Dichroic filters with features as small as 5 µm show better alignment and temperature resistance than devices made with a metal-masking process.
Jim Lane and Philip Buchsbaum
It is common for optical-processing technologies to use techniques and equipment from the domain of microelectronics fabrication. Many times, "last year's" wafer-fabrication process or deposition technique has been adapted with great success by optical technologists, but this has largely been a unidirectional knowledge transfer. Nonetheless, this assimilation has enabled the design and production of new optical systems that have been miniaturized and optimized to a degree never before possible by traditional optical design and manufacturing methods.
Until recently, however, optical coatings has been one area that existed primarily in the macro realm; entire optical surfaces could be coated quite easily with a bandpass filter, for instance, but precise deposition of patterned optical-filter coatings was limited by the use of metal masking. This masking could easily withstand the in-vacuum process heat needed to produce hard, durable dielectric multilayers, but was expensive to manufacture, difficult to align with the substrate, and unable to produce a deposited pattern that could be cleanly aligned edge-to-edge with existing patterns without gaps or overlapping. Similarly, the dicing and bonding together of individual filters to form an assembly is a tedious process at best, with miniaturization limited by handling and dicing constraints.
Now, however, a new production methodology combines modern optical thin-film-deposition techniques and microlithographic procedures, enabling the precision placement and patterning of optical thin-film coatings on a single substrate. This unified process allows for multipatterned arrays of different optical filters for such applications as dense wavelength-division multiplexers, microelectromechanical system (MEMS) devices, and optical waveguide-based devices. The technology also can improve on existing multipart bonded filter applications in products such as digital data projectors and charge-coupled-device (CCD) cameras (see Fig. 1).
FIGURE 1. Devices that can be made with patterned filter coatings include a monolithic filter wheel (background) and (clockwise from top left) a patterned full-color photographic image for projection, a substrate with several color filters for a charge-coupled-device camera, and a red-green-blue dichroic filter array for liquid crystal displays.
A wide variety of optical coatings can be patterned, including all dielectric multilayer reflectors, bandpass filters, dichroic edge filters, and broadband antireflection coatings. In addition, enhanced metal reflectors, low-reflectivity opaque metals, and electrically conductive transparent patterns can be deposited by this technique.
Production process
The production of a patterned optical multilayer coated element begins with application of a release layer to the surface to which a photoresist layer is then applied. Generation of a pattern is performed via mask alignment, photoresist exposure, and development, creating a release/resist pattern on the surface to be coated. The prepared substrates are placed into a vacuum chamber for deposition of the desired multilayer filter coating, under automatic control. After deposition of the filter, the patterned coating is rinsed in solvent, removing the multilayer and the resist from the unwanted areas. Finally, the release layer is chemically removed, leaving the desired patterned filter coating (see Fig. 2). This sequence can be repeated as desired, allowing multiple filters to be deposited.
FIGURE 2. To make a patterned filter, a photo-release layer and photoresist are applied to a substrate and then exposed to light, the dichroic coating is applied, and the unwanted layers are stripped away to expose the finished pattern.
There are several advantages derived from the use of this process to generate patterned optical multilayer coatings. Because the process relies on precision microlithography instead of cut metal masks to pattern the deposited coatings, features (coated areas) as small as 5 µm can be produced, with spatial registration to adjacent coated areas within 1 µm. Undesired "shadowing" or thickness dropoff of the coating at the pattern edges, unavoidable with cut masks, is eliminated due to the clean pattern edge break produced by the liftoff process. Intricate coating patterns of any shape or size can be manufactured without the limits imposed by practical machining limits of the metal masks. In addition, the cost of lithographic tooling does not increase greatly with pattern complexity; lithographic tooling has a longer usable life than cut metal masks, which must be cleaned often to remove coating deposits, and can be easily damaged during loading, unloading, and storage.
The patterned dichroic-filter coatings exhibit optical and physical properties similar to those of their traditional, nonpatterned counterparts. The patterned films have a very high inherent resistance to environmental conditions such as humidity and temperature. Because the optical-filter coatings are directly applied to the substrate or device, mechanical resistance to shock and vibration is improved over bonded discrete filter windows.
Many devices possible
As optoelectronics technologies are miniaturized and integrated with MEMS and microelectronic systems, the ability to selectively deposit multilayer optical structures directly onto the component enables the design engineer to include wavelength-selective filtering structures early in the design process. 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 or optomechanical devices. Optical bandpass filters, for example, can be directly deposited onto waveguide structures or active photodetector regions to create microscopic wavelength-selective detectors. Multilayer red-green-blue color filters can be produced as a single filter array for use with devices such as CCD cameras and liquid-crystal-display panels.
Selected bandpass filters can be combined in arrays for use with multispectral detector systems or patterned in a ring structure for use with fiberoptic devices in industrial and medical instruments. Patterned bandpass filters also can be used in dense wavelength-division multiplexing 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/or beamsplitter coatings on micromechanical mounts.
This advance in optical-coating technology provides an effective and precise means by which optical thin-film coatings can be integrated into the design and manufacture of optomechanical and optoelectronic devices. Multiple depositions of different filter types and patterns can be aligned with micron-scale precision and repeatability. The similarities to microelectronics-fabrication techniques enable the designer of electronic or optoelectronic circuitry and devices to extend the application base of patterned optical-filter technology into future applications.
JIM LANE is engineering manager and PHILIP BUCHSBAUM is general manager at Ocean Thin Films, 8060A Bryan Dairy Road, Largo, FL 33777; e-mail: [email protected].