The requirement for image placement accuracy on photomask has been rising. The ITRS road map says that we need
to achieve 4.3nm accuracy in 2012 for HP 36nm device with single exposure process, further more we must achieve
3.0nm accuracy if double patterning process is selected.
Fig.1 shows the today's performance of image placement accuracy. Some sample photomasks which have same pattern
shape are produced during 3 months, and the mask to mask overlay accuracy of them was measured. The average of
them was 3.6nm. This data shows the possibility to achieve the accuracy of photomask for HP 36nm devices. The image
placement accuracy of actual device pattern during same period is showed in Fig.2. The image placement accuracy of
actual device pattern is worse than that showed before. We categorized these data according to the pattern density, low
density pattern and the high density pattern. The density of the test pattern is categorized into very low density pattern.
The results are showed in Fig.3. We can see the degrading of image place accuracy according to the pattern density.
This degrading of image placement accuracy is caused by resist charging effect. In photomask production process,
electron beam writer is mainly used as lithography tool. Each pattern on photomask is formed by step by step exposure
of electron beam. The surface of resist film will be charged with exposed electron beam, and electric field will be
generated around that charged area. So the orbit of electron beam for next exposure will be bended by the electric field
which generated by previous beam shot, and image placement accuracy will degrade. To achieve the demanded image
placement accuracy, we need to remove the error caused by this phenomenon.
We researched in resist charging effect for correcting it, and we studied that this phenomenon have so complex feature.
After that we tried to research in it on EBM-7000, newly developed electron beam writer, and we found out the reduction
of the image placement error caused by the resist charging effect on EBM-7000.