One of the flattest materials, and the source will surprise you

hey everyone today we're looking at flat surfaces like what's the flattest and smoothest substrate that we can find around the home shop or easily accessible online during the first atomic force microscope video i showed a scan of a precision ground gauge block and it really surprised people by how pretty gnarly it looked it had lots of grinding marks and was not all that smooth at least not what you'd expect from something that's precision ground and to be fair it was a cheap gauge block but it does ring together and it has a pretty nice surface finish so since then there's been a lot of questions on all the follow-up videos about different relatively flat items like lenses and mirrors and people were curious how it compares to stuff like the gauge block so today we're looking at six different items that have varying degrees of flatness and we'll look at them from the most rough to the most flat so to start the comparison we'll look at a brand new gauge block it's still in its grease paper so i haven't used it yet so all the defects that are on the surface are purely from the grinding process itself so like the first gauge block this one has some pretty distinct grinding marks and the surface roughness of the sample is about 11 nanometers rms which puts it right at the kind of high precision ground surface so it's what you'd expect from an item like this the next item up is a first surface mirror this is a precision ground piece of glass that's then coated with i believe it's aluminum in this case a protected aluminum surface and this one is a moderately nice mirror it's not the finest quality i got on clearance from edmonds so it is a lower quality than kind of like the premium mirrors we can still see grinding marks here but they're much more shallow and a whole lot smaller and the whole surface itself is just averaged out to be a much smoother and flat surface rms is only three nanometers here as opposed to the 12 on the gauge block next up is this fused silica mirror blank this is a piece of quartz that's been polished on one side and is ready to become a mirror but hasn't actually been coated yet the specs on this mirror are lambda over 20 meaning that peak to trough no part of this mirror should be more than a wavelength of light divided by 20 in nanometers and so if you take the standard which is like a green wavelength of 630 ish nanometers divided by 20 that's about 30 31 nanometers so peak to trough no point on this mirror should be more than 31 nanometers from highest to lowest if we look at the stats on the scan we can see that the highest peak is about 29 nanometers and the biggest pit is about seven or eight nanometers so that's i'll round up and say that's within tolerance of the spec that puts the rms around 1.9 nanometers and it's interesting because you can see a little bit of texture here where it's not grinding marks like we saw on the prior two examples but they're long streaks but you can kind of see a periodic roughness where the grinding has happened it's not perfectly smooth now amusingly the next flattest sample that i have is this cheap glass slide it's probably just soda lime glass it probably costs less than a penny to make and it has a little bit better surface roughness than the precision lambda over 20 quartz mirror blank now interestingly the surface is both better and worse in different ways so the average roughness is a little bit smoother than the quartz but you can see there's these large particles that are kind of scattered around and they're small enough that i don't think it's just dust and i did clean these all pretty well and tried to keep them dust free so i think these are actual particles on the glass surface and so the deviation is quite a bit higher here than on the precision optics but the average roughness just barely beats it out it's a little bit smoother my theory for this situation is that this is just a cheap piece of float glass where the glass is floated on a liquid metal surface while it's cooling and that gives you a remarkably flat surface because it's just relying on gravity to flatten everything out but you don't have a whole lot of control over the peaks and valleys and so the optical properties are not quite as good as something that's been precision ground now it's probably a good time to pause and talk about the difference between flatness and roughness i've been kind of treating them the same saying like flat or smooth or flat and rough and at this scale that's mostly true when you're looking at a 10 by 10 micron area they're pretty equivalent but in optics and you know the macro world that we live in flatness and smoothness are very different properties flatness is how planar the whole surface is whereas smoothness or roughness is kind of the little deviations in between so the smaller micro uh changes in the flatness and so when you buy a precision optics and you get something that's ground to lambda over 20 you know that's for the whole surface and so the whole surface is both smooth and flat whereas something like a float glass it might be very smooth at a micro scale but the flatness might be super wavy all over the place that's something to keep in mind technically the precision optics probably should be placed higher but i just found it amusing that the cheap cheap piece of glass could have a better average roughness the second flattest item that i have around the shop is the silicon wafer so it is single side polished on one side and it's basically a mirror rms roughness is 1.6 nanometers so this is a pretty smooth piece of material is actually a pretty interesting material because it natively forms a very thin oxide layer a layer of glass essentially on top of the silicon and so when you scan just the wafer like i did it has at least several nanometers of silicon dioxide on top and the silicon dioxide is pretty smooth like we've seen with the the glass samples so far but it's not like super crystallinely smooth just the way the glass forms on top but if you were to strip that oxide off and keep it under an inert gas or in a vacuum and scan it the silicon crystal layer actually has a very very smooth surface and you can get some pretty impressively flat scans although it does have kind of a terraced or stepped nature to it just based on how the crystal was formed so you can kind of see that here some of this modeling is probably from the slight unevenness of the silicon dioxide layer that's on top of the silicon and not necessarily the silicon itself although some of it could be due to the grinding process as well i'm not sure what the specs on this wafer are as far as like flatness and smoothness and finally we get to the flattest material available to me which is a piece of mica and unlike the silicon which is highly refined and grown in a lab to be super pure this is actually just a mineral that's pulled out of the ground and made available and the reason mica can be so so flat is because it can be cleaved along its crystal lattice in a way that gives you atomically flat surfaces so what that means essentially is when you take a piece of mica and put a razor to the edge you can easily separate the mica into two sheets and the way the mineral breaks is that it splits along the edge of the crystal lattice such that you get a continuous crystal face and the face along that crystal is just a set of very flat atoms and so you get what's known as an atomically flat surface where you really have a whole bunch of atoms that are rigidly held in position all at the same height and there's no debris or particulate or any other kind of contamination because you're just splitting a pure crystal in half and just the nature of this crystal is that it cleaves very easily it's not very rigidly held together so you can take a small mica piece like this and get 10 or 15 individual sheets out of it pretty easily without trying too hard the piece of mica that i scanned has a rms of 500 pico meters which is a pretty small number and the peak to trench deviation is only four to five nanometers in either direction so this is a very flat sample and some of it is probably just due to noise or my inexperience with scanning because this was actually probably the hardest sample that i've had to scan yet which is ironic in that there's nothing here to scan it's just a flat surface but because it's so flat and there's no features other than the flatness you really start to pick up any noise that's inherent in the system or vibrations around the room or even air currents or just even the pi controller not being tuned quite right so you start getting oscillations off the really super flat surface so this was actually a real challenge for me to dial in and get a good scan and i suspect someone that knows how to use this system better than i do could get an even flatter and less noisy scan than than this i think it's interesting to see all of the scans kind of in comparison to each other so so far all the scans have been scaled to make it easier to see the individual features of that scan but if we set them all to the same scale so 0 nanometers is the darkest value 100 nanometers is the brightest value you can really see the kind of magnitude difference between the mica and the gauge block for example so it's pretty cool just to see the the difference here and see what you think is flat is really not that flat and then what is unexpectedly flat splitting a mineral in half ends up being atomically flat mica is used very frequently in atomic force microscopy and sometimes in scanning electron microscopy because it's so flat this is important for items like nanoparticles right if you're putting 20 or 50 nanometer particles onto a substrate you want to make sure that any bump you see on the scan is actually the nanoparticle and not just some defect in the substrate itself like if we were to go back and look at using the glass slide or even the precision optic glass some of those bumps could be confused for very small nanoparticles and that's not what you want when you're trying to characterize the nanoparticles itself so mike is often used because you can cleave it and get a fresh surface that you know is devoid of any contaminants and it's super flat and then anything you see is your substance that you're trying to measure okay cool well i think that's all i got for you today i hope you enjoyed that this was a fun little diversion to just see how flat is actually flat and i hope you enjoyed thanks for watching i'll see you next time

Channel: Breaking Taps

Views: 150,608

Rating: 4.9569588 out of 5

Keywords: atomic force microscope, afm, atomic force microscopy, icspi, mica, silicon, wafer, gage block, flat, smooth, roughness, mirror, quartz, atomically smooth, silicon wafer, atomic force microscopy images, atomic force microscopy tapping mode

Id: 2cZ0XJWxyH0

Length: 10min 41sec (641 seconds)

Published: Mon Jul 19 2021

Reddit Comments

[u/paxplantax]

They clearly didn't have acess to my butt.