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Reduction of drift in Complementary Metal Oxide-Semiconductor (CMOS) Ion-Sensitive Field-Effect Transistor (ISFET) pH sensors is demonstrated using monolayer and multilayer graphene sheets. Graphene blocks the ion penetration in the CMOS passivation layers and provides the physisorption sites needed for electrical double layer formation allowing sensing. With an in-house polymer-assisted graphene transfer (PAGT) process, monolayer and multilayer graphene sheets were manually transferred on top of the sensing membrane of CMOS ISFET sensors on a 2 by 4 mm chip. Experiments with pH buffers on five different chips were performed to extract the average performance parameters of capacitive attenuation, trapped charge, sensitivity, drift and noise. The stretched exponential function, that describes dispersion processes in amorphous solids such as silicon dioxide and silicon nitride, was modified to model the dynamic drift behaviour and analyse the effect of graphene on the performance of the sensors. The results show that on average the graphene coated ISFET sensors experience about 50% reduction in drift amplitude, up to 3 times slower surface modification and perform overall better compared to the plain unmodified devices.

In this work, we investigate the impact of successively removing the passivation layers of ISFET sensors implemented in a standard CMOS process to improve sensing performance. Reactive ion etching is used as a post-processing technique of the CMOS chips for uniform and accurate etching. The removal of the passivation layers addresses common issues with commercial implementation of ISFET sensors, including pH sensitivity, capacitive attenuation, trapped charge, drift and noise. The process for removing the three standard layers (polyimide, Si3N4 and SiO2) is tailored to minimise the surface roughness of the sensing layer throughout an array of more than 4000 ISFET sensors. By careful calibration of the plasma recipe we perform material-wise etch steps at the top and middle of the nitride layer and top of the oxide layer. The characterisation of the ISFET array proves that the location of the trapped charge in the passivation layers is mainly at the interface of the layers. Etching to the top of the oxide layer is shown to induce an improvement of 80% in the offset range throughout the array and an increase in SNR of almost 40 dB compared to the non-processed configuration. The performance enhancement demonstrates the benefit of a controlled industry-standard etch process on CMOS ISFET array system-on-chips.

Reactive Ion Etching (RIE) is used to improve the performance of commercial Complementary Metal Oxide Semiconductor (CMOS) Ion-Sensitive Field-Effect Transistors (ISFETs) by thinning the top passivation layers inherent of the CMOS fabrication process. Using a combination of O2 and SF6 in 50% ratio, both polyimide and Si3N4 layers are etched in one etching step. Etching for different times we find the right remaining layer thickness for best ISFET performance to be ∼1 μm of SiO2. The results show an increase in pH sensitivity of 125%, a 5700% increase in passivation capacitance and a 96% reduction in capacitive attenuation. The RIE etch recipe can be used on multi-project wafers (MPW) to boost CMOS sensor performance.

Ammonia (NH3) is a toxic gas released in different industrial, agricultural and natural processes. It is also a biomarker for some diseases. These require NH3 sensors for health and safety reasons. To boost the sensitivity of solid-state sensors, the effective sensing area should be increased. Two methods are explored and compared using an evaporating pool of 0.5 mL NH4OH (28% NH3). In the first method an array of Si nanowires (Si NWA) is obtained via metal-assisted-electrochemical etching to increase the effective surface area. In the second method CVD graphene is suspended on top of the Si nanowires to act as a sensing layer. Both the effective surface area as well as the density of surface traps influences the amplitude of the response. The effective surface area of Si NWAs is 100 × larger than that of suspended graphene for the same top surface area, leading to a larger response in amplitude by a factor of ~7 notwithstanding a higher trap density in suspended graphene. The use of Si NWAs increases the response rate for both Si NWAs as well as the suspended graphene due to more effective NH3 diffusion processes.

Graphene is attractive for gas sensing due to its large surface area to volume ratio. Gas molecules adsorbed on the surface of graphene change its electrical conductivity, which can be measured as a resistance change as a function of time. Normally, only one side of the graphene is exposed to the gas [1]. Exposing both sides for molecule adsorption should increase its sensitivity as the effective surface area is increased. Graphene can be suspended on mesas of a patterned substrate [2]. The maximum distance across which graphene can be suspended without substantial deformation is 100 μm [2]. Contacts need to be defined aligned to the mesa structures to avoid breaking the graphene layer. We suspend graphene over a large area (~cm2), namely on top of a of silicon nanowire array (SiNWA) [3]. This approach removes the need for aligned contacts. In addition we found that it increases the sensitivity to gas molecules dramatically. We present results of the conductivity variation of suspended graphene on Si NWAs in response to NH3 molecules evaporating from 1 ml of aqueous NH4OH in a glass flask [4] in a closed container. The SiNWA was fabricated using metal assisted chemical etching (MACE) [5]. A p-type Si substrate, with resistivity ρ = 1-5 Ω cm, was processed in three consecutive steps. First was the nucleation of Ag nanoparticles (NPs) on the surface using 0.06M AgNO3:28.2M HF:H2O for 10 min. Subsequently, the NWs were etched in 0.6M H2O2:28.2M HF:H2O for 10 min. Finally, the Ag NPs were removed in 67% HNO3:H2O. CVD grown, multilayer graphene on Ni was transferred to the SiNWA via a PMMA support layer. To ensure good adhesion between graphene and SiNWA, the PMMA was dissolved in vapour rather than liquid acetone [3]. The SEM images (see Figure) show that graphene is suspended on the SiNWA over a large area. The resistivity of the graphene measured with a four-point-probe (4PP) set-up was ρ = 10-3 Ωcm. The SiNWA support was found to be non-conductive in the direction parallel to the substrate. To compare the influence of the increase in effective surface area, NH3 adsorption measurements were carried using both graphene-on-SiNWA (GrNW) and graphene-on-SiO2 (GrOx). In the figure, the resistivity is plotted as a function of time while NH3 was evaporating from NH4OH. All measurements were carried out at room temperature. Graphene behaves as a p-type semiconductor, due to ambient H2O and O2 influencing electron and hole mobilities [6]. Since, NH3 molecules are electron donors [7], the conductivity decreases when NH3 molecules adsorb onto graphene. When the current saturates, the NH4OH was removed and the lid of the container was opened. The reaction time of GrNW to NH3 is 5 times larger than for GrOx. There is a 30% increase in resistivity for GrNW compared to only 6% for GrOx, showing a clear increase in sensitivity of the suspended graphene layer. Both samples recover slowly to the initial resistivity value when NH4OH is removed. GrNW needs more time to saturate and recover, probably due to NH3 molecules entrapment in the SiNWA. Results from further experiments with different NWA densities will also be presented at the conference to analyse this phenomenon. References [1] L. Kong, A. Enders, T. S. Rahman, and P. a Dowben, J. Phys. Condens. Matter, vol. 26, no. 44, p. 443001, 2014. [2] C. Li, M. T. Cole, W. Lei, K. Qu, K. Ying, Y. Zhang, A. R. Robertson, J. H. Warner, S. Ding, X. Zhang, B. Wang, and W. I. Milne, Adv. Funct. Mater., vol. 24, no. 9, pp. 1218–1227, 2014. [3] C. Panteli, D. Liu, O. Sydoruk, K. Fobelets, MNE 19-23 Sept. Vienna, Austria (2016). [4] K. Fobelets, Meghani M.; Li C., IEEE Trans. Nanotechnol. 13(6), 1176-1180 (2014). [5] Z. Huang, N. Geyer, P. Werner, J. De Boor, and U. Gösele, Adv. Mater., vol. 23, no. 2, pp. 285–308, 2011. [6] I. Silvestre, E. A. De Morais, A. O. Melo, L. C. Campos, A. M. B. Goncalves, A. R. Cadore, A. S. Ferlauto, H. Chacham, M. S. C. Mazzoni, and R. G. Lacerda, ACS Nano, vol. 7, no. 8, pp. 6597–6604, 2013. [7] C. Li, C. Zhang, K. Fobelets, J. Zheng, C. Xue, Y. Zuo, B. Cheng, and Q. Wang, J. Appl. Phys., vol. 114, no. 17, pp. 5–9, 2013. Figure 1