高精細X光圖像感測器之主動畫素電路的研究

Abstract

隨著X光設備的快速發展，下一世代的儀器結合了X光照相術和平面顯示器的技術，稱之為數位平板感測器。為了達到更低X光劑量與更高影像品質的目標，平板感測器中的畫素感測電路正快速的改善中。最近有一個新的主動畫素感測電路(APS)被提出，它移除了汲極偏壓線，因此每個畫素只具備一條水平線與一條垂直線。本篇論文的焦點主要集中在這個新的高精細主動畫素感測電路在實際應用上的可行性。為了確認此電路的感測功能，我們拿它和先前的電路做比較，利用實驗證明了兩者的感測功能一致，但新的電路可藉由移除偏壓線來消除漏電流所造成的誤差。此外，我們利用模擬對此電路做更進一步的分析。考慮到電路中的固定模式雜訊(FPN)，我們證明了臨界電壓(VTH)是最重要的因子。另一個關鍵的議題是IR drop，此現象主要是由水平供應輸出電流的方式所造成。對於無關於影像的IR drop而言，我們提出雙掃描主動畫素感測電路搭配儲存電容(Cst)校正的方法來解決。對於隨影像改變的IR drop而言，我們則找到誤差最嚴重的情況。最後，我們對電路做雜訊分析，並且依其結果將操作條件最佳化。

With the rapid improvement of X-ray equipments, the next generation is a combination of the X-ray photography and the flat panel display technique, which is called digital flat panel detectors. For lower X-ray dosage and higher image quality, the pixel sensor circuit of the digital flat panel detector is evolving. Recently, a novel active pixel sensor (APS) circuit is proposed, which removes the drain bias bus and is equipped with one horizontal bus and one vertical bus per pixel only. The focus of this thesis is the feasibility of this new fine pitch APS circuit for application. To confirm its sensing function, the new APS circuit is directly compared with the previous one. We experimentally verify their sensing functions are the same, while the new APS eliminates the leakage error by eliminating the drain-source voltage. Moreover, we further analyze the new APS circuit by simulation. Considering the fixed pattern noise (FPN), it is verified that the threshold voltage (VTH) variation for APS is the most important factor. Another critical issue is IR drop, which is resulted from the unique way of providing output current horizontally. For image independent IR drop, we propose the dual-scanning APS circuit with storage capacitance (Cst) adjustment to solve this problem. For image dependent IR drop, the worst case is found. Finally, the noise of the circuit is analyzed and its operating condition is optimized accordingly.