Film cameras were traditionally manufactured by discrete assembly, which means each component was fabricated as an individual item, tested if necessary, and then assembled into the final working product. The advent of solid state imaging did little to change this approach. The film was merely replaced by a light-sensitive electronic component. The only significant change was that the mechanical shutter was rendered obsolete and its action generated electronically within the imager die.
Film and solid state cameras co-existed happily until 2001 when the first mobile phone incorporating a camera was introduced. Solid state cameras are now found in cars, trucks, toys, laptops, net books, machine tools, security systems, etc. In less than 10 years, the number of solid state cameras produced annually has exploded from tens of thousands to billions.
By far the largest market for solid state cameras remains mobile platforms, principally cell phones and compact digital still cameras. These markets are extremely fashion-conscious and the current vogue is for extreme thinness. While the discrete approach to manufacturing camera modules has thus far been able to deliver smaller and cheaper, that may not be possible in the future. The laws of physics limit the dimensions to which the camera module can be shrunk, while there is a finite amount of cost that can be squeezed out of a supply chain that manufactures a plethora of discrete parts and assembles them into a product. The solution to both of these problems is to switch to wafer level manufacturing.
A wafer level camera has three principal components: the image sensor, the housing for the image sensor, and the optical train.
Image sensors used for wafer-level cameras differ from conventional camera sensors in one important regard – pixel dimensions. Camera module height is strongly influenced by pixel size. In modern camera phones, sensors with 1.4μm pixels are common, and most imager companies show roadmaps out to at least 0.9 μm.
One consequence of using imagers with very tiny pixels is that camera module yield can suffer badly from particle contamination. Even a Class 10 clean room environment, working to specification, contains particles that are large enough to cause black pixels if they get lodged on the sensor surface. The solution to this problem is to enclose the sensor die in a protective housing as the very first step of camera module manufacture. Discrete packaging would be prohibitively expensive. However, with approximately 3,000 die on an imager wafer, a wafer level approach results in a package cost of a few cents per die.
Wafer Level Packaging
Wafer-level packaging of image sensors is conducted as follows (Figure 1). First, a picture frame seal of adhesive is placed around the optically active area on each die. Next a cover glass is attached. Finally the wafer is diced to free individually packaged die.
The wafer-level package provides two very important benefits to camera phones. Obviously, the package provides protection from the ambient environment (humidity, salt etc.) and any dirt that does land on the cover glass is removed from the focal plane, causing no defect in the image. Second, the perfectly flat glass lid of the package is an ideal substrate for attaching the camera optics.