Research On Key Technologies Of Chip-scale Cesium Atomic Clocks

With the development of technology, more strict requirements on the accuracy and stability of time and frequency reference are put forward for high end electronic device and equipment. Due to its high accuracy and long-term stability, traditional atomic clock is widely used in these fields such as GPS(Global Positioning System) time service, navigation and positioning, missile and satellite positioning, astronomical observation, precision instrument calibration, telecommunication, high-speed traffic management, geo-physics surveying etc. However, due to its large volume, high power consumption and high cost, traditional atomic clock is not a good choice for handheld and mobile electronic devices. The chip-scale atomic clock(CSAC) provides a new alternative for the traditional one.Chip-scale atomic clock is mainly made up of three parts: physical package, microwave radio frequency module and micro-controller servo loop control module. The most important part is physical package, which consists of the vertical cavity surface emitting laser(VCSEL), micro optical lens group, MEMS cesium vapor cell, indium tin oxide heater and photoelectric detector. The micro-wave radio frequency module is realized with the voltage controlled oscillator(VCO) constructed by film bulk acoustic wave resonator(FBAR). The micro-controller servo loop control module mainly consists of four parts such as temperature control servo loop, VSCEL laser current control servo loop of vertical cavity surface emitting laser, microwave radio frequency module power and frequency servo loop.Compared to traditional small atomic clock, chip-scale atomic clock improves its performance by three modifications to the traditional one. First, the rubidium lamp or cesium lamp is replaced by miniaturized vertical cavity surface emitting laser. Second, the microwave resonant cavity with huge volume is replaced by stable microwave source directly modulating the vertical cavity surface emitting laser. Third, the big glass cell containing rubidium or cesium material is replaced by micro MEMS cesium vapor cell. These improvements sharply reduce the power consumption and volume of chip-scale atomic clock for the equivalent performance. The power consumption reduces from more than 10 W to 100 mW. The volume reduces from more than 230cm3 to 10cm3. These improvements enlarge the application fields of chip-scale atomic clock and bring us many new technology challenges at the same time. Therefore, in this thesis, the key technologies of chip-scale atomic clock are studied. The main achievements are as following:(1) Finish the design of film bulk acoustic wave resonator for the requirements of chip-scale atomic clock: The material of piezoelectric layer is based on aluminum nitride. A film bulk acoustic wave resonator with center frequency at 4.6GHz and the quality factor of 754 is successfully designed by theoretical analysis, numerical modeling, processes optimization and simulation, etc.(2) Put forward the new way of MEMS cesium vapor cell fabrication, which resolves the gas leakage problem of MEMS cesium vapor cell: MEMS cesium vapor cell with buffered gas enclosed is successfully sealed by loading the specific voltage and specific temperature of anodic bonding at the same time. The consistent cesium absorption line of MEMS cesium vapor cell produced by this method is observed after about one year, which proves the gas leakage problem is resolved.(3) Finish the design of the fabrication processes of transparent indium tin oxide thin film and resolving the local heating problem with the special requirement of transparency: The indium tin oxide thin film with high transparency is prepared by the optimized parameters of radio frequency magnetron sputtering. The heater using this thin film possesses the transparency of 82.3% for 895 nm laser and takes less than 420 s to heat a non-vacuum physical package to 85℃.(4) Finish the design of the low power consumption vacuum assembling structure and realize the physical package of chip-scale atomic clock: The assembling structure consists of a vertical cavity surface emitting laser, a quarter-wave plate, a MEMS cesium vapor cell, an indium tin oxide heater and a photoelectric detector. This assembling structure is bonded firstly on PCB board and then is packaged in vacuum environment. By filling the heat-isolated material with a small thermal conductivity to reducing the heat transfer, the power consumption of the physical package reduces to 300 mW. The cesium absorption line and microwave modulated cesium absorption line are observed clearly for the tested vacuum physical package, which proves that our developed physical package for chip-scale atomic clock can work well.