Abstract |
A new generation of microbolometers were designed, fabricated and
tested for the NASA CERES (Clouds and the Earth's Radiant Energy
System) instrument to measure the radiation flux at the Earth's
surface and the radiant energy flow within the atmosphere. These
detectors are designed to measure the earth radiances in three
spectral channels consisting of a short wave channel of 0.3 to 5
μm, a wide-band channel of 0.3 to 100 μm and a window channel
from 8 to 12 μm each housing a l.5×1.5 mm²
microbolometers or alternatively 400×400 μm²
microbolometers in a 1×4 array of detectors in each of the three
wavelength bands. The microbolometers were fabricated by radio
frequency (RF) magnetron sputtering at ambient temperature, using
polyimide sacrificial layers and standard micromachining techniques. A
semiconducting YBaCuO thermometer was employed. A double micromirror
structure with multiple resonance cavities was designed to achieve a
relatively uniform absorption from 0.3 to 100 μm
wavelength. Surface micromachining techniques in conjunction with a
polyinide sacrificial layer were utilized to create a gap underneath
the detector. The temperature coefficient of resistance (TCE) was
measured to be 2.8%/K. These devices have successfully demonstrated
voltage responsivities over 103 V/W, detectivities above
108 cm Hz1/2/W NEP less than
4×10-10W/Hz1/2 and thermal time constant
less than 15 ms. The second part of the talk describes a new
micromachined microbolometer array structure that utilizes a
self-supporting semiconducting YBaCuO thin film thermometer. The
YBaCuO thermometer is held above the substrate only by the electrode
arms without the need of any underlying supporting membrane. This
represents a significant improvement in the state-of-the-art for
microbolometers by eliminating the thermal mass associated with the
supporting membrane. The reduced thermal mass permits lowering the
thermal conductance to the substrate to obtain increased responsivity
or having a shorter thermal time constant to allow for higher frame
rate camera. Devices were fabricated by growing YBaCuO films on a
conventional polyimide sacrificial layer mesa. Subsequent etching of
the sacrificial layer provides the air gap that thermally isolates the
microbolometer. The measured TCE was 3.1%/K at room temperature. The
measured responsivity and detectivity approached 104 V/W
and 108 cm Hz /W. The micromachining techniques employed
are post-complementary metal-oxide-semiconductor (CMOS) compatible,
allowing for the fabrication of focal plane arrays for IR cameras.
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