Deposition of Fine-Pitch Indium Bumps on Single Die

Abstract

Photolithographic lift-off at wafer level is a wellestablished method for depositing indium bumps onto pixels of electronic components such as sensors and application-specific integrated circuits (ASIC). These indium bumps form interconnects between pixels of the components during flip-chip bonding to make devices such as radiation detectors. Such components are not always available on wafer-scale, and single dies need to be processed in small-scale production, R&D, or for Cd(Zn)Te detector material. Photolithography on single dies is ineffective due to the edge-bead of photoresist that compromises perfect indium deposition onto pixels near the periphery of the die. To utilize the maximum surface area of a die, a rigid mask with apertures (shadow mask) is an adequate substitute for the photoresist. Different mask designs and production techniques for fine-pitch indium bump arrays are investigated by testing electroformed stencils or micromachined silicon (Si) membranes as shadow masks. For the indium deposition, the sensor and ASIC dies are clamped into custom-made jigs that include shadow masks. Alternatively, dies can be adhered to shadow masks with soluble adhesive. The apertures of a mask are aligned to the pixel array of the detector component. Bump arrays with 250μm- and 100μm-pitch were demonstrated with electroformed stencils. For smaller pitch (55μm) and bump diameter of ~20μm, specialized masks from Si membranes are tested. Components with successfully deposited indium bump arrays were subsequently flip-chip bonded at room temperature without reflowing the indium. A bond yield of at least 99.9% pixels for such flip-chip bonded radiation detector was shown using the read-out signal from exposure with a flat field X-ray radiation of an Am-241 sealed source.