Solid state image sensors require protection from the ambient atmosphere. First-generation imagers were housed in standard semiconductor packages with glass lids. These were hermetic and delivered exceptional protection for the die, but were also bulky and expensive to manufacture. The introduction of wafer-level packaging, where the costs of the fabrication processes are shared among the good die on a wafer, resulted in a substantial reduction in cost and nearly an order of magnitude reduction in package thickness. Innovation in materials, assembly processes and semiconductor technology has enabled these trends to continue. Modern solid state imagers are commodity items, having a total package thickness of just a few hundred microns, yet they will pass automotive reliability specifications.
Solid State Imagers
Solid state imagers are fairly conventional semiconductor die, built on silicon wafers, where each die has a light-sensitive area. Image sensor die require protection from the ambient atmosphere in order to ensure their longevity. The principal failure mechanisms are corrosion, mechanical damage and obscuration.
Air, or more accurately the moisture contained in normal atmospheric air, is highly corrosive towards semiconductor die. Functional semiconductors are complex, multi-layer assemblies, where the outer most layers are very fine bus bars of aluminium. Aluminium possesses a very high electrode potential so that rapid corrosion ensues when placed in contact with other metals and in the presence of moisture to complete a galvanic circuit. The external interconnects to the semiconductor are, by necessity of function, made of metals other than aluminium, so it is essential to keep moisture away from image sensor die at all times.
The light-sensitive area of an imager is covered by an array of minute, hemispherical lenses. These lenses serve to focus the light that falls on each picture element (pixel) in the imaging area onto the light-sensitive regions of the semiconductor. The remaining area of the imager is not sensitive to light, because there are some electronics and wiring lines associated with each pixel. These micro-lenses are made of soft polymers and are extremely delicate. Any physical contact would result in catastrophic damage to them.
On a modern solid state imager, individual pixels will typically measure 3μm on a side or smaller. This is tiny in comparison with dust particles in normal ambient air. The micro lenses that sit over each pixel are not only soft and easily damaged, but are also slightly tacky, due to a combination of electrostatic charging and their surface chemistry. This means it is neigh impossible to remove a dust particle once it lands on a pixel. Clearly, if the size of the dust particle approaches that of the pixel, it will block the incident light, resulting in a black pixel in the image. The human eye is very sensitive to static defects in an image, and even a single dead pixel is an annoyance.
One effective solution to all the above failure mechanisms is to house each imager in a package. The package is sealed to prevent moisture from reaching the die, while a glass lid allows the light passage to the sensitive area, but protects against mechanical damage and particle contamination.
Next: Ceramic Image Sensor Packaging