近年來可攜式電子產品在隨著半導體製程技術日新月異的進步下，不斷地追求更多工的性能與更輕薄短小的外觀設計，然而在功能增加與體積縮小的同時，也產生了高功率的消耗與容易過熱的問題。而微型熱電元件可利用溫差發電以及直流電致冷等兩種功能，除了具有體積小、無污染、低噪音與高可靠性等優點，且由於元件構造簡單，可藉著微機電製程技術將其微小化以進而應用於各種電子產品中，作為可攜式電源或者局部散熱器等之用。
微型熱電元件是由數以千計的熱電偶(thermocouple)所組成，有發電與致冷兩種功能；而熱電偶一般是由P型與N型熱電材料作為熱電腳(thermolegs)，並以金屬作為連接P/N型熱電腳之兩端電極。當在熱電腳兩端電極分別施加冷熱溫差時，元件便可利用熱電效應將此溫差轉換成可用之電源，稱為熱電發電元件；或者在元件正負端施加電壓使電流流經此元件，便可使其上下兩端電極產生吸熱和散熱之效果，稱為熱電致冷元件。
若依熱點腳的結構來劃分，傳統的微型熱電元件可分為垂直式結構與水平式結構兩種，平面式結構之微型熱電元件是利用薄膜製程於基板上沉積微米厚度之熱電薄膜，並以黃光微影/蝕刻製程技術將其定義成所需尺寸規格之熱電腳，其優點為熱電腳可經由光罩設計而輕易達到所需之長度，以避免因熱回流現象造成兩端溫差達成熱平衡而導致元件發電或致冷能力下降的問題，但缺點是熱流不僅會流經熱電偶結構也會藉由基板而散逸，造成不必要的能量損失並降低元件的熱電轉換效率。而垂直式結構之微型熱電元件其熱電腳是以電鍍之方式於基板上垂直成長柱狀結構之熱電腳，故熱能僅透過熱電偶作傳遞，其熱能量損失較小；但缺點是若要提升熱溫差值則必須增加熱電腳之高度，但同時也增加了製程上的難度，且製程也難以和傳統IC製程相符合。
有鑑於此，本論文利用微機電製程技術中之面型微加工技術，以二氧化矽薄膜作為元件之犧牲層結構，並以低壓化學氣相沉積技術(Low Pressure Chemical Vapor Deposition, CVD)沉積低應力之多晶矽薄膜於二氧化矽犧牲層上，以離子擴散佈值(Ion implantation)技術將多晶矽薄膜定義為N/P型高優質熱電薄膜，最後再由濕式蝕刻方式蝕刻犧牲層來釋放元件結構，製作出創新式懸浮橋式結構之微型熱電元件，可讓熱電偶僅靠兩端接觸基板部位作為支撐，成功讓熱腳與上端金屬懸浮於基版上，如此一來熱能便可直接流經懸浮之橋式熱電偶結構，而不至於流失至基板當中；此外本研究可精確控制熱電腳懸浮長度於100~200微米而不至於塌陷或黏附基板，因此不僅同時擁有平面式與垂直式之微型熱電轉換元件之優點，也一併改善兩者之缺點。本論文所開發之懸浮橋式結構之熱電元件與基板間之懸浮高度為2.5μm，包含2,160 ~ 16,128對之熱電偶；在實驗室的製造環境下，其製程良率高達75%，而主要的製程步驟一共包含七次薄膜沉積與五次黃光微影/蝕刻製程。
經由本研究自行開發之量測系統以及商用高放大倍率熱像儀之量測顯示，本論文所提出之微型熱電發電元件在基板與空氣溫度為10°C的環境下，其冷端與熱端的溫差為4.11°C，每平方公分最大的開路電壓值可達到2.99 V，並產生1.21mW之輸出功率；而在微型熱電致冷器元件散熱效果方面，在驅動電壓為9伏特的情況下，其上下金屬電極最大溫差值可增加0.81°C。根據此熱電特性，本論文所開發之微型熱電發電元件可應用於可攜式電子產品之中，將元件冷熱兩端之溫差轉換成可用之電能輸出，可提供一部分之行動能源；另外所開發之微型致冷元件也非常適合應用於微型電子元件或光電元件中作為局部散熱器之功用，可以解決器材因高熱而衍生的種種問題。

This dissertation describes the development of two novel suspension bridge-type micro thermoelectric devices: micro thermoelectric generator (μ-TEG) and micro thermoelectric cooler (μ-TECs) using surface micromachining techniques. The presented micro thermoelectric devices are combined by suspension bridge-type thermocouple; each thermocouple is constructed by a pair of n/p bridge-type polysilicon thin-film thermolegs and a pair of cold- and hot-side Cr/Au metal plane. In addition, a novel serial/parallel-connected thermocouple is designed to enhance the temperature difference and can gain a large output power and cooling performance. The main fabrication processes adopted in this research are including seven thin-film deposition processes and five photolithography processes. The implemented suspension bridge type thermopile has a 2.5 μm-height air-gap separation from substrate and having 2,160 ~ 16,128 thermocouples in one centimeter square chip area.
The measured maximum temperature difference between the cold/hot sides of the proposed μ-TEG is about 4.11 °C, a maximum open-circuit voltage of 2.99 V/cm2 and output power of 1.21 mW/cm2 can be obtained. Furthermore, a 0.81 °C maximum increase temperature difference can be achieved in the bridge-typeμ-TEC with serial/parallel-connected thermocouples under 9 V driving voltage.
For thermoelectric generator application, by integrating the tens of thousands of micro-thermocouple in a centimeter square area, the temperature difference between the hot plane and cold plane of the presented μ-TEG can be converted into a useful electrical power. The thermoelectrically transferred output electrical power is suitable for recharging various mobile communication products. For thermoelectric cooler application, the bridge-type the cross-section area to length (A/L) of polysilicon thin-bridges was minimized to reduce its thermal conductance, can achieve a large temperature difference between hot/cold-side and are suitable for localized hot-spot cooling applications.

論文審定書 I
誌謝 III
中文摘要 V
Abstract VII
Contents IX
List of Figures XII
List of Tables XVIII
Chapter 1 INTRODUCTION 1
1.1 Review of Micro Thermoelectric Generators 1
(a) Vertical-Type Thermocouple 6
(b) Lateral-Type Thermocouple 9
1.2 Review of Micro Thermoelectric Coolers 15
1.3 Motivations and Objectives 21
1.4 Overview of Dissertation 22
Chapter 2 THEORY DESCRIPTION 23
2.1 Thermoelectric effect 23
2.1.1 Seebeck Effect 24
2.1.2 Peltier Effect 25
2.1.3 Thomson Effect 28
2.2 Thin-film Thermoelectric Material 29
2.3 Thermoelectric properties 33
2.3.1 Thermoelectric Generation and the figure-of-Merit 35
2.3.2 Thermoelectric Refrigeration and the Coefficient of Performance 39
2.3.3 Basic Unit of Thermoelectric Devices 40
Chapter 3 DEVELOPMENT OF MICRO THERMOELECTRIC GENERATORS WITH BRIDGE-TYPE THERMOCOUPLE 42
3.1 Theoretical Formulation of Thermocouple 42
3.2 Optimization of the Suspending-Bridge Type Thermocouple 47
3.3 Fabrication of µ-TEG with Bridge-Type Thermoelectric Generator/Cooler 50
3.4 Measurement System 55
Chapter 4 RESULTS AND DISCUSSION 57
4.1 Thermoelectric Characteristic of Polysilicon Thin-film 57
4.2 Microstructural Inspection by SEM 58
4.3 Thermoelectric Performances of Bridge-Type μ-TEG Device 62
4.3.1 Environment Conditions of Measurement System 62
4.3.2 Conversion Performance of Bridge-Type μ-TEG Device 70
4.3 Thermoelectric Performances of Bridge-Type μ-TEC Device 75
Chapter 5 CONCLUSION AND FUTURE WORK 83
5.2 Future Works 85
Reference 86

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