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Optical Design & Engineering

Scanning near-field infrared radiometry for sub-wavelength thermal imaging

A scanning near-field infrared radiometry system, based on tapered silver-halide fibers, for making sub-wavelength temperature measurements.1 It may be used in the future for thermally imaging electronic devices or biological cells.

7 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0017

There is wide interest in measuring the true temperature and emissivity of infrared (IR) emitters2–5 whose dimensions are less than the wavelength of the radiation they emit. There are already several ways to measure temperature with high spatial resolution,2–3 but they are slow or cumbersome. What's needed is a non-contact, fast, and reliable technique for making these measurements.

Radiometric thermometry 6 is based on measuring the intensity (I) emitted from a body of temperature Tbody (i.e., for a blackbody I = σTbody).4 For a body near room temperature (Tbody ∼ 300 K), the measurement should be done in the spectral range of 3–14μm (according to Wien's displacement law λmaxTbody @ 3000μmK).

This measurement can be done using a lens-based IR microscope, but its resolution is limited by diffraction to 20–30μm. Microscopy in the visible and near-IR is also limited by diffraction, which can be overcome using scanning near-field optical microscopy (SNOM). This technique involves transmitting or collecting light through optical fibers whose ends are tapered to apertures with diameters smaller than the wavelength being measured.

There have been few attempts to develop a scanning near-field infrared microscope (SNIM) for the mid-IR spectral range 3–30μm, because it is difficult to find suitable mid-IR fibers, to taper them and to form very small apertures at their tips.

We have already developed core-only and core-clad fibers made of AgClxBr1-x (0<x<1) that are flexible, non-toxic and insoluble in water. These silver-halide fibers are highly transparent in the 3–30μm spectral range, with losses of roughly 0.2dB/m at 10.6μm. We used the fibers for radiometric measurements of temperatures, 7–10 and they seem ideal for SNIM applications.

To build a SNIM system we used chemical etching to form tapered AgClBr fibers with tip diameters of 1–10μm, and then coated the sides of a tapered end with a 1μm-thick layer of silver. We determined the temperature and spatial resolution of the system using the calibration set-up shown in Figure 1.  A ZnSe window, half coated with a 1μm-thick layer of silver, was placed in front of a highly blackened plate, whose temperature was controlled. The tapered fiber was mounted on an XYZ motorized stage and its tip was brought within 2μm of the ZnSe window. IR radiation emitted from the plate was collected by the tapered tip of the fiber and transmitted to a HgCdTe IR detector with a spectral range of 2–13.5μm.

Figure 1. The calibration measurement setup: IR radiation emitted from the high emission plate is modulated by the chopper and passes through the ZnSe window. The IR radiation is then collected by the tapered fiber, which delivers it to an IR detector.

Measurements were carried out for various temperatures of the high-emission plate. For each, the tapered fiber scanned the ZnSe window in the X axis (in steps of 1μm), from the coated area to the uncoated area, and the IR-detector signal was monitored. Using the knife-edge technique, we found that the spatial resolution of the system was 5μm. We also found that the IR signal depended linearly on temperature, enabling us to generate a calibration curve.

Next we tried to determine the temperature distribution of miniature IR emitters of 30μm × 30μm. We applied a voltage to the emitter element, causing it to heat up and emit IR radiation. We used a tapered fiber of tip diameter 5μm, held 2μm from the surface of the element. We scanned the emitter in the X-Y plane, in steps of 5μm. At each point, we measured the IR signal in two spectral ranges: 2–13.5μm and 8.5–13.5μm. Using the calibration curve we were able to convert the IR detector signal to temperature and so derive the thermal image of the IR emitter in Figure 2.

Figure 2 The thermal image of the miniature IR emitter as measured in two spectral ranges: (a) 2–13.5μm; (b) 8.5–13.5μm (sub-wavelength resolution).

The high-temperature region (∼185°C) was confined to an area of 30μm × 30 μm at the center of the element. The thermal image for the 2–13.5μm band was clearer and less noisy than that for the 8.5–13.5μm band, but both spectral ranges provided an image of the emitter.

This work shows that it is possible to acquire a thermal image of a miniature IR emitter whose dimensions are ≈ 3λ, with sub-wavelength resolution. It relies on silver-halide tapered fibers, whose resolution can be further improved by tapering the fiber ends to form smaller apertures. The technique can also be used to measure spectral characteristics of miniature emitters and, in multi-wavelength mode 11, to determine the true surface temperature of an element, even if it is covered with oxide or nitride layers 4–5. The tool may be useful for measuring hot spots on ICs and to study biological objects.

Sharon Sade, Lev Nagli and Abraham Katzir
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics and Astronomy, Tel Aviv University
Tel Aviv, Israel

Sharon Sade received the BSc and MSc degrees in physics in 1997 and 2000, respectively, from Tel-Aviv University, and is currently pursuing the PhD degree. His area of research includes optoelectronics, IR radiometry, IR spectro-radiometry. He presented several papers at Photonics West 2004 and at SPIE's 47th Annual Meeting.

1. S. Sade, L. Nagli, A. Katzir, Scanning near field infrared radiometry for thermal imaging of infrared emitters with sub-wavelength resolution,
Appl.Phys.Lett., Vol: 87, no. 10, pp. 101109, 2005.
2. J. Altet, S. Dilhaire, S. Volz, J. M. Rampnoux, A. Rubio, S. Grauby, L. D. Patino Lopez, W. Claeys, J. B. Saulnier, Four different approaches for the measurement of IC surface temperature: application to thermal testing,
Microelec. J., Vol: 33, no. 9, pp. 689-696, 2002.
3. A. Trigg, Applications of infrared microscopy to IC and MEMS packaging,
IEEE. Trans. Electronic. Pack. Manufact., Vol: 26, no. 3, pp. 232-238, 2003.
4. E. Takasuka, E. Tokizaki, K. Terashima, S. Kimura, Emissivity of liquid silicon in visible and infrared regions,
J. Appl. Phys., Vol: 81, no. 9, pp. 6384-6389, 1997.