由於半導體技術的蓬勃發展，大型元件陣列的控制和驅動電路，在製程上成本變得更加便宜且元件密度更高，這也使得空氣耦合超音波傳感器陣列能夠應用於超音波觸覺回饋和聲懸浮等領域。現階段市售的空氣耦合超音波傳感器體積相當龐大，這讓產品難以商業化的應用，因此壓電式微型超音波傳感器被提出（pMUT）用來取代傳統的超音波傳感器。目前鋯鈦酸鉛（PZT）薄膜是最穩定的壓電材料，也是 pMUT 最廣泛使用的壓電層，但現有的 PZT 製造方法過於昂貴或需要稀有金屬，為了解決這個問題，本研究提出了使用不同PZT薄膜的空氣耦合pMUT，可以將超音波傳輸到空氣中。
一開始本論文提出了一種體積小、功耗低的壓電微機械超音波換能器（pMUT）來取代傳統換能器，所提出pMUT 的諧振頻率為 40 kHz，元件半徑透過圓盤模型和有限元素模型設計。為了獲得更好的性能，選擇鋯鈦酸鉛作為壓電層並透過射頻濺鍍製造，而pMUT 的空腔是透過深度反應離子蝕刻(ICP)背蝕刻完成。 此pMUT 的實際諧振頻率為 32.9 kHz，與模擬結果接近。單一 pMUT 元件的聲壓在 70 Vpp 時為 0.227 Pa。這個研究成功展示了 pMUT製程的平台，包括優化的設計程序、元件特性量測和製備方式，並展示了 pMUT 陣列在超音波觸覺應用中的潛力。
接著，本論文提出了第一個使用溶膠-凝膠 PZT 薄膜的空氣耦合 pMUT，可以將超音波傳輸到空氣中應用。首先將PZT薄膜的沉積條件優化使其具備高Pr值，再來使用圓板模型設計了諧振頻率接近 40 kHz 的空氣耦合 pMUT，根據模擬結果製備了半徑為 600 μm 至 775 μm 的 pMUT，以評估聲音輸出壓力。其中半徑為725μm的pMUT在10Vpp驅動時，在高度三公分處實現了最大聲壓輸出4.42Pa，諧振頻率為40.48kHz。我們使用 k-Wave 工具箱計算了由半徑為 725 μm 的基於溶膠凝膠 PZT 的 pMUT 組成的陣列輸出壓力，當聚焦在其上方 3 cm 處時，11 × 11 pMUT 陣列的輸出壓力達到 365.62 Pa。這項結果顯示本研究所提出的 pMUT 陣列的輸出壓力可以滿足大多數空中超音波應用的要求。
最後，我們整合前述的兩種製備方式，提出了一種經濟高效且可靠的 PZT 薄膜製造方法。透過沉積商用溶膠-凝膠溶液作為種子層，然後進行射頻濺射，得到了 PZT 薄膜的殘餘極化強度為 113.35 μC/cm2，矯頑力場為 211.6 kV/cm。我們將所提出的 PZT 薄膜製成了高通量空氣耦合 pMUT 和 pMUT 陣列，並且透過有限元素法模擬來最佳化 40 kHz pMUT 的結構參數。為了評估元件的性能，使用標準麥克風測量 pMUT 和 pMUT 陣列上方三公分處的聲壓，而 pMUT 產生的聲壓為 0.764 Pa/V，諧振頻率為 52.2 kHz，pMUT 陣列的最大聲壓為 87.4 Pa。這是目前在100kHz以下，所有已知研究中的最高輸出聲壓，這也凸顯出本論文所提中的PZT 薄膜沉積方法，應用於超音波觸覺回饋和聲懸浮的潛力。

Due to the development of semiconductor technology, control, and driving circuits for large phased arrays have become cheaper and more compact, enabling air-coupled ultrasonic transducer arrays to be applied in areas such as ultrasonic haptic feedback and acoustic levitation. However, current air-coupled ultrasonic transducers are pretty bulky, which prevents the commercialization of these applications. Hence, some researchers proposed piezoelectric micromachining ultrasonic transducers (pMUT) to replace conventional ultrasound transducers. PZT, lead zirconate titanate, is the most reliable and efficient piezoelectric layer for pMUT. However, existing PZT fabrication methods are too expensive or require rare metals. Hence, this thesis proposed several reliable and cost effective PZT fabrication methods and demonstrated their capability on fabricating air-coupled pMUT.
Initially, this work proposes a piezoelectric micromachined ultrasonic transducer (pMUT) with a small size and low power consumption to replace traditional transducers. The proposed pMUT has a resonance frequency of 40 kHz and a radius designed through the circular plate and finite element models. Lead zirconate titanate was selected as the piezoelectric layer and fabricated via RF sputtering for better performance. The cavity of the pMUT was formed by releasing a circular membrane with deep reactive ion etching. The resonance frequency of the pMUT was 32.9 kHz, which was close to the simulation result. The acoustic pressure of a single pMUT was 0.227 Pa at 70 Vpp. This work has successfully demonstrated a pMUT platform, including optimized design procedures, characterization techniques, and fabrication process, and it has also shown the potential of pMUT arrays for ultrasound haptics applications.
In addition, this work demonstrated the first air-coupled pMUT using sol-gel PZT thin film that could deliver ultrasonic waves to mid-air. First, this study optimized the deposition conditions for making PZT thin film with high remanent polarization. Then, air-coupled pMUTs with resonance frequencies close to 40 kHz were designed using the circular plate model. According to the design, pMUTs with radii measuring 600 μm to 775 μm were fabricated to evaluate the acoustic output pressure. Among these, the pMUT with the 725 μm radius achieved a maximum sound pressure output of 4.42 Pa at 3 cm above when driven with 10 Vpp, and the resonance frequency was 40.48 kHz. Finally, the output pressure of a phased array consisting of sol-gel PZT-based pMUTs with a 725 μm radius was calculated using the k-Wave toolbox. The output pressure of the 11 × 11 pMUT array reached 365.62 Pa when focused at 3 cm above it. This result revealed that the output pressure of the proposed pMUT array could fulfill the requirement for most mid-air ultrasound applications.
Finally, this work proposed a cost-effective and reliable PZT thin film fabrication method. By depositing a commercial sol-gel solution as a seed layer followed by radio frequency sputtering, a PZT thin film with remnant polarization of 113.35 μC/cm2 and a coercive field of 211.6 kV/cm was demonstrated. Moreover, this research made high-throughput air-coupled pMUTs and pMUT arrays based on the proposed PZT thin film. Finite element method simulations were conducted to optimize the structure parameters for the 40 kHz pMUT. The author used a standard microphone to measure the acoustic pressure 3 cm above the pMUT and the pMUT array. The pMUT generated an acoustic pressure of 0.764 Pa/V with a resonance frequency of 52.2 kHz, and the pMUT array’s maximum acoustic pressure was 87.4 Pa, which is the highest known acoustic output pressure among sub-100 kHz air-coupled pMUT arrays. It highlights the potential of using the proposed PZT thin film deposition method for high-pressure mid-air ultrasonic applications such as ultrasonic haptic feedback and acoustic levitation.

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