#homecmos IRC log for Friday, 2013-02-15

diginetsivoais: what are you doing here?01:17
sivoaishaha, hey diginet! I'm an engineer. I like engineering things.02:00
diginetI'm not an engineer (lol), but I've talked with azonenberg a little about homecmos02:38
diginetI think having a DIYable chip fab would be just about the greatest thing ever02:39
diginetwe were talking earlier actually (in PM), and I thought of something that seemed odd and he didn't know either, maybe you can answer02:40
diginetsivoais: why is tungsten (often) used for vias in CMOS? I know that copper has issues with electromigration, but why not aluminum?02:43
sivoaisdiginet: no idea. I'm not a materials person. Would be a good thing to look through the literature for02:46
diginetgood idea02:49
Action: diginet googles02:49
Sync_diginet: I guess it has to withstand KOH etches and be easily depositable05:37
diginetSync_: yeah, I looked up some papers and they basically said something similar05:38
Sync_but it seems cu is possible after all05:39
Action: Sync_ is going to kill his prof....05:40
azonenbergSync_: Nobody uses KOH for CMOS05:40
azonenbergTMAH maybe05:40
azonenbergbut Cu survives exposure to low-concentration TMAH used as developer05:40
azonenbergAl is slightly attacked05:40
diginetyou know, I was thinking, wasn't the whole point of silicon that it has a (relatively) good native oxide? If high-k dielectrics are replacing SiO2, why even both anymore05:43
diginetthe problem is finding an alternative that isn't made of unobtainium or something stupid like Arsenic05:43
Sync_I thought KOH was still used for bulk etching05:45
Sync_diginet: germanium is the answer05:45
diginetreally? why do you think so05:46
Sync_becaue I work in it05:46
Sync_we're doing Ge on Si05:46
Sync_and GeSi stuff05:46
Sync_which is easily integratable in current cmos processes05:47
Sync_your phone does it05:47
diginetI wasn't questioning you, I was just curious05:47
diginetI don't have a phone :P05:48
diginetI knew it was used for RF stuff, but could it replace silicon in general?05:48
Sync_well, that's the hope05:48
Sync_because slowly making ever so slightly smaller devices is getting interesting05:49
diginetbut isn't germanium a lot more fragile than silicon?05:50
Sync_to a degree yes but that's why the GeSi stuff is so interesting05:50
Sync_but really, the person who finds a way to integrate optoelectronics in cmos will take the deal05:51
Sync_that'd solve a lot of interconnect issues05:51
diginetoptical innerconnects?05:51
diginetas I understand it, the main issue right now is parasitic capacitance, no?05:52
diginetwhat about GaAs?05:53
Sync_that's not really compatible with cmos05:53
diginetisn't germanium like insanely expensive though?05:53
Sync_that's why GeSi + C HEMTs are so cool for HF05:53
diginetC, like graphene?05:54
Sync_because you can put them on die with the rest of the chip05:54
Sync_no just plain carbon05:54
diginetHEMT are the one's which use 2d electron gas right?05:54
Sync_yeah germanium is spendy, that's why you take a Si wafer and deposit stress free germanium05:54
diginetwhat is their main usage? in theory could you use them to speed up a regular chip like a CPU?05:55
Sync_mobile phones, RF stuff05:55
Sync_they have transition frequencies over 500GHz now05:55
diginetokay this is weird, according to wikipedia, SiGe has lower junction leakage because it has a /lower/ band gap05:57
diginetI thought it was a /higher/ bandgap which would lead to lower leakage?05:57
Sync_magic of sige05:57
Sync_no, really it does some really strange things05:57
diginetwait, how does that even work though?05:58
Sync_in hemts you can do strange stuff like vary the ge concentration through the channel05:58
diginetif the energy required to jump from the valence band to the conduction band is lower, shouldn't that mean an electron is more likely to make the jump?05:59
diginetwhy are HEMTs not used for non-RF stuff then?05:59
Sync_price and that cpu speed is as you said limited by the intereconnects05:59
diginetbut they are otherwise superior in nearly every way?06:00
Sync_but the main thing about them is that you can integrate them cheaply in a cmos process06:05
diginetwhat makes them more expensive than regular FETs?06:07
Sync_putting ge in?06:08
Sync_sorry, exam in 1h by brain is a bit wonky06:09
diginetno problem :P06:09
berndjdiginet: electromigration goes with atomic mass. aluminium is worse than copper because it's lighter. tungsten is better than both. maybe you need high current densities at contacts, so you use tungsten?20:47
diginetAl is worse than Cu? Huh. I didn't know that20:48
berndj[citation needed] and all that. don't take my word for it20:50
berndjbut yeah, as far as i understood it people moved from Al to Cu because Cu could take higher current density, which mattered as feature sizes got smaller20:58
diginetdoes anyone here know a good place to buy tantalum pentoxide?22:17
diginetdoes not need to be high purity, ~97-98% is fine22:18
berndjazonenberg, was using it some time back. if you're lucky the channel logs might find it for you?22:28
azonenbergdiginet: You can get a solution of it from Emulsitone22:49
azonenbergtantalum chloride in ethanol22:49
azonenbergspin coat and bake and you get a film of Ta2O522:49
azonenbergIt cost me $500 for 4 ounces22:49
azonenbergI wanted to use it as a MEMS hardmask22:49
azonenbergI might still try that once i solve my photoresist adhesion issues22:49
azonenbergHMDS might help with that22:49
diginetsurely there have to be cheaper sources of Ta2O522:51
azonenbergIn what form22:52
azonenbergIn my issue the issue was the minimum order22:52
azonenbergi only needed a few drops22:52
azonenbergthey wouldnt sell me less than 4oz of the 16% solution22:53
azonenbergWhat are you trying t odo with it?22:53
azonenbergoptical coating?22:53
diginetno, I decided to use it for my WORM storage thing22:53
azonenbergWhat advantages does it have over SiC22:53
diginetI would buy the metal, but it's fairly easy to refine, and supposedly a lot cheaper22:53
diginetno, Ta for the metal22:53
azonenbergSo you really want metallic Ta22:53
azonenbergHow much, what purity?22:54
diginetbut metallic Ta costs more, so I figured I'd save some money and DIY the refining, I've been meaning to make an FFC reactor anyway22:54
diginetnot high, 95+ would work22:54
diginetmaybe 10g? (10g metal, not sure how much oxide I'd need for that)22:54
diginettantalum has such good properties, probably the best for my purposes of any metal that isn't some insane 10-component superalloy22:55
azonenbergWhich properties22:56
diginetcorrosion resistance and high melting point namely22:56
diginettungsten melts at a high temperature as well, but it oxidizes at above 300 C22:56
azonenbergHow about Au or Pt or Pd?22:56
azonenbergIgnoring cost, just thinking mechanical properties22:57
diginetmore expensive22:57
azonenbergor Nb22:57
diginetand Ta is better than all of those22:57
diginetNb is an option22:57
azonenbergLooking up corrosion resistance22:57
diginet /troll22:57
azonenbergits very similar to Ta, wiki says it melts at 2477C22:57
azonenbergSlightly less corrosion resistant22:58
azonenberg10 grams of Nb in 1/4" pellets, 99.95% purity22:58
diginetthat's kind of hard to beat22:59
azonenberg$16 for 7.2 grams22:59
diginetlook at hastelloy C23:01
azonenbergbeer, lol23:01
azonenbergcan you imagine being in a lab evaluating the corrosion resistance of materials to beer?23:02
azonenberghydrofluoric acid...23:02
azonenbergratred 3 against wet chlorine gas23:02
diginethastelloy C?23:02
azonenbergYou need to be able to do fine patterning of it too though23:02
azonenbergTa can be patterned precisely23:02
azonenbergsorry, i meant 223:02
diginetwhat about Nb?23:03
azonenbergAlloys may not be the best bet23:03
azonenbergI havent looked at Nb but Ta is used in IC fabrication as a barrier/adhesion layer for Cu23:03
azonenbergSo you can get deep sub-nanometer patterning and fine grain sizes23:03
diginetapparently alloying Nb with Zr and W can substantially improve it's qualities23:04
azonenbergBear in mind you need to deposit the alloy as a thin film somehow, right?23:05
azonenbergNot all deposition processes are amenable to this23:05
azonenbergevaporation for example does not work well with alloys23:05
azonenbergi think sputtering does23:05
diginetit's not thing film though23:06
azonenbergisnt it a thin film on a glass substrate?23:06
diginetthat wasn't my intent23:06
diginetI was going to cast an approx 5mm thick plate23:06
azonenbergAnd read back how?23:07
azonenbergthe benefit of metal film on a clear substrate is easy optical readout23:07
diginetwith a microscope23:08
diginetthis isn't a digital medium23:08
azonenbergI'm just saying, tiny variations in surface topography23:08
azonenbergare less good contast than presence or absence of an opaque material on a clear substrate23:08
azonenbergYea, but I pictured your design as being a film a few nm thick23:09
azonenbergof metal23:09
azonenbergmaybe a micron at most23:09
diginetthe plate is cast seperately23:09
azonenbergthen a thick substrate and a slightly thinner cap23:09
diginetbecause it gives me more options on the composition23:10
azonenbergI'm just thinking in the end you have to be able to read it23:10
diginetyou can, the HD rosetta process, while it is a film on glass, is all nickel23:10
digineti.e. you can't see through the Ni film23:11
azonenbergI was envisioning something similar to standard chrome-on-glass semiconductor masks23:11
azonenbergexcept with a second layer on the top surface for protection23:11
diginethaha, no, lol23:12
azonenbergSuch that you could backlight it and read out optically23:12
azonenbergwith extremely high contrast23:12
diginetremember, I'm planning on making it holographic though23:12
diginetthe read out will be with a laser23:13
diginetI won't be directly writing the pixels to the plate, rather I will computer the diffraction gratings and then draw those out23:13
azonenbergoh, i see23:13
azonenbergI was talking about if you wanted direct optical imaging23:13
azonenbergof either binary data or text or graphics23:14
azonenbergsomething like that could be read with nothing but a microscope23:14
diginetyeah, but I think a non-holographic version of mine could as well23:14
diginetI could fill in the troughs with something else23:14
diginetto increase contrast23:14
azonenbergThe ideal structure for my idea would be a clear SiC wafer or similar with a very thin patterned metal film23:15
azonenbergthen more23:15
azonenbergTa2O5 might actually be a good surface coating lol23:15
azonenbergAlmost nothing but HF will corrode it23:15
azonenbergand even HF does so very slowly23:15
diginetthat's why I decided against W, a lot of stuff corrodes W23:15
azonenbergweeks of immersion in hot KOH23:16
azonenbergetch rates were too slow to measure23:16
azonenbergThat was why i decided to use it as a hardmask for KOH etches23:17
azonenbergmy problem was, it was TOO etch resistant23:17
azonenbergi couldn't pattern it23:17
azonenbergmy photoresist would peel off the surface before it etched through23:17
azonenbergHMDS might help with that23:17
azonenbergon my "future research" list23:21
azonenbergtoo many other things going on lol23:21
--- Sat Feb 16 201300:00

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