A Novel 3D Encapsulation Structure Based on Subwavelength Structure and Inserted Pyrex Glass for RF MEMS Infrared Detectors

Abstract

A novel wafer-level three-dimensional (3D) encapsulation structure was designed for radio-frequency microelectromechanical system (RF MEMS) infrared detectors and investigated by using the finite element method (FEM) simulation. A subwavelength structure with a circular array of coaxial apertures was designed to obtain an extraordinary optical transmission (EOT) on top of a silicon substrate. For perpendicular incident light, a maximum transmission of 56% can be achieved in the long-wave infrared (LWIR) region and the transmission bandwidth covered almost the full LWIR region. Moreover, the maximum transmission could be further promoted with an increase in the incident angle. The vertical silicon vias, insulated by inserted Pyrex glass, were used to generate electrical contacts. With the optimized structure parameters, a feed-through level lower than −82 dB, and a transmission coefficient of one single via of more than −0.032 dB were obtained at a frequency from 0 to 2 GHz, which contributed to the low-loss transmission of the RF signals. Due to the matched thermal expansion coefficients (TECs) between silicon and Pyrex glass, the proposed via structure has excellent thermal reliability. Moreover, its thermal stress is much less than that of a conventional through-silicon via (TSV) structure. These calculated results demonstrate that the proposed 3D encapsulation structure shows enormous potential in RF MEMS infrared detector applications.Electronics 2019, 8, 974 2 of 12 planar interconnect packaging [6][7][8][9][10][11]. There are two primary types of wafer-level 3D packaging. One is film encapsulation, which utilizes an epitaxial highly doped silicon film to seal devices and vertical silicon feedthroughs to transmit electrical signals [6,7]. Although the film encapsulation can contribute to the miniaturization of devices, its micro-fabrication process is very complicated. Another type is typically based on through-silicon vias (TSVs) and wafer-level bonding [8][9][10][11]. This technique utilizes TSVs to make vertical interconnections, which embed the vertical metal feedthroughs and SiO 2 films in bulk silicon to transmit electrical signals and act as the insulation, respectively. However, this technique has some shortcomings including mismatched thermal expansion coefficients (TECs) between the metal and silicon, large dielectric loss, insulation failure, a complex process, and high cost [9]. In addition, the wafer-level bonding between the cap and device wafers is critical to achieve long-term operation as well as high thermal insulation for RF MEMS infrared detectors. The common bonding methods such as Cu-Sn bonding and Au-Sn bonding [11,12] are not cost-effective and easily cause large residue stress in the systems. The silicon wafer with inserted Pyrex glass is a promising 3D packaging structure, which utilizes vertical silicon feedthroughs to transmit electrical signals [13][14][15]. The inserted Pyrex glass acts as the electrical insulation and the anodic bonding material, which also has a matched TEC wi...