Wafer Scale Integrated Optics
By restricting the camera optics to just one element it is possible to switch the lens manufacture to wafer scale processing and realize a fully integrated optical component.

Wafer scale assembly is inherently much cheaper than discrete assembly because the materials and process cost are divided among the good parts on the wafer, which can number many thousands on a 200mm diameter wafer.

By using semiconductor-based processes and equipment the optics can be made with sufficient precision that they can be mated with the imager without the need for manual adjustment of focus, resulting in significant savings in assembly cost.

The optical element is typically fabricated in two parts by replication processes, where an optical surface is formed on each side a flat substrate. This does not mean that the substrate has to be optically passive. For example, it can provide a filter action like infra-red cut. Another possibility is to apply to the substrate a thin layer of metal, containing a hole, which then functions as an aperture

Alternatively, both apertures and infra-red filter coatings can be applied to one or both of the optical surfaces. The net result is a completely integrated optical component that has all the functionality necessary for a solid state camera, an example of which is shown in Figure 2 below.

Figure 2. Exploded and (inset) perspective view of an integrated optical component. It comprises two optical surfaces, aperture disc and a substrate incorporating an infra-red filter. Note the two lenses can have different diameter and may be made of different materials. This type of optical is expensive to manufacture using discrete components but is easily realised in high volume at low cost using wafer scale manufacturing techniques. Source: Tessera.

As a result of wafer level manufacturing, the lens is compact, rugged, low cost, with highly predictable optical performance and negligible part-to-part variation.

Through judicious choice of the materials chosen for the integrated optical component, it is possible for the VGA single lens camera to survive the lead-free solder reflow thermal cycle. Conventional camera modules are unable to withstand this thermal excursion and must be interfaced to the phone by a flexible lead and connector.

Figure 3. Example of an integrated optical component for a VGA single lens camera, manufactured at the wafer scale. Source: Tessera.

Notably, this arrangement is mechanically not robust and interconnect failures are one of the leading causes of camera phone returns. A reflowable camera (Figure 3, above) can be attached and interconnected to the main printed circuit board of the handset at the same time as the components using a standard surface mounting process.

The benefits are improved reliability and decreased piece part and assembly costs. This interconnect technology is best suited to small die with low numbers of interconnects, so favoring VGA over higher resolution cameras.

Conclusions
Future growth in mobile phone handset volume will come from the secondary and tertiary markets. In these geographic areas, the way the phones are used and the expectations of consumers are very different. In particularly, the majority of still and video images are only ever viewed on the screen of a handset.

The low resolution of these screens makes it possible to engineer compatible VGA format cameras using a single lens. Because handset cost is a key factor in this subscriber base, conventional camera module manufacturing techniques are inappropriate.

Instead, by using wafer scale manufacturing it is possible to produce a fully integrated optical component containing two optical surfaces, aperture and infra-red filter. This can be married to an image sensor to realize a reflowable camera module that is extremely compact, robust and above all, very low cost.

Humpston is Director, Applications and Yehudit Dagan is VP Marketing, Wafer-Level Camera, at Tessera.