雷射退火再結晶低溫複晶矽薄膜電晶體之特性研究

Abstract

高性能低溫複晶矽薄膜電晶體之應用在顯示器上被廣泛的探討，此論文中探討了多項製程，針對於雷射退火再結晶低溫複晶矽薄膜電晶體之特性研究。其前處理清洗製程利用濃度為0.1%的氫氟酸清洗，並配合多項改善方法及技術以期進一步提昇低溫複晶矽薄膜電晶體之電性，首先包含了多項非晶矽薄膜的前處理製程方式，而其中又以非晶矽薄膜的結晶技術為主要重點。首先我們探討了以不同的前處理製程並利用準分子雷射退火結晶方式製作出的低溫複晶矽薄膜電晶體的各項電性，並從材料分析的結果推究電性與前處理製程條件以及準分子雷射製程條件上的關係。其中以前處理製程表面氧化以及雷射能量密度對複晶矽薄膜的晶粒形狀與低溫複晶矽薄膜電晶體的電性影響最大，其前處理製程包含了幾種不同方法，第一種是利用濃度20ppm臭氧水當作表面清洗，第二種是利用濃度30%過氧化氫當作表面清洗，第三種是利用波長254nm的紫外光照射當作表面清洗，而這些過程都會促使非晶矽薄膜表面的氧化，差別只在於時間的長短。在非晶矽薄膜表面得到完全氧化後經過準分子雷射退火結晶方式製作出的低溫複晶矽薄膜的晶粒大小約0.3微米，形狀類似四方形且均勻性一致，不過這些情況必須搭配最佳的雷射能量密度而製作出n型通道與p型通道低溫複晶矽薄膜電晶體的場效載子移動率可分別達到 110 cm2/V*s與90 cm2/V*s。至於門限電壓會因為非晶矽薄膜的表面氧化程度不同而有不同偏向。 而改變雷射掃描頻率以及雷射掃描的能量對於複晶矽薄膜的晶粒大小形狀與低溫複晶矽薄膜電晶體的電性沒有影響。在不同的準分子雷射退火結晶製作環境也會得到不同效果，先前所提的製程環境是在氮氣的氣氛下進行雷射退火結晶，而現在則利用低濃度的氧氣通入製程環境中，其氧濃度為2000ppm。從材料分析，我們觀察到低氧製程環境中經過雷射退火結晶所產生的複晶矽薄膜表面粗糙度與先前製程結果一樣，最大表面粗糙度大約80nm，平均表面粗糙度大約9nm，但形狀卻不同，低氧製程環境中所產生的複晶矽薄膜粗糙形狀為柱狀，由於形成位置剛好是複晶矽晶粒與晶粒的邊界，位能障壁較大，但接近閘極，當閘極加一電壓時靠近閘極電場較大，使位能障壁降低，門限電壓也同時降低，所以低氧製程環境中所產生的低溫複晶矽薄膜電晶體門限電壓較低，其n型通道與p型通道低溫複晶矽薄膜電晶體門限電壓介於正2與負2伏特之間。 本論文的最後則探討連續波雷射結晶的技術，利用波長為532nm連續波雷射在不同功率不同掃描速度下的結晶情況，我們探討了多項影響結晶大小的因素，包含了掃連續波雷射時移動玻璃基版的平台速度控制配合不同功率的連續波雷射，以及玻璃基版的正面反面經過連續波雷射掃射後在不同的速度下的結晶情況。由於連續波雷射光束能量分佈為高斯曲線分佈且連續波雷射光束掃射在玻璃基板大小會受到透鏡以及功率大小的影響，我們選用焦距為600mm的凸透鏡將雷射光束聚焦在玻璃基板直徑大約150um，當功率愈大掃射在基板上之直徑愈大，反之則小。利用連續波雷射結晶方式製作出的低溫複晶矽薄膜的晶粒大小約3um，形狀類似長條狀，不過大的晶粒只有集中在雷射掃過條狀區中間帶大約60um，且受不同狀態影響，大的晶粒區也會跟著變小。以此條件所製作出n型通道與p型通道低溫複晶矽薄膜電晶體的場效載子移動率可分別達到298 cm2/V*s與210 cm2/V*s。由於結晶過程在一般的空氣下所以門限電壓會因為環境因素的影響而偏大。

It was extensively discussed the high-performance low temperature polycrystalline silicon (LTPS) application in monitor. In this thesis, many processes of crystallization means were discussed and the characterization of low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) were studied, in which the HF with the concentration of 0.1 percent was utilized by pre treatment clean process. Many methods and techniques had been proposed to further improve the performance of LTPS TFTs, which include means of prior clean treatment of amorphous silicon thin films, while most of the effort was focused on crystallization of amorphous silicon (a-Si) thin films. First, the electrical characteristics of LTPS TFTs fabricated by excimer laser annealing (ELA) of a-Si thin films which was used different pre treatment processes. From the results of material analysis and device characterization, the relation between electrical characteristics of LTPS TFTs and pre treatment processes conditions with laser annealing conditions had been identified. It was found that caused the surface oxidation of a-Si thin films by pre treatment process and laser energy density had a deep influence on the resulting poly-Si grain structure and electrical characteristics of LTPS TFTs. It was included the different methods of pre treatment process, the first method is surface cleaning with O3 water which concentration was 20ppm, the second method was surface cleaning with H2O2 water which concentration was 30%, the third method was surface cleaning with UV light exposure which wave length was 254um, and all of these methods can advance the surface oxidation of a-Si thin films, it had the difference at long time or short time treatment. When surface of a-Si thin films was completely oxidized by pre treatment process then treated by ELA, the LTPS thin films will be fabricated with grain size about 0.3um, grain shape like square and uniform distribution on surface. In this situation, the LTPS TFTs fabricated by ELA with optimization laser energy density, the field effect mobility of 130 and 90 cm2/V*s could be achieved for n-channel and p-channel ELA LTPS TFTs, respectively. The thresholds voltage had different shift owing to the difference surface oxidation on a-Si thin films. Changing laser scan frequency and energy couldn’t improve the crystalline quality and uniformity of crystallized poly-Si thin films and electrical characteristics of LTPS TFTs. In different ambiance of ELA crystallization had different efficiency, before mention the process of ELA crystallization had ambiance of N2 gas, now the concentration of low O2 with 2000ppm was utilized in ELA crystallization ambiance. From the results of material analysis, the LTPS thin films fabricated by ELA crystallization in ambiance of low O2 concentration had surface roughness like before mention process, the maximums surface roughness was about 80nm and average surface roughness was about 9nm, but the shape had some difference, the LTPS thin films surface roughness of low O2 concentration status had the shape like the cylinder in which the position was grain boundary and potential barrier was larger than grain area. The cylinder top surface were close to the gate bottom, when gate applied the forward bias, it would enhance the high electric file near the gate then induced the average thresholds voltage degradation. LTPS TFT’s fabricated by ELA crystallization in ambiance of low O2 concentration had lower thresholds voltage than N2 ambient condition, the thresholds voltage of +2 and –2 V could be achieved for n-channel and p-channel ELA LTPS TFTs, respectively. At last part of this thesis, the crystallization of continuing wave laser (CW laser) technology would be discussed. Crystallization of amorphous silicon (a-Si) thin films utilized the wavelength 532nm of CW lasers with different power and scan speed. Many factors of influence for grain size were discussed. It included the speed control in stage with different power during laser scanning; front and backside scan on substrate at different scanning speed. Owing to the laser beam energy distribution was the Gauss shape and laser beam size was affected by focus lens and laser power, we employ the focus lens with focus 600mm and focus the laser beam on substrate where the beam diameter was about 150um, when laser power more high the diameter more long. The LTPS thin films were fabricated by CW laser with grain size about 3um, grain shape like long bulk and large grain was aggregated in center area of laser beam on crystallization surface and the LTPS TFTs fabricated at large grain size area, the field effect mobility of 298 and 210 cm2/V*s could be achieved for n-channel and p-channel LTPS TFTs, respectively. The threshold voltages were shifted to 7V owing to the crystallization ambiance was atmosphere.