準分子雷射矽結晶化機制及複晶矽薄膜電晶體應用之研究

Abstract

本研究探討了半高斯波(Semi-Gaussian)準分子雷射應用於矽結晶化。半高斯波準分子雷射單擊(Single shot)實驗發現，結晶化區域的晶粒尺寸因能量分佈而有差異；雷射能量對複晶矽晶粒尺寸的關係，基本上可區分為低能量因部份熔化造成的細晶粒區、適當能量之近完全熔化的側向經成長大晶粒區，以及高能量均質成核的細結晶區；多擊(Multiple shot)實驗發現，因最高雷射能量區落在前次雷射束造成的小晶粒複晶矽區，致使最大晶粒小於單擊試驗產生的晶粒。 本研究也探討了基板溫度效應對非晶矽雷射退火結晶化特性及薄膜結構的影響。實驗結果發現，在均質成核發生前，基板溫度的增加有助於結晶度的提升，獲得最大結晶度的雷射能量密度隨基板溫度的升高向低能量區偏移，最大晶粒尺寸隨能量密度增加而增加；基板溫度升高會改變複晶矽薄膜的表面粗糙度，在部份熔化區，非晶矽結晶化將使表面粗糙度大幅增加，而且基板溫度越高粗操度越大，薄膜表面粗糙度在大型側向成長晶粒發生時快速下降，進一步增加雷射能量密度至均質成核區，薄膜表面粗糙度則沒有太大的改變。 為了改善雷射退火複晶矽薄膜晶粒尺寸差異大的問題，本研究採用了超高真空化學氣相沉積系統製備的直接沉積複晶矽薄膜進行雷射退火，不同於傳統上使用非晶矽當起始材料的情形，直接沉積複晶矽薄膜經低能量雷射退火呈現結晶度下降的結果，最大晶粒尺寸出現在部份熔化區，其最大尺寸小於以非晶矽進行雷射退火的結果，薄膜表面粗糙度儘輕微的減小，與非晶矽在大型側向成長晶粒發生時快速下降有所不同。 為了探討矽薄膜結晶度對雷射退火結果的影響，本研究亦以不同能量搭配的二次雷射退火對薄膜結構的影響，實驗結果顯示，低於起始能量密度的第一次雷射退火造成的小晶粒薄膜會在第二次高能量的雷射退火中熔化及再結晶，但高能量密度的第一次雷射退火造成的大型側向成長晶粒薄膜，在第二次高能量的雷射退火中會造成晶界處產生小晶粒群凸出，此特性亦反應在其元件特性，低能量與高能量的兩次雷射退火製備的薄膜電晶體有較好的電子遷移率與較小的次臨界波動移，但高能量雷射退火後的複晶薄膜再經第二次雷射退火，其電子遷移率與次臨界波動則會造成負面影響。

In this study, the crystallization of a-Si with semi-Gaussian excimer laser was investigated. After the single-shot excimer laser process, the poly-Si region showed grains with a wide range of sizes corresponding to the Gaussian distributed laser energy. From the view of laser energy, three crystallization regimes were found on the ELA a-Si films: (1) partial-melting, (2) near-complete-melting and (3) complete-melting regimes. Large super-lateral-grain-growth (SLG) grains were observed in the near-complete-melting regime. The grain size of poly-Si film using multiple shot laser annealing was constrained by the Gaussein distributed laser energy. The large grains were suppressed due to the small grains formed in the first shot. In addition, the influence of substrate temperature on the properties of polysilicon films prepared by excimer laser annealing was studied. As the substrate temperature was elevated, the maximum crystallinity and grain size increased, while the laser energy needed to obtain the maximum crystallinity of polysilicon films decreased. The elevated substrate temperature also changed the surface roughness of polysilicon films. In the partially melting regime, the surface roughness increased with laser energy and substrate temperature. The surface roughness dropped pronouncedly before reaching the super-lateral-grain-growth regime. Further increasing energy to homogeneous nucleation regime did not change much of the surface roughness. Furthermore, the influence of laser energy on the properties of excimer-laser-annealed (ELA) amorphous silicon (a-Si) and as-deposited polycrystalline silicon (poly-Si) films has been studied too. For the ELA poly-Si films, in the low energy region, the crystallinity decreased with the energy. After reaching the minimum, it increased to the maximum, and then dropped down. No SLG grains were found in the near-complete-melting regime. The largest grains were observed in the partial-melting regime. The largest grain size (100 nm) of ELA poly-Si was less than that of ELA a-Si (130 nm). Finally, the effects of energy on the microstructure of amorphous silicon (a-Si) films annealed by two-step laser process were systematically investigated. For the low-crystallinity / small-grain films, which were formed after the first low-energy laser crystallization, the grain size decreased and then increased with the energy of second laser annealing. In contrast, for the high-crystallinity films, i.e. SLG-grain films, the grain size monotonously decreased with second laser energy increased. Two-step laser annealed poly-Si films revealed that fine grains were formed and extruded at the grain boundary after the second high-energy laser annealing. High performance poly-Si TFTs can be fabricated from the poly-Si films crystallized by low-energy annealing followed by second high-energy laser annealing. When the results were compared, the poly-Si TFT using the poly-Si film crystallized by single high-energy laser annealing showed poorer mobility and subthreshold swing.