Background: The THz sensors using microbolometers as a sensing element are reported as
one of the most sensitive room-temperature THz detectors suitable for THz imaging and spectroscopic
applications. Microbolometer detectors are usually fabricated using different types of the MEMS technology.
The patent for the detection system presented in this paper describes a method for microbolometer
fabrication using a standard CMOS technology with advanced micromachining techniques. The measured
sensitivity of the sensors fabricated by the patented method is 1000 V/W at an optimal frequency and is
determined by the performance of a double-dipole antenna and quarter-wavelength resonant cavity.
Method: The paper presents a patented method for fabrication of a microbolometer system for radiation
detection in the THz frequency range (16). The method is divided into several stages regarding the
current silicon micromachining process. Main stages are fabrication of supporting structures for micro
bridge, creation of micro cavities and fabrication of Aluminum antenna and Titanium microbolometer.
Additional method for encapsulation in the vacuum is described which additionally improves the performance
of bolometer. The CMOS technology is utilized for fabrication as it is cost effective and provides
the possibility of larger sensor systems integration with included amplification. At other wavelengths
(e.g. IR region) thermistors are usually also the receivers with the sensor resistance change
provoked by self-heating. In the THz region the energy is received by an antenna coupled to a thermistor.
Depending on the specific application requirement, two types of the antenna were designed and
used; a narrow-band dipole antenna and a wideband log-periodic antenna.
Results: With method described in the paper, the microbolometer detector reaches sensitivities up to 500
V/W and noise equivalent power (NEP) down to 10 pW/√Hz. Additional encapsulation in the vacuum
improves its performance at least by a factor of 2, therefore the sensitivity reaches approximately 1000
V/W and NEP down to 5 pW/√Hz. The thermal response time of bolometer is 0.5 µs. The thermistor biasing
current drops with its resistance (defined by microbolometer active area), but the sensitivity rises.
Typical value of biasing current is 300 µA at 680 Ω of resistance, where the sensitivity reaches highest
level. Air pressure decrease highly influences the sensitivity due to lower thermal dissipation to surrounding
air. The sensitivity is therefore doubled when packaged in the high vacuum (0.1Pa).
Conclusion: The main advantage of the presented approach is that the detection devices can be fabricated
by a standard silicon micromachining process. Their overall dimension is defined by the receiving
antenna and they do not need any additional optic source for the operation. They are robust and appropriate
for mass production and can be easily embedded or merged with other vision system in use.
The developed microbolometer is highly sensitive, its noise is low and it operates at a room temperature
with no additional cooling system at a normal atmospheric pressure. The output of the THz detector
connected to a discrete low-noise amplifier increases the total sensitivity up to 106 V/W with no
impact on the noise equivalent power of 5 pW/√HZ.