交流濺鍍高介電係數氧化物鈦酸鍶鋇和鉭鈮酸鍶鉍薄膜在記憶體電容器之研究

Abstract

本研究以物理氣相交流濺鍍方法成長二種高介電常數薄膜材料：鈦酸鍶鋇和鉭鈮酸鍶鉍。在文中探討了介電薄膜的物性和鈦酸鍶鋇在動態隨機存取記憶體及鉭鈮酸鍶鉍在非揮發性記憶體上儲存電容器之電性及可靠度分析。且深入瞭解薄膜退化與鬆弛現象和薄膜內部缺陷的關係，進而對缺陷定量，並與薄膜的漏電流現象相互關連解釋。 本論文首先以不同 O2 和 Ar 氣氛混合比例（OMR）濺鍍在不同基座溫度及白金底電極上所成長之鈦酸鍶鋇層，探討其電性和物性，薄膜沉積後在爐管及快速退火爐中以 O2，N2 和 N2O 中退火效應，結果發現在高氧氣的氣氛中沈積鈦酸鍶鋇薄膜可以提高結晶性，並且也可產生較高介電常數的薄膜。介電常數會隨著氧氣氛比例增加而增加，在 50% OMR 時有最大值；漏電流會隨著氧氣氛比例增加而減少，在 40% OMR 時有最小值。使用壽命可以長達十年以上，且更沒有疲勞（fatigue）效應。對於鈦酸鍶鋇薄膜漏電流機制，主要是取決於氧空缺的含量。在低氧氣氛下沈積的鈦酸鍶鋇薄膜，包含有蕭基特（Schottky emission (SE)）和普法蘭克（Poole-Frenkel (PF)）的機制；但在高氧氣氛下沈積的鈦酸鍶鋇薄膜，只有蕭基特（Schottky emission (SE)）的機制。並且在氧氣氛下做熱處理可以得到較好的結晶性、高介電常數、低漏電流。反而在氮氣氛下做熱處理所得結果沒有比在氧氣氛下來的好。 在鈦酸鍶鋇薄膜中主要的漏電流機制為蕭基特（Schottky emission (SE)）與普法蘭克（Poole-Frenkel (PF)）傳導機制，而缺陷會對漏電流機制產生影響 。在低氧氣氛下(0% ~ 25%)，電場低於490 kV/cm，為蕭基特Schottky emission (SE) (electrode limited conduction)傳導機制；電場高於490 kV/cm，為普法蘭克Poole-Frenkel (PF) transport (bulk limited conduction)傳導機制。但是在高氧氣氛下(40% ~ 60%)，不管高低電場皆為SE傳導機制。主要是由於缺陷密度的影響，在低氧氣氛下成長鈦酸鍶鋇薄膜由於薄膜內部與界面皆有缺陷，故其漏電流傳導機制有SE與PF。而在高氧氣氛下成長之鈦酸鍶鋇薄膜，由於內部缺陷被氧氣補償，使得PF機制不明顯，故漏電流機制反而只顯現SE。在純Ar氣氛下成長之鈦酸鍶鋇薄膜，因薄膜缺陷較多，載子傳輸速度慢，故由CV量測之介電常數（static dielectric constant）非常接近IV量測之介電常數（dynamic dielectric constant）。在有氧氣氛下成長之鈦酸鍶鋇薄膜，因薄膜缺陷漸漸被補償，載子傳輸速度漸漸增快，故IV量測之介電常數（dynamic dielectric constant）會介於CV量測之介電常數（static dielectric constant）與光學介電常數（optical dielectric constant）之間，且漸漸接進光學介電常數（optical dielectric constant）。 接下來我們把鈦酸鍶鋇薄膜沈積在白金（Pt）、釕（Ru）、氧化釕（RuO2）、銥（Ir）、氧化銥（IrO2）等不同的底電極，以瞭解不同電極對鈦酸鍶鋇薄膜在物性及電性上的影響，結果發現銥（Ir）底電極提供最好的附著性、熱穩定性、平坦度、物性及電性。並且探討鈦酸鍶鋇薄膜在直流電壓應力下，所產生的退化效應。從不同 O2 和 Ar 氣氛混合比例濺鍍在不同基座溫度及不同底電極上所成長之鈦酸鍶鋇層的性質探討，可以得知在 50% OMR 氣氛中沈積鈦酸鍶鋇薄膜於銥（Ir）底電極上，具有最佳性質。另外，為瞭解鈦酸鍶鋇薄膜缺陷所扮演角色與在系統中所代表的等效電路，則使用複數平面來分析，更深入瞭解薄膜退化與鬆弛現象和薄膜內部缺陷的關係，進而對缺陷定量。 利用複數平面（complex plane analysis）分析，在 27℃~150℃ 操作溫度與頻率量測下，深入瞭解鈦酸鍶鋇薄膜缺陷所扮演角色與在系統中所代表的等效電路，並建立薄膜退化（degradation）與鬆弛（dielectric relaxation）現象和薄膜內部與界面缺陷的關係，進而對缺陷定量。而在鈦酸鍶鋇薄膜中主要的缺陷為金屬與鈦酸鍶鋇薄膜界面缺陷（interface defect）與晶界缺陷（grain boundary defect）。薄膜中的缺陷會造成介電鬆弛，使得介電常數隨頻率增加而降低、介電損失隨頻率增加而增加。利用不同氧氣氛濺鍍鈦酸鍶鋇薄膜，缺陷濃度會隨氧氣氛比例增加而減少，在 40% 氧氣氛其缺陷濃度為 1.3x1016 cm-3。 最後探討鉭鈮酸鍶鉍鐵電薄膜的電性及可靠度分析。鉍氧化物薄膜在鐵電記憶體之應用，因沒有疲勞（fatigue）效應，故為一個理想材料，其中以鉭酸鍶鉍薄膜為代表，即使經歷1012切換週期，也沒有疲勞現象，且殘留極化下降變化小於5%。但是鉭酸鍶鉍薄膜有一些缺點，例如小的殘留極化；低的居里溫度；必須在750℃高溫製程溫度下，才可得到結晶性好的鉭酸鍶鉍薄膜，對於CMOS製程整合有直接的困難。如果使用後續熱處理來降低製程溫度，卻得不到較好的結晶性與鐵電特性薄膜。在鉭酸鍶鉍薄膜中摻雜鈮元素，可以降低結晶溫度，使得製程溫度可以降低至600℃以下，且改善鐵電特性，有大的殘留極化與高的居里溫度。鉭鈮酸鍶鉍薄膜以不同 O2 和 Ar 氣氛混合比例濺鍍在銥（Ir）電極的矽基板上，發現在 40% OMR 的氣氛中沈積鉭鈮酸鍶鉍薄膜可以得到較高結晶性，並且也可產生較高極化（Polarization ）的薄膜。厚度 400nm 之鉭鈮酸鍶鉍薄膜在 570℃ 基座溫度成長與 40% 氧氣氛下，再經 598℃ 後續熱處理，可以得到介電常數 752、漏電流6x10-6 A/cm2 、損失因子 0.035，殘留極化（2Pr）40 mC/cm2 ， 矯頑電場(2Ec) 77 kV/cm。並且在正常工作電壓有超過10年的使用壽命，經歷 1011 切換週期也沒有疲勞現象。而鉭鈮酸鍶鉍鐵電薄膜之殘留極化可以媲美PZT。

In this study, we present the physical and electrical properties of BST films deposited by RF magnetron sputtering. The effect of deposition temperature, the sputtering oxygen partial pressure, the bottom electrodes and film thickness on the physical properties (grain size, orientation, compostion, crystallinity) and electrical ( dielectric constant, leakage current, conduction mechanism, breakdown voltage, polarization, dielectric degradation, dielectric relaxation, aging effect, fatigue effect) properties were investigated in details. In addition, a post-deposition thermal treatment by furnace and rapid thermal annealing in O2, N2 and N2O ambient was used to improve the electrical characteristics. At first, thin films of (Ba,Sr)TiO3 (BST) on Pt/SiO2/Si substrates were deposited using rf magnetron sputtering at various substrate temperatures and O2/(Ar+O2) mixing ratios (OMR). The crystallinity of the films improved significantly as the OMR increased. The dielectric constant increased with increasing OMR and reached a maximum value at 50% OMR. The leakage current density decreased with increasing oxygen flow, but it had the minimum value at 40% OMR. The results for the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The film deposited at 450 ℃ and 50% OMR exhibited good surface morphology and had a dielectric constant of 375, tangent loss of 0.074 at 100 kHz, leakage current density of 7.35x10-9 A/cm2 at an electric field of 100 kV/cm with a delay time of 30 sec, and charge storage density of 49 fC/mm2 at an applied field of 150 kV/cm. The 10 years lifetime of time-dependent dielectric breakdown (TDDB) studies indicate that 50% OMR sample has a longer lifetime than the 0% OMR sample. The BST thin films were grown on Pt/SiO2/Si substrates by rf magnetron sputtering with various OMR ranging from 0% to 60% to change the concentration of oxygen vacancies on the as-deposited films. The current-voltage behavior of the films was analyzed in terms of Schottky emission (SE) and Poole-Frenkel (PF) mechanisms. The SE conduction and PF transport mechanisms are observed below and above the transition electric field of 490 kV/cm, respectively, in the BST films prepared at low OMR (0% ~ 25%); and while SE mechanism dominates both below and above 490 kV/cm in the films deposited at high OMR (40% ~ 60%). These results show that concentration variation of the oxygen vacancies induced by the film deposition at various OMR significantly affects the conduction mechanism in the films. The static dielectric constant calculated from C-V data are in reasonably good agreement with dynamic dielectric constant obtained from I-V data. Therefore, we suggest that the static dielectric constant should be used to describe the carrier transport in 0% OMR BST film. The combination of static and optical dielectric constants should be employed to describe the carrier transport in other higher OMR % films, since the value of the dynamic dielectric constant was between the static dielectric constant and the optical dielectric constant. And the dielectric constant and the leakage current density of BST thin films deposited on Pt bottom electrode before and after annealing in O2, N2 and N2O ambient were investigated. The improvement of crystallinity was observed in post-annealing processed BST films. The refractive index, dielectric constant, and leakage current of the films were also strongly dependent on the post-annealing conditions. The annealed BST films have a higher dielectric constant and lower leakage current in O2 ambient than those in N2 ambient. Secondary, the dielectric constant and the leakage current density of (Ba,Sr)TiO3 (BST) thin films deposited on various bottom electrode materials (Pt, Ir, IrO2/Ir, Ru, RuO2/Ru) before and after annealing in O2 ambient was investigated. The improvement of crystallinity of BST films deposited on various bottom electrodes was observed after post-annealing process. The dielectric constant and leakage current of the films were also strongly dependent on the post-annealing conditions. BST thin film deposited on Ir bottom electrode at 500 ℃, after 700 ℃annealing in O2 for 20 min, has a dielectric constant of 593, a loss tangent of 0.019 at 100 kHz, a leakage current density of 2.1x10-8 A/cm2 at an electric field of 100 kV/cm with a delay time of 30 sec, and a charge storage density of 53 fC/mm2 at an applied field of 100 kV/cm. From the viewpoint of the dielectric constant, leakage current and reliability, the optimum material for the bottom electrode with post-annealing is Ir. And Ru electrode is unstable because the interdiffusion of Ru and Ti occurred at the interface between the BST and Ru after post-annealing. The 10 years lifetime of time-dependent dielectric breakdown (TDDB) studies indicate that BST on Pt, Ir, IrO2/Ir, Ru and RuO2/Ru have long lifetime over 10 years on operation at the voltage bias of 1 V. The influence of bottom electrodes (Pt, Ir, Ru, Ir(600 ℃)(with 600 ℃ annealing in O2 at 20 min)) on the degradation of (Ba,Sr)TiO3 (BST) thin films under DC stress conditions was investigated. The current-time (I-t) and current-voltage (I-V) measurement results indicated that the BST thin films deposited on Ru have faster degradation than those deposited on Pt, Ir and Ir(600 ℃). The degradation was considered to be caused by the deterioration of Schottky-barrier. Under DC stress conditions, the dielectric relaxation current in the BST dielectric films probably enhances the deterioration. The breakdown time was found to be approximated by an exponential function of an electric field [tB = a exp(-bE)] for DC stress. The value of the exponential factor b for BST deposited on Pt, Ir and Ir(600 ℃) was about a quarter of that for BST deposited on Ru. The different value of b observed under DC stress indicates that the degradation of BST on Ru would be more serious than on Pt, Ir and Ir(600 ℃). The BST film on Pt has a longer lifetime than those on Ir, Ru and Ir(600 ℃), which was ascribed to smaller grain size, higher Schottky barrier, and less polarization enhanced electron transport in the film on Pt. Hence, the optimum conditions of BST deposited by using rf sputtering are the 50% OMR and Ir bottom electrode with O2 post-annealing to get the high quality BST thin films. Third, the dielectric relaxation and the defect analysis of (Ba0.47Sr0.53)TiO3 (BST) thin films deposited on various bottom electrodes and various OMR, such as Pt, Ir, IrO2/Ir, Ru, RuO2/Ru before and after annealing in O2 ambient was investigated. Through the measurement of dielectric dispersion as a function of frequency (100 Hz £ f £ 10 MHz) and temperature (27 ℃ £ T £ 150 ℃), we studied the trapping dielectric relaxation and defect quantity of the films, and proposed an equivalent circuit on the basis of the capacitance, admittance and impedance spectra. A shallow trap level located at 0.005-0.01 eV below the conduction band was observed from the admittance spectral studies in the temperatrue range of 27 to 150 ℃. The origin of dielectric relaxation and defect concentration was attributed to the existence of the grain boundary defect, interface defect and the shallow trap level in the films. An equivalent circuit was established which can well explain the ac response and identify the contribution of the defects on the electrical properties of BST thin film. From the viewpoint of the trapping phenomena and dielectric relaxation analyses, we propose Ir as the best material for the bottom electrode to withstand the post-annealing treatment. This thesis also describes the dielectric and electrical properties of BST thin films deposited by rf magnetron sputtering with various OMR and various bottom electrodes. Correlation of the diffusion of oxygen vacancies with the ionic/electronic conductivity in the films was presented. Through the measurement of conductivity as a function of frequency (100 Hz £ f £ 10 MHz), the diffusion coefficients of oxygen ion at 27 ℃ for BST on Pt were evaluated to be 10-12 ~ 10-13 cm2/sec. The oxygen ion diffusion coefficient was considered to be proportional to the oxygen vacancy density, and the difference in the diffusion coefficients of oxygen ion represents the relative difference in the oxygen vacancy density. The majority of electrical currents was carried by electrons, and hence the BST is an electronic conductor (tele~1). Finally, thin films of Sr0.8Bi2.5Ta1.2Nb0.9O9+x (SBTN) on Ir/SiO2/Si substrates were deposited using two-target off-axis rf magnetron sputtering at various O2/(Ar+O2) mixing ratios (OMR). And the effects of 30 min annealing process at 598 ℃, 640 ℃, and 673 ℃ in an environment with a pressure of 10 mTorr after SBTN thin films deposited, are investigated in terms of the ferroelectric properties of the films. The annealing was found to be effective in obtaining a large remanent polarization. The 400 nm sputtered SBTN fims deposited at 570 ℃ in 40% OMR with 598 ℃ post-annealing exhibited remanent polarization (2Pr) of 40 mC/cm2 and coercive field (2Ec) of 77 kV/cm at an applied voltage of 3 V. The remanent polarization, dielectric constant and leakage current of SBTN thin films with 598 ℃ post-annealing increased with increasing OMR and reached a maximum value at 40% OMR. The results for the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The SBTN film deposited at 570 ℃ and 40% OMR with 598 ℃ post-annealing exhibited good surface morphology and had a dielectric constant of 752, tangent loss of 0.035 at 100 kHz, leakage current density of 6x10-6 A/cm2 at an electric field of 50 kV/cm with a delay time of 30 sec. The 10 years lifetime of time-dependent dielectric breakdown (TDDB) studies indicate that 50% OMR sample with 598 ℃ post-annealing has a longer lifetime than the other OMR samples. The SBTN films demonstrated fatigue free characteristics up to 1011 switching cycles under a 3 V bipolar 1 MHz square wave. And the polarization dispersion of SBTN is low (less 0.5% per decade) up to 240 min.