Wafer inspection as alternative approach to mask defect qualification

Abstract

Defect inspection is one of the major challenges in the manufacturing process of photomasks. The absence of any printing defect on patterned mask is an ultimate requirement for the mask shop, and an increasing effort is spent in order to detect and subsequently eliminate these defects. Current DUV inspection tools use wavelengths five times or more larger than the critical defect size on advanced photomasks. This makes the inspectability of high-end mask patterns (including strong OPC and small SRAF's) and sufficient defect sensitivity a real challenge. The paper evaluates the feasibility of inspecting the printed wafer as an alternative way for the high-sensitivity defect inspection of photomasks. Defects originating in the mask can efficiently be filtered as repeated defects in the various dies on wafer. Using a programmed-defect mask of 65-nm technology, a reliable detection of the printing defects was achieved with an optimized inspection process. These defects could successfully be traced back to the photomask in a semi-automated process in order to enable a following repair step. This study shows that wafer inspection is able to provide a full defect qualification of advanced photomasks with the specific advantage of assessing the actual printability of arbitrary defects.

Optimal mask characterization by Surrogate Wafer Print (SWaP) method

“…Consequently, mask defect review becomes more cumbersome because all defect events found on the wafer need to be assessed and addressed with repair or clean. AMTC has investigated the use of SWaP specifically for defect characterization and reported this work at SPIE BACUS 2007 [1]. This work demonstrated, using a 65 nm DRAM test design as the vehicle, that direct mask inspection nominally captures more defects (critical and non-critical) than wafer inspection but, SWaP is more sensitive (defect size resolution) for some feature types and particularly for mis-sizing defects.…”

“…In the past, the prospect of executing this concept has generally been summarily discarded as technically untenable and logistically intractable. The AMTC published a paper at BACUS 2007 successfully demonstrating the performance of SWaP for the characterization of defects as an alternative to traditional mask inspection [1]. It showed that this concept is not only feasible, but, in some cases, desirable.…”