低溫製程技術應用於動態隨機存取記憶體之鈦酸鍶鋇薄膜電容器之研究

Abstract

本論文將研究以先進低溫製程技術製備之鈦酸鍶鋇( (Ba, Sr)TiO3, BST) 薄膜電容器之特性。本研究中，Pt/TiN/Ti/Si 基板被應用於所有的樣本上，用以模擬動態隨機存取記憶體 (DRAM) 之capacitor over bit-line (COB) 結構。 本實驗使用雙靶射頻磁控濺鍍系統在低基板溫度下 (<450oC)來成長鈦酸鍶鋇(BST)薄膜電容器。環境氣體壓力、濺鍍鎗功率和氧氣/氬氣混合比 (O2/(Ar+O2) mixing ratio, OMR)為此鈦酸鍶鋇薄膜製備過程之主要參數。電氣特性與材料特性分析顯示濺鍍環境氣氛之氧氣/氬氣混合比 (O2/(Ar+O2) mixing ratio, OMR)扮演最重要的角色。電漿光譜顯示BST薄膜之成長速度和品質將被OMR之值所嚴重影響，而在5% OMR 之下可以得到薄膜之最高介電常數值。較高之OMR可提升時間相對之介電崩潰限度(TDDB)，換言之、即BST薄膜之可靠度因為薄膜中氧空缺補償之故而大幅提升。此外導電電流分析顯示在高電場下Pt/BST/Pt薄膜之導電機制將被Poole-Frenkel emission (PF) 所主導, 而低電場下則為蕭特基發射(Schottky emission, SE) 所主導，當OMR增加時蕭特基發射之機制影響亦增加。在本文中將以能帶圖來分析探討此導電電流機制。 BST薄膜於極低溫沉積完成後，低溫後處理將被應用於改善薄膜之品質。應用248nm波長之KrF準分子雷射(ELA) 於低基板溫度300oC下對此薄膜進行熱處理，以強化此膜之結晶性與電介質特性。實驗結果顯示結晶特性及電介特性將被顯著地強化。光學及熱傳分析顯示BST薄膜對248nm波長之準分子雷射之光吸收深度只有20nm，因此下層電極和元件將不會被此製程所損傷。雖然準分子雷射能於BST陶瓷薄膜之上表面進行快速有效的”淺層加熱”，但卻也容易破壞薄膜表面、形成大量之氧空缺，因之漏電流也隨之上升。此外、薄膜之優選晶向也會被雷射熱處理所影響，卽處理完後由 (mm0) 轉變成 (m00) 與 (mmm)，此將影響薄膜之光學與高頻電性。製程參數中, 入射能量、薄膜厚度, 基板組成結構和溫度都會明顯地影響此雷射熱處理之效能。 在氬離子環境中濺鍍與ELA處理所造成之BST表面氧空缺損傷會引起高的漏電流。本實驗使用氧氧離子電漿 (oxygen plasma) 後處理來改善此現象。氧離子電漿後處理可有效地鈍化與補償薄膜表面之氧空缺以降低漏電流，並提高薄膜之可靠度。但電漿處理時間過長將會引起電漿損傷，所以時間與入射能量等參數必須被謹慎地控制。 此外、超薄奈米尺度之鉻金屬夾層被應用於鈦酸鍶鋇薄膜電容器上，形成BST/nano-scaled Cr/BST之三明治夾層事之結構。此三明治夾層結構可以大幅降低漏電流與增強電容與色散特性之熱穩定性，這些有趣的特性將十分有助於實際DRAM電容器之元件整合與應用規範需求。實驗結果顯示漏電流與熱穩定性與鉻金屬夾層之厚度呈相依函數之關係，且2nm厚度之鉻金屬夾層可得最低漏電流。此外，在不同操作溫度下(室溫到150oC)的溫度電容係數(Temperature coefficient of capacitance, TCC)由單層BST電容400nm之35%降至BST(200nm)/Cr(2nm)/BST(200nm) 三明治複合多層電容之5%以下。微觀分析指出非理想介面之散射行為、異常空間電荷場形成與金屬氧化物串接電容為此低電流與熱穩定性之形成機制。 透過低溫製程技術之整合，鈦酸鍶鋇(BST)薄膜電容元件將可成為Giga-bit世代DRAM電容器之最佳候選元件。本文中將綜合結論鈦酸鍶鋇薄膜電容器之最佳特性可由製程參數控制、後處理方法應用與特殊多層膜結構之整合而得到完全低溫低熱預算之製程技術。

The characteristics of (Ba, Sr)TiO3 thin films, prepared by novel techniques of low temperature treatments, were systematically studied in this thesis. Pt/TiN/Ti/Si substrates were applied on each sample to simulate the real capacitor over bit-line (COB) structure of dynamic random access memory (DRAM). (Ba, Sr)TiO3 (BST) films were sputtered by radio frequency (RF) magnetron system with dual targets at low substrate temperature, lower than 450oC (340oC at sample surface), and the effects of the process parameters were also investigated. The work pressure, the sputtering gun power and the gas-mixing ratio are the important parameters in the BST film deposition. Material analyses and electrical testing show that the low temperature BST films are significantly affected by those process parameters. The O2/(Ar+O2) mixing ratio (OMR) is a most critical parameter during BST film sputtering. Plasma emission spectra indicate that the deposition rate declines at a higher OMR due to oxide formation on the target surface. The dielectric constant of the BST films can reach a maximum of 364 at 5% OMR. The ten-year lifetime of the time-dependent dielectric breakdown (TDDB) implies that the reliability of the capacitor can be enhanced at a higher OMR due to the compensation of oxygen vacancies and smaller grain sizes. Current-voltage analysis indicates that the leakage current of the Pt/BST/Pt capacitor is limited by Schottky emission (SE)/Poole-Frenkel emission (PF) at a lower/higher applied field, accordingly. The applied field boundary between SE and PF shifts toward higher field as OMR increases. Moreover, an energy-band model was proposed and this leakage mechanism was also discussed. Post low-temperature treatments were applied on the BST films to further improve their crystallinities and electrical properties. A novel process, KrF excimer laser annealing (ELA) at the wavelength of 248 nm, had been undertaken to implement BST films at a process temperature of 300o C to avoid the steep thermal gradient in thin films. The dielectric constant of the amorphous (α) BST film was remarkably enhanced from 80 to over 250 after ELA treatment. The optical testing and the heat conduction analyses indicate that the underlayer films and devices cannot be damaged during ELA treatment due to a very shallow light absorption depth (20 nm) of the BST film at the wavelength of 248-nm. Besides, the laser energy fluence and film thickness greatly influence the thermal conduction and the temperature distribution within the BST films. In the meanwhile, the as-deposited films revealed (mm0) preferred orientation, and, intriguingly, the preferred orientations changed into (m00) and (mmm) after ELA treatments. The optical and the high frequency properties may be affected by this preferred-orientation change. However, although the ELA can perform “shallow-depth annealing” for BST thin film, the degradation of upper surface is strongly influenced by the laser energy fluence. Hence, the leakage current will be significantly affected by the energy fluence of the laser. The leakage current of the ferroelectric film increases after sputtering process and post ELA treatment, but post oxygen plasma treatment can effectively improve the BST film surface to suppress the leakage at low processing temperature of 250oC. According to the analysis results in this thesis, the oxygen plasma treatment can effectively passivate the oxygen vacancies of BST films, decreasing the leakage currents. The leakage current can be reduced as many as two orders of magnitude under proper control of plasma conditions. The characteristics of the dielectric reliability, TDDB, can be also improved by this treatment due to the compensation of the surface oxygen vacancies. In addition, a nano-scaled chromium (Cr) layer is applied onto (Ba, Sr)TiO3 (BST) thin film capacitor as an inter-layer to enhance thermal stability of capacitance and suppress leakage current. Temperature coefficient of capacitance (TCC) using this BST/Cr/BST (200nm/2nm/200nm) multifilm can achieve 30% lower than that using BST mono-layer (400nm) film. Besides, the leakage current can be also greatly suppressed by applying this nano-scaled Cr layer onto BST thin film capacitor. TCC and leakage current behave as functions of Cr thickness, so the optimal properties can be obtained with the Cr thickness of 2nm. Microstructure analysis suggests that the interfacial continuity strongly influences the TCC and leakage property due to scattering centers and series capacitance formed at imperfect interface. The correlated mechanisms between electric and material properties are systematically investigated in this work. BST thin film can be the most promising candidate for Giga-bit generation cell capacitor, because the low temperature processes can be compatible to the IC’s integration. In this thesis, the optimal properties of the BST films can be obtained by adjusting process parameter, applying post treatments and using thermal stabilization structures to achieve thorough low-temperature processes.