In the second talk of the morning in the EUV session, Andrew Liang of Lam Research showed how much work it takes to optimize a new process, and how that work can pay off. Local critical dimension uniformity (LCDU) is a term that refers to stochastic-induced variation in CD. Conventionally, CDU looks at the variation of CD across a chip, exposure field, wafer, and lot caused by things like mask CD variation, film variations across the wafer, focus control across the exposure field, hotplate temperature uniformity, and many other factors. The length scale of these variations tend to be quite large compared to the pitch of the patterns being printed, so that two feature next to each other are assumed to be largely affected in the same way by all of these variations. Stochastic variations, on the other hand, have a length scale (called the correlation length) that is small compared to the feature size so that we can understand its impact by looking at any features, even ones right next to each other. By measuring the CDU of a small group features (a 7X7 array of contact holes, for example) we can isolate the stochastic impact on CD uniformity from the other CDU factors. This is the idea behind LCDU.
Liang optimized the hard mask below the resist by switching to a thinner PECVD film, optimized the lithography process to maximize the image log-slope, and optimized the etch process using atomic layer etching. The last item is the most interesting to me, since it looks like it is possible to use an etch rate that varies as a function of aspect ratio to compensate for resist CD variation. When the resist CD is too small, the aspect ratio of the hole is higher. For a typical etch process, this higher aspect ratio would cause shadowing of the etchant and a reduction in etch rate, making the small hole even smaller. But etching can also involve polymer deposition on feature sidewalls to slow etching down. If that polymer deposition slows down with higher aspect ratio, maybe it is possible to increase the etch rate when contact holes are too small, thus improving the LCDU. To me this seems like magic, but only in the sense of Arthur C. Clarke’s third law, “Any sufficiently advanced technology is indistinguishable from magic.” Others have reported on this very exciting possibility, and I am looking forward to learning more.
Ravi Bonam of IBM collected a large amount of data from a programmed roughness mask, a mask that contained an added rectangle (jog) along the feature edge of varying size and frequency. By measuring the mask and the wafer after printing, something can be learned about the optical transfer of roughness from the mask to the wafer, and the ability of wafer metrology to see roughness at specific frequencies. Unfortunately his data analysis and presentation left me unable to grasp a single lesson learned from his data. I’ll have to wait for the manuscript.
Tom Wallow gave a comprehensive overview of sources of metrology variation for the case of metrology used for OPC model calibration. His two laments were the same as from every lithography model developer. First, customers want models to fit the data better than the data uncertainty justifies. Second, models that are based on physics require data that has accuracy, not just precision. Historically, metrologists have focused on precision for the simple reason that accuracy is just too hard a problem to comprehend. Tom, I hope people absorb your lessons, but don’t hold your breath.
I presented my last paper at 2pm, and then I was mostly done for the day. After giving my last demo of Fractilia’s new MetroLER software, socializing with friends that I rarely see at other times throughout the year became my last priority of the conference. I sampled only about 10 – 15% of the papers in the symposium, and I learned a tremendous amount from them. I dub this year’s major theme to be stochastics, and I am glad for the attention that it is finally receiving. I’ll go home with many ideas to investigate and try out. For anyone interested in my papers and presentations, I’ll be posting them soon on my lithoguru website, and also on the new Fractilia website. But first I’m going home to relax.