Method for monitoring focus in EUV lithography

摘要

This invention relates to a method of obtaining optimal focus for exposing a photoresist in an EUV lithography with an EUV mask containing a pattern with an assist feature. The invention also relates to an EUV mask with a special focus test target for monitoring focus in EUV lithography, and a method of fabricating this EUV mask by designing the special focus test target. The EUV mask contains a repeating pattern, wherein the repeating pattern has two different pitches, i.e. a first pitch and a second pitch, and contains an assist feature between main features. Because the two different pitches have different focus offsets, the difference between linewidths of said gratings provides a calibration curve which is a measure of focus. The method for monitoring focus is performing an EUV exposure using a focus position with a pre-determined focus position as calibrated using the linewidth difference between the two gratings. The EUV mask for monitoring focus of present invention is applicable to both test and product masks.

主权项

1. A method for monitoring focus in an EUV lithography comprising:
providing a substrate containing a photoresist layer; exposing, with EUV radiation, and printing the photoresist layer at different focus positions with a mask comprising a repeating pattern of two different pitches which include a first pitch and a second pitch, wherein the repeating pattern of at least one of the two different pitches comprises an assist feature; determining a first relationship between printed linewidths of the photoresist layer and the different focus positions under the repeating pattern of the first pitch in the mask; determining a second relationship between printed linewidths of the photoresist layer and the different focus positions under the repeating pattern of the second pitch in the mask; establishing a calibration curve which comprises printed linewidth differences of the two different pitches at the different focus positions; and performing EUV exposure on photoresists or other photo sensitive materials using a focus position which is pre-determined as a focus position on the calibration curve.