So, picking up where we left in the last video,
so in the last video I talked about this deep trenches.
So we saw the Mariana trench which is the
deepest trench in the world, has a aspect ratio of.
37 and we also talked about the
tallest skyscraper in the world which is Burj Khalifa and it has a aspect
ratio of 8.2.
And later I showed that the etches or the structures that we are making
and our semi conductor technology are exceeding the aspect ratio that we
see in the deepest trenches and in the tallest building we saw that the stacked
capacitor in DRAM has in aspect to ratio of 66 or more.
A lot
of the STI
nine flash, they are having a spector show of for 10 or more.
So, this is, this looks you know, very nice, these tall buildings.
And the trenches, what the problem comes in
when you are actually make or manufacture these things.
So if you're a trench this deep, I want to make a building this tall.
So that is what
I want to intend to do in this video. I want to expose you to some of the.
Challenges which come into play when you try to make these structures.
So the first thing I want to talk about relates to the farming of these deep
trenches and that require you to do etch. Step and
in an ideal world the way you would want this edge to function is you would
want these uh,so shown here is is this one of these high aspect ratio structures.
in this case I'm showing you the STI
edge which is required for the NAND flash memory
So in a ideal world, I would want this this etch to be as vertical as possible.
So I want it to be having an annular for 90 degree or being perpendicular to my
substrate. I also want it to be as as anisotropic
as possible, so I want this etch to be highly anisotropic.
So that then you know, I look up in my text books.
I look in my tutorial or
textbook, and I find this arrangement over here, and I found some basic stuff.
How etch process works.
So in an etch process, an isotropy is essentially defined the amount of
undercurrent depending, and divided by the amount to which vertically.
So I want an isotropy to be exactly one, so I want
no undercurrent, and I want a very vertical profile.
That would give me an isotropy of one and, so
I look, you know, I go back again to my 212 textbooks, so how could I do that.
So, you could do a weight processing but it has a very, very bad isotropy,
it etches, it produces a large undercut so I rule that out.
I could do Ion men milling which has an isotropic of one but again it produces
a lot of damage and is not suitable for etching semi conductors or oxide.
So then I'm left with you know, what is
typically used in the industry, that is a plasma process.
And it comes with its pros and cons so
the way this typically works how you produce an
isotropin plasma process, if you look at your 212 book.
As you deposit something which is known as side walls which
is inhibited, so you when you etch at the same time use
[UNKNOWN]
deposit this inhibited on the side walls and that prevents these
sidewalls from etching, and you get an isotropic staircase profile.
But you always get this V shaped profile whenever you do a plasma etch, because
this inhibitor or this sidewall thickness, it keeps on increasing as you.
keep on etching
higher and higher. So you always get these kind of grooves of
not exactly vertical profile. Another phenomenon which causes this is
this flux that is Suppose you are having this plasma which has a certain flux.
So when you are etching something which is Not
so deep you get a flux from a large angle.
So you, if you're etching somewhere in
your etch process you're over here you get this
flux from this much angle let's say 30 degrees.
versus if you are etching something very deep, so if you're etching over here
you get only a flux from this limited angle which reaches over here so you
[INAUDIBLE].
, you get a flux from an angle of only 15 degrees.
So your edge rate as well as it it keeps on
decreasing as you keep on etching further, and that also
[UNKNOWN]
contributes to this v-shaped nature of this profile.
Another thing, another phenomena that you get is a, is a charging.
Which again, if you are etching a structure this deep, they would be
there are especially if you are etching
dielectric charges which build up over here.
And they essentially deflect the
incoming ion or the incoming species, or deflect them back, and
that also contributes to this narrowing of this profile as you as
you go further. What I'm showing here is some
image of one of the experimental edge profile,
and you can see clearly the aspect ratio dependent
on the etch rate. So if you have a feature which is wide,
you see that it has been etched pretty deep.
Worse still if you have a feature which is narrower, it is it is etched
to a, to a smaller height. And again you can this is the part
because of that flux angles. If you have a high aspect ratio structure.
You get a only limited angle of flux,
and So you get a very layout-dependent etch profile.
And one where you avoid that is using these high-density plasmas, which
tend to avoid some of these aspect ratio-dependent issues.
But we need an even higher, aspect ratio so we
really need like, for example in a true cylindrical
[INAUDIBLE]
used for packaging and treaty. I see these edges could be
easily as deep as 200 microns so way, way deep edge.
And these we want to keep these TSVs as thin as possible so they avoid
they occupy the minimum area on your on your on your actual die.
So we want them to be, you know, be
no more than five are you know, a few microns.
And so this results in a very high aspect ratio, and also it's a very deep edge.
So we want something which is fast, and low cost, and
at the same time we want to maintain this vertical profile.
So then, you know, the traditional semiconductor industry looked
at you know, where else has been, this been done before.
To
a MEMS based, MEMS industry. Our MEMs have
been using this kind of etches this high
expect ratio etches for a while. And how this
is achieve is One of the way it's achieved as shown over here.
It's called a Bosch process. Bosch is a way it's a big MEMS company and
this process etch process is a named after them.
And it's a two step process.
it involved the essentially a, a plasma or a.
etch step and it involves deposition of this inhibitor side wall.
So this is, as you can see over here, you can
clearly see these two steps, so what you do is you
deposit these side walls and, then what you come, and essentially you
come and etch it off, and you etch one one groove more.
And then what you do is again you do something which you
know, sputters off this substrate and deposit a side-wall on the side.
So you get these groovy kind of profile-like
you know, like it looks like the surface of a screw.
And so this is used to etch these very high aspect ratio TSV
and I'm showing here a image so which is as you can see a TSV which has a, a
width of two micron and you can see
these groove patterns resulting from from the Bosch etch.