Parámetros operativos del evaporador con serpentín

The bottom electrodes are dened in the nickel-graphene layer by means of ion beam etching. It is important that in the etched areas the nickel layer is completely removed to make sure that there are no current paths between dierent bottom electrodes. It is no problem to etch deeper than strictly necessary, so the etching time in which all nickel is removed is roughly determined, but not too much time is spent to optimise this. The resistance between dierent bottom electrodes was measured to make sure that they are not connected. The found etching time is used thereafter and diered between wafer 1 and wafer 2 due to the dierent thickness of the nickel layer.

The resist layer as well as the contact holes dened in this layer are of signicant importance, because the goal is to obtain well dened junctions at the locations were the contact holes are made without other current paths existing between bottom and top electrodes. On the one hand it is important that the resist layer is insulating. On the other hand the contact holes should allow for good contact between the bottom electrode and the top electrode. Throughout this project dierent types of resist have been used depending on the method used to dene the contact holes (photolithography or EBL). The insulating properties of the resist layer have been tested by fabricating devices without contact holes and measuring the resistance between top and bottom electrodes. Diluted photoresist forms an insulating layer, but for PMMA this is not always the case. The standard spin speed of 4,000 rpm resulted in a resist layer with a nite resistance. Fabricating and testing devices with PMMA layers of dierent thickness by varying the spin speed during spin coating showed that reducing the spin speed to 2,000 rpm is sucient to obtain an insulating layer for most bottom electrodes. However, a later test sample with the same fabrication conditions showed a nite resistance for several bottom electrodes. Time was

useful for alignment.

lacking to investigate this in more detail and completely solve the problem. Figure 5.17 shows AFM images of the edge of the insulting photoresist layer on top of a bottom electrode and of a photolithographically dened contact hole. The height of the bottom electrode is a bit more

(a) Edge of photoresist on top of a bottom

electrode fabricated with wafer 1. (b)hole. Photolithographically dened contact Figure 5.17 AFM images. Fits along the lines indicated in Figure 5.17(a) show the bottom electrode to have a height of a bit more than 300 nm and the photoresist to have a thickness of 0.6

µm next to the bottom electrode and 0.5 µm on top of the bottom electrode. From a t along the

line indicated in Figure 5.17(b) the depth of the contact hole is determined to be 0.5µm.

than 300 nm, as expected, because etching is done a bit deeper to make sure that the nickel layer is completely etched away. The photoresist layer is half the thickness of undiluted OiR 906/12 and it is observed that planarisation of the photoresist takes place to some extent. The depth of the contact hole is in agreement with the thickness of the photoresist. The walls are not very steep.

The alignment of photolithographic contact holes with the bottom electrodes is checked by means of an optical microscope. The alignment accuracy of∼ 1 µm determines the maximum

possible size of a contact hole for a given width of the bottom electrodes. Raman measurements showed signatures of amorphous carbon at the edges of the bottom electrodes, so to be on the safe side the contact holes should be at least 4 µm smaller than the bottom electrodes.

Figure 5.18 shows an optical microscopy image and a SEM image of a nished device with photolithographically dened contact holes. In the optical image the purple area is the Si/SiO2 substrate, the yellowish areas are the bottom electrodes, the large green area is photoresist and the top electrode is visible in white at the bottom of the image. The SEM image indicate the contact hole after deposition of the top electrode to be about 450 nm deep, which is a bit less than before the deposition, so the top electrode is a bit thicker in the contact hole than on top of the photoresist. The top electrode does not seem to be continuous everywhere at the edges of the contact hole. The ridges in the wall of the contact hole visible in the SEM image are caused by the standing wave pattern in the exposure step in photolithography. The dierent layers in the top electrode have an intended total thickness of 47.5 nm, but the thickness can not be inferred from this image.

Contact holes dened by means of EBL are checked with SEM in the EBL set-up. The result is shown in Figure 5.18(c). The size of the contact hole is 124 nm, which is a bit smaller than the intended 150 nm. The intended total thickness of the top electrode for this device is 50 nm, but it is not possible to determine the actual thickness from this image.

(a) Optical microscope (b) Scanning electron microscope

(c) Scanning electron microscope

Figure 5.18 Images of nished devices. Figure 5.18(a) and Figure 5.18(b) are for a device with photolithographically dened contact holes. The optical image shows all dierent parts of the device. In the SEM image the contact hole is depicted. Figure 5.18(c) shows a contact hole dened by means of EBL.