Fabrication of a Solution-gated Indium-Tin-Oxide-based One-piece Transistor Enabling Sensitive Biosensing

The scope of this research is to demonstrate a simple and rapid method for fabricating a solution-gated one-piece transistor for pH sensing and highly sensitive biosensing. To increase the detection sensitivity of solution-gated FET biosensors, the technologies of miniaturization and lower dimensioning of material have been made used. We have found that a solution-gated FET can be easily fabricated by simply etching a portion of conductive ITO thin film to a thickness that exhibits semiconductivity, providing an all-by-ITO technology.

Using this method, a solution-gated FET can be fabricated more easily than with other protocols, and the fabricated device shows a steep subthreshold slope, which is related to sensitivity. The proposed method provides an all-by-ITO technology, leading pH sensing and highly sensitive biosensing, because the source, drain electrodes, and the channel are fully integrated without any interfaces. To begin, wash the glass substrate by sonicating it in acetone, methanol, and water for five minutes each.

Using a nitrogen gas blower, dry the substrate thoroughly and bake it on a hot plate at 110 degrees Celsius for more than five minutes to ensure it is completely dry. Spin-coat the dried glass substrate with an OFPR-800 layer at 500 RPM for five seconds, followed by 3, 000 RPM for 30 seconds. Pre-bake the coated substrate on a hot plate at 110 degrees Celsius for five minutes.

Allow the substrate to cool down to room temperature before proceeding. Position the photomask film on the photoresist-coated side of the substrate and use tape to fix it securely in place. Then expose the substrate to ultraviolet light using a photolithography machine for 40 seconds.

Now, take the substrate out of the photolithography machine and carefully remove the photomask from the surface. To develop the photoresist, immerse the substrate in NMD-3 developer for one minute and verify that a sharp pattern is formed. Then dip the substrate in water to wash away the developer.

After drying the washed substrate by blowing nitrogen gas, bake it on a hot plate at 110 degrees Celsius for five minutes. To begin deposition, fix the substrate onto a substrate holder using tape and introduce it into the vacuum chamber of a sputtering machine. Pump down the sputtering chamber pressure to below 10 to the power of 3 pascals.

Then deposit a 100 nanometer thick layer of indium tin oxide via radiofrequency sputtering at four nanometers per minute under argon without substrate heating. To lift off the photoresist, sonicate the substrate in acetone, methanol, and then water for five minutes. Then blow the substrate dry using a nitrogen gas blower, and bake it on a hot plate at 110 degrees Celsius until fully dry.

To spin-coat the clean substrate with SU-8 3005, spin at 500 RPM for five seconds, followed by 6, 000 RPM for 30 seconds. Pre-bake the SU-8-coated substrate on a hot plate set to 95 degrees Celsius for five minutes. Then place the photomask film on the photoresist-coated substrate and secure it with tape to prevent any movement during exposure.

Expose the SU-8-coated substrate to ultraviolet light for seven seconds using a photolithography machine. Then remove the substrate from the machine and carefully detach the photomask. Post-bake the exposed substrate first at 65 degrees Celsius for two minutes, then at 95 degrees Celsius, and continue baking for five minutes.

Immerse the substrate in SU-8 developer for three minutes under strong agitation to develop the photoresist. Rinse the substrate in 2-propanol for one minute, and dry the developed substrate using a nitrogen gas blower. To begin forming the semiconductive indium tin oxide channel, prepare 0.1 molar hydrochloric acid solution.

Then connect the source and drain electrodes to a semiconductor parameter analyzer. Turn on the B1500A device, and start the EasyEXPERT software. Then open the workspace interface to begin setup.

On the analyzer interface, select the Classic Test Tab, then choose the IVT Sampling option. Set the sampling Interval to 0.5 seconds. Apply a voltage of one volt between the source and drain electrodes, and record the initial current.

Using a micropipette, place a 30 microliter drop of the prepared hydrochloric acid solution directly onto the exposed indium tin oxide channel area. Monitor the current between the source and drain electrodes continuously, and observe the conductivity change during etching. Continue the process until the current drops to 10%of the initial value.

Then immediately rinse the etched indium tin oxide channel with deionized water to stop the etching once the current reaches 10%of the initial current. Dry the one-piece transistor by gently blowing nitrogen gas over the rinsed substrate using a nitrogen gas blower. Using a test fixture, connect the source and drain electrodes of the one-piece transistor to the semiconductor parameter analyzer.

Place a silicone O-ring around the surface of the indium tin oxide channel to form a liquid well. Add 30 microliters of phosphate buffer or another standard pH buffer solution into the silicone O-ring well while making full contact with the indium tin oxide channel surface. Then insert a silver or silver chloride reference electrode into a saturated potassium chloride solution, which is connected to the gate electrolyte by salt bridge.

Connect the silver or silver chloride reference electrode to the semiconductor parameter analyzer, so that it functions as the gate electrode. On the semiconductor parameter analyzer, select the Classic Test tab, then choose the I/V Sweep mode. Connect the source electrode to ground to establish a reference potential.

Set the applied drain voltage to a constant value of one volt. Set the sweep range of gate voltages from minus 0.8 volts to plus 0.8 volts and adjust delay, integration time, and measurement interval. Then set the number of iterations to 10 cycles and use the data from the final cycle for analysis.

Simultaneously, ensure that the leak current between the source and gate electrodes is recorded and start the measurement. On the semiconductor parameter analyzer, select the CMOS category in the Application Test tab, then choose the Id-Vd mode. Set the gate voltage to increase sequentially from zero volts to 0.8 volts in 0.1 volt intervals.

For each gate voltage value, set the drain voltage to sweep from zero volts to one volt in 0.01 volt intervals and start the measurement. Now, remove the silicone O-ring from around the channel surface. Then rinse the channel area thoroughly with deionized water.

Dry the one-piece indium tin oxide device using a nitrogen gas blower. The indium tin oxide ion-sensitive field effect transistor demonstrated a steep subthreshold slope of approximately 81 millivolts per decade, indicating near ideal switching behavior at 25 degrees Celsius. The gate leakage current remained four orders of magnitude lower than the drain current, even when the channel was directly exposed to the electrolyte solution.

The drain current increased with the drain voltage and showed saturation behavior at each gate voltage, confirming good field effect transistor characteristics. The on/off current ratio exceeded 10, 000 for channel thicknesses below 20 nanometers, but dropped drastically at greater thicknesses. The one-piece indium tin oxide ion-sensitive field effect transistor responded linearly to pH changes, exhibiting a voltage shift of approximately 51 millivolts per pH unit.

The pH sensitivity increased slightly after five days of wet storage, approaching the ideal Nernstian limit, and was retained even after 16 days.