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Expert tailors thin-films to diverse applications

15 Jun 2007

Thin-film coating and filter manufacturer Barr Associates has developed products for applications as wide-ranging as the study of the planet Pluto to pinpointing early signs of cancer. OLE catches up with technical experts at the US company to find out more.

Not many engineers can say that the products they helped to develop are going to the planet Pluto – but Tom Mooney can. He is principal engineer at filter manufacturer and thin-film coating specialist Barr Associates, US.

Barr has supplied spectral filters for the New Horizons mission, which was launched in 2006 and will fly past Pluto in 2015. Pluto is the only planet in our solar system unexplored by space probes. New Horizons will take important images that will help us to understand this mysterious, icy planet.

The probe will also take many images of other planets and moons during its journey to Pluto. If it wasn't for Barr's spectral filters, these images would be in black and white. "Without spectral filters, important spectral information would be missing from these images," said Mooney. "The more colours on the filter, the more colours can be detected."

One way of acquiring images over many spectral bands is to incorporate a multiple position filter wheel in the optical train. But the filter wheel imposes a large mass burden on the system and the mechanical nature of the wheel could compromise reliability. Filter wheels are still in use today – Barr supplied filters for the Hubble space telescope. "But when flying to Pluto, you need to conserve fuel," explained Mooney. "The size of the optics, including the filter, had to be reduced when compared with the conventional wheel design."

To achieve this, a multicolour filter can be placed directly over an area array, multiple-line detector or focal-plane array. These multicolour filters are relatively simple optical systems and can incorporate many colours. Using thin-film coatings, Barr makes patterned filters and assembled array filters.

Patterned filters consist of multiple individual filter coatings deposited on a single substrate in well-defined zones, which may or may not overlap. A wide variety of processes exist to produce patterned arrays, most of which involve some form of photolithography. "Yield is a serious issue with patterned filters," said Mooney. "It often limits the practical number of colours to about three or four, but for many systems this is adequate."

The New Horizons mission uses an assembled filter array from Barr that has six colours. Each colour is used to look for a specific material or substance either in the atmosphere or on the surface of a planet.

Unlike patterned filters, assembled arrays can be made from a variety of substrates, using a variety of coatings and can give a large number of colours. Barr's record to date is an 18-element multispectral filter micro-array where each element is a high-precision thin-film filter that is only 0.032 mm wide. "This is an exceptional number of elements," said Mooney. "The usual number for space applications is between four and 10."

The manufacturing of these arrays is extremely challenging. Using thin-film coating techniques, filters are first coated onto appropriate substrate materials of a relatively large area. These are then sliced using a precision saw into tiny individual filters that are assembled together with other filters from other coating batches to make an array. An epoxy-based glue is used to stick the filters together.

"The saw we use was originally designed to cut ceramics for the read-head of hard-disc drives and can give us tolerances of around 10 µm," said Mooney. "The challenge is to assemble the filters within mechanical tolerances and to align them with a predetermined detector architecture."

While this seems like a convoluted way of manufacturing a filter, it does give a large degree of flexibility in terms of design. For example, some arrays can cover the visible through to the thermal infrared wavelength range   a difficult task for a patterned filter.

But not all filters that Barr makes need to be complicated. The company has also supplied simple, low-cost filters to a UK company that makes microsatellites. These small imaging satellites are used for disaster monitoring by countries that ordinarily cannot afford to have a space programme.

Mooney believes that one of the company's strengths is that it does not limit the range of its customers. "Barr works on a wide range of applications, for a wide range of people and makes filters in a wide range of sizes," he added. For example, Barr has produced a tiny filter to go onto the end of an endoscope that will be used to detect early signs of cancer. The company is also capable of making large (0.5 m diameter) filters for ground-based astronomers.

Mooney recalls that one of the earliest projects he worked on at the company was some low-cost filters for a graduate student. "That graduate student went on to become the principal investigator on the Hubble telescope and that contact helped open the door onto a much larger project," commented Mooney.

Working across markets also means innovative ideas can be used in different applications. "We developed a 16-colour filter for a paint manufacturer for a low-cost colour matching system," he said. "Some customers from the space industry saw this in our labs and realized that this could be used for their application. Until they saw it at Barr, they didn't realise it could be done."

• This article originally appeared in the May 2007 issue of Optics & Laser Europe magazine.

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