(Invited) Low-Temperature Particle- and Printing Based Wafer Level Bonding Processes
Authors: Wiemer Maik , Schroeder Tim , Wuensch Dirk , Seifert Tobias , Roscher Frank , Gross Eduard , Otto Thomas 
Published: February 2020

The production costs of micro systems are strongly correlated to the involved packaging technologies like wafer bonding. Well-established wafer bonding technologies need to be optimized to ensure the functionality and reliability of these micro systems. The requirements on this technologies regarding hermiticity, electrical contact, mechanical and thermal resistance are high. Typical wafer bonding technologies are anodic or direct wafer bonding using surface activation or such with intermediate layers like solder materials, polymers, glass layers or pure metal layers. The bonding process is defined by parameters like temperature, tool pressure, hermeticity and substrate properties (surface quality, device surface patterning), the used intermediate layer and the device dimensions.
Glass frit bonding is a cost-effective, industry related wafer bonding technology with intermediate layers manufactured by screen printing. This technology provides hermeticity without high demands regarding surface quality and allows lateral electrical interconnects through the bonding interface. However, conditions like high process temperature in the range between 400°C and 500°C, the dielectric properties of the resulting bond frames and high bond frame width often prevent a footprint reduction of the microsystems.
As an alternative, wafer bonding processes using pure metal intermediate layers manufactured by physical vapour deposition or electrochemical deposition can reduce the bond frame width. However, typical bonding conditions are high process temperatures in the range between 300°C and 500°C and high bonding pressures up to 800 MPa, assumed that a high surface quality is given.
With the usage of metallic nano particles, both bonding temperature and pressure can be dramatically reduced. In addition, demands on surface properties are low and the potential for 3D-integration on chip and wafer level is high. If the pore size after sintering is small enough, a hermetic bond could be enabled. The intermediate layers can be deposited using printing processes like screen printing or aerosol jet printing.
This work will give an overview regarding state of the art approaches, describes capabilities of the additive material transfer technologies aerosol jet printing and screen printing with the purpose of intermediate layers manufactured for chip or wafer level bonding (see Figure 1). For realization of these applications, the deposition process and the related material morphology, the bonding with nano particle based intermediate layers as well as the characterization methods are described.
Screen printed SnAgCu and Au micro- and nanoparticle intermediate layer were used to bond Au and Cu metallized wafers. The bonding experiments are performed using Cu and Au plated 6” silicon, as well as glass wafer in a standard wafer bonder. Different adhesion layers, layer thicknesses, as well as process parameters (pressure, atmosphere and bonding time) and their influences on the resulting bonding strength are investigated. The bonding capability using different widths of bonding frames on structured wafers is evaluated.
The interface between nano particles and substrate was investigated. For bonding result evaluation, bonding frames and occurring bonding interfaces were analysed 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.
Our results and international publications shows that successful bonds are possible by direct printed sub-micron Au and SnAgCu intermediate layers. But it is not possible to create hermetic bonds because the pore size is too high or flux is needed to obtain an oxide-free surface. An alternative is to use the so-called fluxless soldering, using gaeous flux (formic acid).
Increasing bonding pressure correlates to higher tensile strength for the sintered bond frames. Compared to glass frit bonding, the sub-micron Au particle interface enables 3 times higher bond strength at bonding temperatures as low as 200°C. Grain growth and densification could be enhanced by increased bonding pressure. Results are strongly related to the printed pattern morphology. The process is capable to be adapted to chip level or can 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.

Figure 1