單光子崩潰二極體之元件尺寸與串音效應

Abstract

為了能提升空間解析度，元件尺寸的微縮成為光偵測器結構設計的趨勢之一。過往實驗室已設計出直徑8 μm的元件，雖然有低雜訊的優勢，但在光靈敏度上卻大幅降低。本論文藉由台積電 0.18 μm高壓製程，製做單光子崩潰二極體(SPAD)，並設計不同主動區大小的元件來進行量測，藉此討論元件尺寸對其特性的影響。透過TCAD模擬，我們觀察到衝擊離子化密集區在主動區所佔的比例，會隨著元件主動區的微縮而變小。我們推論衝擊離子化密集區比例的微縮，是造成小尺寸元件光靈敏度低於預期的主因。透過元件尺寸效應的研究，我們有利於未來設計適合陣列化的元件。在此同時我們設計兩種不同結構2x2 SPAD陣列：一是外部電極環(Electrode-ring)的共用，二是陰極井(Cathode-well)的共用和矩形主動區圓角化。此二陣列可以作為串音之研究。與過去實驗室的SPAD array相比，本論文提供填充因數(Filling Factor, FF)高達34%之元件，雖然暗計數(DCR)略為提高，但仍具備低雜訊數與高光響應的優勢。在元件間距的微縮下，串音的現象不能再被忽略，因此我們藉由此二元件與工研院提供的2x8 SPAD array來進行串音之研究。透過量測，我們觀察到串音現象符合距離平方的衰減趨勢，因此我們推論直接路徑是光學串音的主因。此趨勢有利於設計元件陣列的最小間距。

To improve the spatial resolution, the scale-down detector structure design becomes the recent research trend. Our group has developed a small size single-photon avalanche diodes (SPAD) with 8 μm diameter active region. Though it owns extremely low dark counts, its sensitivity is much lower. In this work, we design SPADs in standard 0.18 μm high-voltage CMOS technology provided by TSMC. By measuring the SPADs with various sizes, we analyze and investigate the size effect. With TCAD simulation, we observe that the high impact ionization area and its proportion to active region become smaller when SPADs scale down. We infer that decreasing impact ionization area proportion to active region may be the main reason to the lower sensitivity of small-size SPADs. With Size effect research, we can provide the information which is beneficial to SPAD array design. Besides, we provide two 2x2 SPAD array structure design: the one is the SPAD array with electrode-ring sharing, the other is cathode-well sharing SPAD array. Comparing to the conventional SPAD array, our work provides high filling- factor(FF) advantage, which FF is up to 34 %. Though our device design has higher DCR, it still owns the low dark counts and high detection efficiency. Since the SPAD array intensity increases by reducing the space between devices, optical crosstalk problem becomes the main issue. We attempt to research device crosstalk with the 2x8 SPAD array provided by ITRI. Through the measurement, we observe that the crosstalk decay follows the inverse ratio to the distance square. We infer direct optical path dominates optical crosstalk, which benefits us to design the minimum spacing between SPADs.