Virtual Laser-micromachining of MEMS Components

Transcription

1 Virtual Laser-micromachining of MEMS Components M.R.M. Rejab, T.T. Mon, M.F.F. Rashid, N.S.M. Shalahim and M.F. Ismail Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Kuantan, Malaysia / Abstract Finite element models were developed to virtually carry out laser-micromachining of MEMS components. Four different MEMS components micro-bridge, microcantilever array, micro-mirror and micro-come- were considered in the virtual work. Heat flux propagation and temperature plots generated by virtual lasermicromachining were compared and discussed. The important factors to obtain the realistic results from the virtual work were highlighted. The results are generally consistent with the expected laser machining mechanism. This virtual work provides the important information on laser-micromachining parameters and could lead to the possibility of replacing the conventional method with lasermachining in fabricating micron-size components such as MEMS. Index Terms MEMS, finite element modeling, lasermicromachining, virtual technology I. INTRODUCTION The use of laser in micromachining has been a breakthrough technology since various types of laser were commercially available. Laser-micromachining has many technological advantages compared to conventional technologies, including design flexibility, production of complex shape and possibility of rapid prototyping. Particularly, fabrication of MEMS using laser has been gradually taking place of conventional methods because of its unlimited design complexity [1-7]. Several types of laser have been tried out for micromachining in the past study. The excimer laser and femtosecond laser, for instance, were tested in lasermicromachining of silicon for MEMS components and it was successful to a certain extent [8-11]. Numerous parameters are involved in laser processing: pulse energy, pulse duration, beam intensity, repetition rate and moving velocity. These are important process parameters if the laser is used to create a designated geometry. Machining mechanism may depend on factors, particularly the type of laser and material being machined. Major mechanisms are melt transition when cutting metals with CO 2 laser, vaporization especially with machining of silicon and polymers using short pulse laser, and stress cracking when processing glass [4-5, 8]. Considerable amount of research has been reported in the past on lasermicromachining of silicon. In these reports, machining mechanism and the effect of process parameter on surface finish were comprehensively presented. Details can be found in [8-11]. 105 Typical problems that may be faced in lasermicromachining are laser-induced debris, large heataffect zone and laser penetration depth [6-11]. Frequently, high quality components are obtained by chance or at the expense of time and money due to inaccessible machining dimension, improper set of process parameter and high uncertainty in the process itself. To tackle these problems, virtual lasermicromachining with the aid of computational model is greatly desirable. Furthermore, now with the development of advanced virtual technology and CAD/CAM/CAE system many realistic designs, analysis and simulations can be done on the computer prior to actual manufacturing. This will even open a new era in MEMS fabrication such as micro gear, micro-bridge, micro-cantilever array etc. In this paper, laser-micromachining of various MEMS components was virtually carried out in finite element environment. In order to do so, novel computational models were developed using finite element modeling technique as this technique has been matured enough to develop reliable models [12, 13]. The model will help to identify the inherent problems in laser-micromachining and determine appropriate process parameters that would produce the high quality micro-size products. II. MEMS COMPONENTS DESIGN To date, variety of designs for MEMS components has been reported ranging a very simple cantilever to complex assembly of microgears and actuators [14, 15]. Among them micro-bridge, micro-cantilever, micromirror and micro comb are very common. Consequently in this research only these four major component designs are considered. Figure 1 illustrates the size and geometry that may be seen in reality. The following section will present the implementation of FE modeling for these components. Micro bridge Micro cantilever array

2 Micro mirror Micro comb Figure 1. Selected MEMS components [14, 15] III. FINITE ELEMENT MODELING Four FE models were developed representing selected MEMS components shown in Figure 1. The model geometries were created in Solidwork. In order to keep the outline of MEMS components during meshing as this would be used for laser path later in virtual machining, parts were created separately and assembled in such a way that one part had a hole made by outline of the MEMS components and the other was solid that would fill the hole. The size of MEMS components in the first three models were exactly the same as the real ones, yet their computational domain was 1x1x0.5 mm. On the other hand, the whole size of the fourth model was created exactly like the real ones as the detailed information was available. Figure 2 depicts the complete FE model of each MEMS component used for virtual laser micromachining. Despite being 3D models, Figure 2 shows one preferred view of each model where MEMS geometry can be seen clearly. Transient heat transfer analysis was chosen to predict time-resolved temperature distribution due to laser pulse in each model. Due to limited space, related theoretical background was omitted, which can be found in [13-16]. Currently, material for all models was chosen to be acrylic and necessary material properties were taken from built-in material library available in the FE package. Material was assumed to be isotropic. The element type was 8-node brick element. The most important part of the FE modeling was mesh design by which process parameters such as laser type, pulse width and moving velocity along the designated path can be defined. Particularly, the element size or the nodal distance along the laser path controlled the major process parameters, and proper node ordering would define the geometry to be generated by the laser as it would follow these nodes. Consequently, it was essential that certain nodes in the computational domain were ordered in sequence along the designated MEMS geometry as demonstrated in Figure 2. The nodal distance of these ordered nodes was modeled to be the length of 50 um each in the model 1 to 3 and 500 nm in model 4. This length was in accordance with the laser type used and moving velocity. It was assumed that laser source of 10 us pulse duration, 0.1 mj pulse energy and 100Hz repetition rate in the models 1 to 3 while 10 ns, 100 uj and Hz in the model 4 were used. The moving velocity was defined to be same for all as 1 mm/s. Laser power was represented by nodal heat flux and activation time which was interlinked with load curve defined under analysis parameter. Load curve was modeled to have the time interval numerically same as the nodal distance for laser path so that pulse duration, repetition rate and pulse width were lumped into it. The presence of cooling gas was represented by a very high convection coefficient assigned on the surface that would be exposed to the laser beam direct. Initial nodal temperature of all parts was assigned to be at 25 o C assuming the laser processing at room temperature. Model 1 Microbridge Model 2 Microcantilever Model 3 Microcomb Model 4 - Micromirror Figure 2. FE models of selected MEMS components 106

3 IV. VIRTUAL LASER-MICROMACHINING Virtual laser-micromachining was carried out in FE environment. Numerous results can be extracted from this virtual work. Some useful results are temperature plots, heat flux propagation and total machining time. All these results for each model are presented and discussed in this section. In this virtual work, the machining time can be taken from the analysis parameter set. In fact, the analysis time by itself represents the machining time as it should be defined according to the total time to reach the last node of the designated path which in turn depends on the moving velocity from node to node. As such, the machining time for model 1 was 1.5 s, that for model 2 was 2.1 s and so forth. Since the same velocity was set for all models, the longer time means the longer distance for the laser to travel. Figure 3 illustrates the heat flux propagation in models 1 to 3 as it goes along the designated geometries. The propagation trends appear almost same for all indicating the maximum intensity of heat concentrated at the node and the heat propagates at evenly reduced rate to the surrounding material. This phenomenon is a good sign of possibility of fabricating micron-size component by laser. However, the maximum temperature induced was very high and different from model to model. As shown in Figure 4, the maximum temperature was as high as 1200 o C depending on the model geometry being cut. This numerical figure is not a realistic value for the acrylic as it is about ten times higher than the melting point of the acrylic. Since the machining mechanism of acrylic by laser is most probably melt transition, the maximum temperature should be around the melting point [4-6]. The huge error in the temperature value is due to the inaccuracy of convection heat coefficient and improper use of laser type. This implies that by adjusting these two parameters, one can get reasonable results. Figure 4 also demonstrates change of material phase as it interacts with the laser. The material is in melting state for a very short period, indeed in micron second which is the most responsible for cutting. Negative temperature on both sides of the peak indicates sudden-solidification of the neighbor material after melting. Furthermore from the heat flux propagation plot, the size of heat-affected zone can be estimated. Figure 5 and 6 show the heat flux propagation in the surface and through the depth respectively. The size of heat-affected zone is considerably high for the surface geometry as it would damage the principle part whereas it is negligible in depth-wise. As seen in Figure 5, it occupied about 50% of the principle part that is microbridge. This indicates that the laser type chosen is not appropriate. Model 1 Model 2 Model 3 Figure 3. Heat flux propagation in models 1 to 3 Model 1 107

4 Temperature (11) (deg C) Time (s) Model 2 Figure 6. Heat propagation in 3D (Model 1) Model 3 Figure 7 illustrates the heat flux propagation in Model 4. As the laser type used for this model was different from others, its results are presented and discussed in isolation rather than direct comparison with the results of others. Here in laser-micromachining of the required geometry, ultrafast laser was tried out. As a result, laserprocessing time was very short as sub-micron second. The maximum temperature was around 450 o C (Figure 8). Again, this value is not realistic in regard to the melting point of the acrylic. Nevertheless, the size of heataffected zone is acceptable to make such component. Figure 4. Maximum operating temperature in model 1 to 3 Figure 5. Close-up view of heat propagation (Model 1) 108

5 incorporated in FE modeling, laser parameters should be varied in wider range and virtual results should be compared with some experimental results. Figure 7. Heat flux propagation in Model 4 as a whole (top), close up view in surface (middle) and 3D (bottom) Temperature (638) (deg C) Time (s) Figure 8. Maximum operating temperature in Model 4 V. CONCLUSION Virtual laser-micromachining of acrylic material for various MEMS components has been successfully carried out in finite element environment. This virtual work helped to identify the appropriate laser-micromachining parameters to fabricate micron-size components like MEMS in convenient and economical way. It also points out the possibility of replacing conventional method with laser in making micron-size components. The accuracy of the outcome of virtual laser-micromachining principally depends on the accuracy of material properties, appropriateness of material model and the convection heat coefficient. The important information that can be obtained from virtual laser-micromachining are laser type and moving velocity that is suitable for MEMS components and processing time. With reference to the simulated results, generally short pulse lasers and short processing time are preferred for laser-micromachining of MEMS component regardless of the material and component geometry. In future, more realistic material model should be 109 ACKNOWLEDGMENT The authors would like to thank Faculty of Mechanical Engineering, Universiti Malaysia Pahang for funding this research. REFERENCES [1] Ph. Bado, A. A. Said, M. Dugan, Manufacturing of High Quality Integrated Optical Components by Laser Direct- Write, Proceedings of ICALEO 2003, USA. [2] C.-Y. Chien, M.C. Gupta, A. Kruger, P. Feru, B. Craig, Pulse Width Effect in Ultrafast laser Micromachining, Proceedings of ICALEO 2003, USA. [3] Z. Fan, J. Wang, S. Achiche, E. Goodman, R. Rosenberg, Structured Synthesis of MEMS using Evolutionary Approaches, Applied Soft Computing 8, Elsevier, 2008, pp [4] N. Rizvi, Telecom s Cutting Edge, SPIE s OE Magazine, November 2001, pp [5] J. Meijer, Laser machining of materials, technote RA02, Aug 2001, University of Twente, The Netherlands. [6] K. Chen, Y.L.Yao, V. Modi, Gas jet-workpiece interactions in laser machining, Journal of Manufacturing Science and Engineering, ASME 122(2000): [7] K. Chen, Y. L. Yao, V. Modi, Gas dynamic effects on laser cut quality, Journal of Manufacturing Processes 3(1) (2001): [8] J.-P. Desbiens, P. Masson, ArF Eximer Laser Micromachining of Pyrex, SiC and PZT for Rapid Prototyping of MEMS Components, Sensors and Actuators A 136, Elsevier, 2007, pp [9] A.J. Pedraza, J.D. Fowlkes, Y.-F. Guan, Surface Nanostructuring of Silicon, Applied Physics A 77, Material Science and Processing, 2003, pp [10] N. Barsch, K. Korber, A. Ostendorf, K.H. Tonshoff, Ablation and Cutting of Planar Silicon Devices using Femtosecond Laser Pulses, Applied Physics A 77, Material Science and Processing, 2003, pp [11] Andreas Ostendorf, Christian Kulik, Niko Bärsch, Processing thin silicon with ultrashort-pulsed lasers: creating an alternative to conventional sawing techniques, Proceedings of ICALEO 2003, USA. [12] G. Michael, Finite element method: applications in solids, structures, and heat transfer, Boca Raton: Taylor & Francis Group, [13] M. Gad-el-Hak, MEMS: Design and Fabrication, 2nd Ed The MEMS Handbook, CRC Taylor & Francis, [14] A. Liu, J. Wu, C. Lu, C.D. Reddy, MEMS Technology and Devices, Pan Stanford Publishing, [15] Modeling in ALGOR, 2008, ALGOR, Inc. [16] G.F. Naterer, Heat Transfer in Single and Multiphase Systems, CRC Press, USA, 2003.