So this through this Silicon Via technology it like many
other good things in life, it comes in different flavors.
And so you could make this Through Silicon Via, at the
very start of your process flow that's called as Via First.
You could make it in the middle of your process flow.
It says via middle.
You could actually do it after you're done making your
semicondutor chips. So that would be via last.
So let's, let's look into these different flavors of TSV
and and let's look at the different challenges in each of those.
So one flavor of making this TSV is to
make it the very first of the process flow
So what you do, is before you, make, any transistors on your chip.
You first do this etch.
So you etch this TSV and you fill it up.
with your fill material and you polish it out so you do this
TSV process at the way first of your at the way beginning of your
process flow and then after that you make your transistors so you do
your front end processing. Remember that most of the highest thermal
budgets in making your chips out of encounter when you make your transistor.
Or when you do front end processing.
So there, this involves anneals for Making your junctions and other high
temperature anneal, so most of the thermal budget or the high temperature processes
are involved when you make your transistor or in the front end of your chip.
Then after you are done making your transister, than most of your back end.
End of the line processes are typically less than 400 degrees centigrade.
Or you can also call it low temperature or
low thermal budget.
So in this TSV, via first, what you do is at the very first
Even before you make your transistor you make these via's.
And then you do your front end processing, and then you do your back end processing.
And finally, you thin down your wafer from the back until it reaches its TSV.
So, one of the main challenges with the, this Via first approach
as now since you are making this via at the very beginning.
This, we have whatever material that you are
filling it say you could fill it up with
copper or you could fill it up with
tungsten or you could fill it up with poly-silicon.
So some of these so whatever you fill it up with this fill material and this
whole TSV we'll see this high temperature coming
from your front end proces flow. So some materials
for example copper they have very high diffusivity and
they can easily diffuse into your silicon at these temperature.
So it prohibits using some of these options for your for filling up your TSV.
Another way you can make your TSV is to
do this via in the middle of the process flow.
So, what I mean in the middle, is do it
after your front end. So do your front end first.
Then do your TSV process and then do your back end process after that.
So this TSV process flow is in between your front end and your back end.
So this is how, this is cartoon depicting the
same so you first do your front end process flow.
So most of your
high temperature aneals occur over here and then after they are done then you etch out
this, your trench for TSV then fill it up with your TSV material.
So this could be copper in the case.
And then you do this back end process flow.
Where you make your interconnects and routing signals and,
then so on.
And then finally, you thin down your wafer from
the back till it touches your through silicon via.
So advantage of doing this doing this scheme of integration is
that you now avoid, you now avoid this TSV material
from this high temperature anneal steps which are involved in your front end.
So, your TSV material only sees
the back end thermal budget, so it sees this back end terminal budget.
But since there's less than 400 degree
centigrade, it doesn't damage your TSV fill material
so you could fill it up with any material, you can fill it up with copper.
It's okay to fill it up with copper and it does not diffuse at these back end temperatures.
A third integration scheme which is the Via Last scheme
would be that you do this TSV integration at the very last.
So you do your front end then you do your back end and
then when everything is done you come and you make your TSV processor.
So this is a cartoon depicting that.
So you first make your front end, so
you do your high tempurature on these processors here.
Then you also make your back end so you do your 400 degree centigrade
process flow here.
Finally, you thin down your wafer, and you thin it down before you make your TSV.
So, it's important to thin it down to the correct depth, and then
you then you start from the back and then etch out the TSV layer.
And then fill it up with this TSV fill material, which is copper.
So,
a challenge in this case is to align this this backside here with your frontside, so
you know you Made most of your transistor
then your your backend of the line already.
So, important thing is to, align when your doing this backside
processing align this TSV with the transistors that you have already made.
So that it lines
up at the exact position that you want it to be.
So there could be these different integration scheme.
I'm not favoring.
The jury is still out actually, which one is which one is which one will be
adopted by the industry but here is a brief scorecard.
So I've tried to make this scorecard. Where you
could evaluate the Via First, your Via Middle, your via last and you could
evaluate in terms of ease of
different processes flows involved in making a TSV.
So for the etching then to put their oxide then to deposit your seed layer.
And then to do your lithography.
So as I mentioned one of the show stoppers for this Via First approach is
that is the, the Via First approach.
This Via sees a high temperature or it sees the high thermal budget.
So it restricts the options of materials that you can fill it up with.
So it's, it's always impossible for copper, to fill it up with copper.
Because copper will diffuse into your into your silicon at these temperatures.
It's also
difficult to deposit it with tungsten so this is
one of the reasons that via first approach.
It almost it's being ruled out because it's not possible for this TSV
fill material to survive these high
thermal budgets involved in your front-end processing.
Via Middle is one of the strongest contenders, it, it's,
it's, it's, it's seems like an approach. Which has a lot of, lot of things going for
it, mostly greens, some yellows but mostly it's, it's a very strong approach.
Via last approach again, one of, one of the, one of the grey, one
of the yellow areas for this technology is to do this lithography from the back.
And align it to the transistors transistors
made on the front side. But this is also a strong container,
so this is, this is also an option in play for for the TSV integration.