Novel pH Sensor Based on out-of-Equilibrium Body Potential Monitored in Silicon on Insulator with Metal Contacts
Authors: Alepidis Miltiadis , Bouchard Aude , Delacour Cecile , Bawedin Maryline , Ionica Irina 
Published: July 2021

In the field of bio-chemical sensors, the ISFETs (Ion Sensing Field Effect Transistors) occupy an important position since 1972 [1]. ISFETs are based on the conductivity modulation when charges are placed close to the conductive channel or on the gate of the transistor. The majority of chemical and biochemical FET sensors monitor a change in the device current at constant gate voltage (VG) or a threshold voltage (VT) shift [2]. In both cases, the device is considered to be in a static state and its electrical response in “quasi-equilibrium”, even though the bio-chemical environment is modified in real-time. The originality of our work is to demonstrate the possibility of detection based on a measurement in out-of-equilibrium state of the device. Such out-of-equilibrium phenomena were already studied in silicon on insulator (SOI) devices and their presence is due to the fact that the body of the transistor is floating. When sweeping fast the device from accumulation to inversion or vice-versa, an SOI transistor shows an out-of-equilibrium body potential [3], which was exploited for memory applications [3]. The same phenomenon was shown in SOI substrates measured with metallic probes placed on the top silicon film and in which the substrate is used as back gate; DNA detection in this configuration was successfully demonstrated in [4].
This paper shows the possibility to detect pH with a simple fabricated SOI structure with metal contacts deposited on the top, using the out-of-equilibrium body potential measurement. The out-of-equilibrium body potential (VB) is related to the absence of carriers in the Si film, while the VG is swept [5]. When the carriers are injected, the device reaches an equilibrium state and VB sharply drops to the “static” value. Interestingly, VB appears for low absolute gate-voltage values in which the current through the device is almost zero. Additionally, the out-of-equilibrium region is related to the VT value of the device, which is modulated by the presence of charges on the top Si, as in an ISFET. Hence the return to equilibrium will occur for different VG values, depending on the quantity of charges to-be-detected.
Square mesas were etched on SOI wafers with 70nm silicon thickness and 30nm buried oxide (BOX) thickness. A lithography step allowed to define contact areas, where, the native silicon oxide was removed and a 300nm chromium (Cr) layer was deposited by sputtering at 120oC. The contact patterns were revealed after a lift-off step.
The devices used for this study have two Cr pads on the edges (Fig.a). The pads were used as source (grounded) and as drain (for current measurement) or body contact. For the detection, a PDMS chamber with a 3mm diameter opening was placed in the center of each device. Initially, a reference curve without any solution was traced. Then the chamber was filled with solutions of pH values of 4, 7 or 10. The potential of the liquid was set to zero volts with an Ag/AgCl pseudo-reference electrode (RE), inserted through the upper limit of the liquid in the pH solution.
Figure b and c show the drain current ID and the out-of-equilibrium VB versus VG of the samples under test when the solution was in contact with their top surface. Figure d illustrates the VT values extracted from the ID-VG curves and the VG values when VB=0.5V during the sharp drop. The results indicate a VB shift with various pH values. VB shift seems to be stronger than VT probably because it is traced when the device is depleted and hence the free carriers have an exponential dependence on the voltage. Additionally, the strong VB response occurs for low VG values (under the threshold), in a region where ID is small and more difficult to measure. This promises a more power-efficient detection method (VB based) compared to the commonly used modulation of conductance or VT shift. Explanation of the phenomena involved and more experimental results including a benchmark with wafers with thinner Si will be presented in the conference.
In conclusion, devices based on SOI with Cr contacts are promising candidates for detection based on the out-of-equilibrium body potential reading. The dynamic reading method provides a robust voltage signature for lower operation voltages compared to the presently used “static” current measurements. The preliminary results imply a more pronounced shift to the presence of charges. In addition, the studied devices, which require very few and low temperature process steps, provide a naturally large sensing area, ideal for direct placement of charged species. Finally, the proximity of the charges to the conductive channel enhances the sensitivity of the devices.

References

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[2] Syu Y-C, Hsu W-E, Lin C-T. Review—Field-Effect Transistor Biosensing: Devices and Clinical Applications. ECS J Solid State Sci Technol 2018;7:Q3196–207. https://doi.org/10.1149/2.0291807jss.
[3] Bawedin M, Cristoloveanu S, Flandre D, Udrea F. Dynamic body potential variation in FD SOI MOSFETs operated in deep non-equilibrium regime: Model and applications. Solid-State Electron 2010;54:104–14. https://doi.org/10.1016/j.sse.2009.12.004.
[4] Benea L, Popescu M, Bawedin M, Simion M, Ionica I. Novel DNA biosensor based on the out-of-equilibrium body potential method in silicon-on-insulator. IEEE Sens J 2020:1–1. https://doi.org/10.1109/JSEN.2020.3008133.
[5] Benea L, Bawedin M, Delacour C, Ionica I. Out-of-equilibrium body potential measurements in pseudo-MOSFET for sensing applications. Solid-State Electron 2018;143:69–76. https://doi.org/10.1016/j.sse.2017.11.010.

Figure 1