A new correction method for dry etch loading effect in photomask fabrication

The wet etching process in photomask fabrication could not meet requirements for small features for high density devices having optical proximity correction (OPC) features. Therefore, a dry etching process should be employed in mask making. However, there are some critical issues in dry etching process: one of the most important issues is critical dimension (CD) variation across the mask due to the loading effect during dry etching. CD errors caused by dry etch process are known to be originated from non-uniformity of plasma, non-vertical resist profile, and loading effect due to the various pattern densities. In case of loading effect, we are concerned about only macro-loading effect rather than micro-loading effect because the aspect ratio in the photomask is very small compared to that in the wafer. After dry etch, the patterns surrounded with large chrome area have large CDs because of the lower etch rate. We found that the macro-loading effect was about 5-10nm. And the effective range of the loading effect is amount to more than 10mm. In order to improve the CD variation caused from loading effect, we should optimize process conditions based on plasma uniformity, etch selectivity, resist profile, etc. However, it is a time consuming job. In this paper, we present a new method to improve loading effect, which is used for correcting e-beam proximity effect. Once we know the degree of loading effect, we can compensate it by allocating various doses at necessary locations.