<sivoais>
haha, hey diginet! I'm an engineer. I like engineering things.
<diginet>
I'm not an engineer (lol), but I've talked with azonenberg a little about homecmos
<diginet>
I think having a DIYable chip fab would be just about the greatest thing ever
<diginet>
we were talking earlier actually (in PM), and I thought of something that seemed odd and he didn't know either, maybe you can answer
<diginet>
sivoais: why is tungsten (often) used for vias in CMOS? I know that copper has issues with electromigration, but why not aluminum?
<sivoais>
diginet: no idea. I'm not a materials person. Would be a good thing to look through the literature for
<diginet>
good idea
* diginet
googles
<Sync_>
diginet: I guess it has to withstand KOH etches and be easily depositable
<diginet>
Sync_: yeah, I looked up some papers and they basically said something similar
<Sync_>
but it seems cu is possible after all
<diginet>
yeah
* Sync_
is going to kill his prof....
<azonenberg>
Sync_: Nobody uses KOH for CMOS
<azonenberg>
TMAH maybe
<azonenberg>
but Cu survives exposure to low-concentration TMAH used as developer
<azonenberg>
Al is slightly attacked
<diginet>
you 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 anymore
<diginet>
the problem is finding an alternative that isn't made of unobtainium or something stupid like Arsenic
<Sync_>
I thought KOH was still used for bulk etching
<Sync_>
diginet: germanium is the answer
<diginet>
really? why do you think so
<Sync_>
becaue I work in it
<Sync_>
we're doing Ge on Si
<Sync_>
and GeSi stuff
<Sync_>
which is easily integratable in current cmos processes
<diginet>
really
<Sync_>
your phone does it
<diginet>
I wasn't questioning you, I was just curious
<diginet>
I don't have a phone :P
<diginet>
I knew it was used for RF stuff, but could it replace silicon in general?
<Sync_>
well, that's the hope
<Sync_>
because slowly making ever so slightly smaller devices is getting interesting
<diginet>
but isn't germanium a lot more fragile than silicon?
<Sync_>
to a degree yes but that's why the GeSi stuff is so interesting
<Sync_>
but really, the person who finds a way to integrate optoelectronics in cmos will take the deal
<Sync_>
that'd solve a lot of interconnect issues
<diginet>
optical innerconnects?
<Sync_>
yes
<diginet>
as I understand it, the main issue right now is parasitic capacitance, no?
<Sync_>
yes
<diginet>
what about GaAs?
<Sync_>
that's not really compatible with cmos
<diginet>
ah
<diginet>
isn't germanium like insanely expensive though?
<Sync_>
that's why GeSi + C HEMTs are so cool for HF
<diginet>
C, like graphene?
<Sync_>
because you can put them on die with the rest of the chip
<Sync_>
no just plain carbon
<diginet>
HEMT are the one's which use 2d electron gas right?
<Sync_>
yeah germanium is spendy, that's why you take a Si wafer and deposit stress free germanium
<diginet>
what is their main usage? in theory could you use them to speed up a regular chip like a CPU?
<Sync_>
mobile phones, RF stuff
<Sync_>
they have transition frequencies over 500GHz now
<diginet>
okay this is weird, according to wikipedia, SiGe has lower junction leakage because it has a /lower/ band gap
<diginet>
I thought it was a /higher/ bandgap which would lead to lower leakage?
<Sync_>
yes
<Sync_>
magic of sige
<Sync_>
no, really it does some really strange things
<diginet>
wait, how does that even work though?
<Sync_>
in hemts you can do strange stuff like vary the ge concentration through the channel
<diginet>
if 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?
<diginet>
why are HEMTs not used for non-RF stuff then?
<Sync_>
price and that cpu speed is as you said limited by the intereconnects
<diginet>
but they are otherwise superior in nearly every way?
<Sync_>
yup
<Sync_>
but the main thing about them is that you can integrate them cheaply in a cmos process
<diginet>
what makes them more expensive than regular FETs?
<Sync_>
well
<Sync_>
putting ge in?
<Sync_>
:D
<Sync_>
sorry, exam in 1h by brain is a bit wonky
<diginet>
no problem :P
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<berndj>
diginet: 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?
<diginet>
Al is worse than Cu? Huh. I didn't know that
<berndj>
[citation needed] and all that. don't take my word for it
<diginet>
gotcha
<berndj>
but 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 smaller
<diginet>
yeah
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<diginet>
does anyone here know a good place to buy tantalum pentoxide?
<diginet>
does not need to be high purity, ~97-98% is fine
<berndj>
azonenberg, was using it some time back. if you're lucky the channel logs might find it for you?
<azonenberg>
diginet: You can get a solution of it from Emulsitone
<azonenberg>
well
<azonenberg>
tantalum chloride in ethanol
<azonenberg>
spin coat and bake and you get a film of Ta2O5
<azonenberg>
It cost me $500 for 4 ounces
<diginet>
yikes
<azonenberg>
I wanted to use it as a MEMS hardmask
<azonenberg>
I might still try that once i solve my photoresist adhesion issues
<azonenberg>
HMDS might help with that
<diginet>
surely there have to be cheaper sources of Ta2O5
<azonenberg>
In what form
<azonenberg>
In my issue the issue was the minimum order
<azonenberg>
i only needed a few drops
<diginet>
powder
<azonenberg>
they wouldnt sell me less than 4oz of the 16% solution
<azonenberg>
What are you trying t odo with it?
<azonenberg>
High-K?
<azonenberg>
hardmask?
<azonenberg>
optical coating?
<diginet>
no, I decided to use it for my WORM storage thing
<azonenberg>
Oh
<azonenberg>
Hmm
<azonenberg>
What advantages does it have over SiC
<diginet>
I would buy the metal, but it's fairly easy to refine, and supposedly a lot cheaper
<diginet>
no, Ta for the metal
<azonenberg>
Oh
<azonenberg>
So you really want metallic Ta
<diginet>
yeah
<azonenberg>
How much, what purity?
<diginet>
but 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 anyway
<diginet>
not high, 95+ would work
<azonenberg>
sec
<diginet>
maybe 10g? (10g metal, not sure how much oxide I'd need for that)
<diginet>
tantalum has such good properties, probably the best for my purposes of any metal that isn't some insane 10-component superalloy
<azonenberg>
Which properties
<diginet>
corrosion resistance and high melting point namely
<diginet>
tungsten melts at a high temperature as well, but it oxidizes at above 300 C
<azonenberg>
How about Au or Pt or Pd?
<azonenberg>
Ignoring cost, just thinking mechanical properties
<diginet>
more expensive
<azonenberg>
or Nb
<diginet>
and Ta is better than all of those
<diginet>
hmm
<diginet>
Nb is an option
<azonenberg>
Looking up corrosion resistance
<diginet>
DON'T YOU MEAN COLUMBIUM YOU COMMIE
<diginet>
/troll
<azonenberg>
its very similar to Ta, wiki says it melts at 2477C