New method for observation of polyimide adhesion on non-planar surfaces

Many microelectronic devices are nonplanar structures with a complex three-dimensional geometry that includes steps and cavities. The reliability of the electrical properties of such devices depends heavily on good adhesion between the encapsulating coating and the device surface. Nevertheless, characterization of coating-device boundary is often a difficult problem. A method for direct visualization of the internal surface of a polymer (polyimide in this case) coated on an InP-InGaAs bipolar transistor (HBT) is presented. This method is based on etching the whole device, leaving a shell of polyimide for subsequent probing by microscopy (optical, SEM or any other method). The polymer surface is compared with the HBT, thus demonstrating the nonperfect filling of horizontal cavities. The described method is generic and can be implied to detect coating defects in a large variety of microelectronic devices