|Contractor(s):||XENICS (BE), National Optics Institute (CA)|
The ability to quantify Earth’s optical properties in the thermal infrared spectral region is a frequent requirement for remote sensing science and monitoring missions. Measuring thermal infrared spectra from space has traditionally been achieved using semiconductor photon detectors, which require moderate cryogenic operating temperatures. However, cryogenic operation, even at moderate temperatures ranging between approximately 50K and 150K, is costly and represents a major drive towards increased instrument and spacecraft system complexity.
Advances in uncooled thermal detectors over the last decade, both in North America and Europe, are changing the landscape of TIR detectors for space applications. An increasing number of space missions, for both Earth Observation and Astronomy, are currently flying, integrating, planning or envisaging payloads designed around room temperature microbolometer (RTμB) arrays. The sensing element in a resistive microbolometer usually consists of a thin film micro-bridge: a thermometer-absorber thermally isolated by being suspended above the underlying substrate. The substrate is mostly manufactured in standard CMOS technology and incorporates the read-out circuit (ROIC). The materials of the planar micro-bridge currently competing on the market today are, essentially: Vanadium (di)oxide or VOx, amorphous Silicon or α-Si, and Silicon-Germanium or Si-Ge.
This activity aims at further pushing the microbolometer VOx technology and capabilities for push-broom and 2D imaging to meet generic goal requirements for future EO missions. The goal of this development project is to achieve a 2D megapixel array format demonstrating performance parameters equivalent or better than state-of-the-art uncooled microbolometer sensors worldwide. Xenics and INO are responsible for the ROIC design and manufacturing and for the microbolometer design and fabrication, respectively.
The micro-bolometer pixel results were good and it was possible to demonstrate that 17μm pixel-pitch developed during the course of the project are in line with pixel-level requirements and thus support the FPA system level requirements.
The active test pixels were found to be compliant to the requirements, and provided for a thermal time constant of 6.73 ms and a responsivity of 912 kV/W. The 1/f noise coefficient of active and reference pixels were measured to be of the order of 1 X 10-12.
Overall, several roadblocks in the development of high resolution micro-bolometer based FPAs have been identified, brain-stormed and solved during this development project. The experience gained should help in reducing the pixel pitch below 17μm or increasing the array resolution or both to enable future EO space missions. Here are some of the roadblocks and takeaways:
a) A state-of-the-art micro-bolometer 17μm pixel-pitch was designed, fabricated and verified. Future lots are expected to show similar performance, but at the nominal temperature of -10°C.
b) It was observed that novel ROIC architectures and readout mechanisms were needed to go beyond VGA resolution of micro-bolometer FPAs. This is linked to way micro-bolometer arrays are operated and their variability across the array.
c) High resolution ROICs need exhaustive and much more robust verification methods than planned.
A new re-spin of the ROIC with improved bias generation circuits is planned; together with promising results achieved within the project by INO, the authors are confident to be able to demonstrate and fulfil the initial requirements for NEP.
Enable future EO space missions.
Most of the bolometers are manufactured for commercial and light-weight defence applications; bolometers camera are also used for the presence detection in case of fire, hot spot detection in industrial environment; and surveillance of gasoil pipelines.
Due to the malfunctioning of the ROIC, a respin of the ROIC is necessary in order to test the u-bolometer sensor and demonstrate performance, NEP in particular.