鎳金屬誘發非晶矽薄膜側向結晶-成長機制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，探討拉伸應力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利用鎳金屬壓印誘發結晶方法來製作低溫複晶矽薄膜電晶體。並結合鎳金屬誘發非晶矽薄膜側向結晶與準分子雷射退火來製作高效能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氣方法來降低鎳金屬誘發複晶矽薄膜中的鎳金屬殘留。 首先，鎳金屬誘發非晶矽薄膜側向結晶過程包括三個階段: (1)NiSi2金屬矽化物之生成，(2)在NiSi2金屬矽化物生成結晶矽之核，(3)藉由金屬矽化物NiSi2之移動來產生非晶矽薄膜結晶與結晶成長。有相關報告指出，拉伸應力會縮短鎳金屬誘發非晶矽薄膜側向結晶的潛伏時間。然而，其詳細的機制並不清楚。在本實驗中，利用一簡單的彎曲設備來研究拉伸應力對鎳金屬誘發非晶矽薄膜側向結晶之影響。基於實驗結果發現，拉伸應力並不會影響NiSi2金屬矽化物與結晶矽核之生成，但是，會影響結晶矽之成長。並發現壓應力並不影響鎳金屬誘發非晶矽薄膜側向結晶之速率。 鎳金屬壓印誘發結晶法比傳統鎳金屬誘發非晶矽薄膜側向結晶法存在著許多更好的特性。在本實驗中，利用<111>與<112>不同成長方向之針狀結晶來製作低溫複晶矽薄膜電晶體。<111>成長方向之針狀結晶是利用傳統鎳金屬誘發非晶矽薄膜側向結晶來製作，而<112>成長方向之針狀結晶是利用鎳金屬壓印誘發結晶方法來製作。結果發現，<112>成長方向之薄膜電晶體在元件電特性優於<111>成長方向之薄膜電晶體，有2.6倍高的電子遷移率，4倍高的開/關電流比與2.4倍低的漏電流。 雖然，可以利用鎳金屬壓印誘發結晶之方法來製作元件特性較佳，均勻性較好之低溫複晶矽薄膜電晶體。但是，在複晶矽針狀結晶間仍然存在著許多的缺陷與未結晶的區域，這些缺陷會使元件的特性劣化。結合準分子雷射與鎳金屬壓印誘發結晶薄膜，來降低缺陷密度。並比較其薄膜電晶體之特性，當雷射能量在345 mJ/cm2時，晶粒大小從50奈米增加到250奈米。結合準分子雷射退火之電晶體，由於有較大的晶粒與較少的晶粒間之缺陷，其電子遷移率為413 cm2/Vs，開/關電流比為4.24×106，遠優於單純壓印結晶之電晶體。 最後，發展出兩種吸氣的方法來降低鎳金屬誘發側向結晶複晶矽薄膜中的鎳金屬殘留。第一，利用厚度為100奈米的非晶矽薄膜來當作吸氣層，與厚度為30奈米的氮化矽來當做蝕刻停止層，在溫度為550°C下進行退火90小時來降低鎳金屬殘留。第二，利用鍍覆非晶矽薄膜之吸氣基板與鎳金屬誘發結晶之複晶矽薄膜接合並進行退火。其複晶矽薄膜中的鎳金屬殘留可以被大大的降低。

In this thesis, Ni-metal induced lateral crystallization (NILC) of amorphous silicon (α-Si) has been studied. The influence of tensile stress on the growth mechanism of NILC is investigated. Furthermore, we fabricate the LTPS TFTs by Ni-metal imprint-induced crystallization method. Combine NILC and excimer laser annealing (ELA) method to produce high-performance LTPS TFTs. Moreover, in order to solve this issue of NILC poly-Si film, we develop an effective gettering method to reduce the Ni-metal impurity contamination of the NILC poly-Si films.

Initially, three stages have been identified in the NILC process: (1) the formation of NiSi2 precipitates, (2) the nucleation of crystalline Si (c-Si) on NiSi2 precipitates, and (3) the subsequent migration of NiSi2 precipitates and growth of c-Si. It has been reported that the incubation time could be reduced by tensile stress. However, the detail has still not been clarified. In this study, a simple bending fixture was used to investigate the effects of tensile stress on the growth of NILC. Base on the results of this study, it was found that tensile stress did not enhance NiSi2 formation and c-Si nucleation stages, but enhanced the c-Si growth stage. It was also found that compressive stress did not change NILC rate.

The Ni-metal imprint-induced crystallization method exhibited many superior characteristic over traditional NILC method. In this study, for the LTPS TFTs fabricated using <111> and <112> needle grains have been investigated. They were fabricated by traditional NILC and Ni–metal imprint-induced crystallization method. It is found that the performance of 112-TFT was far superior to that of 111-TFT. The device transfer characteristics of 112-TFT include 2.6-fold-higher field-effect mobility (μFE), 4-fold-higher on/off current ratio (ION/OFF), and 2.4-fold-lower leakage current (IOFF) compared with those of the 111-TFT.

The improved performance and good uniformity LTPS TFTs have been fabricated using Ni-metal imprint-induced crystallization method. However, the polycrystalline silicon film contained many intra-grain defects with some un-crystallized regions between poly-Si needle grains. These defects degrade the transfer characteristics of TFT devices, including the field effect mobility (μFE) and the leakage current. In this study, combine the excimer laser crystallization method to reduce the defect density. To compare the performance of IMPRINT and IMPRINT-ELA TFT, upon increasing the laser energy to 345 mJ/cm2, the grain size increased from 50 to 250 nm and the performance of IMPRINT-ELA-TFT was found to be far superior to that of IMPRINT-TFT due to larger grains and fewer intra-grain defects of the IMPRINT-ELA poly-Si film than that of the IMPRINT poly-Si film. The mobility of the IMPRINT-ELA-TFT was 413 cm2/Vs, which was 31.7 times higher than that of the IMPRINT-TFT. The on/off current ratio of the IMPRINT-ELA-TFT was 4.24 ×106, which was 2 orders magnitude higher than that of the IMPRINT-TFT.

Finally, develop two gettering methods to reduce the Ni contamination within the NILC poly-Si film. First, using α-Si films with a thickness of 100 nm as a Ni-gettering layer, silicon-nitride (SiNx) films with a thickness of 30 nm as the etching stop layers and annealed at 550°C for 90 h to reduce the Ni-metal impurity within the NILC poly-Si film. Second, an α-Si-coated wafers used as Ni-gettering substrates then bonding the gettering substrate and NILC poly-Si film together. The Ni-metal impurity within the NILC poly-Si film was greatly reduced.