Hello, I'm Nan Jokerst at Duke University and I'm joined by my Ph.D. student, Aditi.
Hey, Nan.
In this video, Aditi will demonstrate the operation of the thermal evaporator.
The fabrication tools, including thermal evaporator,
are located inside the clean room.
So, first we have to gown up to enter the clean room.
We are now wearing the proper attire to enter the clean room. Let's go.
The thermal evaporator is used to deposit thin films of
materials onto the surface of a substrate or sample.
Thermal evaporation is one of several methods for depositing thin films.
The deposition method used will depend on a number of
factors including the specific material being deposited,
how much material is available,
and even the resulting subtle differences and microstructure of
the deposited film which can vary with the deposits and method used.
Today, we will demonstrate
thermal evaporation using a small amount of gold source material.
We will use a silicon wafer as our sample.
This is a thermal evaporator.
Let's open the door to the vacuum chamber.
To do this, we need to force-spring the vacuum chamber to
atmospheric pressure which is called venting system.
So, we open the control panel.
You can think of this as opening a vent that lets air into the system.
We usually add nitrogen to the system when we vent
the system to keep the chamber as clean as possible.
The previous user unset process by pumping down the system and leaving
the chamber under vacuum because that keeps the chamber cleaner too.
Press the vent button.
First, replace our wafer on the sample holder.
The wafer is secured to the holder with springs.
After securing the wafer to the holder,
we are ready to place the wafer holder into the vacuum chamber.
Okay.
Let's open the system now.
Here are the gaskets that provide the vacuum seal for the door when we pump down.
Next, we need to place our source material into the sample boat.
The source material is the material we wish to deposit.
Today, we are evaporating gold.
Gold is often used in electronics because it does not corrode or oxidize.
We will use a tungsten boat to hold the gold since
tungsten melts at a higher melting temperature than gold.
The boat is held between two electrodes within the vacuum chamber.
We install the boat by loosening these screws and
placing the boat under the washers and tightening the screws down.
We then place a small amount of gold in the boat.
This is high-purity gold.
See, the label shows us that it's 99.999% pure.
We use high-priority materials because
some materials are incompatible with certain processes on materials.
Even a small amount of unwanted material can ruin a device.
Via the sample holder to the vacuum chamber,
we now have our source material in place as well as our wafer.
During evaporation, the boat is heated using electrical current.
The gold will melt and begin to evaporate.
The evaporated metal will travel towards the wafer
at the top of the chamber and condense on the surface of the wafer,
forming a thin film of gold.
We can actually measure the thickness of the gold film as it is being
deposited using an instrument called a thickness monitor.
This monitor uses a vibrating crystal connected to the electrical monitoring equipment.
As more material is deposited,
the vibration frequency of the crystal changes.
The controller then calculates corresponding material thickness.
The user can see in real time how much material is being deposited.
Also note the vacuum boat at the rear of the chamber.
This leads to the turbo pump.
If you look closely,
you can see the turbine fins.
We are now ready to close the vacuum chamber and begin the pump-down sequence.
First, we rough down the system down to a few torr.
Then, we start the double-pump so that high vacuum can be achieved.
The turbo pump uses a mechanical pump as a vacuum pump.
After about 90 minutes,
the pressure in the chamber is down to about five times ten raised to minus six torr.
This is the level we need to be to ensure a high quality evaporated film.
Now, we can start the evaporation process.
This process is automated so the computer controls the heating
of the boat and monitors the thickness using thickness monitor.
To correctly sense the deposited material thickness,
we need to enter what material we are depositing into
the thickness monitor computer program as well as our desired thickness.
Then, we simply select the desired recipe and press start.
When the desired thicknesses is reached,
the process is ended and the boat cools down.
So, let's now begin the process.
Shortly after pressing start,
the current flowing through the boat has heated it red hot.
The gold has melted and is now evaporating.
As mentioned, we can view the thickness of
the deposited film in real time with the thickness monitor.
It only takes a few minutes to reach our desired thickness.
In this case, we deposited 75 nanometers of gold.
At this point, the automated process is halted and the boat is allowed to cool.
We now must vent the vacuum chamber so that the door can be opened.
This means we bleed nitrogen gas into
the chamber until the pressure inside reaches atmospheric pressure.
That is the same pressure in the room.
After a few minutes of bleeding in nitrogen gas,
the chamber is fully vented and we can open
the vacuum chamber door and retrieve our wafer.
Our wafer is now coated with 75 nanometers of gold.
Let's look at our wafer before and after the gold deposition.
We now close the vacuum chamber and pump it
down so that it will be ready for the next to user.
We keep all our vacuum systems pumped down when not in use.
This ensures that they remain as clean as possible.
I hope that you enjoyed the demo of thermal evaporation.
Thank you for joining us today.
