Microsystem Technologies 7 (2001) 196±202 Ó Springer-Verlag 2001

Application of electroplating in MEMS-micromachining exemplified by a microrelay M. Becker, D. LuÈtke Notarp, J. Vogel, E. Kieselstein, J.-P. Sommer, K. BraÈmer, V. Groûer, W. Benecke, B. Michel

196

Abstract Electrodeposition processes were proposed for the fabrication of micro-electro-mechanical systems (MEMS) long time ago. Patterned layers of nickel, nickel alloys, gold alloys, silver and copper are used to realize micro switches, relays, valves, pumps, coils and gyroscopes. The metal layers have to meet the highest requirements on homogeneity, as well as on mechanical, electric or magnetic characteristics. The application of microelectroplating has to cope with speci®c effects in¯uencing the electroplating conditions in the micrometer range. So, many efforts concentrate on the control and standardisation of microelectroplating processes. The suitability and application of electroplating methods will be exempli®ed by two concepts of a microrelay assigned

for current loads of up to several amperes. The development of these microrelays is object of the project ``MikroRel'', partially funded by the German ministry for education and research (BMBF).

1 Introduction Micro-electro-mechanical-systems (MEMS) are increasingly used in industrial sensor and actuator applications. Even in the age of semiconductor power switches, mechanical relays are of relevance in technical applications due to EMC-aspects, mechanical separation of current lines or electrical isolation of control and load circuit. Automotive and communication systems require a high number of mechanical relays and therefore provide a promising market for electrical contacts based on microsystem technologies (MST). MEMS-relays could be M. Becker (&), D. LuÈtke Notarp, W. Benecke favoured for size reduction, SMT-capability, integration Institut fuÈr Mikrosensoren, with microelectronics and improved dynamic behaviour -aktuatoren und -systeme, and reliability. University of Bremen, Bibliothekstr. 1, D-28359 Germany Several electromagnetically driven microrelays have been reported [2, 5, 11, 12]. However, due to the miniaJ.-P. Sommer, V. Groûer, B. Michel turisation of MEMS, actuation principles different from Fraunhofer Institut fuÈr ZuverlaÈssigkeit und Mikrointegration, traditional relay technologies, which are based on currentBerlin, Germany driven inductor coils, may be considered like thermal E. Kieselstein [7, 10] or electrostatic drive [1, 8, 9]. Chemnitzer Werkstoffmechanik GmbH, After presentation of general aspects concerning miChemnitz, Germany croelectroplating, the focus will be on the development phases in the project MikroRel. In order to choose the J. Vogel, adequate contact materials, theoretical aspects concerning Angewandte Micro-Meûtechnik GmbH, contact resistance have been investigated. Next, the conRangsdorf, Germany tact resistance of several contact materials has been meaK. BraÈmer sured for forces down to 1 mN representing the minimum TU Chemnitz, assumed contact force of the actuator. A contact system Chemnitz, Germany without integrated actuator was fabricated. For charac1 terisation of this contact system, a hybrid combination The authors wish to thank the partners in the project MikroRel for their cooperation. The characterisation of material properties consisting of the contact system and an inductor coil was and the measurement of the contact resistance were carried out realised. A microrelay of reduced size and volume with Chemnitzer Werkstoffmechanik GmbH. FEM-analysis was external electromagnetic actuation is proposed. In order to performed at Fraunhofer Institut fuÈr ZuverlaÈssigkeit und distinguish from traditional relays, the ®nal step was to Mikrointegration. The development of microelectroplating was integrate an actuator on the chip. Using electrostatic acsupported by Blasberg Ober¯aÈchentechnik GmbH. tuation, the device aims at high-current applications of 1 several amperes at low power dissipation. Thereby, the Partners in the project MikroRel Institut fuÈr Mikrosensoren, -aktuatoren, und -systeme (Bremen) device size could be reduced to 1 or 2 mm in height.

Robert Bosch GmbH (Stuttgart) Blasberg Ober¯aÈchentechnik GmbH (Solingen) Chemnitzer Werkstoffmechanik GmbH (Chemnitz) Fraunhofer Institut fuÈr ZuverlaÈssigkeit und Mikrointegration (Berlin) Hengstler GmbH (Wehingen)

2 Microelectroplating Electroplated layers are of interest for several reasons in MST. Metal layers serve as wiring layers, contact materials,

sacri®cial layers or functional layers in movable elements. The focus with respect to the application of the electroplated layer is on different mechanical and electrical material properties. Thereby, a speci®c material could serve for several functions mentioned above. In order to achieve the desired layer quality, the deposition process could be transferred into a model and system of characterisation and standardisation [6]. Starting from a basic theory distinguishing the deposition of alloys or analloys, a model in¯uenced by chemical, electrical or geometric factors can be derived. After characterisation of the deposition, parameters can be fed back in order to veri®cate and adopt the input data. Therefore, fundamentals regarding mass transfer, ®eld and ¯ow theory and chemical reactions have to be examined. Parts of these results have been presented [6] or will follow. The practical approach in the course of the project MikroRel is depicted in Fig. 1. Deposition parameters, material characterisation and chemical analysis of the electrolytes are brought in correlation, in order to ®nd out about the in¯uences on the desired material properties for use in the microrelay.

3 Contacts for a microrelay The maximum current load of the microrelay to be developed depends on actuator features like displacement or contact gap, switching dynamics or contact force as well as on material properties. Depending on the mechanical load on the contact system (plastic or elastic deformation), the contact resistance correlates to different parameters and formulas [4]. 1

Rcontact;plastic …H; . . .† / FC 2

…1†

Fig. 1. System of standardisation for MEMS-development using microelectroplating

1

Rcontact;elastic …E; . . .† / FC 3

…2†

H is the hardness of the contact materials, E the Young's modulus, FC the contact force. Concerning contact resistance, elastic processes and a high number of contact spots are preferred. Concerning wear, abrasion or contact sticking, hard surfaces are favourable. So, material characterisation was carried out to support the choice of suitable contact materials, at ®rst. Roughness, hardness and Young's modulus of coating materials like AuCo, AuNi, PdNi and Ag electroplated in patterned deposition were measured (Figs. 2 and 3). Hardness and Young's modulus were measured by registering nanoindentation test. The roughness was measured using LaserScanning-Microscopy. In the next step, the suitability of these contact materials concerning contact resistance was characterized at forces of 50 mN down to 1 mN (Fig. 4). The test equipment, allowing measurements below the region of 1 mN contact force, was developed and fabricated by Chemnitzer Werkstoffmechanik GmbH (CWM). The upper limit of 50 mN is due to the comparison with measurements of standard relay testing. All examined coatings exhibit contact resistances lower than 10 mW at 10 mN contact force. Especially AuCo and Ag offer favourable contact resistances with low changes at 10 mN contact force (Fig. 4). In the next step, a contact system consisting of two ®xed contacts and a movable shorting bar was realised using microelectroplating. Thereby, the surface of the contact system is covered with the coating materials tested before. Copper is used as sacri®cial layer for the movable shorting bar consisting of Ni. As there is no integrated actuator, the contact system had to be actuated by external electromagnetic forces (Fig. 6) using a hybrid con®guration with a micro inductor coil (Fig. 5). Contact systems have been tested switching up to 2 A current-switching at 12 V. In Fig. 7, switching cycles at

Fig. 2. Measurement results of mean hardness (by nanoindentation test) and mean roughness (by Laser-Scanning-Microscopy) of electroplated layers with different thickness

197

198

Fig. 3. Measurement results of mean Young's modulus (by registering nanoindentation test) of electroplated layers with differ- Fig. 5. Con®guration of micro inductor coil and contact system without integrated actuator for switching characterisation ent thickness

Fig. 4. Contact resistance of various coating alloys at contact forces down to less than 1 mN

50 Hz are depicted. Turn-on and turn-off times of about 700 ls were measured (not indicated) and no bouncing was observed. The minimum driving power was about 250 mW.

4 Microrelay with integrated actuator 4.1 Principle of electrostatic actuator The ®nal development step is to integrate the contact system with an actuator on chip. MEMS-relays can bene®t from actuation principles different from traditional relay technologies. The reduction in device size is favourable for electrostatic actuation resulting in a microrelay with low power consumption. The actuator proposed consists of electroplated cantilever beams, which bend out of plane due to internal layer stress gradients and which form the movable electrodes for electrostatic actuation (Fig. 8).

Fig. 6. Contact system (Fig. 5) consisting of two ®xed areas and a shorting bar

While pulling down the beam to the ®xed counter electrode on the substrate by electrostatic forces, the minimal air gap beginning at the suspension sets forth along the beam, until the beam is pulled to the substrate at pull-in voltage completely. This con®guration is suitable for high tip diplacements at lower electrical voltages compared to actuators based on plate capacitors. In the course of basic beam designs, the effect of interdepending parameters like beam bending, stiffness, airgap or overtravel on features like de¯ection, contact force or pull-in voltage have been derived analytically. The pull-in voltage can be calculated from the mechanical and electrical energy stored in the system [3].

199

Fig. 9. Cantilever beams bending out of plane due to stress graFig. 7. Switching cycles of contact system in hybrid con®guration dients in the layer stack of PECVD-oxide and electroplated Ni with micro inductor coil (CH1: driving system, CH2: contact system)

stress. The beams are up to 4 mm in length, 750 lm in width and 25 lm in height. De¯ections of more than 100 lm occurred depending on the layer stresses, thicknesses and beam designs (Fig. 9). At various actuator beams with substrate electrodes, pull-in voltages from 30 to 40 V were measured for beam de¯ections of 20 lm. These voltages are higher than acceptable and are predominantly due to the electrode gap of about 4 lm resulting from the sum of thicknesses of the sacri®cial layer and of the PECVD-oxide layer. When the cantilever beam actuator is combined with the contact system, the pull-in voltage can be reduced by fabricating the beams without PECVD-oxide. The beams then will consist of a unique metal layer, completely electroplated of nickel or nickel alloys, where stress gradients will be in¯uenced by the electroplating conditions. Simultaneously, bimetallic effects on the beam can be avoided. Fig. 8. Electrostatic cantilever beam actuator for out-of-plane actuation

Upull-in

s cm deff 1 ˆ R2 e0 b

…3†

cm is the stiffness, e0 the dielectric constant, R the bending curvature, b the beam width, deff the effective air gap. For the actuator considered, this is expressed in (3). The pull-in voltages increases with higher mechanical stiffness cm, which is determined by the geometry and material properties, increases with smaller bending curvature R expressed by higher tip de¯ections, and decreases with smaller effective gaps between the electrodes. In order to examine the intended beam bending, cantilever beams without contact system and without substrate electrode were fabricated. The required stress gradient was produced by a layer sandwich consisting of PECVD-oxide with compressive stress and Ni with tensile

4.2 Actuator design In order to achieve current loads of up to 5 A at contact resistances less than 20 mW, speci®cations concerning the major characteristics were formulated like contact gap more than 30 lm, contact forces more than 10 mN and pull-in voltages less than 24 V. The basic con®guration of the contact system like in the hybrid con®guration consisting of two ®xed contacts and a shorting bar is maintained as well as the chip size of 6 ´ 6 mm2. In the latest device design, of the integrated microrelay, the ®xed contacts are surrounded by several actuation beams carrying the shorting bar, which is linked to the actuator at reserved positions (Fig. 10). FEM-analysis of this structure yielded displacement vectors, contact forces and force distribution for given actuation voltages and varying technology parameters. The surrounding actuator beam con®guration effects a homogeneous force application on each of the contact areas from all edges of the contact system.

200

Fig. 11. FEM-analysis of applied driving voltage versus contact force for the microrelay design of Fig. 10

Fig. 10. Microrelay design with surrounding electrostatic cantilever beam actuator

In Fig. 11, the calculated contact force on the two ®xed contacts versus driving voltage is depicted. As expected, a hysteresis between the closing and opening stage occurs for increasing and decreasing driving voltages. Furthermore, the applied forces on the two contacts are different due to the perpendicular orientation of the contact system to the actuator beam. The essential prediction of the analysis is, that contact forces of 10 mN at pull-in voltages about 20 V are achievable at initial de¯ections of 30 lm, initial air gaps about 500 nm and overtravels of 1.5 lm.

4.3 Technology The system elements of the microrelay are fabricated in surface micromachining (SMM) dominated by microelectroplating. The contact system is coated by the contact materials discussed in Sect. 3. The shorting bar and the cantilever beams consist of Ni. Electroplated Ni exhibit favourable mechanical properties. The electroplating procedure can be modi®ed to in¯uence mechanical stress factors in order to bend the actuator beams out of plane. Beside variation of deposition timing or bath compounds, the use of rotational deposition units give possibilities for controlled variation of layer stress. For instance, the shorting bar sandwich can be shaped with a concave bow in order to establish electrical contact in the centre of the contact system ®rst. As electrostatic actuation in MEMS requires small airgaps, the sacri®cial layer for the movable elements is electroplated of copper with less than 1 lm layer thickness. A modi®ed procedure of patterned deposition without plating mould ensures uniformities about 2% and smooth edges, which is elementary for the electrostatic pull-in. The removal of the copper sacri®cial layer can be

achieved wet-chemical without sticking effects and without affecting any materials used. The establishment of contact force depends on the stiffness and overtravel of the contact spring. In order to build up contact force, the actuator has to be pulled down by the overtravel, after the contact system has been in temporary forceless contact. In SMM, a difference of sacri®cial layer thickness would be needed, where the sacri®cial layer of the actuator beam would have to be thicker than that of the contact system. As thin air gaps are needed for the electrostatic actuation, this difference of two sacri®cial layers for achieving an overtravel can not be satisfactory. The use of electroplating allows to deposit the ®xed contacts after removing the sacri®cial layer under the contact shorting bar and the actuator beams, effecting that the overtravel can be de®ned by the height of the ®xed contacts (Fig. 12). Thus, the use of a single sacri®cial layer shared for the movable shorting bar and for the actuator beams of less than 1 lm is possible. The device is intended to be encapsulated by bonding a bulk-micromachined cap wafer onto the device wafer with electroplated or sputtered lead-free solder alloys. Waferbonding on plane substrate surfaces is supported by electrical contacting from the chip backside, which seems necessary regarding a low line resistance (Fig. 13) and due to the surrounding beam con®guration covering the contact system, anyway. Therefore, the vias for the contacting are from the chip backside before removal of the sacri®cial layer. By means of stability, the vias are ®lled up partly using electroplating before (Fig. 12). At last, the encapsulated chip can be put onto a board contacted by re¯ow soldering resulting in a device height on board of about 1 mm (Fig. 13). The status regarding technology can be summarized as follows. The electroplating procedures concerning sacri®cial layers, the fabrication of the contact system and the ®lling up of bulk-micromachined vias are established or successfully tested. Electroplating procedures for in¯uencing layer stress and for fabrication of the ®xed contacts after removal of the sacri®cial layer are under development. The development of the waferbonding procedure with lead-free solder alloys is on the way.

Fig. 12. Concept for contacting from the chip backside and fabrication of ®xed contacts after removal of the sacri®cial layer

5 Conclusions In order to build up a system of standardisation in the application of electroplating depositions for MEMS-fabrication, extensive characterisation of electroplated layers as well as of speci®c effects in electroplating procedures is necessary. We work on collecting these data to join the in¯uence of moulding formations and geometries, deposition conditions, material parameters and electrolyte chemistry. The proposed system for the development and control in microelectroplating is built up in cooperation with partners in industry and research2. It is not intended to be limited for the microrelay development, but aims in general at the improvement of microelectroplating processes for application in MEMS, thus becoming competetive to traditional silicon-based micromachining. The use of electroplating procedures and electroplated materials as functional layers, sacri®cial layers and contact materials for fabricating microrelays in SMM has been exempli®ed by two systems. The development started with the choice of materials, which was supported by material characterisation and the measurement of contact resistances of various alloys at forces lower than 1 mN. A hybrid con®guration with micro inductor coil has been used for switching characterisation of a contact system fabricated by microelectroplating. Encouraged by the characterisation results, we are working on improvements, which could be identi®ed at the contact system, the coil-chip-interface and magnetic system, in order to realize an applicable microrelay in a hybrid con®guration. Concepts for characterisation of switching dynamics, lifetime, wear and abrasion of our contact system are available and will be executed subsequently with the improved system. Furthermore, a microrelay with integrated electrostatic cantilever beam actuator is proposed. Fabrication results of the beam actuator, simulations of the microrelay design and technological aspects of the device have been discussed. 2 Partners in the project MikroRel Institut fuÈr Mikrosensoren, -aktuatoren, und -systeme (Bremen) Robert Bosch GmbH (Stuttgart) Blasberg Ober¯aÈchentechnik GmbH (Solingen) Chemnitzer Werkstoffmechanik GmbH (Chemnitz) Fraunhofer Institut fuÈr ZuverlaÈssigkeit und Mikrointegration (Berlin) Hengstler GmbH (Wehingen).

Fig. 13. Concept for chip encapsulation by waferbonding and placement on board

At this stage, we gathered fundamentals concerning characterisation of electroplated materials intended as functional or contact layers as well as concerning processing microrelays based on microelectroplating. The hybrid microrelay will represent a miniaturised fast traditional relay with reduced package volume for direct device replacement. The integrated microrelay will represent a new sort of ``high-current'' microrelay of minimized package volume for low power applications.

References

1. Adamschik M; Schmid P; Ertl S; Gluche P; FloÈter A; Kohn E (2000) Performance of high speed diamond micro switch. Proc 3rd Int Conf Micro Materials, Dresden, ddp Goldenbogen pp. 863±866 2. Fullin E; Gobet J; Tilmans HAC; Bergqvist J (1998) A new basic technology for magnetic micro-actuators. Proc IEEE Micro Electro-Mechanical Systems, 25±29 January 1998, Heidelberg, Germany, pp. 143±147 3. Hesse SK; Hanke M; Schlaak HF (1998) Dimensionierung elektrostatischer Mikrorelais-Antriebe; F & M 106 7-8, pp. 546±548, MuÈnchen, Carl Hanser Verlag 4. Holm R (1967) Electrical Contacts. Springer, Berlin, Heidelberg, New York 5. Hosaka H; Kuwano H; Yanagisawa K (1994) Electromagnetic microrelays: concepts and fundamental characteristics. Sensors and Actuators A A40: 41±47 6. LuÈtke Notarp D; Becker M; Benecke W (2000) Control in electroplating depostions. Proc 3rd Int Conf Micro Materials, Dresden, ddp Goldenbogen, pp. 1087±1090 7. Matoba H; Kim C-J; Muller RS (1994) Fabrication of a bistable snapping microactuator. Micro-system technologies/

201

4th Int Conf on Micro-Electro-, Opto-, Mechanical Systems and Components, 19±21 October 1994, Berlin 8. Schiele I; Huber J; Evers C; Hillerich B; Kozlowski F (1997) Micromechanical relay with electrostatic actuation. Int Conf Solid-State Sensors and Actuators, Chicago, 16±19 June 1997, Vol. 2, pp. 1165±1168 9. Schimkat J; Kiesewetter L; Gevatter H-J; Arndt F; Steckenborn A; Schlaak HF (1994) Moving wedge actuator: an electrostatic actuator for use in a microrelay. Micro System Technologies 94 Conference, Berlin 1994, pp. 989±996

202

10. Sun X-Q; Farmer KR; Carr WN (1998) A bistable microrelay based on two-segment multimorph cantilever actuators. Proc IEEE Micro Electro-Mechanical Systems, 25±29 January 1998, Heidelberg, Germany, pp. 143±147 11. Taylor WP; Brand O; Allen MG (1998) Fully integrated magnetically actuated micromachined relays. J Microelectromech Syst 7(2) 181±191 12. Wright JA; Tai Y; Chang S (1997) A large-force, fully integrated MEMS magnetic actuator. Proc Transducers 1997, Chicago, IL, Vol. 2, pp. 793±796