鋯鈦酸鋇Ba(Zr,Ti)O3鐵電薄膜材料擁有隨不同外加直流偏壓而改變介電常數的特性，稱之為調諧性(Tunability)。運用此特性可製作變容器、調變濾波器(Tunable Filters)與相位移器(Phase Shifter Devices)等元件。本研究著重在兩個方向，包括平行板式(parallel structure)與共平面式(coplanar structure)電極的鐵電薄膜變容器結構，而討論主題分三點，1.底電極效應對平行板式變容器之影響；2.界面層對高阻值矽基板上製作之鐵電薄膜變容器的影響；3.高介電、低損耗之緩衝層與退火條件對高阻值矽基鐵電變容器之影響。分述如下：
在底電極效應的主題中，利用白金、鎳酸鑭/白金與鎳酸鑭/金/白金三種導電電極，並於其上成長鋯鈦酸鋇鐵電薄膜研究其特性之變化。由實驗結果發現增加白金電極的厚度在高頻與微波頻段皆可有效減少整體損耗，提升變容器的優異值(FOM)。此外，導電氧化物電極(鎳酸鑭)有助於鋯鈦酸鋇鐵薄膜之優選指向成長， 提升薄膜崩潰電壓，進而提升變容器調變率，使元件優異值大幅上升。
在界面層對矽基變容器影響的主題中，藉由加入二氧化矽層與否來研究矽基變容器的界面層機制與其特性探討。包括鐵電薄膜直接鍍製於高阻值基板與二氧化矽緩層之高阻值矽基板兩種，利用阻抗分析與網路分析儀量測其特性。研究結果發現共平面的矽基變容器主要的高頻調變性來自於矽基半導體的載子累積與空乏作用，並且可由兩個背對背MOS電容串接而得到。此外，過厚的二氧化矽層會使得背對背MOS電容調變率消失。而由分析設備間接證明出正電荷的形成與累積，是造成微波頻段正向調變性的原因，並提出等效線路加以說明。
在高介電、低損耗之緩衝層加入之影響中，藉由氧化鉭與氧化鈦摻雜之氧化鉭兩種不同介電值之緩衝層來探討其對矽基鐵電變容器的提升效果。高頻的背對背MOS電容中發現緩衝層厚度越厚與介電值越低，則變容器之調變性越低。此現象可藉由高頻等校線路與兩個背對背MOS電容的重疊原理加以說明。因此發現薄的緩衝層可提升高頻調變性。另外，從研究結果中發現，氧化鉭的還原與氧空缺的形成造成微波頻段調變性改變，由前個主題中的正向調變性轉變成負向調變性，形成的原因也可由等校線路中加以說明。只要是由於在界面層中的累積電壓對矽基板之吸引的能力，使得累積於矽表面的載子對微波屏蔽的程度變化，造成最終電容是否與矽基板電容並聯而有變大或變小。而在退火條件之影響中發現，氧氣氛退火會減少氧空缺的量，與造成鋯鈦酸鋇薄膜中陽離子氧化，使得界面層中的正電荷，進而降低對矽基表面載子之吸引，因而喪失調變性。在氮氣氛退火實驗中，發現還原氣氛會使氧空缺大量生成，且鋯鈦酸鋇薄膜亦容易產生大量陽離子還原，皆增加變容器之漏電。此部分亦可由電滯曲線之形成來佐證。

The dielectric constant of ferroelectric material, Ba(Zr,Ti)O3, (BZT) changes with external dc biasing voltage. Based on this tunable characteristic, it is useful to fabricate the varactors, tunable filters and phase shifter devices. In this research, two kinds of ferroelectric varactors were investigated, i.e. structures made with parallel and coplanar electrodes, and the study includes three topics: (1) the influences of bottom electrodes in parallel structure, (2) the influences of interfacial layer in the ferroelectric varactors fabricated on high-resistivity silicon (HR-Si), (3) the influences of high-k and low loss buffered layer and annealing condition in the ferroelectric varactors fabricated on high-resistivity silicon.
In the first topic, three kinds of bottom electrode with different thickness were studied; including Pt, LaNiO3/Pt and LaNiO3/Au/Pt. Increasing the Pt thickness reduces effectively the total loss and raises the figure-of-merit (FOM) value. The conductive oxide, LaNiO3 (LNO) layer, enhances the textured crystallinity and increases the breakdown voltage, resulting in higher tunability of capacitance and FOM value.
In the second topic, the BZT films deposited on HR-Si with or without tetraethylorthosilane (TEOS) SiO2 as buffer layer were studied. The influences of electrical characteristics were measured by impedance and network analyzer at high and microwave frequencies, respectively. The results at high frequency show that the tunability of coplanar varactors results from the depletion and accumulation of carriers in Si, which can be modeled by two back-to-back MOS capacitors in series connection. At microwave frequencies, positive tunability, i.e. [C(V)-C(0)]/C(0) > 0, was observed and can be explained by an equivalent circuit in relation to the formation of positively-charged defects at the interfacial layer and the modulation of carrier accumulation at the interface in Si.
In the third topic, the Ta2O5 and (Ta2O5)1-x(TiO2)x layers were used to investigate the influences of high-k and low loss buffer layer. The results show that the thicker buffer layer and lower dielectric constant result in lower tunability. Moreover, the reduction of Ta5+ and formation of oxygen vacancies induce the change of carrier distribution at interface in Si, giving rise to different tunability at microwave frequencies. The positively-charged defects in the interfacial layer attract the electrons accumulated on the surface of silicon which screen the microwave from penetrating into the HR-Si, and affect the capacitive effect of HR-Si in series connection to the varactor. In addition, annealing in O2 annihilates the oxygen vacancies and oxidize the cations of BZT films causing the reduction of carrier accumulation and tunability. Annealing in N2 increases the oxygen vacancies and the reduction of cations in BZT films causing the raise of charged defects and injected carriers giving rise to hystersis behavior.

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