Near-Infrared Bandpass Filters for Astronomy

Marcus Wallace and Henry Orr, NDC Infrared Engineering Ltd.

Near-infrared filters operate within a wavelength range of 800 to 5000 nm and typically consist of multiple thin-film interference layers of silicon/ silicon compounds deposited on a suitable optical substrate. They can be manufactured in narrow- and wideband filters and in cut-on/-off varieties, and they have applications in both terrestrial-based IR telescopes and in satellite IR measurement payloads.

Orion Nebula as captured by VISTA in visible light. Image by ESO; J. Emerson, VISTA; and R. Gendler. Courtesy of Cambridge Astronomical Survey Unit.

Filter manufacturing techniques

Near-IR filters generally are produced using vacuum deposition techniques. The choice of the technique determines the structure of the thin films being deposited.

Traditional vacuum deposition methods involve thermal evaporation of the bulk material, with electron beam excitation being the most popular. The optical material is heated in vacuum until it sublimes or becomes molten, to produce a dispersed vapor within the vacuum process chamber. Each layer is built up progressively, as the vapor condenses uniformly across the surface of the prepared optical filter substrate.

Figure 1. Pulsed-DC magnetron sputtering system. Images courtesy of NDC Infrared Engineering Ltd.

An alternative method is pulsed-DC magnetron sputtering. One technique uses a single-fixed-source sputter target of high-purity silicon and a high-power-pulsed-DC power supply¹ (Figure 1). By choosing different sputter gas combinations (argon, nitrogen or oxygen), various filter layers can be produced. Key filter parameters, indicated in Figure 2, are peak wavelength and transmission, bandwidth and blocking region.

Figure 2. Near-infrared narrowband filter characteristics.

Bandwidth is controlled by the design of the filters and the material properties. This is achieved by optimizing the manufacturing process to deliver the same material properties repeatedly for each filter. Peak wavelength and bandwidth are determined by selecting the appropriate number, composition and thickness of filter layers deposited, and the substrate used. In this way, a wide range of filters can be manufactured with peak wavelengths ranging from 400 to 5000 nm and bandwidths from 1.0 to 14 percent full width half maximum.

Blocking filters are established by coating the back surface of the filter substrate with another multilayer of dielectric wideband filters, which typically have bandwidths upward of 14 percent.

Filters for IR telescopes

Instruments such as those on WFCAM² (Wide-Field Camera on UK IR Telescope on Mauna Kea in Hawaii) and VISTA³ (Visible & Infrared Survey Telescope for Astronomy, European Southern Observatory in the Atacama Desert of Chile) require filters operating from 0.8 to 2.4 μm. WFCAM is composed of four Hawaii-II 2048 x 2048 x 18-µm-pixel array detectors from Rockwell Scientific, with a pixel scale of 0.4 in. (f/2.4) and a field of view per exposure of 0.21 square degrees.

Each detector is divided into quadrants, with each of those divided into eight channels of 128 x 1024 pixels. The detectors are spaced at 94 percent of the detector width. Four exposures are required to survey a contiguous area (tile) of 0.8 square degrees. WFCAM has eight filter housings, including the broadband set Z/Y/J/H/K, and two narrowband filters, H2 and Br-Gamma. The eighth filter housing is blanked for darks. The Y, J, H, K, H₂ 1-0 S1 and Br-Gamma band filters are manufactured by NDC Infrared Engineering Ltd. using the pulsed-DC magnetron sputtering technique. The filters’ specified bandpasses are shown in Table 1.⁴

Table 1. Bandpasses of NDC Infrared Engineering filters for WFCAM.

A major challenge to producing narrowband filters in this region of the spectrum using thermal evaporation techniques is loss of transmission efficiency,¹ particularly noticeable for Si/SiO filters at wavelengths in the 1200- to 1300-nm range. Typical transmission efficiencies at these wavelengths can be on the order of 60 percent, compared with 70 to 80 percent at higher wavelengths.

For high-refractive-index semiconductor film materials such as silicon and germanium, the packing density in films produced by thermal evaporation is less than the bulk or crystalline form of the material. Films deposited in this way can have a poorer definition of the absorption edge and are generally less transparent. The reasons are associated with the fact that the substrate temperature is significantly lower than that of the vapor, and there is insufficient energy in the vapor molecules to anneal the film structure into a dense solid similar to that of the bulk form of the material.

Although things can be improved somewhat by raising the temperature of the substrate, the pulsed-DC magnetron sputtering technique offers some significant advantages. It produces a vapor with much higher energy, enabling films to be deposited with well-ordered structures and near-bulk packing density, and, by choosing different sputter gases, silicon-monoxide, silicon-dioxide and silicon-nitride films also can be deposited, offering improved transmission at wavelengths down to 700 nm.

The benefits of this approach are illustrated in the case of the 1185-nm filters used on VISTA. At the heart of VISTA is a 3-ton camera containing a 16-detector array. Figure 3 shows a comparison of in-band transmission for the 1185-nm filter produced using a sputtered Si/Si₃N₄ film stack process and a similar filter produced using a conventional evaporated Si/SiO film process.

Figure 3. Comparison of in-band transmission for sputtered versus evaporated filters of similar wavelengths.

The sputtering process produces a filter with not only better transmission but also a narrower bandwidth. Even more challenging was the requirement by Oxford Astrophysics, a department of the University of Oxford in the UK, for matched sets of filters to operate at 975 and 985 nm on VISTA.1 The center wavelength of these filter sets falls within the spectral range most affected by silicon film optical absorption, and the specification (Table 2) requires a narrow bandwidth of approximately 1.1 percent of center wavelength.

Table 2. Summary specifications by Oxford Astrophysics for 985- and 975-nm filter sets on VISTA.

To optimize transmission at these low wavelengths, the narrowband “peak” coating was designed using a Si₃N₄/SiO₂ multilayer structure, which minimized the in-band optical losses. The blocker coating design was Si/Si₃N₄, but with some modified gas parameters for the silicon layers, which introduced a partial oxidation state. The result was excellent in-band transmission of ~80 percent for the 975- and 985-nm bands.

Another significant benefit offered by the sputtering technique is the reduced spectral shift exhibited by the sputtered filters under cryogenic conditions.

Filters for satellite payloads

Besides the application to shorter wavelength filters shown above, the pulsed-DC magnetron sputtering technique has been applied to filters for longer IR wavelengths, encompassing the upper limits of the InSb detector spectrum. It will be used on the INSAT-3D weather satellite, which is due for launch by 2011.⁵

The INSAT-3D instrument is an advanced infrared geostationary meteorological satellite being developed by the Indian Space Research Organisation Space Applications Centre for high-resolution monitoring of temperature and trace chemical species in the regions between the troposphere and stratosphere.

The instrument comprises a six-channel imaging radiometer designed to measure radiant and solar reflected energy from areas sampled on Earth, and a high-resolution infrared sounder to measure vertical temperature profiles, humidity, surface and cloud-top temperatures, and ozone distribution. Six short-wavelength infrared filters for the sounder covering the wavelength center range of 3.75 to 4.57 nm were produced using the pulsed-DC magnetron sputtering technique.⁶

The reduced spectral shift with temperature for filters produced in this way has contributed to the filters passing the environmental and spectral aging stability requirements for the mission according to standard ESA-PSS-01-702.

Meet the authors

Marcus Wallace is optics supervisor and Henry Orr a consultant, both at NDC Infrared Engineering Ltd. in Maldon, UK; e-mail: mwallace@ndcinfrared.co.uk; Email: hjborr@ndc.com.

References

1. H.J.B. Orr et al (2008). Near-infrared bandpass filters with improved transparency for 1000nm spectral region using sputtered silicon compound films. Proc. SPIE, Vol. 7018, pp. 701830-701830-12.

2. http://www.roe.ac.uk/atc/projects/wfcam

3. VISTA: http://www.eso.org/public/teles-instr/surveytelescopes/vista/camera.html

4. WFCAM instrument filters: http://www.ukidss.org/technical/technical.html

5. INSAT-3D:http://www.reading.ac.uk/infrared/research/projects/ir-isro.aspx

6. G.J. Hawkins et al (2008). High-performance infrared narrow-bandpass filters for the Indian National Satellite System meteorological instrument (INSAT-3D). Applied Optics, Vol. 47, No. 14.