Low Temperature Thermo Compression Bonding with Printed Intermediate Bonding Layers
Authors: Wiemer Maik , Roscher Frank , Seifert Tobias , Vogel Klaus , Ogashiwa Toshinori , Gessner Thomas 
Published: February 2020

The manufacturing costs of microelectronic components like Micro Electro Mechanical Systems (MEMS) are strongly correlated to involved packaging technologies like wafer bonding. Typically wafer level packaging of MEMS is done by direct bonding with activated surfaces, field assisted bonding or bonding with intermediate bonding layer (IBL). Typical intermediate layers could be solder materials, polymers, glass layers or adhesion promoters coated with a pure metal layer.
The compatibility between bonding processes and considered MEMS process flow is often defined by process parameters like temperatures, tool pressure, hermeticity level and by substrate properties like surface quality (roughness, flatness), patterning of samples surface, the used intermediate bonding layers and sample dimensions. Glass frit bonding as a cost-effective technology based on IBL manufactured by screen printing (SP), provide hermeticity and tolerance to surface properties (roughness) and allows lateral electrical feed throughs through the bonding interface. Other conditions like its high process temperature in the range between 400 °C and 500 °C, glass frits dielectric properties or limited bond frame width often prevent a foot print reduction of MEMS. As an alternative, metal based thermo compression bonding (TCP) can replace the glass frit bonding process, but typically the bonding process is performed at a temperature range of 350 °C to 600 °C using high bonding pressures up to 800 MPa, assumed that a high surface flatness is given. To reduce bonding process temperature, having lower demands to surface properties and higher potential for 3D-integration applications, metallic nano particle based IBL are promising to be used for bonding processes on chip and wafer level. Supported by temperature and bonding pressure, these nano particle based intermediate bonding layers enable a hermetic and robust interface due to sintering effects like densification and grain growth between nano particles itself and diffusion effects between nano particle and adhesion promoter. These intermediate bonding layers can be deposited using printing processes like Screen Printing (SP) or Aerosol Jet Printing (AJP).
This work describes capabilities of the additive material transfer technologies Aerosol Jet Printing and Screen Printing with the purpose of IBL manufacturing for Chip-to-Chip, Chip-to-Wafer and Wafer Level Bonding. For realization of these applications, the deposition process and the related material morphology, the bonding with nano particle based IBL as well as the characterization methods are described. AJP was used to deposit Ag as IBL to bond Cu and Au metallized wafers. Screen printed Au nano particle IBL layers were used to bond Au metallized wafers. The interface between nano particles and substrate were investigated. For bonding result evaluation, bonding frames and occurring bonding interfaces were analyzed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and focused ion beam (FIB). Mechanical properties were qualified using compression shear tests and tensile tests.
There are state-of-the-art-approaches on silicon wafers, which contains dry etched rim structures around the active chip structure to precisely define widths of bonding frames manufactured by SP. Also lithography patterned, glass based carrier wafers are used as an intermediate step to establish a precise pattern transfer of printed sub-micron Au particles based bonding frames. International publications and this work shows that successful bonds are possible by direct printed sub-micron Au particle IBL. In contrast, we present a process flow were the bond frames are directly deposited to the substrate and no additional intermediate step is needed as shown in Figure 1.
Increasing tool pressure while bonding leads to increasing tensile strength. Compared to glass frit bonding, the sub-micron Au particle interface enables 3 times higher bond strength at bonding temperatures of 200°C. Grain growth and densification could be enhanced by increased bonding pressure. Results are strongly related to the printed pattern morphology. The cross sections of a sub-micron Au particle bond frame after bonding shows 2 typical zones (i) an inner contact zone responsible for the joint with almost no porosity and (ii) the outer contact zones, produced by the printing process but not in contact with the counter substrate. The process is capable to be adapted to chip level or be expanded to 3D integration applications. Not only process-related costs could be saved, but also new possibilities could be realized, like metallization of MEMS at stages of manufacturing, where no wet chemical processes, no electroplating or no lithography steps are suitable anymore. Furthermore the metallization patterns can be easily generated by CAD data.

Figure 1