Implementing registration measurements on photomasks at the Nanometer Comparator

Due to the introduction of double patterning schemes in the production processes of the semiconductor industry and the corresponding stringent specifications of the placement errors of the optical images of mask features (1.2 nm (3 sigma)) in the lithographic projection process, significant improvements of the registration measurements on photomasks are required. The Nanometer Comparator of the PTB, a one-dimensional vacuum-interference length comparator, has already demonstrated measurement reproducibility at this level on line scales and linear encoders, and therefore this comparator should also be applied for placement metrology on photomasks. Because a photomask is a relatively thin plate which is supported almost at its ends, the bending influence is much larger than for line scales and precautions had to be taken to reduce the influence of the sample support on the measurements results. A new sample mount was built, which exhibits mechanical alignment aids. In addition the position deviations caused by the mask bending were corrected by means of finite element calculations. Measurements performed with the Nanometer Comparator on a high-quality mask showed a maximum deviation of measured feature positions from the common mean of below 0.5 nm and a reproducibility of 0.2 nm. These results suggest that the use of a one-dimensional calibration of mask measuring machines allows us to perform traceable registration measurements with sufficient accuracy.