Verification of Total Band Gap in 2D Si-based Phononic Crystal

In recent years, there are studies aimed at phononic crystals, both experimentally and theoretically. However, the smallest scale of the phononic structures considered is in the millimeter scale and the frequency is limited in the MHz range. For the purpose toward the applications of phononic crystals to micro electromechanical system (MEMS) related components, it is necessary to reduce the lattice size to micrometer or even in nanometer scale. Moreover, for further integrating with the complementary metal-oxide semiconductor (CMOS) processing techniques, silicon is chosen to be the base material of the two dimensional phononic crystals in this thesis. From the simulated results by plane wave expansion (PWE) method, the total band gap of 2-D Air/Si phononic crystal appears at high filling fraction. This result inspires us to employ not only partial band gap but also total band gap in MEMS related components. The purpose of this thesis is to verify the phenomenon of total band gap of 2-D Air/Si phononic crystal by MEMS process. Then comparing the insertion loss of surface wave (SAW) passing through phononic crystal structures within/without the total band gap frequency. In order to generate SAW in the silicon substrate, piezoelectric thin film is sputtered on top of the wafer. Moreover, the 2-D phononic crystals is fabricated by ICP etching (inductively coupled plasma-reactive ion etching) process. Finally, the total band gap of 2-D air/silicon phononic crystals in micrometer-scale are successfully verified by the layered IDT devices, and this experimental result agrees with the theoretical prediction by PWE method.Acknowledgements I Abstract…. II List of Notations III Table of Contents V Table of Figures VII List of Tables IX Chapter 1 Introduction 1 1-1 MOTIVATION 1 1-2 LITERATURE REVIEW 2 1-3 CONTENTS OF THE CHAPTERS 3 Chapter 2 Analysis of Acoustic Waves in 2-D Phononic Crystal 6 2-1 CONCEPT OF 2-D PHONONIC CRYSTALS 6 2-2 EQUATION OF MOTION 7 2-3 MASS DENSITY AND ELASTIC CONSTANTS 7 2-4 DISPLACEMENT VECTOR 9 2-5 SURFACE AND BULK WAVES IN 2D PHONONIC CRYSTALS 10 Chapter 3 Simulations of Phononic Crystal and Experimental Design of Total Band Gap 19 3-1 SIMULATION OF TOTAL BAND GAP IN 2-D PHONONIC CRYSTAL 19 3-1.1 Air/Silicon Square Lattice 19 3-1.2 The Dispersion Relation of 2-D Air/Si Phononic Crystal 20 3-2 SIMULATIONS OF SAW OF LAYERED STRUCTURES 20 3-3 EXPERIMENTAL DESIGN ABOUT VERIFICATION OF TOTAL BAND GAP 24 Chapter 4 Fabrications and Experimental Results about Verification of Total Band Gap 37 4-1 GROWTH OF ZNO THIN FILM ON SILICON SUBSTRATE 37 4-1.1 Deposition of ZnO Thin Film 37 4-1.2 Analysis of Thin Film properties 38 4-2 FABRICATIONS OF LAYERED IDT AND 2-D PHONONIC CRYSTAL 39 4-2.1 Fabrication Process of IDT/ZnO/Silicon Layered SAW Devices 39 4-2.2 Fabrication Processes of 2-D Air/Silicon Phononic Crystal 41 4-3 EXPERIMENTAL RESULTS OF LAYERED IDT 43 4-3.1 Time Gating Approach 43 4-3.2 Comparisons of simulation and experimental results 44 4-4 EXPERIMENTAL RESULTS ABOUT VERIFICATION OF TOTAL BAND GAP IN 2-D AIR/SI PHONONIC CRYSTAL 45 Chapter 5 Conclusions and Future Works 67 5-1 CONCLUSIONS OF THIS THESIS 67 5-2 FUTURE WORKS 67 References 6