(Invited) Parylene C Based Adhesive Bonding on 6” and 8” Wafer Level for the Realization of Highly Reliable and Fully Biocompatible Microsystems
Authors: Selbmann Franz , Baum Mario , Meinecke Christoph , Wiemer Maik , Otto Thomas , Joseph Yvonne 
Published: December 2020

For the realization of advanced bio-microsystems for medical applications such as implants, fabrication processes require the usage of biocompatible materials only. Especially for the encapsulation and hermetic sealing, e. g. of microfluidic structures, a biocompatible wafer bonding process is necessary. Additionally, the ongoing integration of new and temperature sensitive materials in micro systems, such as biomolecules, requires bonding processes with low bonding temperatures only.

However, when choosing the wafer bonding process, additional criteria are a high reliability, since the bonding is critical for the operation of the chip, as well as the applicability on 6” and 8” wafer level in order to fabricate a high number of devices at low costs.

In order to fulfill all these requirements, we propose an adhesive bonding process, which uses Parylene C as intermediate layer. Parylene C is a thermoplastic polymer, which is ISO 10993 certified as biocompatible and biostable, chemically inert against all common acids, bases and solvents, as well as a low permeable and optical transparent. In addition and in contrast to other established bond adhesives, such as SU8, the Parylene layers are deposited by CVD, and offer a very high three dimensional conformity as well as a very high homogeneity without any pinholes. Furthermore, due to its chemical inertness, Paryleneis compatible with a variety of established microtechnologies.

Previous studies prove that adhesive bonding using Parylene C is feasible. However, these works are limited to 4” wafer level and the bonding provides only low tensile strengths, e. g. 10 MPa. [1-7]

Our presented Parylene based adhesive bonding technology uses a 0.5 µm to 10 µm Parylene C layer, which is deposited on 6” and 8” wafers and can be patterned optionally by an oxygen plasma without leaving residues. A subsequent thermal and adhesive bonding process at temperatures of 280°C to 300°C as well as moderate wafer contact pressures are performed to successfully bond the wafers. Within our study, different materials combinations for the bond interface, e. g. silicon - silicon, silicon - silicon dioxide, silicon – glass, silicon – silicon nitride, and silicon – aluminum were investigated. Doing so, only one wafer was coated with Parylene. Additionally, bonding processes were performed with two Parylene coated wafers, and hence, a Parylene - Parylene bond interface. Since almost all materials, established in microsystem technologies can be coated with Parylene, this enables the usage of this bonding process for a high variety of material combinations and particularly materials, which were not investigated by our study. The focus of the experiments was the investigation and optimization of a feasible parameter range in respect of the bonding time, bonding temperature, wafer contact pressure, and Parylene thickness.

The characterization of the bonding process was performed using Parylene test structures of squared geometry and different width. The bonding process was analyzed by IR imaging to determine voids and defects. Afterwards the wafers were diced into chips, centering the Parylene frame. The investigation of tensile and shear strengths proved a high reliability of the bonding but also a strong dependency on the bonding parameters. The tensile strength reached up to 35 MPa was improved by a factor of 3 compared to previous studies, the shear strength reached up to 80 MPa Furthermore, the impact of thermal and mechanical shocks as well as long-time load on the tensile strength was investigated. Finally, cross-sections of the bonded chips were analyzed by SEM. The results prove that the Parylene frame structure was still intact after bonding and not flattened due to bonding. Furthermore, the hermetic properties of the Parylene bonds were investigated. Doing so, the Helium leakage rate was determined to < 1∙10-7 mbar ∙ l/s. The study was completed by the investigation of the bonding process on three dimensional structures, which refers to bonding over patterned metal but also bonding of cavities without previous pattern of the Parylene. Finally, after successfully bonding the test frames, the established bonding process was applied on a real microsystem.

References:

[1] M. Kurihara, et al., IEEE MEMS 2012 (Paris, France) pp 196-9

[2] Y.-C. Yen, et al., IEEE MEMS 2012 (Paris, France) pp 381-384

[3] Q. Shu, et al., International Conference on Solid-State and Integrated-Circuit Technology 2009 (Beijing, China)

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[6] D. Ziegler, et al., J. Microelectromech. Syst. 15 (6), 2006, 1477–82

[7] H.-S. Noh, et al., J. Microelectromech. Syst. 14, 2004, 625–31