Control of Direct Bonding Behavior by Interlayers
Authors: Eichler Marko , Dillmann Helena , Nagel Krees , Klages Claus-Peter 
Published: February 2020

For MEMS manufacturing as well as MEMS operation the surface contact behavior for substrates and integrated components plays an important role. In the area of the bonding frame high bond strengths are required. In MEMS with membranes, gyrating masses, and valves, on the other hand, bond frames are frequently located in close vicinity to areas where anti-sticking properties are needed. The deposition of thin layers allows to adjust adhesion by surface energy or roughness. Using the atmospheric-pressure plasma tool SELECT surfaces can be subjected to a patterned treatment, for example they can be coated locally by PECVD or a specific topography can be generated by plasma etching. A particular challenge is the characterization of the bonding behavior of PECVD layers during and after annealing. Therefore an electrode is used which limits plasma treatment to a 46 mm wide strip with a gap distance (electrode to substrate) of 70 μm over the full length in the middle of a 100 mm wafer. The 46 mm wide plasma-modified strip will be used to characterize the bonding behavior in comparison to non-modified areas beside the strip. The bond strength (surface energy) can be determined by crack opening test in situ during annealing for increasing temperature. Investigations have shown that PECVD coatings of fluorocarbons as well as an etching of the surface with fluorine-containing etching gas enables non-sticking properties. For the treatment of non-structured surfaces in particular wet-chemical processes are suited. An interesting application is the coating of nanometer-thick polyelectrolyte multilayers as interfacial layers. The growth behavior of these layers has an influence on the surface roughness and thus the bonding behavior. Furthermore, in certain cases it could be shown that several ten nanometer thick polyelectrolyte multilayers allow bonding of rough substrates. The pH value of the polyelectrolyte solution determines the growth mode of polyelectrolyte multilayers with weak polyelectrolytes because it influences the degree of ionization. If polyelectrolytes are almost fully charged, only a relatively small bilayer thickness in the order of 1 nm can be achieved during the deposition of films. Larger bilayers thicknesses can be obtained with decreasing degree of ionization of polyelectrolytes. Bonded wafer pairs containing polyelectrolyte interlayers show initially an increased value of surface energy starting at room temperature. Thus they are suitable for low-temperature bonding. Further coatings using polyelectrolytes have been developed which are suitable for temporary bonding of silicon wafer pairs: The surface free energy of the bonded wafer pair increases as temperature rises. However, once a certain temperature is reached, bond strength falls abruptly and remains low even after cool-down, which means that the wafers can be separated easily. The strength level can be controlled within the range between 100 °C and 250 °C by selection of the temperature during annealing. This means that in the first annealing step substrate wafers can be bonded firmly onto a carrier wafer and then processed. Following a second annealing step the wafers can then be easily separated from each other at room temperature.
This paper will discuss new approaches for whole-surface and patterned-surface modifications having the potential to simplify standard MEMS manufacturing process steps.