A RF mass-sensitive SAW sensor with U-groove structure

A novel mass-sensitive SAW sensor with U-groove structure which uses the mass-loading effect of surface acoustic waves is proposed. This sensor works at RF frequency range from several decade MHz to several hundred MHz. There is a 10 /spl mu/m deep U-groove structure in the middle of the surface acoustic wave path on ZnO/SiO/sub 2//Si piezoelectric film substrate. A SAW oscillator, with input and output IDTs, is fabricated on each side of the U-groove. The U-groove is used to contain a certain amount of the liquid mixture for testing. After evaporation of the tested liquid mixture, the remaining material is deposited on the bottom of the U-groove to give the required mass-loading on the surface acoustic wave path. The loading material reduces the center frequency of the SAW sensor and increases its insertion loss. The loading mass surface density and the density in the liquid can be calculated by analyzing the shifted frequency response according to the mass-loading frequency equation. A SAW oscillating circuit is connected to the input IDT and output IDT to cause the oscillation and increase its Q value. It increases the sensitivity of the frequency shift via loading material density. When an amplifier is connected to the input and output IDT, an 11 dB decrease of insertion loss is obtained also. A prototype device has been fabricated on a LiNbO/sub 3/ wafer with a central frequency of 165 MHz. A sensitivity of 1 ppm frequency shift per 0.04 micrograin/cm/sup 2/ loading mass density is obtained. This device will be competitive in real-time liquid components analyzing and will be an excellent unit cell for an addressable biochip.