Angle Position Monitoring Technology of Micro-mirror Array for Freeform Illumination Module in Immersion Lithography

摘要

In the 28nm and below nodes lithography machine, the freeform pupil illumination has been widely used with the application of source mask optimization technology. As one of the most important implementations, the freeform illumination module is equipped with micro-mirror array (MMA), which can generate arbitrary' pupil by adjusting angle position of individual mirrors. It is necessary to monitor the angle position of MMA real time for its significant function. However, there are several difficulties in the monitoring: 1) The monitoring unit could not disturb the working light path. This determines that the monitoring light should be glancing incident onto the micro-mirror; 2) The size of micro-mirror is relative small, and its rotation angle range is relative large in two dimensions; 3) There are several thousands of micro-mirrors. The crosstalk should be avoided in the monitoring method. In order to find a suitable monitoring method, the imaging and Fourier transform methods are studied. In the imaging method, the reticle is imaged onto the detector with the reflection of a single micro-mirror. The center of the reticle image is calculated to represent the angle position. In the Fourier transform method, the angular distribution of the light reflected by the micro-mirror is detected. And the angular distribution centroid is used to evaluate the angle position. In order to verify the feasibility and compare the performance of the two methods, the influences of alignment error and the scattered light are analyzed. The simulation results show that the Fourier transform method is insensitive to the scattered light and is independent to the relative position of the micro-mirror and Fourier transform lens.