INDUSTRY NEWS

THE MICRO INTERVIEW

TSMC's Burn Lin

PHOTO COURTESY OF TSMC

Burn Lin has been extending the limits of optical lithography for more than 30 years. The list of articles he has written—many of them the first of their kind on certain topics—reads like a what's what of advanced litho technology: deep-UV, multilayer photoresist systems, depth-of-field (DOF) and resolution scaling equations, k1 reduction, optimum numerical aperture (NA) in projection printing, and many more. But in his role as senior director in TSMC's micropatterning technology division over the past couple of years, Lin has been focused on 193-nm immersion, becoming one of its most fervent champions. What was once a lithographic curiosity has become the optical extension method of choice, due in no small part to Lin's scientific and diplomatic skills.

Before joining TSMC, Lin held various technical and managerial positions in the microlithography field at IBM. An SPIE and IEEE fellow, Lin has received numerous honors over the course of his career, including SPIE's first Fritz Zernike Award for Microlithography in 2004, TSMC's outstanding innovation award (also in 2004), and Taiwan's outstanding engineer award in 2002. He is editor in chief of the Journal of Microlithography, Microfabrication and Microsystems. He has also been president of his own company, Linnovation, since 1992.

I spoke with Lin during the SPIE Microlithography week in San Jose in early March. Our conversation covered many aspects of the IML phenomenon. He recalled how the presentation he gave at a 157-nm workshop in mid-2002 initiated the urgent discussions that eventually led to a sea change in the industry's lithography roadmap. He  recounts the difficulties he faced convincing the chipmaker and supplier companies to join the immersion camp, as well as the continuing evolution of the technical challenges posed by the wet-litho approach. After detailing some of TSMC's own IML roadmap, he ponders the technology's ultimate extendability.—TC

MICRO: I'd like to start by going back a few years. I can remember when I first heard you talk about immersion you were a voice in the wilderness, and there weren't that many people paying serious attention to the idea or thinking that this would be a production-worthy technology, or even be used. Can you look back and discuss some of those days, and then when you saw the idea really start to take hold?

LIN: The earliest I wrote about it and that I presented about it was in 1987. I mentioned immersion as a way, probably as a last means, to extend the depth of focus. But then there was no need for it. Now when you look at the roadmap, it goes very clearly from 193 nm to 157 nm and EUV and so forth; it's very continuous and there's no gap where you can insert immersion. For example with 157, the only place may be immersion after 157 and so forth. But I noticed that if you tried to do immersion 157, it's very, very difficult. Actually the technology itself in the dry form is already very difficult. So what's the way out of it? Maybe we should do immersion earlier than that. So I looked at 193, and then I looked at the properties of water and all of a sudden, when I did some calculations, it turned out that wavelength is equal to 134 nm, even shorter than 157. So that was the key point that convinced me that we should go 193 immersion instead. Because it's easier and the potential is higher and so forth. And I brought this point up at a 2002 meeting. It was actually a 157-nm workshop. I think I was the only invited speaker in that meeting to talk about immersion, and people thought I was going to talk about 157-nm immersion. I stood up and I told them, "Hi guys, unfortunately I'm in the 157-nm workshop but I don't think it's going to work. It should be 193 immersion." And I explained why. I had many slides explaining, mainly from that 134-nm concept. I think at that meeting I woke a lot of people up. When they realized the potential of 134 nm, quite a lot of people woke up.

The reaction was that after my talk people were murmuring, discussing, debating, whatever, on whether 193-nm immersion was going to be usable. I don't think too many people remembered they were in the 157-nm workshop. Then Sematech had this very urgently formed workshop, an immersion workshop, that I think took place either December that year or January the following year. In that workshop, we laid out many things that we should work on to make immersion more viable. From that point on, the momentum just kept building.

The other obstacle actually was not technology, but it was financial or economic. Because a lot of companies have invested in 157-nm technology, they don't want to give it up because first they want to recover their investment before doing something else. So there was a lot of inertia there.

MICRO: Internally, how much work did you have to do to convince your bosses and everyone else that TSMC also needed to participate in this and to convince them of the value of immersion 193?

LIN: In TSMC it was not too difficult. Since people sort of look at me as a leader in lithography, they had a lot of trust in my judgment. One thing that really surprised me was that even internally I was only starting to advocate immersion lithography. And our company president, in one of our outside lithography technology symposia, announced that TSMC is going to go for immersion lithography.

MICRO: He didn't tell you this before he made the announcement?

LIN: No! (laughs) So that was a pleasant surprise to me.

MICRO: What do you see as some of the key points of the evolution and the progress of immersion, from the point in time of that emergency workshop forward?

LIN: One key point is that we had to convince the equipment suppliers and the material suppliers to change their roadmaps. Of course, it was not easy. Fortunately, TSMC is a big customer of ASML, but still it was not easy because we had a lot of discussions between ASML and us about going into immersion. The argument I heard most often, almost from all suppliers, is that they had spent so much money on 157, hundreds of millions of dollars, so I asked them, "Are you still spending it?" They said, "Sure."  So, I said, "The sooner you stop, the sooner you stop wasting your money." That was my counterargument.

Now, in order to convince the suppliers, actually I had to line the users up. It was no good if TSMC was the only company that said they needed that. All the users and all the infrastructure would need to be there. So I did have to spend a lot of time talking to other semiconductor companies to convince them that that's the way to go. Of course, it was not an easy task. But then we got the positive feedback [from one company], won another company over, which helped me to win over another one, and also it helped to convince the suppliers and so forth.

MICRO: What about the list of challenges that was to be addressed? Looking back, when you examined that list when it was first put together, how has it turned out so far, comparing what you thought then to what the research actually found out after the work started?

LIN: There were several points. First of all, we had to look at the long-term potential. People were worrying that if we go to such a high NA, would we lose the advantage of immersion lithography. Then the ultimate potential will not have been as high as I thought. I did have to spend a lot of time trying to get a satisfactory answer, even for myself. When we go to a larger angle, we do lose some contrast due to polarization dependence, straight line, and so forth. So it's a matter of how I quantify those things. I spent a lot of time, and fortunately I got the answer. I thought up the methodology to simulate those effects, so I did a very thorough simulation and looked at the future of the technology to see how far it could go. That was good.

The other thing is that people always worry about depth of focus. And depth of focus is always a very difficult term to define. I think because of immersion I got to straighten out several points about depth of focus. I also used it to clarify and make sure that I'm comparing apples to apples. Of course, we're always worried about bubbles, defects, and whether we can keep the fluid homogeneous enough, whether it would introduce any aberration in the optical system and so forth. So those things came one by one.

Up to now, defect is still an issue. We're still studying, trying to understand where all those sources of defects are and trying to eliminate them one by one. I think bubbles are a much better picture. Since day one, we have known that bubbles can be dissolved in degassed water, so, first of all, the bubbles that are smaller than 50 nm wouldn't matter even if they were there. The intermediate-sized bubbles, if they can be dissolved very quickly in water, shouldn't matter either. But there will be sizes that are large enough that don't dissolve in water quickly and that would influence the imaging.

So it doesn't turn out to be that bad. I did put up some analysis on where the bubbles come from and how you can deal with them one by one. I think right now, the bubbles come from the design of the immersion head, so to speak. If it's not hydrodynamically designed, and not sufficiently hydrodynamic, there's a chance that you create turbulence, create bubbles. Also there's an issue of the surfaces matching, like whether they are hydrophobic, hydrophilic, and so forth. One really has to make sure and sort those out to minimize the bubbles. And it can be process induced, completely independent of whether we're using an immersion tool or not. We can create identical types of bubble from a dry tube if we don't carefully control our process. So we gradually understand all this, and again we think that the bubble issue is easier than the defect issue.

MICRO: With the defect issue, obviously, certain questions won't be answered until there's some production-sized quantities of wafers going through the tools. It's not something you can do just a few shots at a time.

LIN: Yes. Exactly.

MICRO: What about the lens. What kind of progress do you see the lens making, in terms of hitting 1.0 NA?

LIN: The lens is probably the easiest part, except that it's very expensive. It's not the cheapest, but it's probably countable. You can sort of predict how far it can go. I think the concern is whether one can really go to a lens beyond 1.35 NA. The reason being, at that NA, it's limited by the refractive index of water, so you almost have to raise the index of the fluid.

MICRO: With nonaqueous or aqueous fluids?

LIN: You can have a high index with an aqueous solution.

MICRO: Right. I've spoken to resist people, and they have this mysterious HIF1 or something like that. When I ask them, what's the secret sauce, they say, "We can't tell you that!"

LIN: Well, that, of course, is key: Who holds the rights to that liquid, holds the future! Well, actually, there are two stages of the liquid. If the liquid goes to a refractive index of about 1.55, the requirement is only that you have that index and reasonable transmission may be comparable with water. That would be sufficient. But let's say a lot of people claim they have 1.6, 1.65, whatever; once it's higher than 1.55, then you want to worry about the transmission of that fluid. It has to be higher transmission than even water. Therefore the requirement is twofold: refractive index and transmission, if you're talking about an index higher than 1.55.

MICRO: What is your view on the progress being made in the area of immersion photoresists?

LIN: We, as a semiconductor manufacturer, being concerned with cost and cycle time and so forth, we definitely do not want to see a topcoat there. We've let our suppliers know very clearly that we don't want a topcoat. In the interim, we will do the topcoat because if we don't know enough, we don't want to damage the lens or do something crazy. So in the short term, we live with a topcoat; in the long term, we don't want a topcoat. But either way, we have seen very promising materials.

MICRO: What about line-edge roughness?

LIN: The line-edge roughness shouldn't be worse than dry. It's universal.

MICRO: And what about CD uniformity and the like?

LIN: CD uniformity is a good topic. We just published a paper in the symposium that talks about our CD uniformity results. Two things: One is we compared the CD uniformity in the dry and immersion tool, and we did it in focus and out of focus. The in-focus one is very comparable because we are comparing with the same NA. So you don't expect anything to be better than the other. But when it goes out of focus, the CD uniformities for the immersion tool are much better than those from the dry tool. Again, that is quite obvious because the immersion process has larger depth of focus. So if you're out-of-focus at the same defocus, you're expecting to get better CD...

MICRO: Those go hand in hand.

LIN: Yeah. We also had an interesting way to compare the depth of focus. We qualified the depth of focus using yield. We exposed a wafer that's going for yield testÉwe exposed an exposure and defocus matrix. And then we [analyzed] the wafer, once with the dry tool and once with the immersion tool. Clearly we saw that the high-yield region was much larger for defocus with the immersion tool.

MICRO: What about the cost side? What are you seeing so far in terms of what you expect with the cost of ownership and productivity of the immersion tools?

LIN: We are seeing the potential throughput of the immersion scanner approaching that of the dry scanner, even though now it's not the case. But we're not seeing any particular problem that [would prevent it from] approaching that output of the dry scanner. But the tool itself is a little bit more expensive. It's more expensive than we had calculated. If you think about it, there's no expensive extra part in the scanner. The lens has the same elements, and they cut the last element by half and fill it with water, so big deal. There's the water head and so forth, a few mechanical, machined parts, and there's no tremendous high precision required in those. We don't see a reason for extra cost. But it is more expensive. I think probably because of [the suppliers] trying to recover the development costs of immersion and maybe also trying to recover the development costs of 157! (laughs)

Now the other cost factor would be when you push to a very high NA. The lens itself would be more expensive. Ounce for ounce, a 0.9 NA lens would be much less expensive than 1.1 or 1.3. In the good old days, the same thing, right? A 0.75 was more expensive than 0.65 and so forth. The other thing is that when you go to very high NA, and you want the extra advantage of polarization, then of course that adds to the cost.

MICRO: When the first production tools come in, they will obviously be used for critical layers. In a typical fab, how many immersion tools do you think you will need?

LIN: I cannot really tell you very precisely, but I can give you a feeling. So a typical fab covers about one or two generations. And the output of those fabs accounts for, let's say, a few tens of thousands of wafers per month, OK? Then you can work out the throughput of the scanners and so forth. Thinking about scanner throughput, it's about 100 wafers per hour, but it has to go through the scanner many times because there are many layers. So let's say typically if you have to go through 30 layers, some of them are critical and some of them not so critical, so there will be some less-expensive tools to go through and some more-expensive tools to go through as well. Also, on some of the not extremely critical layers, we will want to use the dry tools, because they are less expensive. So typically the number of scanners in a fab would be in the tens.

MICRO: So let's talk a little bit about your company's roadmap and how it looks at this point. What are the various insertion points or key development points?

LIN: Right now, we're using the 0.85 NA tool to develop 65-nm manufacturing technology. Hopefully we will have it in a very dependable operating mode by the beginning of 2006. That's our target. We hope to have debugged everything, have a very dependable process by the beginning of 2006. And from that point on, if that is our yardstickÉfor the 65-nm node, then let's say if we're on a very aggressive schedule for Moore's Law, that would be a two-year [cycle]. But in reality, you never know. If the demand is there, we may speed it up. If it's not there, we'll slow it down. But let's take two years as a norm. Then the 45-nm node will be beginning of 2008, then [32 nm in] 2010, and so forth.

MICRO: So you're pretty much sticking to that as long as you hit your target for the beginning of next year?

LIN: Yes.

MICRO: And ASML's own roadmap obviously has to be in step with yours.

LIN: Yes.

MICRO: Are you confident that they're going to be able to hang in there with you?

LIN: I think so...[at least] up to the 32-nm node.

MICRO: So, how far do you think immersion can be extended?

LIN: We definitely can count on it for the 32-nm node, which is equal to 45-nm half pitch. When we go to 32-nm half pitch, or 22-nm node, [that's] a scary node. The 193-nm immersion will still be possible but there will be more restrictions on the design, the throughput will be slower because we may have to go to double exposure, and so forth.

And of course, we have to see how EUV and some of the other NGLs are coming along.

MICRO: There are some people who question whether it can be extended to 32 nm. You seem very confident.

LIN: Getting to 32 is fine, at least from my simulation. I've been very carefully simulating. That means I have incorporated all those stray lights, everything, in my simulation. We do put some specifications; for example, at that time, we will need so many nanometers of depth-of-focus tolerance from the tool. And we will need, maybe, 100 nm of resist thickness. So the resist supplier has to give us 100-nm resist, but if they say they can only give us 150-nm resist, then we're out of luck. So everybody has to chip in a little bit.

MICRO: How much of your time is taken up with the immersion program?

LIN: At this moment, not that much time. Mostly, my people are spending time on it.

We have put quite a lot of people on it. We have the equipment people, we have the process development people, we have the metrology people, the defect team, and so forth.

MICRO: How big is the team?

LIN: Oh, I really haven't counted. At one time, there was a reporter who wanted to take a picture of the people who were related. It was about 15 or something, but I'm not sure. There are people who work on it who are not full-time and so forth. It's not easy to really count.

MICRO: Do you have some final thoughts on any short- or longer-term challenges that need to be overcome so that immersion can move forward at the rate that you hope for or that the roadmap calls for?

LIN: It's similar to the problem statement. We need to get rid of the particles, and I'd like to see a resist that doesn't need a topcoat, a very dependable non-topcoat resist. And, slightly longer term, I'd like to see that magic [high-index] fluid.