Critical dimension (CD) targeting is one of the key process parameters for the disposition of photomasks. The
specifications are tightened at a constant rate over the years and are currently in the range of 3 nm for the most
critical layers. Many efforts have been put in prediction of the critical dimension that is targeting the actual
product based on data of previous products and also using resist data for further analysis and correction cycles.
So far the tool of choice was the CD SEM (scanning electron microscope) with significant shortcomings due to
altering the resist and its defect criticality during resist measurement.
Here we present data of long term resist CD monitoring on an n&k CDRT 5700 scatterometer system
measuring standard mask patterns in the non-active field. Presented are results for one resist on two different
photomask stack materials. The data is compared with the final CD measurement by CD SEM. The data is
correlated accounting for tool variances in the manufacturing process and the mask clear field loading. The
resulting model is still fairly simple with only 4 parameters for each process of record, one of them for the slope
of CD values between CD SEM and n&k and three offsets for different process variables.
The data shows stable model behavior over close to one year including several resist lot changes and significant
drifts in the front end process. The maximum forecast error is slightly above 2 nm and the process has a 95%
capability to predict mean to target values better than 2 nm. Furthermore, the defect level has shown to be
constant during that time frame with not a single incidence of particles due to the usage of the scatterometer. The
total cycle time impact is minimal because only 4 points are recorded thus loading and unloading the mask to the
tool is the actual cycle time adder. The described method is capable to significantly improve the CD targeting
performance due to better partitioning of processes.
Jan Richter, Jan Richter,
"CD forecasting in resist by means of scatterometry", Proc. SPIE 7545, 26th European Mask and Lithography Conference, 754509 (15 May 2010); doi: 10.1117/12.864431; https://doi.org/10.1117/12.864431