Direct bonding of organic polymeric materials

Direct bonding of organic polymeric materials can be realized when their surfaces are prepared in such a way that they are clean, smooth and susceptible to direct-bonding. In the surface-preparation process, tribo-chemical polishing is an essential step. Polymeric materials such as polymethylmethacrylate (PMMA), polyarylate, polyimide and polycarbonate were bonded either to themselves, to another polymer or to an inorganic material such as silicon or fused silica. The surfacial bond energy of the room temperature bond is surprisingly high: 0.1–0.2 J/m². Heating strengthens the direct bond; for example, for a bonded PMMA/PMMA wafer pair annealed at the glass-transition temperature of PMMA (105°C), the surfacial bond strength increases to 7.8 J/m². This indicates that the bonded surfaces are fused and are interlinked by chemical bonds. When polymers are bonded to low-thermal-expansion materials such as Si and fused silica, during annealing treatments, thermal stresses can induce fracturing of the inorganic part of the bonded wafer pair. By limiting the maximum annealing temperature or the size of the bonded area, fracturing can be avoided.