Dual Ring Discs

Also tested were dual ring disc electrodes with a view to producing electrodes capable of making several different measurements simultaneously with pH generation, such as a pH generation/detection feedback mechanism with simultaneous trace metal analysis.

5.4.

Results and Discussion

5.4.1. Electrode Fabrication

All stages of material growth were performed by Element Six (Harwell, Oxford, UK). Typically insulating substrates were 1 mm thick and mechanically polished on the growth face to a surface roughness of <2 nm. Structures of the desired geometry were machined using a high power laser micromachiner (E- 355-H-3-ATHI-O, Oxford Lasers, UK), before acid cleaning as described in

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Chapter 2. The substrates were then overgrown with BDD using MW-CVD,

excess BDD removed by polishing, and acid cleaned for a second time. This process is illustrated in Figure 5.4. Electrical contact was made to the rear of the electrode using laser micromachined holes as described in section 5.4.1.1.

Figure 5.4: Schematic illustrating the process for creating BDD ring disc electrodes sealed in an insulating diamond substrate. Insulating diamond is laser micromachined into trenches of the desired geometry, overgrown with BDD, then polished back to reveal the electrode faces.

5.4.1.1. Laser micromachining and back contacting

Contacts to the back of the BDD electrode were formed via laser micromachining. When machining holes to back contact all-diamond devices there are several important points to consider. Firstly, the hole must be deep enough to reach the back of the BDD but not so deep as to go all the way through the front surface. Secondly, to maintain individual addressability, the holes for individual electrodes must not overlap with other electrodes or holes. In order to do this the laser must initially be focused and the parameters calibrated to produce holes of the desired depth.

Firstly, the laser is focused on the back surface of the insulating diamond in a blank area, and the depth per pass is subsequently measured and calculated as described in Chapter 2. From this the approximate depth per pass is used to set the z-step, this is especially important for back contacting as over-lasering could result in holes through the thin BDD layer whilst under-lasering will not

Page | 116 contact the electrode, poor laser focus can also cause these issues. Finally, the number of passes (N) necessary to reach the back of the BDD is calculated for each electrode on the substrate, and N-2 passes are applied with the above parameters. The depth of the hole is once again measured via interferometry and the final layers are lasered and tested sequentially with a multimeter until contact is achieved, illustrated in Figure 5.5. The final hole depth is confirmed using interferometry. A circular cutting program at frequency = 10 kHz, 100% power, is used to laser individual devices out of the substrate in 3 mm diameter cylinders, with the electrodes in the centre.

Figure 5.5: Illustration of the process of laser back contacting the disc of an all diamond ring disc electrode. A circular laser program is applied for 1 pass, before the depth is measured using interferometry, this is repeated and the depths used to calculate the number of laser passes needed to reach the rear of the BDD electrode (shown in the final step).

The electrodes were acid cleaned once more. In order to form an ohmic contact to the BDD a Ti/Au layer was sputtered into the lasered contact holes. Upon annealing TiC is formed at the interface between the Ti and the BDD, the Au protects the outer Ti surface from oxidation; this has been found to produce well defined ohmic behaviour. It is important to maintain the individual addressability of the electrodes when sputtering, as such circular holes are lasered in an adhesive kapton mask in the same arrangement as those in the all-diamond device. The mask is aligned over the back of the electrode with the help of a dissection microscope (Motic, US) and secured in

Page | 117 place, leaving the contact holes clear but ensuring the rest of the diamond is covered, this process is illustrated in Figure 5.6. The all-diamond devices are then sputtered with a 10/300 nm layer of Ti/Au and subsequently annealed at 400 °C for 5 hours.

Figure 5.6: Schematic illustrating the Ti/Au sputter contacting of a back contacted all- diamond ring disc device. Firstly, a Lasered kapton mask is placed over the rear of the electrode, leaving only the lasered back contact holes clear. A 10 nm layer of Ti is sputtered over the entire surface, followed by a 300 nm layer of Au. The mask is peeled back revealing the Ti/Au sputtered contacts; the electrode is now ready to be annealed.

Once annealed and cooled to room temperature, the all-diamond electrodes were tested using a multimeter to ensure electrical contact is achieved and the individual electrodes remain isolated. The electrodes are placed face down on gel pak inside a cylindrical Teflon mould. Individual cores of a multi core Cu wire (small enough to fit in the contact holes) are placed in the laser micromachined back contacting holes with the aid of a dissection microscope and micropositioners, and secured in place using conductive silver epoxy (Circuitworks, ITW Chemtronics). This is left to dry for 24 hours. The teflon mould is half-filled with a 2 part mixture of non-conductive epoxy (RX771C/NC, Robnor, UK), ensuring the silver epoxy is coated but the free ends of the wires are left uncovered, and left to dry for a further 24 hours. A ~0.5 cm section of insulation is stripped from both ends of insulated Cu wires,

Page | 118 these are soldered to the bare ends of those attached to the electrode ensuring no contact between electrodes. Finally, the mould is filled completely with non-conductive epoxy such that the soldered wire connections are completely covered, and left to dry for 24 hours. Once dry, the electrode is removed from the mould and the front surface is polished with alumina before testing.