EUVL Focus

Monthly Archives: March 2017

Further Thoughts from the 2017 SPIE AL EUV Lithography Conference

By Vivek Bakshi, EUV Litho, Inc.

Stochastics, Lent, Reporting on Conferences, Reality of Things, and a New SEMATECH

In the previous blog, I listed technology status and would now like to discuss a couple of topics in detail. During last year’s SPIE AL conference, the message for EUVL was “Not If, but When.” This year the message was “Not If, but When and How Much Volume.” It was nice to see the technology that I bet on so long ago coming so far and doing so well.

Stochastics and LWR – Why This is not the End of EUVL and Optical Projection Lithography

The stochastics of photons and material were in the focus during the conference. One presenter even called it the cause of the “end of EUV and lithography.” Line width roughness (LWR), or the non-uniform and wiggly shapes of lines that form tiny electrical wires, affects the electrical properties of the circuits that we are working to produce in the end. Although these properties are better for circuits made with EUV compared to multiple patterning, EUV has a serious stochastics challenge as there are 14x fewer photons. I believe that that stochastics will be addressed in some ways and we must remind ourselves that our goal is not “patterns on resists,” which is an intermediate step, but to make “tiny patterns” in the material under the resist. We can beat the apparent limit of physics in how nicely we can transfer the image from mask to resist by finding solutions after the intermediate steps, as I elaborate on below.

First, LWR is not a new story. Back in the nineties, when 193 nm lithography was being developed, I was working in the ATDF fab at SEMATECH developing etch processes. I was told by many that LWR would kill 193 nm litho, as printed lines indeed looked terrible on resist, as well as when those images were transferred to the material below to form lines and contacts. I went to the library (yes, in those days you actually went to the library) trying to figure out the source of this problem, but did not get any clues. In the end by trial and error, I found that a special post-resist patterning etch, initiated before the main etch, could clean up the pattern and drastically reduce LWR. I published a paper on it and did not think much of it at that time. This post-processing of resist patterns, now combined with post-litho rinses, new underlayers, new resist chemistries, litho- dep- etch optimization, flexible pupil illuminations, and innovative mask optical proximity correction (OPC) tricks, are among several knobs available to turn down LWR. Again, remember that resist image is only an intermediate step to what we are trying to do.

At this point, we need to remind ourselves that the Rayleigh criterion of resolution limits how small we can print using a given wavelength. However, we would not be printing what we can today if we had stopped at this resolution criterion. We have been overcoming this limit via OPC and other tricks, and the factor that quantifies our capability to print smaller is called k1. A whole industry emerged around how to take k1 as low as possible. When we approached the diffraction limit of 0.25 for k1 value, it was overcome by multiple patterning and the process continued. Eventually, to continue to print smaller and smaller in the quest toward atomic-level patterning, Litho needs to work together with deposition, etch and metrology. This is already happening, as demonstrated by several papers that showed joint development with etch suppliers.

I propose that the industry develop another factor like k1 (maybe call it s1) to measure how much we can reduce the effect of stochastics, with our goal being low s1 processes.

It is worthwhile to say a few things about Moore’s Law and its projected end by many. I believe that Moore’s Law in its true spirit is not only about physical scaling of the transistors, but also about the scaling of technology to allow ever-increasing information processing. I see transistors as “units of information processing.” We will get to the limits of the current mode of scaling at atomic level patterning with circuit parameters that cannot be gainfully further improved, but that is not an end to scaling of the speed of information processing. In the end, we must switch to different technologies like quantum computing to continue the pace. However, I see no end in the next decade for the current form of scaling. Let us not forget that developing technology for scaling is not cheap, and not without lots of effort.

Lent 

Ash Wednesday usually falls during the SPIE AL conference. Cathedral Basilica of St. Joseph is around the corner from the Fairmont in San Jose, where I usually stay. It has a beautiful interior and is worth a visit. I usually walk the blocks around the church until I get my required steps on Fitbit, and ponder on what I am going to give up this Lent – things that I very much enjoy and have not worked for me. This year, it surprisingly appeared to me that the trade press was also observing Lent and had given up mockery and negative coverage on EUVL, which usually starts on Sunday after Nikon’s Lithovision meeting, even before the start of the actual SPIE AL conference. It was unusually quiet this year on reporting. 

Reporting on Conference News

Toward the end of the week, there were some press reports which contained some inaccuracy. During the conference, one keynote speaker complained to me that he was incorrectly quoted by the media. Another keynote speaker was widely quoted as saying something that was not said in the presentation.

I do not blame the press fully for this, as there is an inherent difficulty in news reporting of technical conferences. Those who are familiar with scriptures know this is a challenge that humanity has faced since ancient times –reporting on complicated things from a distance. The Bhagwat Gita starts with the inquiry of the blind king Dhritrashtra, who asks an expert, Sanjay, to report on what is happening in the far away battlefield (dhramshetra kurushetekimkurvat Sanjaya – in the battlefield at Kurushetra, dear Sanjay, tell me what happened?). It’s interesting to note that Sanjay himself was not at the battle and had to rely on other means to tell the story – such is the case for many of us who are basing their reports on what is being told by someone in the conference. I cannot blame the press too much, having myself missed on a couple of points now and then. 

Reality of things – Lessons from Zen with Relevance to our Industry

As my Zen teacher says, We like the idea of things but not the reality of things. Ordinary coping is an attempt to shape our experience to always match our idea of things. If our experience maps onto our precious idea of things, this is called ‘happiness’ or ‘satisfaction’— getting what we want. This, we are taught, is the purpose of our lives and where we will find real meaning— it is the foundation for enjoying success.” In Zen training we practice turning toward and engaging with the bare reality of things. He further adds, “We are not continually trying to shape ourselves or the world to fit our idea of things. We are meeting things just as they are and yet working with them as skillfully as we can. Zen practice encourages and supports this skillfulness.”

When the industry got to immersion lithography, the biggest challenge was how to get rid of bubbles in the water. We certainly need to do a lot more and solve problems on many technical and infrastructure fronts. EUVL indeed is complicated, as it not only involves a new type of scanner but also changing the infrastructure for mask, resist and modeling. Materials, high temperature plasmas, lasers, contamination, fabrication and metrology— you name it. Moore’s Law did not say that scaling is going to be easy or inexpensive– it just said that it will happen.

I may be the only person in the world who believes that “EUVL IS NOT LATE,” and that “WE HAVE DONE WELL” with EUV technology development. Let us not forget how much time it took us to get immersion fully working, even though we had many fewer problems. The investment now is going to pay off. Chip makers know best, and so have decided on EUVL.

New SEMATECH

One last thing. During the conference I ran into Mark Melliar-Smith, ex-CEO of SEMATECH. it was a nice surprise, as I thought he had retired some time ago. I had just finished putting up an award on my office wall that I got from him many years ago. Seeing him reminded me of good old days of the semiconductor industry, when companies got together to address infrastructure challenges and consider technical challenges where success was not guaranteed. We saved money and tackled big challenges. It had to be done then, and later on we got away from the idea. It may be time to think about a new SEMATECH regarding efforts to extend Moore’s Law. In my previous blog, I listed many things that a New SEMATECH (if we ever have it again) could do, like considering stochastics at the 5 and 7 sigma levels, new resist chemistries, and new types of sources such as those proposed by PSI. We will not get zero defect mask blanks without considering new materials, ultrafine polishing techniques and contamination control options. Chip makers (it is usually Intel which spends more than others) cannot do this alone, and suppliers cannot afford to look at these challenges on their own, either. If we want to pursue new frontiers to continue pushing Moore’s Law forward, we need a new consortium like SEMATECH. I do not mind wishing for things, as that is the first step for things to happen! I leave you with a favorite quite from the Persian poet Hafez:

“I should not make any promises right now,
But I know if you
Pray
Somewhere in this world –
Something good will happen.”
― Hafez

 

2017 SPIE Advanced Lithography – EUVL Conference Update

By Vivek Bakshi, EUV Litho, Inc.

To simplify the vast amount of information from the 2017 SPIE AL EUVL Conference for my blog, I have adopted a new format. It includes a short summary of EUVL Status, a list of notable updates, and additions to the current list of EUVL Challenges (previously published on this site). An additional commentary will follow this blog.

  1. Current EUVL Status 

Source: Current power of 148 W corresponding to 104 wafers per hour (WPH) scanner throughput in-house at ASML. Stable 130 W noted in field. 375 W in lab EUV sources in burst mode at 50 KHz. 200 W of stable power is possible in field in 2017. Source power now meeting requirements for introduction of NXE3400. Current source availability at 75%, needed at >90%. Droplet generators and collector lifetime improving but need further improvement.

Scanners: Fourteen EUVL scanners in field. Four shipped in 2016. 0.3 nm critical dimension uniformity (CDU) and 1.8 nm overlay. 148 W and 104 WPH with increase of 8 wafers per hour (WPH) achieved via increase of stage speed at the same source power.

Masks: Mask blank defects acceptable for now via defect avoidance and repair.

Mask Pellicles: Mask defect addition during manufacturing is still a concern for chip makers. Pellicles are at 125 W and need to be ready for 250 W by 2H 2017. 

Mask Defect Inspection: Samsung has made its own AIMS tool and plans to use it for high volume manufacturing (HVM). Tool is using HHG based EUV source and a scanning zone plate. Zeiss is now shipping its first AIMS tool. Actinic Patterned Mask Inspection (APMI) tool still missing. Mask defect inspection via wafer inspection for now, at a cost and with lower yield. APMI is only red flag item for 7 nm insertion of EUVL. 

Resist: Lots of talk about stochastics, but I believe it will be addressed and it is not a showstopper. Important to note that resist image is only an intermediate step and there are still several knobs available to improve the performance of the final circuit – which is what matters.

  1. Notable Updates 

Scanner and imaging

  • Increase of throughput by 8 WPH via greater stage speeds is first such increase, with more to come. Now expect source power of 210 W to give 125 WPH instead of 250 W (increase in throughput via stage speed improvement)
  • Extension of EUVL to low k1 may be more difficult than for 193i. Discussion of various factors and how to address them has started.
  • More enthusiasm for high NA scanner, as it can help with line width roughness (LWR) and extension to lower k1. Detailed checklist of High NA challenges from Samsung.
  • Data showing that around 5 sigma errors deviate from standard distribution. We do not understand error distribution behavior at 5 to 7 sigma (it is no longer a normal distribution). We now need to print one trillion vias in one exposure with no open! 3 sigma is no longer enough.
  • Closer cooperation among litho, etch and deposition is the way to reduce EPE and address stochastics. Work has already started.
  • Scanner to scanner variation higher for EUVL than for 193i. How to address this in optical proximity correction (OPC)? Will this lead to scanner specific EUV masks?

Source

  • Need to better understand source power requirements for 3 nm and beyond. How much additional help we will get from scanner for increasing throughput? Is 500 W enough? Will we need additional power?
  • Power scaling to 500 W is still lots of work and not a done deal as conversion efficiency decreases at higher pulse energy (favored method for power scaling). 

Resist

  • Introduction at 7 nm planned at 20 mJ dose
  • Micro bridging (aka nano bridging) of resist is a new challenge reported by several people. Its relationship to dose, type of resist and LWR is not clear. Some said that this may become bigger than LWR issue. Papers showing OPC and Litho- etch optimization can help reduce this effect.
  • Continued work on chemically amplified resists (CAR), metal based inorganic resists and molecular resists to support 7 nm and beyond.
  • Out of Band (OOB) filter in now in scanner that also acts to keep resist outgassing products out.
  • Sigma alone may be insufficient to characterize LWR. New additional variables needed? 

Mask

  • Replacement of current mask absorbers by Ni to improve imaging. Continued review of new mask structures with improved imaging potential, but patterning challenges exist for these new stacks.
  • Need source mask optimization to address 3D mask effects.
  • High sensitivity to Pellicles defects for small pupil fills imaging
  • Need for further analysis and reduction of defects in the scanner, that end up on masks, generated during manufacturing.
  • Current fixed pellicle design needs to evolve to provide future solutions.
  • New carbon nanotube based pellicles from IMEC
  • New APMI design from PSI/ETH, supported by small synchrotron based EUV source.

New terms heard at SPIE AL

  • Etch Color, CD healing, Black Swans (at seven sigma), Vote-taking Lithography (resurrection of a 1986 idea to move away from 100% defect free mask requirements), nano bridging and micro bridging of resists, and Tone inversion.

Most Interesting Papers

  • Couple of papers on stochastics – Line edge roughness (LER) performance targets for EUVL (10143-10) by Tim Brunner of GlobalFoundries and Lithographic Stochastics – extrapolating to 7 sigma (10143-31) by Robert Bristol of Intel.
  1. Additions to existing list of challenges for EUVL

7 nm

Nothing new

5 nm

Micro bridging of resists

Error distribution at 5-7 sigma 

3 nm

Power scaling to 500 W and beyond

Micro bridging of resists

Error distribution at 5-7 sigma