Physical characteristics and electrical properties of SrBi2Ta2O9 ferroelectric thin films based on transistor structure
|關鍵字:||鐵電陶瓷薄膜;鍶鉍鉭;電晶體;Ferroelectric thin film;SBT;transistor|
|摘要:||在本研究中，探討添加物對於鍶鉍鉭鐵電薄膜在Pt/Ti/SiO2/Si和Al2O3, ZrO2, SiO2/Si基版與不同退火溫度下的鐵電特性、漏電流與記憶窗之影響。且利用SEM與TEM對其表面型態與橫截面研究添加物與退火溫度對微觀結構所造成的變化，發現添加15 mol%釩與20mol/%鈦在金屬-鐵電薄膜-金屬(MIM)電容中其150Kv/cm的極化值上升最大分別為30.5與26.3μC/cm2，且漏電流在100kV/cm時降低5 x 10-8 A/cm2 and as 2 x 10-8 A/cm2兩項優點，並且對於原本鍶鉍鉭鐵電薄膜良好的疲勞效應，在不超過20mol%的添加量時並不會發生衰減的現象。接著將鍶鉍鉭鐵電薄膜整合在具有金屬-鐵電薄膜-絕緣層-半導體 (MFIS) 這樣的閘極電容中，討論鍶鉍鉭鐵電薄膜對於成分變動在退火效應上所造成的變化，並研究三種不同介電常數的介電層：氧化鋁、氧化矽、氧化鋯在不同厚度時所造成的影響，進而以添加物模擬介面的情況，觀察其鐵電特性。發現在900oC的退火溫度會有較大記憶窗和較低的漏電流，而鉍量的改變在不同退火溫度下的最佳值也會跟著變化，在4nm時氧化鋁介電層在900oC時外加電壓5V有較佳記憶窗4.8V，而在10nm時氧化鋁介電層的表現也優於氧化鋯，而因為氧化鋁介電層具有較佳的記憶窗與較低的漏電流，為了後段的製程處理，本研究也探討MFIS結構的電容經過還原氣氛處理後，其鐵電效應的變化，發現隨著還原氣氛處理的溫度增加，氫的入侵將會造成介電常數的急速下降，但是並不會對記憶窗造成太大的影響，且漏電流會隨的下降；由於本研究所發展的鐵電薄膜製程可以在MFIS電容上具有優異的記憶特性，配合相關的半導體製程，證實所發展的MFIS閘極結構可以進一步地應用在真實的記憶元件上。|
ABSTRACT Doped-Sr0.8Bi2Ta2O9 (SBT) ferroelectric thin films were deposited on Pt/Ti/SiO2/Si and insulators (Al2O3, ZrO2, SiO2)/Si substrates to its related electrical properties including ferroelectric, leakage current and memory characteristics. The effect of annealing temperature and insulator on the electrical properties of doped-SBT were investigated in this investigation. In addition, both SEM and cross-sectional TEM microstructure were studied to clarify the role of dopant and annealing temperature in the electrical properties of doped-SBT. The related results are summarized and described in the following sections. Vanadium-doped strontium bismuth tantalate Sr0.8Bi2.3(Ta2-xVx)O9 (SBTV) and titanium-doped strontium bismuth tantalate Sr0.75Bi2.35(Ta2-xTix)O9 (SBTT) have been prepared on Pt/Ti/SiO2/Si substrates to form MFM capacitors. The effects of V and Ti substitution on the microstructure evolution and ferroelectric properties of Sr0.75Bi2.35Ta2O9 films were investigated. The maximum remanent polarizations (2Pr) of the SBTV (x=0.15) and SBTT (x=0.2) films were 30.5 and 26.3μC/cm2 both larger than the un-doped SBT films annealed at 800oC at an electrical field of 150 kV/cm. Furthermore, the SBTV(x=0.15) and SBTT(x=0.2) can effectively reduce the leakage current density to 5 x 10-8 A/cm2 and as 2 x 10-8 A/cm2 at an applied electric field of 100 kV/cm. The substitution of limited vanadium(x=0.2) and titanium(x=0.2) for tantalum does not cause appreciable fatigue. In addition, the annealing temperature dependent integrity of Sr0.8Bi2Ta2O9 on ultra-thin 4nm SiO2 and Al2O3 buffered Si to form Metal-ferroelectric-insulator- semiconductor (MFIS) capacitors was investigated. Although the capacitance-voltage characteristics show hysteresis loops in both cases, the memory window of Sr0.8Bi2Ta2O9/Al2O3 capacitor is larger than that of Sr0.8Bi2Ta2O9/SiO2 capacitor. As increasing annealing temperature from 800 to 900oC, the memory window increase but the leakage current in both structures becomes smaller. Sr0.8Bi2+xTa2O9 thin films with x=0–0.8 were prepared to investigate annealing temperature effect, especially at high temperature, on physical characteristics and electrical properties of SBT films on Al2O3 (10 nm)/Si and ZrO2/Si. Depending on the physical characteristics and thickness of the insulator, the SBT film on ultra-thin Al2O3(10 nm)/Si shows the good memory properties and similar leakage current as compared with those of SBT/ZrO2(10 nm)/Si with increasing annealing temperature. Furthermore, excess Bi is also helpful for further reducing leakage current. Moreover, Sr0.8Bi2Ta2O9 ferroelectric film constructed on Al2O3/Si was prepared to study the degradation behavior of SBT films under forming gas annealing (FGA). Although the diffusion of hydrogen ions leads to the rapid decrease in the dielectric constant has been detected during FGA treatment, but shows no apparent influence on the memory window of Sr0.8Bi2Ta2O9/Al2O3/Si capacitors. It was found that FGA shows positive effect on the leakage current of MFIS in contrast to that of metal/ferroelectric/metal (MFM) structure. Finally, the memory effects of SBT/Al2O3 one transistor memory have been studied. The ferroelectric effect improves as increasing Al2O3 thickness from 40 Å to 65 Å and decreasing the leakage current. However, the SBT/Al2O3 MOSFET memory under charge injection mode has a large memorized ION/IOFF of >2 orders of magnitude even at a 10 □m gate length, while this device under ferroelectric mode has fast switching time of ~ 5 □s.