Fabrication of mechanically-adjustable devices with two opposing graphene electrodes

The goal of this project is to fabricate mechanically-adjustable devices with two face-to-face graphene electrodes. The design of the devices will allow us to perform electrical measurements of molecules spanning between the graphene electrodes while varying the distance between the graphene electrodes and also simultaneously carrying out optical microscopy/spectroscopy through the graphene. Microfluidic channels are designed to bring in liquid solutions of interest to the space between the opposing graphene sheets. This report presents the structure of the device, the fabrication scheme, and our recent progress.

People Involved

Wan Li

Summary

Figure 1 shows the schematic of our device. The central part of this device consists of two opposing suspended graphene sheets initially separated by a distance of 100s of nm. The graphene sheets are contacted by metal wires for electrical measurements, and suspended over small holes to allow for simultaneous optical microscopy and spectroscopy. We also incorporate microfluidic channels connected with inlet and outlet ports, which will enable us to bring in liquids of interest to the space between the opposing graphene sheets. A previous study [1] demonstrated that with the application of external pressure, suspended graphene sheets can be deflected by over 150 nm. Our device will combine this ability with the opposing graphene geometry, deflecting the graphene sheets together with external pressure to vary the distance between them in a controlled manner. Novel types of experiments will be enabled by this design while taking advantages of the unique high conductivity, transparency, and mechanical strength of graphene.

Figure 2 illustrates the process flow that we are developing to realize the device. The devices are fabricated with two wafers that respectively support the top and bottom graphene sheets. These two wafers are prepared separately, and then bonded together to achieve the final opposing graphene geometry. To prepare the wafer supporting the bottom graphene, we start with a 4 inch silicon wafer coated with 300 nm low stress silicon nitride on both sides. We first pattern the silicon nitride on one side of the wafer and use it as an mask for KOH etching to suspend a silicon nitride membrane on the other side of the wafer and create 100 100 µm2 windows (Fig. 2a). Following this, we define multiple metal leads on the wafer using photolithography (Fig. 2b). Small holes to be used for suspending the graphene are then etched on the suspended silicon nitride membrane using CHF3/O2 plasma (Fig. 2c), producing hole diameters ranging from 4 to 6 µm. At this stage, the wafer is ready for graphene transfer. To increase the yield of suspended graphene and maintain a relatively clean graphene surface, CVD graphene grown on copper foil is transferred onto the wafer using the method used in Ref. [2], which removes the PMMA on graphene by thermal annealing instead of organic solvents (Fig. 2d). The wafer supporting the top graphene is prepared following similar procedures, except that additional windows are opened in the wafer during the KOH etching step to expose the metal leads on the bottom wafer after bonding, and also to permit the introduction of microfluidic inlets/outlets (Fig. 2f). No metal leads are defined on the top wafer before bonding.

After the two wafers are prepared, SU-8 is spin-coated on the bottom wafer. Microfluidic channels are patterned into the SU-8 layer using photolithography (Fig. 2e), and the two wafers are bonded together using SU-8 as the adhesive layer (Fig. 2i). As the last step of fabrication, the top graphene sheet is connected with metal leads by e-beam evaporation using shadow masks (Fig. 2j).

Figure 3 shows our current progress on fabrication. Up to now, we have succeeded in obtaining relatively clean graphene sheets suspended over holes on silicon nitride membranes (Fig. 3a) with high yield. The contact resistance between the metal leads and graphene is measured to be ~250 Ohms (Fig. 3b). We also developed a bonding recipe which can bond two blank wafers using SU-8 patterned with microfluidic channels (Fig. 3c). We are now working on applying this recipe to bond the wafers that support the top and bottom graphene sheets. Figure 3d shows the optical image of a test device which is made by bonding a completed bottom wafer with a transparent, blank wafer.


Figure 1
Figure 1: Device schematic: mechanically-adjustable device with opposing graphene sheets.


Figure 2
Figure 2: Fabrication Process flow.


Figure 3

Figure 3: Current Progress: a. Scanning electron micrograph of a suspended graphene sheet on device, inset: diffraction pattern taken on a piece of graphene prepared in the same method as the one on device; b. Current-voltage curve measured between two metal leads contacting with the same piece of graphene; c. Optical micrograph of the a microfluidic channel made by bonding two blank wafers with patterned SU-8; d. Optical image of a test device which was made by bonding a completed bottom wafer to a transparent blank wafer.

References

  1. J. S. Bunch et al. Nano Lett. 2008, 8, 2458-2462.
  2. A. M. van der Zande et al. Nano Lett. 2010, 10, 48694873.


Last updated: 22-Jul-2011

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