本論文提出一套新的自動對焦系統-「差動式疊紋對焦系統」，其利用兩個線性光柵配合顯微鏡架構，來分析待測物的位置變化時反應至偵測面上的成像大小，並依據疊紋光形的變化情形，來判斷待測物的位置。本論文之發明，可提供面板產業所使用的自動光學檢測機(automatic optical inspection, AOI)以及雷射修補機(laser repair, LR)所需的自動對焦問題，以此為基礎，提供一個穩定、簡易、且高解析度的自動對焦系統。
我們依據光學成像和疊紋效應原理，配合顯微鏡架構，開發出一套可觀測與自動對焦的系統。本論文的研究方法如下:以一準直光源通過第一道光柵，經物鏡聚焦於待測物上，再由待測物之表面反射回到光路系統後，經目鏡成像於第二道光柵上;疊紋是由兩組或兩組以上間距相近的光柵所交疊形成的低頻條紋，當成像大小不變(放大或縮小)時，將可觀測到斜率為0(正或負)的線性疊紋。如此可藉由疊紋光形變化，搭配疊紋判讀技術與迴路控制演算法，來判斷待測物離焦程度使之自動化對焦。
根據實驗結果，本系統在光柵間距0.1mm時，其靈敏度為1.6*10-2(um-1)，光柵間距0.2mm時，其靈敏度為1.3*10-2(um-1)。因此使用者可在固定物鏡的規格下，擁有彈性的量測能力。在優化靈敏度的設定中，我們可藉由偵測位置的選定，以提升系統的靈敏度；調整兩光柵間夾角，使系統具有判斷離焦方向的能力。
將優化條件設定於不同的物鏡系統，根據其實驗結果，本系統在低倍物鏡下，工作距離最長可達200 um，其精度為5 um；在高倍物鏡下工作距離最長為15 um，其精度可達0.2 um。由此可知，使用者可依據不同的使用需求，選擇需要的規格，達到對焦的目的。
本系統利用疊紋效應原理，以及偵測光強差動訊號的方式，可達到較為精密的對焦。更進一步，配合新式的演算法，使得非同調光源系統的對焦穩定性提升，以達到高解析度、穩定、且簡易的對焦系統。

In this thesis, a novel auto-focus system based on a differential technique and the moiré method is presented. Two linear gratings and the structure of the microscope are used for detection of the sample’s defocus situation. The system basically solves the auto-focusing problems in the AOI (Automatic Optical Inspection) and LR (Laser Repair), and is a simple, cheap, and useful to apply.
The system is based on optical images, the moiré effect principle and the architecture of the microscope, and is able to shows the image and auto-focus at the same time. The moiré fringe is formed from two similar gratings with the lower frequency. If the size of the projected image size is the same as the (bigger or smaller) grating, a horizontal linear moiré pattern (having a positive or negative angle) is detected along the horizon line. We can determine whether the sample is located on focus or not from the detected moiré patterns. Finally, we combine the moiré detection system with P control and a feedback algorithm.
The simulation and experimental results show our system to have a higher sensitivity with a smaller grating period and a higher magnification objective lens. We also provide advanced setting function to improve the system’s sensitivity by choosing the exact detected pixels on a Linear CCD. The features distinguish between in focus or far focus by rotating the gratings. All the advanced choices are set for objective lenses with different magnifications. The experimental results show that we can obtain an accuracy of about 0.2 um with a 50X objective lens, or a long detection range of about 200 um with a 10X objective lens.
Our system has a high resolution and simple structure, which is based on the moiré technique, the differential signal, and a novel feedback algorithm. It has the ability to enhance stability with a non-coherent light source focusing system.

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