Apparatus for euv lithography and method of measuring focus

A method of controlling an extreme ultraviolet (EUV) lithography process, the method comprising:

• measuring, in operation of an EUV lithography apparatus comprising an EUV source, a roughness parameter of a test pattern on an exposure wafer, the test pattern being made of photoresist and comprising circular islands or holes and the roughness parameter comprising standard deviation of a perimeter of the circular islands or holes;
• determining whether a focus of exposure by the EUV source is within an acceptable focus range based on the measured roughness parameter;
• and
• performing, in response to a determination that the focus is outside the acceptable focus range, a control task comprising (a) stopping the EUV lithography process, or (b) obtaining focus within the acceptable focus range by iteratively changing the focus of exposure, measuring the roughness parameter and determining whether the roughness parameter is within an acceptable roughness parameter range.

An apparatus for extreme ultraviolet (EUV) lithography, the apparatus comprising:

• a EUV radiation source;
• a wafer holder configured to hold a wafer to be exposed to EUV radiation from the EUV radiation source;
• a controller configured to control the apparatus for EUV lithography based on a quality of focus of exposure on the wafer;
• a metrology unit operatively connected to the controller and configured to measure parameters of a pattern on the wafer;
• and
• a non-transitory computer-readable storage comprising instructions configured to cause the controller to:
• cause the metrology unit to measure a roughness parameter of a test pattern formed of a photoresist prepared by multiple exposures of EUV at different foci of exposure and estimating a function representing a dependence of the roughness parameter on the focus, the test pattern comprising circular islands or holes and the roughness parameter comprising a standard deviation of perimeter of the circular islands or holes,
• estimate a best focus based on an extremum of the function,
• cause the metrology unit to periodically measure the roughness parameter for the test pattern on the wafer obtained by exposing the wafer at the best focus, and
• determine an abnormality in focus based on the measured roughness parameter and the function.