以準分子雷射結晶方法製作低溫複晶矽鍺薄膜電晶體之研究

Abstract

本研究針對利用準分子雷射結晶法製備低溫複晶矽鍺薄膜的機制做深入的探討，並配合製程流程的設計與改良，製作出高性能之低溫複晶矽鍺薄膜電晶體。 低溫複晶矽薄膜電晶體已被廣泛的應用在高密度記憶體以及高效能顯示器上。為了配合高速電路的運作，薄膜電晶體必須具備有更高的載子移動率。基於高效能元件的需求，準分子雷射已被普遍的研究於如何製備出具有良好結晶性的高品質複晶矽薄膜。另一方面，為了突破製程技術上的限制，新的材料也不斷的被提出;目前，由於複晶矽鍺擁有相當高的載子移動率，故被視為最有可能取代複晶矽薄膜做為電晶體主動區的材料。 複晶矽鍺薄膜在高效能的半導體元件上已有相當多方面的應用。近幾年來，由於具備可降低製程熱預算及提高載子移動率的優點，複晶矽鍺薄膜電晶體引起相當程度的注意。目前已有多種製備複晶矽鍺薄膜電晶體的技術被提出，包括固相再結晶法及快速熱退火法…等。然而，利用準分子雷射技術製作複晶矽鍺薄膜電晶體確鮮少被研究。 本論文首先針對非晶矽鍺薄膜的沉積以及準分子雷射結晶法的機制做深入的研究。實驗結果發現，由於鍺具有催化的效應，故相較於非晶矽薄膜沉積，利用低壓化學氣相沉積法可於較低的溫度下沉積非晶矽鍺薄膜。且隨著GeH4/SiH4的氣流比例增加，薄膜沉積的速率將提高，而薄膜由非晶相轉換成複晶相的溫度則會下降，如此一來，便可降低沉積製程的熱預算。 本研究利用準分子雷射對非晶矽鍺薄膜進行再結晶，藉由多種材料分析的技術可以發現，非晶矽鍺薄膜可於準分子雷射的照射下達到相當程度的結晶 ; 然而，由於矽跟鍺兩種元素熔點上的差異，造成在利用準分子雷射結晶的同時發生了鍺偏析的現象。此外，相較於傳統利用準分子雷射結晶法所製備的複晶矽薄膜，複晶矽鍺薄膜經準分子雷射結晶後呈現較差的結晶性。因此，利用準分子雷射直接對非晶矽鍺薄膜進行再結晶時，將會遭遇製程上所引起的問題 (如:鍺偏析，結晶性差..等)，進而降低元件的電特性。 為了避免利用準分子雷射對矽鍺薄膜直接進行再結晶時所遭遇到的製程問題，我們提出兩種製程改善方法來製備高效能的複晶矽鍺薄膜電晶體。為了降低鍺偏析的效應，我們於經準分子雷射接晶後的複晶矽鍺薄膜表面再覆蓋上一層矽薄膜，接著再進行第二次準分子雷射的再結晶，如此一來，將可以有效的降低鍺偏析的情形，進而改善元件的特性。然而，受限於複晶矽鍺的本身較差的結晶性，元件的電特性表現仍然不符合需求。 因此，我們進一步提出了利用準分子雷射摻雜鍺之複晶矽鍺薄膜電晶體來達到高效能元件的需求。論文中，我們提出了兩種機制來描述此種元件的電特性。由於主動區域具有良好的結晶性加上藉由鍺摻雜所提升的載子移動率，利用準分子雷射摻雜鍺之複晶矽鍺薄膜電晶體於小尺寸元件上呈現出相當優秀的元件特性。而相較於利用準分子雷射製備的複晶矽薄膜電晶體，利用準分子雷射摻雜鍺之複晶矽鍺薄膜電晶體於載子移動率及驅動電流上分別提升了41%及52%。故藉由本論文中所提出的製程改善，利用準分子雷射結晶法可製備具有高效能的低溫複晶矽鍺薄膜電晶體，並符合未來製程技術的發展及高效能元件的需求。

Low-temperature poly-Si thin-film transistors (LTPS TFTs) have been used in various applications including high-density memories and high-performance displays. TFTs with high carrier mobility are required for high-speed circuit operation. To fabricate devices with high carrier mobility, excimer laser crystallization (ELC) technology was extensively studied to create high-quality poly-Si films with large sized grain and good crystallinity. On the other hand, new materials were also widely investigated to overcome the existing technology limitations. At present, polycrystalline silicon-germanium (poly-Si1-xGex) is an excellent candidate as an alternative to poly-Si for the channel active layer due to the high carrier mobility of Ge atom. Poly-Si1-xGex films have been used in various applications for high-performance semiconductor devices. In recent years, low-temperature poly-Si1-xGex TFTs has attracted many attentions due to the lower process thermal budget and carrier mobility enhancement. Several crystallization technologies including solid phase crystallization (SPC) and rapid thermal annealing (RTA) have been reported to fabricate poly-Si1-xGex TFTs. However, little investigation has been done on poly-Si1-xGex TFTs fabricated by excimer laser crystallization. In this thesis, we first explored the mechanisms of both deposition and excimer laser crystallization of amorphous Si1-xGex thin films. The experiment results demonstrate that a-Si1-xGex thin films could be deposited at lower temperature by LPCVD than those of a-Si thin films. As the GeH4 to SiH4 gas flow ratio increases, deposition rate of a-Si1-xGex thin films increases while the transition temperature for amorphous-to-polycrystalline deposition decreases. The reduced thermal budget of deposition process could be attributed the catalytic effect by Ge incorporation. Excimer laser irradiation was performed to crystallize the a-Si1-xGex films in this study. Physical characterizations results indicate that a-Si1-xGex can be effectively crystallized by excimer laser irradiation. However, due to the difference of the melting point of Si and Ge atoms, Ge segregation occurs during ELC process. Furthermore, ELC poly-Si1-xGex films exhibit worse crystallinity in comparison with ELC poly-Si films. As the result, TFTs fabricated by direct laser crystallization of a-Si1-xGex thin film would suffer from these deteriorated process issues, resulting in poor device performance. To avoid the process-related issues of ELC poly-Si1-xGex films, we proposed two novel modified ELC processes to fabricate high-performance poly-Si1-xGex TFTs. The ELC poly-Si1-xGex TFTs with a Si capped layer were introduced to alleviate Ge segregation during ELC process. Although the device performances were improved by introducing a Si capping layer, the worse crystallinity of ELC poly-Si1-xGex active layer still limited the electrical properties of the devices. Therefore, a novel Ge-doped ELC poly-Si1-xGex TFT with lower Ge concentration and better crystallinity in the active layer was proposed to manufacture high-performance devices. Two competing mechanism were established to illustrated the electrical characteristics of Ge-doped ELC poly-Si1-xGex TFTs. Since the good crystallinity and carrier mobility enhancement by Ge incorporation, Ge-doped ELC poly-Si1-xGex TFTs exhibit excellent carrier mobility and high driving current in short channel devices. The mobility and drain current of the Ge-doped ELC poly-Si0.91Ge0.09 TFTs with W/L=2μm/2μm were enhanced by 41% and 52% than those of the conventional ELC poly-Si TFTs. By using the modified process proposed in this thesis, novel Ge-doped ELC poly-Si1-xGex TFTs demonstrate excellent performance in short channel devices that would meet the requirements for the trends of low-temperature polycrystalline TFT technology developments.