低溫多晶矽薄膜電晶體與軟性電子之研製與分析

Abstract

我們在非晶矽下方及上方分別成長矽氧氮化物熱吸收層及金屬柱雷射反射層，研究使用準分子雷射製備可控制位置的側向成長多晶矽，發現當成長385奈米厚汽相沉積比0.83的矽氧氮化物及700奈米高的鋁金屬柱時，可達到最遠1.78微米的側向成長多晶矽。此外，以此方法形成的多晶矽結晶性較傳統方法所製成的多晶矽來的好。如果將鋁金屬柱排列成週期性分怖，經過高能量準分子雷射退火後，多晶矽可成長為均勻的正方形晶格，接著去除掉金屬柱之後，採用低能量雷射，將原本在金屬柱下方的非晶矽結晶為多晶矽。此低能量雷射以不破壞原本的正方形晶格為目標。此種方法製作的多晶矽應用於薄膜電晶體上，可達到450 cm2/V-sec的載子遷移率，以及開關比大於107。此外，我們發現，在通道寬度及長度較小的情況下，由於通道內的晶粒邊界及缺陷較少，所以薄膜電晶體可以表現出較小的次臨界擺幅。 我們研究以氟化氪準分子雷射製備低溫多晶矽薄膜電晶體，探討當通道寬度可相比或是小於多晶矽晶粒時的現象。接著利用橫截面掃描電子顯微鏡及TCAD軟體，觀察等效元件通道寬度及模擬通道區域內的電子密度分怖。我們發現，擁有10條40奈米寬的多通道薄膜電晶體有較好的電特性，例如：大於107的電流開關比、8.8×10-14安培的低漏電流、較少的晶粒邊界缺陷以及較小的次臨界擺幅(0.45 V/dec)。 此外，我們也研究以氟化氪準分子雷射在聚亞醯胺基板上製備低溫多晶矽薄膜電晶體。我們採用兩階段式雷射法，第一發雷射先將非晶矽氫膜中的氫原子去除，第二發雷射再將非晶矽結晶為多晶矽，完成的元件可以有相當好的特性，也就是370 cm2/V-sec的載子遷移率及電流開關比大於106。 而在聚亞醯胺基板上製做微晶矽薄膜電晶體方面，其元件的電特性可達到電流開關比大於6個次方、最大0.2 cm2/V-sec的載子遷移率，及臨界電壓16.6伏特。我們認為此高臨界電壓的原因，是因為利用電漿輔助化學汽相沉積成長微晶矽時，矽烷的流量相當低，所以剛開始需要相當高的功率來產生電漿，接著再調回所設定的成長功率。因此，一開始的微晶矽薄膜成長速率相當快，以至於閘極絕緣層氮化矽與微晶矽界面上，存在著許多缺陷密度。 在聚亞醯胺基板上製做非晶矽薄膜電晶體方面，我們製作通道寬度及長度為200和40微米的元件，其電流開關比可大於105、0.59 cm2/V-sec的載子遷移率及臨界電壓5.8伏特。 在完成以上三種在聚亞醯胺基板上製作的元件後，我們將此三種元件分別貼在曲率半徑為1、2及4公分的彎曲基板上測量其電特性的變化。在非晶矽薄膜電晶體方面，只有在曲率半徑為1公分的情況下，載子遷移率及臨界電壓才有明顯的衰退，分別為3~5%及3.6 ~ 4.4%。而低溫多晶矽薄膜電晶體上，曲率半徑及元件特性間則沒有明顯的相關性。

A location-controlled lateral growth of polycrystalline silicon grain with underneath SiON heat absorption layer and top metal pads laser reflector fabricated by excimer laser annealing are studied. It is found the maximum lateral growth length of poly-Si can reach 1.78 μm by using 385 nm thick SiON (Xg=0.83) layer and 700 nm thick Al metal pads. Besides, the crystallinity of poly-Si prepared by this method is better than that of the conventional method. If the metal pads are periodically arranged, the poly-Si can grow to regular square grains following the high power excimer laser annealing (ELA). After removing the metal pads, the low power laser shot transfers the a-Si:H underneath the original metallic pads to poly-Si without destroying the already formed square grains. The TFTs fabricated by this method achieve a field effect mobility of 450 cm2/V-sec and an on/off current ratio exceeding 107. It is found that the TFT with smaller channel width and length results in a better subthreshold swing because it contains fewer grain boundaries and defects in the channel region. Low temperature (<500oC) multichannel polysilicon thin-film transistors prepared by KrF excimer laser annealing with channel width comparable or smaller than the poly-Si grain size are investigated. The cross-sectional scanning electron microscope is used to measure the effective channel width and TCAD software is used to simulate the electron density distribution in the channel region. It is found that the thin film transistors with ten 40 nm wide multichannels have superior electrical characteristics, including higher ON/OFF current ratio (>107), lower leakage current (8.8x10-14A), less grain boundary defects density and a better subthreshold swing (0.45 V/dec). Low temperature poly-silicon TFTs prepared by KrF excimer laser annealing of a-Si:H are fabricated successfully on polyimide substrate. Two step laser shot is used to dehydrogenate and crystallize of the a-Si:H into poly-Si on polyimide substrate. The completed poly-Si TFT on polyimide shows a very good performance with field effect mobility of 370 cm2/V-sec and on/off current ratio > 106. For microcrystalline silicon TFTs on polyimide substrate, the electrical characteristics can reach on-off current ratio more than six order of magnitude, maximum mobility of 0.2 cm2/V-s, and threshold voltage of 16.6 V. Since the gas flow rate of SiH4 is relative low when depositing μc-Si film by PECVD, it needs a high power to generate plasma, then, adjusts the rf power to desire power density. Hence, at the beginning, the growth rate of μc-Si film is very fast. Consequently, many trap densities exist at the interface between SiNx/μc-Si. The bottom gate a-Si:H TFTs with W/L = 200/40 μm/μm on the polyimide exhibits Ion/Ioff current ratios larger than 105, field effect mobility = 0.59 cm2/V s, and threshold voltage = 5.8 V. After measuring three kinds of TFTs on bent substrate with 1, 2, and 4 cm radius of curvature, the μc-Si TFTs show apparent degradation in mobility and threshold voltage when the radius of curvature decreases. As for a-Si:H TFTs, only when radius of curvature is equal to 1cm, the mobility and threshold voltage degrade apparently 3~5 and 3.6~4.4 %, respectively. The poly-Si TFTs exhibit no significant correlation between electrical characteristics of devices and bending condition.