CMOS Monolithic Electrochemical Gas Sensor Microsystem Using Room Temperature Ionic Liquid

Abstract

The growing demand for personal healthcare monitoring requires a challenging combination of performance, size, power, and cost that is difficult to achieve with existing gas sensor technologies. This paper presents a new CMOS monolithic gas sensor microsystem that meets these requirements through a unique combination of electrochemical readout circuits, post-CMOS planar electrodes, and room temperature ionic liquid (RTIL) sensing materials. The architecture and design of the CMOS-RTIL-based monolithic gas sensor are described. The monolithic device occupies less than 0.5mm² per sensing channel and incorporates electrochemical biasing and readout functions with only 1.4mW of power consumption. Oxygen was tested as an example gas, and results show that the microsystem demonstrates a highly linear response (R² = 0.995) over a 0 - 21% oxygen concentration range, with a limit of detection of 0.06% and a 1 second response time. Monolithic integration reduces manufacturing cost and is demonstrated to improve limits of detection by a factor of five compared to a hybrid implementation. The combined characteristics of this device offer an ideal platform for portable/wearable gas sensing in applications such as air pollutant monitoring.

Keywords: electrochemical; gas sensor; monolithic microsystem; room temperature ionic liquid.

Fig. 1.

Block diagram of an amperometric electrochemical sensor system.

Fig. 2.

Conceptual illustration of the CMOS monolithic RTIL-based electrochemical gas sensor microsystem.

Fig. 3.

Schematic of the monolithic microsystem, consisting of the amperometric instrumentation circuit and the RTIL-based sensor. Green rectangle block illustrates the clock waveform.

Fig. 4.

Die photographs of fabricated CMOS electrochemical instrumentation chip; (a) chip as received from foundry with functional blocks labeled, (b) chip after post-CMOS fabrication of (top) on-CMOS electrodes and (bottom) on-CMOS RTIL sensor/electrolyte layer.

Fig. 5.

Post-CMOS pocess flow for on-chip RTIL-based sensor fabrication: Photoresist is spin-coated (a) and developed (b). Titanium and gold is deposited by PVD (c). Photoresist is then rinsed off to leave the electrode (d). CMOS chip with on-chip electrode is wire bonded to package (e). A droplet of RTIL is casted on the electrodes (f).

Fig. 6.

The electrochemical test setup.

Fig. 7.

The R2rop output voltage range and linearity error as a function of input voltage. The absolute error of the output voltage is less than 0.05% over the range from 0.03 V to 4.90 V.

Fig. 8.

Amperometric readout output range and linearity error as a function of input current when f_s = 100 kHz, Vref₂ = 1.3 V. In the range of −1 μA to 1 μA, the absolute linearity error is less than 4 nA.

Fig. 9.

Output of amperometric readout circuit for on-CMOS RTIL-based sensor response to oxygen. Oxygen concentration varies from 0 – 21% with a step of 2.1%. Oxygen exposures were 10 s in duration, preceded and followed by exposure to pure nitrogen for 30 s. The amperometric circuit was set to V_ref1 = 2.1 V and V_ref2 = 1.3 V (the sensor was applied at −0.8V respectively), and fs=100 kHz.

Fig. 10.

Oxygen calibration curve for monolithic RTIL-based gas sensor using data extracted from Fig. 9. A high level of linearity (R² = 0.995) was achieved.

Fig. 11.

Repeatability test. Constant-potential current response measured over five cycles of alternate exposure to 4.2% oxygen and pure nitrogen flow at an applied bias of −0.8 V. The standard deviation of the five test cycles is 89 μV and the corresponding repeatable LOD of oxygen is 0.06%.

Fig. 12.

Off-CMOS and on-CMOS sensor response time characterized by the amperometric readout circuit. The response time saturates around 1 sec.