Porcentaje de cultivos Asociación Asaraty En el gráfico 4, observamos,

The fabrication of devices starts with4 inchSi wafers on which Ni, graphene and a protective re-

sist layer are already applied. For this project, the wafers of two dierent suppliers are used. Part of the wafers is externally bought from graphene-supermarket.com, a web shop from Graphene Laboratories, Inc., a company which produces various graphene products. Other wafers are pro- duced by Derya Ataç (post-doc in the NE-group) in the cleanroom of the MESA+ institute. The production processes used by both suppliers are comparable. They both use a Si wafer on which a lm of Ni is deposited. The graphene is deposited using chemical vapour deposition (CVD). In this process, the sample is heated to about950◦Cand exposed to a mixture of hydrogen and

methane gas. The Ni layer acts as a catalyst which causes the methane to decompose and the C atoms to be absorbed into the Ni. When the sample is cooled down again, the C atoms migrate to the surface, leaving typically between 1 and 7 layers of graphene. The graphene arranges itself in patches with dierent thickness. The size of each patch is about 3-10 microns. Finally

a layer of protective photoresist is spin coated before dicing the sample in chips of 11×11 mm.

This resist layer is applied to protect the graphene surface against wafer fragments during the dicing step. [29] [30]

4.1.2 Transport measurements

To test the insulating properties of the SiO2 layer, bottom and top contacts which can be connected to a PCB need to be dened. A schematic representation of the chip layout can be found in gure 4.2. This design was made by Elmer van Geijn. The size of the bottom electrode is3×11 mmand the three top electrodes are1.2×7.2 mm. These dimensions are large compared

to structures that are previously made in this research[11][12]. This is because having a large overlap area between the top and the bottom electrode gives statistically better information about possible leakage through the SiO2 layer (if there is no conduction for a large overlap area, then it is very unlikely that there will be conduction in a sample with a much smaller overlap area). The bottom electrode is dened by photolithography with a positive resist. At the exposed parts, the Ni and graphene are etched away. Then a layer of SiO2 is sputtered, covering the whole sample. Finally, the top electrodes are deposited using electron beam evaporation with a shadow mask. These electrodes consist of an Al layer with a layer of Cu on top. The reason that Al as well as Cu are applied is that, for the nal devices described in chapter 5, a thin Al layer can be used for Tedrow-Meservey measurements when it is made superconducting. This layer needs to be very thin to be able to withstand high magnetic elds while retaining its superconducting state. Because the top electrode also needs to be a good conductor when there it no superconductivity an extra layer of Cu is applied. This layer also protects the Al against oxidation and makes ensures that there are no gaps in the lm (especially at step edges). The top and bottom electrodes can be connected to a PCB chip using Al wires which are attached using a wirebonder. To contact the bottom electrodes the wirebonder needs to pierce through the SiO2 layer. This is generally not very hard because this layer is relatively thin.

Al/Cu

Si/SiO2

Ni/Graphene

Figure 4.2 Layout of a chip for testing the SiO2layer. The sample consists of a bottom electrode

(grey) out of Ni covered with graphene. Over the whole sample, a SiO2 layer is applied (dotted in

black). Finally three top electrodes (green) are deposited. The resistive behaviour of the SiO2layer

can be tested by applying a voltage dierence between a top and the bottom electrode.

The fabrication starts with removing the protective photoresist layer using acetone. Then a layer of positive OiR 907/17 photoresist is spin coated onto the sample. To dene bottom electrodes, photolithography is performed. For simplicity a printed overhead sheet was used

in stead of a glass mask. After exposure, the sample is baked on a 120◦C hotplate for one

minute and developed in OPD-4262, leaving the resist only at the unexposed parts. Then ion beam etching is performed to etch away the Ni and graphene around the bottom electrodes. The bombardment of the photoresist with Ar during this step, cuts the length of the polymer chains. This makes the top layer of the photoresist less sensitive to solvents like acetone. This layer is removed by a 30 second oxygen plasma treatment in the TePla 300E. The remaining

resist is removed by cleaning the sample with acetone. The sputtering of the SiO2 is done using the TCOater. The thickness of the deposited layer can be determined using ellipsometry. The thickness of this layer is varied as well as the amount of O2 that is added to the chamber during the sputtering. Finally, the top electrodes are applied by electron beam evaporation. In this machine the sample is covered with a shadow mask in which the top electrodes are dened. A layer of Al and Cu are evaporated onto the sample to form these electrodes. The nished sample is placed on a PCB chip and wires are bonded to the electrodes.