Polyimide coating on non-planar microelectronic devices: characterization of vacuum drying effects by a new ‘flip–paste’ back-etching method

Abstract

Abstract Many microelectronic devices are non-planar 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 contact and adhesion between the encapsulating coating and the device surface. Nevertheless, characterization of the coating/device boundary is often a difficult task. A new method for direct visualization of the internal surface of a polymer (polyimide in this case), coated on an InP–InGaAs heterojunction bipolar transistor (HBT), was used. This method is based on etching the whole device, leaving a shell of polyimide for subsequent probing by microscopy (optical, scanning electron or any other). The polymer surface was compared with that of the HBT, thus demonstrating the non-perfect filling of horizontal cavities. Replacing thermal drying of the polymer solution film with a vacuum-drying step was found to be very effective in overcoming this problem.