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入射光的偏振态通常采用斯托克斯参量表示,根据斯托克斯公式可知,通过采集入射光经过不同方向线偏振片和相位延迟器调制后的强度信息就可以反演出入射光的偏振信息[3]。对于文中涉及的线偏振成像系统,只需采集入射光经过不同方向线偏振片调制的强度信息即可计算出入射光的线偏振信息。研究表明,将线偏振片设置为0°、45°、90°、135°四个方向是最大化信噪比和最小化系统误差的选择[4-5],所以采用如图1所示的光栅阵列偏振片排布模式。
线偏振光栅采用硅基多层金属-介质亚波长光栅,具体结构如图2所示。光栅设计基于严格耦合波理论[6]与等效介质理论[7],并采用时域有限差分(Finite Difference Time Domain, FDTD)技术[8]对光栅各层金属厚度、周期、占空比以及介质层厚度等结构参数在中红外波段进行优化设计,结合实际加工工艺条件得到光栅结构参数如表1所示。
Figure 2. Schematic diagram of silicon base multilayer metal-dielectric sub-wavelength grating structure
Parameter Value Grating width, a/nm 150 Grating interval, b/nm 150 Grating period, T/nm 300 Dielectric thickness, h0/nm 270 Depth of grating groove, h1-h3/nm Au: 100
Pd: 50
Ti: 10Duty ratio of grating, f 0.5 Table 1. Design parameters of grating structure
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所设计的中波红外偏振光栅为透射光栅,光栅周期较小,光栅基本单元尺寸为微米量级,光栅基本单元中含有四个偏振方向的光栅,基于上述光栅的特点,采用纳米加工技术实现光栅阵列制备。光栅层为金、钯、钛多层金属光栅,光栅图形化采用电子束直写光刻及干法刻蚀技术实现,具体技术实施如图3所示。
Figure 3. Fabrication process of silicon base multilayer metal-dielectric sub-wavelength polarization grating
所设计的红外偏振光栅为透射光栅,且最终光信号需要被光敏单元接收,因此光栅基底需要透光,且衬底材料在中波红外波段需要有较好的通过率,所以选择在单晶硅基底上实现光栅的加工。然后在硅基上采用物理溅射方法依次制备介质层与各金属光栅层。对于掩蔽层,由于光栅均在纳米尺度,后续干法刻蚀需要有较高的选择比以及陡直的图形形貌,因而采用硬掩蔽方法,即在光栅层上再次利用物理溅射制备氧化硅材料,在氧化硅材料上进行图形光刻。由于设计光栅尺度较小,采用电子束直写光刻方案直接在掩蔽光刻胶上进行直写曝光,完成图形制备。紧接着采用等离子体干法刻蚀的方法制备光栅结构,实现图形转移,相较于其他光栅结构制备方法,等离子体干法刻蚀方法可以实现对光栅形貌与尺寸的最好控制。最后通过干法刻蚀去除剩余掩蔽层材料,完成光栅制备,制备得到的光栅形貌图如图4所示。
Preparation of medium wave mercury cadmium telluride infrared polarization focal plane detector (Invited)
doi: 10.3788/IRLA20211008
- Received Date: 2020-11-20
- Rev Recd Date: 2020-12-22
- Available Online: 2021-01-22
- Publish Date: 2021-01-22
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Key words:
- mercury cadmium telluride /
- infrared focal plane detector /
- polarization
Abstract: In order to meet the needs of accurately detecting and recognizing many kinds of high-value targets (such as stealth), the continuous development of detection technology, and realizing high probability true or false target recognition and high precision target detection, location and tracking in complex battlefield environment, it is of great significance to research stealth and weak feature target detection and anti-jamming detection in complex battlefield environment, the high integration of polarization focal plane infrared detector technology is one of the important direction. Focusing on the development of an integrated MW 256×256 HgCdTe polarization focal plane infrared detector, the research progress from the integration of polarization detectors to design and preparation of polarization structures were introduced, as well as the performance testing of polarization detector. A subwavelength metal grating array was designed and fabricated, the polarization detector was integrated by flip chip, and the infrared polarization performance was tested and evaluated on MW256×256 HgCdTe focal plane device.