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Lock-In Thermography Imaging Workstation

Photo of a silver rectangular cabinet sitting on top of a small table that contains electronic components.  The cabinet is open on the front. The interior walls of the cabinet are black.  A metal frame in the cabinet holds a light source on the top and a sample stage on the bottom.

Thermography unit that is one of the Stand-alone Measurements and Characterization tools in the Process Development and Integration Laboratory.

This page provides details on the lock-in thermography imaging workstation in the Stand-Alone Measurements and Characterization bays of the Process Development and Integration Laboratory. This capability is currently operational.

Lock-in thermography uses modulated excitation to periodically excite carriers or apply voltages. When triggered at the unique excitation frequency, the infrared camera's sensitivity is considerably improved. Such sensitivity allows detection of very small changes in infrared radiation or transmission of a sample and can be used to (1) detect shunts where relatively large currents heat the sample or (2) sense electron concentrations by the amount of infrared energy the electrons absorb or emit.

Applications:

  • Dark lock-in thermography is used to image shunts/shorts by applying either a reverse bias to concentrate current in shunts or a forward bias to sense shunts/weak diodes that are active at the operating point of the solar cell. Illuminated lock-in thermography uses light, instead of voltage applied by contacts, to induce a voltage and drive currents through the shunts. Variations of these techniques can also be applied to sense heat where currents are affected by nonuniform series resistance.
  • Carrier density imaging is used to image minority-carrier lifetime and carrier trapping when excess carriers are generated by absorption of light.
    • When the background is warmer than the sample, the infrared camera images heat being transmitted through the sample. Excess carriers increase absorption in regions where their concentration is high, which corresponds to where their lifetimes are long, or their lifetimes are prolonged by trapping.
    • When the sample is warmer than the background, infrared emission of the excess carriers is imaged. Carrier concentration, such as emitter doping, can also be monitored with such infrared detection.

Special features:

Image showing a dark background with scattered yellow and orange dots of light.

Dark lock-in thermography image of a finished silicon solar cell. The bright areas indicate heated regions of high current in reverse bias.

  • The system uses an indium antimonide (InSb) infrared camera with 640 x 512 pixels, 3.6 to 5.1 µm spectral response, and thermal sensitivity <20 mK. The camera detector is cooled to about 77 K.
  • Entire sample in field of view, or use a zoom lens for about 5-µm spatial resolution
  • Capture thermal images and movies, and use lock-in where a Fourier analysis is carried out at the pixel level on a series of images. This results in an amplitude image and a phase image.
  • Illumination is from a laser diode with 810-nm wavelength and up to 60 W of power to spread out over a 6" x 6" area to achieve at least 1-sun intensity
  • Can image a wide variety of photovoltaic materials including single-crystalline silicon (Si) wafers, multicrystalline Si wafers, copper indium gallium diselenide thin films, and cadmium telluride thin films
  • Can transfer a sample between the table-top (stand-alone) enclosure and the vacuum chamber located on the Integrated Measurements and Characterization cluster tool.

Contact Steve Johnston for more details.