EVOLUTION OF COATING PROCESSES
Meeting Special Optical Requirements
Today’s coating processes provide the capability to produce anti-reflective (AR) coatings, bandpass filters, separation filters, high laser damage threshold coatings, mirror coatings, etc. with high precision and high yields. The production environment is based on batch-coating processes and component sizes that range from mm to meter. Generally, one function such as limited wavelength coverage is provided by the standard coating process.
The desire to incorporate optical coatings at the wafer-level stage was inspired by semiconductor manufacturing practices, and has led to an evolution in specialty optics requirements. Development of processes that would enable complex multi-layer coatings to be incorporated into wafer manufacture called for new thinking. It allowed for the reduction of deposition temperatures and incorporation of photolithographic steps to define smaller coated areas.
Working at the wafer level can mean significant cost reduction as many devices can be handled at once. A single wafer can incorporate as many as 10s to 1000s of devices. Adding complex precision optical filters directly to the detector eliminates handling, weight, thickness and part count, but is not without peril. The spectral filter is one of the last process steps and hence the wafers have a high value compared to a clean piece of glass. Frequently, the wafer has areas that need to be protected from optical coating and patterning is required. Often the structures are not coplanar, complicating the photolithography and deposition.
In addition to the spectral requirements, the cosmetic specifications for imaging applications are rigorous. When filters are placed in close proximity, or directly on a pixilated detector such as a micro bolometer or a CCD, any defect that is comparable in size to that of the detector element results in loss of information. In Materion’s clean room facility in Westford, MA, they typically coat wafers up to 200 mm for the Ultraviolet, the Visible and the Long Wave Infrared spectral regions. The filters range from antireflection coatings of a few layers to complex multiband filters of hundreds of layers. Once the filters have been applied, the wafers can be bonded, producing a large number of hermetic packages that can be diced into finished devices.
In early 2000, promise of high density, high speed communications and data networks inspired the development of DWDM (dense wavelength division multiplexing) filter deposition and subsequent mm-sized dicing, with associated mounting and testing techniques. Currently, the proliferation of miniaturized spectral analytical instrumentation requirements and their expanding applications have inspired the introduction of wafer-level manufacturing processes for band-pass filters. This satisfies the needs of low-cost miniature non-scanning spectrometer applications.