Investigation of the Processing Behavior and Stability of Different Glass Frit Materials
Authors: Knechtel Roy , Wenig Micaela , Svoboda Stefan , Seyring Martin , Göbelt Manuela , Schwarz Uwe , Seifert Tobias , Roscher Frank , Wiemer Maik 
Published: February 2024

Although glass frit bonding has long been a well-established process in the industry, research and development efforts are continuously improving it for application in modern MEMS products. First, it should be mentioned that the main advantage of the glass frit bonding is easily integration of this process module into even the most complex technologies. Using screen printing, the glass frit paste can be applied in a structured manner to one of the two wafers to be bonded, no subsequent structuring (e.g., by lithography and etching) is necessary. Subsequently, the paste is transformed into a compact glass in a batch process (the organic binder is burned out in a furnace and the glass particles are melted). The bonding is performed in conventional wafer bonders. The glass frit, which is located as an intermediate layer between the wafers to be bonded, is melted, and joined by surface wetting to form a heretic seal (vacuum or defined gas pressures can be included). During cooling the glass solidifies and forms a very strong and reliable bond. Virtually all surface layers utilized in microsystems technology can be bonded. The surface roughness of the wafers does not play any role here and even rather high topographies (e.g. metal lines in the bond interface) can be well covered and sealed [1]. Related to this process flow, the glass frit bonding is very cost attractive, e.g., compared to Al-Ge eutectic bonding.
The research and development activities of the last years, the requirements that led to them and the corresponding solutions are to be presented and discussed here.

Bond frame width: screen printing is mostly associated with very coarse and broad structures and in fact a few years ago, the bond frame width determined the chip size. In the meantime, bond frame widths of 100 µm and smaller can be realized, which is hardly larger than with other bonding processes and allows the economical production of small chips. To realize this, special new screen materials are of key importance.

Wafer layout of the Glass Frit layer: with the smaller chips, sealing rings at the wafer edge and support structures in the scribe line, the degree of coverage during screen printing increased significantly, which makes the release of the screen and thus a defined printing process much more challenging. These requirements can be achieved with the new screen materials and measures for keeping the back of the printing screen clean.

Glass frit in complex manufacturing chains: Glass frit wafer bonding is not only utilized at the very end of the process to encapsulate MEMS structures; it is now also part of complex wafer level packaging technologies for realizing SoC (system on chip) solutions. After wafer bonding, many different process steps are implemented to create through-silicon-vias (TSVs) and solderable metallizations, for example. The above-mentioned sealing rings at the wafer edge are an important requirement for bonded wafers in such processes, in which mainly wet chemicals are used, therefore it is also essential that the glass frit is chemically resistant and thermally stable up to a certain temperature. This was investigated for different glass frit materials, and it can be concluded that there is a good stability against water and solvents. The resistance to bases varies from material to material and acids usually destroy the glass (Figure 1). Regarding the temperature, it can be stated that up to 50 °C below the bonding temperature, the Glass frit can be considered as solid.

Lead-free glass frit materials: until now, lead in glass frit materials was exempt from the legal regulations governing their use. These exemptions will expire in the foreseeable future [2]. Therefore, the detailed evaluation of lead-free glass frits is of very high importance. These materials are currently being investigated in terms of the entire process chain in preparation for industrial application. This evaluation faces some challenges (crystallization tendency of lead-free glasses) but also advantages (e.g., significantly low bonding temperatures are possible with Te-based glass solders).

Even though glass frit bonding appears at first glance to be a well-understood process that is easy to apply. There are several modern manufacturing processes, with new challenges that can be mastered by understanding the materials and the processes. However, this requires extensive research and optimization.
Literature:
[1] Knechtel, Roy. "Glass frit bonding: an universal technology for wafer level encapsulation and packaging." Microsystem technologies 12.1-2 (2005): 63-68.
[2] DIRECTIVE 2011/65/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL

Figure 1