Development of human IgE biosensor using Sezawa-mode SAW devices

• Sezawa-mode SAW device is fabricated with ZnO thin films.
• Sensitivity of the Sezawa-mode SAW device is calculated and presented.
• A maximum sensitivity of the Sezawa-mode is 4.44 × 106 cm2/g for IgE detection.

This paper reports Sezawa-mode surface acoustic wave (SAW) devices with via-isolated cavity to construct the allergy biosensor. To fabricate Sezawa-mode SAW devices, the RF magnetron sputtering method for the growth of piezoelectric ZnO thin films are adopted and influences of the sputtering parameters are investigated. The optimal substrate temperature of 300 °C, RF power of 120 W and sputtering pressure of 2 Pa were used to deposit piezoelectric ZnO films with a smooth surface, uniform grain size and strongly c-axis-orientated crystallization. A back-etched SAW resonator is used in this study. The wet etching of (100)-oriented silicon wafers is used to form a back-side cavity which is critical to the formation of a hopper cavity for holding bio-analytes. The remaining membrane structure silicon thickness was 25 μm. In this report, the chrome (Cr, 12 nm)/gold (Au, 66 nm) layer was initially deposited onto the sensing area of SAW devices as the binding layer for biochemical sensor. The resonance frequency of the Sezawa-mode SAW device is 1.497 GHz. The maximum sensitivity of the Sezawa-mode is calculated to be 4.44 × 106 cm2/g for human immunoglobulin-E (IgE) detection. The stability for human IgE detection is calculated to be 80% and the variation of the stability ±3% was obtained after several tests.