Focus control enhancement and on-product focus response analysis methodology

With decreasing CDOF (Critical Depth Of Focus) for 20/14nm technology and beyond, focus errors are becoming increasingly critical for on-product performance. Current on-product focus control techniques in high volume manufacturing (HVM) are limited; ASML's focus metrology (Yieldstar) and new target design have madeon-product focus errors measureable. HVM fabs can now optimize their process based on the focus response of a specific layer for a specific product. Next to developments in imaging and focus control capability of scanners and general tool stability maintenance, on-product focus control improvements have become key to meet on-product imaging specifications. In this paper, we discuss focus monitoring, wafer (edge) fingerprint correction and on-product focus budget analysis through diffraction based focus (DBF) measurement methodology. Several examples will be presented showing significantly improved focus response and control on product wafers. Also, a method will be discussed for a focus interlock automation system based on product for an HVM environment.