Research On MEMS Atomic Vapor Cells And Integration Of Chip Scale Atomic Clock Physics Package

The time（frequency）standard has two development trends——one towards ultra precision and the other towards miniature.The dissertation focuses on developing and improving the technology of miniature atomic clock.The goal of the dissertation is to combine microelectromechanical system（MEMS）technology with advanced atomic interrogation technology to produce atomic clock with better performance which is suitable for many new applications.Atomic vapor cell is currently recognized as the most convenient device that can realize quantum state manipulation.It stores gaseous atoms in transparent material vessels to form a closed ensemble,and performs the production and detection of atomic quantum states through laser.The device which can manipulate spin-state of atomic ensemble is the core of atomic clock.The atomic clock based on coherent population trapping does not need microwave cavity and therefore was not restricted by the volume of microwave cavity.Cooperating with specific integrated circuit,the device can realize chip scale atomic clock,which requires using MEMS technology instead of the traditional glass blowing method to fabricate alkali atomic vapor cells.However,the traditional dispensing method is not compatible with the existing MEMS process due to the extremely active chemical properties of alkali metal elements.Due to the limitation of alkali dispensing,buffer gas filling and cell’s bonding method,most MEMS atomic vapor cells are fabricated by compound dispensing,but the quality is poor due to the influence of residual impurities.The fabrication of high-quality MEMS atomic vapor cells is the main bottleneck for the miniaturization of atomic clock physics package.In view of the above problems,the dissertation systematically studied the analytical method and the model of chip scale atomic clock MEMS atomic vapor cells and carried out the research of the dependence of vapor cell’s performance on its size and analyzed the limiting factors of life span in vapor cells.And elaborated that frequency stability of the millimeter scale MEMS atomic vapor cell can break through 10^-10τ^1/2,then the dissertation explored the mechanism of alkali element consumption and the control steps of element consumption rate,which provides theoretical guidance for the design and fabrication of MEMS atomic vapor cells and the design of optical system in chip scale atomic clock physics package.In order to meet the requirements of high reliability batch fabrication of chip scale atomic vapor cells,the dissertation carried out the research to develop controllable dispensing technology of wafer-level alkali metal substance compatible with MEMS fabrication process.In view of the low yield of wafer bonding due to the unknown failure mechanism of the alkali vapor cells,the reaction process between the alkali and the amorphous oxides in the glass window during high temperature bonding was studied,and the alkali consumption at high temperature was identified as the main reason for the failure of the atomic vapor cells.Using self-developed technology and equipment to strictly control the amount of water and oxygen in the encapsulation process and the low temperature process to effectively restrain alkali reaction between the amorphous oxide of the glass window in the bonding process,10 mm³ volume is implemented without side reaction cavity MEMS atomic cells of wafer level batch fabrication,the buffer gas pressure was precisely controlled,the yield of wafer preparation is higher than 80%,and the vapor cells leakage rate is better than 1×10^-10 mbar·L/s.The lifetime of alkali vapor cells was evaluated by accelerated aging experiment.In order to improve the performance of vapor cells and meet the needs of future anti-relaxation coating at the same time,the dissertation chose gold and indium with low melting point as an intermediary forming Au In binary alloy as the final bonding layer,used the characteristics of transient liquid phase bonding of isothermal solidification which achieved high performance and air-tight atomic vapor cells which has processing temperature less than 200℃while can tolerate remelting temperature higher than 350℃.Based on the integration requirements of chip scale atomic clock physics package,the dissertation used finite element simulation to analyse the physics package model and simulated the mechanical performance and heat dissipation.An adiabatic suspended structure was proposed to meet the reliability requirements and it was fabricated by MEMS process.The method is to fabricate the atomic vapor cells with a volume of 10mm³ by MEMS process,and then integrate the atomic vapor cells,VCSEL,photodetector,temperature control chip,optical components and so on to form the core of the physics package.Then it carries on adiabatic encapsulation with physics package’s volume less than 0.6 cm³,which dissipated less than 30 m W of power and reached for the impact resistant more than 1500 g,first-order natural frequency above 2000 Hz.To perfect the microfabrication process of atomic vapor cells and characterize the performance of the atomic clock,the dissertation determined the initial dispensing amount of alkali metal in vapor cells by conducting the experiment of alkali element consumption rate.The methods to inhibit chemical reaction and reduce the consumption of alkali elements through condensation orientation pads and diffusion barrier layer were proposed,which lay the foundation of suppression technology of alkali element consumption.The performance and reliability of chip atomic clock physics package are characterized with the volume of 2.3 cm³after implement assembly into the clock,and short-term frequency stability of 1.4×10^-10,and the precision better than 1μs in timekeeping of one day performance were realized.The use of MEMS technology reduces the size and power consumption of the miniature atomic clock by more than 100 times compared with that of other atomic clocks.Combined with the mass production ability of MEMS micro-manufacture,the technology opens a new way for the application of atomic clock technology.