Study on Fabrication of ZnO Waveguide Layer for Love Wave Humidity Sensor Based on Magnetron Sputtering.

Changbao Wen,Yue Ma,Nan Gao,Feng Ru,Taotao Niu

doi:10.3390/s18103384

Abstract

The ZnO waveguide layer for the Love wave humidity sensor was fabricated by radio frequency (RF) magnetron sputtering technique using ZnO as the target material. To investigate the effect of RF magnetron sputtering temperature on the ZnO waveguide layer and Love wave device, a series of Love wave devices with ZnO waveguide layer were fabricated at different sputtering temperatures. The crystal orientation and microstructure of ZnO waveguide was characterized and analyzed, and the response characteristics of the Love wave device were analyzed by network analyzer. Furthermore, a humidity measurement system is designed, and the performance of the Love wave humidity sensor was measured and analyzed. The research results illustrate that the performance of the ZnO waveguide layer is improved when the sputtering temperature changes from 25 °C to 150 °C. However, when the sputtering temperature increases from 150 °C to 200 °C, the performance of the ZnO waveguide layer is degraded. Compared with the other sputtering temperatures, the ZnO waveguide layer fabricated at 150 °C has the best c-axis orientation and the largest average grain size (53.36 nm). The Love wave device has the lowest insertion loss at 150 °C. In addition, when the temperature of the measurement chamber is 25 °C and the relative humidity is in the range of 10% to 80%, the fabricated Love wave humidity sensor with ZnO waveguide layer has good reproducibility and long-term stability. Moreover, the Love wave humidity sensor has high sensitivity of 6.43 kHz/RH and the largest hysteresis error of the sensor is 6%.

Highlights

Love wave, which is one of the shear horizontal waves, can be excited by the inter-digital transducer (IDT) deposited on a semi-infinite piezoelectric substrate and propagates in the waveguide layer [1,2]
In the Love wave device, the direction of the particle vibration of the Love wave is parallel to the surface of the substrate, and the energy is mainly concentrated in the waveguide layer
The Love wave humidity sensor requires that the shear velocities and densities of the waveguide layer materials are smaller than those of the piezoelectric substrate materials, and the waveguide layer materials were required to have good elastic properties and weak acoustic wave absorption

Summary

Introduction

Love wave, which is one of the shear horizontal waves, can be excited by the inter-digital transducer (IDT) deposited on a semi-infinite piezoelectric substrate and propagates in the waveguide layer [1,2]. Because the coupling effect between the substrate surface and the waveguide layer load is very poor [3,4], the Love wave device can be used as a gas material sensor, and can measure the humidity in the presence of liquid or gas-liquid coexistence, which other humidity sensors cannot do [5,6,7]. The Love wave humidity sensor has the advantages of fast response and small size. It has been widely used in biosensor and analytical chip [8,9,10]. The Love wave humidity sensor requires that the shear velocities and densities of the waveguide layer materials are smaller than those of the piezoelectric substrate materials, and the waveguide layer materials were required to have good elastic properties and weak acoustic wave absorption

Methods

Results

Conclusion