molecules is activated over different annealing steps, so called post exposure bake (peb). Using SU-8 can achieve a high bonding yield. In addition, the substrate flatness, clean room conditions and the wettability of the surface are important factors to achieve good bond results. Procedural steps The standard process (compare to figure "Schematic bonding process") consists of applying SU-8 on the top wafer by spin-on or spray-on of thin layers (3 to 100 µm). Subsequently, the structuring of the photo-resist using direct UV light exposure is applied but can also be achieved through deep reactive-ion-etching (DRIE). During coating and structuring of the SU-8 the tempering steps before and after exposure have to be considered. Based on thermal layer stress the risk of crack formation exists. While coating the photoresist the formation of voids due to the layer thickness inhomogeneity has to be avoided. The adhesive layer thickness should be larger than the flatness imperfection of the wafer to establish good contact. The procedural steps based on a typical example are: Cleaning Top wafer Thermal oxidation Dehydration Spin coating the SU-8 Softbake 120 s at 65 °C 300 s to 95 °C Cooling down Exposure with 165 to 200 Post-exposure-bake 2 to 120 min at 50 to 120 °C to room temperature relaxation time development rinsing and dry spinning hard bake at 50 to 150 °C for 5 to 120 min For non-planar wafer surfaces or free standing structures, spin-coating is not a very successful SU-8 deposition method. As a result, spray-on is mainly used on structured wafers. The bonding takes place at the polymerization temperature of SU-8 at approximately 100 °C. The soft-bake allows that high residual solvent content minimizes intrinsic stress and improves cross-linking. The SU-8 layer is patterned using soft contact exposure followed by post-exposure bake. The non-exposed SU-8 is removed by immersing in, e.g. propylene glycol methyl ether acetate (PGMEA). Ensuring void-free bonding a homogeneous layer thickness of the SU-8 over the wafer surface is important (compare to cross section photo). To ensure good contact of the wafer pair a constant pressure between 2.5 and 4.5 bar during bonding is applied. The frames should be kept above the non-flatness value of the wafer, based on the fact that defects usually are caused by the curvature of the wafer. A shear strength of the bonded wafer pair of about 18 to 25 MPa is achievable. Examples Adhesive bonding using