by Franklin Kalk, Toppan Photomasks
September 21, 2011 – The March 11, 2011 Tohuku earthquake and tsunami caused many deaths and much property destruction, bringing parts of Japan to a virtual standstill. One casualty was Photomask Japan 2011 (PMJ), which was originally scheduled to take place in Yokohama in April but was cancelled after the horrific events. PMJ has a sister conference, SPIE Photomask Technology (known in the vernacular as BACUS), annually staged in Monterey, CA in September, and this year BACUS has been extended by one-half day to allow the "10 best" PMJ papers to be presented in a special kick-off session. (The earthquake and tsunami occurred more than a month before the scheduled PMJ conference, a time when many authors had not yet finished their manuscripts, much less their slide presentations; nonetheless, 10 other papers are queued for the September 20th poster session. This is a testament to the indomitable Japanese spirit.)
Most of the papers focused on mask reliability and defect management, but a few examined the effect of masks on wafer print quality. Kei Mesuda of Dai Nippon Printing began the special session by discussing the impact of mask blank optical properties on printing quality in argon fluoride (ArF) wafer lithography. Surveying a range of available real-world set of optical indices for the mask blank absorber, Mesuda-san and his colleagues used modeling to identify two regions that might improve on current state-of-the-art materials. Interestingly enough, one area had high k (~2.5) and low n (~0.5) — clearly a binary absorber candidate — while the other area had low k (~0.5) and high n (~2.5), which is very similar to conventional molybdenum silicide (MoSi)-based attenuators. They then built and tested a material closely resembling MoSi. In 12 percent transmission (%T) form, the material offers better exposure latitude than 6%T MoSi on simple patterns. Mesuda-san promised future reports on this material.
James Oberschmidt of IBM India presented results of his group’s evaluation of process variations and their effects on wafer printing accuracy. Using a baseline optimized mask design, they systematically varied several parameters, including mask critical dimension (CD), illuminator partial coherence, resist n and k, and others. They found that mask CD has the strongest effect on print accuracy. I guess there’s no rest for the weary.
Itaru Kamohara of Nihon Synopsys explored the use of ellipsometry to characterize topography in double patterning. Using the example of the litho-freeze-litho-etch (LFLE) process, he showed how the detailed topographic information available from ellipsometry can be used to optimize and control the contact hole lithography process.
Mask pattern accuracy drew a couple of ably-presented papers. Erez Graitzer showed how Carl Zeiss’ RegC mask registration correction system can be used to improve the image placement accuracy of masks. The technology deforms the mask placement semi-locally by creating small, non-imaged damage sites inside the fused silica substrate with a femtosecond laser. The damage site layout can impose asymmetric deformation on the mask. A nice example is one of a mask with two areas of asymmetric placement errors that are orthogonal to each other (one error in X, one in Y). The tool can render the placement error symmetrical over the entire imaged area; this "scale" error is then readily corrected by the wafer exposure tool.
Aki Fujimura of D2S discussed the use of model-based data prep to improve the accuracy of variable-shape beam e-beam mask writers while simultaneously reducing shot counts (and therefore cost and cycle time). Standard e-beam proximity effect correct (PEC) algorithms can maintain required pattern accuracy over scale lengths greater than, say, 1