Rapid 3D reconstruction of photomasks based on differential structured illumination microscopy

The quality of the photo-masks directly affects the accuracy of the photolithography, therefore the detection of defection of photo-masks is necessary. The existed detection methods mainly include two-dimensional (2D) detection methods and three-dimensional (3D) detection techniques, of which 3D detection methods can obtain richer information compared with 2D detection methods and thus receive much attentions. Structured Illumination Microscopy (SIM) is an established method for accurate 3D reconstruction. However, this method needs to vertically scan the sample which is time-consuming. A method named Differential Structured Illumination Microscopy (DSIM) is proposed in this paper for fast 3D reconstruction of photo-masks. In this method, the sinusoidal patterns formed by computer are illuminated onto the sample surface through digital micro-mirror devices. The reflected images are captured by the DSIM imaging system. Before the measurement, the relationship between the differential modulation value and the height is calibrated through combing spatial domain phase shift method. The 3D shape of the sample can be determined according to the differential modulation differential and the established calibration results. The experiment results conducted on a photomask indicate that the measurement accuracy can reach 10nm and the reconstructing time of an imaging field only needs 0.02s. This method can achieve an accurate measurement with efficiency. This method has the potential to be applied in the domain of 3D defect detection of masks.