EDIT: This is the pic I was asking about:
BUT, the image is also made worse by how the image is captured. The picture you are looking at is a FIB (Focused Ion Beam) cut cross-section of a transistor which is then imaged by a SEM (scanning electron microscope). Probably in one of their fancy dual source beam devices. Depending on how you cut with the FIB, you can also push material downwards into lower layers and create a waterfall effect that will blur edges. It's can be very noticeable in rookie cuts but doesn't look like a major impact here.
Imaging with the SEM itself can also cause distortion and blurring as trapped charge accumulates on the surface and deflects the electron beam slightly. You can see this where the edges get increasingly bright because charges gather and become trapped at the boundary of the conductive and insulating layers. The image isn't sharp enough for me to tell if that is poly-silicon or aluminum contact traces.
Thanks for the detailed response. Might you have a link to one of those other posts or some other resources that talks about how these processes tend to produce rounded structures?
If so, that's not an SEM issue, that's just what it looks like. Photo exposure by it's very nature doesn't edge things 100% properly, diffraction or tiny misalignments causes stuff to end up being 'soft'.
Also, keep in mind that was from 1997. Fab tech has gotten way better; although we don't have a scale reference, I imagine if you were to make the same device on modern tech everything would look a lot 'sharper'. You still can't get away from some softness though.
Yes to both of these questions. "Soft" is probably a better way to articulate this, thanks. I'm not following the explanation of this being caused by photography however. Is this specific to photography at this scale?
>"I imagine if you were to make the same device on modern tech everything would look a lot 'sharper."
I don't have an example handy however I have seen more recent micrographs and they display the same "softness" and lack of true 90 degree angles etc.
As for 'modern tech' vs. old tech, I'm referring to if you made the same size silicon structure on newer tech. When you get down to 7nm or whatever, the same problems show up (or even new ones) because the sizes get smaller. I wouldn't be surprised if the image from 1997 is 180nm or larger nodes.
Also, unlike the other replier, I don't think there's any meaningful diffraction (causing softness) by the actual SEM images... they don't use glass lenses like traditional cameras.
Yes. A smaller scale makes it worse, a larger scale makes it better.
If you reduce things enough, everything will become so blurred that you won't be able to see anything (that's why we can't photograph atoms using visible light), and if you make things large enough your camera will become the bottleneck so a very small amount of blur is added to any object, whatever the size.
The resist has a finite thickness (often comparable to the size of the feature you are trying to make).
The developer solution works downwards through the exposed resist, but also outwards at the edges (where you will have some proximity effect in the exposure).
Deposited metal or insulator sometimes sticks to both the bottom of the pit and the top of the resist without a clean break, or contains chunks or grains that make the coating uneven.
Wet chemical etchants don't etch straight down, and often create rounded corners in cross-section.
Every aspect of this has decades of optimization behind it, and its success in any one fabrication run is vulnerable to a frustrating number of variables. If there's an SEM cross-section of a device in a paper, it's likely because fabricating it was an accomplishment, and/or the design is in some way new (not ruling out other reasons of course: there's also "we have an SEM and not a lot else to put in the paper").
TL;DR: I agree. These things are very small and if we could make the corners sharper, that would remove one major barrier to making them even smaller. And then they wouldn't look so tidy anymore.
I didn't quite understand what you meant here. Could you elaborate on this? What are some of those limits?
>"These things are very small and if we could make the corners sharper, that would remove one major barrier to making them even smaller. And then they wouldn't look so tidy anymore."
Aren't these contradictory? Did you mean to say "And then they would look so tidy"?
A few factors:
There's always some finite transition region between the fully-exposed area and the fully-unexposed area of the polymer resist area. With optical lithography, to get the smallest features possible, we're basically trying to make a shadow edge that's perfectly sharp and vertical through the thickness of the resist, by pressing a transparent mask with a pattern of thin metal on it right against the resist.
The thicker the resist, the blurrier the shadow down at the substrate. This blur takes the form of partially-exposed resist, which will develop faster than unexposed resist but more slowly than exposed resist.
With electron-beam lithography, there's the focus and control of the e-beam writer, but the sharpness and precision of the exposed pattern also depend on electron scattering and charging effects in the sample being exposed. Again, thicker resist makes the pattern less sharp as it needs a higher electron dose and will suffer more scattering.
We use a chemical to dissolve the exposed resist (there are processes where it's the unexposed resist that dissolves instead, but that's a tangent here). It takes time to dissolve the resist right down to the substrate, and as it's doing so, it's attacking the edge of the pattern too, albeit more slowly, making corners rounder and changing the vertical edge profile of the resist.
Say we get a decent pattern in our developed resist, and the next step is a liquid chemical etch. The etchant will dissolve the substrate at a certain rate. But it won't only etch straight down. If it etches equally in all directions, it will round the corners and etch under the resist, which makes the etched-away areas bigger than the pattern in the resist, with the rounded vertical profiles we often see in device cross-sectional images. This is assuming perfect resist adhesion.
A really tiny wet-etched pit has to start with a really, really tiny resist pattern and may well consist mostly of rounded undercut.
Fluid dynamics and chemistry in confined spaces often mean some areas etch faster than others and lines can get wobbly (this can be a factor in development too).
If, instead of etching, we want to deposit material like insulator or gate material, the developed edge of the resist has to have a profile that the deposited film cannot just run continuously up, or when we try to "lift it off" in the unwanted areas by dissolving the remaining resist, the patterned film will tear unevenly (at best). An overhanging resist profile is best for this. It's very difficult to lift off a deposited film that's thicker than the resist, so in general if you're depositing a film, you're starting with the challenge of a thicker resist layer.
Now imagine you've done a couple of steps on the devices already. You have perhaps some etched features, ohmic contacts, and an insulator, and now it's time to pattern some gates on top. The devices are now 3D. The resist is thicker in some places than others. The insulator tends to charge up in the electron beam. It's all the same principles again but a little more complex.
There is a plethora of techniques and chemistries to mitigate the issues encountered at every step, and you mix and match the things you are allowed to do and can afford to do and the skill level of available personnel to get the best result you can. The more automated you can get it, the better, because it's crazy how tightly every environmental variable has to be controlled to make a complex process reproducible, but in academic labs you still have students with gloves and tweezers and beakers and stopwatches, tweaking their process and device design to make something new happen.