Metal Oxide Photoresists Lower Roughness
by Bryon Moyer
April 08, 2014 at 12:00 PM
One of the huge challenges of advanced-node patterning is roughness. There are actually two flavors of this: line-edge roughness (LER) and line-width roughness (LWR). Almost the same, but not quite.
The first is visible by looking at one edge of a line, and it’s hard to use any other word besides “rough” to describe what this means. It’s not long meanders in the line (which you might not expect from a mask, but might get with something like directed self-assembly (DSA); it’s the really small granular stuff. If you were to extract the frequency content of the edge, you’d be looking for the high-frequency mode, not the low end.
LWR is similar, but takes into account both sides of a line. Each edge has its own roughness, but, if by some effect, the two sides managed to track exactly, each jogging right or left when the other did, then the width of the line would be exactly constant. So even though the edges might be rough, in this idealized case, there would be no LWR (and no changes in impedance, although the constant impedance would be impacted by scattering at the rough edges).
In practice, the left side might jog left while the right side jogs right, creating a wide spot (lower impedance) in the line. Or vice versa, pinching the line down (higher impedance). This gives roughness to the line. You can think of LER as common mode and LWR as differential mode if you want. Yes, there’s a mathematical definition; we don’t need it for our purposes here.
So, obviously, the thinner your lines become, the more that roughness – if it remains constant – becomes a problem, because the deviations from perfection become an increasing percentage of the line width. And I used the word “granular” above on purpose, since it is the granularity of the resist that contributes to the roughness.
Standard resists are polymers – chains of monomers of some substance. The smallest unit available here is the monomer itself – break that and you end up with a different chemical substance. And, according to Inpria, a startup offering new resists, these monomers are in the range of 4 to 6 nm in length. That’s a non-trivial percentage of the feature size at a 10-nm node (even though we shouldn’t take that 10 too literally).
In order to smooth out the resist, a smaller grain is needed. And this is what Inpria is proposing to bring to the table: metal oxide-based resists. They’re delivered in an organic medium, but that organic nature gasses away, leaving only something that resembles a ceramic more than a plastic. And the grain size is on the order of 1 nm. Inpria claims the smallest, smoothest sub-10-nm lines yet.
Its hardness is another benefit. Many times “hard masks” are created by filming the wafer with a “hard” material and then patterning that with resist. These metallic oxide resists are hard enough to act as their own hard masks, eliminating those extra steps.
Challenges remain. Dose sensitivity is still being optimized, although it’s apparently better than conventional resists, which Inpria says require a high dose. Anything reducing the needed dose goes directly to improving exposure throughput, which we know is the biggest remaining EUV hurdle. (At this point, I have no information as to whether the double-exposure trick reported earlier applies here… no reason to think it would, since the chemistry is different.)
More challenging, perhaps, is reducing defectivity. This is where much of Inpria’s activity is focused now.
I suppose it’s too early to declare victory – new materials and new vendors of something so fundamental will be viewed cautiously by fab managers. You can imagine metals and their oxides that might play havoc if set loose in a cleanroom, so there’s something of a convincing job. But hafnium and tin are top of the list at this point, with good performance and good fab compatibility.
We’ll be able to watch progress even as EUV results are demonstrated. You may notice in the numbers that companies like ASML present, in some cases they’ll show results both for conventional and for Inpria resists. If you’ve ever wondered what that means, well, now you know.