Another place where advanced packaging technology is used inside our smartphones
is in this field of CMOS image sensors. So in this image sensor
this advanced packaging, which is now enabling an
improvement in the performance of your camera actually.
And it specially improves the, your
performance of your camera in a dimly lit or darkly lit
conditions. So this is shown the traditional
technology, which was used in a CMOS image sensor, which is also known as front
side illumination. So what used to happen
was you would have the silicon, silicon based
image sensor, which would capture your incoming light.
And your incoming light will have to transfer through this
silicon chip, which would have this silicon plane for capturing the light.
But it will also have these metal lines,
which are used for signal routing and so on.
So this these metal lines they they
block partially block this incoming photons from reaching
the silicon also they scatter this incoming light.
So especially the pixel size is is at is continuous to
decrease currently you know we use a pixel size of somewhere between.
1.4 to 1.6 so 1.7 micron as and you know as
you know, as , as, the pixel size continues to decrease, a lot
of these incoming photons are now blocked
by these metal lines scattered by these metal lines.
So by advancement in packaging in this
area by using this technology called backside illumination.
So the way backside illumination works is you flip the silicon chip around.
So now,
you have this silicon, which is facing the incoming photons.
So, your incoming photons are now absorbed in
the silicon without being scattered by this meta lines.
And all these metal lines for routing are now residing at the back.
So they don't, scatter or hinder your incoming photons.
And this specially helps in dark light condition, when you have less
number of incoming photons. So this was, you know, introduced
somewhere in your, the iPhone 4S or the later generations of iPhones.
And it has really become main stream starting last year.
So shown here is is one of the
old image sensor, which uses front side illumination technology.
For example
your iPhone, iPhone 3 would have an image sensor like this and
you can see the lens assembly here, so there'd be lens cell here.
They will focus here incoming photons to
the focus your incoming photons to this small image sensor over here.
And you can see that there are these metal routing lines and they scatter
a lot of incoming light and this degrades the performance
of your image sensor. And shown here is
the latest image sensor with the back side illumination.
So this is image sensor which uses the back side illumination technology.
And what you can see here is now you have this silicon chip, which is facing
the incoming lights so here is the lens assembly.
And your incoming light is focused to this silicon without being scattered.
And so the way this image backside illumination technology
is enabled, is shown depicted in the series of cartoon over here.
So, the pro, the way this process flow works is you
start with a SOI wafer and then you grow a Epitaxial layer on top of it.
And then you do your rest of the processing in in
a similar fashion so you, you place all these metal one, metal two.
Metal three line, which are used for routing and
what you do after that is you flip this chip.
And you bond it to a
carrier wafer there is a carrier wafer you bond
this to you bond this you process image sensor to that.
Then you then you polish the back side of this wafer.
And then you just leave that epitaxial layer, which you grew.
So you polish it back till you reach that epitaxial layer.
And so what you have now is this is this
epitaxial layer directly facing the incoming light.
And then, of course, you put some, anti reflection coating.
You put your color filters
. Your color filters.
You put your lenses.
And then this chip is flip chip bonded to your PCB, and many cases
you use through silicon via process where you have these, these through silicon via.
Which would be taking this signal out from these metal lines.
And some cases you in fact have image processor,
which is directly bonded to the back of, the back plane of this image sensor.
And you know, there is a lot of use
of advance packaging technology to enable this back side illumination.