Automatic mask alignment without a microscope

In this paper we propose a two step approach for obtaining coarse and fine alignment without using a microscope. The new approach Is based on moire technique. The alignment marks on the mask and the wafer are in the form of coarse and fine gratings. The mask to wafer placement accuracy for the present system, is estimated to be ±40 μm. This is well within the capture range of the coarse moire system of pitch 1000 pm. The coarse moire system gives automatic alignment within ±8 μm, which is within the capture range of the fine moire system of pitch 25 μm. The final alignment accuracy is better than ±50 nm. Thus very high alignment accuracy has been obtained automatically, in two steps, without using the microscope and avoiding the dependance on the expertise of the operator