#homecmos IRC log for Wednesday, 2011-07-06

lekernelazonenberg, what's the difference between epitaxy and evaporation/LPCVD?11:00
azonenberglekernel: I'm not too fami.liar with the details of epitaxy but it forms a single-crystal coating12:12
azonenbergThe others form a polycrystalline coating from a vapor, the difference is the source of the vapor12:12
azonenbergevporation is purely physical (heated chunk of the material)12:12
azonenbergCVD (and variants like PECVD, LPCVD, MOCVD) pump in reactive gases that combine or decompose in some way12:13
azonenbergAnd sputtering is also a physical process but there's an inert gas present to do the sputter12:13
azonenbergReactive sputtering is a hybrid - vapor is formed by normal sputtering processes but before it hits your sample it reacts with another gas12:14
azonenbergfor example using an Ar / N mix to sputter Ti12:14
azonenbergthe Ar knocks Ti atoms off the target, which then react with N to form a film of TiN12:15
azonenbergEpitaxy, on the other hand, you are depositing either the same material as your substrate or something with a compatible crystal lattice12:16
azonenbergin either case it forms a single-crystal coating where the atoms are locked to the lattice of your substrate12:16
azonenbergAnother important difference is the pressure12:20
azonenbergSputtering has an inert gas present and CVD has source gases, so both happen at relatively low vacuum (few hundred Pa ish)12:21
azonenbergMean free path is short and you get a pretty even coating since the atoms bump into each other and are moving pretty randomly12:21
azonenbergEvaporation is done at much low pressure (1E-6 torr ish)12:21
azonenbergMean free path is comparable or greater than the distance from the source to your sample12:21
azonenbergWhich means atoms evaporating from your source travel in pretty much a straight line (not hitting any gas molecules etc) before hitting the sample12:22
azonenbergAs a result the coating has some of the characteristics of spray painting - bumps in the surface cast shadows, edges perpendicular to the source are coated thinly if at all, etc12:23
azonenbergwhich is bad for some processes (filling vias for example) but very good for others (lift-off)12:23
lekernelso, epitaxy is a particular case of evaporation?12:25
azonenbergThe source of the vapor for epitaxy can vary12:28
azonenbergI'm not familiar with the usual sources12:28
azonenbergthis isnt a process i've considered using either for my fab, or needed for anything at work etc12:29
azonenbergThe other processes i've actually studied in some detail12:29
azonenbergIn any case i'm heading up to campus now, i've got work to do12:29
azonenbergAnd a 10:00 (local time) appointment with a SEM :)12:30
azonenbergExpect pictures this evening12:30
azonenberg_worklekernel: back13:17
azonenberg_workOn the SEM now taking pics17:05
azonenberg_workprobe tips are sharper than i thought17:06
azonenberg_work200nm ish dia flat tip17:06
bart416:D17:08
bart416azonenberg, btw did you think about the destructive stm test?17:08
lekernelepitaxy might be interesting as well - think of microwave transistors, diodes, etc. though I don't know the difficulty level to have equivalent performance with the commercial ones that need an interview to get a quote for17:12
azonenberg_worklol18:00
azonenberg_workworth looking int but prob difficult18:00
bart416lekernel, any ideas for scraping a single layer of atoms of a sample?18:42
lekerneler... maybe with those tunnel microscopes?18:42
bart416Yeah, that's the thing18:48
bart416It's too slow to remove the layer from a large surface18:48
bart416I suggested it somewhat unserious yesterday to use a STM to scan a sample layer by layer18:50
bart416But the more I think about it the more it makes sense18:50
bart416An STM could break a crystal latice18:50
bart416It'd take several millenia to do that for a large surface :p18:54
lekernelwhat do you need atomic precision for?18:58
lekernelespecially on a large surface ...18:58
bart416I want to scan layer per layer with a stm :P18:59
bart416Somebody in ##electronics had a nice idea18:59
bart416Using a pulsed laser with a kerr lens18:59
lekernelmh, maybe the stm isn't the best tool for that18:59
lekernelor maybe you could make it faster by increasing the voltage and the distance between the probe and the sample? (just guessing here)19:03
bart416Maybe19:08
azonenberg_workPics coming later today19:48
azonenberg_workSome interesting stuff but the guy had to leave before he could show me how to use EDS19:49
azonenberg_workSo i have to come back tomorrow19:49
bart416EDS as in what sense?20:01
azonenberg_workenergy dispersive spectroscopy20:03
bart416ah20:03
bart416That should show interesting results20:04
azonenberg_workusing x-rays emitted from the sample to determine what elements are present20:04
azonenberg_workWhich will let me tell whether the stuff i'm looking at is hardmask (tantalum based), photoresist (organic), or a mixture (some of each)20:04
bart416If you're willing to do some math you should be able to get a pretty good formula for it20:04
azonenberg_workhttp://i.imgur.com/R5o3m.jpg is a quick shot of what i did today (low res, original is 2x in both dimensions)20:04
azonenberg_workThis is an etched die (F7) seen at 2000x20:05
azonenberg_workThe thin layer on top is hardmask residue (not removed completely by liftoff), the silicon can be seen extending outwards below it to the etched surface in the background20:05
azonenberg_workDiagonal surfaces are <111> planes, the background is a <100> plane20:06
azonenberg_workAnd the microscopy tools have the software to identify most of the peaks in the chart20:07
azonenberg_workI just dont know how to use it or how to install the detector (it's not installed by default)20:07
bart416heh20:07
bart416read the manual!20:07
azonenberg_workiirc you need to physically remove the backscatter detector to install this one20:07
azonenberg_workkeeping the everhart-thornley secondary electron detector for aiming20:07
azonenberg_work(and imaging)20:07
berndjif you forego atom-size res, could you push target atoms around faster than normal with an STM?21:13
berndjjust thinking of that destructive test procedure you guys were talking about21:13
berndj(ab)using the STM probe tip as a mechanical cutting tool, in effect21:13
bart416berndj, yeah but a good stm top doesn't come cheap :(21:21
bart416*tip21:22
berndjoh - i was assuming DIY STM with broken lightbulb filament probe!21:22
bart416Those won't do well for atom moving I fear21:23
berndjno?21:23
berndjdo you need different / better tips for atom moving than for imaging?21:25
bart416well, moving an atom is a lot harder21:29
bart416Precision becomes more important21:30
azonenberghttp://i.imgur.com/DljuL.jpg22:41
azonenbergColorized version of another etched die (G3)22:41
azonenberggreen is unetched Si, blue is etched down by around five microns22:42
azonenbergyellow is hardmask that wasn't completely stripped, plus possibly other surface contamination22:42
--- Thu Jul 7 201100:00

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