The Evolution of Pixel Structures for Consumer-Grade Image Sensors

Materials integration in semiconductors, wafer fabrication process development, and device packaging have seemingly evolved at an exponential pace over the last decade. While microprocessor and memory chip manufacturers are the leading drivers of innovation, several other technology sectors benefit from the technologies that enable Moore's law scaling. Image sensor manufacturers, in particular, have realized many advancements from the selective use of advanced wafer fabrication and packaging developments. The motivations for the imaging industry to pursue advanced technology generation scaling are comparable to that of the broader semiconductor industry. In addition, image sensor companies seek a reduction of the camera module form factor, an increase in camera resolution, and an increase in pixel array performance. The pixel size of recent camera phone sensors has shrunk to 1.12 μm. This is about half the pixel size of leading edge devices of six years prior, and yet mobile imaging sensors have dramatically increased in performance. Design innovation continues to have an increasing contribution to the performance of leading edge pixels; however, to date, it has been fabrication process development that has substantially enabled the continuous breakthroughs in digital imaging. Current image sensor fabrication process flows mark a significant departure from conventional CMOS logic processes. Beyond silicon foundry processes, digital imaging companies must also concern themselves with the optical systems, packaging solutions, and image processing chips required to integrate their silicon devices with the consumer electronics supply chain. Chipworks, as a supplier of competitive intelligence to the semiconductor and electronics industries, monitors the evolution of image sensor technologies as they come into production. Chipworks has obtained charge-coupled devices and CMOS image sensor chips from leading manufacturers and performed structural, compositional, and design analyses to benchmark the successful technologies employed by the market leaders.