鎳金屬誘發非晶矽薄膜側向結晶－成長機制、金屬捉聚與低溫複晶矽薄膜電晶體效能之研究

Abstract

在本論文中，主要的研究是鎳金屬誘發非晶矽薄膜側向結晶。其中，探討快速熱退火技術對於鎳金屬誘發非晶矽薄膜側向結晶成長機制之影響以及探討氮化矽薄膜對於鎳金屬誘發非晶矽薄膜側向結晶成長機制並製作低溫複晶矽薄膜電晶體。此外，為了解決對於鎳金屬誘發複晶矽薄膜來說，非常重要的鎳金屬殘留問題，因而發展出有效的吸鎳方法來降低鎳金屬誘發複晶矽薄膜中的鎳金屬殘留，論文中提出兩種有效的鎳金屬捉聚方式。

首先，我們探討了以RTA與CFA這兩種不同的退火方式成長的NILC複晶矽的成長機制、微結構以及所製成的薄膜電晶體的傳導特性。在成長速率方面以RTA方式成長的NILC其速率為CFA成長的5倍，原因可能來至於光催化與熱應力效應導致NILC成長時活化能被降低。而到達飽和時間與飽和長度RTA試片則明顯的降低，原因可能則來至於RTA過程中複晶矽的固相結晶的活化能較低而較容易形成固相結晶，這使得以RTA方式成長的NILC提早達到飽和長度。由於RTA試片的NILC複晶矽較小而造成晶界變多以及NILC複晶矽區域有固相結晶的產生使得RTA-TFT元件的特性較CFA-TFT元件特性來的差。

我們發現當覆蓋在非晶矽上的氮化矽薄膜愈厚時則NILC的飽和長度愈長，這是由於固相結晶被抑制所致。藉由SEM的觀察我們證實了氮化矽薄膜確實能抑制固相結晶形成。在薄膜應力的分析上，證實氮化矽薄膜抑制固相結晶的應力來源應該是來自於大量氫原子脫離而對非晶矽造成壓應力。薄膜電晶體的電性表現上，不同薄膜與厚度所製作的TFT元件之間並無太大的差異這是由於覆蓋的氮化矽與氧化矽薄膜在NILC的成長過程中並不會影響NILC結晶的結構。

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.