交流濺鍍高介電係數氧化物薄膜之電學及光學性質研究

Abstract

本研究以物理氣相交流濺鍍的方法成長鈦錫酸鋇及鈦酸鍶鋇薄膜。文中探討鈦錫酸鋇的物性以及在動態隨機存取記憶體應用上電學和可靠度分析，並且深入了解薄膜退化與鬆弛的現象和薄膜內部缺陷的關係，進而對缺陷定量。此外文中同時也探討氧氣氛處理對鈦酸鍶鋇的物性、光學及光電特性上的影響。 本論文首先在Pt/TiO2/SiO2/Si(100) 基板上以不同的溫度濺鍍鈦錫酸鋇薄膜，並探討其物性及電學特性。當基板溫度高於650°C時，薄膜具有較佳的結晶性，介電常數為210，在100 kV/cm電場強度下的漏電流密度為3.9x10-8 A/cm2，鈦錫酸鋇薄膜確實符合在動態隨機存取記憶體電容器應用上的要求。此外，研究結果亦指出，介電常數及漏電流密度與XRD圖譜之(111)/(110)繞射峰比值間，有著相當緊密的關係存在。 透過複數平面分析，在27°C £ T £ 125°C操作溫度與100 Hz £ f £ 10 MHz量測頻率間，深入了解缺陷在鈦錫酸鋇薄膜所扮演的角色，同時推演出其所代表的等效電路，以建立薄膜退化與鬆弛現象和薄膜內部與界面缺陷的關係，進而對缺陷定量。實驗結果指出，在鈦錫酸鋇薄膜中，缺陷主要來自金屬與薄膜界面缺陷 (interface defect) 以及晶界缺陷 (grain boundary defect)。這些缺陷造成介電鬆弛的現象，使得介電常數隨著頻率增加而降低，同時介電損失會隨著頻率增加而增加。但經由不同的氣氛處理，可以有效降低其缺陷濃度。 此外，我們也探討氣氛處理對鈦錫酸鋇薄膜物性及電學特性的影響，經由氧氣及笑氣(N2O)氣氛的快速高溫處理後，不僅可以提高其結晶性，同時也改善其介電特性，降低介電鬆弛的現象。在笑氣氣氛的處理下可以得到結晶性較佳、高介電常數、低漏電及可靠度較好的薄膜。若與未進行氣氛熱處理薄膜的比較亦發現，造成介電鬆弛最主要的原因是來自於金屬與介電膜介面的缺陷。依時介電崩潰(TDDB)分析的結果亦指出，透過E-模型及1/E-模型的分析，發現崩潰時間與外加電場之間為線性相依，且崩潰時間與薄膜厚度的相依性非常低。厚度介於50至100奈米的鈦錫酸鋇薄膜，在外加偏壓為1.5伏特時，其生命期在十年以上。經由氣氛處理後，其介電崩潰強度度不僅增加，生命期也跟著同時增長。 本文最後探討不同溫度下的氧氣氛處理對鈦酸鍶鋇薄膜的晶體結構、晶粒成長以及光學和光電子學特性的影響。根據所得的結果發現，填充密度、晶粒大小及折射率會隨著熱處理溫度的提高而增加。當氧氣氛熱處理溫度提高時，薄膜呈現較緻密的成長以及展現出趨近於塊材的光學特性。同時也發現，光學能隙的大小會受到晶粒大小的左右，具有較小晶粒的薄膜其光學能隙會變大。此外也引用了Wemple 和DiDomentico利用單振子色散模型所導出的關係式來進行折射率色散 (dispersion) 現象的分析。實驗的結果指出，經由750°C 的氧氣氛處理後，Wemple振動能量Eo (?1.49Egap) 及實驗因子 b ? 0.23 與相關報導所得的塊材結果 (Eo ? 1.5Egap and b ? 0.26 ± 0.04 eV) 相當接近.

In this study, we present the physical, electrical and optical properties of Ba(Ti0.8Sn0.2)O3 and (Ba0.7Sr0.3)TiO3 films deposited by RF magnetron sputtering. The effects of deposition temperature and film thickness on the physical properties (grain size, orientation, composition, crystallinity), electrical (dielectric constant, leakage current, conduction mechanism, breakdown voltage, polarization, dielectric degradation, dielectric relaxation) properties were investigated in details. In addition, a post-deposition thermal treatment by rapid thermal annealing in O2 and N2O ambient was used to improve the electrical characteristics. At first, the Ba(Ti0.8Sn0.2)O3 paraelectric thin films were successfully prepared on Pt/TiO2/SiO2/Si(100) substrates by RF magnetron sputtering deposition. The films deposited at 650 oC exhibited good crystallinity and had a dielectric constant of 210 and a leakage current density of 3.9x10-8 A/cm2 at an electric field of 100kV/cm with a delay time of 30 seconds. This indicates that the BTS films can be applied to the dielectrics of DRAM capacitors. It was also found that the dielectric constant and leakage current of the films closely correlated with the peak intensity ratio of (110)/(111) in the XRD patterns of the films. Secondary, the dielectric relaxation phenomenon and the defect analysis of RF magnetron sputtered Ba(Ti0.8Sn0.2)O3 thin films, were investigated using complex capacitance, impedance and admittance analysis as a function of frequency (100 Hz £ f £ 10 MHz), and temperature (27°C £ T £ 125°C). An equivalent circuit was proposed to well explain the AC response of Ba(Ti0.8Sn0.2)O3 thin films in the frequency range between 100 Hz to 1 MHz. However, at frequencies above 1 MHz, the films exhibit resonance in capacitance. The complex admittance measurements proved to be very useful to identify the presence and contribution of defect states on the frequency dependence resistance which in turn influence the electrical properties of Ba(Ti0.8Sn0.2)O3 thin films. On the basis of the complex plane analysis, we envisage that the effects of shallow defect traps are negligible whereas the grain boundary and interface defects provide significant contribution for the origin of dielectric relaxation. Present studies also demonstrate that dielectric relaxation of Ba(Ti0.8Sn0.2)O3 films is less pronounced compared with the other dielectric films. Third, the dielectric properties of Ba(Ti0.8Sn0.2)O3 films have been studied with and without post-annealing. Dielectric constant of the films increased with increasing thickness and with post-annealing. The improvement in film crystallinity attained through the post-annealing process is envisaged to be responsible for the better dielectric properties of the BTS films. The dielectric dispersion with the capacitance, which is due to the dielectric relaxation, has been observed for the as-deposited film and the effects of this phenomenon can be suppressed by the post-annealing in the O2 and N2O ambient. From comparison of the electrical characteristics of as-deposited and post-annealed films, it is concluded that electrons from oxygen vacancies in the interfacial layer are the origin of the dielectric relaxation. The leakage current and time dependent dielectric breakdown (TDDB) properties were investigated for the reliability of BTS thin films with and without post-annealing. The leakage current decreases with increasing film thickness and with post-annealing. Both tBD versus E and tBD versus 1/E plots showed straight lines, independent of the film thickness, and projected a lifetime longer than 10 years at 1V for 50 to 100 nm BTS films. Furthermore, the post-annealed film showed a longer lifetime than the as-deposited film leading to a longer dielectric breakdown time and higher dielectric breakdown field. Finally, rf-sputtered thin films of Ba0.7Sr0.3TiO3 (BST) were prepared on the SiO2/Si substrates with radio frequency magnetron sputtering. This work also examined how the O2 atmosphere annealing temperature affects the crystalline structure, the grain growth and the optical and optoelectronic properties. According to our results, the packing density, the grain size and the refractive index of the films increased with an increasing annealing temperature. The films exhibit the highest dense packing growth and near-bulk optical properties as the O2 atmosphere annealing temperature increases. The optical band gap energy depends on the grain size and becomes smaller for films with a larger grain size. The dispersion of the refractive index was also analyzed in terms of the parameters Eo and Ed using the single oscillator dispersion model of Wemple and DiDomentico. Our experimental results about Wemple oscillator energies Eo (?1.49Egap) and the empirical factor b ? 0.23 of the 750oC annealing films closely correspond with reported bulk data (Eo ? 1.5Egap and b ? 0.26 ± 0.04 eV).