Electrical properties and crystal structure of (Ba,Sr)TiO3 films on metallic oxide bottom electrodes
Chung Ming Chu
|Keywords:||鈦酸鍶鋇;動態隨機存取記憶體;高介電;金屬導電性氧化物;薄膜;鈣鈦礦;鎳酸鑭;釕酸鋇;BST;DRAM;High Dielectric;Metallic oxide;Thin films;Perovskite;LaNiO3;BaRuO3|
在第二章中，以(Ba1-xSrx)TiO3 , x = 0.3 ~ 0.7靶材濺鍍BST薄膜，x=0.5時所鍍BST薄膜(Ba+Sr)/Ti成份比為1.007、Ba / Sr成份比為0.42/0.58計量比最佳，電性也最好。以(Ba0.5Sr0.5)TiO3成份靶材、Pt為電極濺鍍BST薄膜，薄膜結晶性、介電常數、漏電流密度隨鍍膜通氧比、鍍膜溫度升高而增大；氧混合比OMR=50%、鍍膜溫度550℃時，可得介電常數450、等效SiO2厚度0.76nm之BST薄膜，150KV/cm電場下漏電流密度約為 ~5×10-7A/cm2。
在第三章中，以濺鍍法製備LNO薄膜，在300℃鍍膜溫度下即可得到高(100)優選方位之LNO薄膜(阻值~0.5 mΩcm)。並以LNO為電極濺鍍BST薄膜，在Td=250℃時可得(100)優選方位之BST薄膜，Td= 300℃時介電常數即達250，但介電常數隨鍍膜溫度增加的速度十分緩慢，且在Td=350℃時出現了異常下降的情形，因為鍍膜溫度Td≧350℃時，在LNO底電極與BST間會有擴散發生，形成寄生電容降低介電常數。在LNO電極上濺鍍BST薄膜，鍍膜溫度250~350℃所鍍薄膜，漏電機制為蕭特基模式，在外加電壓±5伏特下，其電容漏電流密度都遠低於1×10-7A/cm2以下。
在第四章中，以濺鍍法成長BRO薄膜，在400℃鍍膜溫度下可得到具有(110)優選方位之BRO薄膜(阻值~0.8 mΩcm)。以BRO為電極濺鍍BST薄膜，在Td=200℃的鍍膜溫度下可得(110)優選方位之BST薄膜，Td= 300、500℃時介電常數分別為240、325。以高解析穿透式電子顯微鏡做BRO/BST薄膜界面研究，可以觀察到區域磊晶成長的現象，並可看到在BRO、BST界面有輕微擴散的情形發生，此界面擴散層，在缺氧狀態是半導體，而在充分的氧化態下則為絕緣體，所以薄膜電容以500℃ N2氣氛熱處理30分鐘電性不變，但在400℃ O2氣氛熱處理30分鐘後介電常數下降。Td = 400℃、薄膜厚度350埃時，BST薄膜介電常數160，等效氧化矽厚度為0.78nm，外加電壓0.0-3.0伏特，電容漏電流密度都在1×10-7 A/cm2以下。
There are three part of discussions were included in this study. In chapter 2, (Ba,Sr)TiO3 (BST) films on Pt electrode were fabricated with different Ba/Sr ratio BST ceramic as targets by radio-frequency (rf) magnetron sputtering at various substrate temperatures and O2/(Ar+ O2) mixing ratios (OMR). In chapter 3, BST films were deposited on (100) prefer orientation LaNiO3 elctrodes, Which were prepared by rf magnetron sputter deposition, the effects of LaNiO3 electrode on BST electrical and structure characteristics were discussed. In chapter 4, BST films were deposited on (110) prefer orientation BaRuO3 elctrodes, Which were prepared by rf magnetron sputter deposition, the effects of BaRuO3 electrode on BST electrical properties and crystal structure were discussed. We have some comparsions and conclusions in chapter 5. In chapter 2, We were using different Ba/Sr ratio BST ceramic as targets, (Ba1-xSrx)TiO3, x = 0.3 ~ 0.7, to fabricate BST films on Pt electrode by rf sputtering. As x=0.5, The BST films compositions was, (Ba+Sr)/Ti=1.007, Ba/Sr=0.42/0.58, with good composition stoichio-metric and dielctric properties. The crystallinity, dielctric constant, and leakage current characteristic of BST films were inceased with deposition OMR and deposition temperature (Td). The film deposited at Td= 550℃ and 50% OMR had a dielectric constant of 450, equivalent silicon-oxide thickness was 0.76nm, and the leakage current density was about ~5×10-7A/cm2 at an electric field of 150KV/cm. In chapter 3, BST films on a LaNiO3 underlayer were prepared by radio frequency magnetron sputter deposition. The onset of crystallization of the BST films occurred at a deposition temperature (Td) as low as 200℃. Good crystallinity was observed at Td □ 350℃. A moderate high dielectric constant (□r ~ 170-250) of the films was obtained at relatively low Td ~ 200-300℃. The value of □r showed a much slower increase with Td □ 350℃ than that expected from the progress of crystallinity. The deviation was attributed to an interdiffusion layer formed between the film and underlayer, which also caused an abnomal contact potential barrier and decilne of the □r value of the samples postannealed at 350℃. Annealing at 300℃ under O2 atmosphere lessened the oxygen deficiency in the film and enhanced the dielectric constant without forming an interfacial layer. In chapter 4, BST films were synthesized on BaRuO3 (BRO) using radio-frequency magnetron sputter deposition. Conductive (~1mΩ-cm) BRO layers of (110) texture were produced at a deposition temperatured 400℃, about 200℃ lower than that of SrRuO3. The BST (100nm) deposited on BRO showed a (110) prefer orintation and crystallization formation at temperatures as low as 200℃. An interfacial layer and the local epitaxial growth of BST on BRO layers within columnar grains were confirmed by high-resolution transmission-electron microscopy studies. Uniform I-V characteristics (J < 5×10-7A/cm2 ) for bias voltages in the range 0.0-3.0V of all the BST films deposited within Td=300-500℃ were observed. The dielectric constants of the BST films (e.g., □r = 300 at Td = 400℃) are considerably higher than generally achievable for the same Td by using Pt bottom electrodes. The dependence of the annealing atomsphere (N2 and O2) of the dielectric constants was studied and attributed to a diffused BST/BRO interfacial layer which is semiconductive in a reduced state and insulating in an oxidized state. In summery, Dielectric properties of the BST films were found to be closely correlated with the film’s structure or its crystallinity. A significant improvement in crystallization and electrical properties of BST films which were deposited at low temperature (Td = 350℃) through the perovskite type LNO and BRO electrodes. The interfacial diffusion layer on BST/LNO, and BST/BRO interfaces were avoidless, because the crystal structures and ionic radius of BST were similar to LNO and BRO. So the control of processes temperature were very important.
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