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By Jim Belsky
Sensors Unlimited, Inc., part of Goodrich Corp.
Optical microscopy has long been a major tool for the scientist and visible microscopy has been the standard wavelength region used by the industry. Advances in imaging techniques and detectors now extend microscopy beyond the reaches of visible wavelengths, which has embraced the age of modern electronic imagers and cameras. New applications using the basic refractive lens principles have grown considerably, most notably with ultraviolet (UV) microscopes for biologists, and near-infrared (NIR) microscopes for materials scientists. This article will examine the use of microscope cameras sensitive in the shortwave infrared (SWIR) range.
The Camera and Microscope Relation
Most modern optical microscopes use refractive glass to examine the specimen at magnifications of up to several hundred fold and suitable to visible wavelengths from nominally 400 to 700 nm. Silicon cameras match well to this range, having natural sensitivity through the visible up to ~1100 nm. A shortwave infrared camera operates from ~900 nm to 1700 nm (0.9 to 1.7 µm), thereby taking over approximately where silicon gives up. An advantage of SWIR wavelengths is that many materials have unique optical properties in the range, and that optical glass remains transmissive in this region (as long as there are no specific IR-blocking coatings present). Because SWIR cameras often operate with standard glass optics (that is, they often do not require exotic optical materials or optimized optics) they can, conveniently, be used with the many existing lab and quality control microscopes in the field (specialty SWIR lenses and objectives are available and can offer better contrast and throughput to assist in more difficult investigations). Beyond this range, midwave-IR (MWIR) and longwave-IR (LWIR) cameras require the use of reflective specialty optics to pass the wavelengths, and their use becomes a larger capital goods acquisition.
The modern SWIR camera is generally available with a focal plane of nominally 320 x 240 or 640 x 512 pixels and is available with a C-mount optical interface, matching the interface of camera-compatible microscopes. The pixel sizes range from 25 µm to 40 µm square, translating to an array of roughly a 10mm-to-21mm diagonal (the larger dimension pushing near the limits of the C-mount design and still well-illuminated by the microscope). The SWIR camera pixels are generally larger than pixels of a typical silicon camera, and the difference can be compensated by proper objective magnification selection. Once installed into a microscopy system, the SWIR cameras can be operated similar to silicon B&W cameras with little additional operator training required.
Applications: Peering into Silicon
The silicon semiconductor crystal, and associated III-V variants with a band gap near 1.11 eV, will start transmitting light beyond 1100 nm (1.1 µm). The band gap of indium gallium arsenide (InGaAs, a SWIR detector semiconductor material) is significantly lower, making it an excellent sensor for SWIR wavelengths up to 1700 nm (1.7 µm). Further, the source of SWIR photons for probing the silicon is readily available in a standard tungsten filament light bulb typically found on a microscope: Using a color temperature of 2400K–2600K (a somewhat dimmed “brownish” bulb), the light output actually peaks for this bulb in the SWIR near 1200 nm (1.2 µm), with ample power for the InGaAs camera.
The maturity of silicon technology has progressed to the point that structures can be made on two sides of a polished wafer, allowing more complex devices to be fabricated. As such, a critical measurement involves the alignment top-to-bottom of these structures to a given tolerance. The SWIR microscope can measure the position readily and in real-time with no lag in the image through the use of alignment keys or other fiducial points deposited during the fab.