introduced a microactuation technology that combines magnetic thin films ... taminating the conventional microelectronic and micromechanical fabrication steps.
FABRICATION PROCESSES FOR MAGNETIC MICROACTUATORS WITH POLYSILICON FLEXURES
Jack W. Judy and Richard S. Muller Berkeley Sensor & Actuator Center (BSAC) Department of EECS, University of California, Berkeley, California 94720-1770 (U.S.A)
ABSTRACT Electroplated magnetic films have been integrated with silicon-based surface micromachining to fabricate magnetically actuated microflexure structures. Both the “frame-plating” technique and also a less complex onemask plating process have been used to electrodeposit NiFe onto polysilicon flexures coated with Cr-Cu seed layers. The microactuators are released by removing the underlying sacrificial layer in a hydrofluoric-acid etch. The microactuators described in this paper are potentially useful in microphotonic systems as well as other applications.
INTRODUCTION Magnetic forces have been demonstrated to be very effective at achieving large forces and large displacements in micromachined actuators [1-7]. However, many magnetic microactuators are constructed in a non-batch fabrication process [1-3]. In 1994, we introduced a microactuation technology that combines magnetic thin films with surfacemicromachined flexures in a relatively simple batch-fabrication process [5-6]. This combination of materials and fabrication technologies creates new, useful, and relatively highforce microactuation possibilities. DEVICES Cantilever microstructures, shown in Figure 1, were used as an initial test of the technology [5-6]. In these devices, the tip of an 800 µm-long cantilever-test structure, consisting of a 400 µm-long NiFe plate attached to a 400 µm-long polysilicon cantilever, was displaced more than 1.2 mm and rotated more than 180° in a magnetic field of ~5 kA/m. Torsional microstructures that have potentially useful applications in microphotonics, such as for optical-beam chopping, scanning, and steering, have also been investigated [7]. A torsional device, shown in Fig. 2, with a 400 µm-long NiFe plate attached to a pair
of 400 µm-long torsional beams has been driven out of the plane of the wafer into a vertical position by an external magnetic field of ~8 kA/m. A theoretical model [7] has been formulated that takes into account the affect of the demagnetizing field and magnetic anisotropy on the magnitude and direction of the net magnetization vector inside the magnetic material. As shown in a plot given in Fig. 3, the theoretical model accurately predicts the experimental angular deflection as a function of external magnetic field. Although the microactuators are driven by an off-chip source for the magnetic field, it is feasible to integrate an on-chip source. For example, a 100-turn coil of 5 µm-wide wires with 5 µm-wide spaces and a 1 mm inner diameter can theoretically produce a magnetic field of ~5.5 kA/m when conducting a current of 100 mA. MATERIALS The microactuators discussed in this paper are constructed in a fabrication process that combines electroplated NiFe films with surface-micromachined phosphorus-doped LPCVD polycrystalline silicon. Polysilicon and NiFe are proven thin-film materials with well-known deposition and processing techniques. Polysilicon has been used extensively in both high-Q and highly compliant microelectromechanical systems because of its excellent mechanical properties. In particular, polysilicon has a fracture strain of 1 to 2% and a very long lifetime. Permalloy is well known for its use in thin-film magnetic recording heads and in variable-reluctance magnetic microactuators [2-4] because of its excellent magnetic properties. Specifically, permalloy offers a higher saturation magnetization (Ms = 1.1 T) than nickel (0.7 T) and is less susceptible to stress-induced magnetic effects because it has low magnetostrictive properties. However, magnetically hard films, specifically Co and its alloys, may have certain advantages in these microactuators and thus are a subject of present investigation. FABRICATION The microactuators are produced by adding fabrication steps to a process that had been developed at BSAC to produce polysilicon resonant microstructures [8]. All new steps, many of which are borrowed from the magnetic-recording head fabrication process, are added near the end of the fabrication sequence, just prior to the micromechanicaldevice “release” step. This processing sequence prevents the magnetic material from contaminating the conventional microelectronic and micromechanical fabrication steps. Beginning with a silicon substrate, a 2.0 µm-thick phosphorus-doped silicon dioxide (PSG) layer is deposited by low-pressure chemical-vapor deposition (LPVCD). The PSG layer is called the sacrificial layer because it will be selectively removed at the end of the fabrication process to release the micromechanical devices. After a layer of photoresist is spun-on and patterned, the PSG layer is etched through to the substrate and the photoresist is stripped. Next, a 2.0 µm-thick LPCVD polycrystalline silicon (polysilicon) layer and a
subsequent 0.2 µm-thick LPCVD PSG layer are deposited. Since the polysilicon layer will be used to form the released micromechanical structure, a low mechanical stress gradient in the polysilicon is needed. To achieve this low stress gradient, the wafer is annealed at 1000 °C for 1 hour. The upper PSG layer is then removed in a wet hydrofluoric-acid etch and, after a layer of photoresist is spun on and patterned, the polysilicon layer is etched to define the micromechanical structure and flexures. Except for the final “release” step, the silicon-based surface micromachining is complete. An adhesion layer (10 nm of Cr) and an electroplating seed layer (100 nm Cu) are then deposited by evaporation (Figs. 4a and 5a). The fabrication of our initial devices (Fig. 1 and Fig. 4) is completed by using a “frame-plating” technique, described by Liao [9], to insure a uniform electrodeposit thickness, deposition rate, and composition. After a thick layer of photoresist (~10 µm) is spun on and patterned to form the plating mask (Fig. 4b), NiFe is electroplated onto the exposed areas of the seed layer using the procedure described by Ahn, Kim, and Allen [2] (Fig. 4c). A second layer of photoresist is spun on and patterned to protect the areas of the electrodeposit inside of the photoresist frame. The unprotected NiFe is removed by a subsequent wet-chemical etch which also removes the revealed areas of the seed layer (Fig. 4d). After all the remaining photoresist is stripped, the portions of the seed previously protected by the photoresist frame are removed by a brief wet-chemical etch. The devices are then released by removing the sacrificial-PSG layer in a concentrated hydrofluoric-acid etch (Fig. 4e). The hydrofluoric acid does not appreciably attack the NiFe electrodeposit. Recently we simplified the fabrication process by implementing a one-mask plating sequence (Fig. 5). In this process, a much thinner layer of photoresist (2 µm thick) is spun on and patterned, and NiFe is electroplated onto the exposed areas of the seed layer (Fig. 5c). The magnetic material can be electroplated to a thickness many times that of the photoresist plating mask, causing the electrodeposit to “mushroom” over its edge (Fig. 6). The spreading of the NiFe does not adversely affect this actuation technology because abrupt vertical edges are not important, unlike in many other microactuator designs [2-3,8]. After the plating-mask photoresist is stripped, the remaining exposed seed layer is removed by either wet-chemical etching or sputter etching (Fig. 5d). The devices are then released using a brief etch in concentrated hydrofluoric acid (Fig. 5e). Although forming the magnetic component of this microactuator with the frameplating process yields operable devices, the added processing complexity associated with the two thick-film photolithographic steps is unnecessary for this actuation technology since a single thin-film photolithographic step can be used instead. In addition, the frameplating process limits the thickness of the electrodeposit to approximately that of the photoresist plating frame. Limiting the thickness of the electrodeposit can limit device performance because the forces generated by this actuation technology are approximately proportional to the thickness of the electrodeposit. The one-mask plating sequence does not have such a strong constraint on the thickness of the electrodeposit.
CONCLUSIONS This research has demonstrated that electroplated magnetic films can be integrated with silicon-based surface micromachining to fabricate magnetically actuated microflexure structures. Both the “frame-plating” technique and also a simpler one-mask plating process have been used to electrodeposit NiFe onto polysilicon flexures coated with Cr-Cu seed layers. Cantilevered and torsional microactuators have been fabricated that achieve displacements greater than 1.2 mm and angular deflections of more than 180° in a magnetic field of ~5 kA/m. This combination of materials and processing techniques makes feasible new actuation possibilities that are applicable to microphotonic elements, such as scanning mirrors and scanning gratings. REFERENCES [1] [2]
[3]
[4]
[5]
[6] [7]
[8] [9]
B. Wagner, W. Benecke, G. Engelmann, and J. Simon, “Microactuators with moving magnets for linear, torsional, or multiaxial motion,” Sensors and Actuators A (Physical), Vol. A32, no. 1-3, pp. 598-603, 1992. C. H. Ahn, Y. J. Kim, and M. G. Allen, “A planar variable reluctance magnetic micromotor with fully integrated stator and wrapped coils,” Proceedings of IEEE Micro Electro Mechanical Systems (MEMS ‘93), IEEE Catalog #93CH3265-6, Fort Lauderdale, FL (February 7-10, 1993), pp. 1-6. H. Guckel, T. R. Christenson, H. J. Skrobis, T. S. Jung, J. Klein, K. V. Hartojo, and I. Widjaja, “A first functional current excited planar rotational magnetic micromotor,” Proceedings of IEEE Micro Electro Mechanical Systems (MEMS ‘93), IEEE Catalog #93CH3265-6, Fort Lauderdale, FL (February 7-10, 1993), pp. 7-11. C. H. Ahn and M. G. Allen, “A fully integrated micromagnetic actuator with a multilevel meander magnetic core,” Tech. Dig. IEEE Solid-State Sensor and Actuator Workshop (Hilton Head ‘92), IEEE Catalog #92TH0403-6, Hilton Head Island, SC (June 22-25, 1992), pp. 14-18. J. W. Judy, R. S. Muller, and H. H. Zappe, “Magnetic microactuation of polysilicon flexure structures,” Tech. Dig. Solid-State Sensor and Actuator Workshop (Hilton Head ‘94), Hilton Head Island, SC (June 13-16, 1994), pp. 43-48 (http://wwwbsac.eecs.berkeley.edu/archive/conference/hh1994/jjudy/). J. W. Judy, “Magnetic microactuators with polysilicon flexures,” Masters Report, Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, 1994 (http://www-bsac.eecs.berkeley.edu/archive/masters/jjudy/). J. W. Judy and R. S. Muller, “Magnetic microactuation of torsional polysilicon structures,” International Conference on Solid-State Sensors and Actuators Digest of Technical Papers (Transducers ‘95), Stockholm, Sweden, (June 25-29, 1995), pp. 332-335. (http://www-bsac.eecs.berkeley.edu/archive/conference/trans1995/jjudy/). W. C. Tang, T.-C.H. Nguyen, M. W. Judy, and R. T. Howe, “Electrostatic-comb drive of lateral polysilicon resonators,” Sensors and Actuators A (Physical), vol. A21, no. 1-3, pp. 328-331, 1990. S. Liao, “Electrodeposition of magnetic materials for thin-film heads,” IEEE Transactions on Magnetics, vol. 26, no. 1, pp. 328-332, 1990.
400 µm
400 µm
Magnetic Plate
Polysilicon Cantilever
Anchor
Hext
Figure 1. Prototype cantilever magnetic microactuator (a) at rest and (b) actuated with Hext ~ 5 kA/m.
Magnetic Plate 400 µm Polysilicon Torsional Beam
Anchor
400 µm
Anchor
Hext
Figure 2. Torsional magnetic microactuator (a) at rest and (b) lifting out of plane with Hext ~ 8 kA/m.
90°
Permanent Magnet Theory
80°
Angular Deflection
70°
Experiment
60° 50°
M < Ms
40°
M = Ms
30°
Theoretical Model
20° 10° 0° 0
5
10
15
20
25
External Magnetic Field (kA/m) Figure 3. Comparison of theoretical calculations for angular deflection versus external field and experimental data.
Electroplating Seed Layer (a)
Silicon
Polysilicon
P-doped SiO2 (PSG)
Photoresist Frame
(b)
Electroplated NiFe
(c)
Protective Photoresist
(d)
Sacrificial-Layer Etch (e)
Figure 4. Integration of the frame-plating process: (a) metallic adhesion and seed-layer deposition; (b) photoresist frame definition; (c) magnetic-material electrodeposition; (d) protective photoresist definition and magnetic-material and seed-layer etch; (e) photoresist removal and sacrificial-layer etch.
Electroplating Seed Layer (a)
Silicon
Polysilicon
P-doped SiO2 (PSG)
Photoresist Plating Mask
(b)
Electroplated NiFe
(c)
Sputter Etch
(d)
Sacrificial-Layer Etch (e)
Figure 5. Integration of the one-mask plating process: (a) metallic adhesion and seedlayer deposition; (b) photoresist plating-mask definition; (c) magnetic-material electrodeposition; (d) photoresist removal and seed-layer etch; (e) sacrificial-layer etch.
Figure 6. Scanning-electron microscope image of the electrodeposit mushroomed over the thin plating mask.
