Advances in Process Overlay on 300 mm wafers

Overlay budgets are getting tighter within 300 mm volume production and as a consequence the process effects on alignment and off-line metrology becomes more important. In a short loop experiment, with cleared reference marks in each image field, the isolated effect of processing was measured with a sub-nanometer accuracy. The examined processes are, Shallow Trench Isolation (STI), Tungsten-Chemical Mechanical Processing (W-CMP) and resist spinning. The alignment measurements were done on an ASML TWINSCAN(TM) scanner with the ATHENA(TM) sensor and the off-line metrology measurements on a KLA Tencor. Mark type and mark position dependency of the process effects are analyzed. The mean plus 3 a of the maximum process induced overlay offset after correcting batch average wafer parameters is used as an overlay performance indicator (OPI). 3 sigma residuals to the wafer-model are used as an indicator of the noise that is added by the process. The results are in agreement with existing knowledge of process effects on 200 nun wafers. The W-CMP process introduces an additional wafer rotation and scaling that is similar for alignment marks and metrology targets. The effects depend on the mark type; in general they get less severe for higher spatial frequencies. For a 7(th) order alignment mark, the OPI measured about 12 nm and the added noise about 12 nm. For the examined metrology targets the OPI is about 20 nm with an added noise of about 90 nm. Two different types of alignment marks were tested in the STI process, i.e., zero layer marks and marks that were exposed together with the STI product. The overlay contribution due to processing on both types of alignment marks is very low (smaller than 5 nm OPI) and independent on mark segmentation. Some flyers are observed for the zero layer marks. The flyers can be explained by the residues of oxide and nitride that is left behind in the spaces of the alignment marks. Resist spinning is examined on single layer resist and resist with an organic Bottom Anti-Reflective Coating (BARC) underneath. Single layer resist showed scaling on unsegmented marks that disappears using higher diffraction orders and/or mark segmentation. Resist with a planarizing BARC caused additional effects on the wafer edge for measurements with the red laser signal. The effects disappear using the green laser of ATHENA(TM).