Average CD of CD SEM and scatterometry CD (OCD) have been adopted for advanced CD control. The advantages and
disadvantages of these two CD metrologies have been well discussed. The target of CD uniformity (CDU) for
advanced technology has been driven to 1 nm, i.e. about three and half the size of a silicon atom, which is 0.29 nm. In
the real production environment, engineers need to face sub-nanometer (< 1 nm) CD variations and do the necessary
process corrections to meet the 1-nm CDU requirement. In other words, advanced CD process control has already been
in the world of atomic scale. It turns out that methodology to ensure the accuracy of sub-nanometer CD has become
essential for advanced CD control.
In this paper, we introduced a methodology to produce 0.25, 0.5, and 1 nm programmed pitch offsets through mask
design. These offsets are attainable with current process capability. Pitch offsets instead of line/space width offsets were
used because the pitch is relatively process insensitive. The pitch has already been widely used as a CD SEM
magnification calibration standard, e.g. Hitachi m-scale 240-nm pitch and VLSI 100-nm pitch standards. We produced
large and small pitch splits to meet different magnification linearity requirements. We also used optical CD to verify the
programmed pitch offset. Using the raw spectrum of OCD, systematic pitch signal changes in 0.25-nm steps can be
detected, ensuring that 0.25-nm pitch offset standards are meaningful. Interestingly, 0.25 nm is smaller than the 0.29-nm
We also used this standard wafer to do the sampling size or data quality evaluation for different CD SEM measurement
methodologies, e.g. 150K by 150K or 80K by 35K magnifications that in turn dictates the sample size. Pitch sensitivity is
strongly related to the sampling size and line-edge roughness (LER). For example, 0.25-nm pitch sensitivity needs a
larger sampling size than those of 0.5-nm and 1- nm pitch sensitivities.
By means of this standard wafer, we can easily quantify metrology quality as well as choose the right metrology and
sampling size for advanced process control.