EXPERIMENTAL EVALUATION OF THE 2-STATE ALIGNMENT SYSTEM

In previous papers we have proposed the principle of the two-state alignment method and studied the effect of process coatings on the alignment signal. In this paper, we experimentally evaluate the performance of the two-state alignment system. We found the alignment signal sensitivity to be larger than 3 V/mu-m with a noise level of < 10 mV, which gives a noise equivalent error smaller than 0.003-mu-m. The alignment signal time response is < 10 ms. In a repeatability experiment, we obtained an alignment standard deviation 3-sigma = 0.036-mu-m from 500 alignment procedures. We also explored the effect of mask-wafer gap change on alignment signals. The results show that the alignment signal is tolerant to gap changes in a range of several microns. The details of experiment and experimental data analysis are described in this paper. The possible error sources to the alignment accuracy are also discussed. On the basis of the analysis, we can expect the alignment accuracy to be much better than the 36 nm we have achieved.