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Hello, I am Shree Prakash Tiwari, and currently I am working as a postdoctoral fellow in Professor
Bernard Kippelen’s research group. I’m working on organic field effect transistors;
basically fabricating them and studying the performance and stability of those devices
What I’ll talk about today is the fabrication of doped source-drain organic field effect
transistors. Let me explain about the organic field effect
transistor device structure. We have a top contact and a bottom gate. The bottom layer
is heavily doped n-type silicon which acts the gate.
The first layer which is deposited on the bottom is the gate contact, which is a titanium
gold layer: 2 nanometers or 5 nanometers of titanium and 100 nanometers of gold on the
bottom. In between you see a gate dielectric layer
which is essentially a silicon dioxide layer, preferably 200 nanometers thick in this case.
We passivate the silicon dioxide layer with a surface treatment or buffer layer, which
is OTS, which is Octadecyltrichlorosilane. Or for N-channel transistors we have various
polymer layers like benzene cyclobutane (BCB) or polystyrene.
We passivate the dielectric layer to prevent any hydroxyl groups to come on the surface
and degrade the performance. This structure shows a semiconductor layer
which is widely known pentacene, which is vacuum evaporated. But this layer could be
any spin coated polymer which can act as a semiconductor.
Here I show the actual device which has been fabricated .
You can see two parallel electrodes which act as source and drain, and there are various
channel lengths, ranging from 200 microns up to 25 microns, which is facing between
two electrodes. And we have some capacitive structures, which
are circular structures with varying areas, which are used to characterize the capacitance
density. So to fabricate them, basically we have silicon
substrates which are heavily doped, and one side is polished and the other side is rough.
To make the devices we need gate electrodes, heavily doped gate silicon gate electrodes.
But to connect the gate electrodes you need some connecting metal, like gold, and hence
we deposit gold electrodes on the bottom. So to do this we need to protect the front
side with a photo resist. You can see the color on this wafer. So basically we protect
the front surface to protect the silicon dioxide which acts as a gate dielectric , and on the
back side we remove the silicon dioxide and put a titanium gold layer. And titanium acts
as an oxide layer for this gold. So here I explain how we start with the substrate.
We cut it down, either can we dice in a properly nice manner one inch by one inch. But if you
just need small substrates you can break it into pieces. These are substrates having gold
on the bottom and photo resist on the top. SPIN COATING
So this is a nitrogen glove box, and these organic semiconductors are very sensitive
to moisture and oxygen. So that’s how why we have a controlled environment in these
boxes where we control O2 and H2O up to a 0.1ppm level.
So basically we have the substrates ready, and we transfer these substrates inside these
glove boxes. And then before spin coating we need to have the semiconductor in controlled
ambient so they don’t degrade and we have better devices. So let’s go and see how
we start the spin coating process. These boxes, basically the pressure inside the box is higher
than what is outside. That’s why the arm kind of things where you put your arms they
are always outside. So the higher pressure inside helps keep the oxygen not to go inside
while we are doing the process. There is a button which help us to reduce
the pressure inside. So while I’m putting my hands inside I need to press that button
so that for some time the pressure inside becomes low and I can put my hands inside
the glove box. Before processing some solutions, we have
some gloves inside; we need to protect these rubber gloves with proper gloves which are
more chemical resistant. So now coming to the spin coating of organic
semiconductors. Basically we have the semiconductor material dissolved in a solvent ; in this
case we have chlorobenzene solvent for pcbm c-60 which is a commonly known electron transport
material. We dissolve this material and we stir it all night to make a uniformly dissolved
solution. We can change this chuck which is used for spin coating, we have some different
sizes, and this chuck should be able to hold the substrate by vacuum.
So to start I’ll have a substrate which is silicon dioxide with a buffer layer of
a polymer BCB and then I’ll place this substrate in the middle of the chuck.
And the next thing I’ll do is take some solution in the syringe from this bottle,
and after that I will use a filter which in this case is a 0.2 micron filter so that I
get a nice and clean and uniform film on this substrate. Then I will put some solution from
this bottle and put some solution in the middle of the substrate. And the solution should
be enough to cover the substrate entirely. So basically right now we are spin coating
this semiconductor at 1000 rpm for 60 seconds, and the speed and time depends on what kind
of thicknesses you want. So in this case we have around 80-90 nanometers of PCBM. But
generally we can change this speed to get thicker films or thinner films.
Now we have a nice film on this and you can see the slightly blue color of the PCBM film
on the substrate. The next thing will be to place the substrate
for metal deposition so we’ll transfer these substrates to the antechamber to the next
nitrogen glove box. MASKING
After we have finished the spin coating procedure we have to transfer the samples to the next
glove box. This is the chamber where we pull the samples from the other side, then we transfer
the substrate, and then we close the transfer chamber.
We have some mask holders where we put our masks which are used for top source-drain
electrode deposition. The first one is a mask used for transistors. On one mask we can do
nine different substrates. This metal film has openings where the metals go through in
the deposition system and it gets deposited on the substrate. I’ll put this metal mask
on a substrate mask holder. You have to match these metal clips properly.
Now we can take the substrates and tape it from the backside. The semiconductor comes
on the bottom and from this side you deposit the metal. Once you have substrates taped
on the mask you can transfer the substrates to the metal deposition chamber which is attached
to this box so you don’t expose the substrate to the air again. The next process is metal
deposition. CHARACTERIZATION
After the fabrication of the devices, we have to transfer the devices to another glovebox
where we can characterize the devices. We have a nitrogen cylinder which is airtight
and we keep the devices inside with nitrogen ambient so it is not exposed to air. This
is the fabricated device we have transferred. This is a chuck where we have a metal plate
on the bottom. And in the device we have a gold gate electrode on the bottom which is
placed on the chuck. Now we have two probes for source and drain.
The movement of the probe is controlled by three knobs. One *** moves the probes in
an upward or downwards motion. And then you have one *** on the back where you can move
in a forward or backward direction, and one *** which can move it left or right.
You can also move the chuck to make it centered while you move the probes, and then you can
probe the electrodes one by one. We can actually pattern these devices to have
less parasitic leakages. You can scratch the device in a way where the semiconductor layer
is disconnected from anything else. We can just use one probe to basically pattern
the semiconductor, and we can just move all around the device and cut through the semiconductor.
Gently we need to touch these electrodes, we don’t need to push very hard.
Chuck #1 is contacting to the gate which is on the bottom of the substrate. And then you
can touch the two probes to the source and drain.
This transistor has three terminals: two are the source and drain electrodes, and one is
the gate electrode, which actually controls the device to go on and off.
Now we are going to characterize the devices. The transistor has three electrodes: gate,
source and drain. And there are two characteristics that we are interested in. First is transfer
characteristics where we scan the gate voltage from negative to positive voltage and the
current flows from source to drain electrode, and we record that current.
And then at a certain point you can see the transistor switching on which is the turn-on
voltage which in this case is slightly higher than one volt. And then after a certain time
you see it going toward the saturation region. We take the square root of this drain current
and then we can find out the threshold voltage. We take the slope of this curve and wherever
it intersects the x axis, that voltage is called threshold voltage. And with this slope
we can calculate electron or hole mobility. The next one is where we fix the gate voltage
and then scan the voltage difference between source and drain and that is called output
characteristics. I’ll explain now what is output characteristic.
On the x-axis you see the drain voltage, and at various gate voltages there are different
currents, from zero, two, four, eight and ten. The transistor starts and this is scanning
at zero gate voltage. And zero gate voltage means the device is off, and you can see that
current. As the voltage increase to two the devices
is slightly on. After the threshold voltage you can see the current increasing, but after
a certain time there is a saturation and that is what we need in our transistor device.
Now I’ll show how the transfer characteristic is done.
The device is off in a certain region when the gate voltage is zero, or low, or on the
negative side. When you apply a positive gate voltage the
device is on. When you characterize at a certain drain voltage you could see the device turning
on from an off state. You can see the scan is going through and coming back. This is
to find out hysteresis in the devices. These devices do not show any hysteresis. But if
you had some dielectric you would see a gap in going and coming down and both scans have
a kind of distance that is called hysteresis.