Luận văn Simulation And Fabrication Of Piezoelectric Mems Inkjet Print Head

MASTER OF SCIENCE SUPERVISOR LEE. JAICHAN SIMULATION AND FABRICATION OF PIEZOELECTRIC MEMS INKJET PRINT HEAD A Thesis Presented by PHAM VAN SO Department of Materials Science and Engineering Graduate School of SungKynKwan University MASTER OF SCIENCE SUPERVISOR LEE. JAICHAN SIMULATION AND FABRICATION OF PIEZOELECTRIC MEMS INKJET PRINT HEAD A Thesis Presented by PHAM VAN SO Submitted to the Graduate School of SungKynKwan University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Materials Science and Engineering June 2007 Department of Materials Science and Engineering Graduate School of SungKynKwan University i SIMULATION AND FABRICATION OF PIEZOELECTRIC MEMS INKJET PRINT HEAD by PHAM VAN SO ABSTRACT Microelectromechanical systems (MEMS) have played an increasingly important role in sensor and actuator applications. And its key contribution is that it has enabled the integration of multi-components (i.e., electronics, mechanics, fluidics and etc) on a single chip and their integration has positive effects upon performance, reliability and cost. Compared to conventional electrostatic, thermal or magnetic actuating schemes, piezoelectric MEMS inkjet has the advantages of lower power consumption, lower voltage operation and relatively larger driving force. Based on the primary design and fabrication of piezoelectric MEMS inkjet (1st version-InkjetVer1) done in our STD Lab, the computer simulation and validation of inkjet have been investigated, and then the 2nd version (InkjetVer2) with the modified nozzle shape was fabricated and characterized. In details, firstly the simulation of piezoelectric MEMS inkjet with the electro- mechanical-fluid interaction has been performed. In order to verify the simulation results, a fabrication and characterization of actuator part consisting of PZT-based actuating membrane and ink chamber was carried out. These treatments are to determine how much “dynamic force”, in terms of membrane’s maximum displacement, maximum force and driving frequency, can be produced by the actuator membrane. Secondly, a simulation of microdroplet generation in inkjet has also been done. This work gives an understanding about the droplet generation process, and the effects of driving characteristics, fluid properties and geometrical parameters on droplet generation. Especially, this simulation helps to predict how much “dynamic force” is required to generate mirodroplets. The combination of both results (i.e., how much “dynamic force” produced and required) gives an effective guideline in designing inkjet structure. Thirdly, in the experimental work, the fabrication of InkjetVer2 was carried out based on MEMS techniques. And then its electrical, mechanical characteristics as well as possibility of ink ejection were also tested. Finally, the feedback information from these simulation and experimental work helps to suggest a new design (3rd version - InkjetVer3) which is expected to produce enough “dynamic force” and possibly generate microdroplets. Then, mask design and fabrication of InkjetVer3 have also been proceeding. ii ACKNOWLEDGMENTS First, I would like to thank my supervisors, Prof. Dr. Jaichan Lee and Assoc.Prof.Dr. Dang Mau Chien for their professional guidance, constructive criticism and, last but not least, for giving me a good opportunity to study at the Semiconductor and thin film devices Lab, Department of Materials Science and Engineering, SungKyunKwan University. I would also like to thank PhD candidate Sanghun Shin and MSc. Jangkwen Lee for sharing their knowledge on MEMS processing with me as well as for their useful discussions. Furthermore, I would like to thank Prof. Minchan Kim and Dr. Dongwon Lee in Jeju National University for their generous assistance on my simulation work. And I’m so grateful to KIST, KITECH and other labs for sharing all the equipments available for my experimental work. I would like to thank all STD lab’s members: Dr. Leejun Kim, Dr. Teakjib Choi, Dr. Juho Kim, MSc. Cho Ju Hyun, MSc. Chul Ho Jung; PhD candidates Phan Bach Thang, Do Duc Cuong, Ong Phuong Vu and Eui Young Choi; Master candidates Hyun Kyu Ahn, Jihyun Park, Sukjin Jong and Byun Jun Kang; and lab’s secretaries for their invaluable help during my MSc course. And my thanks send to my friends in SKKU, N.T.N. Thuy, N.T. Tien, N.T. Xuyen and N.D.T. Anh, for their helpful discussion and argument about my results. Finally, I want to thank my parents and relatives for their constant encouragement and support. iii DEDICATION To my parents Mr. Pham Van Vinh and Mrs. Le Thi Anh iv Table of contents ABSTRACT ......................................................................................................................... i ACKNOWLEDGMENTS ...................................................................................................... ii Table of contents.............................................................................................................. iv List of figures .................................................................................................................. vi List of tables .................................................................................................................. viii CHAPTER 1. INTRODUCTION ............................................................................................ 1 1.1 Piezoelectricity ...................................................................................................... 2 1.1.1 Piezoelectric effect ........................................................................................ 2 1.1.2 Lead zirconate titanate (PZT) ......................................................................... 3 1.2 Piezoelectric MEMS inkjet print head................................................................... 5 1.3 Numerical simulation ............................................................................................ 7 1.3.1 Role of numerical simulation ...................................................................... 7 1.3.2 General principle of numerical simulation ..................................................... 8 1.3.3 Numerical simulations of piezoelectric MEMS inkjet with CFD-ACE+ ........ 9 1.4 References ........................................................................................................... 10 CHAPTER 2. NUMERICAL AND EXPERIMENTAL STUDY ON ACTUATOR PERFORMANCE OF PIEZOELECTRIC MEMS INKJET PRINT HEAD ........................................................11 2.1 Introduction ......................................................................................................... 12 2.2 Modeling and simulation settings........................................................................ 13 2.3 Experimental procedure....................................................................................... 16 2.4 Results and discussion ......................................................................................... 17 2.4.1 Performance characteristics of PIPH actuator in air................................... 17 2.4.2 Performance characteristics of PIPH actuator in liquid .............................. 18 2.5 Conclusion........................................................................................................... 20 2.6 References ........................................................................................................... 21 CHAPTER 3. SIMULATION OF MICRODROP GENERATION IN PIEZOELETRIC MEMS INKJET PRINT HEAD ...................................................................................................... 26 3.1 Introduction ......................................................................................................... 27 3.2 Modeling and simulation settings........................................................................ 27 v 3.3 Results and discussion ......................................................................................... 29 3.3.1 Microdrop generation process....................................................................... 29 3.3.2 Effect of actuating characteristics ................................................................. 29 3.3.3 Effect of fluid properties ............................................................................... 30 3.3.4 Effect of geometrical parameters .................................................................. 32 3.4. Conclusion.......................................................................................................... 32 3.5 References ........................................................................................................... 34 CHAPTER 4. FABRICATION AND CHARACTERIZATION OF PIEZOELECTRIC MEMS INKJET PRINT HEAD ...................................................................................................... 38 4.1 Introduction ......................................................................................................... 39 4.2 Experiments ......................................................................................................... 39 4.3 Results and discussion ......................................................................................... 41 4.4 Conclusion........................................................................................................... 42 4.5 Rerefences ........................................................................................................... 44 CHAPTER 5. CONCLUSION AND SUGGESTION ............................................................... 50 5.1 Conclusion........................................................................................................... 50 5.2 Suggestion (new design)...................................................................................... 50 Appendix A. Python Source Script for simulation of microdroplet generation (effects of driving characteristics and fluid properties) ................................................................... 52 Appendix B. Pattern conditions for fabrication of Inkjetver2 ....................................... 54 Appendix C. Dry etching conditions ............................................................................. 55 vi List of figures Fig.1-1. Direct piezoelectric effect in open circuit (a) and in shorted circuit (b). ............ 2 Fig. 1-2. Converse piezoelectric effect: (a) free displacement and blocking force and (b) static and dynamic operation............................................................................. 3 Fig. 1-3. Structure of PZT unit cell: (a) Cubic (T≥Tc) an (b) tetragonal (T< Tc). ............ 4 Fig. 1-4. Phase diagram for the PbZrO3-PbTiO3 system. C: Cubic, T: Tetragonal, RI: Rhombohedral (high temp form), RII: Rhombohedral (low temp form), A: rthorhombic, M: MPB, and Tc: Curie temperature. ........................................... 4 Fig. 1-5. Deformation mode of piezoelectric inkjet actuator: (a) squeeze, (b) bend, (c) push and (d) shear mode.................................................................................... 6 Fig. 1-6. A typical approach to MEMS application from concept to devices................... 7 Fig. 1-7. Steps of overall solution procedure.................................................................... 8 Fig. 1-8. Modeling settings for design of piezoelectric MEMS inkjet. Computations are performed using CFD-ACE+ package software. .............................................. 9 Fig. 2-1. Model of a piezoelectric inkjet print head (PIPH) structure: (a) design and (b) CFD-ACE+ symmetric model with meshing grids. ................ 23 Fig. 2-2. Flowchart of fabrication process (a) and SEM images (b) of PIPH actuator... 23 Fig. 2-3. Maximum displacement of PIPH actuator membrane (300 um): (a) simulation and (b) experiment. Simulation was extended with membrane width of 500- 600 um............................................................................................................. 24 Fig. 2-4. Dependence of actuator performance on geometrical parameters: (a) maximum displacement vs. thickness ratio (PZT/support layer) and (b) maximum force (Fmax) and maximum displacement (δmax) vs. membrane width...................... 24 Fig. 2-5. Resonance frequency (in air) of PIPH actuator membrane: (a) FEMLAB simulation and (b) experiment with HP4194A impedance analyzer. .............. 24 Fig. 2-6. Deflection shape of actuator membrane interacting with liquid: (a) & (b) dome shape with one peak at low frequencies and (c) & (d) unexpected shape with more than one peak at higher frequencies (above 125 kHz < 379 kHz - resonance frequency in air ). ........................................................................... 25 Fig. 2-7. Resonance frequency (in liquid) of PIPH actuator membrane: (a) simulation and (b) experiment. ......................................................................................... 25 vii Fig. 3-1. Inkjet head geometry, (a) Three dimensional (3D) and (b) 2D symmetric section in CFD-ACE+. .................................................................................... 35 Fig. 3-2. Microdrop generation process at driving displacement with amplitude of 5 μm and frequency of 30 kHz. ................................................................................ 35 Fig. 3-3. Droplet properties: no-droplet, single droplet and satellite droplets at various driving displacements (2~5um, 50 kHz). ........................................................ 36 Fig. 3-4. Time duration for droplet generation at various actuating characteristics: (a) amplitude and (b) frequency. Droplets are generated in one cycle or several cycles. .............................................................................................................. 36 Fig. 3-5. Time duration for droplet generation with fluid properties: (a) surface tension and (b) viscosity. High surface tension or viscosity makes cohesive forces predominant..................................................................................................... 36 Fig. 3-6. Geometrical parameters: (a) relative chamber X1/X2, (b) aspect ratio d/h and (c) diffuser. ...................................................................................................... 37 Fig. 3-7. Time duration for droplet generation vs.: (a) relative chamber size (A-type) and (b) aspect ratio (B-type & C-type). ................................................................. 37 Fig. 3-8. Time duration for droplet generation vs. driving characteristics of the selected structure (B-type). Microdroplet can be generated at an applied voltage of 9V- 21V and frequency above 15 kHz. .................................................................. 37 Fig. 4-1. Schematic of piezoelectric inkjet print head structure (side view): (a) Inkjet version 1 and (b) Inkjet version 2 with the modified nozzle shape at locations marked 1 &2.................................................................................................... 45 Fig. 4-2. Masks used for fabrication of PIPH : M1-M6 (wafer 1) and M7- M10 (wafer2). ......................................................................................................................... 45 Fig.4-3. Fabrication process flow of PIPH: (a) wafer 1-actuator and chamber and (b) wafer 2-channel and nozzle. Both wafers are bonded by Eutectic bonding method. ............................................................................................................ 46 Fig. 4-4. SEM and optical micrographs of the fabricated PIPH structure...................... 47 Fig. 4-5. Preparing for ejection test: (a) 4-inkjet heads on 1 cell and (b) PCB-wire bonding and tube attachment........................................................................... 48 Fig. 4-6. Ejection testing by high speed digital camera system. .................................... 49 Fig. 4-7. Meniscus vibration under an applied voltage of 10V-40 kHz. ........................ 49 Fig. 5-1. Model of InkjetVer3 (3-silicon wafers). .......................................................... 51 Fig. 5-2. Masks used for fabrication of InkjetVer3. ....................................................... 51 viii List of tables Table 2-1. Fluid properties............................................................................................. 22 Table 2-2. Support layer properties ............................................................................... 22 Table 2-3. PZT properties (PZT 52/48 ) ......................................................................... 22 Table 2-4. The displacement at various driving frequencies (voltage=5V) ................... 22 Table 2-5. Summary of actuator performance characteristics....................................... 22 1 CHAPTER 1. INTRODUCTION INKJET printing is familiar as a method for printing computer data onto paper or transparencies as well as industrially printing information on cans or bottles. Recently it has been used as free-form fabrication method for building three dimensional parts (maskless fabrication) and is also being used to produce arrays of proteins and nucleic acids. The objective of this thesis is to investigate the piezoelectric MEMS inkjet print head from design to fabrication. Therefore, this chapter will briefly summarize the background of piezoelectricity, types of piezoelectric MEMS inkjet head and general principle of numerical simulation. 2 1.1 Piezoelectricity 1.1.1 Piezoelectric effect All polar crystals show piezoelectricity, since any mechanical stress T will result in strain because of the elastic properties of the materials. And the strain will affect the polarization since the polarization is caused by a displacement of the charge centers of the anions and cations. For small changes of the stress T, the relation P=d.T is called the direct piezoelectric effect, where d denotes the piezoelectric coefficient. Once a force (mechanical stress) is applied to a piezoelectric material, surface charge is induced by the dielectric displacement and therefore an electric field is built up. On applied electrodes this field can be tapped as electrical voltage (Fig. 1-1. (a)). If the electrodes are shorted, the surface charge balances out by a current ((Fig. 1-1. (b)). The direct piezoelectric effect is employed for mechanical sensors. Fig.1-1. Direct piezoelectric effect in open circuit (a) and in shorted circuit (b). Because of the piezoelectric property of polar materials, a converse effect is observed. If an external electrical field, E is applied, a strain S=d.E is observed. If this strain is prevented (blocking totally or partially the material), an elastic tension T occurs. A force F is thereby applied to the device, which prevent to the distortion of the piezoelectric body (Fig. 1-2.(a)). In practice, the converse piezoelectric effect is used in static as well as dynamic operation (Fig. 1-2. (b)) and it is used for 3 mechanical actuators. The first experimental work on piezoelectricity was performed by Pierre and Jacques Curie in 1880. Fig. 1-2. Converse piezoelectric effect: (a) free displacement and blocking force and (b) static and dynamic operation. The piezoelectric effect is exhibited by a number of naturally and synthetically single crystals under two different behaviors. Materials such as quartz exhibited a zero polarization w