Abstract
摘 要 本篇論文主要探討 可以分為兩個部份，一是如何對於一個新購置電子束微影系統校正與萃取 出適當製程參數並且製作出次微米金屬半導體金屬(MSM) 的光偵測器，另 一部份是對於砷化鎵 MSM 光偵測器的模擬分析。 第一部份中我們 首先校正了曝光圖樣的大小與SEM 的放大倍率之間的關係、校準馬達的反 衝（backlash）誤差、樣品的支撐物(holder)的平整性、支撐物的位置與 控制座標的關係、電腦速度的校正、最小的停留時間、停留步伐等和電子 束曝光濃度與停留時間的校正。接下來必須考量電子束能量、PMMA的厚度 、PMMA的分子量、硬烤時間、顯影條件、電子束電流及曝光濃度、金屬厚 度、Proximity effect校正和聚焦等問題找出適當的參數，最後我們成功 的製作出金屬厚度為 1000A 的次微米線寬及不同間距的MSM光偵測器。 另外一部份我們利用二維模型分析砷化鎵的 MSM電脈衝響應，其結果包含 證明電脈衝響應由載子與位移電流所組成、取得不同外部偏壓的電脈衝響 應有一個最小響應時間、以及電場遮蔽效應(screening effect)造成載子 速度的衰減，和位移電流受脈衝寬度、光強度及電壓大小的影響。在模擬 分析中，我們也可以看到當 MSM縮小化增加響應速度的卻同時因為電場的 分佈造成響應較長的尾巴，以及運用低溫與薄膜技術對於響應長尾巴的改 善。我們利用0.5um MSM 的本質響應再加上寄生電路的響應成功擬合出參 考文獻5ps 的 MSM電脈衝響應。關於寄生電路對於響應的延遲與衰減，我 們提出當寄生電路中有很大的電感時必須設計出適當的電容值以減少響應 信號的失真。最後我們利用 20um 的傳輸線結構與半高寬為100fs、spot size 為 5um 的光脈衝，分析非均勻照射法產生大約為 3THz 高頻寬響應 。 Abstract There are two major parts in this thesis. One is to calibrate the newly setup SEM lithography system and to apply it to the fabrication of submicron Metal-Semiconductor- Metal (MSM) photodetectors. The other is to setup a comprehensive model to analyze the photocurrent response of GaAs MSM schottky photodiodes. In the first part, the frame size was first calibrated, and the backlash of the motor stage, the dwell time, the step size, as well as the probe current were then optimized. Other factors such as the thickness of PMMA, the baking condition, the exposure procedure, the metal thickness, the proximity effects, and then the focusing were also investigated. With all these optimization procedure, we succeeded in fabricating MSM photodetector with submicron metal line width and various spacing. In the second part of this thesis, the photoresponse of the MSM photodetectors was analyzed with 2-D simulator. Our simulation of the carrier transport in these structures reveals the strong influence of distance between the finger electrodes, the external voltage, the GaAs layer thickness, the temperature and the excitation intensity on the intrinsic response time and the corresponding frequency bandwidth of these photo detectors.We also calculated the performance for intrinsic devices with 0.5um interdigital spacing with the corresponding equivalent circuit of the photodetector. It is pointed out that parasitic effects may induce a significant distortion and a delay of intrinsic response. These can be minimized by choosing the optimal photodetector capacitance. Our results are consistent with that reported in the literature.Finally, nonuniform illumination to generate electric pulses from the transmission lines was also studied and a 3THz bandwidth was found for the transmission lines with 20um linewidth and spacing.