臭氧前處理及雷射退火對多晶矽特性之研究

Abstract

在低溫多晶矽薄膜電晶體(low temperature poly silicon thin film transistors, 通稱LTPS TFT )的製造技術中，準分子雷射退火扮演舉足輕重的角色，由於非晶矽(amorphous silicon ,a-Si)薄膜對紫外光具有超高吸收率之特性，屬紫外波長輸出的準分子雷射便提供了絕佳的製程方式，在低溫600°C的製程環境下，讓基板上之非晶矽薄膜進行回火結晶而成為多晶矽(Poly-Si)，以提供薄膜電晶體元件作為具有高電子/電洞移動率(相對於a-Si)的通道材料，由於多晶矽薄膜形成品質的優勢，影響LTPS TFT的元件性能至深，因此準分子回火結晶的製程能力，將是左右LTPS TFT發展的重要關鍵。

論文利用電漿輔助式化學氣相沉積 (plasma enhanced chemical vapor deposition，PECVD)製備非晶矽薄膜，先於玻璃基板上成長55nm SiN，接著成長130nm SiO，接著43nm a-Si，形成三層結構。在雷射退火處理之前，先利用臭氧水(濃度18ppm)於非晶矽薄膜層膜進行前處理，為了去除臭氧水前處理在a-Si表面形成的氧化層，過程中亦使用1% HF清除氧化物的污染物。

雷射退火處理是利用波長308nm XeCl 連續波準分子雷射製作低溫多晶矽薄膜，並製作出TFT元件，量取相關之電性。為了探討臭氧水前處理的影響，論文中分別針對有臭氧水前處理之多晶矽試片及無臭氧水前處理之多晶矽試片(對照組)進行分析比較，包括未進行雷射退火前之a-Si薄膜。分析主要是利用X光電子能譜術、歐傑電子能譜術、二次離子質譜術等材料表面分析技術分析表面成分與縱深分布，並利用掃描式電子顯微鏡及穿透式電子顯微鏡觀察多晶矽晶粒之晶粒尺寸、厚度、形貌、結晶性，更進一步以電子背向散射繞射(electron backscatter diffraction, EBSD) 對兩組多晶矽薄膜試片進行晶體結構晶向(texture)分析及顯微組織形貌觀察。

實驗結果發現臭氧前處理有較厚的氧化矽層在a-Si，且經過雷射退火所形成多晶矽薄膜有較大之晶粒與較強之<100>晶向。TFT 電性顯示經過臭氧前處理之元件有較高之遷移率(mobility).兩組試片經過交叉分析比較，發現臭氧前處理可以有效去除碳之汙染，而改善多晶矽之性質。

Excimer laser annealing plays an important role in low temperature poly silicon thin film transistors (LTPS TFT) manufacturing technology. Because excimer laser has ultraviolet wavelengths and amorphous silicon has a high absorption rate of the UV light, excimer laser annealing can be effective to obtain polycrystalline silicon films. After excimer laser annealing of amorphous-Si layer on glass at temperature < 600□C, the crystallized poly-silicon films can be used as channels in TFT devices of high electron and hole mobilities with good stability. Since the quality of poly-silicon film after excimer laser annealing strongly affects LTPS TFT performance, it can be improved by extending the processing technology of excimer laser annealing.

Here, we used plasma-enhanced chemical vapor deposition to form amorphous silicon films on glass substrate. Before deposition of 43nm thick amorphous silicon, the substrate was covered with 55nm SiN and 130nm SiO by PECVD to form a 3-layer structure. The ozone water concentration of 18 ppm was used in the membrane pretreatment of the amorphous silicon thin-film layer, and the surface oxide was removed by the use of 1% HF simultaneously.

For the laser crystallization process, a continuous wavelength 308XeCl excimer laser was employed to produce low-temperature polysilicon thin films. The polysilicon thin films then were processed to fabricate TFT devices in which they were acted as channels. The electric properties were measured from TFT devices. To evaluate the effect of ozone pretreatment, two kinds of samples were prepared, one was poly-Si from ozone-pretreated a-Si, the other was poly-Si without ozone pretreatment. Also, comparison of a-Si films before excimer laser annealing had been made for those with and without ozone pretreatment. Surface analytical techniques of Auger electron spectroscopy, x-ray photoelectron spectroscopy, and secondary I on mass spectroscopy, with Fourier-transform infrared spectroscopy, have been used to characterize the surface composition, depth profiles, and bonding characteristics. Microstructural characterization of polycrystalline silicon were carried out by using scanning electron microscopy and transmission electron microscopy to obtain the distribution of grain size, thickness, surface morphology, and crystallinity. Further, electron backscatter diffraction characterization was performed to compare the textures of poly-Si with and without ozone pretreatment.

The results show that poly-Si obtained from the ozone pretreatment has a larger grain size with a strong <100> texture, and the TFT devices have a high electron mobility. The comparison between both samples with and without the ozone pretreatment suggests that carbon contamination which can be effectively removed by the ozone pretreatment is responsible for the characteristic properties.