Title : Optimization of ISFET structure formation technology for use in biochemical analysis systems
Currently, the creation of biosensors compatible with CMOS IC technology is of practical interest. One of the possible solutions for the creation of microanalytical systems of a new generation is the use of ion-sensitive field-effect transistors (ISFET) as a detecting element, which provides the possibility of integration with semiconductor technologies for the creation of IC.
In this work, we show the concept of creating an ISFET transistor in which the sensitive surface directly in contact with the electrolyte solution is moved outside the transistor body, and the connection between them is carried out using a floating gate. This design makes it possible to fit the sensor manufacturing into the standard route within the framework of post BEOL treatments, bringing the sensitive surface above the signal processing circuit. This saves the area of the entire crystal, while not changing the speed and power consumption of the IC.
The described ISFET manufacturing approach fully fits into the standard CMOS process, with the exception of finishing operations: opening the surface of the aluminum floating gate in the passivation layer and subsequent formation of the dielectric layer, which is the sensitive surface of the biosensor. Our experiments show that the sensor characteristics of the final device are largely determined by the quality of the interface between the dielectric and the aluminum floating gate formed at the CMOS factory. We have developed a surface preparation operation before deposition of a dielectric layer based on argon stripping followed by annealing. The argon stripping operation was carried out in order to activate the surface and form additional bonds for the subsequent formation of dielectric layers using APEX SLR high density plasma chemical vapor deposition system at a temperature of 160 °C for 5 minutes. Then, in the same chamber, low-temperature annealing of structures in an oxygen atmosphere at 160 °C for 10 minutes was carried out in order to passivate the surface and form stable aluminum oxide. After preliminary preparation of the floating gate surface, sensitive surface fabrication was done using the atomic layer deposition method, which allows us to create uniform films with a controlled thickness.
Ta2O5 was chosen as the ISFET sensitive surface material because of its chemical resistance to aqueous solutions and the possibility of creating structures with a low subthreshold swing. The film was deposited thermally using Fiji G2 ALD system, using pentakis(dimethylamino)tantalum(V) (PDMAT) precursor and deionized water at 250 °C. The thickness of the resulting film was controlled by ellipsometry, and the chemical composition was controlled by Auger spectroscopy.
For the proposed floating-gate ISFET design, we selected the subthreshold mode of transistor operation, which is characterized by a higher sensitivity to changes in the surface potential compared to operation in the strong inversion mode. Another advantage of the subthreshold mode is the minimum value of the input capacitance. That is why the means for assessing the quality of the preparation of the ISFET sensitive surface are the analysis of the I-V characteristics, and the determination of the values of subthreshold swing and hysteresis. The proposed method of surface preparation demonstrates an improvement in the subthreshold swing value after the formation of the tantalum oxide film by 7% and a significant decrease of the hysteresis from 20 to 5 mV, which subsequently has a huge impact on the stability and repeatability of biosensor readings.
For the discussed transistor with the tantalum oxide film formed by the ALD method, the pH dependence equal to 55 mV/pH was obtained, which corresponds to theoretical data.
The discussed ISFET designs can be used as structural elements for the creation of highly selective chemical detectors and biosensors with the possibility of use in complex analytical systems such as "lab-on-a-chip" with sample delivery system integration.
The study was supported by the Russian Science Foundation grant No. 21-79-10175, https://rscf.ru/project/21-79-10175/
- A method for optimizing the route of ISFET structure fabrication for use in biochemical analysis systems is described.
- It can be used to improve the critical parameters of ISFET structures: subthreshold swing, hysteresis.
- It is also possible to adapt the method for other sensitive surface materials obtained by atomic layer deposition, such as HfO2 and ZrO2.