Highly accurate optical interference filters can be manufactured using thin film deposition processes. These optical interference filters are used for multivariate optical computing, multiple-band-pass, and the like and can exhibit complex optical spectra defined over a range of wavelengths. These filters are typically constructed by depositing alternating layers of transparent materials where one layer possesses a much larger refractive index relative to the other layer. Theoretically, the proper choice of composition, thickness and quantity of layers could result in a device with any desired transmission spectrum.
Among the simplest devices is the single cavity bandpass filter; i.e., the thin-film form of an etalon. This device consists of three sets of layers. The first stack is a dielectric mirror, a next thicker layer forms a spacer, and a second stack forms another dielectric mirror. The mirror stacks are typically fabricated by depositing alternating transparent materials that have an optical thickness that is one quarter of the optical wavelength of light.
To achieve theoretical optical performance, each layer must possess a precise and specific physical thickness and refractive index. Any nonuniformity in the deposition of the layers can affect the spectral placement and transmission or reflection characteristics of the device. A design that requires very tight manufacturing tolerances over large substrate areas could result in the costly rejection of many devices. Given these manufacturing limits, it would be desirable to analyze the devices after construction and alter the devices that do not meet a predetermined optical transmission or reflection specification by some electrical or mechanical means. For example, if the peak transmission wavelength of a manufactured optical bandpass cavity filter was slightly out of tolerance, it would be desirable to have a mechanism or process for shifting the peak back to the desired spectral location. It is also desirable that the optical filters have precise rejection bands and passbands that are electrically or mechanically selectable.
Mechanical methods of achieving a variable transmission spectrum device are well known. This includes changing a prism or grating angle, or altering the optical spacing between mirrors of an etalon. To overcome the performance, size and cost disadvantages of using mechanical schemes, many have conceived of electrical methods for varying a transmission spectrum. For example, U.S. Pat. No. 5,150,236, issued Sep. 22, 1992 to Patel, discloses a tunable liquid crystal etalon filter. The liquid crystal fills the space between dielectric mirrors. Electrodes on the mirrors are used to apply an electric field, which changes the orientation of the liquid crystal that changes the optical length for tuning. The change in the optical length corresponds to a change in the location of the passband. U.S. Pat. No. 5,103,340, issued Apr. 7, 1992 to Done et al., discloses piezoelectric elements placed outside the optical path that are used to change the spacing between cascaded cavity filters. Furthermore, U.S. Pat. No. 5,799,231, issued Aug. 25, 1998 to Gates et al., discloses a variable index distributed mirror. This is a dielectric mirror with half of the layers having a variable refractive index that is matched to other layers. Changing the applied field increases the index difference that increases the reflectance. The mathematics that describes the transmission characteristics of multilayer films composed of electro-optic and dielectric materials are well known.
Another electrically actuated thin film optical filter uses a series of crossed polarizers and liquid crystalline layers that allow electrical controls to vary the amount of polarization rotation in the liquid by applying an electric field in such a way that some wavelengths are selectively transmitted. However, these electrically actuated thin film optical filters have the characteristic that the light must be polarized and that the frequencies of light not passed are absorbed, not reflected. Another electrically actuated thin film optical device is the tunable liquid crystal etalon optical filter. The tunable liquid crystal etalon optical filter uses a liquid crystal between two dielectric mirrors.
The common cavity filter, such as the etalon optical filter, is an optical filter with one or more spacer layers that are deposited in the stack and define the wavelength of the rejection and pass bands. The optical thickness of the film defines the placement of the passband. U.S. Pat. No. 5,710,655, issued Jan. 20, 1998 to Rumbaugh et al., discloses a cavity thickness compensated etalon filter.
In the tunable liquid crystal etalon optical filter, an electric field is applied to the liquid crystal that changes the optical length between the two mirrors so as to change the passband of the etalon. Still another tunable optical filter device tunes the passband by using piezoelectric elements to mechanically change the physical spacing between minors of an etalon filter.
Bulk dielectrics are made by subtractive methods like polishing from a larger piece; whereas thin film layer are made by additive methods like vapor or liquid phase deposition. A bulk optical dielectric, e.g., greater than ten microns, disposed between metal or dielectric mirrors suffers from excessive manufacturing tolerances and costs. Moreover, the bulk material provides unpredictable, imprecise, irregular, or otherwise undesirable passbands. These electrical and mechanical optical filters disadvantageously do not provide precise rejection bands and passbands that are repeatably manufactured.
In an attempt to avoid some of the foregoing problems, modeling of interference filters can be conducted during on-line fabrication with in-situ optical spectroscopy of the filter during deposition. The current state of the art for on-line correction of the deposition involves fitting the observed spectra to a multilayer model composed of “ideal” films based on a model for each film. The resulting model spectra are approximations of the actual spectra. To use reflectance as an example: the measured reflectance of a stack of films can be approximately matched to a theoretical reflectance spectrum by modeling. Layers remaining to be deposited can then be adjusted to compensate for errors in the film stack already deposited, provided the film stack has been accurately modeled. However, films vary in ways that cannot be readily modelled using any fixed or simple physical model. Heterogeneities in the films that cannot be predicted or compensated by this method cause the observed spectra to deviate more and more from the model. This makes continued automatic deposition very difficult; complex film stacks are therefore very operator-intensive and have a high failure rate. To improve efficiency in fabrication, laboratories that fabricate these stacks strive to make their films as perfectly as possible so the models are as accurate as possible.
As outlined above, many thin films are usually designed in a stack to produce complex spectra and small variations in deposition conditions make it difficult to accurately model in situ film spectra for feedback control of a continuous deposition process because it is practically impossible to obtain full knowledge of the detailed structure of the stack from reflectance, transmittance, ellipsometry, mass balance or other methods. Thus, a thin film interference filter is needed that is less difficult to manufacture, which will address varying refractive indices of thin films and varying absorptions with deposition parameters.