Organic Electrochemical Transitor (OECT)
Here I report on the fabrication of a superhydrofobic device combined with the conductive polymer PEDOT:PSS to create a chemical sensor for the separation and detection of molecules in a droplet
The fabrication consists in five steps, namely mask fabrication, pillars and contact fabrication, pillar connection, PEDOT (Poly(3,4-ethylenedioxythiophene)) deposition and Teflon (Polytetrafluoroethylene) deposition.
Mask Fabrication
The first step consists in the fabrication of two different masks for the fabrication of the golden contact area and for the fabrication of the pillars. The schematic of the two masks are depicted below.
The CAD layout of the first masks consists in 10 square contacts areas of 4 mm2, connected with lines with 200 µm width at the initial trait and 30 µm in the last, horizontal trait. The 5 lines at the left side are separated from the 5 lines at the right side by a gap of around 60 µm. The layout of the second mask consists in the replication of 4 square contacts areas and connections for the purpose of alignment respect to the previous mask, plus a central area of 25 mm2 filled with circles of 10 µm diameter. As shown in figure below, the circles have a perpendicular arrangement, with a pitch of 30 µm center to center, but in the central part of the array they assume a radial arrangement, becoming denser towards the center.
The masks necessary for optical lithogrpahy were fabricated using direct SEM litography. The layout is elaborated and processed by a NPGS (Nanometer Pattern Generation System) software that convert the CAD layout into patterns for SEM (Scanning Electron Microscope) lithography. In this way, each geometrical object is defined with an electron beam exposure time, dose and pitch. We choose an electronic beam current of 1.8 nA with 50 nm pitch for the circles definition, while we choose a current of 6.4 nA and 100 nm pitch for the contact definition. The two masks are prepared starting from conventional photomask blanks, made by a substrate of quartz with a thin layer of chrome on one surface and a photosensitive layer on top. When inserted in the SEM main chamber, the electron beam exposes the areas defined by the initial CAD design with the abovementioned parameters. Once the exposure has ended, the photomask is developed, so that the exposed part is removed. The mask is treated with a solution of chrome etch, to remove the chrome only in the patterned area. Once this process is concluded, the photoresist is completely removed and the mask is complete.
Pillar Fabrication
To fabricate the device, we start from a 500 µm thick p-doped (100) Si wafer substrate with a resisivity of 5-10 Ohm/cm. The wafer is cleaned with Acetone and IPA and then etched with a solution 4% of HF to remove all the contaminants. After etching, the wafer is rinsed in DI water and dried with N2. At this point a layer of S1813 is deposited on the wafer by spin. The spin parameters are 4000 rpm for 60 seconds. Then the wafer is heated at 90° for 4 minutes and brought under the UV lamp of the maskaligner. The first mask, the one with the lines, is loaded on the UV lamp and we expose the pattern on the wafer for 10 seconds in hard contact.
The sample is then developed in MF-319 for 1 minute, drying with N2. It is very important at this point to check under an optical microscope whether the resist has been correctly developed. Any traces on the channels will compromise the gold adhesion, compromising the following steps. So the development must be repeated until no traces (rainbow colored under the microscope) are visible. After that we deposit 70 nm of gold via chemical evaporator (more effevtive respect to the sputtering). With this operation, the golden paths are complete. At this point we operate a lift-off technique, putting the wafer in hot Acetone until the wafer is clean. This procedure can be long, it is useful to inject sometimes the acetone directly to the surface of the sample with a pipette. Also the wafer should be grabbed with a proper tool to flip it over.
When the gold circuits are ready, we prepare the wafer for the second mask. We spin a layer of SU8-25 onto the wafer at 3000 rpm, baking it at 95° C for 30 minutes. We expose the resist under the second mask with the Maskaligner; in this operation we have to align the pattern of the mask with the one on the wafer and we can refer to the small apertures on the mask that match some gold area on the wafer.