半導體元件中介電材料的結構、電性、介電性質以及機械行為之理論分析與模擬研究

First Principles Modeling and Simulations of the Structural, Electronic, Dielectric, and Mechanical Properties of the High-κ and Low-κ Dielectric Materials

陳宗儒(Tsung-Ju Chen)

指導教授 ： 郭錦龍

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02048

摘要

近年來隨著積體電路(IC)不斷的微型化過程中，為了使元件能夠維持同樣的性能及正常運作，傳統IC中的介電層材料二氧化矽勢必面臨被新的介電材料所取代。 在一個IC元件中，介電材料主要被應用的地方有二: 一為在金屬氧化物半導體中做為閘極介電層，而另一則為銅導線之間的介電層絕緣材料。
對於閘極的介電層，目前採取的方式是以高介電常數材料來取代傳統的二氧化矽，主要是因為高介電常數材料可形成較厚的介電層，進而緩解因穿隧效應所產生的漏電流。在眾多高介電常數材料中，二氧化鉿(HfO2)目前是做為MOS元件閘極介電層材料。然而，由於二氧化鉿比二氧化矽容易結晶且常有較多的帶電缺陷存在，其效能常不及傳統的二氧化矽。如欲改善這些問題，則必須對介電材料本身的結構、電子、與介電性質、以及其缺陷的行為有更深入的瞭解。在本論文中，我們即藉由第一原理計算與分子動態模擬來了解參雜矽或鑭元素對二氧化鉿結構可能產生的影響，並進一步分析其與電子和介電性質之間的關連，從而增進我們對此一材料系統的瞭解。
針對矽酸鉿的系統，我們使用“置換−冷卻”方法來產生含有不同矽濃度的非晶質結構模型。根據這些結構模型，我們觀察到矽酸鉿的結構特徵在矽濃度高於50%與低於50%時會有顯著的不同。關於其電子性質，我們也觀察到矽酸鉿的能隙值在矽濃度低於50%時並不會隨結構中矽濃度的增加而有明顯的改變；然而當矽濃度高於50%時，其能隙值會大幅度地增加而呈現非線性的變化。在介電性質方面，我們的結果顯示結構中矽濃度的增加會導致矽酸鉿的靜介電常數值急速下降，因而使得整個過程呈現非線性的變化關係。在氧空位分析方面，我們計算的結果顯示矽酸鉿結構中氧空位的生成能主要決定於其局部鍵結型式，而與矽濃度的變化無顯著的關聯。我們的結果也顯示當矽進入結構後可使氧空位的生成能增加且傾向於呈電中性而不帶電，其將有益於減緩臨界電壓不穩定性以及增加通道上的載子遷移率。此外，在二氧化鉿結構中參雜矽亦可有利於減少漏電流的產生。
針對參雜鑭元素的系統，我們採用“置換−升溫−冷卻”的方法來產生不同含鑭濃度的非晶質Hf1-xLaxO2-0.5x結構模型。根據產生的結構模型，我們發現其密度會隨鑭濃度的增加而呈線性下降的趨勢；其陽離子(鉿和鑭元素)的配位數均會隨鑭濃度的增加而下降，但是氧原子的配位數卻會隨鑭濃度的增加而增加，且在鑭濃度大於50%時，氧原子配位數的分布會與二氧化鉿有明顯的不同。在電子性質方面，我們計算的結果顯示Hf1-xLaxO2-0.5x結構的能隙值會隨鑭濃度的增加而稍微下降，主要是因為傳導能帶的能階往下移所造成的緣故。此外，我們的結果亦說明了實驗中觀察到XPS光譜隨鑭濃度變化所產生的光譜位移乃是因鑭參雜導致組成原子間電荷轉移程度改變所致。在介電性質方面，我們的結果顯示微量的鑭參雜(< 25% )不僅能使二氧化鉿的結晶溫度提高，亦能夠有效提高材料本身 的介電常數；然而，在含鑭濃度較高(25% ~ 50%)的系統，其介電常數並不會隨組成濃度的變化而有明顯的改變，此與實驗中所觀察到的結果一致。在氧空位分析方面，平均氧空位的生成能僅與局部結構氧原子的鍵結型式有關，而與鑭濃度的變化無顯著關聯。另外，與參雜矽元素的系統相反的是含鑭結構中的氧空缺其生成能會較二氧化鉿為低且傾向於帶正電，這意味著參雜過多的鑭元素將有可能造成臨界電壓不穩定性以及平帶電壓位移的現象發生。
對於銅導線間的介電絕緣層材料，目前半導體業界是以降低其介電常數來減少元件因微形化所產生之過多寄生電容，進而減緩其相關之相移電路延遲行為。在眾多低介電常數材料中，有機矽酸鹽玻璃已被認為是最有希望作為下一代銅導線間之介電絕緣層材料，其主要是利用具低極性之有機分子來取代二氧化矽結構中之氧原子來降低材料的離子極化程度以及增加其內部的孔隙率，欲藉此以降低材料本身之介電常數。然而，在此同時也可能影響了絕緣材料的機械強度。目前關於有機分子如何影響矽酸鹽玻璃材料的機械強度仍未有定論，文獻中相關研究的結果仍充滿歧異。因此，我們希望藉由理論計算的分析來闡述有機矽酸鹽玻璃材料的結構與機械性質間之相關性，進而提供一材料設計上之方向。本論文研究是以“置換−回火”的方式來產生含有不同碳濃度的甲基橋以及乙基橋有機矽酸鹽玻璃結構模型。我們的結果顯示此兩種有機矽酸鹽玻璃材料之密度均會隨碳橋濃度的增加而呈現下降之趨勢，但是其孔隙率卻有可能隨碳橋濃度的增加而有不同的變化。在剛性分析方面，兩者的彈性模數均會隨碳橋濃度的增加而呈現下降之趨勢，而我們對於其破裂結構之表面能計算亦呈現相同的變化傾向。倘若根據以上計算的結果以Griffith的脆性破裂理論預測，二氧化矽比兩種有機矽酸鹽玻璃材料均會有較好之機械強度。然而，我們對各種材料進行拉升試驗模擬的結果顯示，雖然甲基橋有機矽酸鹽之材料破裂能仍舊較二氧化矽為低，乙基橋有機矽酸鹽在拉升過程中卻有可能藉由塑性變形與斷鍊重組而增加其結構韌性，因而使得其材料破壞能高過於二氧化矽與甲基橋有機矽酸鹽。此外，本研究亦分析了二氧化矽與有機矽酸鹽玻璃材料之破裂表面可能發生之結構重組。此分析研究將有助於未來了解有機矽酸鹽玻璃經電漿處理後內部可能產生之缺陷結構。

關鍵字

電子元件之介電材料 ； 矽參雜二氧化鉿 ； 鑭參雜二氧化鉿 ； 有機-無機二氧化矽玻璃材料 ； 第一原理計算

並列摘要

With the continued down-scaling of the integrated circuits, the conventional dielectric oxide, SiO2, have been replaced by new dielectric materials, such as the high-κ oxides in CMOS transistors to avoid excess leakage current and the low-κ silicate insulating layers in-between copper wires to reduce the interconnect capacitances. To further control or improve their performance, a detailed understanding of the structural, electronic, dielectric, and the mechanical properties of these new dielectric materials are strongly in demand. In this thesis, we have employed first-principles density functional theory calculations to investigate the important properties of these new dielectric oxide materials in many fundamental aspects.
For gate dielectrics in CMOS transistors, we have investigated the he structural, electronic, and dielectric properties of the Si- and La-doped HfO2, as well as the O vacancy formation and the relevant induced defect states in band gaps over a wide range of chemical compositions. For Si-doped HfO2, i.e. Hf-silicates (a-Hf1-xSixO2), our results show that the Si-rich Hf-silicates possess distinct structural characteristics from the Hf-rich ones and the electronic band gaps were found to vary nonlinearly with the Si content. Different from the previous theoretical studies, our results support a nonlinear dependence of the dielectric constants on the composition of silicates, which is mainly attributed to the rapid reduction of the low-frequency vibrational motions of Hf atoms with increasing the Si content. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in amorphous HfO2 and in Hf-silicates, respectively. Our calculations showed that the formation energies of O vacancy and the positions of the induced defect states in the band gap are largely dependent on the local structures of the vacancy site rather than on the compositions of Hf-silicates. Considering the measured valence band offset between Si and Hf-silicates, a considerable amount of O vacancies are likely to stay in the charge neutral state in Hf-silicates when the Fermi level lies in the band gap region of Si. Furthermore, the concentration of O vacancy in Hf-silicates was found to be much lower than that in HfO2 when the Fermi level lies in/below the mid-gap region of Si. Consequently, the flat band voltage shift and the transient threshold voltage instability can be significantly reduced in Hf-silicates in comparison to that in HfO2, indicating that the presence of a Hf-silicate layer in between the gate oxides and the Si substrate could be beneficial to the performance of the CMOS device.
For the La-doped HfO2 (a-Hf1-xLaxO2-0.5x, x = 0 ~ 0.5), the mass density was found to decrease linearly with the La content, but the electronic band gap only reduces slightly by 0.29 eV as the La content increases from x = 0 to 0.5. Our Bader charge analysis shows that the average atomic charge on Hf (La) atoms can decrease from 2.31 (2.12) to 2.26 (2.06) as the La content increases from 0 to 0.5, which indicates that the ionicity of the Hf (La) atom tends to decrease as the La content increases. This results is consistent with our calculations for the Hf 4f core level energy, which reveal a binding energy shift of around 0.3 ~ 0.6 eV as the La content increases from 0 to 0.5, in good agreement with a recent experimental measurement. Our calculations also show that the dielectric constant of a-Hf1-xLaxO2-0.5x increases rapidly with the La content from x = 0 to 0.25 but it appears to be nearly unchanged as x ranges between 0.25 and 0.5, which is consistent with the recent experiments. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in a-Hf1-xLaxO2-0.5x. Similar to Hf-silicates, the formation energies are largely dependent on the local bonding configurations rather than on the compositions. However, O vacancy was found to be more likely to stay in positively charged states with lower formation energies than that in HfO2, implying that the flat band voltage shift and the threshold voltage instability could be enhanced due to La-doping. Furthermore, the trap energy level was found to decrease gradually with increasing the La contents, which indicates that the trap-assisted conduction can become more seriously in a-Hf1-xLaxO2-0.5x.
For the interlayer insulating dielectrics, we have investigated the structure and mechanical properties of the methyl- and ethyl-bridged organosilicate hybrid glasses (OSG) using first-principles calculations. Our results show that the mass density of OSG can gradually decrease with increasing the carbon content, but the porosity may not necessarily increase with carbon-bridging units in the amorphous bond network. Our results also reveal that the elastic moduli and fractured surface energies of OSG models tend to decrease with increasing the alkylene-bridged units in the silicate bond network, which imply that the crack-resistance of the alkylene-bridged OSG can be much worse than fused silica based on the Griffith theory of brittle fracture. According to the calculations of decohesive energy, the mechanical strength of a-SiO2 still outperforms the methyl-bridged OSG models, indicating that the crack-resistance of these two materials is largely determined by the chemical bond strength. However, the decohesion energy of the ethyl-bridged OSG models turns out to be higher than a-SiO2 though their critical stress for crack propagation predicted by Griffith theory is simply half of that in a-SiO2. Our results further show that the enhanced fracture toughness in the ethyl-bridged OSG model can be mainly attributed to the plastic deformation and bonding reconfiguration induced by the ethyl-bridged units during fracture, which was not observed for the methyl-bridged OSG models and a-SiO2.

並列關鍵字

gate dielectric ； interlayer dielectric ； Si-doped HfO2 ； La-doped HfO2 ； alkylene-bridged organosilicate hybrid glasses ； first-principles calculations

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