Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61875031) and the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 61421002)
Received Date:11 June 2019
Accepted Date:19 July 2019
Available Online:01 October 2019
Published Online:20 October 2019
Abstract:In this paper, a two-dimensional subwavelength periodic titanium (Ti) disk array integrated in micro-bridge structure is proposed to enhance the absorption of terahertz (THz) microbolometer. Based on the rigorous coupled wave analysis (RCWA) method, THz absorption characteristics of Ti disk arrays with different structure parameters in micro-bridge structure arrays are studied. Periodic disk array structure reduces the surface plasmon frequency of Ti, excites the spoof surface plasmons in the THz band and leads to resonance enhanced absorption. The resonance absorption frequency is determined by the structural parameters of Ti disk array including period and diameter while the absorption rate of THz wave is greatly affected by the thickness of Ti disks. The resonant cavity in micro-bridge structure can reduce the resonance frequency and enhance the coupling efficiency. The micro-bridge structure designed in this paper breaks the diffraction limit and traps the THz wave with a small period (37 μm). An absorption of nearly 90% is achieved at 3.5 THz. The structure meets the requirements of small size, high absorption and good process compatibility of the THz microbolometer. Keywords:THz/ microbolometer/ spoof surface plasmon polaritons/ rigorous coupled wave analysis
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2.结构设计与理论计算微测辐射热计型室温太赫兹探测器阵列由许多MEMS微桥结构的像元在焦平面上二维重复排列构成, 每个像元对辐射进行测量. 其基本原理为: 太赫兹辐射被像元中的吸收层吸收后引起温度变化, 进而使氧化钒热敏电阻的阻值变化; 氧化钒热敏电阻通过MEMS绝热微桥支撑在硅衬底上方, 并通过支撑结构与制作在硅衬底上的COMS读出电路(ROIC)相连; 读出电路将热敏电阻阻值变化转变为差分电流并进行积分放大, 经采样后得到热图像中单个像元的灰度值. 为了提高探测器的响应率和灵敏度, 要求探测器像元微桥具有良好的热绝缘性, 同时为保证成像的帧频, 需使像元的热容尽量小以保证足够小的热时间常数. 本文中室温太赫兹微测辐射热计焦平面阵列的探测单元微桥结构如图1(a)所示. 每个微桥结构由桥面(敏感区域)以及支撑桥面的两条桥腿组成. 细长的桥腿同时用作机械支撑、电学和热学通道. 桥面尽量轻、薄以减小热质量. 桥面层自下而上包括氮化硅(Si3N4)支撑层、氧化钒(VOx)热敏薄膜、钝化层(Si3N4)和金属薄膜(Ti)太赫兹波吸收层. 在衬底上制作反射层(Ti), 与桥面之间形成谐振腔(约2 μm), 但由于太赫兹波的波长较长, 该谐振腔并无明显的增强吸收的效果, 在没有太赫兹波吸收层的情况下, 微桥结构的太赫兹波吸收率极低(2.6%—4%). 为了增强太赫兹波的吸收, 在桥面的顶部集成了一层超薄Ti薄膜作为太赫兹波吸收层. 与其他金属薄膜相比, Ti薄膜厚度容易控制且可以通过反应离子刻蚀工艺实现图形化, 具有很好的工艺兼容性. 图 1 吸收结构图 (a) 太赫兹微测辐射热计焦平面阵列的探测单元微桥结构; (b)二维亚波长Ti圆盘阵列俯视图; (c) 周期Ti圆盘阵列剖面图; (d) 增加反射层的吸收结构; (e) 增加反射层及支撑层的吸收结构; (f) 直角坐标系下的入射平面波与吸收结构模型 Figure1. Absorption structures: (a) Pixel structure of THz microbolometer focal plane array (FPA); (b) top view of a two-dimensional subwavelength Ti disk array; (c) sectional view of a periodic Ti disk array; (d) absorption structure with a reflection layer; (e) absorption structure with reflection layer and supporting layer; (f) absorption structure illuminated by a plane wave with a rectangular Cartesian coordinate system attached.
$A = 1 - R - T = 1 - \mathop \sum \nolimits {r_{mn}} - \mathop \sum \nolimits {t_{mn}}.$
Ti在不同频率下的介电常数为${\varepsilon _{{\rm{Ti}}}} = {\left({{n_{{\rm{Ti}}}} + {\rm{i}}{k_{{\rm{Ti}}}}} \right)^2}$, nTi与kTi值如图2(a)所示[30]. Si3N4材料的${n_{{\rm{S}}{{\rm{i}}_3}{{\rm{N}}_4}}}$与${k_{{\rm{S}}{{\rm{i}}_3}{{\rm{N}}_4}}}$值如图2(b)所示[31]. 真空谐振腔厚度为2 μm (nd = 1). 太赫兹波垂直入射到吸收结构上, 则θ = 0, φ = 0, ψ = 90°(TE极化). 图 2 Ti与Si3N4的材料参数 (a) Ti在不同频率下的nTi与kTi值; (b) Si3N4在不同频率下的${n_{{\rm{S}}{{\rm{i}}_3}{{\rm{N}}_4}}}$与$ {k_{{\rm{S}}{{\rm{i}}_3}{{\rm{N}}_4}}} $值 Figure2. Material parameters of Ti and Si3N4: (a) nTi and kTi values of Ti at different frequencies; (b) ${n_{{\rm{S}}{{\rm{i}}_3}{{\rm{N}}_4}}}$ and $ {k_{{\rm{S}}{{\rm{i}}_3}{{\rm{N}}_4}}} $ values of Si3N4 at different frequencies.