Organic Electrochemical Transistor

Detecion and separation of molecules

Combine hydrophobc pillars with polymeric circuits for incredible appplications

oect

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.

CAD schematic for the two masks. On the left, mask defining the metallic contact and the connections. On the right, mask for the pillar definition with 4 contact areas for alignment to the previous mask.

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.

Close up view of the CAD Layout for the second mask. The regular square geometry of the circles changes towards the center of the mask, assuming a radial arrangement.

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.

Photo of the first mask.

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.

First mask inserted in the Maskaligner.

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.

Lift-off of the exposed sample. The Acetone heated by the hot plate etches the polymer and removes the gold above it, leaving only the golden lines on the chip.

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.

Pillar mask inserted in the maskaligner.

The exposition will last 17 seconds under hard contact. After taht, the sample is baked again at 95° C for 10 minutes. Last comes the development; we use SU-8 developer for 3 minutes, placing the sample with the exposed face towards the bottom.

Last development of the sample. It is inserted upside-down to facilitate the resist removal.

At the end we rinse the sample in IPA, and we repeat the development in case of traces. When the pillars are visible, the chip is ready for the SEM.

Chip after two masks exposition. The pillar matrix is visible in the center.

Following, a schematic depicting the operations up to now.

Schematic of the device fabrication. From left to right, the Si substrate is covered by a layer of PMMA, subsequently exposed to the UV while shielded by the first mask. The deposition of gold and the lift off of the PMMA creates the final gold connection. Then a layer of SU8 is deposited and exposed to the UV shielded by the second mask. The development of the SU8 removes the unexposed layer, creating the pillars.

Pillar connection

The final process consists in the fabrication of metallic contacts on the top of the central pillars, connecting them to the golden contact area. This task is performed by EBID (Electron Beam-Induced Deposition) fabrication. This process consists into injecting a precursor gas with Pt-C into the SEM chamber; the gaseous molecules are then hit by the electron beam and precipitated onto the substrate with a high spatial accuracy. For each pillar, the platinum deposition comes into three steps. In the first step, the contacts are deposited on top of the pillars.

Schematic (left) and SEM image of the platinum deposition on top of the pillar.

With a SEM column current of 1.6 nA and voltage 20 kV, we deposit a layer of platinum of around 100 nm with the dimensions indicated in Figure 4, a dwell time of 200 ns and a thickness parameter of 15 µm. the horizontal lines protrude from the pillar, creating a deposition on the bottom useful for a later connection to the gold structure. The second step consists in the connection to the gold track. Since the distance between lines is around 60-70 µm, the process of fabrication becomes considerably longer, so we increase the electronic current and voltage to 6.4 nA and 30 kV respectively.

Schematic (left) and SEM image of the connection to the gold track.

As a last step, we provide the connection on the side of the pillar. For this we turn and tilt the substrate to an angle of 45° in order to deposit on the lateral surface of the pillar. The EBID deposition parameters are the same as in the first step.

SEM image of a full connected pillar.

The EBID process is performed on 5 pillars, all located in the gap between the golden tracks, to provide different probe points for the current measurements. With this operation, we realize the micro-electrodes.

Pedot and Teflon deposition

In order to increase the electrical response from the measurements, we deposit on the device a thin layer of pedot. The polymer is spun on the silicon chip down to a thickness of 80 nm, then it is baked at 140° C for 60 minutes. Last step consists in a deposition of C4F8 on the substrate by means of a ICP – RIE (Inductively Coupled Plasma Reactive Ion Etching) instrument. This steps makes the chip superhydrophobic.

Schematic of a final pillar with micro-electrode.

This chop is being tested with droplet with a solution of CTAB and adrenaline. The device operates the separation of molecules in a super-hydrophobic drop based on their size and charge and thus may resolve biological mixtures in a solution.