臺灣博碩士論文加值系統

本文主要是研究微馬達的設計與製作。在製程考量方面，我們選擇了和半導體製程最為相容的表面微細加工技術，並採用類似Deng的幾何配置，設計製作了一個靜電側邊驅動、晃動式微馬達。我們並對此種設計提出改進之道，搭配上改進的製程，以期能提高微馬達製作的良率並使微馬達的效能提昇。其中有兩個主要的改進：其一是蝕刻阻擋（Etching stop）的製作使得中央固定軸不至於在結構釋放（Releasing）的過程中被破壞，此一蝕刻阻擋並可以形成轉子接地的通路，減低轉子與下方基材間的電位差和吸力。其二是採用混合層來構成軸承間隙（Bearing clearance）。我們利用濕氧化層（Wet oxidation）的高均勻性（Conformality）與電漿輔助化學氣相沈積（PECVD）氧化層不消耗矽之特性來得到厚度為一微米的軸承間隙，此間隙可以有效克服反應離子蝕刻（Reactive ion etching）造成的結構鎖死（Interlocking）並可能提供轉子一個向上的提昇力（Lift force）。在製程方面的努力與設計上的修正使我們製作出馬達主要的結構：分離且可動的轉子、定子、及聯結轉子與錨的軸承。但是由於機台的效能不佳，結構邊壁（Sidewall）的垂直度仍有待提昇。此外，我們亦依循Trimmer的推導，建立了適用於本身設計的力矩數學模型。而利用此數學模型可以估計出在理想操作條件下微馬達的輸出力矩。

This thesis focuses on the design and fabrication of micromotors. By using the surface micromachining technique, we designed and fabricated an electrostatic side-drive wobble micromotor based on Deng's geometric design.. Moreover, we also made improvement on her design and process. There are two major new ideas: the first is the fabrication of the etching stop which protects the bottom of the anchor from being damaged during releasing. With this etching stop, an additional electric path from the rotor to the substrate is also established. This electric path is always effective and would reduce the potential difference between the rotor and the substrate. The second is the 1 mm bearing clearance composed by a mixed-layer, a wet oxide and a PECVD oxide layer. The former has the advantage of high conformality while the latter consumes no polysilicon. This thick bearing clearance can solve the interlocking problem posed by the non-uniform RIE process and may induce a lift force on the rotor. Due to the efforts on process and the modified design, we fabricated a micromotor with the free moving and separated rotor, the stator, and the bearing connecting the rotor and the anchor. But because of the low performance of the RIE machine we've used, the sidewalls were not vertical as expected. Improvement on this process step is required. In addition to the design and the fabrication process, we also followed Trimmer's derivation and established the torque model for our micromotor geometry. By this model we can estimate the driving torque of the micromotor under ideal operation conditions.

Contents
Abstract in ChineseI
Abstract in EnglishII
AcknowledgementIV
ContentsV
List of figuresVIII
List of tablesX
Chapter 1 Introduction1
1.1 Motivation1
1.2 Survey of related researches2
1.2.1 Micromotor fabrication and analysis2
1.2.2 Wet and dry etching techniques3
1.2.3 Stiction in surface micromachining3
1.3 Thesis organization3
Chapter 2 Micromotor modeling5
2.1 Wobbling motion5
2.2 Electrical driving torque6
2.3 Dynamics10
Chapter 3 Micromotor design12
3.1 The prototype12
3.2 Mask design14
3.3 Properties of selected materials18
Chapter 4 An overview of the fabrication process19
4.1 Process overview in text form19
4.2 Process overview in schematic form21
Chapter 5 Fabrication process24
5.1 Considerations on process details24
5.1.1 Starting wafer and the electric shield24
5.1.2 Sacrificial/isolation layer between the stator and the substrate25
5.1.3 Microlithography25
5.1.4 Wet etching of the anchor post26
5.1.5 LPCVD polysilicon layer26
5.1.6 POCl predeposition and drive-in annealing28
5.1.7 Thin oxide film as RIE mask28
5.1.8 Patterning the rotor and the stator by RIE (reactive ion etching)29
5.1.9 Anchor/bearing ring/rotor clearances oxidation29
5.1.10 Hydrofluoric acid releasing30
5.2 Stiction in surface micromachining30
5.2.1 Possible affection due to stiction in micromotors31
5.2.2 Origins of stiction and critical dimensions31
5.2.3 Methods to avoid stiction32
5.2.4 Experiment32
5.3 Process study on plasma etching34
5.3.1 Principles34
5.3.2 Experiment35
5.4 Process study on LP/PE CVD37
5.4.1 Principles37
5.4.2 Experiment38
Chapter 6 Experimental results and discussion40
6.1 Photoresist processing40
6.2 Thin film processing and etching43
6.3 Problems encountered and discussion45
6.3.1 Misalignment45
6.3.2 Nearly isotropic and low-selectivity RIE45
6.3.3 Residual stress in polysilicon47
6.3.4 Stiction due to surface forces47
6.3.5 Huge friction force due to the bawl-shaped surface and structural interlocking48
6.3.6 HF induced degradation of the polysilicon film49
6.3.7 Test structures unavailable50
6.4 Testing methods and results50
Chapter 7 Modified design and process51
7.1 Parameter definitions51
7.2 Design rules and constraints53
7.3 Modified process54
7.4 Fabrication results and testing55
Chapter 8 Conclusions and future directions60
8.1 Conclusions60
8.2 Conductive MEMS components fabricated with a novel one-masked
process for micro-assembly60
References64

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