Development of A Novel Micromachined Magnetostatic Membrane ...
Development of A Novel Micromachined Magnetostatic Membrane Actuator Melvin Khoo and Chang Liu Micro Actuators, Sensors, and Systems (MASS) Group Microelectronics Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 [email protected], Phone: (217) 265-0808, Fax: (217) 244-6375 We present work on the development of a new, micromachined magnetostatic membrane actuator. The unique aspects of this actuator lie in its design and fabrication, and in its operation simplicity. A novel micromachining process was developed for embedding micro-sized pieces of a ferromagnetic material (Permalloy, Ni80 Fe 20 ) into a thin, very flexible, silicone elastomer membrane (PDMS, polydimethyl siloxane). This unique design results in a membrane actuator capable of generating significantly larger membrane displacements than that of typical membrane microactuators. Before we discuss our design, it is important to first review the current technology of membrane actuators, which has found wide application in microfluidic systems. Microfluidic systems[1], in which pumps and valves are key components, is an area of heightened interest in the field of MEMS during recent years. However, membrane actuators used in pumps and valves typically have limited maximum displacements, whether flat-membranes (10−20µm displacements) or even specially-designed membranes with corrugations. This most basic characteristic limits the overall volume flow rate of membrane micropumps. Most membranes utilize silicon[2−5] or siliconbased thin films, which have a high modulus of elasticity and are therefore stiff. Various actuation mechanisms are employed, including thermopneumatic[2], piezoelectric[3], electrostatic[4], and electromagnetic[5]. To enable desirable membrane displacements, these devices invariably require relatively high voltages or high power. We have developed a new membrane actuator to circumvent these limitations. Structurally, the new actuator is composed of a thin PDMS membrane over a silicon cavity, with rectangular pieces of Permalloy embedded within the membrane(Figure 1). Membrane displacement is affected by the interaction between an externally applied magnetic field with the embedded Permalloy pieces(Figure 2). The applied field induces magnetic torque in the Permalloy pieces, thereby causing them to deflect and displace the membrane. Magnetic actuation is favorable as it generates large and long-range forces. In comparison, our membrane is 4-times smaller than the one reported in [5] and achieves 10-times greater displacements. This significantly larger displacement is also affected by the use of PDMS, which has a modulus of elasticity 5 orders of magnitude lower than silicon. Simplicity of operation is achieved by the designed use of an external magnetic field. Fabrication and packaging complexities are reduced. Telemetric operation is possible without any wires for power input, and no precise field-actuator alignment is required. In addition, PDMS is physically stable and chemically inert, making it bio-compatible for biomedical applications. These unique properties of the microactuator is crucial for its designed application, a tetherless micropump for implantable biomedical microfluidic devices. A unique micromachining process for embedding Permalloy pieces within the thin PDMS membrane is developed for the first time. In designing the actuator, we identified and optimized key design parameters using finite element analysis. These parameters include (1) sizes of Permalloy pieces, (2) dimensions of the membrane, and (3) Permalloy pieces placement within the membrane. Computer simulations with ANSYS yielded the optimized design layout shown in Figure 3, where the Permalloy pieces are strategically positioned asymmetrically in the 2×2mm2 , 40-µm-thick PDMS membrane. Membrane tests were conducted to characterize membrane displacements at various applied magnetic fields (Figure 5). The maximum membrane displacement was 211.5-µm under an applied external magnetic field of 0.23Tesla. This displacement can be easily varied by the applied field. ANSYS simulations produced a good match, less than 6% error, between experimental results and theoretical expectations. REFERENCES [1] M. Elwenspoek et. al., Towards integrated microliquid handling systems, Journal of Micromech. & Microeng., v. 4, no. 4, Dec. 1994, pp. 227−245. [2] O. C. Jeong et. al., Fabrication of a thermopneumatic micropump with a corrugated p+ diaphragm, Transducers ’99: Int. Conf. Solid-State Sensors and Actuators, vol.2, Sendai, Japan, June 7−10, 1999, pp.1780−1783. [3] M. Koch et. al., The dynamic micropump driven with a screen printed PZT actuator, J. of Micromech. & Microeng., v. 8, no. 2, June 1998, pp. 119−122. [4] R. Zerlenge et. al., A micro membrane pump with electrostatic actuation, IEEE Micro Electro Mechanical Systems, An Investigation of Micro Structures, Sensors, Actuators, Machines and Robots, Germany, Feb. 4−7, 1992, pp. 19−24. [5] W. Zhang et. al., A bi-directional magnetic micropump on a silicon wafer, 1996 Solid-State Sensor and Actuator Workshop, June 3−6, 1996, pp.94−97.
Figure 1. Schematic diagram of the micromachined magnetostatic membrane actuator (cut across its symmetry plane to illustrate the cross-section).
Figure 2. Actuation principle of the magnetostatic membrane actuator.
Figure 4. Fabrication process for the magnetostatic actuator. Permalloy pieces are embedded by first over-electroplating over the PR mold (d), and then spin-coating a thin layer of PDMS over and around the Permalloy pieces (e). The resultant laterally overplated extension acts as an anchor to hold the Permalloy pieces within the membrane when actuated.
Membrane Deflection at Various Magnetic Fields
0.23 Tesla 0.19 Tesla 0.14 Tesla 0.10 Tesla
Deflection, δ (µm)
250 200 150 100 50 0 0
0.5
1
1.5
2
Position, x (mm) Figure 3. Layout (top view) of Permalloy flaps on a 2mm square membrane shown with pertinent parameters. The A’−A’ line defines the cross-section Figure 5. Plot shows average membrane at which membrane displacements will be measured displacements across the 2-mm window along the A’−A’ cross-section in Figure 3, tested at applied and plotted. magnetic fields of 0.23, 0.19, 0.14, and 0.10 Tesla. The points represent the displacement measured at each corner of each Permalloy flap.
Figure 1. Schematic diagram of the micromachined magnetostatic membrane actuator (cut across its symmetry plane to illustrate the cross-section).
Figure 2. Actuation principle of the magnetostatic membrane actuator.
Figure 4. Fabrication process for the magnetostatic actuator. Permalloy pieces are embedded by first over-electroplating over the PR mold (d), and then spin-coating a thin layer of PDMS over and around the Permalloy pieces (e). The resultant laterally overplated extension acts as an anchor to hold the Permalloy pieces within the membrane when actuated.
Membrane Deflection at Various Magnetic Fields
0.23 Tesla 0.19 Tesla 0.14 Tesla 0.10 Tesla
Deflection, δ (µm)
250 200 150 100 50 0 0
0.5
1
1.5
2
Position, x (mm) Figure 3. Layout (top view) of Permalloy flaps on a 2mm square membrane shown with pertinent parameters. The A’−A’ line defines the cross-section Figure 5. Plot shows average membrane at which membrane displacements will be measured displacements across the 2-mm window along the A’−A’ cross-section in Figure 3, tested at applied and plotted. magnetic fields of 0.23, 0.19, 0.14, and 0.10 Tesla. The points represent the displacement measured at each corner of each Permalloy flap.
