Developing A New Curvilinear Data Format

The data size generated by curvilinear masks could impact turnaround time (TAT) for photomask production and hence the adoption of curvilinear masks. In a previous blog on curvilinear masks, our panel of luminaries discuss some possible solutions in a video discussion. In this seventh video, the panel looks at some ideas to define a new curvilinear data format to reduce file sizes. Aki Fujimura of D2S opens the panel discussion with an explanation of the problem for those not as familiar with the discussion. He illustrates the fundamental challenge for a curvilinear data format in trading off data size versus accuracy and then turns to the panel to address the question of how to compactly represent a curve with infinite resolution by using a mathematical technique.

Noriaki Nakayamada from NuFlare Technology has published numerous papers on the topic of a curvilinear data format and explains the progression over time of the NuFlare mask writer formats shown in figure 1. The latest format, MBF 2.0, introduces holes and B-spline curves to support curvilinear masks. According to NuFlare, MBF 2.0 reduces the data size as compared to MBF 1.1 by a factor of 2-3 with an edge placement error (EPE) of 1nm tolerance.

Fig. 1: NuFlare Technology’s MBF 2.0 data format includes B-spline curve to support curvilinear masks with reduced data size.

Danping Peng from TSMC welcomes industry cooperation on this topic. He believes a reasonable representation of the data with acceptable data size will facilitate acceptance of curvilinear ILT and masks. Ezequiel Russell from Micron Technology agrees but also emphasizes the importance of accuracy noting that EPE budgets are already tight when moving to advanced nodes. So, a new curvilinear format needs to avoid introducing a new source of EPE error. Aki Fujimura further explains that there is a trade-off in the amount of effort to encode a B-spline algorithm for curves versus accuracy. Meanwhile, the work continues in the industry to come up with a widely accepted curvilinear data format.