Hello, everyone. Today we are going to see how we can spin-coat
polymers to obtain flexible substrate. What we are going to do is, we are going to
use a precursor. It is similar to that what you have seen for
a photoresist spin-coating and we will be using a carrier wafer in this case, it is
silicon three-inch wafer. We will spin-coat on that, the precursor,
the polymer that we want to obtain is a flexible substrate. And then we will cure it at two different
temperatures. And finally, we will show you how we can get
a flexible substrate out of the silicon carrier wafer. So, the first step to spin coat any kind of
photoresist or polymer or similar precursor solution is to use a spin coater. Spin coater works on the principle of centrifugal
force. You keep the substrate and the substrate is
placed on a sample holder, the sample holder holds the sample by virtue of vacuum, which
is generated by a dedicated vacuum pump. So, let us now see how the spin coater can
be operated. So, now we will see how a substrate holder
looks like, so we can open the lid of the Spin coater, and then we take out the substrate
holder, so this is how a substrate holder looks like. The black colour ring that you see is the
O-ring, the technical name is O-ring, and there are multiple groups depending upon the
size of the substrate, the smallest one can be for a small wafer pieces, the middle one
can be for a 3-inch silicon wafer, the larger other one can be for 4-inch silicon wafer,
depending on the substrate you want to spin coat on, you can put dedicated O-ring. The O-ring should be a put after applying
vacuum compatible grease, so that the contact between substrate which is silicon wafer and
the O-ring is leak free. So, now we will see how we can program a spin
coater. By programming, we can have multiple steps
for spin coating. Why do we need multiple steps? We will see now, as my colleague will show
you. So, generally, when we spin coat polymers
or photoresist, we give two speeds. One speed is to spread the solvent along with
the precursor and second speed is to dry off the solvent and thin it down, so as we can
see as of now, we have given fifteen seconds of rotation at a particular speed, the speed
we have decided in the RPM: 500 hundred RPM is the first. So, what this particular step will do is,
it will spread the precursor uniformly on the surface and we can give the acceleration
as well. Once that is done, the next step is to give
the higher speed step to ensure that the solvent, which is spread uniformly can now be evaporated
and thin down. Generally, for photoresist, we give 4000 RPM
and for one for 40 seconds, but in this case, we have optimized to get a flexible substrate,
so we are giving it for 60 seconds, and the RPM is around 1000 and acceleration will remain
almost the same, which is around 400. As we had discussed, acceleration speed, but
in some of the Spin coaters, acceleration time is there, so you can do math and calculate
the acceleration time for this, we have calculated to be 5. These spin coaters are almost similar in terms
of functioning, only the software interface is different. So now, we have given the speed and the time
for spin coating first and second step. So, now we are ready to spin coat. Now, there is one more final step, which is
called as ramp-down step. This is to ensure that spin coater does not
stop abruptly, we can generally give it for 10 to 15 seconds depending upon the speed
that we are already in and the RPM can be given as 500, so that it goes down to 0 at
a very gradual speed. Again, this RPM that we are referring to,
depends on the spin coater to spin coater. For some cases, it may require us to feed
the RPM, but in this case, even if we give one step of study state ramp-down, this will
function as a ramp-down. See, so as you can see in the graph plotted
where y-axis is the RPM, X-axis is the time in seconds. We can see there is a ramp up speed and then
it stays there for some time, then again, there is second ramp up, which is thousand
RPM, it stays there for sixty seconds or one minute, and then there is a ramp-down. So, we now have set the spin coating parameters,
let us know spin coat. So now, we will see a hole, the precursor
of the polymer can be spin coated onto a carrier silicon wafer. So, my colleague is showing how to place the
silicon wafer, we need to be very careful. We will place the polish surface exposing
facing us and non-polish surface, in case of single side polish silicon wafer. We need to align the vacuum check with silicon
wafer to ensure that the centrifugal force acting upon the silicon wafer, which ensures,
uniform coating of precursor or any other photoresist matches with the central axis
of the vacuum check. Once that is done, now, we are ready to hold
or to allow the silicon wafer be folded by the vacuum check for that, we have a dedicated
pump. So, as my colleague will switch on the vacuum
pump and you will hear a noise coming out of the pump. Now, before spin coating, it's always good
to make a check by gently moving silicon wafer using tweezer, not moving very fast, just
to see at all if silicon wafer is moving from check or not. This is to ensure that once we start the spin
coating, the wafer does not break because of loss of vacuum, this check needs to be
done very carefully, and vary slight towards, just a touch of that should be applied to
ensure that it is nicely holding it. Now, we have checked it and we are ready for
spin coating. So now, my colleague will use a dedicated
wire for pouring the precursor onto the silicon wafer. Pouring itself requires some amount of practice. We need to ensure that there is no bubble
formation. So, we need to be very careful while pouring
the polymers precursor. We need to pour it very gradually to ensure
that there are no air bubbles form on the silicon wafer. So, as we can see, he is gradually pouring
the polymer precursor on the silicon wafer and now, since it is gradually poured, he
will wipe off the excess of polymer from the wire to keep it clean and so there is no contamination
next time when we spin coat. And now he will close the lid and press the
play button, as we have seen general music player in the similar way, we will press the
play button, which will mean we can start the spin coating process. So, spin coating process, as you can see,
has started and the wafer has started rotating. This is the first step, where the polymer,
the polymer precursor is spreading. Once this step is over, the next step is where
it will increase by ramping up to the speed where it can thin down also evaporating some
of the solvent. So, now we have spin coated and placed the
wafer on a hot plate, which was preheated at the dedicated temperature, which is eighty
degree. For the first step, the question arises, why
are we going for two different steps of heating? The reason is as follows; the first step ensures
that a part of the solvent is evaporated and the substrate is nicely, firmly adhered to
the spin coated polymer film. Also, if we do not increase to very high temperature
immediately, so that there is no thermal stress built up in the polymer film, which might
lead to cracking or a very poor curing of polymer. So, we will cure it at 80 degrees celsius
for one hour, and thereafter we will carry it at a higher temperature for 2 hours, which
will complete the overall curing process. So, in the process of curing, as we discussed,
there are two steps. So, one step for first step of curing is done
now, which was done at 80 degrees celsius. For one hour and now one hour has lapsed. Now, we will keep it for curing at 250 degrees
celsius for 2 hours. This will form the second step of curing. So, now we will gently press, keep the wafer
on the hot plate, which is preheated at 250 degrees celsius. This will be kept for 2 hours and this completes
the curing cycle. So, now 2 hours are left since we gave it,
kept it for second stage of curing, and therefore the wafers can be taken off from the hot plate,
and my colleague will do the procedure. So, this is how we can spin coat photoresist
or polymers such as polyamide or acute on desired substrate, in this case, it was silicon
wafer.
