Ultra fine pitch hybridization of large imaging detectors

Abstract

LETI has always played a major role in the development of Mercury Cadmium, Telluride (CMT) detectors, and in the flip-chip technology that goes with the packaging of such infrared arrays. Today with the increasing demand for better spatial resolution we observe the emergence of very large arrays detectors with a very fine pitch. However the hybridization of such fine pitch detectors onto a readout circuit may be very challenging and represents one of the major limiting factors for the development of these detectors. At LETI we have developed a wafer scale flip chip technology to perform large Infrared Focal Plane Arrays (IRFPA) up to 1024x1024 pixels with pitches down to 15 µm. In this paper we describe this hybridization technique which includes bonding, underfilling and polishing steps. The detectors are interconnected to a CMOS readout circuit using very uniform Indium bumps, then all hybridized onto a wafer in one step using a collective method. With our expertise we achieve very high interconnection efficiency, good functionality and a low cost package. We are also presenting reliability results and the optical IR performance of the detectors to further prove the effectiveness of this flip-chip technology. With the emergence of new packaging challenges regarding size, material, and performance this technology may be used for other applications.

Thin film charged particle detectors

“…The pitch limit for PbSn solder bumping is in the range of 10 ∼ 15μm, due to the precision of galvanic process as well as the mask alignment. The pitch limit for indium bumps is slightly lower [26]. Hybrid pixel technology has access to high-density electronic circuitry, where hundreds of transistors are connected to each pixel.…”

Reflow flip-chip bonding technology for infrared detectors

“…During this process, the dimensions of the UBM and the indium bump are the main factors to determine the morphology of the indium balls. An homogeneous indium ball array plays a major role in interconnection efficiency [7].…”

“…The fatigue life of new reflow flip-chip bonding ( − N f R ) is approximately four times longer than that of thermo-compression ( − N f T ), according to equation (3): f T f R (7) Roughly speaking, the new reflow flip-chip bonding will definitely surpass thermo-compression in terms of reliability.…”

“…And both techniques present specific issues regarding hybridization yield, most of which are related to planarity and warping of the devices to be hybridized [6]. In order to compensate for parallelism or local non-planarity defects, the Integration of Components Laboratory of LETI [7] has developed a reflow process which uses hot welding of indium bumps that presents major advantages to achieve true welding between each pixel of the detector and the readout circuit. The process benefits from the self-alignment of the two components due to capillary forces and leads to a very high interconnection yield, close to 100% on large IRFPAs with 15 μm pitches.…”