Design of a microactuator array against the coupled nature of microelectromechanical systems (MEMS) processes

Design of a microactuator array against the coupled nature of microelectromechanical systems (MEMS) processes Annals of CIRP, vol.49/1, 2000 Abstract S. G. Kim (2) and M. K. Koo Advanced Display and MEMS Research Center Daewoo Electronics, Co., Ltd. 60-8, Kasan-Dong, Kumchun-Gu, Seoul, 153-023, KOREA Piezoelectric micromirror array is developed based on microelectromechanical systems (MEMS) technology. The inherent coupled nature of the thin-film processes generates many problems unless the design of the microactuator is properly uncoupled or decoupled among the functional domain, the physical domain and the process domain. The design of the first generation microactuator array was highly coupled between the functional domain and the physical domain, and could not be implemented successfully over 24 months of fabrication effort. Once the design has been uncoupled, successful arrays could be fabricated within 6 months and could be upgraded easily. The brightest projection display system has been developed with the microactuator arrays. Keywords : micro-actuator, micro-machining, axiomatic design

1 INTRODUCTION Microelectromechanical systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through the utilization of microfabrication technology. [1] While the microelectronic components are fabricated using integrated circuit process steps, the micromechanical components are fabricated using compatible micro-machining processes that selectively etch away parts of the silicon wafer and/or add new structural layers to form the mechanical, optical, chemical or electrical devices. MEMS is a new manufacturing technology to make complex electromechanical systems using batch fabrication processes at a relatively low cost while ensuring the uniformity of a number of repetitive structures. MEMS technology has the big potential to revolutionize the production of microsensors, actuators and their systems, thereby, allowing the development of integrated smart systems. However, the introduction of MEMS into major industry depends on the speed and cost with which MEMS product can be designed and fabricated. As MEMS technology expands its application into more and newer engineering systems, the development tim e and cost should be minimized by the systematic design approach out of the realm of an art at the present time. Design is a series of zig-zagging decomposition process among four domains of design (e.g. consumer domain, functional domain, physical domain, and process domain. There exist two design axioms, which the mapping process among those design domains must satisfy. [2] If a MEMS design violates the axioms, the design would require very costly iterations of fabrication effort or, sometimes, cannot reach the conclusion. The Thin-Film Micromirror Array (TMA) is a new kind of reflectivetype spatial light modulator developed by the authors. TMA uses micromachined thin-film piezoelectric actuators to control the gray scale of the display. It is found that the development time and effort of the new MEMS device are strongly dependent on the quality of design. This finding complies with the design axioms and its theorems, which have been proved in the designs of software, manufacturing systems, and large-scale systems. [2,3,4] 2 COUPLED NATURES OF MEMS PROCESSES In general, surface micromachining processes add thin films on a silicon wafer and selectively etch away parts of the films to pattern 3-dimensional microstructures. However, depending on the sequence of deposition or etching processes, a MEMS design would become coupled or decoupled. Many designers do not know that they have a coupled or decoupled design and randomly change process variables to fabricate MEMS devices. Consequently, the designs become dysfunctional. [2] The spatial distribution of the thickness of a thin-film layer deposited is affected by the undulation of the underlying layer. As a thin film crosses a step of the underlying substrate, it suffers unwanted thinning or cracking. A measure of how well a thin film maintains its nominal thickness across the step is expressed by the step coverage, which is the ratio of the minimum thickness of a film on the step to the nominal thickness of a film on the flat region. Figure 1 shows the scanning electron microscope pictures of the typical poor step coverage cases. The cracked or thinned film may cause the current leakage or electrical short which deteriorates the functioning of microactuators. This is one of the forward coupling natures of the thin-film deposition processes. The thickness of a thin film layer can be affected by the overetching of the upper layer unless the etch selectivity with respect to the underlying layer is not high enough. The microstructure of the underlying thin film layer may also be affected by the thermal, physical and chemical process conditions of the subsequent processes. For example, the polarization characteristics of a thin piezoelectric layer are affected by the temperatures of the subsequent processes which exceed 150 C. These are the typical backward coupling natures of the MEMS processes. stress (a) cracking (b) thinning & high Figure 1; SEM photos of poor step coverage The inherent coupling natures of the MEMS processes would generate many problems in implementing the design unless a MEMS device is properly designed by identifying the forward and

backward couplings of the micromachining processes. The forward and backward coupling natures of the MEMS processes can be represented by a design equation between the physical domain and the process domain as follows. Consider there are three thin film layers to form a MEMS device; layer 1 at the bottom, layer 3 at the top. Assume the selectivities of the etching processes are high enough. The design parameters (DPs) and process variables (PVs) can be chosen to form a design equation (1). The upper left triangular 3 by 3 matrix in equation (1) represents the forward coupling while the lower right reverse triangular 3 by 3 matrix represents the backward coupling of the MEMS process. Since two triangular matrices have opposite direction, the total design matrix is copuled. It can become decoupled only when the sequence of the DP s rearranged properly. The decoupled design equation (2) implies that the right decision sequence of the design is DP6, DP5, DP4, DP1, DP2, and DP3. The change of process variables should be executed accordingly. Otherwise, the design would become coupled. DP1 layer1 thickness DP2 layer2 thickness DP3 layer3 thickness DP4 layer1 microstructure DP5 layer2 microstructure DP6 layer3 microstructure PV1 layer1 patterning PV2 layer2 patterning PV3 layer3 patterning PV4 layer1 add process PV5 layer2 add process PV6 layer3 add process dark room conditions for the acceptable image quality due to their lack of brightness. The Thin-Film Micromirror Array (TMA) [5,6,7] is a new kind of reflective-type spatial light modulator based on piezoelectric microactuator array device. TMA uses micromachined thin-film piezoelectric actuators to control the angular position of each micromirror. The continuous change of the tilting angle of each micromirror controls the gray scale of each display pixel with simple image processing circuitry. Each TMA pixel consists of a mirror and an actuator. High reflectivity and excellent flatness are the functional requirements for the micromirror while the linear and the fast response characteristics and the mechanical and electrical reliability are those for the actuator. The TMA uses thin-film piezoelectric actuators in the form of microcantilevers. A cantilever consists of the supporting layer, bottom electrode, piezoelectric layer, and top electrode. When an electric field is applied between the two electrodes, the piezoelectric layer shrinks to the horizontal direction. Since the neutral plane of the cantilever shifts toward the bottom electrode due to the thickness of the supporting layer, the mechanical strain of the piezoelectric layer causes vertical deflection of the cantilever and the upward tilting of the mirror on top of the actuator, consequently. DP1 DP2 DP3 DP4 DP5 DP6 X O O X O O X X O O X O X X X O O X O O O X X X O O O O X X O O O O O X PV1 PV2 PV3 PV4 PV5 PV6 (1) DP3 DP2 DP1 DP4 DP5 DP6 X X X O O X O X X O X O O O X X O O O O O X X X O O O O X X O O O O O X PV3 PV2 PV1 PV4 PV5 PV6 (2) a)1 st generation design b) 2 nd generation design ( c) 3 rd generation TMA 3 THINFILM MICROMIRROR ARRAY A large enough and clear information display is necessary to have a large audience simultaneously view the same content. At the present time, however, most of the currently available projectors require near Figure 2; Generations of TMA pixel design The design of the TMA pixel has evolved over the years as shown in Figure 2. In the first and second generation TMA designs, the actuator and the mirror

were coplanar, fabricated at the same level over single sacrificial layer. The platinum top layer functions as the mirror as well as the top electrode of the cantilever actuator. When the actuator is driven by the voltage signal, the actuator part deforms with a curvature. Since the mirror and the actuator are coplanar, the curved actuator surface unwantedly scatters the light to deteriorate the image contrast ratio significantly. Reversely, the change of the top surface thickness for enhanced reflectivity affects the modulus of the cantilever and changes the tilting behavior of the cantilever, consequently. Therefore, the top level design equation (3) of the coplanar design is not only physically coupled but als o functionally coupled. In order to improve, drastically, the optical efficiency of TMA and to shorten the time to fabricate, the TMA design needs to be uncoupled. In this respect, the third generation design of TMA was developed to have the mirror on top of the actuator. Fig. 3 shows the cross section of the hidden actuator design of the third generation TMA pixel. The mirror fully covers the actuator with an air gap, and then the top level design equation (5) became uncoupled. FR1 FR2 X O O X DP1 DP2 (5) FR1 light reflection FR2 mirror tilting DP1 cantilever top surface DP2 cantilever sandwich FR1 FR2 The inherent couplings between the physical and process domains further complicate the coupled design during the fabrication. The fabrication of the first generation TMA design had required numerous changes of design parameters and process variables until the designers fully understood the inherent coupling natures of the surface micromachining processes as described in section 2. It took authors to fabricate the first-generation TMA more than 24 months. The second-generation TMA pixel was designed in order to decrease the coupling and to increase the fill factor, which is the ratio of the area for light modulation to the whole pixel area. It has the large mirror part between the twin slim actuators. The mirror is free from bending when the cantilever actuators deform with the same curvature. The functional requirement of the light reflection and the functional requirement of the mirror actuation became lightly coupled (or decoupled) with the second generation design. Even though the optical properties and the mechanical uniformity were improved with the second-generation design, they were far short of the specifications. It took authors 9 months to fabricate the design. FR1 FR2 X X X X X O X X DP1 DP2 DP1 DP2 (3) The fill factor of this design can be as high as 94%. In addition, the hidden actuator design can produce much flatter mirrors and more uniform cantilever actuators by controlling design parameters independently. And, before everything, the successful fabrication of this design was completed within 6 months. Now, the design improve-ments of TMA can be made easily every 4 months. mirror 1st sacrifici al layer 2nd sacrificial layer Cantilever actuator PZT sol coating Plugged Anchor Figure 3; Cross-sectional view of the hidden actuator TMA 4 MICROMACHINING OF TMA The TMA module is monolithically fabricated over a PMOS active matrix by surface micromachining techniques. The size of each mirror is 97 µm x 97µm for the VGA format prototype and 50 µm x 50µm for XGA format TMA modules. The fabrication of the TMA module begins with the completed PMOS active matrix employing a tungsten metallization process in order to stand the high

temperature post processes. The active matrix is a transistor array that addresses the video signal to each pixel. After a circuit protection layer is added on top of the active matrix, a poly-si sacrificial layer is deposited by a low pressure chemical vapor deposition (LPCVD) process. A chemical mechanical polishing (CMP) process is used to provide a flat surface for the subsequent TMA fabrication processes. The degree of planarization does not seriously affects the mirror s flatness since there is another sacrificial layer under the final mirror layer, which uncouples the design. The planarized sacrificial layer is removed later to produce an air gap for the vertical displacement of cantilevers. Subsequent layers comprising the cantilever are deposited on top of the first sacrificial layer; the supporting layer, the bottom electrode, the PZT (PbZrTi) as a piezoelectric layer and then the top electrode. The supporting layer is made of a siliconrich silicon nitride (SiNx) and deposited by LPCVD process. The function of this layer is to convert the expansion or contraction deformation of the piezoelectric layer to the vertical displacement of the cantilever. In addition, it has a role of making the long cantilever flat by controlling its residual stress. The top and bottom electrodes are deposited with Pt. The piezoelectric layer is formed by the sol-gel method. The sol material is first spin-coated onto the bottom electrode, and followed by the heat treatments. A rapid thermal annealing (RTA) process is used to make thin-film PZT crystallized into the Perovskite structure. After the deposition of the top electrode, etching steps of shaping the cantilever are performed in the reverse order. The top electrode, the PZT and the bottom electrode are etched using dry etch processes. The second sacrificial layer is deposited in order to build a flat mirror on top of the actuator structure. A fluidic polymeric substance is spin-coated and hardened to form the second sacrificial layer. The hole that is patterned in the second sacrificial layer form the metal support post which connects the mirror to the cantilever end. The mirror layer is made of aluminum and is sputterdeposited on the second sacrificial layer. The aluminum layer is patterned to make the mirror shape by a dry etching process. Finally, the hybrid sacrificial layers are removed to form the double air gaps. Figure 4 presents the SEM photographs of the completed TMA module after the release process. A real array of mirrors is shown in Fig. 4(a). Note that some mirrors are intentionally removed to show the underlying structures in Fig. 4(b). b) Figure 4; SEM pictures of TMA for VGA resolution 5 CONCLUSIONS Piezoelectric micromirror array is developed based on microelectromechanical systems (MEMS) technology. The inherent coupled nature of the thin-film processes has generated many problems when a MEMS design was coupled among the functional domain, the physical domain and the process domain. The hidden actuator design is an uncoupled design of TMA. The fabrication of the design was completed much faster than that of the coupled design. It provides the highest fill factor and flat enough micromirrors resulting 22% system optical efficiency, which is the highest among all the reflective and transmissive light modulators in the world at the present time. A working projector prototype of 5,400 true ANSI lumen is realized with three TMA modules and a 1 kw Xenon lamp. (Fig. 5) TMA projector prototype is brighter than any projectors at the same lam p power. a)

Figure 5; A photograph of XGA format TMA projection image (200 inch screen) 6 REFERENCES [1] MEMS Exchange, What is MEMS Technology, www.mems_exchange.org [2] N.P. Suh, The Principles of Design, Oxford University Press, 1990 [3] N.P. Suh, S. Sekimoto, Design of Thinking Design Machine, Annals of the CIRP, Vol. 39, No.1, 1990 [4] S.J. Kim, N.P. Suh, S.G. Kim, Design of Software System based on Axiomatic Design, Annals of CIRP, Vol. 40, No. 1, 1991 [5] S. G. Kim, K. H. Hwang, Y. J. Choi, Y. K. Min, and J. M. Bae, "Micromachined Thin-Film Mirror Array for Reflective Light Modulation," Annals of the CIRP, Vol. 46, No. 1, pp.455-458, 1997. [6] Sang-Gook Kim, Kyu-Ho Hwang, Jin Hwang, Myung-Kwon Koo, and Keun-Woo Lee, "Actuated Mirror Array - A New Chip-based Display Device for the Large Screen Display", SID International Display Research Conference, Seoul, September 1998, pp.329-998. [7] Sang-Gook Kim and Kyu-Ho Hwang, "Recent progresses of Thin-film Micromirror Array (TMA)", Proc. of international Display Workshop, Sendai, Japan, December 1999. (Invited Paper)