A method of spatial focus control using electro-optics for camera lens systems

A typical scene under the camera lens of a wafer inspection equipment consists of device features that are at different height relative to each other. To image such a scene, conventional camera systems within the equipment apply a single focus setting to all the features in the field of view. The resulting images consist of blurred regions corresponding to features that are not located at the object distance conforming to the lens focus setting. Such defocused images reduce the accuracy of defect detection process and adversely affect wafer yield. In this paper, we present a method to overcome this limitation using spatial light modulator (SLM). By optimizing the voltage to the cells of SLM, it can be converted into an array of partitioned thin lenses. Using this unique property of the SLM, we describe a process for obtaining “all-in-focus”, high resolution image of the entire field of view. Our simulation results show clearly the benefit of our solution.