Key Technologies Of Film Bulk Acoustic Wave Resonators For Chip Scale Atomic Clocks

The atomic clock based on coherent population trapping effect can greatly reduce the size and is the preferred technical scheme of chip scale atomic clocks(CSACs).Using RF local oscillator(LO)instead of PLL based oscillation circuit is expected to significantly reduce power consumption.Therefore,the core device of oscillation circuit-high Q resonator has become one of the focuses of research.Thin film bulk acoustic wave resonator(BAWR)is an important technical solution for high Q resonator.BAWR based on aluminum nitride(AlN)is a CMOS compatible device,which has attracted extensive attention.Based on the application of Rb87 CSAC,this paper studies the design,preparation,characterization and modulation technology of BAWR for CSAC.The specific work is as follows:1.Relationship between RF LO performance and BAWR parameters.Using the MBVD model equivalent BAWR and combining it with the single port pierce resonant topology circuit,the 3.4 GHz oscillation circuit for Rb87 CSAC is designed.The effects of key parameters such as Qs,Qp,RS and effective electromechanical coupling coefficient(kt2)of BAWR on the key parameters of LO such as phase noise are studied,and the research objectives of BAWR are clarified.The results show that increasing the device Qs and reducing Rs can effectively reduce the phase noise and increase the output power;Increasing kt2 will slightly increase the phase noise,but it can greatly reduce the starting time and increase the output power;Qp has little effect on phase noise.2.BAWR design and optimization.Mason model and finite element simulation are used to design the device.The effects of AlN and Mo thickness ratio,damping coefficient and resonant area on the FOM of the device are studied,and the preliminary design scheme is obtained.The results show that increasing the thickness ratio of AlN can improve the kt2 of the device,but reduce Qs and increase Rs;Reducing the mechanical/dielectric damping of AIN can improve Qs;Increasing the area of the device can reduce the intensity of S1 Lamb wave and improve Qs.The dispersion curve of the device was calculated,and the scheme of suppressing Lamb wave through frame and apodization is discussed.A lamb wave suppression technique based on micro apodization method is proposed,and the parameters suitable for Rb87 CSAC are discussed.3.Preparation and characterization of BAWR.AlN and AlScN films are prepared by sputtering.The process of obtaining high-performance films through insertion layer and buffer layer is studied.The XRD(002)FWHM of 1 μm AlN,1 μm Al0.904Sc0.096N and 1 μm Al0.8Sc0.2N are better than 1.5°,the stress range can be controlled within ± 50 MPa,and the average stress can reach-6 MPa.The piezoelectric coefficient is characterized by piezoelectric force microscope(PFM),and the correction coefficient of d33 is given by finite element simulation.The BAWR device with the structure of 30 nm AIN seed/220 nm Mo/550 nm AlN/175 nm Mo/100 nm AlN was prepared by MEMS process.The S parameters are characterized by network analyzer to calculate the impedance and Q value.The results show that the fs of the device is 3.458 GHz,fp is 3.565 GHz,Rs is 0.239 Ω,Qs is 980,and the kt2 is 7.4%,which is consistent with the design.The simulation results show that the phase noise of the oscillator based on the device is-110 dBc/Hz@10 kHz,which can meet the application requirements.4.Temperature characteristic modulation technology of BAWR.Phosphorus doped silicon oxide(PSG)is inserted into the device.The effects of P ratio on the resonant characteristics and the elastic modulus temperature of PSG are studied.The results show that the elastic modulus temperature coefficient of PSG can reach 200 ppm/℃ when the atomic ratio of P is about 1.4%.Insert nano thickness film into BAWR to explore the temperature characteristic modulation technology of the device.The relationship between the resonant characteristics of the BAWRs and the thickness of the insertion layer is studied.When Ti thickness is about 20nm,the frequency temperature sensitivity of the device can reach-825 kHz/℃,and the TCE of Ti is almost 10 times that of poly crystalline Ti.The phenomenon can be explained by the thickness size effect of elastic modulus.According to this phenomenon,a temperature tuning BAWR for CSAC with Ti insertion layer is designed.According to the calculation result of Mason model,the frequency tuning ability of the device is 3417± 20 MHz under the temperature difference of 40℃,which can meet the working requirements of Rb87 CSAC RF LO.