On Tuesday morning at SEMICON I had the opportunity to sit down with Harry Levinson, Sr. Director of Technology Research and Sr. Fellow at Global Foundries and Michael Lercel, Director of Strategic Marketing at ASML to discuss the state of lithography.
I opened the discussion with a question about how we are going to address lithography from 10nm down to 5nm.
Mike Lercel – two specific directions, control and edge placement, the second theme is how multiple patterning introduces sources of variability. Process simplicity with EUV is beneficial at both 7nm and 5nm. A lot of argon fluoride immersion multiple patterning will stay and EUV will be used for the most challenging layers.
Harry Levinson – from a chip maker’s perspective there has been a lot of concern about EUV maturity for 7nm. They are looking at 7nm as a node that can be done with optical.
I asked a question about Line Edge Roughness (LER) and how much it matters for cut masks. Harry noted that even for cuts you do care about LER, for contacts and vias you do care about regularity. Via or contact on line-end is one of the most critical applications so LER does matter. Mike noted that LER affects where the line ends.
Harry noted that the good news is if we introduce EUV at 7nm we aren’t pushing it too hard. People are working on understanding LER. Shot noise is at the top of the list and you can’t do much other than increase the dose. Photoresists also contributes to LER and you need to control it at a molecular level and even the building blocks of the polymer are important so we need smaller building blocks. Mike – some of the novel materials look interesting because metal and some of the others are different than what we have today.
I asked about smoothing to address LER. Mike said it is spatial frequency dependent, the high frequencies can be smoothed better than the low frequencies. Harry, there is definite potential. Smoothing contact holes is harder than line/space and pesky line ends.
Harry said the 7nm node could be done optically and depending on customer demand could be introduced early with optical and then EUV could come next in 2018.
Mike said ASML systems in the field are at 125 watts and about 85 wafers per hour (wph), ASML’s target is 125 wph at 250 watts. Harry noted that the throughput is based on ASML assumptions and manufacturers have different requirements in terms of fields, dose, etc. Harry went on to say they are struggling to have EUV equivalent to immersion triple patterning on cost and that 5nm will likely be defined how far you can push EUV and still have single patterning. Mike, a true shrink that requires 6 immersion layers versus 3 EUV layers is kind of a cost wash.
Harry, a 2.5nm overlay budget is really hard because you are dealing with angstroms. Mike went on to say that is why you need really good mix and match of EUV to immersion.
Harry said EUV at 7nm would really help because you could learn before you have to really push it.
Mike noted that there are 8 – 3300s out in the field running and generating a lot of cycles of learning. 445,000 wafers have been processed through the tools. Without EUV we could be looking at 100 mask layers in a logic technology slowing down cycles of learning, design verification and manufacturing cycle time. (Authors note, I commented that this really struck me at the Advanced Lithography Conference this year, that for the first time there are multiple EUV systems around the world running wafers in volume and that is what you need for learning).
Harry said we will see contacts and vias done first, then metal blocks. Mask defects are still a problem but contact/vias have a lot of space to cover the defects (dark field with small open area). He is concerned about metal masks and defects (light field with high open area). It would be very desirable if line/spaces could be EUV. At N7 metal is 3 masks but he would like to do a single EUV mask. Mike also pointed out that at N5 you could be looking at a grating and more than 2 block masks.
Harry said that for 7nm contacts/vias productivity is still the main issue, they need 250 watts robust in the field. I asked him if he had a 250-watt high uptime tool today could he do 7nm contact/vias and he said yes. For contacts you have local critical dimension uniformity (LCDU) the contact version of LER and they can hit the specs with high dose, the key is how far you can back off the dose without hitting yield. With respect to mask defects the ITRS specs were based on planar gates, today there is no rigorous metric but to get to metal layers you need lower mask blank defects. Mike agreed, with contacts dark field can cover defects, metal is light field and the can’t cover the defects.
Harry, once you have the power you need to see if there are mask and wafer heating issues. Mike, Samsung saw a mask blister at 40,000 wafers, it wasn’t that long ago that immersion hazing occurred on masks at 35,000 wafers.
I asked about pellicles and Mike said no new announcements today (Mike and Harry were both scheduled to present in a session after our interview). They have run 200 wafers at a customer tool and they continue to run it at a 40-watt level. They still need to improve the pellicle. Harry jumped in to say the pellicles don’t have the transmission we are used to plus may need a filter; we will lose at least 20% of the light.
In closing Harry said that the front end of line (FEOL) has lower density so mask blank defectivity is less of an issue. EUV could enable multiple gate lengths and Mike also noted eliminate multiple cut masks.