CVD-Graphene
The Aixtron Black Magic CVD furnace is dedicated to graphene synthesis and can accommodate multiple pieces or a single 4" wafer, with methane and hydrogen available as process gases. Process recipes have been optimized and are available to the user for the following: Cu foil (available from SNF stockroom or provided by the user), Cu-coated substrates, and Ni substrates. The recipes on Cu have been characterized to give excellent monolayer coverage with minimal areas of bilayers and low defectivity. Ni substrates will give multi-monolayer growth.
Many thanks to Ning Wang and Christopher Neumann for their contributions on this page.
Contact List and How to Become a User
Training to Become a Tool User
Process Monitoring and Machine Qualification
Picture and Location
The tool is located at Exfab.
Left: SNF Black Magic Pro 4” Furnace. Right: Inner process chamber.
Background
Chemical vapor deposition of graphene first involves the introduction of a carbon source (CH4 is most common, although C2H2 and liquid sources have also been utilized) into an enclosed, reducing environment. At sufficiently high temperature, the carbon source decomposes into various radicals, some of which travel to the catalyst surface (Process 1 in the figure below). Upon reaching the surface, the carbon radicals react with the catalytic substrate and nucleate into graphene (Process 2,3,4,5), releasing H2 byproducts (Process 6). As the growth time increases, this reaction continues and the nucleation sites grow to form large grains of graphene which eventually merge to form a continuous layer. Quality is determined by growth conditions (time, temperature, pressure, and gas flow rates) as well as substrate type, cleanliness, and morphology.
Processes involved in CVD graphene growth. Figure adopted from [S. Bhaviripudi et al., Nano Lett., 10, 4128-4133 (2010)]
Process Capabilities
Cleanliness Standard
CVD-Graphene is classified as Gold contamination level in general sense, however, it also has its own classification. Only certain metals used as catalyst for CNT growth are allowed in this system.
The CVD-Nanotube processes take place with a chamber temperature up to 1100C. To prevent contamination and ensure safety, the following materials are not allowed:
- Polymers
- Wet Samples
- Any foreign material with low melting point (below or close to process temperature), or high vapor pressure
- Plastic, including Teflon
- Particles need to check with staff case by case
- No other metals than Copper and Nickel
Performance of the Tool
What the Tool CAN do
- Monolayer graphene on Cu foil (CUFOIL): Most robust and commonly used growth recipe; yields acceptable (D-G ratio < 0.1 under Raman) polycrystalline graphene with 1-10 μm grains depending on substrate pretreatment.
- Monolayer graphene on Cu think film on SiO2 (CUTHINFILM): Experimental recipe which yields high quality (i.e. defect free) graphene for 500 nm Cu evaporated on SiO2 with acceptable coverage. However, graphene transfer is more difficult in this case as the films tend to stick to the growth substrate, and thus we recommend only users experienced with graphene transfer use this recipe.
- Multilayer graphene on Ni Film/Foil (NI): Recipe tuned to yield multilayer graphene with varying layer thickness > 2. This recipe has been characterized the least of the three, and thus we have no guarantee on quality nor layer count. However, growth does yield complete coverage and should be suitable for most needs. IF NI GROWTHS ARE DESIRED, USERS MUST CONTACT THE PROCESS STAFF AND SUPERUSER TO EXCHANGE QUARTZWARE, otherwise the Ni catalyst may contaminate the Cu growths
To maintain reproducibility, only these three growths are allowed. For new growth recipes and/or substrates, users should contact the process staff and superusers and be prepared to purchase spare quartzware.
What the Tool CANNOT do
Contact List and How to Become a User
Contact List
The following people make up the Tool Quality Circle:
- Process Staff: Xiaoqing Xu <steelxu@stanford.edu>
- Maintenance: jhaydon@stanford.edu
- Super-Users: Ning Wang <ningwang@stanford.edu>
Training to Become a Tool User
Operating Procedures
Initialize Tool:
1. Enable the tool on Badger.
2. Ensure that the tool is in proper stand-by state:
a. Pump (behind the furnace) should be on and operational.
b. Water chiller should be off. Check the water level—if it is halfway between the two lines, do not proceed and contact a superuser to refill the chiller.
c. Gas valves (on the right side of the tool) in the OFF position. MFC 2-7 should point down, while MFC 1 should point up.
d. Terminal should only be displaying the desktop. If Aixtron software is open, check to make sure previous user is done—otherwise, close the software and proceed.
3. Turn on processes gases—MFC 1,2,4,5, and 6 should be turned to the right. MFC 3 and 7 are spares and not used.
4. Turn on the water chiller. If an alarm sounds, turn off the chiller and notify a superuser.
5. Open PECVD control software on desktop:
6. Change user to GUEST and log in (password: graphene). Leave “use camera” checked, which will allow you to see the chamber on screen during growth. This can be unchecked if you wish to record video of your growth using the Logitech software—however, this will disable the video feed in the Aixtron software.
7. Software should load and open to standby state shown below. Base pressure (shown above the main video display) should read under 5e-1 mbar—the pump is possibly failing if the pressure is higher. Users should notify superusers and abort processes if this is the case.
Load sample:
1. Close the pump valve, then open the vent valve. Chamber should vent in roughly a minute, and the green LED will light indicating that the chamber is safe to open. Do not attempt to open the door before the green LED is lit as the door is interlocked for safety.
2. Gently lift the chamber door. Afterwards, proceed to unscrew the chamber lid lock and gently lift. The lid has a bit of resistance, so use a gradual, firm movement to lift the lid without damaging the tool.
3. Place sample on bottom heater in the center and avoid touching the top heater. Check also that the IR tube (in the middle of the top heater) is centered, otherwise contact a superuser to adjust the IR tube.
4. Close the lid, making sure that the top thermocouple is in contact with the top heater as the lid is lowered or else the temperature reading will be inaccurate for the top heater.
5. Hand-tighten the chamber lid lock, close the vent valve, and open the pump valve to pump down the chamber. Wait until chamber falls below 100 mBarr.
Growth:
1. Start PROCESS MODE once chamber is pumped down.
2. Click “SHOW WIZARD” to open the recipe wizard. Click load recipe and select desired recipe. Recipe steps will be loaded and displayed.
3. Click “RUN WIZARD” to start growth. The software will ask to save file—click YES and create a new folder or select an existing one to save the run data. The run data will come in handy if your resulting growth is unsuccessful.
4. Recipe will now begin. The tool will sometimes trigger an error with a prompt to hit the reset button—this will occur if the tool isn’t enabled on badger or if it’s being started for the first time. If this happens, press the blue RESET button underneath the monitor and start “RUN WIZARD” again.
5. Wait until recipe gases start flowing—if the gas flow reads 0 despite the line being checked on the left, check the gas lines to make sure the valve is ON. Otherwise, notify superuser and press “PROCESS STOP”. You will not be able to proceed as the recipe is interlocked to make sure gas flow is present before heating to prevent excessive metal deposition in the chamber.
6. Stay until top heater begins to power on (~ 2-3 min). The heater power is displayed on the right (shown as 528W in this image), and should be greater than 0. If top heater never powers on, then the tool is in the incorrect mode and a super user must be notified to change the furnace to the correct mode.
7. Recipe will take ~2 hours to run. Users are not required to stay by the BM Pro, but occasional checks are recommended. If anything appears wrong (gas flow incorrect, heaters not powering), hit PROCESS STOP to abort the recipe and notify super users.
Sample Removal and Shutdown:
1. Recipe will end once chamber is below 150° C. Put furnace into STANDBY mode, close pump valve, and open the vent valve to remove sample. Wait until green LED is lit to open the chamber.
2. Open the chamber as before, and be careful to avoid touching the hot chamber surfaces.
3. Remove sample and close chamber.
4. Close the vent valve and open the pump valve. Wait until tool pumps below 5e-1 mBarr.
5. Close software.
6. Close all gas valves.
7. Turn off the water chiller.
8. Disable tool on Badger.
Process Monitoring and Machine Qualification
Qual Process Overview
Recommendation to users with critical processes
Procedure
Reported Data
EE412 Report:
Black Magic Pro 4” Graphene Furnace Development and Characterization
Epitaxy of GaAs on transferred CVD graphene
Qualification Results
Machine Status States
Red:
Yellow:
Green: