Much of the revolution that took place in the electronics industry in the 20^th century is due to the innovation of the integrated circuit when discrete devices were replaced with an entirely new method wherein multiple devices are fabricated in close proximity while remaining individual entities. The 21^st century may see a similar revolution by applying integration techniques to optics. Like integrated semiconductors, integrated optics can deliver substantial economies of scale, consistency of reproducibility and performance, and new functionality that can not be obtained from discrete optics. An example of this is the ability of diffractive optic elements to distribute light over a very wide space and/or in a precisely defined pattern at very high efficiency. The challenge is whether wafer-level optics can be used to reduce the cost and form factor of camera modules and, ideally, improve performance at the same time.

Wafer-scale packaging of imagers is economically attractive because the fixed process costs are divided among the number of good parts on the wafer. The same argument holds for manufacturing lenses at the wafer scale. By using several wafers of lenses, these can be easily and accurately stacked and bonded together. These stacks can then be singulated. The net result is an optical part with the same functionality as a conventional camera optic, but with greatly reduced costs and very precise and reproducible alignment.

Switching to wafer-scale manufacturing techniques provides new freedoms in materials choice and lens shape. The materials used to make wafer-scale lenses can have a higher refractive index than injection-molded lenses, whereby aiding size reduction. The materials chosen for the construction of the lenses can be selected for high temperature durability, facilitating surface mounting of the camera module and accompanying cost benefits. The precision of layer-to-layer and rotational alignment made possible by wafer-scale manufacturing means the lenses can have re-entrant profiles, diffractive surface features and be asymmetric in shape to match the rectangular format of the imager. These advances in optics permit the lens stack to be reduced in height without sacrificing optical performance.

Wafer-Level Camera Modules
While wafer-level packaging of imager die was originally developed as a means to decrease the cost of camera modules (by eliminating assembly yield loss) a highly fortuitous result is that the cover glass provides an exceptionally uniform surface, spaced an exact distance from the imager die. It therefore makes an ideal substrate or platform onto which a wafer-level optical stack can be attached. Owing to the high precision of the wafer-level imager package and also the wafer-level lens stack, they can be permanently joined without the need for costly, live adjustment of focus. The absence of moving parts and small size makes for an extremely robust product. Because the wafer-level stack has a considerably lower profile than the housing and lens turret it replaces, the resultant wafer-level camera module is appreciably smaller. This is clearly evident in Figure 5, which shows a VGA wafer-level camera with a camera module built from discrete parts. The wafer-level camera is 30 to 50 percent less expensive, has a form factor 50 percent smaller and is surface mountable.

Innovation in imager and optics wafer-level packaging has lead to the development of extremely compact camera modules that can be manufactured at low cost. However, these developments have not significantly altered the optical performance of the camera module. The means by which this can be accomplished are discussed in Part 3 of this article series.

About the Author
Giles Humpston, Ph.D., serves as Director, Research and Development of Tessera. Dr. Humpston has spent his entire professional career working in the field of semiconductor packaging, initially for military applications and more recently for high volume consumer products. He is a metallurgist by profession and has a doctorate in alloy phase equilibria. Dr. Humpston is a cited inventor on more than 75 patents and has co-authored several textbooks on metallic joining processes. His work and technical publications have been recognized by five international awards. Dr. Humpston's current interests are packaging of solid state camera modules and product miniaturization through wafer level technologies.