A Study on Fine-Pitch Convertors for Radiation Detectors with Interposers as an Alternative to Through Silicon Via Technology

Pixel radiation detectors for X-rays and gammarays are commonly built by hybridization of sensor and readout application-specific integrated circuit (ASIC) chips using flip-chip bonding. A typical example is pixelated high-Z semiconductor sensor material (e.g. CdTe) bonded to a matching pixel array on an ASIC to form a hybrid module. Conventionally, the I/O connections of the ASIC for such hybrids are wire bonded to a Printed Circuit Board (PCB) which subsequently is connected to a data acquisition system. The wire bond pads and PCB connections require a considerable amount of the module's footprint and usually protrude at one side of the detector module by approximately 1.5mm. The disadvantage of this practice is that those modules are only suitable for close proximity tiling modules along three sides of the detector (3-side buttable) to form larger sensor areas. In recent years Through-Silicon Via (TSV) technology was introduced to overcome this disadvantage which permits the fabrication of potentially 4-side buttable modules. Commonly a TSV-last process step in the ASIC production is integrated. However, ASICs with TSV are expensive and the TSV-last process step is prone to failure of the TSV metallization. The consequences of the latter are missing I/O connections. Alternatively, 4-side buttable hybrid detector modules can be fabricated using interposers as pitch convertors. Here the sensor has a larger pixel-pitch than the ASIC and therefore the sensor covers a larger area than the ASIC. A Re-Distribution-Layer (RDL) on the interposer facilitates connections to the I/O of the ASIC. Therefore, ASIC and all other wire bonded connections can be placed under the sensor with all four sides of the sensor being the boundary of the full-sized module. This was demonstrated in the past with a conversion from 250um-pitch on the ASIC to a 500um-pitch on the sensor using a layered PCB. In order to achieve detectors with smaller sensor pixel pitch, the layout of pitch re-routing on the interposer has to be considerably reduced in size. As a consequence, the fabrication of such interposers becomes more difficult. This paper studies desirable designs and interposer materials and compares these with the limitations of current fine-pitch interposer fabrication methods and design rules for a pitch convertor interposer. In addition, the paper will also present necessary changes such as indium deposition on individual sensor die instead of wafer-level fabrication that have to be taken into consideration for sensor/ASIC hybridization of fine-pitch radiation detectors.