Fabrication of Microelectrodes and PDMS Molding of Microfluidic Structures - Research Paper Example

Fabrication of Microelectrodes Introduction Electrode, when connected, allow a current to pass through it. Microelectrodes are smaller sized electrodes with tips. Microelectrodes is used extensively in many applications. For example, these can be use in electrochemical reaction mechanisms and kinetics, trace electrochemical analysis, electrochemical reaction in solutions of very high resistance, analytical sensing, in vivo measurements on biological objects, and detection in flowing liquids. There are various methods to fabricate microelectrodes. Photolithography is one of these methods that uses light to generate a pattern on the surface of a substrate. The experiment is carried out in clean-room for micro and nanotechnology. The process begins with the preparation of a clean substrate followed by photoresist application, exposure and developing, etching, and finally photoresist removal. These steps are illustrated in the figure below. Figure ( ): illustrated the steps of photolithography In this experiment we fabricated a microelectrodes array which is suitable for use in studying the AC electrokinetic phenomena of electrorotation and dielectrophoresis. We performed the fabrication using a glass substrate from a gold/chrome film. A seed layer of thin chrome (5nm) is used to promote adhesion of thicker inert gold film (70 nm) to the substrate. The fabrication is performed in a clean-room. In this report we will present the method used, the result of fabrication of electrodes, and also discuss the positive and negative result. Finally we will discuss why the results happened the way they did and how to overcome the negative results. Methodology The first step in our process is to clean the substrate. We took six glass microscope slides and placed them into a staining jar covered with a 100% solution of Decon 90 detergent. Then, the staining jar was placed in the Decone ultrasonic bath for 5 minutes to ultrasonically clean the slides. After that, we removed the staining jar from the ultrasonicator and then, at the sink, the glass slides were removed from the staining jar and it was rinsed thoroughly with hot tap. Individually the slides were dried using the wall blow dryer, hand dryer or hair dryer. Then, we placed the clean slides in a covered Petri dish. The second step we followed was for deposition of metal films by evaporation using an Edwards 306 Turbo Evaporator. We turned on evaporator and set it to the required condition. Then, we placed the holder on a table and inserted the clean slides using the spring loaded clips. After that, we placed the glass slides on a substrate holder and replaced it by aligning the flat on the spindle with hexagonal nut and tightening them. We checked the condition of the chrome filament and tungsten boat. We moved the cover lever to ‘SHUT Cr’ position. The gold pills were placed into the tungsten boat. We lowered the lid carefully until it was seated properly onto the seal. Then, keeping the lid closed, we checked the chrome filament continuity and the gold filament continuity. After that, the evaporator was left to pump down to a vacuum of approximately 2.4*10-6. It took 1 ½ hour to reach the level. In order to evaporate the chrome film, we moved the ‘FILAMENT COVER’ to ‘SHUT Cr’ position, and we used the ammeter to put the current to 1.8A. We moved the ‘FILAMENT COVER’ lever to‘SHUT Au’ to start depositing chrome and we adjusted the current to a deposition rate of 0.1nm/s to get final chrome thickness of 5 nm. The process took 50s. After that, we moved the ‘FILAMENT COVER’ lever to ‘SHUT Cr’ to stop the deposition process. Then, we reduced the current to zero and moved the ‘HT/LT’ switch from the‘LT’ position to the ‘0’ position. Then, we closed the thickness monitor shutter used the red ‘SHUTTER’ button. We followed the same procedure to evaporate the gold film adjusting the current to a deposition rate 0.2- 0.3 nm/s. For gold deposition of 70 nm, it took 3.5 minutes. Finally, we waited for 3 minutes to allow the filaments to cool and then pressed the ‘VENT’ button. We took the slides and placed them in a petri dish. In the third step, we moved to spin coating stage. We placed the chuck onto the spindle noting position of the locating flat on the spindle. Then, we checked the vacuum seal by switching the VACUUM switch to manual and saw the glass slide is firmly held on the chuck. We switched the ‘VACUUM ’control to ‘AUTOMATIC’. We deposited 1ml of resist over the surface of glass slide and pressed START button to start the spinner for 1 minute. We moved the slide and placed on the hotplate preheated to 116 C and left it for 1 minute. Fourth, we moved to the resist exposure stage. We checked the mercury lamp power is stable at 210 watts. We selected the required mask , placed it with the patterned emulsion side down into the mask holder and it was clamped into position. We placed the glass slide onto the chuck with the resist coating face facing upwards. Then we moved the control switch to ‘ALIGN/EPOSE’ to engage the vacuum. We exposed resist for 18-35s and removed the glass slide. Fifth, we moved to the final stage, which is electrode development. We took a glass and placed it in MF319 resist developer for 40s. Then, we took the slide out of the developer and placed it into the beaker of ultrapure water to stop the development process. After that, we placed the dry slide in the aqua regia and agitated gently until all the exposed gold had disappeared. This took 20s.We then placed the slide into the beaker of ultrapure water. Then, we placed the slide into chrome etchant and agitated gently until all the exposed chrome had disappeared. We then removed the slide and placed it into the beaker of ultrapure water and then we dried it. Finally, we used the microscope to examine the electrode. Result and discussion In this section we will present what we observed during the experiment stages and we will discuss it. In the first stage of cleaning the substrate, we observed small bubbles in the surface when we placed the staining jar in Decon ultrasonic bath. This was because of Decon 90 and the heat. In deposition metal films (Cr / Au), we observed that after increasing the current, there was a glow of heat on the wire chrome which meant that the deposition of Cr was nearly completed. The same occurred with Au when it was deposited. Then the slides was removed from the evaporator and tested under the microscope. It was observed that some slides had pin holes which meant that the slides did not clean well with Decon 90, and a slide had had big black spots. This was caused by using of tweezers. However, when coated the slides using spin coating we observed that slides did not coated completely it was possibly because of the shape of slides (rectangular). May be when we use circular shape we will have better results. Then, after when the photoresist was exposed in mask- Aligner the slides was rinsed in MF319 for 40s which is a developer solution. After that, the slides were placed in gold etchant and then in chrome etchant. During etchant process, we observed that the gold as well as chrome which was unexposed was removed by etchant. Conclusion This experiment aimed to fabricate microelectrodes. This goal was achieved by following these steps: clean of substrate, evaporation, resist coating, and resist development. Not only this, we learned how to use chemicals substance in order to fabricate microelectrodes. The main process in this experiment was deposition of gold/ chrome by evaporation and it was achieved successfully. We got clear and good microelectrodes. Read More