Dichroic Filter Array Patented Patterned Coatings Technology

Today's optical coatings technologies have been miniaturized and optimized like never before possible with traditional optical design and manufacturing methods. Now, innovative techniques pioneered by Ocean Optics are ushering in a new generation of optical filter technologies.

Traditional Filter Technologies
Until recently, entire optical surfaces could be coated quite easily with a bandpass filter, although precise deposition of patterned optical filter coatings was limited to the use of metal masking. Metal masking can withstand the in-vacuum process heat needed to produce durable dielectric multi-layers, but is expensive to manufacture, difficult to align with the substrate, and unable to produce a deposited pattern that can be cleanly aligned to existing patterns -- i.e., edge-to-edge, without gaps or overlapping.

Other coatings technologies employed to transmit and reflect wavelengths of light are similarly limited. For example, the dicing and bonding of individual filters to form an assembly is a tedious process at best, with miniaturization limited by materials handling and dicing constraints. Also, materials such as gels and colored glass are not very robust and may not provide adequate transmission or blocking efficiency for some applications.

Optical Coatings with Microlithographic Precision
Ocean Optics has pioneered an optical coatings production methodology that combines modern optical thin film deposition techniques with microlithographic procedures. This patented process enables micron-scale precision patterning of optical thin film dichroic coatings on a single substrate. (A dichroic filter selectively transmits light according to its wavelength.)

With its new process, Ocean Optics can create multi-patterned arrays of different optical filters for use in dense wavelength division multiplexers, micromechanical devices (commonly referred to as MEMS) and optical waveguide-based devices. The process also can be applied to multi-part bonded filter applications common to the manufacture of digital data projectors and CCD camera detectors. In fact, a wide variety of optical coatings can be patterned, including dielectric multi-layer reflectors, bandpass filters, dichroic edge filters and broadband anti-reflection coatings. In addition, the Ocean Optics technique can be used to deposit enhanced metal reflectors, low-reflectivity opaque metals and electrically conductive transparent patterns.

The Ocean Optics Process
The production of a patterned optical multi-layer coated element begins with application of photoresist to the surface. To generate a pattern, we align the mask and then expose and develop the photoresist. This creates a resist pattern on the coating surface. Next, the prepared substrates are placed into a vacuum chamber for controlled deposition of the multi-layer coating.

After deposition of the filter, the patterned coating is rinsed in solvent, which removes any unwanted multi-layer and resist and leaves the desired patterned filter coating. This sequence can be repeated as needed, allowing multiple filters to be deposited.

Advantages of the Precision Patterning Process
Because the Ocean Optics process relies on precision microlithography instead of cut metal masks to pattern the deposited coatings, features (coated areas) as small as 2 µm can be produced, with spatial registration to within 1 µm. There are other advantages as well:

Undesired "shadowing" or thickness drop-off of the coating at pattern edges -- unavoidable with cut masks -- is eliminated due to the pattern edge break produced in the lift-off process. Intricate coating patterns of any shape or size can be manufactured without the machining limits inherent to metal masks. The cost of microlithographic tooling does not increase significantly with pattern complexity. Cut metal masks are less durable than microlithographic tooling. Metal masks must be cleaned often to remove coating deposits, and can be easily damaged in handling.

Patterned dichroic filter coatings have optical and physical properties comparable to traditional non-patterned coatings, and have very high resistance to humidity and temperature. And because the coatings are applied directly to substrates and devices, their mechanical resistance to shock and vibration is an improvement over commonly used bonded discrete filter windows.

Implications for Design Engineers
Photonics technologies are becoming smaller and more commonly integrated into microelectronic and micromechanical systems. With our dichroic filter-making process, engineers can integrate wavelength-selective filtering structures much earlier into the design process. What's more, because the ion-assisted deposition process delivers multi-layer optical structures directly onto a component, it is no longer necessary to make a physical transition from micron-sized, on-chip structures to macroscopic diced and bonded optical filter elements -- and back again -- for wavelength-selective integrated optoelectronic and optomechanical devices.

For example, optical bandpass filters can be deposited directly onto waveguide structures or active photodetector regions, to create microscopic wavelength-selective detectors. This has major implications for the future of Ocean Optics' core spectroscopy and optical-sensing technologies, with new systems such as grating-less spectrometers and multi-function fiber optic sensors resulting from a marriage of technologies.

That's just the tip of the iceberg. Multi-layer RGB color filters can be produced as a single filter array for devices such as CCD camera detectors and LCD display panels, or as a rotating filter wheel for projection display applications. Selected bandpass filters can be combined in arrays for use with multi-spectral detector systems, or patterned in a ring structure in industrial and medical fiber optic instruments. Also, patterned bandpass filters can be used in dense wavelength division multiplexer and photonics-based microprocessor applications. Optical filter coatings can be deposited onto MEMS structures to form "tunable" filter elements, waveguide relays, and switches (from deposited reflector and beamsplitter coatings) on micromechanical mounts.