by M. David Levenson, Editor-in-Chief, Microlithography World
April 21, 2008 - Since any real defect on any photomask is likely to damage every die printed, maskmakers laboriously inspect their products to assure their customers that they are free of printable anomalies. (Meanwhile, the cost of mask inspection has begun to rival that of the mask writing process.) For advanced photomasks, newer inspection tools like the KLA-Tencor TeraScanHR can detect (using pixel sizes down to 72nm at the 4X mask scale) anomalies far smaller than those that can be printed by even the most advanced immersion stepper.
The problem is that such tiny anomalies can sometimes affect the resist image, depending on their type and location. Thus, sorting out the printable defects from harmless anomalies and false defects has been a major challenge for mask makers and mask equipment vendors.
At the just-concluded Photomask Japan, KLA-Tencor introduced Wafer Plane Inspection (WPI), a software suite that takes the high-resolution transmission and reflection images of the TeraScanHR system and computes both the aerial image projected by an exposure tool and exposure pattern in resist. By comparing the aerial and resist images of different die on a reticle (die-to-die mode), printable defects can be separated from the nonprintable, according to a paper presented by Rajesh Nagpal and a team from Intel Mask Operations and KLA-Tencor. Because the final criteria are based on low-resolution stepper-simulation images, 90nm pixel size on the WPI enabled TeraScan captures all relevant defects -- and rejects most false defects, speeding inspection by 40% compared to the 72nm pixel size. Review of suspicious locations found by WPI on a Zeiss AIMS optical aerial image defect review tool (considered the gold standard of photomask inspection) revealed a good correlation (22 of 25 located were indeed printable), according to the authors.
Line and space and contact hole masks were inspected, for a total of 12 advanced node reticles. Hard defects (which always print), soft defects (which only appear at the edges of the focus exposure window) and reticle haze (from airborne contamination) were identified. The longest reported inspection time with 90nm pixels was 89min. The availability of high-resolution reflected and transmitted images facilitated the evaluation of low contrast anomalies, such as haze, according to the paper. Left to itself, the high-resolution system with its 90nm pixel would have reported 319 potential defects, whereas the WPI system located 145, and reported 50 as yield-affecting.
WPI leverages innovations in computational lithography (as used for OPC verification by Brion, etc.) to extract information from the high-resolution reticle images captured by the familiar TeraScan inspection system. But first it must reconstruct the actual photomask structure -- which is not trivial, since phase-related information does not automatically appear in the transmitted or reflected images. Both phase-shifting mask structures and semi-transparent haze affect phase in yield-impacting ways. The exposure tool model (incorporating off axis illumination and other key parameters) propagates the mask pattern to the wafer plane, forming an aerial image. Then a resist model (currently a simple threshold) converts the aerial image intensity into a predicted pattern of circuit features which can be compared to the desired structure. "Defects" are located where the critical dimensions of these predicted wafer patterns differ from those desired by some specified amount.
Thus, the KLA-Tencor WPI detector attempts to do with computation what the Zeiss AIMS and Applied Materials' just-released Aera2 try to do with optics. Success will facilitate meaningful inspection of complicated OPC masks, allowing even more creative solutions to advanced imaging problems to be employed in production. -- M.D.L.