FAST-LH: a manufacturing-environmental friendly method of lens heating monitoring

Lens heating monitoring is crucial for photolithography process control; ineffective lens heating compensation will cause severe focus and image drift on photoresist pattern. Conventional/standard lens heat-test recommended by equipment vendor normally requires long measuring time which is not manufacturing-environmental friendly, and it is designed more to equipment perspective. A fast and accurate method of lens heating monitoring (FAST-LH) is discussed in this paper. Focus drift induced by lens heating is measured using both conventional and FAST-LH; result comparison shows strong correlation of focus drift with the new measuring method. Detailed methodology for the lens heating monitoring is studied; a fine tuned new measuring method is proven to be not only fast but also accurate to monitor lens heating LC compensation rate. Compared to the conventional method, FAST-LH could reflect better the actual focus drift under manufacturing environment. Due to the limitation of transforming the FAST-LH to equipment LH compensation settings, the FASTLH is implemented for periodic monitoring and feedback; whereas the conventional method is used during compensation/corrective action.