鈦酸鍶鋇薄膜之應用性質研究

Abstract

本論文我們提供幾種有效改善鈣鈦礦結構之鈦酸鍶鋇與鈣銅鈦氧化物薄膜之電性、熱穩定性及殘留應力的方法，由於單電晶體單電容之動態記憶體(lT1C; DRAM)的研究上，由原本的多晶矽/絕緣層/多晶矽 (SIS)結構演進到金屬/絕緣層/多晶矽 (MIS)結構和金屬/絕緣層/金屬 (MIM)結構，絕緣層具有作為與半導體間的緩衝層及降低元件操作電壓的功能外，必須同時具備低漏電流的特性以增加元件的記憶時問。所以，絕緣層必須具有高介電常數、低漏電流特性與高熱穩定性。在本研究中藉由種子層的加入，可有效改善鈦酸鍶鋇與鈣銅鈦氧化物薄膜之表面粗糙度、介電損失、漏電流密度、及薄膜熱穩定性。此外，藉由不同濃度的掺雜，亦可有效抑制鈦酸鍶鋇之晶粒成長，藉以改善薄膜之介電特性。 本文首先探討不同奈米種子層的鉻厚度對鈦酸鍶鋇薄膜特性之影響。研究中發現，加入鉻的種子層對鈦酸鍶鋇薄膜的特性有顯著的改善，其中包括：改善介電損失、降低漏電流密度、提高薄膜楊氏係數、減少薄膜殘留應力及結構熱穩定度的增加等。其中，鉻的種子層為2奈米厚度時，介電損失改善59%、薄膜楊氏係數提高41 %、薄膜殘留應力減少28 %及元件熱穩定度增加35 %等。 此外，我們也探討不同鋁含量的掺雜鈦酸鍶鋇鎂薄膜特性之影響。研究中發現，由於鋁離子取代鈦離子時，鋁離子所代表的行為是受體(Acceptor)，一方面可防止鈦離子在製程過程中由四價還原成三價鈦離子，另一方面可吸收及中和在不同鈦離子之間的跳躍電子，因而可大幅減少薄膜漏電流及介電損失。此外，由容限因子(tolerance factor; t)公式來估算，不同鋁含量的掺雜可達到抑制/控制晶粒成長之目的，因而使鈦酸鍶鋇鎂薄膜的結構更穩定。 由於鈣銅鈦氧化物具有極大之介電常數，因而被視為下世代動態隨機讀取記憶體熱門材料之一。因此，探討加入鈣鈦礦結構之鈦酸鍶鋇薄膜作為鈣銅鈦氧化物(CaCu3Ti4O12; CCTO)之奈米種子層特性的影響。研究中發現，加入鈦酸鍶鋇奈米種子層，可有效提高鈣銅鈦氧化物之熱穩定係數及鈣銅鈦氧化物的絕緣特性。 最後為本文之總結，並在各章節中加入本論文研究成果與相關文獻中之研究結果作一比較。

In this thesis, we provide some effective methods to improve the electrical properties, thermal stability, and residual stress of barium strontium titanate oxide and calcium copper titanium oxide thin films. The electrical devices of one transistor and one capacitor dynamic random access memory (1T1C, DRAM) technologies were from polysilicon/insulator/ polysilicon (SIS) structures to metal/insulator/metal (MIM) structures. The insulator served as a buffer layer needs a high dielectric constant to reduce the operation voltage and low leakage current in order to increase the retention time of devices. Therefore, the requirements of the insulator must possess high-k dielectric constant, low leakage current and high thermal stability. In the thesis, the improvements of surface roughness, dielectric loss, leakage current density, and thin films thermal stability of barium strontium titanate oxide and calcium copper titanium oxide thin films by adding a seeding layer. In the thesis, we discovered that a Cr seeding layer can increase Young’s modulus and reduce MIM structure residual stress for BST thin films. Besides, it can suppress the grain growth and improve the dielectric properties of thin films by the different doped concentration. In the beginning, we discuss the effects of the different thickness of the Cr seeding layer to barium strontium titanate films. It is obviously improvable the characteristics of BST films including decreasing the dielectric loss, leakage current, increasing Young’s modulus, and reducing residual stress by added the Cr seeding layer. The dielectric loss, thermal stability (TCC), Young’s modulus, and residual stress of BST films with a 2 nm Cr seeding layer are improved by about 59%, 35 %, 41 %, and 28 %, respectively, compared with BST films without a Cr seeding layer. Besides, the Ba0.5Sr0.5Ti0.95Mg0.05O3 (BSTM) films properties are studied as a function of Al content and have remarkable improvements including dielectric loss, leakage current, and Figure of merit (FOM) as well as films grain sizes. It was observed that Al3+ ions occupy the B sites of Ti4+ in the ABO3 perovskite structure and behave as electron acceptor-like dopants. These acceptors prevent the reduction of Ti4+ to Ti3+ by neutralizing the donor action of the oxygen vacancies. The tolerance factor (t) of Al-doped BSTM perovskite thin films is 0.97 as compared to 0.87 of the undoped BSTM thin films. The increasing of tolerance factor value indicates that the specimens with Al doped BSTM films are more stable than undoped specimens. CaCu3Ti4O12 (CCTO) is regarded the promising material in next generation dynamic random access memories (DRAMs) as a result of the colossal dielectric constant. In the research, a BST seeding layer to the interface between CCTO/Pt structures has remarkable influences on CCTO thin film properties including dielectric properties, insulating characteristics, and the thermal stability (TCC). The experimental results were summarized in the final chapter. Comparisons other researchers' results with this thesis studies were discussed in each chapter.