I bounced all over the conference on Thursday (the last day), from Tim Brunner’s paper predicting focus and overlay performance based on measured wafer bow and thickness, to Derk Brouns’s update on an EUV pellicle. Nelson Felix of IBM discussed early use of EUV lithography for 10-nm and 7-nm node logic devices (as opposed to the high-k1 results reported by Intel on their 14-nm node). The data-packed paper deserves much further study and I hope his proceedings paper contains all the information from his presentation. He mentioned the rule of thumb, commonly discussed this week, that if EUV throughput can get above a reliable 60 wafers per hour, EUV is cost effective compared to combining three 193 immersion patterns. All such calculations assume many things, not all of which I was able to catch from the talk, that greatly affect the outcome. I’m sure that equal yield is assumed (a standard assumption for cost calculations), but we also need to know the dose that was assumed. Nelson mentioned that while printing 36-nm-pitch lines and spaces, going to a dose of 35 mJ/cm2 produced a noticeable yield improvement compared to a 30 mJ/cm2 dose. I wish that we might see more data like this in the future, since we desperately need to understand the yield/dose trade-off.
Jo Finders of ASML gave an excellent talk, emphasizing what many resist companies don’t quite get: the quality of the image coming from the scanner matters a lot. For decades, lithographers have focused on optimizing masks and illumination to maximize the NILS (normalized image log-slope) of the image. Exposure latitude is proportional to NILS, so every little bit of improvement matters. Early EUV work was at larger k1 values, and many people were not yet concerned with process windows, so NILS did not get as much attention. But Jo reminded us of something we should never forget: LER, LWR and the local critical dimension uniformity (LCDU) that is caused by that roughness are all inversely proportional to NILS. Thus, for a given resist material at a given exposure dose, the easiest way to lower LER and LCDU is to increase the NILS, using classical approaches like illumination optimization. Of course, everyone should be following Jo Finder’s advice. We also have to be aware that any roughness measurements must be made at the same NILS to be comparable – something almost no one does. The RLS (Resolution-LER-Sensitivity) trade-off should always be NILS corrected. Another option is to use the LER/LWR resist metric that I proposed, though it seems not to have caught on since I published it in 2014. Still, that metric is also NILS dependent and so NILS must be controlled in order to make comparisons. One way to do so is to use a reference image, such as the interferometric images produced at Paul Scherrer Institut.
On another topic, ASML and Nikon described the performance of their latest 193 immersion scanners. Overlay, focus control, and throughput on these new tools are very impressive. Tool productivity has increased by a factor of 2 in the last 7 years or so, and I wonder where continued productivity improvements will come from. A very difficult problem.
The last paper I attended was on detailed characterization of roughness measurements. My friend and roughness expert Vassilios Constantoudis could not make it this year, so his co-author Hari Pathangi did a good job of delivering the paper for him. Building on the earlier work of Ricardo Ruiz, this paper explored the correlation of roughness from feature to feature for both SAxP and DSA. (I hope you have been reading this blog all week, because as you can see I am now dropping acronyms as if I learned them in elementary school). For these techniques we must look not just at edge-to-edge correlations, but feature-to-feature correlations across multiple pitches.
So what are my impressions of the symposium overall? This was a year of important but incremental progress. Let’s look at how various technologies are trending.
EUV lithography has made important progress over the last year and the mood among many is more positive (especially since two years ago). Now that the 80W sources in the field are running in a moderately reliable fashion, learning in the fabs has begun in earnest. ASML has demonstrated a 200W source, but the delta between lab demonstration and reliable performance in the field is a great one. The key question for the source will be when do customers get their next upgrade? Will it be 100W by the end of the year?
Nanoimprint lithography (NIL) has made lonely progress at Canon (since they bought Molecular Imprints) and Toshiba, with SK Hynix joining the effort to some extent. And while serious investment in NIL came years too late, there is still a good chance they will succeed, at least for flash production.
Stochastic awareness seems to have hit critical mass this year. At the dimensions we are now experiencing, the fundamental stochastics of the world are coming to dominate lithographic behavior. Stochastics are hard to control, but any hope that we might do so will be through greater theoretical understanding and careful experimental measurements.
Directed self-assembly (DSA) progress has been disappointing in the last year, though that could be due to my inflated expectations. It appears that no one is yet using DSA in production, and every head-to-head with a competing method (atomic layer deposition for contact hole shrinks, SAQP for lines and spaces) has favored the incumbent process.
EUV resists switched from excitement about nanoparticle resists to excitement about metal-containing resists. There is much hope that real progress is just around the corner, but results remain depressingly consistent: the only way to lower LER is to raise the dose. Resist developers have not embraced a thorough understanding of stochastics as the foundation of their resist design, and have not internalized Lord Kelvin’s dictum: if you can’t model it, you don’t understand it.
Of course, there were many other things going on at the conference, and I was able to attend only a small fraction of the many talks presented this past week. As always, I am invigorated by the progress and learning that I have seen, and exhausted by the non-stop intellectual challenges that this symposium provides. When I hit that post button for this final summary, I’m going to bed.