Bump deposition techniques for hybrid X-ray detectors

Abstract

Flip-chip bonding is a common method for joining application-specific integrated circuits (ASIC) to pixel sensors in order to build hybrid radiation detectors for X-rays and gamma-rays. STFC-RAL is using two methods for the interconnects between ASIC and sensor pixels. These are either indium bumps which are deposited on ASIC and sensor prior to bonding or alternatively electrically conductive adhesive dots are printed on the sensor pixel array and flip-chip bonded to gold studs attached to each pixel of the ASIC. Conventionally the indium deposition is carried out on wafer-scale using a photolithographic lift-off process. Sensor and ASIC with indium bumps are singulated from wafers afterwards. Some sensor material (e.g. CdZnTe) which is required for high-energy and high-flux X-ray detectors at X-ray Free Electron Lasers (XFEL) or in other scientific experiments is only available as individual die instead of wafers. The stencil printing of conductive epoxy dots onto those sensor dies together with gold ball studding of ASICs is a suitable method for those dies. However, due to the size of printed epoxy dots this method has a limited pixel pitch and is currently only used for sensors with 250 μm-pitch or for larger pitch. A novel method for indium deposition was developed for such dies. A shadow mask with small apertures is optically aligned to the pixel array and mechanically clamped to the sensor die. After indium evaporation onto this assembly and after removal of the mask, indium bumps as small as 50 μm with a height of ∼ 5 μm are deposit onto the pixel array of the sensor. The same is done for a matching ASIC. A comparison of these two methods indicates that indium bumps created by the shadow mask technique are approx. half the size of the epoxy dots and comparable with gold studs. Using this method, at present a pitch of 100 μm for those indium bumps can be achieved and development for further improvement towards smaller pitch is carried out currently. This novel indium deposition method is compared with the conventional wafer-scale indium lift-off method and the epoxy/gold stud flip-chip bonding in terms of interconnect quality, bond yield, and suitability for hybrid radiation detectors.