Capacitive Pixelated CMOS Electronic Nose

Abstract

Although some of the human senses can nowadays be replaced by low-cost electronic sensors such as microphones and image sensors, a compact low-cost electronic nose (E-nose) remains elusive. In this work, an E-nose is presented that can capacitively detect volatile organic compounds (VOCs). The E-nose consists of an array of 1024 capacitive microelectrodes on a complementary metal-oxide-semiconductor (CMOS) chip, functionalized by inkjet printing. The pixels are coated with a UV-curable ink and metal-organic frameworks (MOFs: ZIF-8, MIL-101(Cr), MIL-140A) to create chemically diverse microdomains that generate gas-specific response patterns through adsorption-driven dielectric loading. ZIF-8 exhibits the highest response to 2-butanone, whereas the UV-curable layer responds most strongly to toluene; both show low cross-sensitivity to water vapor, enabling operation under humid conditions. After calibration in pure gases, reproducible responses to controlled binary mixtures of toluene and 2-butanone are observed. The device operates at low power, combines a large 1024-pixel array with CMOS integration, and offers application-specific functionalization by inkjet printing, providing both low cost and versatility. By further extending the range of functionalization materials, the E-nose can be applied to analyze a wide variety of gases, with potential applications in safety monitoring, health, agriculture, and robotics.

Figures (24)

• Figure 1: Pure and mixed VOCs gas sensing setup. The setup comprises three syringe pumps (I--III) that deliver $\mathrm{N}_2$ to the dilution, $\mathrm{VOC}_1$, and $\mathrm{VOC}_2$ channels, respectively. Each VOC channel includes a shut-off valve to select $\mathrm{VOC}_1$, $\mathrm{VOC}_2$, or their mixture. A four-way diagonal valve (shown in pink) switches the flow sent to the sensor between pure $\mathrm{N}_2$ and $\mathrm{N}_2$ mixed with VOCs. The schematic depicts the pure $\mathrm{N}_2$ case. To expose the chip to a $\mathrm{VOC}_1/\mathrm{VOC}_2$ mixture, rotate the diagonal valve by $90^{\circ}$ to enable VOC delivery. The experimental setup is shown in Fig. \ref{['fig:lab-setup']}.
• Figure 2: Working principle and CMOS pixelated sensing chip.(a) Working principle of the capacitive gas sensor. The capacitance changes when exposed to VOCs due to the effect of the gas on the dielectric constant of the sensing ink. A more detailed CMOS device cross-section is shown in Fig. \ref{['fig:Xsection']}. (b) Packaged CMOS pixelated sensing chip mounted on readout PCB (c) Close-up optical image of the CMOS pixelated chip in a ceramic package. (d) Microelectrode arrays of the pixelated capacitive sensing chip. The dimensions of the microelectrodes in the three sensor matrices are shown in Fig. \ref{['fig:chip-microelectrodes']}. (e) SEM top-view image showing the pixelated microelectrode structures. (f) Display of the capacitance distribution over all 1024 pixels, with corresponding ADC values indicated by the colorscale.
• Figure 3: Artist impression, based on SEM images, of cross-section of CMOS sense electrodes by sensing ink. (a) The electrodes, made in the top-metal layer of the CMOS process, are connected vertically by vias and intermediate metal layers to pairs of switching transistors at the bottom (not visible). The middle electrode is selected by toggling (switching) its potential between $0$ and $0.9\,\text{V}$ at high frequency. This makes it the active plate of the selected sense capacitor. Electric field lines emerging from it pass through the green sensing ink and terminate mainly on grounded adjacent electrodes that collectively constitute the counter electrode plate of the selected sense capacitor. Note that Fig. (a) is an artist's impression that has been constructed by cut-and-pasting SEM cross-section images from various parts of the chip. (b) SEM image of a reference bare electrode. (c) SEM image of a functionalized electrode with a UV ink. Note that one ink droplet covers several electrodes. (d) A focused-ion-beam (FIB) SEM cross-section image of a single electrode pixel. Thinner CMOS backend metal layers that connect the pixel to the transistors are visible below the thick top electrode. An ink meniscus is adhering to the edges on both sides of the pixel.
• Figure 4: Graphical summary of the material synthesis, deposition, and characterisation. Various MOF powders, which are selected for their gas sensing potential, were synthesized, characterized, and mixed with ethanol/water solvent to make them printable ink. Later, it was mixed with UV-light curable ink to improve the stiction of the MOFs to the sensor array. Droplets of the mixed inks containing the MOF powder are inkjet printed on the sensor chip, UV cured, and characterized. The details of these steps, and the improvement of the ink formulation, can be found in Sec. \ref{['sec:experimental']}.
• Figure 5: Sensor response of the pixelated sensor matrix to VOCs after functionalization by inkjet printing. (a) Optical microscopy image of inkjet-printed ZIF-8 (ink 1) and UV-curable ink (ink 2) single droplets spaced by 15 $\mu\text{m}$ on part of the CMOS microelectrode array. Details of the microelectrode-array structures for the different sensor matrices are explained in Fig. \ref{['fig:chip-microelectrodes']}. The image shows individual droplets after UV light curing, followed by multiple droplet depositions, leading to their merging as in (b-e). (b–e) Lateral distribution of the capacitance change $\Delta \mathrm{ADC_{output}}$ under gas conditions. The scale bar represents the change in the ADC code (I), where 1 ADC code represents a capacitance change of 3.3 aF as expressed in Equation 1. The dashed lines indicate different ink regions: ZIF-8 ink (white) and UV ink (black). This sensor chip is different from that shown in (a). (b) Response under N$_2$ (reference). (c) Exposure to toluene. (d) Exposure to 2-butanone. (e) Mixed toluene and 2-butanone exposure. (f-g) Selected regions over which ADC values are averaged for determining $\Delta \text{ADC}_{\text{output,$t_0$}}$ for ink 1 and ink 2 functionalised areas and $\Delta \text{ADC}_{\text{output,empty}}$ in the reference region.