本文針對電容式超音波換能器的基礎設計、製作及量測提出整套有系統的研究。首先就電容式超音波換能器的薄膜振動模式、直流偏壓及上電極的附加質量效應予以討論，再研究直流偏壓及上電極附加質量效應，對電容式超音波換能器薄膜共振頻率產生下降的影響。文中採用符合實際的邊界條件求解換能器薄膜的機械阻抗，結合聲學輻射阻抗推導出準確的超音波換能器等效電路，作為電容式超音波換能器的設計依據，並利用薄膜共振頻率及換能器輻射聲場的數值實例計算，建立出電容式超音波陣列的設計準則。
有關製作部分，使用與積體電路製程技術相容的微機電面型微加工技術，製作電容式超音波換能器。這些技術包含低壓化學氣相沉積技術、黃光微影技術、活性離子蝕刻技術、犧牲層濕蝕刻技術、熱阻絲蒸鍍技術及電漿輔助化學氣相沉積技術。製程中影響換能器性能的重要考量，諸如下電極的離子佈植、隔絕層的材料與厚度、犧牲層的厚度與蝕刻比的影響、側向蝕刻孔的尺寸與真空密封、蝕刻道的路徑與形式、振動薄膜的直徑、厚度與材料及上電極的厚度與尺寸等，在文中皆予以討論。
最後將切割完成的電容式超音波換能器黏著於印刷電路板，用以進行特性量測，包括總輸入阻抗量測，其量測值與理論預測值相當吻合；發射與接收特性量測顯示，電容式超音波換能器的接受信號有超過35dB的訊雜比，顯現出其可使用性；超音波陣列輻射聚焦特性量測顯示，超音波陣列具有良好的聚焦性能，證明電容式超音波換能器的製作及應用為可行。

This thesis presents the primary design, fabrication and measurement of the Capacitive Micromachined Ultrasonic Transducer (cMUT). Previous studies have investigated the bias effect, vibration behavior of the cMUT and optimum size of the top electrode; however, these studies did not analyze combining the resonant frequency drop of the cMUT with the effects of bias and added mass. In this thesis, reasonable boundary conditions for solving the modified Mason model, acoustic radiation impedance of medium, bias effect and added mass effect of the top electrode are all utilized to derive an accurate equivalent circuit for cMUT design. Computer simulations of the cMUT are performed and several numerical examples are computed. The modified cMUT model predicts behaviors of the cMUT’s membrane with increased accuracy, especially on the resonant frequency.
The cMUT fabrication uses the full surface micromachining techniques of the Micro Electro Mechanical System (MEMS), which are compatible with integrated circuit fabrication processes, have been further developed over the recent decade. These techniques include Low Pressure Chemical Vapor Deposition (LPCVD), photolithography, Reactive Ion Etching System (RIE) dry etching, sacrificial layer wet etching, metal thermal evaporation coating and Plasma-Enhanced Chemical Vapor Deposition (PECVD). Several important issues regarding fabrication process that the bottom electrode, insulating layer, sacrificial layer, etching hole, etching channel, membrane particular, and metal electrode are discussed for optimizing the performance of the cMUT.
Finally, the input impedance of the cMUT is measured and the measured result agrees with the theoretical prediction having simply supported membrane boundary conditions. The received signal has a 35 dB signal-to-noise ratio indicating that practical applications of the immersion cMUT are feasible and that the radiation pattern measurement of the cMUT array has good beamforming characteristics for underwater imaging.

Contents
Contents Ⅳ
List of Figures Ⅶ
List of Tables Ⅹ?
References and Appendixes ⅩII
Chapter 1 INTRODUCTION
1.1 Introduction 1
1.2 Research motivation 1
1.3 Literature review 5
1.3.1 Review of ultrasonic technology 5
1.3.2 Review of ultrasonic transducer development 7
1.4 Research method and thesis structure 9
1.4.1 Research method 9
1.4.2 Thesis structure 9
Chapter 2 THEORIES
2.1 Equivalent circuit of the cMUT 13
2.2 Membrane properties 17
2.2.1 Variational equation of motion for a stretched
thin plate 17
2.2.2 Mechanical impedance of the cMUT membrane 19
2.3 Acoustic radiation impedance 22
2.4 Characteristic analysis of the cMUT 23
Chapter 3 RADIATING SIMULATION OF THE cMUT
3.1 Circular piston radiation equation 35
3.2 Design of the ultrasonic transmitter array 36
3.2.1 Rectangular transmitter of equivalent sound source 37
3.2.2 Design of two-dimensional ultrasonic transmitter array 39
3.3 Numerical example of radiation pattern of ultrasonic transmitter 41
3.4 Design criteria of ultrasonic transmitter array 42
3.5 Theory of focusing acoustic transducer 42
3.6 Numerical example 45
Chapter 4 FABRICATION OF THE cMUT
4.1 Fabrication processes 61
4.2 Discussion 63
4.2.1 Bottom electrode 63
4.2.2 Insulating layer 63
4.2.3 Sacrificial layer 63
4.2.4 Etching hole 64
4.2.5 Etching channel 64
4.2.6 Membrane particular 65
4.2.7 Metal electrode 65
Chapter 5 MEASUREMENT OF THE cMUT
5.1 Measurement equipment 76
5.1.1 Impedance analyzer 76
5.1.2 Function generator 77
5.1.3 Commercial transducer 77
5.1.4 Wideband preamplifiers 78
5.1.5 Power supplier 78
5.1.6 Oscilloscope 78
5.2 Measurement of the cMUT 79
5.2.1 Input impedance measurement 79
5.2.2 Transmission and receiving experiments 79
5.2.3 Radiation pattern measurement 81
5.2.4 Focusing device measurement 81
Chapter 6 CONCLUSIONS
6.1 Design criteria of ultrasonic transmitter array 93
6.2 Achievement of research 94

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