金屬-氧化鈦鉿-金屬電容於動態記憶體與射頻電路之應用

Abstract

本篇論文中，我們採用高介電常數的二氧化鈦(TiO2)，摻雜具有較大導帶不連續(conduction band offset)和能帶寬度的二氧化鉿(HfO2)，混合而成的鈦化鉿(HfTiO)材料作為金屬-氧化物-金屬(MIM)電容的介電質，來改善金氧金電容特性，以達到動態記憶體以及射頻電路的需求。本論文重點在比較上電極的效應、和介電層組成相關的漏電機制以及應力效應。在Pt/HfTiO/TaN 金氧金電容中，我們得到一個高電容密度約17.5 fF/□m2而所對應到的k值為37，且在電壓-1伏特下得到很低的漏電約3.4x10-8 A/cm2。和Al電極相比，高功函數的Pt電極不只改善漏電，同時可以改良類比特性，像是電壓電容係數(VCC)以及溫度電容係數(TCC)。除此之外，不同的Hf含量對HfTiO介電層的特性改善亦有研究。實驗數據顯示，增加Hf含量從35%至48%可以抑制漏電在電壓-3伏特下約1.5個等級。在改良漏電方面，我們成功發展出氮氣電漿處理HfTiO介電層上的方法，在電壓-3伏特下可以有效降低約兩個等級大小的漏電，同時維持電容密度以及VCC特性。 為了找出降低VCC的方法，深入了解VCC的基本機制是必須的。由於影響VCC特性的原因很多，在此，我們僅針對介電質的厚度效應、介電質成份組成以及堆疊式金氧金結構(laminate MIM structure)三個方向來探討。實驗結果顯示增加Hf的比例、增加介電層的厚度均可降低VCC。然而，使用HfTiO/Y2O3(三氧化二釔)堆疊式電容亦可達到降低VCC的效果，這是由於Y2O3具有負的VCC-α可以扺消具有正VCC-α的HfTiO，進而降低了VCC。 為了要達到高頻電容的需求，我們成功製造出厚度51奈米的HfTiO金氧金電容，電容密度為5.1 fF/um2，且在電壓-1伏特下的漏電為1.3x10-9 A/cm2。此外，在頻率100 kHz下的VCC-α值約40 ppm/V2，此一電容特性已符合2012 ITRS的規格。 最後，我們探討金氧金電容在定電壓應力測試(constant voltage stress)下，基本電性以及電壓電容係數特性的變化。同時，可以觀察到電荷捕捉以及電荷釋放的現象以及小於1%的電容密度變化。使用Pt電極不僅可以降低漏電，同時可以抑制應力產生漏電(stress-induced leakage current)和電容密度變化，並得到較好的VCC特性以確保長時間下的可靠度。

In this thesis, we adopted a hafnium titanate (HfTiO) film as MIM dielectric, which dopes HfO2 with large conduction band offset and wide bandgap into high-k TiO2 (k~50-80) to improve the performance of MIM capacitors for DRAM/RF applications. The attention of the thesis is focused on the effect of electrodes, conduction mechanism, film composition, and stress behavior. Low leakage current of 3.4x10-8 A/cm2 at -1 V and high capacitance density of 17.5 fF/um2, which reflects a dielectric constant of 37, were obtained in a Pt/HfTiO/TaN MIM capacitor. Compared with Al top electrode, using high-work-function and thermodynamically stable Pt metal not only reduces the leakage current largely but also modify the analog characteristics (ex: VCC and TCC). Besides, the effect of Hf content on the performance of HfTiO dielectric was investigated. The experimental results indicate that the leakage current can be suppressed by nearly 1.5 orders of magnitude at gate bias of -3 V as Hf content increase from 35% to 48%. We developed successfully a N2 plasma treatment on HfTiO dielectrics to further lower leakage current by two orders of magnitude at -3 V and no apparent degradation is observed on the capacitance and VCC properties. To find out the solutions for reducing voltage nonlinearity, thickness effect, film composition, and laminate MIM structure are investigated. From the experimental results, we found that the VCC properties can be reduced effectively as the increase of Hf content and thickness of HfTiO dielectrics. Similar effects also can be achieved by using a HfTiO/Y2O3 laminate, which adding Y2O3 results in a drop in VCC-α because of canceling out the effect of positive VCC-α due to with the negative VCC-α□of Y2O3. To meet the requirement of a RF capacitor, a 51 nm-thick Pt/HfTiO/TaN capacitor was fabricated successfully, which a capacitance density of 5.1 fF/□m2 , low leakage current of 1.3x10-9 A/cm2 at -1 V and very small VCC-α value of 40 ppm/V2 at 100 KHz were obtained to achieve the goals of 2012 ITRS. Finally, we investigate the stress behavior of [Pt or Al]/HfTiO/TaN MIM capacitors on electrical properties and VCC characteristics under constant voltage stress (CVS). Charge trapping and trap generation phenomena are observed on the HfTiO MIM capacitors under CVS stress. Capacitance variation less than 1% can be achieved. The use of Pt electrode not only reduces the leakage current but also can suppress the stress-induced leakage current and capacitance variation to get a better VCC □for long-term reliability.