Coating Materials News Volume 11 Issue 2
DWDM... What's all the Fuss About?
A fleeting topic of discussion within many thin-film coating companies is Dense Wavelength Division Multiplexing filters. Once it is learned what is required to manufacture DWDM bandpass filters, however, the discussion rapidly turns to more manageable tasks. Indeed only a handful of companies are capable of achieving economical manufacturing yield. What’s this all about?
A demand for ever-increasing bandwidth exists and is continually being created in the telecommunications area. Rates of 10 Gbps are current, and the number of channels capable of carrying that rate on a single optical fiber will soon exceed 50, resulting in very large total data rate requirements. Sending so many separate channels of communication at these high data rates can only be accomplished with the high frequency nature of light waves. The frequency of light of 1550 nm wavelength (the "C"-band) is 193.4 THz (~10^14 cps). WDM systems have narrow bandpasses (1.6 nm) spaced 200 GHz apart situated around that central frequency. Current systems are spaced 100 GHz apart, and their corresponding FWHM bandwidths are 0.8 nm with edge attenuation reaching 25 dB within 2 nm of the in-band top. Future systems will be at 50 GHz bandwidth to permit even denser packing.
The separation of the dozens of wavelengths squeezed into a C-band system may be accomplished in several ways. One is by generating Bragg diffraction gratings in a fiber surface. We are concerned with the technique that uses individual multi-layer filters to isolate neighboring bandpasses on the input to the fiber and a corresponding output set at the receiving end. Such filters require thin film designs that include more than layers. The layer materials are typically tantala and silica. Both of these oxides exhibit low absorption at 1550 nm wavelength, and good deposition behavior. To meet the requirement of <0.5 db loss in-band, each of the layers must have exceptionally low absorption and scatter. This requires extraordinary control and stability of deposition conditions, materials, and chamber geometry. For example, the temperature and deposition thickness distributions must be constant through the run. Pressures, rates, etc. must be constant to insure that refractive indices do not drift during the run. Magnetron and ion beam sputtering are more often used in production because they offer better control and consistency than e-beam.
The filter designs are Fabry-Perot in nature, with all layers integral Quarter Waves or multiples thereof, and there may be three or four cavities depending on required bandwidth and edge slope. Each cavity consists of a thick spacer layer whose optical thickness is typically 3 or 4 half-waves sandwiched between QW mirror stacks. The series of coupled cavities making up the filter must have the central resonant wavelength of each cavity coincident to provide efficient transmittance for the assembly. The error in the thickness of the multiple HW spacer layers must be kept to <0.05%. This is equivalent to 10å thickness control for a low index.
Fortunately the final filter size is 1 mm sq., so a deposition run that might require more than a day to complete can yield some area of useable filter due to random non-uniformity across the substrate. Figure 1 shows the computed performance of a DWDM filter design as described above, on both transmittance % and dB scales.
With all these considerations in mind, it should be no mystery to the reader why only a handful of companies have made the investment in deposition, monitoring, and measuring equipment and in the coating development necessary to produce filters. Certainly DWDM filters represent a state of the art of our technology not even dreamed of only 10 years ago.
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Low Temperature ITO Coatings
Previous issues of CMN have discussed the important transparent conductor coating tin-doped indium oxide and its widespread uses in optical, electronic, and mechanical applications. Until magnetron sputtered deposition of ITO was developed to large-scale production readiness, thermal evaporation was the method available. At substrate temp. >300° C, good optical and electrical properties were relatively easy to obtain. Sputtering permits low temperature large-area substrates such as polymer film to be coated routinely in web systems. Control of electrical and optical properties to achieve long-term consistency still requires some effort, however. Both e-beam and sputter deposition produce amorphous films with less than optimum electrical properties. An aspect of working with optical materials that depend on doping to determine electrical properties such as conductance is that the performance properties are inversely related. For example, to obtain low resistivity, optical transmittance must be traded and become low, and vice versa. Coaters often rely on test runs and experience to gain control without adequate understanding of the influences of the key variables: partial pressure, rate, power, substrate temp., and starting composition. The first three parameters influence the oxidation state, while the last two are responsible for the activation of the cation (here Sn) and thus the film resistivity. The work reviewed here assists us in gaining some of the needed insight into what makes ITO (and similar materials) work.
The technique used was pulsed laser deposition (PLD) using a KrF excimer laser (248 nm) to evaporate from small dense targets of pure In2O3, and of 5 wt. % and 10 wt. % SnO2 doping . Pulses of energy density 2-5 J/cm2 at a rate of 20 Hz in an oxygen pressure of 1-1.5 E-02 Torr produced a deposition rate of 12 nm / min. The effects of substrate temperature on electrical and optical properties and film structure/ surface morphology were studied. While the PLD technique is not easily scaled at this point in time, some interesting results were obtained that shed new light on ITO film deposition and growth.
At substrate temperatures below 100° C, amorphous films were obtained regardless of the doping concentration. Evidence of initial crystallization was seen as low as 100° C, contrary to the reported crystallization temperature of ITO being 150 – 160° C in the work of Song, et al. . The lower temperature for crystallization might be due to the higher laser fluence used here than by Song, et al. Pure In2O3 films showed the lowest resistivity, ~2 E-04, at temperature <100° c. This is attributed to the creation of oxygen vacancies in excess of the free carrier population in the film. Doping is known not to contribute to carrier generation at low substrate temperature where the film is amorphous.When the pure >2O3 film is deposited at temperatures above 150° C, the film resistance increases, while that for doped films decreases. Evidently the carrier concentration and Hall mobility decrease. The opposite occurs for the doped films, causing the resistivity to decrease. This is explained as due to the ionization (activation) of the Sn to provide free electrons to the conduction band. These free electrons are then available to produce conduction. Above 200° C, there was a rapid increase (6x) in resistivity due to the depletion of oxygen vacancies. The doped film layers showed a decrease in resistivity (to 1/3), however.
The average optical transmission (450 – 800 nm) for all of the 100 nm thick films was above 85% for all substrate temps. This could be increased a couple % when the temp. was above 150° C. The surfaces of the films were smoother at temperatures below 100° C and above 200° C.
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The Need for Alternate Transparent Conduction Oxides
The major applications of ITO are flat-panel displays and thin-film photo-voltaics. Both applications depend on the film having simultaneously high transmission across visible wavelengths and low sheet resistance. Future improvements in devices demand improved transparent conductive oxide (TCO) properties. The large volume of research on ITO materials and deposition processes published over the past 4 decades has essentially reached the limit in improving either transmission or resistivity. Therefore research into alternate TCOs is being promoted. A further justification for finding alternates to ITO is the limited availability of indium. Indium is a byproduct of zinc refining. While the market demand for indium is in flat-panel display production, indium- alloy low temperature solder consumes its share. Thin film solar cells using indium diselenide and ITO panels could require the annual production of several tens of metric tons of indium by 2005. This demand cannot be presently supported by the available indium resources. Finally, the impact on the price of ITO material has been felt by coaters.
It makes sense, therefore, to develop TCOs not based on indium. Among the most promising candidates for the photo-voltaic application are; cadmium stannate (Cd2SnO4) and zinc stannate as well as CdO. CdO films can be prepared by CVD while the others are easily RF sputtered. Cadmium also is a derivative of zinc refining, but its toxicity is of concern. However, greatly improved performance has been demonstrated in PV devices based on cadmium stannate. This material might represent the transition from ITO to newer, less toxic TCO materials that are being developed. Faced with the pressure of energy shortages, we are sure to hear more about these in the near future.
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*Newsletters beginning with Vol. 6 are available from the CMN archives page of this web site. Contact CERAC for printed copies of referenced newsletters prior to Vol. 6.
If you have a question or a topic you would like us to consider for a future issue of CMN, e-mail your requests to firstname.lastname@example.org or fax them to 414-289-9804.
(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)
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