Novel micro-pneumatic actuator for MEMS

Sensors and Actuators A 97±98 (2002) 638±645

Novel micro-pneumatic actuator for MEMS Sebastian BuÈte®sch*, Volker Seidemann, Stephanus BuÈttgenbach Institute for Microtechnology, Technical University of Braunschweig, Alte Salzdahlumer Str. 203, 38124 Braunschweig, Germany Received 13 June 2001; received in revised form 20 November 2001; accepted 22 November 2001

Abstract This paper presents a micro-pneumatic actuator utilizing a new actuation principle for micro-mechanical systems. As a ®rst application, a micro-gripper driven by two bellow-type micro-pneumatic actuators, is described. The basic structures of the micro-pneumatic actuator and the micro-gripper are fabricated by silicon dry etching in a single etching step. The device consists of a Pyrex±silicon±Pyrex sandwich structure which was assembled by anodic bonding. Alternatively, an SU8 UV-depth lithography process was used to realize the pneumatically driven micro-gripper. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Pneumatic actuator; Micro-gripper; SU8

1. Introduction

2. Micro-pneumatic actuator

Reduction of size, weight, and power consumption are ongoing trends in microtechnology, microsurgery, and bioengineering that require the availability of smart micro-mechanical actuators. Therefore, worldwide research is focussing on the development of new actuator concepts and materials. Piezoelectric, electrostatic, and shape memory alloy (SMA) actuators are used to realize complex hybrid microsystems. These actuators have their particular advantages and disadvantages [1,2]. A novel approach is the use of compressed air as driving force in a micro-actuator. A new concept for a micro-pneumatically driven actuator has been developed and realized in our laboratory. This actuation principle has several advantages:

The basic structure of the micro-pneumatic actuator consists of a piston connected to the housing by two spring elements. These spring elements enable the piston to move when a pressure is applied and provide the sealing against the environment similar to a bellow piston (Fig. 1). Two Pyrex wafers are forming the top and bottom part of the cylinder. To enable the movement of the piston and the spring elements, a gap between the Pyrex lids and the sealing structure of some microns has to be realized. During dicing the wafer, the inlets of the cylinder are opened. Capillary tubes with a diameter of about 300 mm are mounted to the inlets of the cylinder for applying the compressed air to the system. The over all dimensions of the cylinder are 6600  4950 mm2 with a thickness of 1500 mm. As a ®rst application a micro-gripper has been realized using the micro-pneumatic actuator. The gripper linkage is an advancement of SMA driven micro-grippers presented earlier [3,4]. To transmit the generated force, the piston is attached to the gripper linkage by a beam shaped ¯exure hinge. This compliant hinge compensates the rotational movement of the input crank of the gripper linkage. Two pneumatic actuators are provided, one for opening, one for closing the gripper. FEM simulations were performed to optimize the shape of the spring elements and the ¯exure hinges (Fig. 2a). The optimization included keeping the produced stress as low as possible to avoid mechanical failure as well as reducing the necessary driving pressure. A critical part of the structure in terms of the occurring stress is the connecting linkage between the two gripper jaws (Fig. 2b).

    

high energy density; large achievable displacements; high generated forces; excellent dynamic behavior; usage of various fluids as driving medium (e.g. important for microsurgery);  usage as final controlling element with continuous action;  high design flexibility. Therefore, this concept has great potential in microactuator technology.

*

Corresponding author. Tel.: ‡49-531-391-9766; fax: ‡49-531-391-9751. E-mail address: [email protected] (S. BuÈtefisch).

0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 4 3 - 3

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Fig. 1. Pneumatically driven micro-gripper.

3. Fabrication process The micro-pneumatic actuator and the gripper mechanism were fabricated in a single step. Two different materials and fabrication processes have been investigated:  silicon reactive ion etching (RIE);  UV-depth lithography with SU8 photoepoxy. Both processes have their particular advantages and disadvantages. 3.1. High aspect silicon RIE The base material for this process is a silicon wafer polished on both sides with a thickness of 360 mm. An anisotropic inductively-coupled plasma (ICP) dry etch process was optimized to form the structure of the micropneumatic actuator and the gripper gear in a single etch step (Fig. 3). Photolithographically structured resist with a sputtered chrome layer as adhesive layer was used as etch mask. For this task an optimized process for the photoresist microallresist MA-P 1240 has been developed, obtaining a thickness of 12 mm with good structural resolution. During the etch process the wafer is electrostatically clamped onto a helium cooled chuck. To achieve in-plane compliant struc-

tures like the spring elements in the piston bellows of the pneumatic actuators and the ¯exure hinges of the gripper linkage, through wafer etching is necessary. This through etching is problematic since the process gases damage the machine chuck when longer over-etch times are necessary due to non-uniform etch rates. Therefore, a backside passivation has been developed and tested to protect the chuck against the process gases. The layer deposited at ®rst onto the wafer backside is a 500 nm sputtered aluminum layer as passivation against the process gases. In order to enforce this layer mechanically a 7 mm thick copper layer is applied by electroplating. After the etching process all layers are removed by wet chemical etching releasing movable compliant structures. Another problem arises from the deviation of the etch rate caused by different mask openings. Thus, for larger mask openings additional structures have been applied, keeping the same mask opening width for all critical structures. These additional structures are only attached to the passivation layer and are removed during the wet chemical of this layer acting as sacri®cial structures. After this process two Pyrex wafers are attached to the structured silicon wafer by anodic bonding forming a Pyrex± silicon±Pyrex sandwich. To enable the movement of the piston and the spring elements a groove of about 10 mm was

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Fig. 2. FEM simulation of the: (a) piston bellow; (b) gripper gear.

etched into the Pyrex wafers before bonding them to the silicon structure. This has been realized using a with photo lithographically structured gold layer and hydro¯uoric acid containing etchant. The chips are separated using a wafer

saw. The inlets of the cylinders are opened in the same process step. The capillary tubes for supplying the actuator with compressed air are applied to the device and ®xed and sealed with epoxy resin.

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Fig. 3. SEM micrograph of the silicon structure.

3.2. UV-depth lithography with SU8 photoepoxy Alternatively, an optimized high aspect ratio SU8 process [5,8] was used to realize the compliant structures of the micro-pneumatic actuator and the ¯exural hinges of the gripper linkage. Film thicknesses between 200 and 600 mm were realized using spin coating with a rotating lid spinning tool, baking on ramped hot plates and a two solution (GBL and PGMEA) development. These measures are important to guarantee uniform ®lm thickness, low stress coats, and high resolution. Aspect ratios of up to 36 were achieved, well suited to realize the spring elements of the pneumatic actuator and the ¯exure hinges of the gripper. For partially releasing the compliant structures from the substrate sputter deposited copper proved to work best as a sacri®cial layer due to its ease to be deposited and etched. After releasing the compliant structures with this sacri®cial layer technique the base substrate (Pyrex) is cut with a wafer saw whereas parts of the substrate remain under the spring elements building the bottom part of the cylinder. The top part is also built by a Pyrex wafer attached to the device by epoxy resin. Main advantage of the SU8 photoresist as base material for the pneumatically driven micro-gripper is the low

Young's modulus of 2.5±5 GPa [5]. This enables a good movability of the compliant structures while keeping the occurring stresses low. Used in micro-mounting facilities the transparency of the material enables the observation of the grasped object. In combination with video cameras and image processing the accuracy of the mounting process can be increased device [7]. 4. Actuator performance and applications To evaluate the actuators' performances a test set up was built. It consists of a tactile 3D force sensor which has been developed at our laboratory [6] and a high precision xyzstage. The actuator is ®xed to the xyz-stage which enables a very ®ne positioning versus the tactile element of the force sensor. The force sensor is mounted to a de¯ection facility consisting of a double beam ¯exure hinge construction. With this de¯ection facility the tactile element of the force sensor can be placed in a very precise way on the part of the actuator where the generated force will be transmitted. First measurements have been performed applying a constant displacement through the force sensor to the actuator. By increasing the pressure and measuring the generated forces the characteristic curve shown in Fig. 4a is obtained. With a

Fig. 4. (a) Generated force versus driving pressure with varying applied displacements. (b) Displacement of the piston due to an oscillating driving pressure.

Fig. 5. Pneumatically driven SU8 micro-gripper.

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Fig. 6. (a) SU8 lid furnished with alignment structures; (b) gripper structure with slot.

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Fig. 7. Grasped micro-mechanical object.

pressure of about 120 mbar displacements of up to 600 mm and generated forces of over 10 mN have been achieved with the silicon piston bellow (Fig. 4a). Dynamic measurements have been performed by applying an oscillating pressure to the piston. Frequencies of over 150 Hz and strokes of 500 mm have been measured (Fig. 4b), whereas the limitating factor was not the piston but the electromagnetic valve for switching the driving pressure. The overall dimensions of the SU8 piston bellow is three times smaller than the silicon device keeping the same minimal structural dimension at 30 mm due to the much smaller Young's modulus of the material. Therefore, the dimensions could be reduced to 2200  1650 mm2 for the cylinder and 6050  6000 mm2 for the complete microgripper (Fig. 5). The generated gripper force was measured with the same set up as described above. Maximum gripping forces of up to 10 mN have been achieved with a driving pressure of up to 400 mbar. In a further approach, the sealing lids have been fabricated in the same process step as the micro-gripper (Fig. 6a). To ensure a good sealing and alignment of the lid a slot was included at a certain distance of the cylinders (Fig. 6b). The lid is furnished with a alignment structure which ®ts to the slot next to the cylinders. The round shaped openings next to

alignment structures are used to restrict large SU8 areas in order to avoid cracking of the polymer. Both parts have been realized using a double layer process [8]. There are two ways to fabricate double layer parts. Firstly, the bottom layer can be processed as described above, i.e. spin on, expose, and develop. Subsequently, the second layer is fabricated in the same manner. The second possibility is to spin the bottom layer on, expose, but omit the development. Then the second layer is added and exposed, whereupon both layers are developed. This allows to save time and cost for the development step. The micro-pneumatic gripper proved its functionality by repeated gripping and holding of micro-parts without failure and is currently in use in a micro-assembly station (Fig. 7). 5. Conclusion A new actuation conceptÐthe micro-pneumatic actuatorÐand a micro-gripper as ®rst application are presented. Two fabrication processes, silicon RIE and UV-depth lithography with SU8 photoepoxy were developed and are demonstrated on the compliant structures of the actuator and the micro-gripper.

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Main advantage of silicon as base material are the excellent material properties like long-term stability and no fatigue. Another advantage is the anodic bond technology as joining technique and the possibility of integrating sensors based on diffused piezoresistors in the device. The SU8 provides a very good movability of the compliant structures due to the small Young's modulus. Besides the low price, the transparency of this material is another advantage, especially for assembly applications where visual control is often necessary to increase the accuracy of the assembly process. The actuators' performance and the gripping forces generated by the pneumatically driven micro-gripper were investigated using a special set up including a new tactile 3D force sensor. Displacements of up to 600 mm and generated forces of over 10 mN at frequencies of over 150 Hz been achieved. Further research needs to be carried out to determine the long-term mechanical behavior of the devices like fatigue and aging proof. Another interesting aspect is the usage of liquids as driving medium in terms of medical applications such as minimal invasive surgery. Investigating the in¯uences of different driving media on the actuator's performance is subject of further research tasks. Acknowledgements The authors would like to thank B. Matheis and M. Feldmann for the help with process development. References [1] M. Madou, Fundamentals of Microfabrication, CRC Press, Boca Raton, FL, 1997. [2] S. BuÈttgenbach, MikromechanikÐEinfuÈhrung in Technologie und Anwendung, 2nd Edition, Teubner, Stuttgart, 1994. [3] S. BuÈtefisch, G. Pokar, S. BuÈttgenbach, J. Hesselbach, A new SMA actuated miniature silicon gripper for micro-assembly, in: Proceedings of the Seventh International Conference on New Actuators (Actuator 2000), Bremen, Germany, June 19±21, 2000, pp. 334±337. [4] S. BuÈtefisch, S. BuÈttgenbach, New differential-type SMA actuator for a miniature silicon gripper, Smart material and MEMS, in: Proceedings of the SPIE, Melbourne, Australia, December 13±15, in press. [5] V. Seidemann, S. BuÈtefisch, S. BuÈttgenbach, Application and investigation of in-plane compliant SU8-structures for MEMS, in:

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Proceedings of the Transducers 2001, Munich, Germany, June 11±14, in press. [6] S. BuÈtefisch, S. BuÈttgenbach, T. Kleine-Besten, S. Loheide, U. Brand, Silicon three-axial tactile sensor for micromaterial characterization, in: Proceedings of the Third International Conference for Micromaterials (Micromat 2000), Berlin, Germany, April 17±19, 2000, pp. 420±427. [7] J. Hesselbach, G. Pokar Assembly of a linear actuator using vision feedback, in: Proceedings of the SPIE on Microrobotics and Micromanipulation, Vol. 4194, Boston, USA, November 5±8, 2000, pp. 13±20. [8] V. Seidemann, J. Rabe, M. Feldmann, S. BuÈttgenbach, SU8micromechanical structures with in situ fabricated movable parts, in: Proceedings of the Fourth International Workshop on High-aspectratio Micro-structure Technology (HARMST), Baden-Baden, Germany, June 17±19, in press.

Biographies Sebastian BuÈtefisch received his Diploma in mechanical engineering from the Technical University of Braunschweig, Germany, in 1997. Since 1997, he has been employed at the Institute for Microtechnology at the Technical University of Braunschweig. His research interests are in the development and fabrication of micro-grippers, micro-mechanical actuators, low-g acceleration sensors, tuning fork gyroscopes and high resolution threedimensional tactile force sensors. Volker Seidemann received his Diploma in mechanical engineering from the Technical University of Braunschweig in 1998. Throughout his studies he specialized in microsystem technologies and worked on related research projects at the Institute for Microtechnology in Braunschweig, the MSMA group of M.G. Allen at the Georgia Institute of Technology and the Corporate Research and Development Department at the Robert Bosch GmbH, Stuttgart. Since September 1999, he is employed as a scientific assistant at the Institute for Microtechnology in Braunschweig to earn the PhD degree. The main focus of his recent work is UV-depth lithography and other fabrication technologies for electro-magnetic sensors and actuators. Stephanus BuÈttgenbach received his Diploma and PhD degrees in physics from the University of Bonn, Germany, in 1970 and 1973, respectively. From 1974 to 1985, he was with the Institute of Applied Physics of the University of Bonn, working on atomic and laser spectroscopy. In 1983, he was promoted to Professor of Physics. From 1977 to 1985, he was also a scientific associate at CERN in Geneva, Switzerland. In 1985, Dr. BuÈttgenbach joined the Hahn-Schickard-Society of Applied Research at Stuttgart as Head of the Department of Microtechnology, where he worked on micro-mechanics, laser microfabrication, and resonant sensors. From 1988 to 1991, he was the Founding Director of the Institute of Microstructure and Information Technology of the Hahn-SchickardSociety. In 1991, he became Professor of Microtechnology at the Technical University of Braunschweig. His current research centers on the development and application of micro-electro-mechanical systems.