Experiment shows that thin film bulk acoustic resonator (FBAR) is feasible to detect gamma irradiation, but the sensing mechanism is not studied deeply. For this problem, different sensing mechanisms are proposed to explain the resonance frequency shift after gamma irradiation according to two different FBAR structures. One FBAR structure is four - layers stacked (metal layer - piezoelectric layer - oxide layer - metal layer). After gamma irradiation, a voltage will be formed in the radiation sensitive layer (oxide layer), which is equivalent to impose a DC voltage to the piezoelectric layer that makes resonant frequency shift. There is a semiconductor layer between oxide layer and piezoelectric layer in the other FBAR structure, which is the difference between the two structures. A voltage formed in the oxide layer after irradiation will change the surface potential of the semiconductor and then change the space charge layer capacitor in semiconductor that makes the resonant frequency shift. The results of two mechanisms are obtained by simulation and compared with those in related literature, it is found that the trends and magnitudes of frequency shift are the same, so the two mechanisms are feasible.
Bulk Acoustic Wave Resonators (BAWRs) have been well developed both as filters and as high sensitivity sensors in recent years. In contrast to traditional megahertz quartz resonators, BAWRs offer significant increases in resonant frequency, typically operating in gigahertz regimes. This translates into a potential sensitivity increase of more than three orders of magnitude over traditional QCM (Quartz Crystal Microbalance) devices. Given the micrometer-scale size of BAW sensor-head, read-out circuitry can monolithic integrated with this GHz transducer is urgently needed to produce small, robust, and inexpensive sensor systems. A BAW sensor read-out circuit prototype based on Pierce oscillator architecture is fulfilled in this paper. Based on the differential measurement scheme, two uniform BAWRs are used to constitute two BAW oscillators as a reference and a measurement branch respectively. The resonant frequency shift caused by the measurand is obtained by mixing and filtering the two oscillator signals. Then, the intermediate signal is amplified, shaped and converted to a digital one. And a FPGA is used for frequency detection. Taking 2 GHz BAW mass sensor as a case study, deign procedure are given in details. Simulation and experimental results reveal a 0-99 MHz frequency shift measurement range. Main factors affecting phase noise of the BAW oscillator (i.e. mainly frequency stability of the BAW sensor readout circuit) are also discussed for further optimizations.