to thedirection of the magnetic force. Once it is stopped in acertain distance by a latch, the rotor is able to be lockedand latching function of the device can be carried out.The maximum force is about 10mN/m when the  L  is400 m m and the  d   is  L /2. The effective length for keepingstable state is about 0.6mm in every unit and the total forceis 72 m N calculated by summating the forces on all 12 units.The requirement of the stable device can be satisfied withoptimizing the structure of magnetic system. In addition,every permalloy core was split into six pieces for thereduction of the eddy loss as shown in Fig. 1. 3.3. Analysis of electrical switch Differing from the common motor, in our design, everywinding consists of two layers of planar coil instead of thesolenoid coil. When a single winding is placed in amagnetic field, reciprocating current would not induceelectromagnetic drive force in the lateral direction. Bylocating two adjacent windings in opposite direction andconnecting all the coils in series as shown in Fig. 1(b), thecurrent is in the same direction between two adjacentpermalloy cores. The magnetic field are uniform in the gapbetween the adjacent permalloy cores in Fig. 3. The lateralmagnetic force is produced from the coils based on theFleming’s rule. In order to ensure the directional coherenceof the electromagnetic actuation, the adjacent fan-shapedpermanent magnetic chips are stuck onto the permalloyrotor in opposite direction and keep the whole devicefirmly as shown in Fig. 2.The corresponding magnetic induction intensity is shownin Fig. 5. Two paths are located at the position of two-layer ARTICLE IN PRESS Fig. 3. Magnetic field distribution of one stable state. -800 -600 -400 -200 0 200 400 600 800-12-10-8-6-4-2024681012      F    m    a    g     (     N     /    m     ) deviation(um) L=200L=300L=400L=500L=600Fmax Fig. 4. The relationship between the magnetic force and the deviation(d)at different lengths(L) of effective side of the permalloy core. S. Fu et al. / Microelectronics Journal 38 (2007) 556–563 558  coil between two permalloy cores. Fig. 6 shows that thevalue of the magnetic induction intensity is about 0.3T atboth of the top and bottom coils. The drive force can becalculated according to Fleming’s rule. F   ¼ nBII  eff  ,where  l  eff   (1.5mm) is the effective length of one unit of coil.Each stator consists of six units of the winding. There are16 coils in each planar winding. So the total effective lengthis about 1.152m. When the current (0.15A) is applied, thetotal drive force is about 51.84mN. 3.4. The stress analysis of the brush A curved contactor of the electrical has the capability of making the contact more stable and increasing the loadcurrent due to the deformation induced by the contactpressure. 3D model of the electrical brush is built up basedon the virtual structure and simulated by the FEM analysissoftware. The main parameters of each leg of the brush areshown as following: 16 m m of thickness, 2mm of effectivelength, 0.4mm of width. When the end of the electrical brushis pressed to 20 m m, the simulated maximum internal stress isabout 247 m N which is much more than the total magneticforce of 72 m N in the stable state. So the pressure of thecontact derives from the magnetic force in the stable state. 4. Fabrication As key parts of the device, both the stator and electricalsystem including contactor and two out-pads are fabricatedby the microfabrication. 4.1. The process of stator Fig. 7 shows the process of one stator. Six windings andpermalloy cores are fabricated on the single crystal softmagnetic substrate.(1) Depositing the chrome/copper (30/50nm) on the singlecrystal soft magnetic substrate as seed layer forelectroplating the microstructures.(2) 15–18 m m positive photoresist is spin coated on the seedlayer, then pre-baked at 60 and 90 1 C for 1h and 2h, ARTICLE IN PRESS Fig. 5. The magnetic field distribution in the gap.Fig. 6. Magnetic induction intensity at the top: (a) and bottom (b) coils. S. Fu et al. / Microelectronics Journal 38 (2007) 556–563  559  respectively, later patterned by UV-LIGA. Then 15 m mcopper is electroplated in the patterned microgroovesto form the bottom layer coil.(3) Positive photoresist is spin coated on the top of the firstphotoresist and patterned by lithography to electro-plate copper pillar to link the upper coil.(4) The third positive photo resist is spin coated afterremoving all the previous photoresist using acetone.Then, the permalloy core was electroplated in thecenter of the bottom coil.(5) After the removal of photoresist, the seed layer isetched away by RIE. Then depositing aluminum oxideon the surface to cover the bottom coil and permalloycore by sputtering.(6) The link pillars of the bottom coils and permalloy coreswere exposed by polishing the surface of aluminumoxide for linking the top coils.(7) Build the top coils using the above steps again fromfirst to sixth step. Finally two out-pads of the windingswas built on the top layer.Finally, one stator with six windings connected inseries is separated from the chip and every windingconsists of two-layer coils and one permalloy core asshown in Fig. 8. 4.2. The fabrication of electrical contact system The electrical contact system consists of the electricalbrush and cover board with two out-pads. The process of the brush is shown in Fig. 9. The difficulty of its fabricationlies in the curved contactor for the contact stability whenthe device is in the ‘‘on’’ state. In general, meltingphotoresist is one of microfabrication techniques toproduce 3D microstructures, but it cannot fabricatemicro-bulking surfaces with a large radius of curvature.A novel technique was adopted to form arbitrary 3Dmicrostructures [29] by a series of melted photoresist linesand then coating photoresist on the lines.(1) 2 m m titanium was sputtered on the glass as sacrificiallayer to release the contact brush. Then 50nm copperwas sputtered on the titanium film as seed layer.(2) Smooth curved surface was fabricated on the seed layerusing the novel melting photoresist technology asshown in the above black pane.(3) Sputtering chrome/copper (30/50nm) again.(4) 20 m m positive photoresist is spin coated onto thecopper film and patterned. Electroplating 10 m m of copper film as the body of the brush, 3 m m of nickel and3 m m gold film for good conductivity. After removal of positive photoresist with acetone and seed layer byRIE, the brush was released from the glass substrateusing hydrofluoric acid. ARTICLE IN PRESS Fig. 7. The fabrication process.Fig. 8. Optical photo of one stator.Fig. 9. The fabrication flow of the electrical brush. S. Fu et al. / Microelectronics Journal 38 (2007) 556–563 560
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