Broadband, Absolutely Calibrated Microbolometer Array Development

Microfabricated electrical substitution bolometers with vertically aligned carbon nanotube (VACNT) absorbers are being integrated into laboratory standards for optical laser power1 and CubeSats for solar irradiance mesurements2,3. We are building on these successes to extend the single bolometer technology to broadband microbolometer arrays with electrical substitution and VACNT absorbers. A linear array of radiometer elements has been fabricated. Each element is an uncooled bolometer with vanadium oxide (VOx) temperature measurement. Unlike existing VOx microbolometer technology where the thermistor film is incorporated into a narrow band cavity and doubles as the absorber, in these microbolometer arrays we use VACNT absorbers. Additionally, each element can be electrically heated to provide an absolute radiometric calibration through electrical substitution.

The array provides two benefits for Earth Science applications that require spectral data across many wavelengths. The first is the electrical substitution calibration on every pixel. Existing instruments are typically calibrated on the ground but then subjected to severe launch and space environments. On-orbit calibration is achieved with a reference such as a blackbody, lamp, or reference material and by ground verification. On-orbit references increase mass and power and may be degraded by the space environment. This technology eliminates the need for on-orbit references by incorporating electrical substitution calibration. By comparing the temperature rise of the bolometer when illuminated to the temperature rise when a known current is injected into a heater of known resistance, a calibrated irradiance is measured.

The second benefit is an extended spectral range. A typical microbolometer response is from 8 um to 15 um, but a VACNT absorber can extend the response to a wider 0.3 um to 100 um range. A broader spectral response in a single array can serve to reduce the number of detectors needed to span a broad wavelength range. The response above 20 µm improves measurement capabilities for applications such as Earth Radiation Budget. The combination of broadband spectral response and integrated calibration capability makes this single technology applicable to a broad range of scientific measurements.

This talk will describe the design, fabrication, and performance of the microbolometers. To date we have achieved a noise floor of ~100 K/sqrt(Hz) at 0.1 Hz. We show the expected performance of one of these arrays incorporated with a small telescope is appropriate for use on a smallsat.

Acknowledgements: This work is funded by the NASA Earth Science Technology Office.

1. M.G. White, et al., “Cryogenic primary standard for optical fibre power measurement”, Metrologia 55(5), 706-715 2018.

2. Erik Richard et al., “The compact spectral irradiance monitor flight demonstration mission”, Proc. SPIE 11131, CubeSats and SmallSats for Remote Sensing III, 1113105 (30 August 2019).

3. David Harber et al., “Compact total irradiance monitor flight demonstration”, Proc. SPIE 11131, CubeSats and SmallSats for Remote Sensing III, 111310D (30 August 2019).