Response Mechanism for Surface Acoustic Wave Gas Sensors Based on Surface-Adsorption

A Finite Element Method (FEM) simulation study is conducted, aiming to scrutinize the sensitivity of Sezawa wave mode in a multilayer AlN/SiO2/Si Surface Acoustic Wave (SAW) sensor to low concentrations of Volatile Organic Compounds (VOCs), that is, trichloromethane, trichloroethylene, carbon tetrachloride and tetrachloroethene. A Complimentary Metal-Oxide Semiconductor (CMOS) compatible AlN/SiO2/Si based multilayer SAW resonator structure is taken into account for this purpose. In this study, first, the influence of AlN and SiO2 layers’ thicknesses over phase velocities and electromechanical coupling coefficients (k2) of two SAW modes (i.e., Rayleigh and Sezawa) is analyzed and the optimal thicknesses of AlN and SiO2 layers are opted for best propagation characteristics. Next, the study is further extended to analyze the mass loading effect on resonance frequencies of SAW modes by coating a thin Polyisobutylene (PIB) polymer film over the AlN surface. Finally, the sensitivity of the two SAW modes is examined for VOCs. This study concluded that the sensitivity of Sezawa wave mode for 1 ppm of selected volatile organic gases is twice that of the Rayleigh wave mode.

Section: Problem Formulationmentioning

confidence: 99%

FEM Analysis of Sezawa Mode SAW Sensor for VOC Based on CMOS Compatible AlN/SiO2/Si Multilayer Structure

Aslam

Jeoti

Karuppanan

et al. 2018

“…In addition, there is a linear relationship between the frequency variation (delta frequency) and gas concentration for both methanol and ethanol. According to [27], the relationship between the frequency shift of SAW sensor and the coated polymer film can be simplified as:

Δ f = (k_{1} + k_{2}) {f_{0}}^{2} h ρ

where

Δ f

is the frequency shift of SAW sensor,

k_{1}

and

k_{2}

are the substrate material constants,

f_{0}

is the original frequency of SAW sensor,

h

is the thickness of the film, and

ρ

is the density of the film. We can assume:

h ρ = m_{p} / A

where

m_{p}

is the weight of the polymer film, and A is the sensing area.…”

Section: Resultsmentioning

confidence: 99%

A Low Noise CMOS Readout Based on a Polymer-Coated SAW Array for Miniature Electronic Nose

Liu

Chiu

et al. 2016

An electronic nose (E-Nose) is one of the applications for surface acoustic wave (SAW) sensors. In this paper, we present a low-noise complementary metal–oxide–semiconductor (CMOS) readout application-specific integrated circuit (ASIC) based on an SAW sensor array for achieving a miniature E-Nose. The center frequency of the SAW sensors was measured to be approximately 114 MHz. Because of interference between the sensors, we designed a low-noise CMOS frequency readout circuit to enable the SAW sensor to obtain frequency variation. The proposed circuit was fabricated in Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 μm 1P6M CMOS process technology. The total chip size was nearly 1203 × 1203 μm2. The chip was operated at a supply voltage of 1 V for a digital circuit and 1.8 V for an analog circuit. The least measurable difference between frequencies was 4 Hz. The detection limit of the system, when estimated using methanol and ethanol, was 0.1 ppm. Their linearity was in the range of 0.1 to 26,000 ppm. The power consumption levels of the analog and digital circuits were 1.742 mW and 761 μW, respectively.

“…By combing Equation (4) and Wohljent’s method, the authors introduced in [ 22 , 23 ] the quantitative relation between gas pressure and the frequency shift of SAW sensor:

where Δ f 0 is the frequency offset of the SAW oscillator caused by a monomolecular layer of absorbed gas molecules covering the detector surface. Equation (5) describes the relationship between the sensor output and gas concentration, which is the theoretical model of the response mechanism for a SAW gas sensor.…”

Section: Theoretical Analysismentioning

confidence: 99%

The Development of Love Wave-Based Humidity Sensors Incorporating Multiple Layers

Wang

Liu

2015

Self Cite

A Love wave humidity sensor is developed by using a multilayer structure consisting of PVA/SiO2 layers on an ST-90°X quartz substrate. The theoretical result shows that the sensor with such a two-layer structure can achieve a higher sensitivity and a smaller loss than the structures with a single polymer layer. Comparative experiments are performed for the sensor incorporating PVA/SiO2 layers and the sensor incorporating a PVA layer. The experimental results agree well with the theoretical predication.