Method for monitoring focus in euv lithography

An EUV mask for monitoring focus in an EUV lithography comprising:

• a mask substrate;
• a multilayer reflector on the mask substrate;
• and
• an absorber over the multilayer reflector, the absorber comprises a repeating pattern of two different pitches which include a first pitch and a second pitch, wherein the first pitch and the second pitch are selected from a set of pitches inspected to give a largest or sufficient difference at their best focus positions for a printed photoresist pattern, and the repeating pattern of at least one of the two different pitches comprises an assist feature.

A method of making a test pattern in an EUV mask 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 a set of pitches, wherein the repeating pattern of at least one pitch comprises an assist feature;
• determining relationships between printed linewidths of the photoresist layer and the different focus positions under the repeating pattern of the set of pitches;
• identifying best focus positions based on the relationships;
• and
• selectively etching or depositing an absorber of an EUV mask to form the repeating pattern of two different pitches, wherein a first pitch and a second pitch are selected from the set of pitches inspected to give a largest or sufficient difference at their best focus positions for a printed photoresist pattern.

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;
• identifying a reference focus position where a printed linewidth of the first pitch is same as a printed linewidth of the second pitch;
• and
• performing EUV exposure on photoresists or other photo sensitive materials using a focus position with a pre-calibrated offset from the identified reference focus position.