For some applications or process steps in microsystems technology, the wafers being used are so large and thin in the meantime that they would break. What is known as temporary wafer bonding is used in these cases. For example, silicon wafers are secured on a handling wafer for processing. Subsequently, it must be possible to separate them again with no mechanical damage. Polyelectrolyte intermediate layers are used at Fraunhofer IST to control the adhesion of the wafers.
Bonding with conventional adhesive films
Microsystems technology is the leading field of application for temporary bonding. In particular, selecting the right adhesive is of crucial importance here. Both the mechanical properties and the thermal and chemical stability of the adhesive have to meet the material requirements that apply for subsequent processing.
Intermediate layers made of waxes and thermoplastic materials as well as UV-activated adhesive films have proven themselves for temporary bonding. Usually the films are applied by spin coating or rolled on as laminates. The adhesive force can be controlled by increasing the temperature or using UV radiation so that the wafers can be mechanically separated again afterwards. The separation process becomes more demanding as the wafers get thinner. Elaborate cleaning of the wafers is generally required prior to further processing.
Bonding with polyelectrolyte intermediate layers
Coatings using polyelectrolytes have been developed at Fraunhofer IST in cooperation with the Institute for Surface Technology of Braunschweig Technical University. They are suitable for temporary bonding and replace the previously mentioned adhesive layers and adhesive films. Polyelectrolytes are polymers with positively or negatively charged ionic groups. In watery solutions, they generally very highly adsorbing on surfaces with opposite charges. This creates single-layer polyelectrolyte layers. Polyelectrolyte multilayers (PEM) are formed through the successive adsorption of positively and negatively charged polyelectrolytes. The benefit of using polyelectrolytes is that they only cover the surfaces as monolayers, which means they grow up evenly and are easy to remove again.
Separation by tempering
Temperature-specific studies of bonding strength on silicon wafers with polyelectrolyte intermediate layers have shown that the strength increases with the temperature. But when a certain temperature is exceeded, the strength drops sharply and remains low enough even after cooling that the wafers can be easily separated. This strength can be adjusted by selecting the temperature for tempering in the range of 100 °C to 250 °C. In a first tempering step, substrate wafers can be firmly bonded to a carrier wafer and then processed. Subsequently, the wafers can be easily separated at room temperature following a second tempering step.
Future work focuses on controlling the separating process across many different temperature ranges by using other polyelectrolyte combinations. These are to be optimized for different wafer materials. Fundamental studies to understand the separating process are planned as well.