Temperature stability analysis of LGS for SH-SAW sensor applications

New piezoelectric materials such as Langasite (La₃Ga₅SiO₁₄, LGS), are of particular interest for bio-chemical sensor applications because of their temperature compensated characteristics. For liquid phase sensing applications, acoustic devices operating in shear horizontal surface acoustic waves (SH-SAW) mode are desired due to their low wave attenuation. A LGS substrate with an Euler angle (0°, 220°, 900°), which supports SH-SAW and also exhibits zero-temperature coefficient of frequency (TCF), is chosen and electrically characterized. In this paper, the temperature characteristics are analyzed over a range from 50°C up to 720°C. The first three harmonics of a SH-SAW delay line (DL) device are analyzed in the experiments. The DL devices are also tested with a polymer coating layer. The polymer layers are used to investigate and assess the effect of temperature fluctuations in typical biochemical sensor applications. Variations in the frequency-temperature-characteristics (FTC) are observed, indicating the effects of thick polymer layers and high temperatures on the device response. The results show that the coated devices exhibit larger fractional frequency changes near room temperature. As a result, the use of a temperature compensating system or the use of differential measurements scheme with a dual delay line is necessary when using this substrate for biological and chemical sensing applications.