A sub-micron metallic electrothermal gripper

Transcription

1 Microsyst Technol (2010) 16: DOI /s TECHNICAL PAPER A sub-micron metallic electrothermal gripper Daniel Sang-Won Park Arun Kumar Nallani DonKyu Cha Gil-Sik Lee Moon J. Kim George Skidmore Jeong-Bong Lee Jeong-Soo Lee Received: 6 February 2009 / Accepted: 21 October 2009 / Published online: 8 November 2009 Ó Springer-Verlag 2009 Abstract We report the design, fabrication, and characterization of a multiple bent beam, sub-micron metallic electrothermal gripper. A bottom electroplating mold for electrodes was patterned using electron beam lithography in an SU-8, followed by nickel electroplating. A top electroplating mold for a sub-micron metallic gripper with high aspect ratio bent beams (thickness of 1 lm, width of 350 nm) was prepared using electron beam lithography in a polymethyl methacrylate (PMMA), followed by nickel electroplating and dry release of the top and bottom molds. The sub-micron gripper was characterized using a nanomanipulator system installed in a dual column scanning electron microscopy/focused ion beam system. The ability of the jaw to close up to 1.39 lm displacement with high precision and reliability has been reproducibly observed at an applied current of 28 ma, corresponding to the maximum power consumption of 11.2 mw. Finite element modeling displacement results performed using ANSYS for effective bent beam widths of 370 nm showed a good agreement with the measured displacement results. The sub-micron gripper demonstrated herein will enable the reproducible manipulations with nano-scale resolution displacement and could provide an effective means of D. S.-W. Park A. K. Nallani D. Cha G.-S. Lee M. J. Kim J.-B. Lee J.-S. Lee Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas, Richardson, TX 75083, USA G. Skidmore Zyvex Corporation, Richardson, TX 75081, USA J.-S. Lee (&) Department of Electronic and Electrical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk , Republic of Korea ljs6951@postech.ac.kr interface between nano-scale objects and the micro/macro scale robotic systems. 1 Introduction Microgrippers have been widely used for manipulation and transport of mechanical and biological objects with high precision and reliability in micro-assembly, micro-optics, and biological applications. A variety of materials including polycrystalline silicon (poly-si) (Kim et al. 1992; Millet et al. 2004) and metals (Carrozza et al. 1998) have been used as structural materials in the fabrication of microgrippers using microelectromechanical systems (MEMS) technologies such as surface micromachining, bulk micromachining, and the LIGA techniques. The typical actuation principles of microgrippers are electrostatic (Kim et al. 1992; Millet et al. 2004), piezoelectric (Carrozza et al. 1998), and electrothermal (Que et al. 2001; Lai et al. 2004; Enikov and Lazarov 2003; Kim et al. 2004) actuations. Among them, electrothermal actuation is a good candidate for microgrippers with controllable precision and reliability owing to their large displacements and moderate gripping force at low actuation voltage. The electrothermal microgrippers have been investigated in various materials such as highly doped silicon (Que et al. 2001), poly-si (Lai et al. 2004), and metals (Que et al. 2001; Enikov and Lazarov 2003; Kim et al. 2004). Because of the relatively large thermal expansion coefficients of metals, metallic electrothermal grippers could be actuated at lower actuation voltages for larger displacement with lower steady-state power consumption over silicon or poly-silicon electrothermal microgrippers (Enikov and Lazarov 2003). With the emergence of nano-scale science and technology, demand for physical manipulation of nano-scale

2 368 Microsyst Technol (2010) 16: objects with nano-scale resolution displacement has been greatly increased. Scaling down of MEMS technology to nanoelectromechanical systems (NEMS) or a bottom-up manufacturing approach such as self-assembly is a feasible way to achieve nano-scale manipulation devices. A pair of nano-scale tips with electrostatic actuators by a top down approach of wet anisotropic etching and inductively couple plasma reactive etching (ICP-RIE) was demonstrated for precise manipulation of nano particles with a relatively high actuation voltage (Mita et al. 2003). Nanogrippers were developed utilizing a bottom-up method of aligned multiwall carbon nanotubes along with their characterization of electromechanical properties (Jang et al. 2008). Even though such nanogrippers can be used for nano-scale manipulation with low actuation voltage, high temperature processing of carbon nanotubes for nanogrippers was inevitable to overcome the failure of plastic deformation. With a goal of developing nano-scale precision manipulation devices, we have recently demonstrated sub-micron metallic hot arm cold arm actuators which showed reproducible displacements in the nano-scale range with low actuation voltages using low temperature fabrication process (Lee et al. 2005). In this paper, we extend our previous effort into design, fabrication, and characterization of submicron high aspect ratio (*3:1) metallic electrothermal grippers for repeatable nano-scale resolution displacement with CMOS fabrication compatibility. Design aspects including in-plane actuation, operating temperature range for reliable actuation, and amount of displacement due to physical fabrication variations of the bent beam widths were studied in detail along with finite element modeling (FEM) using ANSYS. Process development efforts using electron beam lithography and electroplating techniques for high aspect ratio metallic sub-micron bent beam structures are described. A method of controlled characterization of sub-micron grippers using nanomanipulators in a dual column scanning electron microscopy (SEM)/ focused ion beam (FIB) system is also described. opened gripper. When an electric current is applied across the two electrodes of the gripper, the bent beams are elongated due to resistive heating and push against each other to provide the linear motion of the apex of the bent beams upward along the axis of symmetry, thus resulting in the displacement of the gripper jaw to be closed. The ratio of the lateral dimension (width) to the vertical dimension (thickness) in the bent beams is crucial in the laterally moving sub-micron gripper. Bent beam structures with high aspect ratios would provide the desired linear motion preferably in the lateral direction for in-plane displacement and suppress out-of-plane displacement in the vertical direction. Based on our fabrication capability for sub-micron structures, bent beam structures were designed with an aspect ratio of approximately 3:1 using a bent beam thickness of 1 lm and width of 350 nm, which will give much less out-of-plane displacement than in-plane displacement by a factor of 9. The length of the bent beams and the width of the center bar were designed to be 45 lm and 1.5 lm, respectively. The initial jaw opening of the sub-micron gripper was designed to be 1.6 lm. A series of three dimensional, electro-thermo-mechanical FEM were performed using ANSYS TM simulation in order to investigate the behaviors of the sub-micron gripper. The material properties of the electroplated nickel used in the FEM simulation were as follows: resistivity of 15 lx-cm obtained from the four-point probe technique for the electroplated nickel, Poisson s ratio of 0.31 (Lee et al. 2005), Young s modulus of 200 GPa (Fritz et al. 2002), thermal expansion coefficient of 17 ppm K -1 (Safranek 2 Design of a sub-micron electrothermal gripper The design of the multiple bent beam, sub-micron electrothermal gripper was based on a chevron or V shaped bent-beam actuator (Que et al. 2001; Lai et al. 2004) and the widths of the bent beams were scaled down for submicron structures in order to realize the nano-scale resolution displacement. Figure 1 shows a schematic diagram of the sub-micron electrothermal gripper. The chevron shaped sub-micron gripper includes two banks of ten bentbeams for electrothermal actuation at an angle of 6 with respect to the horizontal line and a gripper located at the concave side of the bent beams, operating as a normally Fig. 1 A schematic diagram of the sub-micron metallic electrothermal gripper with the geometrical parameters

3 Microsyst Technol (2010) 16: Fig. 2 The temperature distribution results from ANSYS simulation for the sub-micron electrothermal gripper with an applied current of 29 ma 1986), and thermal conductivity of 91 W m -1 K -1 (Safranek 1986). In the FEM simulation, the material properties of the electroplated nickel were assumed to be constant over temperature and the fabrication process variations. Electrothermal grippers should be operated in an acceptable temperature range for reliable actuation in order to avoid localized material redistribution and local plastic deformation. While an upper temperature limit for electrothermal actuation of metallic microstructures in nickel was reported to be about 350 C (Que et al. 2001), our submicron metallic electrothermal actuators in nickel showed an upper temperature limit of about 200 C (Lee et al. 2005). The ANSYS FEM simulation was performed in a vacuum environment to investigate temperature distribution of the sub-micron gripper as a function of an applied current. Thus the allowable applied current for the upper temperature limit of about 200 C was obtained, below which the sub-micron gripper can be safely operated without undergoing the irreversible plastic deformation. Figure 2 shows the temperature distribution of the submicron gripper in a vacuum environment with the beam width of 350 nm. The FEM result indicated that the maximum temperature of about 200 C would be generated around the center region of the bent beams in the gripper with an applied current of 29 ma. Although the bent beams widths in the chevron shaped sub-micron gripper were designed to be 350 nm, it was expected that the beams would be widened near the large structures such as the center bar and the electrode region due to the proximity effect during the electron beam lithography process. In addition, due to the electron scattering effect of the electron beams to the exposed resist, the bottom width of the sub-micron cavities for bent beams would be slightly widened after resist development. Such process-induced non-ideal factors were taken into account during the electro-thermo-mechanical ANSYS FEM simulation to predict the displacements of the sub-micron gripper as a function of an applied current. Different bent beam widths were simulated in the range of 350 * 400 nm to find the effective bent beam width in a controlled vacuum environment, and a fixed bent beam width of 350 nm was simulated to study the effect of heat conduction to displacement of the sub-micron gripper in air. The FEM simulation results with the designed bent beam width of 350 nm at an applied current of 21 ma showed that the gripper would generate a jaw closing of 900 nm in vacuum, whereas it would only generate a jaw closing of approximately 250 nm in air (Fig. 3). When the sub-micron gripper is operated in air, the surrounding air is thermally conductive and a large amount of heat conduction through air to the substrate occurs. Therefore, it was expected that significantly less amount of displacement with the same input power would be generated in the air environment. The sub-micron grippers with the bent beam width of 350 nm in air (denoted by circles with a dotted line) and the bent beam width of 400 nm in vacuum (denoted by triangles with a solid line) showed comparable displacements of the jaw (Fig. 3).

4 370 Microsyst Technol (2010) 16: Fig. 3 The ANSYS simulation results for the sub-micron gripper with different bent beam widths and surrounding environments 3 Fabrication of a sub-micron electrothermal gripper For controllable operation of the sub-micron electrothermal grippers, reproducibility of the fabrication sequence for the dense bent beams with sub-micron scale widths in the gripper is very critical. Electron beam lithography and electroplating techniques have been successfully demonstrated for the reproducible fabrication of the single, hot arm in the sub-micron electrothermal actuators (Lee et al. 2005). The same fabrication sequence has been used with further process optimization in the electron beam lithography and electroplating for the fabrication of the dense bent beams with sub-micron scale widths in the sub-micron gripper, which is shown in Fig. 4. A commercially available negative-tone resist, Nano TM SU (MicroChem, Newton, MA, USA), was used for the electroplating bottom mold of electrodes and as a sacrificial layer for sub-micron grippers. After electron beam evaporation of an electroplating seed layer of 5 nm chromium (Cr)/25 nm gold (Au) on an oxidized silicon wafer, Omnicoat TM (MicroChem, Newton, MA, USA) was spin-coated as an adhesion promoter between the seed layer and SU-8 resist and was baked at 200 C for 1 min on a hotplate. Then, SU-8 resist was spin-coated and prebaked using a multi-step baking of 65 and 95 C on a hot plate to obtain a resist thickness of approximately 1.2 lm. Fig. 4 Fabrication sequence of a sub-micron metallic electrothermal gripper: a SU-8 electron beam lithography, b nickel electroplating for electrodes, c PMMA electron beam lithography, d nickel electroplating for a sub-micron gripper, and e released sub-micron gripper with electrodes

5 Microsyst Technol (2010) 16: SU-8 samples were exposed to form the SU-8 resist mold by using the EBM XIV, a Texas Instrument customdesigned electron beam lithography station setup operating at an acceleration voltage of 20 kv (Nallani et al. 2003). The exposed SU-8 samples were post-baked using the same multi-step baking method as pre-baking, developed in Nano TM SU-8 developer, rinsed in isopropyl alcohol (IPA), and dried in air. Next, the SU-8 mold samples were further cleaned by reactive ion etching (RIE) for 30 s in 100% oxygen plasma at 150 mtorr with a low incident power of 50 W. This RIE cleaning rendered a clean seed layer surface, thus providing effective growth of metal during electroplating process (Fig. 4a). Nickel electroplating was performed at a temperature of 55 C with continuous agitation to form metallic structures for the electrodes through SU-8 mold (Fig. 4b). The electroplating bath consisted of Ni(SO 3 NH 2 ) 2 (nickel sulfamate) 450 ml, H 3 BO 3 (boric acid) 37.5 g, and C 12 H 25 NaO 4 S (sodium dodecyl sulfate) 3 g in 1 l of de-ionized water. A commonly used electron beam lithography positivetone resist, 950 PMMA C7 (MicroChem, Newton, MA, USA), was selected for the fabrication of the dense bent beams in the sub-micron grippers. After electroplating of nickel in the SU-8 mold for electrode fabrication, the second electroplating seed layer of 5 nm Cr/25 nm Au was deposited using electron beam evaporation. Polymethyl methacrylate (PMMA) with a thickness of 1 lm was spin-coated, followed by baking at 170 C for 30 min in a convection oven. Electron beam lithography for PMMA was performed with a LEO 1570 SEM equipped with the nanometer pattern generation system from JC Nabity Lithography Systems, operating at an acceleration voltage of 30 kv. A beam current of approximately 40 pa was used to adjust the electron exposure dose by controlling the beam dwell time and the step size of 15 nm. Optimization of the electron beam exposure dose for sub-micron bent beam structures was carried out by varying the dose from 135 lc cm -2 to 275 lc cm -2. A more detailed description of the electron beam lithography process for the sub-micron high aspect ratio PMMA structures can be found elsewhere (Lee et al. 2005). After electron beam exposure, the samples were developed in 1:3 MEK: IPA solution for 7 min (Fig. 4c). The developed PMMA molds were further cleaned by RIE using the same condition for the SU-8 structures. Prior to the electroplating process, the samples with submicron PMMA top molds were immersed in the nickel plating solution and air bubbles trapped in the PMMA mold were removed using a vacuum pump. This step ensures the initiation of nickel electroplating in the submicron scale high aspect ratio PMMA mold. Next, electroplating of nickel was performed (Fig. 4d). The control of the intrinsic stress during the electroplating process is crucial for the formation of sub-micron metallic structures with minimal deposit stress, which will in turn provide controllable operation of the electroplated, sub-micron electrothermal grippers. A constant electroplating temperature of 55 C was maintained during the electroplating process, which helped minimize the intrinsic stress in the electroplated sub-micron structures. Applied current density also plays a key role in minimizing the intrinsic stress, so a current density of 2 ma cm -2 was selected with a deposition rate of 2 lm h -1. Such current density of 2mAcm -2 would produce Young s modulus close to 200 GPa (Fritz et al. 2002), which was used in the ANSYS FEM simulation as described in the previous section. After PMMA removal by RIE, the widths at the top and bottom of the bent beam structures in nickel were inspected under SEM. The bottom widths of the bent beams in the nickel structures showed dimensional increases as the electron beam exposure dose increased due to higher electron scattering effect. With the optimal exposure dose of 135 lc cm 22, the bottom width of the bent beams in the nickel structures confirmed the smallest variation of 13% compared to the top width of the bent beams. The top metallic seed layers of gold and chromium were removed in potassium iodide-based gold etchant and chromium etchant (a solution of 20 g: 20 g: 100 ml of NaOH: K 3 Fe(CN) 6 :H 2 O), respectively. Finally, microwave barrel plasma (300 series MW plasma system, PVA Tepla America, Inc., Corona, CA, USA) was used to remove SU-8 sacrificial layer at 500 W with 80% O 2 and 20% CF 4, followed by removal of the bottom metallic seed layers, resulting in the released submicron featured metallic electrothermal grippers (Fig. 4e). Figure 5 shows the SEM images of the sub-micron gripper, suspended in air 1.2 lm from the substrate. Closer inspection of the air suspension gaps around the submicron gripper showed uniform gap spacing between the Fig. 5 Close-up SEM images of a sub-micron electrothermal gripper. The width of the bent beam is 350 nm (inset)

6 372 Microsyst Technol (2010) 16: metallic structures and the substrate, confirming the optimized minimal stress process during electroplating of submicron grippers. The chevron shaped bent beams with a top width of 350 nm are shown in the inset of Fig Characterization of a sub-micron electrothermal gripper An FEI Nova 200 Nanolab system (Hillsboro, OR, USA) was used for characterization of actuation of the submicron metallic electrothermal grippers. The system is a versatile, high performance dual column SEM/FIB, equipped with the S100 nanomanipulator system (Zyvex Corp, Richardson, TX, S100.html) including four manipulators with 10 nm positioning resolution, and provides ultra-high resolution images as well as the controlled nano-scale characterization environment. Hewlett Packard 4155A semiconductor parameter analyzer was employed to apply currents to the two electrodes of the sub-micron gripper through the two probes of the S100 system. Figure 6 shows an SEM image of the nanomanipulator probes and the sub-micron gripper inside the SEM/FIB chamber. A series of still images for the jaw of the sub-micron gripper before and after applying current were taken and the exact amount of the displacement in the jaw at any applied current was accurately determined from the SEM images. Figure 7 shows a graph of the amount of displacement (jaw closing) as a function of the applied current Fig. 6 An SEM image of two nanomanipulator probes and a sub-micron gripper inside the SEM/FIB chamber Fig. 7 The amount of displacement (jaw closing) of the sub-micron electrothermal gripper as a function of the applied current with ANSYS simulation results with effective bent beam widths of 370 nm in vacuum and in air for the sub-micron gripper with ANSYS simulation results in vacuum and in air for the effective bent beam width of 370 nm. Reproducible displacements have been achieved up to 1.39 lm with an applied current of 28 ma, which corresponds to the applied voltage of 400 mv and the input power of 11.2 mw, respectively. The minimum displacement of approximately 50 nm has been obtained with the applied current of 5 ma. It was found that ANSYS simulation results with the effective bent beam width of 370 nm matched well to the measured values in the vacuum (Fig. 7). When the applied current increased above 30 ma, plastic deformation was found in the bent beam region. In order to provide a wider range of displacements without approaching plastic deformation, the bent beams can be designed longer and the angles of the bent beams smaller (Que et al. 2001). To demonstrate the physical gripping actuation of the sub-micron gripper, a platinum solid cylinder with a diameter of 1 lm and thickness of 2 lm was deposited between the gripper jaws using FIB deposition. A trimethyl-methylcyclopentadienyl-platinum (C 9 H 16 Pt) was used as a platinum precursor. Ga? ion energy of 30 kev and a beam current of 10 pa were used for the platinum column deposition. Figure 8 shows the SEM images of the sub-micron gripper which show successful gripping of the FIB deposited platinum column with an applied input current of 25 ma. By reducing the spacing between the end-effectors in the gripper design, the gripper could achieve full range of actuation with much lower current.

7 Microsyst Technol (2010) 16: a better understanding of the acceptable range of gripping forces depending on the nano-scale objects of interest. A scaled-down version of the sub-micron gripper demonstrated herein will enable the reproducible manipulations of very small (down to sub-micron in dimension) objects with high precision and nano-scale resolution displacement. In addition, the sub-micron gripper could provide an effective means of interface as end-effectors between nanoscale objects to be manipulated and the micro/macro scale robotic systems. Acknowledgments This work was supported by the National Institute of Standards and Technology Advanced Technology Program (NIST-ATP) (#70NANB1H3021) and partly by WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (Project No. R ). The authors would like to thank PVA TePla for equipment support (TePla 300 microwave plasma etch system). The authors would also like to thank the members of the Micro/Nano Device and Systems (MiNDS) Laboratory and Cleanroom staffs at the University of Texas at Dallas. References Fig. 8 SEM images of the sub-micron gripper grabbing a Pt solid cylinder: a before and b after applying a current of 25 ma 5 Conclusions The fabrication of sub-micron metallic electrothermal grippers has been successfully demonstrated using electron beam lithography and electroplating techniques. The amount of displacement of the sub-micron grippers has been characterized using a dual column SEM/FIB system equipped with nanomanipulators. Reproducible jaw closing of up to 1.39 lm with an applied current of 28 ma has been achieved with high precision and reliability. The results were in good agreement with those of the finite element analysis using ANSYS for effective bent beam widths of 370 nm. Further characterization of sub-micron metallic electrothermal grippers could be done, including FEM and physical measurements of gripping forces, which will lead to Carrozza MC, Menciassi A, Tiezzi G, Dario P (1998) The development of a LIGA-microfabricated gripper for micromanipulation tasks. J Micromech Microeng 8: Enikov ET, Lazarov K (2003) PCB-integrated metallic thermal micro-actuators. Sens Actuators A 105:76 82 Fritz T, Cho HS, Hemker KJ, Mokwa W, Schnakenberg U (2002) Characterization of electroplated nickel. Microsyst Technol 9:87 91 Jang JE, Cha SN, Choi Y, Kang DJ, Hasko DG, Jung JE, Kim JM, Amaratunga GAJ (2008) A nanogripper employing aligned multiwall carbon nanotubes. IEEE Trans Nanotechnology 7(4): Kim C-J, Pisano AP, Muller RS (1992) Silicon-processed overhanging microgripper. IEEE J MEMS 1(1):31 36 Kim K, Nilsen E, Huang T, Kim A, Ellis M, Skidmore G, Lee J-B (2004) Metallic microgripper with SU-8 adaptor as end-effectors for heterogeneous micro/nano assembly applications. Microsyst Technol 10: Lai Y, McDonald J, Kujath M, Hubbard T (2004) Force, deflection and power measurement of toggled microthermal actuators. J Micromech Microeng 14:49 56 Lee J-S, Park DS-W, Nallani AK, Lee G-S, Lee J-B (2005) Submicron metallic electrothermal actuators. J Micromech Microeng 15: Millet O, Bernardoni P, Régnier S, Bidaud P, Tsitsiris E, Collard D, Buchaillot L (2004) Electrostatic actuated micro gripper using an amplification mechanism. Sens Actuators A 114: Mita M, Kawara H, Toshiyoshi H, Ataka M, Fujita H (2003) An electrostatic 2-dimensional micro-gripper for nano structures. In: Transducers 03, the 12th international conference on solid state sensors, actuators, and microsystems, pp Nallani AK, Park SW, Lee J-B (2003) Characterization of SU-8 as a resist for electron beam lithography. Proc SPIE 5116: Que L, Park J-S, Gianchandani YB (2001) Bent-beam electrothermal actuators part I: single beam and cascaded devices. IEEE J MEMS 10(2): Safranek WH (1986) The properties of electrodeposited metals and alloys, 2nd edn. American Electroplaters and Surface Finishers Society, Florida