Thermal detectors include thermopiles, pyroelectric detectors, and bolometers. They have lower manufacturing costs than photon detectors. Micromachining technology has greatly improved their quality. Recently, uncooled arrays of these detectors operating in the infrared region have become commercially available in thermal imaging cameras. Thermal detectors detect infrared radiation by measuring changes in temperature or electrical resistance.

1. Thermal detectors
BY: A.M. NASEEFA.
AZEEZ. MYMOONA. NASEEFA

2. Thermal detectors
• Thermopiles
• Pyroelectric detectors
• Bolometers
# Metallic and thermistor bolometers
# Semiconductor and micromachined bolometers
# Superconducting bolometers
AZEEZ. MYMOONA. NASEEFA

3. Thermal detectors are of low manufacturing costs in comparison to
photon detectors.The quality of these detectors was greatly
improved after introduction of micromachining technology.
Recently the uncooled arrays of these detectors working in the IR
region (thermovision cameras) are commercially available.
Electrical scheme for a thermal detector
AZEEZ. MYMOONA. NASEEFA

4. Thermopiles
A thermopile deposited on the membrane.
Two pixels of an array with thermopile
structures.
SiN absorber, heating the appropriate junction,
covers nearly the whole pixel.
Thermoelectrodes are made from materials of
very high figure of merit.
Read-out circuitry is manufactured in a silicon
substrate.
AZEEZ. MYMOONA. NASEEFA

5. Pyroelectric detectors
One uses the dependence of spontaneous polarization on temperature for the
pyroelectric material and also the dependence of elecrical permittivity on
temperature (dielectric bolometer).
Pyroelectric detector (shown in a form of an electrical equivalent circuit) at
the input of transimpedance amplifier.
Voltage sensitivity of a pyroelectric detector as a function of frequency. Both
electrical and thermal time constants are important.
AZEEZ. MYMOONA. NASEEFA

6. In order to obtain small thermal time constant, pyroelectric detectors are
manufactured onto suspended membranes .
Bottom electrode and a mirror form a resonant cavity for IR radiation
(thickness λ/4). By splitting upper electrode two capacitors are formed, which
connected in series enable reducing of noise caused by mechanical vibrations.
AZEEZ. MYMOONA. NASEEFA

7. Bolometers
Bolometer is a resistance element with a high TCR (temperature coefficient
of resistance) and a small heat capacity. Absorbed infrared radiation
changes the temperature and then the resistance of a bolometer.
Metallic and thermistor bolometers
Bolometers in a form of thin metallic films (Ni, Bi, Sb) with moderate TCR
(typ. 0.3%/K) but high stability and low noise are still manufactured.
Thermistor bolometers (Co, Mn, Ni,V oxide sinterings) have one order of
magnitude higher TCR but also higher current noise.
To increase the detectivity one uses sometimes immersion lenses as radiation
concentrators. In this case the signal to noise ratio increases n2 times due to
n-th increase of a detector area
(n- refractive index of a lens material, eg. Si, Ge).
AZEEZ. MYMOONA. NASEEFA

8. Bolometer in a bridge circuit with compensation of ambient temperature variations
Bolometer with immersion lens
AZEEZ. MYMOONA. NASEEFA

9. Semiconductor and micromachined bolometers
High detectivities are obtained by using semiconductor bolometers (Ge, Si) at
cryogenic temperatures. At very low temperatures the relative changes of
semicondutor resistance are higher and absorbing samples are thicker (lower
specific heat) what increases absorption. In a far infrared region these
detectors have detectivities comparable to those of photonic detectors, hence
their applications in astronomy, spectroscopy etc.
AZEEZ. MYMOONA. NASEEFA

10. Application of micromachining enabled manufacturing of
microbolometers with a high thermal resistance (1x 108 K/W) , close
to the theoretical limit caused by radiation.
Bipolar CMOS input amplifiers are used in readout circuits.
AZEEZ. MYMOONA. NASEEFA

11. Superconducting bolometers
Very high variation of resistance for a small change in temperature is
obtained for superconductors in the vicinity of critical temperature Tc.
The development of this technology was possible due to the discovery of
high temperature semiconductors, HTSC.
The typical HTSC material is YbBa2Cu3O7-x (YBaCuO{Yttrium barium
copper oxide}) with transition temp. ca 90 K.
Using this material and micromachining technology it was possible to obtain
bolometers with detectivities close to
1010 cmHz1/2/W (better than for photonic detectors at 77 K).
AZEEZ. MYMOONA. NASEEFA

12. Superconducting microbolometer in micromachining technology
AZEEZ. MYMOONA. NASEEFA

13. AZEEZ. MYMOONA. NASEEFA