Sony Curves Images Sensors & TSMC Stacks Them

Two papers at an upcoming engineering conference promise to take image sensors in new directions.

The image sensor market leader Sony Corp. is due to report on an improved CMOS image sensor that uses a curved substrate to improve the image fidelity and reduce the dark current. At the same event, the Symposium on VLSI Technology, which takes place June 9-12 in Honolulu,engineers from TSMC will report on a CMOS image sensor with a 3D stacked architecture.

Sony's curved sensor is back side illuminated, with the curvature matched to the curved depth of field that comes from an integrated lens close to the surface of the chip. The use of a flexed substrate doubles the light sensitivity at the edge of the image and increases it at the center by a factor of 1.4, according to the abstract of the paper. This provides options to relax the lens design in terms of F number. At the same time, the tensile stress of curving the substrate widens the energy band gap and thereby lowers the dark current.
TSMC's paper reports on replacing the conventional BSI carrier wafer with an ASIC wafer, which contains a part of periphery circuit and is connected to the sensor wafer through bonding technology. The engineers are due to report that, with appropriate layout design and process control, the impact of through-silicon-vias (TSVs) on the 1.1-micron BSI CMOS image sensor's performance can be minimized.
The abstract to paper 21.3 states that the stacked sensor exhibits comparable pixel performance to conventional BSI. It points out that this allows separate optimization of the sensor processes and should lead to improvement of dark current performance.