微波處理於異質磊晶鍺元件之研究

Abstract

本論文研究將鍺(Ge)作為場效電晶體之通道材料，研究使用微波退火於閘極堆疊、合金材料性質及場效電晶體之特性探討。 閘極結構方面，使用多種微波氧化退火條件研究微波於鍺基材之氧化能力、介面特性及製作電容結構分析其電特性。我們使用微波氧化退火(microwave thermal oxidation) 於鍺基板上形成二氧化鍺(GeO2)薄膜，降低介面能態密度，隨後堆疊三氧化二鋁(Al2O3) 製作出 TiN/Al2O3/GeO2/n-Ge 結構。使用電導法(conductance method)萃取，能隙中央之介面能態密度約為 1.2 × 1012 cm-2-eV-1。而 TiN/Al2O3/GeO2/n-Ge 之電容結構，可以更進一步調整製程細節，藉由成長二氧化鍺與三氧化二鋁後再進行後微波氧化退火(postdeposition microwave thermal oxidation)，電導法萃取出之介面能態密度約為6.7 × 1011 cm-2-eV-1，改善幅度為51 %。由材料分析結果可以得知，透過X 射線光電子能譜(X-ray photoelectron spectroscopy, XPS)分析確認，隨著氧化功率與時間的增加，二氧化鍺能有效形成。 接續討論使用低溫微波退火形成金屬合金於量子井-矽鍺矽基板(Quantum well Si/Ge/n-Si substrate)結構上，製作蕭特基接面二極體與場效電晶體。為了比較傳統快速升溫熱退火(RTA)與微波退火(MWA)元件表現，由二極體電性觀察，隨著退火溫度或瓦數的增加，二極體逆偏電流的電壓相依性會隨之增加。使用低溫微波退火之電特性表現上優於傳統快速升溫熱退火，二極體之逆偏電流、蕭特基能障、理想因子及等效串聯電阻均表現甚佳。以二極體電性之探討為基礎，將之引入量子井結構製作出蕭特基場效電晶體，利用微波退火製作出之元件電性表現也優於傳統快速升溫熱退火製作之元件。由X 光繞射能譜(X-ray diffraction, XRD)之結果判斷，微波退火所製作出的合金比快速升溫熱退火所製作出之合金相對均勻，進一步驗證低溫微波退火能有效形成晶相均勻之合金，同時也能改善蕭特基介面特性的電性表現。 針對微波退火製作出的合金做探討，我們將之與摻雜偏析技術整合於p 型鍺基板上。首先，調變微波退火瓦數探討合金的材料變異性，由原子力顯微鏡技術(Atomic force microscopy)觀察可知，雖然退火溫度相較於傳統快速升溫熱退火的製程溫度低，但隨著微波退火瓦數的增加，表面粗糙度仍會隨之逐漸劣化；進一步由X 光繞射能譜(X-ray diffraction, XRD)分析，由晶體訊號之半高寬萃取晶粒大小，驗證表面粗糙度劣化的原因源自晶粒的改變。了解微波退火於製作合金之材料特性變化後，我們搭配摻雜偏析於二極體及n 型鍺場效電晶體探討電性變化，為了比較不同退火技術間的差異，一致使用微波退火於鍺基板上形成NiGe 合金後，接著打入磷原子，分別使用微波退火與快速升溫熱退火進行摻雜偏析載子活化，實驗結果顯示微波退火除了能抑制摻雜的擴散，也能改善金屬原子擴散，進而優化二極體與場效電晶體之特性。 了解微波退火對於合金的材料基本特性與元件電性的變化後，著手引入結構工程探討金屬與合金的轉變現象對元件造成的影響。我們於n 型鍺基板上依序堆疊白金(Pt)、鎳(Ni)，改變白金的厚度並使用微波退火，由實驗結果發現，對比過去文獻中所探討的三元合金NiGePt，此晶相可以藉由低溫之微波退火形成。因此我們更進一步，針對白金層厚度的改變，觀察退火後合金間的差異，從穿透式電子顯微鏡(transmission electron microscopy)影像可以發現，隨著白金層的厚度增加，合金分層效應(alloy-separation phenomenon)越來越明顯，形成雙層合金層於鍺基板上。同時也比較快速升溫熱退火與微波退火製作出的合金差異，我們發現相同退火時間與溫度下，微波退火能於相對低溫的製程條件，能有效的形成品質較為均勻的合金，而快速升溫熱退火製作出的合金表現相對粗糙。針對微波退火製作出的合金，我們交叉比對材料分析與電性分析，利用奈米電子束繞射技術(nano-beam diffraction pattern)分析材料雙層合金的成分與結構，結果發現下層合金為固溶態NiGe(Pt)、上層合金為固溶態PtGe(Ni)。由電性部分觀察出分層越明顯的結構下，電性表現也相對優異，隨著白金層厚度增加，能有效改善二極體逆偏漏電流，等效串聯電阻也可有效降低。而蕭特基場效電晶體的表現，隨著白金厚度的增加製作之蕭特基金屬接觸，能使電性有明顯的改善。此外，白金層引入的結構工程，能有效抑制鎳金屬的擴散，改善元件特性。 基於使用微波退火應用於閘極堆疊與合金製作之相關元件經驗，我們整合鍺薄膜於矽基板並製作立體式短通道元件。我們使用全微波退火技術製作出無接面場效電晶體，並引入合金結構工程，探討合金分層效應於場效電晶體之表現。由實驗之n 型及p 型無接面場效電晶體之電特性觀察可知，合金分層效應可以有效降低源汲極電阻進而改善元件驅動電流。

In this dissertation, we have investigated the feasibility in using Ge as channel material of CMOS devices, studying the gate stacks, alloys properties, and characteristics of MOSFET with using treatment of microwave annealing. On structure of gate stacks, various conditions of microwave thermal oxidation was utilized to investigate the ability of oxidation, properties of interface, and the electrical characteristics of MOSCaps. We used the microwave thermal oxidation to form GeO2 on n-Ge substrate to reduce the density of states at the interface, and deposited Al2O3 to perform the structure of gate stack (TiN/Al2O3/GeO2/n-Ge). The density of interface states (Dit) was 1.2 × 1012 cm-2-eV-1 near the midgap extracted through conductance method. To discuss further with adjusting the processes of fabrication, after depositing GeO2 and Al2O3, the post-deposition microwave thermal oxidation (PDMTO) was introduced to improve the interface quality. The Dit value was about 6.7 × 1011 cm-2-eV-1 near the midgap, which was around 51% improvement as compared with that without PDMTO treatment. Furthermore, from the results of X-ray photoelectron spectroscopy (XPS), the GeO2 could be formed with increasing the annealing power and time of microwave. Subsequently, alloy formation through technique of microwave annealing was carried out to investigate the Schottky junction and pMOSFET on the starting wafer of Ge quantum well (Si/Ge/n-Si) substrate. Comparing the treatments of RTA and MWA, the diode characteristics performed that the off-leakage currents were degraded as increasing the process temperature of RTA or annealing power of MWA. However, the performance with using MWA exhibited better Schottky barrier height, ideality factor, and effective series resistance. Based on the results of Schottky junction, the processes were introduced to fabricate Ge quantum well pMOSFET with Schottky S/D. The electrical performance of Schottky pMOSFET forming through MWA was superior to RTA. The XRD spectrum performed that the crystalline with using MWA was relatively uniform as compared with RTA, which further proved that low temperature annealing of MWA could form the uniform quality of alloy and improve the electrical characteristics of Schottky junction/pMOSFET. Focusing on the discussion of microwave-annealed alloys, we integrated dopant segregation technique through microwave annealing on p-Ge substrate. First, modulation of MWA annealing powers were discussed with material analyses. The result of atomic force microscopy (AFM) showed that the surface roughness would degrade with increasing the annealing power of MWA even the annealing temperature was lower than the traditional RTA. Furthermore, according to the analysis of XRD, the grain size could be extracted from the full width at half maximum of XRD spectrum, which proved that the surface degradation was caused by the change of grain size. With the experiences of alloy properties formed through MWA, it was introduced to the diode and nMOSFET for further comparing the electrical characteristics between MWA and RTA. After forming alloy through MWA, the dopants were implanted and activated with using MWA and RTA, respectively. The experimental results exhibited that MWA could improve the dopant and metal diffusion for further optimizing the electrical characteristics. Based on the fundamental material and electrical results related with MWA processes, structure engineering was introduced for studying the transformation of alloys. We deposited various thicknesses of Pt and Ni on the n-Ge substrate for forming alloys. It was observed that the ternary-phase alloy could be formed with using low-temperature MWA, which was different from previous results. To further study the differences of alloy, with varying the Pt thickness, alloy-separation phenomenon will become obvious as increasing Pt thickness to form bilayer alloy on Ge substrate. Subsequently, after comparing the results of alloys forming through MWA and RTA, we observed that MWA could form the more uniform and smooth alloys than RTA with the same process temperature and time. To make a detail discussion of microwave-annealed alloys, the cross comparisons of material and electrical analyses were demonstrated. In material part, nano-beam diffraction pattern was utilized to analyze the bilayer alloy, which differentiated that top and bottom layers were PtGe(Ni) and NiGe(Pt), respectively; In electrical part, the off-leakage of diode could be improved as increasing the thickness of Pt, and the introduction of Pt could further inhibit the Ni diffusion for achieving better performance. Finally, based on the previous experiences of fabricating gate stacks and metal alloys, integrating Ge thin film on Si substrate and performing Ge junctionless MOSFETs have been demonstrated with full microwave annealing processes. Considering the advantage of alloy-separation phenomenon with introduction of structure engineering, bilayer alloy is integrated to the MOSFET as metal S/D for enhancing the driving current.