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University of California, San Diego
Mechanical and Aerospace Engineering
MAE 156B: Senior Design Project
Background
Background
Graphene is an exciting new nanomaterial comprised of a one atom thick sheet of carbon atoms. Some of its properties of note include its high tensile strength, thermal conductivity, and electron mobility.[1] Due to these properties, it has many potentials in electronics, but is still being experimented on by researchers, namely the sponsor of this project: Professor Oscar Mena of the UCSD Nanoengineering Department.
Stacks, cut from a sheet
One current method for obtaining graphene is to purchase it in sheets, sandwiched between a layer of copper and polymethyl methacrylate (PMMA). Graphene comes this way due to the method in which it is manufactured, known as chemical vapor deposition (CVD). In CVD, carbon atoms precipitate out of a gas onto a copper foil until it is covered in a layer of graphene. [1] The top side of the graphene is then coated with a thin layer of PMMA to protect it. This three-layer sandwich is known as the "stack."
To test the graphene, the copper foil underneath the graphene must be removed to apply it to device substrates like PCBs. The current process, detailed below, makes heavy use of the "fishing" technique. It can take up to five hours.
The stack is added to a bath of ammonium persulfate to dissolve the copper, and then fished out with a glass slide.
It is added to and fished out of repeated consecutive baths of deionized water to wash away trace residues of ammonium persulfate.
Once the graphene visually appears clean enough, it is fished out of the final bath and slid onto the substrate.
Fishing
This project aims to automate the fishing process. In the current process, the need to constantly tend to the it takes away valuable time from the members of the lab that could be used for other tasks. Frequent human interaction also leads to a high rate of failure. The single atom thick layer of graphene is quite fragile and any slip of the hand when transferring the stack between baths can tear it.
This project also aims to provide the Professor with insight on how the cleanliness of the graphene affects its material properties, as he currently has no scientific monitoring of the solutions involved in the process.
Design Solution
Design Solution
The proposed solution uses a set of baskets, with a tank and a pump system, to dissolve the copper foil in an ammonium persulfate solution and clean the sample with deionized water. A conduction probe keeps track of the dissolved ions, and when the sample meets a threshold for cleanliness, it is laid onto the final substrate to dry.
Mechanics of the Process
Mechanics of the Process
Rather than holding onto the stack and moving it up and down with motors, to ensure the stack is handled as delicately as possible, the level of the fluid is used to control the vertical position of the stack. A capacitive level switch stops the flow of deionized water into the tank at a predetermined level such that the graphene layer never floats over and out of the top of the basket. In between baths, the fluid level drops and the graphene rests temporarily on top of the substrate. After testing done by the team, it was found that if the next bath is pumped in soon afterwards, the graphene does not have time to dry and stick to the substrate, and it will float back up to allow for further cleansing of the underside.
Simplified view of design
The basket
The Basket
The Basket
A basket is used to hold both the chip substrate and the graphene layer throughout the process. The geometry of the basket allows the substrate to slide in and act as the bottom. Gaps on the bottom allow fluid in and out. Due to the sponsor's desire to process multiple stacks at once, the project uses a modular design where any number of baskets can hooked onto the edges of the tank to allow for any number of stacks to be cleansed. These baskets can be created quickly with a resin printer found at the UCSD Makerspace in the Design and Innovation Building. In order to accommodate the possibility of varied device substrate and stack dimensions, a CAD file has been given to the lab with an easily-editable list of parameters that can be modified to regenerate a basket of any size.
The Pumps and Drains
The Pumps and Drains
Rather than transferring the graphene between different baths, this solution seeks to keep the graphene in one tank as the fluid is pumped in and drained out. Ammonium persulfate is a highly corrosive solution, so material compatibility was of great concern. To minimize the cost of the project, rather than sourcing a pump resistant to corrosion by ammonium persulfate, the user will pour the first bath themselves. This still maintains the requirement that the user can start the process in one sitting and then leave it unattended. Gravity is used to drain the baths out of a solenoid at the bottom of the tank. The solenoid, tank walls, and drain piping were all chosen out of stainless steel, acrylic, and PVC, which were all determined to be chemically compatible with ammonium persulfate. [2][3]
Conductivity Probe
Conductivity Probe
Deionized water has a very low conductivity. As ions are added, the conductivity increases. [4] This relationship, when measured by a conductivity probe, gives a controller insight into the status of the process. When it is determined that the copper is done dissolving, the ammonium persulfate bath can be drained out. As the repeated baths of deionized water are pumped into and drained out of the tank, when the conductivity settles at a threshold minimum, the graphene can be considered clean and the process stopped.
By adjusting that final threshold value and doing subsequent testing on graphene, the sponsor hopes to characterize the properties of graphene as a function of its cleanliness.
Results
Results
IMG_5204.MOVTimelapse video of functional test of process
Testing shows
The copper is effectively dissolved inside the baskets by the persulfate solution.
The subsequent cleaning baths are successfully able to pump in and out without damaging the graphene layer.
The Arduino microcontroller communicates to an already existing lab computer using a serial connection, and the computer logs the data using a terminal software like PuTTY.
The process is automatically completed when the conductivity probe reads a cleaning bath that stays under the 50 μS/cm threshold.
References
References
[1] https://www.sciencedirect.com/science/article/pii/S0008622310000928?via%3Dihub
[2] https://www.calpaclab.com/stainless-steel-chemical-compatibility-chart/
[3] https://www.calpaclab.com/pvc-polyvinyl-chloride-chemical-compatibility-chart/
[4] https://apureinstrument.com/blogs/what-is-salinity-meter-and-how-does-it-work/
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