Multi-modal Thermal Detectors Based On Metasurface Microbolometers

Uncooled infrared detection technology has made great progress since 1990s,including microbolometer based on thermal-sensitive effect,pyroelectric detector based on pyroelectric effect and thermopile detector based on Seebeck effect.Compared with cooled infrared photon detectors,uncooled infrared detectors can work at room temperature without additional cost of refrigeration,so they have unique advantages in performance,volume,weight and power consumption.However,the working principle of uncooled infrared detector is based on the thermal effect of infrared light to measure the temperature changes,so it cannot respond to any other information such as spectrum and polarization besides intensity.Infrared detector should rely on filter and polarizer for spectral and polarization,which leads to the system volume increase,instability increase,slow imaging speed and other problems.Polarization detection of the reflected or radiated light of objects can provide information on surface topography and scattering characteristics,so it is widely used for target detection in national defense and biomedical fields.Spectral detection with high resolution can provide material composition,so it can be used for food quality assessment,art certification and other applications.Separate spectrum or polarization information is not enough to identify and distinguish the target.When spectral detection and polarization detection are performed at the same time,more information on surface and material can be obtained.This technology is called polarization spectral imaging,which has a wide and important application in remote sensing and target recognition.In order to solve the problem that traditional uncooled infrared detectors cannot respond to the spectral and polarization information at the pixel level,a new scheme with high integration density is needed.Metasurface is a kind of man-made periodic two-dimensional array with a subwavelength scale,which can realize flexible control of electromagnetic wave amplitude,frequency,phase,polarization and other characteristics through proper design.Thanks to the improvement of semiconductor processing technology,the machining size of metasurface is becoming smaller and smaller,and the precision is becoming higher and higher,which greatly promotes the development and application of metasurface.In this paper,a metasurface integrated microbolometer infrared detector with response to specific spectrum and polarization is proposed.The main research results are as follows:（1）Genetic algorithm was applied to the optimization and design of metasurface spectral and polarization selective absorber.The first designed metasurface absorber achieved an average absorption coefficient of 0.734 for TM mode and a polarization extinction ratio of31.17 between 3μm and 5μm.The second designed metasurface absorber achieved an average absorption coefficient of 0.773 for TM mode and a polarization extinction ratio of37.21 between 8μm and 12μm.The number of division of the antenna was optimized in genetic algorithm.The thickness of the dielectric layer was discussed.And the two designed spectral and polarization selection metasurface absorbers were fabricated.The samples were tested by scanning electron microscope and Fourier Transform Infrared Spectroscopy.The results showed that the spectral and polarization properties were consistent with the design,which confirmed that the correctness of the design and fabrication of the dual-band polarization selective metasurface absorbers.（2）The feasibility of integrating dual-band polarization selective metasurface absorber into microbolometer was verified by simulations.The problems that may occurred in this architecture were discussed,such as pixel size,pixel spacing,incident angle,thermal analysis,et al.The simulation results showed that the metasurface absorbers could maintain their properties at a pixel size of 8μm and a pixel spacing of 400 nm.When the incident angle was less than 10°,the 3～5μm metasurface absorber could achieve an average absorption coefficient above 0.73 for TM mode and a polarization extinction ratio above 30,while the8～12μm metasurface absorber could achieve an average absorption coefficient above 0.76for TM mode and a polarization extinction ratio above 33.Thermal analysis showed that the extinction ratio of the temperature rise caused by different polarization state could reach 39.31at a single wavelength.The supporting structure of the integration process was discussed.The sacrificial layer and back release processes were introduced.In the end,large pixel and back release methods were selected according to status of process level.（3）Dual-band polarization selective metasurface microbolometer was fabricated.The main processes included photolithography,plasma enhanced chemical vapor deposition,electron beam evaporation,etching,and electron beam lithography.The spectral and polarization response of the samples were measured,and the temperature coefficient of resistance were all about-0.012 K^-1.Then,the chip was vacuum packaged,and metasurface integrated microbolometer for spectral and polarization detection was finally developed.（4）The performance of the fabricated metasurface microbolometers was verified and its application prospect was analyzed.The spectral response curve was consistent with the absorption coefficient curve.The two detectors achieved a peak responsivity of 1.00 V/W and1.46 V/W,respectively.The square of correlation coefficient between the polarization response curve and the theoretical fitting curve was more than 0.92.The time constants tested using chopper and lock-in amplifier for the two detectors were 1.67 ms and 1.78 ms,respectively.Detector 1 achieved the highest detectivity of 6.94×10⁵ cm Hz^1/2W^-1 at a modulation frequency of 11 Hz.Detector 2 achieved the highest detectivity of 9.95×10⁵ cm Hz^1/2W^-1 at a modulation frequency of 36 Hz.