The Design And Fabrication Of Infrared Microlens Array Used For Pyroelectric Detectors

Uncooled Infrared Focal Plane Array(UIRFPA) can be used for staring imaging of infrared objects. As cryogenic operation is not requested for UIRFPAs, the overall weight, power consumption and price of the imaging system can be decreased. For this merit, the UIRFPA has found various applications in both civil and military fields. For pyroelectric UIRFPA, the thermal crosstalk between adjacent pixels can hamper the detector performance significantly. Therefore, thermal isolation of the detector pixels is mandatory, which however gives rise to a decreased fill factor of the detector and thus a low energy usage ratio. This can be solved by integrating a microlens array to concentrate the incident irradiation into the active area of each pixel. In general, integrating the detector with a microlens array can improve its performance greatly in terms of responsivity, signal-noise ratio, etc.Based on the above-mentioned application, both the design and fabrication of infrared microlens array are studied in this work. To obtain microlenses with large focal numbers and high quality, the fabrication process, i.e. thermal reflow and ion beam milling, are mainly investigated. The content of this work can be concluded as followings:1. The favorable parameters of the microlens array for its application in pyroelectric detectors are clarified.2. To improve the focal number of microlens fabricated by thermal reflow method, the underlying physical and chemical mechanisms of the reflow process were studied. It is found that gravity and thermal crosslinking of the photoresist material are critical factors that limit conventional thermal reflow method to achieve high focal numbers. Based on this, a novel thermal reflow method with an additional flood exposure process and an upside-down reflow configuration is proposed in this work. By this method, the maximum focal number of the fabricated microlenses can be improved from 2.4 to 9.7. Besides, the microlens profile can be tuned precisely by adjusting the flood exposure dosage.3. To transfer the photoresist microlens to silicon wafer with high fidelity, the profile evolution process during ion beam etching was studied, as well as the impact of etching selectivity, facet effect and trenching effect. By optimizing the incident angle of the ion beam to 40 degree, the detrimental impact of trenching and facet effect can be eliminated. In this case, the profile error of the obtained microlens with respect to an ideal spherical cap can be limited to be less than 0.27 μm. Besides, the obtained microlens array has a high uniformity and surface smoothness.