IGZO薄膜電晶體之光學低溫退火與金屬接觸界面處理

Abstract

本論文意在發展出非晶銦-鎵-鋅-氧薄膜電晶體(a-IGZO TFTs)的光學低溫退火製程。未經退火處理的a-IGZO TFT在在電性上的表現是很不穩定的，而後段退火處理則可以促進其在的電性上的再現性。由射頻濺鍍系統 (radio-frequency sputter) 所成長的非晶IGZO主動層在晶格結構上是混亂的並可能導致介電層和主動層之間載子捕捉缺陷的存在。元件操作的時候常伴隨嚴重的臨界電壓飄移現象，推測應是載子捕捉效應所造成。一般常見的退火方法為本於熱效應的高溫爐管，但其並不適合被使用在具有溫度限制的電路系統上。由本研究所發展的低溫退火技術為以脈衝雷射(Nd:YAG, 266nm)及紫外燈(氙準分子燈, 172nm) 為光源設備，提供另外一種有別於傳統高溫爐管的退火方法。本論文中呈現，經由脈衝雷射或紫外線燈處理後的a-IGZO TFT ，其在穩定度獲得改善的同時也保有一定的元件電性，例如:載子遷移率(雷射:6.23 cm2/Vs, 紫外燈:3.49 cm2/Vs)、臨界電壓(雷射:0.3 V, 紫外燈:0.2 V)、次臨界擺幅(雷射:0.32 V/dec., 紫外燈:0.2 V/dec.)還有開關比(107)，足以和350ºC下爐管退火後的元件相較(移動率:6.51 cm2/Vs, 臨界電壓:1.8 V,開關比:108)。除滿足低溫的訴求外，此光學方法還可以節省時間並具有可局部化應用的能力。在連續量測下所觀察到的臨界電壓位移是一個最直接的參數來反應由載子捕捉所造成的不穩定性。未經退火的元件，其臨界電壓的飄移量通常可達10V左右。脈衝雷射和紫外線燈可以有效降低臨界電壓的飄移量到0.5伏以下 (雷射:0.5V, 紫外燈:0V)。本篇論文之研究，是興周政偉學長及陳蔚宗學長共同進行開發的。 在本研究中也同時證明，使用脈衝雷射對非晶IGZO薄膜做表面處理將可以減少薄膜電阻率進而改善電極及主動層間的電子注入。其實，在雷射處理後所造成的具有高導電率的IGZO區域已足夠直接作為源/汲極之電極，這提供光處理之頂閘極-自對準電晶體實現的可能性。以光照射方式取代離子佈植在製程簡化或成本效率上將可具有很大的優勢。

A low temperature fabrication method based on light illumination for amorphous Indium-Gallium-Zinc-Oxide thin film transistors (a-IGZO TFTs) was developed in this study. The as-fabricated a-IGZO TFTs without post annealing are quite unstable and demand post-annealing to get adequate repeatability in electric characteristic. The crystal structure of IGZO channel layer deposited by radio frequency sputter (rf sputter) in as-fabricated TFTs may be disorder and result in some trapping defects near interface between dielectric and active layers. Carrier trapping effect during device operation is the main source that causes instability of IGZO TFT. The general annealing method is furnace that base on thermal effect and may be not compatible with some temperature limited systems. Light illumination based on pulse laser (Nd:YAG, 266nm) or UV lamp (Xe excimer lamp, 172nm) developed in this study provides another effective annealing approach without heating the substrate obviously. In this thesis, the stability-improved a-IGZO TFT treated by pulse laser and UV lamp also have accepted criteria, such as mobility (Laser:6.23 cm2/Vs, UV:3.49 cm2/Vs), threshold voltage (Laser:0.3 V, UV:0.2 V), subthreshold swing (Laser:0.32 V/dec. UV:0.71 V/dec.) and On/Off ratio (107) which are comparable with the one annealed by 350ºC furnace (mobility:6.51 cm2/Vs, Vth:1.8 V, subthreshold swing:0.75 On/Off ratio:108). Moreover, illumination could also be a timesaving and localizable method for further application. The threshold voltage shift during sequent transfer curve measurement is the most direct parameter to reflect the instability caused by trapping effect. The threshold voltage of as-fabricated a-IGZO TFT generally could achieve 10.8 V. Pulse laser and UV lamp annealing could reduce the threshold voltage shift to 0.5V and 0V respectively. This work was studied with my senior classmate, Cheng-Wei Chou and Wei-Tsung Chen. This study also demonstrates that pulse laser also could be used to treat the a-IGZO surface to reduce the resistivity and then improve the carrier injection in contact. This provides an effective approach for contact treatment or even direct contact pattern. In this study, the light only patterned a-IGZO TFT perform comparable performance as compared the one with metallic contact. Therefore light illumination seems a possible method to replace ion implantation for valuable top gate self-aligned structure.