非晶硅薄膜厚度及特性对准分子激光晶化的影响

Effects of Thickness and Characteristics of Amorphous Silicon Films on Excimer Laser Crystallization

目的增加准分子激光晶化最优能量密度的工艺窗口以及提高晶化后多晶硅晶粒尺寸的均匀性,并最终改善低温多晶硅薄膜晶体管(Low Temperature Poly Silicon Thin Film Transistor,LTPS TFT)的特性。方法采用PECVD技术在玻璃基板上沉积不同厚度及折射率的非晶硅薄膜(Amorphous Silicon Film)。利用高温退火炉脱氢后进行准分子激光退火(Excimer Laser Annealing,ELA),完成非晶硅到多晶硅的转变。通过扫描电镜、原子力显微镜对多晶硅晶粒尺寸以及表面粗糙度进行分析,后续完成薄膜晶体管后利用I-V测试机台对器件特性进行测试。结果随着非晶硅薄膜厚度的增加,准分子激光晶化的最优能量密度(Optimal Energy Density,OED)以及工艺窗口(OED Margin)均增加,当膜层厚度大于等于47 nm时,OED Margin均为25 mJ/cm^2。当膜厚为47 nm时,多晶硅晶粒尺寸均匀性为0.64,也处于较优的水平。非晶硅薄膜折射率为4.5时,形成的多晶硅晶粒尺寸均匀性为0.45,远优于折射率为4.38时的多晶硅晶粒尺寸均匀性。折射率为4.5的非晶硅薄膜形成未掺杂的LTPS TFT(PMOS)迁移率为120.6 cm^2/(V?s),阈值电压为-1.4 V,关态电流为53 pA;折射率为4.38时的迁移率为112.4 cm^2/(V?s),阈值电压为-2.0 V,关态电流为71 pA。结论当非晶硅膜厚为47 nm时,准分子激光晶化的OED Margin以及晶化后的多晶硅晶粒尺寸均处于较优的水平。另外,提高非晶硅薄膜折射率同样有利于改善多晶硅晶粒尺寸均匀性以及薄膜晶体管转移特性。

The work aims to improve optimal energy density process windows of excimer laser crystallization and enhance grain size uniformity of polycrystalline silicon, and finally improve characteristics of low temperature poly silicon thin film transistor （LTPS TFT）. Amorphous silicon （a-Si） films of different thickness and refractive index were deposited on glass sub- strates by adopting PECVD technology. After dehydrogenation in high temperature annealing furnace, excimer laser annealing was performed to complete transition from amorphous silicon to polycrystalline silicon （P-Si）. Grain size and surface roughness of potycrystalline silicon were analyzed with scanning electron microscope and atomic force microscope. After completion of thin-film transistors, I-V testing machine was used to test device characteristics. Both the optimum energy density （OED） and process window （OED margin） of excimer laser crystallization increased with the increase of amorphous silicon thin film thick- ness. When the film thickness was ≥47 rim, OED margin was 25 mJ/cm^2. Provided with film thickness of 47 nm, grain size unitbnnity was 0.64 and at optimal level. When refractive index of amorphous silicon film was 4,5, the grain size uniformity of polycrystalline silicon was 0.45, which was much superior to that of polycrystalline silicon grain with refractive index of 4.38. For un-doped LTPS TFT（PMOS） made from amorphous silicon film with refractive index of 4.5, mobility ratio was 120.6 cm^2/（V.s）, threshold voltage -1.4 V, and off-state current 53 pA; for that made from amorphous silicon film with refractive in- dex of 4.38, mobility ratio was 112.4 cm^2/（V.s）, threshold voltage -2.0 V, and off-state current 71 pA. Provided with amorphous silicon film thickness of 47 nm, OED margin of excimer laser crystallization and grain size uniformity of crystallized p-Si are both at superior level. In addition, improvement of refractive index of amorphous silicon film is also beneficial to enhancement of P-Si grain size uniformity and LTPS TFT transfer characteristics.