<azonenberg>
So I want to start exploring alternate hardmasks
<azonenberg>
Ta2O5 is still worth looking into for when vacuum isnt available but i think i can get better results with evaporated Ni
<azonenberg>
also patterned by liftoff
<azonenberg>
Unfortunately i dont have a working evaporator yet
<berndj>
what are you hoping to achieve with this hardmask stuff?
<berndj>
and, does it relate to that PR "peeling" problem you were having?
<azonenberg>
berndj: Yes
<azonenberg>
A hardmask of some sort is necessary because photoresist is attacked by KOH and will disintegrate in seconds
<azonenberg>
So you need a material that you can pattern using photoresist (by either lift-off or etching) and then use that material as a mask for the KOH etch
<azonenberg>
Normally people use silicon nitride, iirc it's patterned by an HF wet etch
<azonenberg>
But my resist is not very good against HF
<azonenberg>
So I tried using lift-off, which works but i am having issues with so far
<berndj>
KOH is to get the oxide off?  or to actualy "dig into" the silicon?
<azonenberg>
KOh is for etching the silicon
<azonenberg>
To significant depth (for MEMS applications)
<azonenberg>
as in several microns to a few hundred microns
<berndj>
was about to ask if this was for MEMS
<azonenberg>
Yes, thats the main application
<berndj>
didn't think you'd absolutely need that for making just circuits
<azonenberg>
No, you typically dont
<berndj>
for accelerometer type devices: how do you get "under" the probe mass?
<azonenberg>
There are a couple of methods depending on the process
<azonenberg>
I was going to use bulk micromachining and actually have it free hanging
<azonenberg>
etch from the back up until the center of the die is only a few microns thick
<azonenberg>
then do another etch from the top down with my target pattern
<azonenberg>
Another is based on surface micromachining and involves depositing a sacrificial spacer followed by polysilicon
<azonenberg>
you make the moving parts out of poly-si then dissolve the spacer
<azonenberg>
presumably at some point in between you'd put support structures through the spacer layer to hold your device up
<berndj>
isn't MEMS an order of magnitude harder to DIY than just "normal" devices (just circuits)?
<azonenberg>
Not for something simple like a comb drive
<azonenberg>
because CMOS is so sensitive to trace metal contamination
<berndj>
i'm only assuming so because it's a decade or two or three further along in history
<azonenberg>
mems dont really care
<azonenberg>
And making really complex mems usually needs RIE
<azonenberg>
But for simpler stuff, not necessary
<berndj>
does NaOH also work to etch Si?
<azonenberg>
Yes
<berndj>
works too well?
<azonenberg>
But KOH is apparently more repeatable
<azonenberg>
In mixed signal CMOS+MEMS devices they usually use TMAH which is purely organic and has no metal ions
<berndj>
TMAH?  geez, another james bond movie death-chemical!
<azonenberg>
berndj: Lol
<azonenberg>
What do you mean?
<azonenberg>
It can be toxic but i'm not aware of it having been used in film :p
<berndj>
just looking at its wikipedia article, that makes it sound like another one of those death-on-contact chemicals like HF
<azonenberg>
It can definitely be nasty, i'm not planning to use it without more research
<azonenberg>
Which is one of the reasons i am sticking with mems for the time being
<azonenberg>
since i definitely dont need it
<azonenberg>
i want to explore alternative photoresist developers, preliminary research suggests straight household ammonia might work
<berndj>
if you can tell me a household-chemicals means of making PR (for pcb type resolution) i'll give you a wet sloppy kiss
<azonenberg>
I'm not aware of any, chemistry isnt my main strong point lol
<azonenberg>
i'm into this mostly for the engineering of devices
<azonenberg>
the process development is a necessary step to make that happen
<berndj>
maybe i'll just dick around sometime with indoor varnish + UV, see if anything looks workable
<berndj>
my outdoor timber suggests there's some UV action, but i don't want to sit with multi-year exposures!