有機薄膜電晶體與非晶矽薄膜電晶體在偏壓與光照下之可靠度分析

Abstract

在論文中，我們探討了低溫製程下有機薄膜電晶體和非晶矽薄膜電晶體的可靠度問題。首先在有機薄膜電晶體的部份，元件在偏壓下以及照光下的劣化機制被深入的探討。在直流偏壓的實驗中，我們首次發現利用汲極電壓調整通道載子濃度，元件臨界電壓漂移量和通道中累積的載子濃度成正比，我們並修改了舊有的偏壓壓力(Bias Stress)模型，使模型可以完整呈現閘極和汲極電壓對有機薄膜缺陷產生機制以及元件臨界電壓的影響。在交流偏壓的實驗中，我們發現臨界電壓漂移量會受施加偏壓的頻率所影響，負閘極偏壓的頻率效應可以用元件通道累積電洞的反應時間來解釋，等效電路模型和非晶矽薄膜電晶體使用的相同。在更進一步的研究中，我們探討絕緣層表面狀態以及環境對於元件可靠度的影響。從實驗結果中我們發現，利用具有氫氧基的絕緣層製作出來的元件在含水氣的環境中，元件劣化速度是較快的。另外，也發現五苯環素薄膜電晶體通道中可移動的電子，並非由汲極源極注入，而是由環境光或是因有機介電層中的氫氧基與水氣反應所提供。最後，我們探討元件照光的反應以及五苯環素光電晶體的光偵測能力。實驗中發現光引起的電子濃度是可以由電場來控制且也可以利用電場來增加有機薄膜中的光致電子電洞對，我們稱此效應為光電場效應。我們利用此光電場效應來增強元件對光的靈敏度，也詳細探討光強度、電場強度、元件尺寸等參數對光靈敏度以及測光時可偵測範圍(detectable range)的影響。利用光電場效應可以使五苯環素電晶體在藍光下的光響應高達92 A/W。 在非晶矽薄膜電晶體的部份，我們量測並分析由工研院製作在軟板上的低溫非晶矽薄膜電晶體，元件是製作在聚亞醯胺薄膜上且製程溫度在160 oC之下。元件的基本電特性與傳統溫度下(300 ~ 350 oC)製作出來的元件並無太大差異，只有元件的可靠度受低溫沉積的影響而下降。在我們的研究中首度發現，同時施加長時間閘極和汲極偏壓後，元件的臨界電壓漂移會隨通道寬度變大而增加且會使可靠度模型失效。這是因為元件產生了自我加熱的效應，導致元件通道中的溫度上升，當使用導熱差的塑膠基板時，自我加熱效應比玻璃基板上更為嚴重，我們也延伸可靠度模型來萃取等效的通道溫度，並和傳統的自我加熱模型計算出的溫度比較，傳統的等效熱電阻模型並無法解釋臨界電壓漂移與通道寬度的關係，因為熱對流以及熱輻射等散熱方式並沒有被傳統的等效熱電阻模型考慮。因此，我們利用可靠度模型來推估元件通道溫度，此方法不需要建立複雜的等效電路與量測材料的熱特性，而且，此方法首度探討並解釋了自我加熱效應和偏壓壓力效應的交互影響。最後，我們首度將元件置放在彎曲的載台上進行量測並觀察其自我加熱效應，我們發現若沒有自我加熱效應產生，則元件可靠度不受彎曲影響；若有自我加熱效應產生時，可靠度會隨彎曲程度增強而劣化。這可能是因為薄膜內的矽氫鍵結在有應力情況下變得較脆弱，自我加熱效應會使這些較脆弱的鍵結斷裂，使缺陷態更容易產生。

In this thesis, the reliability issues of low-temperature process organic-based TFTs and a-Si:H TFTs are discussed. Firstly, for organic-based TFTs, the degradation mechanisms of device under bias stress or under prolonged illumination are carefully investigated. In the steady-state bias stress experiment, we use drain bias to adjust the carrier concentration in the channel and find that the accumulated carrier concentration is proportional to the threshold voltage shift. This verifies the relationship between the defect generation and the accumulated carrier concentration in organic thin film for the first time. In the pulsed bias stress experiment, under positive and negative bias, the threshold voltage shifts have the different pulse width dependence. These results show that there is obvious difference in hole and electron accumulation rates. On the other hand, the influences of dielectric surface states and environmental conditions on the reliability of device are studied. Based on the experimental result, the device fabricated on the dielectric with hydroxyl groups in the moisture environment has more serious reliability issues. Additionally, since electron injection from Au to pentacene is difficult, it is found that the accumulated electrons are provided from light-induced electrons or from the negative-charge states produced when OH groups react with moisture. Finally, the influence of illumination on OTFT and pentacene-based organic phototransistors (OPTs) are studied. Using bias to adjust the channel carriers can control the light-induced threshold voltage shift. Also, electric field is found to enhance the dissociation of light-induced excitons. This is named as the photoelectric field effect in this dissertation. We use the photoelectric field effect to enhance the sensitivity of OPTs for the first time. We also investigate the influence of light intensity, wavelength, bias, channel dimension, and illumination time on the light-induced threshold voltage shift. Under blue light illumination, the photoresponsivity of pentacene-based TFTs reaches 92 A/W using the photoelectric field effect. For a-Si:H TFTs, we analyze devices fabricated by Industrial Technology Research Institute, Taiwan. Devices are fabricated on polyimide substrate and process temperature is kept at 160 oC. The basic device parameters such as mobility, threshold voltage and threshold slope do not differ from device fabricated in the conventional process temperature but the reliability issue becomes more serious. In our studies, after applying simultaneous gate and drain bias stress, it is found that the threshold voltage shift has the channel width dependence and can not accurately predicted by the original reliability model due to the self-heating effect. According to the equivalent thermal resistant circuit, when changing glass substrate to polyimide substrate, the smaller thermal conductivity of polyimide substrate cause the low cooling capacity to accumulate higher channel temperature. We also firstly observe a relationship between the self-heating effect and the bias-stress effect, particularly when devices have wide channel width. Conventional thermal resistant circuit can not explain the channel width dependence. Therefore, we use the reliability model to fit the experimental data and extract the effective channel temperature. This is a new methodology to discuss the self-heating effect without calculate detailed thermal resistance model or simulate thermal flow of devices. Finally, we also firstly find that tensile stress may further accelerate the generation of defects when self-heating effect occurs.