What could be better than spending a week in September in Monterey, California.\u00a0 This is especially true if you are from Austin, Texas, where the temperatures are still reaching 100 F and we just came through one of the hottest Augusts on record.\u00a0 Yesterday\u2019s sunrise (experienced while out for a run along the coast) was spectacular, as is the beer at Fieldwork.\u00a0 But what makes my week even better is that I get to spend it thinking, talking, and dreaming about lithography!\u00a0 (Does that make me weird?)\u00a0 I\u2019m here for SPIE\u2019s Photomask Technology + Extreme Ultraviolet Lithography conferences.<\/p>\n\n\n\n
Since the late 80s I have been attending (irregularly) the Photomask\nTechnology conference, which everyone calls the Bacus conference (long story). Three years ago, the Extreme Ultraviolet\nLithography Symposium was brought into the SPIE fold and collocated with Bacus,\nwhich has proven to be a great idea. The\ncritical challenges of EUV masks and related technology are getting the\nattention they deserve through joint sessions of the two conferences. For the ten years prior to the merger I didn\u2019t\nattend either conference. But bringing\nthem together has made it worthwhile for me, and since I have been coming every\nyear.<\/p>\n\n\n\n
The three and a half days of conference this week have 140 papers for both conferences (78 orals, 62 posters), including something new this year: two sessions devoted to stochastics. This topic is now well recognized as the primary challenge for the success of EUV lithography. The joint plenary session featured a talk by Mark Phillips of Intel, where he provided a good review of the progress and remaining challenges for EUV lithography, including the growing effort to develop high-NA systems and infrastructure. My favorite tidbit was the insight (obvious in hindsight) that the need for a pellicle in EUV manufacturing depends on your die size. When there are many die on one reticle, living without a pellicle may make sense. In this scenario, wafer (print die) inspections are used to find reticle defects (\u201crepeaters\u201d on the wafer) after they occur. Despite efforts to keep the inside of the EUV scanner clean, 20% of the time reticles at Intel developed particle adders between inspections, killing yield for those die. On the other hand, the use of a pellicle, which today has a one-pass transmission of 83%, results in a 30% loss of scanner throughput. (Note that using a pellicle also requires the use of a membrane just above the wafer to block out-of-band radiation, and this membrane has about 90% transmission. Overall light intensity is reduced by 0.9*0.83*0.83.) Which is more expensive, the lost scanner throughput due to low pellicle transmittance, or the lost yield due to printing reticle defects? That will depend on the die size.<\/p>\n\n\n\n
Marcel Mastenbroek, ASML\u2019s NXE:3400 product manager, gave a\ntalk on that tool\u2019s progress the way a proud parent talks about their children. After 20 years of industrialization efforts\nat ASML, both Samsung and TSMC are now producing chips, being shipped to\ncustomers, with at least one layer printed with EUV. Granted, this is \u201crisk production\u201d, with\nthose same layers also being printed for some wafers with conventional 193i\nlithography, but that doesn\u2019t take away from the importance of this accomplishment. ASML deserves to be proud. He noted that the next generation pellicle is\nspec\u2019ed to have a one-pass transmission greater than 88%, to be available next\nyear if all goes well. We\u2019ll hear more\nabout pellicle progress later in the week.<\/p>\n\n\n\n
One of my favorite quotes of the day came from Andreas\nFrommhold of imec: \u201cSchrodinger\u2019s\ncontact hole\u201d. This is a contact hole\nthat, due to stochastic variations, looks good at the top of the hole but doesn\u2019t\ndevelop all the way to the bottom (some middle region doesn\u2019t get enough\nphotons or doesn\u2019t generate enough acids to make the resist soluble). The top-down SEM image ADI (after develop\ninspect, meaning after lithography but before etch) looks normal, but after\netch the hole is missing. He also noted\nthat etching both increases the number of missing holes observed and the number\nof merged holes observed compared to ADI. \nMore work on etch process optimization is obviously required.<\/p>\n\n\n\n
On Tuesday I heard a talk by Claire van Lare of ASML on\nmaking attenuated phase-shifting masks (attPSM) for EUV. It is interesting to watch a new generation\nof lithographers learn the lessons of the past as she grappled with the\nproblems of \u201chigh reflectance\u201d attPSM such as sidelobe printing and reticle\nbarcode reading, things that were experienced 20 years ago with high-transmittance\nattPSM at the 248nm wavelength. I shudder\nto think about how OPC will deal with stochastic sidelobe printing.<\/p>\n\n\n\n
It is rare now days to have any semiconductor manufacturer give\na technical talk at a conference like this. \nSo I was very happy to see Intel give two talks this week. Robert Bigwood talked about the role of edge\nplacement error (EPE) in process definition, and he illustrated the value to\nIntel and to the industry of giving such talks. He described the thought processes used by\nIntel to develop an EPE budget and to make process choices based on that\nbudget. In doing so he was able to\nchallenge the audience with his needs. \nFirst on the list was a sufficiently predictive full-chip etch model,\nsince not every litho EPE will transfer into a complex film stack. I also learned that stochastic variations of\nresist feature height are a critical issue for them. I had not given this issue much thought\nbefore, but I will now.<\/p>\n\n\n\n
During the resist session three companies talked about\nincreasing resist absorption as a way to reduce absorbed photon shot noise\nwithout increasing dose. All three companies\nshowed that this is easier said than done. \nZeon made the most progress, with a 30% reduction in dose-to-size for\nsimilar contact hole local critical dimension uniformity (LCDU, a good measure\nof contact hole printing stochastics). Zeon\nstill has a way to go, however, since their non-chemically amplified resist\nstill uses a higher dose than the more conventional chemically amplified\nresists. Still, their progress was quite\nimpressive.<\/p>\n\n\n\n
I gave a talk on Monday afternoon, going back to my roots\nand the ideas that got me interested in stochastics more than 13 years\nago. I\u2019ve been trying to develop a\nsimple, analytical framework for predicting how the major factors affect\nroughness. For a long time I\u2019ve been\nstymied by quencher, a complication that I could not overcome mathematically. I made some progress (as always, by making a\nsimplifying assumption), and I am hopeful that my new derivations will prove\nuseful.<\/p>\n\n\n\n
The conferences are halfway through, and I\u2019ll report more at the end of the week. But I\u2019ll end this post with a tribute to Tony Vacca. Veterans of the Photomask Technology conference know that the highlight of the week is always the Bacus enterainment<\/a>, a series of skits with singing and dancing that make fun of the people and players in our industry. The all-volunteer cast works for months writing and rehearsing, with a frantic push at the end to make the show come together just in time. For the last ten years or so this crew of \u201cBacanalians\u201d (as we are called) was led by the amazing Tony Vacca. Starting four months out he would line up the volunteers, solicit skit ideas, cull the list, flesh them out, edit (mercilessly cutting to make each skit shorter and funnier), manage music and videos, and then rehearse. For an amateur actor like myself it was immensely fun, though incredibly stressful at the end. But the stress that Tony was under was always immense. Last year at the end of the show I thought Tony was going to have a heart attack from the pressure, and he announced that he was retiring from the Bacus entertainment. No one stepped up to replace him, and this year we have no entertainment. So tonight, when the conference banquet goes to the Monterey aquarium, I will be raising a glass of wine silently to Tony, with a smile on my face for all the wonderful Bacus entertainment memories that he enabled. Thank you, Tony.<\/p>\n\n\n\n