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Thinfilm technologies to fabricate a linear microactuator
Sensors and Actuators A 91 (2001) 145±149
Thin®lm technologies to fabricate a linear microactuator M. FoÈhse*, T. Kohlmeier, H.H. Gatzen Institute for Microtechnology, Callinstr. 30 A, Hanover University, D-30167 Hanover, Germany
Abstract The design selected for the microactuator is a three phase variable reluctance (VR) stepping motor with teeth. The stator consists of a ¯ux guide, three groups of poles with teeth, and meander type coils for exciting each group of poles. The system's traveler is built up by a ¯ux guide and teeth. For a good actuator's performance the electroplated magnetic structures' properties have to be optimized. To reach a small air gap that all VR-motors require, the microactuator's surface needs a very low roughness. Therefore, to fabricate the microactuator in thin®lm technology the most important steps are electroplating the structures and embedding these structures for ®nal planarization. # 2001 Elsevier Science B.V. All rights reserved. Keywords: MEMS; Linear motor; Electroplating; Embedding; Planarization
1. Introduction Fabricating a linear microactuator in thin ®lm technology poses interesting technology challenges [1]. The ®rst is in the area of magnetic ®lm deposition. To achieve a maximal driving force, thick magnetic layers with good magnetic properties are required. A technology lending itself to ful®ll these requirements is galvanic microforming which combines electroplating and the use of photoresist moulds. The second challenge is to minimize the air gap. In general, a linear actuator consists of an active and a passive element; in our case, the active element is a stator system, the passive one a traveler, i.e. the equivalent of a rotor of rotational motors. To minimize the air gap between the two, planarization techniques have to be applied. 2. Principle The actuator's design is a three phase variable reluctance (VR) step motor as described in [2]. Fig. 1 depicts a schematic view of the VR-type linear microactuator. The stator consists of three groups of poles with teeth (only one pole each is shown) called phases. For each phase, the tooth location is offset from the other two by one third or two thirds of a tooth pitch, respectively. The traveler, shown at the top, consists of a ¯ux guide and teeth of the same tooth *
Corresponding author.
pitch as the ones on the poles. For the coils to energize each phase, a simple meander design was chosen. For the phase energized, stator and traveler teeth will align. By executing an appropriate energizing sequence, the teeth' offset between the three phases will result in a forward or backward motion of the traveler. Fig. 2 depicts a schematic of the motor technology. The stator's ¯ux guides, poles and teeth were made of electroplated permalloy (NiFe 81/19), the meander coil of electroplated copper. A sputter deposited alumina ®lm isolates the coil from the ¯ux guides, for technology reasons this ®lm also appears at the bottom between the teeth (and originally also on top of the teeth). The traveler's ¯ux guides and teeth are also electroplated. As mentioned before, to control the gap between stator and rotor, both the stator's and the traveler's pole faces have to be planarized by a mechanical or chemical mechanical process and coated with a tribological ®lm de®ning the `air' gap dimension. To be able to execute this process which puts substantial stress on the motor's thin ®lm structure, both the coil area (stator only) and the gaps between the teeth (both stator and traveler) have to be ®lled with embedding material. Furthermore, all materials at the pole faces have to lend themselves to being coated with a tribological ®lm. Besides the pole face surfaces have to be capable of being coated with a tribological ®lm whose thickness de®nes the dimension of the `air' gap between stator and rotor. The ®rst part of the paper discusses the steps taken to optimize the magnetic properties of the permalloy, the second one elaborates on the embedding technologies developed.
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 5 0 9 - X
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M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
Fig. 1. Schematic view of a VR-type linear microactuator with three phases.
Fig. 3. Pulse sequence of pulsed power supply.
Table 1 Test parameters
Fig. 2. Schematic view of the motor technology and the layers' structure.
3. Permalloy electroplating Electroplating was executed in a `paddle cell', i.e. a galvanic cell where agitation of the electrolyte is accomplished by a paddle moving back and forth throughout the deposition process. Agitation of the galvanic bath compensates for local depletion of the electrolyte due to the plating process and is a precondition to achieve a consistent plating rate. Furthermore, the electrolyte concentration was held constant by appropriately replenishing the materials depleted due to the plating process. For Ni this was accomplished by using a Ni anode, for Fe iron was added regularly to the bath. The ratio between the permalloy's Ni and Fe was controlled by using a pulsed power supply and optimizing the pulse ratio. Permalloy has optimal magnetic properties at a composition of approx. 81 at.% Ni and 19 at.% Fe. At this compositional range, the materials magnetostriction, expressed by the magnetostrictive factor l, has a zero crossing. A ®lm with minimal magnetostriction typically has optimal coercivity and permeability properties. Therefore, by varying the permalloy composition the magnetic ®lm properties may be adjusted. For depositing permalloy, a pulsed power supply was used. Fig. 3 depicts a pulse sequence with total duty cycle length T and forward and reverse pulses lengths Tfwd and Trev, respectively. While the pulse cycle length was held at 10 ms, both the duty cycles of Tre/T wand the reverse current were varied. Table 1 shows the test parameters. Subject to variation were the duty cycle ratio Tfwd/Trev, the current density S and the current level of the reverse pulse. Besides the power supply related variation, the agitation and the temperature were varied. Fig. 4 summarizes the results. The permalloy composition changes when either the agitation or
Parameter
Standard
Test variation
Tfwd/Trev S (mA/cm2) Irev (mA) Paddle frequency (Hz) Temperature (8C)
8/2 13 760 0.35 30
1/1±9/1 8±15 0±800 0.1±0.8 25±45
the temperature or the reverse current level were varied. All ®ve parameters do in¯uence the permalloy composition. The total current density substantially in¯uences on the composition of the permalloy layer, but to maintain a good surface quality this parameter should not be varied widely. The most effective way to control the ®lm composition is to vary the absolute value of the reverse current. This way it becomes possible to control the ratio between Ni and Fe in a wide range and depositing ®lms of a high surface quality at the same time. Other important electroplating-parameters are the bath temperature and the agitation of the solution [3]. These parameters have to be held at constant values for obtaining a good depositing surface and ®lm composition. Fig. 5 depicts the magnetic properties of several electroplated ®lms with different compositions. The optimized deposited permalloy layers reach a saturation ¯ux density Bs of 1.1 T, a remanence Br of 0.25 T A/m and a relative permeability mr greater 500.
Fig. 4. Film composition dependent on pulsed power supply's and other parameters.
M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
147
Fig. 5. BH-loops for different film compositions.
Fig. 6 presents a cross-section of electroplated teeth measured with a stylus surface pro®ler. At the teeth' corners, the electric ®eld is concentrated, resulting in a preferred material deposition in the corner regions. The ®gure emphasizes the need to planarize the tooth faces. 4. Embedding and planarization As discussed previously, to allow a pole face planarization both the gaps between the poles as well as between the teeth had to be ®lled; to withstand the forces applied during planarization without disintegration of the ®lms, the resulting structure had to have a good integrity. Otherwise, the forces occurring during planarization may tear off the ®lms. Fig. 7 depicts the embedding and planarization process. For embedding the coil area, an organic material was used; the gap between the teeth was ®lled with copper. For this purpose, excellent results were achieved with a thick negative tone photosensitive epoxy resin (SU 8). It possesses intrinsic adhesion characteristics superior to conventional thick resists. In a crosslinked state when cured
Fig. 6. Measured surface profile of an electroplated NiFe-pole with teeth.
above 1008C, this epoxy not only is chemically resistant but also thermally stability up to above 2508C. The cured epoxy resin lends itself to be used for structural microparts with a thickness in excess of 40 mm. Processing the photosensitive epoxy resin starts with spin-coating it onto the dehumidi®ed wafer at 2000 rpm, resulting in a ®lm thickness of about 20 mm. The softbake takes place at 958C, followed by a cooling with a low temperature gradient. After the relaxation exposure, the resist is subjected to a ®nal hardbake process. For ®lling the gaps between the teeth at both stator and traveler, electroplated copper was used. To deposit the copper in the desired region, a photomask was created which covers all stator or traveler areas, respectively, except the one between the teeth. In the consecutive electroplating step, copper is deposited. As mentioned before, at the stator both the gaps between the teeth as well as the teeth tops are covered with alumina. Therefore, the ®lling of the gaps with copper originates from the tooth sidewalls. The alumina at
Fig. 7. Principle of embedding: (a) starting with electroplated stator; (b) teeth-embedding; (c) SU-8-embedding; (d) planarization.
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M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
Fig. 8. Planarized surface of a traveler in detail.
the bottom between the teeth is not desirable, but does not compromise the structural integrity of the embedded tooth structure. Both the embedding of the coils as well as ®lling the gaps between the teeth resulted in ®lm structures with great integrity. Both materials lend themselves to be planarized [4,5] and after planarization to be coated with a tribological ®lm. Fig. 8 shows a detailed view of a planarized traveler's surface analyzed by white light interferometry. For the given example, a CLA roughness value RA of only 25 nm was achieved which satis®es the actuator's requirements.
5. Conclusions and outlook Two of the key challenges in fabricating a VR type linear actuator were to create electroplated thick magnetic ®lms with appropriate magnetic properties as well as embedding techniques allowing to planarize both the stator's and the traveler's pole faces. The best way to controlling the magnetic properties of thick electroplated permalloy ®lms, is to use pulse plating and to appropriately adjusting the reverse current level. For allowing the pole faces to be planarized, the coil was embedded in photosensitive epoxy resin and the gaps
Fig. 9. SEM micrograph of a microactuator's stator system; the meander coils (copper) appear brighter than the NiFe-structures.
M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
between both stator and traveler teeth was ®lled by electroplated copper. The resulting structure had a good physical integrity and withstood the forces applied during planarization. However, for eddy current reasons it ultimately will be desirable to ®ll the gaps between the teeth with a dielectric material. Applying the chosen technologies resulted in a microactuator ful®lling its magnetic as well as physical material requirements. Furthermore, the application of these techniques is not limited to VR motor designs. It will also be usefull for fabricating hybrid motors which tend allow for greater forces than VR motors (Fig. 9). Acknowledgements The authors would like to thank A. Kariazine for the development of planarization process. This research is supported in part by the German Research Foundation. References [1] R. Belmans, K. Hameyer, Electromagnetic and design aspects of magnetic mini-actuators, Proc., Actuator (1998) 537±540. [2] H.H. Gatzen, H.-D. StoÈlting, S. BuÈttgenbach, H. Dimigen, A novel variable reluctance micromotor for linear actuation, Proc., Actuator (2000), Bremen (accepted).
149
[3] V.C. Kieling, Parameters influencing the electrodeposition of Ni±Fe alloys, Surf. Coatings Technol. 96 (1997) 135±139. [4] H.H. Gatzen, J.C. Maetzig, Nanogrinding, precision engineering, J. ASPE 21 (2/3) (1996) 134±139. [5] H.H. Gatzen, J.C. Maetzig, M.K. Schwabe, Precision machining of rigid disk head sliders, IEEE Transac. Magnetics 32 (3) (1996) 1843± 1849.
Biographies M. FoÈhse (1970) received his Dipl.-Ing. degree in electrical engineering from the Hanover University in 1999. He is currently a fellow of the Institute for Microtechnology at Hanover University with the aim at graduation. His research interests include the design and characterization of micro electro mechanical systems (MEMS) fabricated in thin film technology. T. Kohlmeier (1971), VDI, received his Dipl.-Ing. degree in mechanical engineering from the Hanover University in 1998. He is currently with the Institute for Microtechnology at Hanover University with the aim at graduation. His research interests include MEMS actuators fabricated in thin film technology. H.H. Gatzen (1943), IEEE, VDI, received his Dipl.-Ing. (MS equivalent) degree in Mechanical Engineering from the Technical University Munich and his doctoral degree from the Technical University, Aachen. His industrial career centered on computer peripherals with tenures at Siemens, Munich, Seagate Technology, Scotts Valley, USA and Conner Peripherals, San Jose, USA. In 1992, he became professor at the Hanover University and founding director of the Institute for Microtechnology. His research work is centered on the design and fabrication of MEMS, ultraprecision machining, and engineering concepts.
Thin®lm technologies to fabricate a linear microactuator M. FoÈhse*, T. Kohlmeier, H.H. Gatzen Institute for Microtechnology, Callinstr. 30 A, Hanover University, D-30167 Hanover, Germany
Abstract The design selected for the microactuator is a three phase variable reluctance (VR) stepping motor with teeth. The stator consists of a ¯ux guide, three groups of poles with teeth, and meander type coils for exciting each group of poles. The system's traveler is built up by a ¯ux guide and teeth. For a good actuator's performance the electroplated magnetic structures' properties have to be optimized. To reach a small air gap that all VR-motors require, the microactuator's surface needs a very low roughness. Therefore, to fabricate the microactuator in thin®lm technology the most important steps are electroplating the structures and embedding these structures for ®nal planarization. # 2001 Elsevier Science B.V. All rights reserved. Keywords: MEMS; Linear motor; Electroplating; Embedding; Planarization
1. Introduction Fabricating a linear microactuator in thin ®lm technology poses interesting technology challenges [1]. The ®rst is in the area of magnetic ®lm deposition. To achieve a maximal driving force, thick magnetic layers with good magnetic properties are required. A technology lending itself to ful®ll these requirements is galvanic microforming which combines electroplating and the use of photoresist moulds. The second challenge is to minimize the air gap. In general, a linear actuator consists of an active and a passive element; in our case, the active element is a stator system, the passive one a traveler, i.e. the equivalent of a rotor of rotational motors. To minimize the air gap between the two, planarization techniques have to be applied. 2. Principle The actuator's design is a three phase variable reluctance (VR) step motor as described in [2]. Fig. 1 depicts a schematic view of the VR-type linear microactuator. The stator consists of three groups of poles with teeth (only one pole each is shown) called phases. For each phase, the tooth location is offset from the other two by one third or two thirds of a tooth pitch, respectively. The traveler, shown at the top, consists of a ¯ux guide and teeth of the same tooth *
Corresponding author.
pitch as the ones on the poles. For the coils to energize each phase, a simple meander design was chosen. For the phase energized, stator and traveler teeth will align. By executing an appropriate energizing sequence, the teeth' offset between the three phases will result in a forward or backward motion of the traveler. Fig. 2 depicts a schematic of the motor technology. The stator's ¯ux guides, poles and teeth were made of electroplated permalloy (NiFe 81/19), the meander coil of electroplated copper. A sputter deposited alumina ®lm isolates the coil from the ¯ux guides, for technology reasons this ®lm also appears at the bottom between the teeth (and originally also on top of the teeth). The traveler's ¯ux guides and teeth are also electroplated. As mentioned before, to control the gap between stator and rotor, both the stator's and the traveler's pole faces have to be planarized by a mechanical or chemical mechanical process and coated with a tribological ®lm de®ning the `air' gap dimension. To be able to execute this process which puts substantial stress on the motor's thin ®lm structure, both the coil area (stator only) and the gaps between the teeth (both stator and traveler) have to be ®lled with embedding material. Furthermore, all materials at the pole faces have to lend themselves to being coated with a tribological ®lm. Besides the pole face surfaces have to be capable of being coated with a tribological ®lm whose thickness de®nes the dimension of the `air' gap between stator and rotor. The ®rst part of the paper discusses the steps taken to optimize the magnetic properties of the permalloy, the second one elaborates on the embedding technologies developed.
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 5 0 9 - X
146
M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
Fig. 1. Schematic view of a VR-type linear microactuator with three phases.
Fig. 3. Pulse sequence of pulsed power supply.
Table 1 Test parameters
Fig. 2. Schematic view of the motor technology and the layers' structure.
3. Permalloy electroplating Electroplating was executed in a `paddle cell', i.e. a galvanic cell where agitation of the electrolyte is accomplished by a paddle moving back and forth throughout the deposition process. Agitation of the galvanic bath compensates for local depletion of the electrolyte due to the plating process and is a precondition to achieve a consistent plating rate. Furthermore, the electrolyte concentration was held constant by appropriately replenishing the materials depleted due to the plating process. For Ni this was accomplished by using a Ni anode, for Fe iron was added regularly to the bath. The ratio between the permalloy's Ni and Fe was controlled by using a pulsed power supply and optimizing the pulse ratio. Permalloy has optimal magnetic properties at a composition of approx. 81 at.% Ni and 19 at.% Fe. At this compositional range, the materials magnetostriction, expressed by the magnetostrictive factor l, has a zero crossing. A ®lm with minimal magnetostriction typically has optimal coercivity and permeability properties. Therefore, by varying the permalloy composition the magnetic ®lm properties may be adjusted. For depositing permalloy, a pulsed power supply was used. Fig. 3 depicts a pulse sequence with total duty cycle length T and forward and reverse pulses lengths Tfwd and Trev, respectively. While the pulse cycle length was held at 10 ms, both the duty cycles of Tre/T wand the reverse current were varied. Table 1 shows the test parameters. Subject to variation were the duty cycle ratio Tfwd/Trev, the current density S and the current level of the reverse pulse. Besides the power supply related variation, the agitation and the temperature were varied. Fig. 4 summarizes the results. The permalloy composition changes when either the agitation or
Parameter
Standard
Test variation
Tfwd/Trev S (mA/cm2) Irev (mA) Paddle frequency (Hz) Temperature (8C)
8/2 13 760 0.35 30
1/1±9/1 8±15 0±800 0.1±0.8 25±45
the temperature or the reverse current level were varied. All ®ve parameters do in¯uence the permalloy composition. The total current density substantially in¯uences on the composition of the permalloy layer, but to maintain a good surface quality this parameter should not be varied widely. The most effective way to control the ®lm composition is to vary the absolute value of the reverse current. This way it becomes possible to control the ratio between Ni and Fe in a wide range and depositing ®lms of a high surface quality at the same time. Other important electroplating-parameters are the bath temperature and the agitation of the solution [3]. These parameters have to be held at constant values for obtaining a good depositing surface and ®lm composition. Fig. 5 depicts the magnetic properties of several electroplated ®lms with different compositions. The optimized deposited permalloy layers reach a saturation ¯ux density Bs of 1.1 T, a remanence Br of 0.25 T A/m and a relative permeability mr greater 500.
Fig. 4. Film composition dependent on pulsed power supply's and other parameters.
M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
147
Fig. 5. BH-loops for different film compositions.
Fig. 6 presents a cross-section of electroplated teeth measured with a stylus surface pro®ler. At the teeth' corners, the electric ®eld is concentrated, resulting in a preferred material deposition in the corner regions. The ®gure emphasizes the need to planarize the tooth faces. 4. Embedding and planarization As discussed previously, to allow a pole face planarization both the gaps between the poles as well as between the teeth had to be ®lled; to withstand the forces applied during planarization without disintegration of the ®lms, the resulting structure had to have a good integrity. Otherwise, the forces occurring during planarization may tear off the ®lms. Fig. 7 depicts the embedding and planarization process. For embedding the coil area, an organic material was used; the gap between the teeth was ®lled with copper. For this purpose, excellent results were achieved with a thick negative tone photosensitive epoxy resin (SU 8). It possesses intrinsic adhesion characteristics superior to conventional thick resists. In a crosslinked state when cured
Fig. 6. Measured surface profile of an electroplated NiFe-pole with teeth.
above 1008C, this epoxy not only is chemically resistant but also thermally stability up to above 2508C. The cured epoxy resin lends itself to be used for structural microparts with a thickness in excess of 40 mm. Processing the photosensitive epoxy resin starts with spin-coating it onto the dehumidi®ed wafer at 2000 rpm, resulting in a ®lm thickness of about 20 mm. The softbake takes place at 958C, followed by a cooling with a low temperature gradient. After the relaxation exposure, the resist is subjected to a ®nal hardbake process. For ®lling the gaps between the teeth at both stator and traveler, electroplated copper was used. To deposit the copper in the desired region, a photomask was created which covers all stator or traveler areas, respectively, except the one between the teeth. In the consecutive electroplating step, copper is deposited. As mentioned before, at the stator both the gaps between the teeth as well as the teeth tops are covered with alumina. Therefore, the ®lling of the gaps with copper originates from the tooth sidewalls. The alumina at
Fig. 7. Principle of embedding: (a) starting with electroplated stator; (b) teeth-embedding; (c) SU-8-embedding; (d) planarization.
148
M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
Fig. 8. Planarized surface of a traveler in detail.
the bottom between the teeth is not desirable, but does not compromise the structural integrity of the embedded tooth structure. Both the embedding of the coils as well as ®lling the gaps between the teeth resulted in ®lm structures with great integrity. Both materials lend themselves to be planarized [4,5] and after planarization to be coated with a tribological ®lm. Fig. 8 shows a detailed view of a planarized traveler's surface analyzed by white light interferometry. For the given example, a CLA roughness value RA of only 25 nm was achieved which satis®es the actuator's requirements.
5. Conclusions and outlook Two of the key challenges in fabricating a VR type linear actuator were to create electroplated thick magnetic ®lms with appropriate magnetic properties as well as embedding techniques allowing to planarize both the stator's and the traveler's pole faces. The best way to controlling the magnetic properties of thick electroplated permalloy ®lms, is to use pulse plating and to appropriately adjusting the reverse current level. For allowing the pole faces to be planarized, the coil was embedded in photosensitive epoxy resin and the gaps
Fig. 9. SEM micrograph of a microactuator's stator system; the meander coils (copper) appear brighter than the NiFe-structures.
M. FoÈhse et al. / Sensors and Actuators A 91 (2001) 145±149
between both stator and traveler teeth was ®lled by electroplated copper. The resulting structure had a good physical integrity and withstood the forces applied during planarization. However, for eddy current reasons it ultimately will be desirable to ®ll the gaps between the teeth with a dielectric material. Applying the chosen technologies resulted in a microactuator ful®lling its magnetic as well as physical material requirements. Furthermore, the application of these techniques is not limited to VR motor designs. It will also be usefull for fabricating hybrid motors which tend allow for greater forces than VR motors (Fig. 9). Acknowledgements The authors would like to thank A. Kariazine for the development of planarization process. This research is supported in part by the German Research Foundation. References [1] R. Belmans, K. Hameyer, Electromagnetic and design aspects of magnetic mini-actuators, Proc., Actuator (1998) 537±540. [2] H.H. Gatzen, H.-D. StoÈlting, S. BuÈttgenbach, H. Dimigen, A novel variable reluctance micromotor for linear actuation, Proc., Actuator (2000), Bremen (accepted).
149
[3] V.C. Kieling, Parameters influencing the electrodeposition of Ni±Fe alloys, Surf. Coatings Technol. 96 (1997) 135±139. [4] H.H. Gatzen, J.C. Maetzig, Nanogrinding, precision engineering, J. ASPE 21 (2/3) (1996) 134±139. [5] H.H. Gatzen, J.C. Maetzig, M.K. Schwabe, Precision machining of rigid disk head sliders, IEEE Transac. Magnetics 32 (3) (1996) 1843± 1849.
Biographies M. FoÈhse (1970) received his Dipl.-Ing. degree in electrical engineering from the Hanover University in 1999. He is currently a fellow of the Institute for Microtechnology at Hanover University with the aim at graduation. His research interests include the design and characterization of micro electro mechanical systems (MEMS) fabricated in thin film technology. T. Kohlmeier (1971), VDI, received his Dipl.-Ing. degree in mechanical engineering from the Hanover University in 1998. He is currently with the Institute for Microtechnology at Hanover University with the aim at graduation. His research interests include MEMS actuators fabricated in thin film technology. H.H. Gatzen (1943), IEEE, VDI, received his Dipl.-Ing. (MS equivalent) degree in Mechanical Engineering from the Technical University Munich and his doctoral degree from the Technical University, Aachen. His industrial career centered on computer peripherals with tenures at Siemens, Munich, Seagate Technology, Scotts Valley, USA and Conner Peripherals, San Jose, USA. In 1992, he became professor at the Hanover University and founding director of the Institute for Microtechnology. His research work is centered on the design and fabrication of MEMS, ultraprecision machining, and engineering concepts.