Research On Laser Displacement Sensor Based Wafer Inspection And Focusing Technology

Optical wafer inspection technology plays an irreplaceable role in the entire product lifecycle of semiconductor process development,production yield improvement,and mass production monitoring.The wafer surface is out of focus with respect to the focal plane of the objective lens due to factors such as surface undulation and thickness deviation of the wafer,which seriously affects the wafer inspection accuracy.In view of the above problems,based on the characteristics of laser displacement sensor such as good repeatability,high accuracy and non-contact measurement,this paper proposes a research plan for using laser displacement sensor to achieve focusing in wafer defect inspection equipment and conducts experimental verification of it,the main research content includes.1.Explain the optimal feasibility of using laser displacement sensors for fine focusing.The software and hardware design scheme for focusing using laser displacement sensor is introduced,and the principle of laser displacement sensor focus detection measurement,focus control principle and focus working principle are elaborated to provide the theoretical basis for the implementation of the focusing scheme.2.Design the focusing workflow.The focusing workflow proposed in this paper mainly consists of wafer facet data measurement,facet data filtering,facet fitting and compensation of off-focus volume.In this paper,we combine global coarse focusing with local fine focusing,and use laser displacement sensors to scan and measure part of the wafer face shape data on the basis of coarse focusing in order to fit an accurate full wafer face shape.According to the fitted full-slice surface shape data and the focusing principle,off-focus volume of any point on the wafer surface relative to the focal plane of the objective lens can be known,and the off-focus volume can be compensated by the focusing mechanism to realize the local fine focusing of the wafer.To ensure the reliability of the sampled data,and considering the real-time and sensitivity requirements of the wafer inspection system,this paper selects a composite filtering algorithm combining limit filtering and sliding average filtering.The experimental results demonstrate that the measurement error of the laser displacement sensor is effectively reduced by 3.3μm after using the composite filtering algorithm.In order to improve the accuracy of full-slice surface shape fitting,a grid fitting algorithm is proposed in this paper.Compared with the polynomial fitting algorithm used in the traditional process,the fitting error of this algorithm is reduced by 3μm,which can effectively improve the wafer inspection accuracy.3.Validation of the focusing study protocol.The dynamic and static repeatability of the laser displacement sensor is analyzed experimentally,and the results show that the dynamic and static repeatability is good and meets the practical requirements.The vertical face shape error of the system is analyzed,and the vertical face shape difference between the two stations of the laser displacement sensor and the CCD camera is compensated by fitting the third-order plane to further improve the focusing accuracy.Finally,the Tenengrad gradient method is used to evaluate the image sharpness of the system before and after focusing.The experimental results show that the line width of 1μm can be distinguished after using the focusing scheme in this paper,thus confirming the effectiveness of the focusing scheme.