Ni-Metal Induced Lateral Crystallization of Amorphous Silicon－Growth Mechanism, Metal Gettering and LTPS TFTs Device Performance
|關鍵字:||薄膜電晶體;捉聚;金屬誘發結晶;金屬誘發側向結晶;Thin film transistor (TFT);gettering;metal induced crystallization (MIC);metal induced lateral crystallization (MILC)|
在本論文提出兩種捉聚方式。第一種為利用晶圓鍍覆非晶矽的方式作為捉聚基板，將捉聚基板與複晶矽薄膜接合退火，我們成功的把複晶矽中殘餘鎳捕捉至捉聚基板中。可以發現到捉聚後聚集在晶界的NiSi2蝕刻孔洞明顯減少，而由SIMS所偵測到的鎳濃度也降低為捉聚前的1/30。捉聚前後的複晶矽薄膜製做NILC TFT與GETR TFT兩組元件，從電性上經過捉聚後的GETR TFT因為鎳雜質的減少，漏電流進而降低的34倍，開關電流比提高了近9倍。
第二種捉聚方式，是藉由濃度梯度的擴散使得複晶矽中殘餘鎳能成功的透過接觸窗捕捉至上層捉聚層。可以發現上層非晶矽由於鎳金屬的擴散而成長出NILC複晶矽。並且發現到捉聚後聚集在晶界的NiSi2蝕刻孔洞也明顯減少。將捉聚前後的複晶矽薄膜製做NILC TFT與GETR TFT兩組元件，從電性上經過捉聚後的GETR TFT因為鎳雜質的減少，而使得電性獲得提升，其中包括電子遷移率、次臨界斜率、以及開關電流比。|
In this thesis, Ni-metal induced lateral crystallization (NILC) of amorphous silicon (α-Si) has been studied. The influence of pulse rapid thermal annealing on the growth mechanism of NILC is investigated. And the influence of capped silicon nitride film on growth mechanism of NILC is also investigated. We fabricate the LTPS TFTs by those methods. Moreover, in order to solve this issue of NILC poly-Si film, we develop two effective gettering methods to reduce the Ni-metal impurity contamination of the NILC poly-Si films. Initially, two annealing methods, pulse rapid thermal annealing (RTA) and conventional furnace annealing (CFA), were used to fabricate NILC polycrystalline silicon (poly-Si). It was found that the growth rate of RTA-POLY was 5 times higher than that of CFA-POLY, which is due to the photon-assisted and/or free energy released by sudden heating during RTA. RTA-TFT contained not only NILC but also SPC poly-Si grains. Moreover, the width and length of RTA-GRAIN were smaller than those of CFA-GRAIN, indicating that RTA-GRAIN had more defects than CFA-GRAIN. As a result, the performance RTA-TFT was not as good as CFA-TFT. It was found that the NILC saturation length increased as the thickness capped silicon nitride films increased, which was due to solid phase crystallization (SPC) of a-Si film retarded. The retardation of SPC was investigated by scanning electron microscopy (SEM).And in the thin film stress analysis, we investigated the SPC retardation effect was due to the compression stress from silicon nitride film. There was a large number of hydrogen atom escaped from silicon nitride film during annealing process, which resulted in compression IV stress on a-Si film. The performance of the TFTs was almost the same. We develop two gettering methods to reduce the Ni contamination within the NILC poly-Si film. The first one, we proposed using a-Si-coated wafers as Ni-gettering substrates. After bonding the gettering substrate with the NILC poly-Si film, the Ni-metal impurity within the NILC poly-Si film was reduced. It was found that the silicide-etched holes at NILC-POLY grain boundaries were greatly reduced after the Ni-gettering process. The Ni concentration within NILC-POLY was reduced to 1/30. The device transfer characteristics of GETR-TFTshowed an 9-fold increase in the ON/OFF current ratio and a 34-fold decrease in the minimum leakage current compared with those of NILC-TFT. In the second one gettering method, a-Si film was coated on the top of contact holes as Ni-gettering layer. Ni atoms were diffused from NILC-POLY to the gettering layer due to the concentration gradient. Ni atoms were diffused from NILCPOLY through the contact holes and reach the a-Si layer and then transformed the a-Si into needlelike poly-Si grains. It was also found that the silicide-etched holes at NILC-POLY grain boundaries were greatly reduced after the Ni-gettering process. The GETR-TFT exhibited an enhanced field-effect mobility, steeper subthreshold swing, higher on/off current ratio.
|Appears in Collections:||Thesis|