Graphene Fabrication

Mechanical Exfoliation

Graphene samples are fabricating using the standard “mechanical exfoliation” technique. This involves cleaving Kish graphite by placing it on Scotch tape and repeatedly folding the tape over the graphite and opening it up again. Once the tape is coated with a relatively thin layer of graphite, it is pressed onto a SiO2-covered wafer. The back of the tape is rubbed lightly with a pencil eraser for 1-5 minutes. To get the pressure correct, I suggest applying lots of pressure at one end, and gradually decreasing the pressure until it is very low at the other end; then look under the microscope to see what level of pressure gave the best yield. On a good day, you should be able to get graphene samples with dimensions of 10s of microns.

The following optical image shows a typical sample demonstrating how the visible contrast varies with graphene thickness (on 90 nm SiO2). The thinnest region is a monolayer; nearby are regions with 2, 3, and many layers.

graphene sample

Optical Contrast

In order to see the graphene optically, it must be deposited some sort of thin film with an interference condition giving it optimal contrast. We use SiO2 on Si, with a thickness of 90 nm or 285 nm (for optimal contrast around 550nm). Graphene on 90 nm of SiO2 is easier to see, but has a greater likelihood of gate leakage. Below is a contrast plot of graphene on SiO2/Si as a function of wavelength and SiO2 thickness.

Visible Contrast of Graphene on SiO2 on Si

CVD Graphene Growth and Transfer

Part 1: Prepare wafer for growth

  1. Start with a silicon wafer with 500 nm Oxide
  2. Clean the wafer using Hot Piranha
  3. Evaporate 300 nm Nickel on the wafer slowly (1-1.5 A/s). This will take a while.
  4. Dice wafer into appropriately sized pieces for McEuen Group furnace (less than 22.5 mm)

Part 2: Grow Graphene on Ni

  1. Heat to 1000C with 1 SLM Argon should take ~ 20 Min to equilibrate
  2. Anneal for 1 hour 1 SLM Argon, 0.1 SLM H2
  3. Change Gas Flows to: 0.08 SLM CH4, 0.1 SLM H2, 0.300 SLM Ar
  4. Flow for 2 Min
  5. Turn off CH4, H2. Change Ar flow to 1.5 SLM.
  6. Turn off heat
  7. Cool at controlled rate. Aim for 10 - 20 C/min

Part 3: Transfer graphene onto arbitrary substrate.

  1. Spin thick PMMA layer onto graphene covered Nickel. 11% Anisole-PMMA, 4000 RPM for 45 seconds
  2. Bake at 175 C for 2-3 minutes
  3. Etch oxide to release PMMA membrane. Place chip in BOE 6:1 until membrane comes free.
  4. PMMA Membrane with graphene and Nickel will float in BOE solution. Scoop out with extra silicon piece.
  5. Transfer into Nickel Etch solution
  6. Leave in Nickle etch for 20 minutes
  7. Scoop PMMA graphene membrane into DI water. Repeat into new DI water
  8. Membrane should float on the surface of water, with the PMMA side up, and the Graphene side down.
  9. Scoop membrane out of liquid one more time with desired final substrate. The graphene should be in contact with your surface.
  10. Let chip and membrane dry for a day
  11. Dip Chip in Acetone to remove PMMA. Graphene layer should remain on chip.

CVD On Copper Foils - Park Set up

  1. Cut 25 μm copper foils into ~ 2 cm x 2 cm strips
  2. Clean the strips of Cu with acetone and IPA
  3. Load Cu into the furnace and lower the pressure to under 10 millitorr
  4. Anneal Cu with H2 at 50 sccm at 1000C for 40 minutes
  5. Flow CH4 into the furnace at 880 sccm for 13 minutes
  6. Turn off furnace, and let everything cool down
  7. Turn off H2 and CH4 flows, and turn on Ar at 500 sccm
  8. After the system is depressurized, remove samples

CVD On Copper Foils - McEuen Set Up

“Standard growth”

  1. Cut a piece of Alfa Aesar 25 μm copper foil into a long strip, such that it will fit on the quartz boat
  2. Optional: place copper foil in acetic acid for 10 minutes, then rinse with water and blow dry with nitrogen gun
  3. Place the copper foil on the quartz boat and load it into the furnace; see Furnace Use for how to operate the furnace
  4. Program the furnace with the following values (slow ramp to avoid overshoot):
SP1 T1 SP2 T2 SP3 T3 SP4 T4 T5
940 0 940 10 980 30 980 >anneal+growth time off
  1. Flow hydrogen at 60 sccm at 980 C for 20 minutes (anneal)
  2. Continue flowing hydrogen during growth and cool down
  3. Flow methane at 36 sccm for 20 minutes (growth)
  4. Manually shut off the furnace when finished
  5. Prop the furnace open with the small metal piece
  6. Let cool to 450 C, then open the furnace completely
  7. Allow to cool below 200 C, stop flowing hydrogen
  8. Unload your sample (refer to Furnace Use for details)
  9. Cut the copper foil into smaller pieces, and keep track of which side is the “top” (e.g. cut a small notch in the lower right hand corner)

Remember to send Isaac (irs9~at~cornell.edu) your Raman spectra and/or SEMs, so he can make sure the furnace is still producing good graphene.