This invention relates to a sensor, particularly to an accelerometer, its fabrication method and acceleration sensors which includes such accelerometer.
Nowadays, accelerometers have been used in various applications, such as, measuring the magnitude of earthquake and gathering seismic data, detecting the magnitude of collision during a car collision, and detecting the tilting direction and angle of a mobile phone or a game console. As the micro-electro-mechanical systems (MEMS) technology continues to progress, many nano-scale accelerometers have been widely commercially used.
In general, the accelerometers can be categorized into two kinds, one is parallel plate accelerometer, such as Chinese invention patent with publication No. CN102768290A. The parallel plate accelerometer measures the acceleration through the parallel plate capacitor formed between the top cap, the mass, and the bottom cap. When there is an acceleration, the frame displaces towards the direction of acceleration, but due to inertia, the displacement of the mass is relatively small causing the distance or the area of projection between the top cap, the mass, and the bottom cap to change. The capacitance between the top cap, the mass, and the bottom cap also changes. Integrated circuits calculates the direction and magnitude of the acceleration based on the change of capacitance.
Another type of accelerometer is comb structure accelerometer, such as Chinese invention patent with publication No. CN1605871. Comb structure accelerometer detects acceleration by measuring the change in capacitance of two spaced apart comb structures. The comb structure comprise movable teeth provided on the mass, and fixed teeth adjacent to the movable teeth. As the mass displaces due to acceleration, the movable teeth also displaces; thus the distance or the area of projection between the movable teeth and the fixed teeth changes, leading to a change in capacitance. Integrated circuits calculates the direction and magnitude of the acceleration based on the change of capacitance.
In a parallel plate accelerometer, the mass is relatively large, and the relation between the measurement accuracy and the mass is shown in:
      Acceleration    ⁢                  ⁢    due    ⁢                  ⁢    to    ⁢                  ⁢    noise    ⁢          :        ⁢                  ⁢                  a        ¨            _        =                              F          n                _                    A        1              =                                        F            n                    _                m            =                                    4            ⁢                                                  ⁢                          k              B                        ⁢            T            ⁢                                                  ⁢                          ω              0                                mQ                    
where kB represents Boltzmann constant, T represents temperature, ω0 represents resonance frequency, Q represents quality factor, m represents mass. Therefore, when the resonance frequency and the quality factor are fixed, increasing the mass reduces the effect by noise. The capacitance formed between the mass and the cap is also relatively large, which means the sensitivity is high. However, during fabrication, parallel plate accelerometer has a high squeeze-film damping force; thus it requires vacuum environment for packaging, which dramatically increases the packaging and fabrication cost. In comparison, the comb structure accelerometer has a low squeeze-film damping force. Based on the book “Analysis and Design Principles of MEMS Devices” the coefficient of damping force in MEMS chip can be calculated by:
            c      rec        =                            μ          ⁢                                          ⁢                      LB            3                                    h          3                    ⁢              β        ⁡                  (                      B            L                    )                      ,            where      ⁢                          ⁢      L        ⪢    B    ,      β    =    1    ,            β      =      0.42        ;  
For example, the coefficient of damping force of 1000 um×1000 um accelerometer with 100 pairs of 500 um×20 um comb teeth is 1.5% of the coefficient of damping force of 1000 um×1000 um accelerometer without comb teeth. Therefore, comb structure accelerometers can be packaged under non-vacuum environment, which means the packaging cost is low. However, due to the characteristics of comb structure, the mass is relatively small, and the capacitance in a comb structure accelerometer is smaller than parallel plate accelerometer. Thus, the sensitivity of comb structure accelerometer is lower compared with parallel plate accelerometer. Furthermore, comb structures are fabricated by using photolithography and etching. The spacing between the movable teeth and the fixed teeth is limited by the etching process to 2 um. On the other hand, parallel plate accelerometers are fabricated by bonding, the spacing between the mass and the caps can be controlled in 1 um. However, the accuracy of bonding technique is lower than photolithography and etching. In conclusion, both parallel plate accelerometers and comb structure accelerometers have their own advantages and disadvantages.