Advanced MEMS Packaging based on Parylene: One Material Enabling Encapsulation, Wafer and Chip Bonding, as well as Ultra-thin Flexible Printed Circuit Boards

Micro-Electro-Mechanical Systems (MEMS) are key enablers for emerging smart technologies, including the Internet of Things (IoT), autonomous driving, and Industry 4.0. [1] To enable the next generation of MEMS with respect to improved performance and increased miniaturization, novel and advanced packaging approaches are required. Doing so, the integration of new materials with unique properties is crucial. One such material is the polymer family known as Parylene (Poly[p-xylylene]). Parylene provides a unique combination of outstanding properties, including excellent dielectric characteristics, optical transparency for both infrared and visible wavelengths, biostability, and biocompatibility in accordance with ISO 10993 standards. It also features low permeability to moisture and gases, sufficient mechanical strength even at minimal layer thicknesses, and a high durability. Additionally, Parylene is deposited by chemical vapor deposition (CVD) at room temperature, ensuring high conformity and the absence of intrinsic mechanical stresses. It can be patterned on a wafer scale using oxygen plasma in combination with a resist mask or by laser ablation. [2]

Parylene has been extensively studied for its use in MEMS packaging as well as a structural and functional material in MEMS. Doing so, various functionalities of Parylene have been identified. This paper aims to summarize the multiple applications of Parylene in microsystems and their packaging.

Utilizing its excellent dielectric properties, including a high breakdown voltage, Parylene is suitable for applications such as TSV insulation and passivation. [3] Furthermore, it can encapsulate MEMS and electronic devices, protecting them against environmental factors by leveraging its effective barrier properties. Parylene encapsulations are particularly beneficial in scenarios where biocompatibility is essential, such as in smart medical implants. [2, 4]

A bonding process utilizing Parylene as an adhesive has been developed and thoroughly characterized for wafer and chip-scale applications. This process is established for three different types of Parylene - Parylene C, Parylene N, and Parylene F - each designed for varying bonding temperatures. Using the Parylene adhesive bonding high mechanical strengths of up to 38 MPa (tensile strength) and 85 MPa (shear strength), respectively, can be achieved, which ensure the compatibility with subsequent processes such as dicing, grinding, or polishing. Additionally, these bonding strengths, combined with the used standard wafer sizes of 150 mm and 200 mm in diameter, facilitate the scalability of the process for industrial applications. The bonded wafers have been evaluated for bond reliability and have demonstrated good hermeticity and stability against thermal and mechanical shocks, constant loads, and aging, respectively. [5, 6]

Parylene’s compatibility with established microfabrication technologies such as physical vapor deposition, lithography, and wet etching allows for the creation of metallic redistribution layers (RDL) on its surface. By using Parylene as a dielectric for electrical insulation between two RDLs, these layers can be stacked to produce an ultra-thin Parylene-based printed circuit board (PCB) with multiple RDLs. This Parylene PCB can achieve a total thickness of 20 µm or even less while offering a high flexibility. Doing so, Parylene functions as an ultra-thin substrate, dielectric, and encapsulation layer. The electrical performance of this Parylene PCB was characterized to be excellent under both, flat and bent conditions. Various established technologies, including soldering and wire bonding [7], were adapted for integrating discrete components, and new integration methods, such as the transfer of discrete components, have been developed [8].

In conclusion, Parylene unites multiple functionalities for MEMS packaging, which are established and demonstrated within the presented work. These include barrier layers, dielectric layers, adhesive layers for wafer and chip bonding as well as ultra-thin substrates. By combining its different functionalities with other emerging processes such as micro transfer printing, Parylene is a highly promising material for advanced packaging including the realization of new architectures.