增強壓電超晶格極子天線之電磁輻射能力

Abstract

微型化低頻天線的演進過程，往往有尺寸限制且伴隨著品質因子、頻寬的問題，研究者們除改變傳統電小天線的形狀，達到整體電流長度不變又能縮小體積的效果外，逐漸發展的機械式天線也成為近年鑽研的項目，其原理是透過應力致動壓電或壓磁材料，產生電磁輻射效果，但傳統壓電機械式天線的缺點是機械波與電磁波耦合量較低。近年來發展成熟的壓電超晶格成為微型化天線的方式，經由結構中週期性的調變，有著禁帶與通帶的特性，內部則有機械波波傳的聲子晶體、電磁波波傳的光子晶體，以及兩者耦合而成的極子，而本研究使用鈮酸鋰做為壓電超晶格天線的材料，經微機電製程使晶體結構產生週期性的正反兩種極化方向，透過機械波與電磁波的耦合性質激發出特定頻率的電磁波。 由於壓電超晶格的共振頻取於週期寬度，本論文將工作頻率設定在80 MHz，並探討改變鈮酸鋰晶片之厚度對其電磁輻射的影響，選用500 µm及1000 µm兩種不同厚度的晶片進行製作，極化方式採用高壓電極化法，極化時饋入電壓與材料的厚度成正比，為解決極化過程所發生的金屬電極造成之電場集中與縱向極化深度不足等問題，進行光罩改良等製程相關研究，接著使用膜厚儀、光學顯微鏡以及電子顯微鏡進行表面與剖面檢測，確認極化深度與範圍，確立晶片極化的完成度。 基於壓電超晶格的機械波與電磁波的耦合效應，針對設定頻率附近的頻段進行遠場量測，比較之週期極化鈮酸鋰晶片尺度包含3吋晶片以及經切割後的12 mm × 10 mm微型晶片，量測方式以網路分析儀的S11、S21以及頻譜分析儀在80 MHz的輻射效率為主，結果顯示厚度為1000 µm之鈮酸鋰晶片表面積與厚度為500 µm的相同，但在80MHz附近有著較高的電磁輻射接收能力，除驗證壓電超晶格可以有效率的耦合內部機械能與電磁能以及發射和接收電磁輻射的能力，也因厚度變為兩倍，其電磁輻射接收能力有約2.5倍的增幅，有助於改善微型化天線效率不足的問題。

The evolution of miniaturized low-frequency antennas is often limited in size and accompanied by quality factor and bandwidth problems. In addition to changing the shape of traditional electric small antennas to achieve the effect of maintaining the overall current length and reducing the volume at the same time. Mechanical antenna has also become a research project in recent years. The principle is to generate a dipole potential difference through applied stress on piezoelectric or piezoelectric materials, so that can achieve the effect of the antenna. However, the disadvantage of mechanical antenna is the electromechanical coupling is weak. In recent years, piezoelectric superlattice has become a way of miniaturized antenna. Through periodic modulation in the structure, it has band gap, and there are phononic and photon for mechanical wave and electromagnetic wave propagation. Lithium niobate is used as the material of the piezoelectric superlattice antenna, and the structure is produced in periodic positive and negative polarization through the MEMS process. Electromagnetic waves of specific frequencies are excited through the strong coupling of mechanical waves and electromagnetic waves. Since the resonance frequency of the piezoelectric superlattice depends on the period width, the working frequency is set at 80 MHz in this thesis, and the influence of changing the thickness of the lithium niobate wafer on its electromagnetic radiation is discussed. We selected two different thicknesses, 500 µm and 1000 µm. The polarization method adopts the high-voltage electric polarization method. The feeding voltage is proportional to the thickness of the material during polarization. In order to solve the limitation of the wafer in polarization, the process-related research such as mask improvement is carried out, and then a film surface profiler is used , optical microscope and SEM for surface inspection of finished products to establish the completeness of wafer polarization. Based on the electromechanical coupling effect of piezoelectric superlattices, microwave measurements are carried out for the frequency band near the set frequency. For the accuracy of the experiment, the measurement and comparison of the wafer size includes a complete 3-inch wafer and a cut 12 mm × 10 mm. The measurement method is mainly based on the S11 and S21 of the network analyzer and the radiation efficiency of the spectrum analyzer at 80 MHz. The results show that the lithium niobate wafer with a thickness of 1000 µm has a higher electromagnetic radiation efficiency. Piezoelectric superlattices can efficiently couple internal mechanical energy and electromagnetic energy, as well as the ability to emit and receive electromagnetic radiation, and also have different electromagnetic radiation efficiencies due to different thicknesses, which helps to improve the problem of insufficient efficiency of miniaturized antennas.