Hi all ...
I've looked around quite a bit for an answer to this question, since I'm
sure it exists, but I must not be using the correct search terms.
We are building a surface-micromachined structure that requires electrical
connectivity to the suspended surface. When thinking of different ways to
obtain this connectivity, two options come to mind. The first is to deposit
the sacrificial layer, build the structure on top of that, etch down
through all the layers to some contact pads and attempt to sputter metal
into the vias to create the electrical contact posts. The second is to
electrodeposit the posts before laying down the sacrificial layer, then
deposit the sacrificial layer, etch through it to expose the posts, and
build the suspended structure on top of that.
However, I see drawbacks to both methods. In sputtering, it might be
difficult to get good connection and fill in a via that is 4-5um deep and
5-10 um square (not to mention that's a large quantity of metal to
sputter). And in the case of depositing the posts first, it would seem that
you would have problems with planarity at the top of the post. Chances are
you'd either have the sacrificial layer taller than the post or the post
taller than the sacrificial layer, even with good reflow. CMP seems like it
may be an option to planarize, but how would you know exactly how much to
remove to end up with planar post tops?
It seems like you'd end up with structures such as: (need a mono-spaced
font to view)
______
_____/ \_______
| | POST | SAC |
| | | LYR |
or
_____ ______
| \______/ |
| | | |
| | POST | |
or what seems even more likely (to me) is that there is sacrifical layer on
top of the posts that you have to etch away, ending up with a crater edge
of sorts:
/\______/\
_____/ | | \______
| | | |
| | post | |
| | | |
We have been considering using gold posts and BPSG as the sacrificial layer
due to its better reflow properties over PSG, although we're open to other
materials still.
Can anyone comment on what is the most appropriate method for making
electrically connected surface micromachined structures? Direct help or
pointers to literature are greatly appreciated.
Thanks,
Todd