Chapter 07 Thermal detectors

Thermal detectors
CHAPTER 07
1 Thermopile detectors
1-1
1-2
1-3
1-4
1-5
1-6
Features
Structure
Characteristics
How to use
New approaches
Applications
2 Bolometers
2-1 Operating principle and structure
2-2 Characteristics
1
Thermal detectors
Thermal detectors have an absorption layer that absorbs and converts light into heat, and provide an electric signal output that
represents the change in absorption layer temperature. Because thermal detectors have no wavelength dependence, they can
serve as infrared detectors when used with a window material such as Si that transmits infrared light.
Thermal detectors are mainly classified into: (1) thermopile detectors that change in electromotive force, (2) bolometers that
change in resistance, (3) pyroelectric detectors that change in dielectric surface charge, and (4) diodes that change in voltagecurrent characteristics. Hamamatsu manufactures two types of thermal detectors: thermopile detectors and bolometers. These
two types of thermal detectors are different in terms of operating principle, structure, and characteristics.
Thermopile detectors have a structure in which a large number of thermocouples are serially connected on a silicon substrate
and their sensitivity increases as more thermocouples are used. This means that the larger the photosensitive area, the higher
the sensitivity, because the number of thermocouples is proportional to the size of the photosensitive area.
In bolometers, the photosensitive area uses a bolometer resistance made up of thermoelectric conversion materials, so the
resistance temperature coefficient is the primary cause in determining bolometer sensitivity. Since bolometer sensitivity does
not depend on the size of the photosensitive area, detectors can be fabricated that have a small photosensitive area yet no drop
in sensitivity.
Thermopile detectors are usually manufactured as single-element detectors with an ample photosensitive area or arrays with
a small number of elements, while bolometers are manufactured as arrays with a larger number of elements than thermopile
detectors.
Hamamatsu thermopile detectors and bolometers
2
Product name
Multi-element array
Sensitivity
enhancement
Supply current
Package
atmosphere
Rise time
Thermopile detector
Possible with larger pixel size
(pixel size: 200 × 200 µm or larger)
Possible
Not required
(thermal
electromotive force)
Nitrogen
1 ms or more
Bolometer
Possible with smaller pixel size
(pixel size: 75 × 75 µm or smaller)
Possible
Required
Vacuum
2 ms or more
[Figure 1-1] Thermally isolated structure
(thermopile detector)
Thermocouple
Thermopile detectors are thermal detectors that utilize
the Seebeck effect in which a thermal electromotive
force is generated in proportion to the incident infrared
light energy. Thermopile detectors themselves have no
wavelength dependence and so are used with various
types of window materials for diverse applications such as
temperature measurement, human body sensing, and gas
analysis.
Hot
junction
Metal A
Metal B
Cold
junction
ΔV
Hot
junction
• Operates
• Spectral
ΔT
Features
1-1
ΔT
Thermopile detectors
1.
at room temperature
Cold
junction
n × ΔV
response characteristics that are not
KIRDC0046EA
dependent on wavelength
• No
Our thermopile detector structures differ according to the
type of device, namely, single/dual/quad element types
and linear/area arrays.
optical chopping is required, and voltage output
can be obtained according to input energy.
• Low
cost
• Long
Single/dual/quad element types
life
1-2
Structure
In order to obtain a large output voltage, Hamamatsu
thermopile detectors have many thermocouples that
are serially connected on a silicon substrate to magnify
the temperature difference between the hot and cold
junctions. The hot junction side (photosensitive area) is
designed to be a thermally isolated structure on which
an infrared absorption film is attached. To make the
thermally isolated structure, MEMS technology is used
to process the membrane (thin film) to make it float in a
hollow space. Our thermopile detectors use materials that
have a large Seebeck coefficient (thermal electromotive
force) and are easily formed by the semiconductor
process.
When infrared light enters a thermopile detector having
the above mentioned structure, the hot junction on
the membrane heats up and produces a temperature
difference (ΔT ) between the hot and cold junctions
accompanied by generation of a thermal electromotive
force (ΔV).
Single/dual/quad element types have large photosensitive
areas and are manufactured by bulk processing technology
with high workability. Etching is performed from the
backside of the substrate to form the photosensitive area
in a membrane state so that the hot and cold junctions
are thermally isolated from each other to achieve high
sensitivity.
[Figure 1-2] Cross-sectional view
(single/dual/quad element types)
Infrared
absorption film
Cold junction
Photosensitive area
Isolation
layer
Substrate
Hot junction
Thermocouple
KIRDC0106EA
[Table 1-1] Hamamatsu thermopile detectors
Type
Number of elements
Window material
Spectral response range
Package
Main applications
Single element
1
Anti-reflection coated Si
3 to 5 µm
TO-18
Gas analysis, temperature measurement
Dual element
2
3.9 µm, 4.3 µm
TO-5
3 to 5 µm
TO-8
Band-pass filter
Quad element
Linear array
4
16, 32
5 µm long-pass filter
Area array
Gas analysis
8×8
Flat package
Temperature measurement
TO-8
Temperature measurement, human body sensing
5 to 14 µm
3
Temperature characteristics
To manufacture linear and area arrays, the gap between
each element must be made narrow in order to reduce
non-sensitive areas. To do this, the portion directly under
each photosensitive area is selectively bored by surface
processing technology so that the photosensitive area
becomes a thin membrane-like structure. CMOS process
technology is utilized to lay out the signal processing
circuits on the same chip where the thermopile section is
formed.
[Figure 1-4] Temperature characteristics of sensitivity
(single element type T11262-01, typical example)
60
58
56
Sensitivity (V/W)
Linear and area arrays
[Figure 1-3] Cross-sectional view
(linear and area arrays)
Thermocouple
Cold junction
Hot
Infrared
junction absorption film
Isolation
layer
54
52
50
48
46
44
Etching
hole
42
Metal
wiring
40
-20 -10
0
10
20
30
40
50
60
70
80
Element temperature (°C)
Signal processing
circuit
Substrate
Thermopile section
Thermopile section
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KIRDB0522EA
[Figure 1-5] Temperature characteristics of element
resistance (T11262-01, typical example)
150
Characteristics
Sensitivity
Thermopile sensitivity (Rv) is determined by the number
of thermocouples as expressed by equation (1).
140
Element resistance (kΩ)
1-3
130
120
110
Rv =
η:
n:
α:
G:
ω:
τ:
ηn α
G
1 + ω2 τ 2
[V/W] ……… (1)
100
-20 -10
emissivity
number of thermocouples
Seebeck coefficient
thermal conductivity
angular frequency
thermal time constant
0
10
20
30
40
50
60
70
80
Element temperature (°C)
KIRDB0523EA
Linearity
Noise
Thermal noise called Johnson noise in the element
resistance is predominant in thermopile detector noise.
Noise (VN) is expressed by equation (2).
VN =
k :
T :
Rd:
Δf :
4
4k T Rd Δf
[V rms] ……… (2)
Boltzmann’s constant
absolute temperature
element resistance
bandwidth
Figure 1-6 shows an example of the relation between the
input energy and output voltage. Thermopile detector
output voltage is proportional to the input energy.
[Figure 1-6] Output voltage vs. input energy
(T11262-01, typical example)
[Figure 1-7] Frequency characteristics
(Typ. Ta=25 ˚C)
2
(Ta=25 ˚C)
10-1
1
0.3 × 0.3 mm
0
Relative output (dB)
10-2
Output voltage (V)
0.25 × 0.25 mm
10-3
10-4
-1
-2
2 × 2 mm
-3
1.2 × 1.2 mm
-4
10-5
-5
0.1
10-6
10-5
10-4
10-3
10-2
1
10
100
1000
10-1
Frequency (Hz)
Input energy (W/cm2)
KIRDB0561EA
KIRDB0560EA
Spectral response
Frequency characteristics
Since thermopile detectors have no wavelength dependence,
their spectral response is determined by the transmittance
characteristics of window materials. Spectral transmittance
characteristics of typical window materials are shown in
Figure 1-8.
Figure 1-7 shows the frequency characteristics of thermopile
detectors each having a different photosensitive area.
Frequency response tends to decrease as the photosensitive
area becomes larger.
How to use
1-4
Single/dual/quad element types
(1) Circuit not using thermistor
In cases where the ambient temperature is constant or
high precision measurement is not required, thermopile
detectors can be used with a circuit that does not include
a thermistor.
[Figure 1-8] Spectral transmittance characteristics of window materials
100
Anti-reflection coated Si
T11262-01
90
5 μm long-pass filter
80
8 to 14 μm band-pass filter
Transmittance (%)
70
3.9 μm band-pass filter
T11722-01 (reference light)
60
Si
50
4.3 μm band-pass filter
T11722-01 (CO2)
40
30
4.4 μm band-pass filter
20
10
0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
25
Wavelength (μm)
KIRDB0512EA
5
• Dual-polarity
power supply type
[Figure 1-9] Amplifier circuit
(dual-polarity power supply type)
C1
R2
-V
4
R1
2
method using a microcontroller is more common.
Figure 1-11 shows a circuit example where the thermistor
output signal is fed into the amplifier circuit. This type of
circuit is used when high measurement accuracy is not
required. The circuit shown in Figure 1-11 applies to both
cases where the thermistor is externally connected to the
thermopile detector or the thermistor is built into the
thermopile detector.
[Figure 1-11] Amplifier circuit with thermistor
-
6
Vout
3 +
+V
+V
7
Ra
Thermopile
detector
3 7
+ 6
2-
+V
Thermopile detector
R1
Rb
Gain = 1 + (R2/R1)
fhigh = 1/(2πC1 R2)
Vout
4
Thermistor
Rth
-V
R2
C1
Rd
Gain = 1 + (R2/R1)
fhigh = 1/(2πR2 C1)
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• Single
KIRDC0051EB
[Figure 1-10] Amplifier circuit
(single power supply type)
C1
R2
GND
R1
+V
2
3 +
Finding the resistance values (Ra, Rb, Rc, Rd) for the
circuit with a thermistor
 To find the resistance value of Rd at which the thermistor
output Vth is linear in the operating temperature range
1) Determine the operating temperature range (Tmin to Tmax).
2) Find the resistance value (Rh) of the thermistor Rth at Tmax.
3) Find the resistance value (Rl) of the thermistor Rth at Tmin.
4) Find the resistance value (Rm) of the thermistor Rth at
an intermediate temperature between Tmin and Tmax.
5) Find the resistance value of Rd from equation (3).
Rd =
Rh Rm + Rl Rm - 2Rh Rl ………
(3)
Rh + Rl - 2Rm
4
6
Vout
7
+V
Thermopile detector
 To measure the thermopile output voltage Vout and
check the voltage range where the thermistor output
voltage (Vth) varies in the operating temperature range
(when the measurement object's temperature is 25 °C)
1) Measure the thermopile detector output voltage
(Voutmin) at Tmax.
2) Measure the thermopile detector output voltage
(Voutmax) at Tmin.
Gain = 1 + (R2/R1)
fhigh = 1/(2πC1 R2)
R4
KIRDC0050EB
(2) Circuit using thermistor
Output signals of a thermopile detector are temperature
dependent. When detecting the temperature of an object
in locations where the thermopile element temperature
may drastically fluctuate, some means of making the output
signals constant is required to ensure stable temperature
detection. There are two methods to compensate for
the temperature: one is to directly input the thermopile
detector and thermistor signals into a microcontroller,
and the other is to feed the thermistor output signal into
the amplifier circuit. If high accuracy is necessary, the
6
Vth
power supply type
When using an op amp that operates from a single power
supply, an error occurs near ground potential which is
caused by the op amp’s offset voltage and nonlinearity. To
cope with this, the thermopile detector is operated with
one terminal biased. In the circuit shown in Figure 1-10,
the op amp supply voltage is biased with dividing resistors
R3 and R4.
R3
Rc
‘ To find the resistance values of Ra, Rb and Rc
1) Find the voltage drop (Vb) of Rb and the voltage drop
(Vc) of Rc, from the simultaneous equations (4) and (5).
Voutmin =
Vb Rd
+ Vc ……… (4)
Rh + Rd
Voutmax =
Vb Rd
+ Vc ……… (5)
Rl + Rd
2) Find the voltage drop (Va) of Ra.
(b) T11264 series
V: supply voltage
3) Determine the Rb value which should be smaller than
Rth + Rd by at least two orders of magnitude.
4) Find the Ra and Rc values from the simultaneous
equations (7) and (8).
Temperature sensor (+)
Vdd
Vref
Az_in
Naz_hold_in
Naz_on
Thermopile detector
Video
LPF
Hsp
Vclk
Reset
Hclk
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(1) Driving method
V Rc
……… (8)
Ra + Rb + Rc
Vc =
Vsp
Horizontal shift register
Preamplifier
V Ra
……… (7)
Ra + Rb + Rc
Va =
Vdd GND
Vertical shift register
Temperature sensor COM
Va = V - Vb - Vc ……… (6)
Linear and area arrays
Linear and area arrays consist of a one- or two-dimensional
thermopile array, shift registers and temperature sensor, and
to which a preamplifier is hybrid-connected to amplify the
output signal. Since the preamplifier is built into the same
package, this reduces external noise and also simplifies the
circuit configuration connected subsequent to the sensor.
The voltage to linear and area arrays is supplied from a 5 V
single power supply. The timing pulse to each terminal is
input as shown in the timing chart below so that the signal
(Video signal) from each element of the thermopile array
is output in a time series.
[Figure 1-13] Timing char t (T11264 series)
Vsp
Vclk
Hsp
Hclk
Reset
Az_in
Naz_hold_in
Naz_on
[Figure 1-12] Block diagram
1
2
3
4
5
6
7
8
(a) T11263 series
Vsp
Vclk
Hsp
Hclk
Reset
Az_in
Naz_hold_in
Naz_on
AD_trig
Temperature sensor
Temperature
sensor (+)
Thermopile detector
Address switch
Shift register
LPF
Video
Preamplifier
Video
1
2
1
2
3
4
5
6
7
8
1
1
2
3
4
5
6
7
8
9
Naz_on
Naz_hold_in
Vref
Az_in
Reset
Phinor
Sp
Clk
Vdd
GND
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(2) Temperature sensor
KIRDC0112EA
In linear and area arrays, a temperature sensor is mounted
on the same chip where the thermopile is formed. When a
constant current flows in this temperature sensor, a voltage
signal can be obtained that is inversely proportional to
the temperature. A Si diode is used in the temperature
sensor, which typically has a temperature coefficient of
approximately -2.2 mV/°C.
[Table 1-2] Digital input description (linear and area arrays)
Digital input
Description
Vsp, Hsp
Input logic signals needed to start the vertical/horizontal shift register scans. These inputs are required to scan the shift registers.
Vclk, Hclk
Input logic signals needed to switch the vertical/horizontal shift register channels. The shift register scan speed
can be adjusted by changing the clock rate.
Reset
Input logic signal for setting the video output to a fixed potential while the pixel output signals are not read out
Video
This outputs the thermopile detector signals in synchronization with the vertical/horizontal shift register scan timing.
Az_in
Naz_hold_in
Logic signals for driving the internal amplifier
Naz_on
AD_trig
Timing signal for acquiring the video signal into an external A/D converter, etc. This is used in synchronization with the
logic signals to the input terminals.
7
[Figure 1-14] Temperature sensor forward current vs.
temperature (T11264 series, typical example)
0.9
Forward voltage (V)
0.7
0.6
0.5
0.4
0.3
0.2
2-1
0.1
0
0
20
40
60
80
Temperature (°C)
KIRDB0563EA
New approaches
We are further enhancing our in-house CMOS and MEMS
technologies to develop thermopile detectors with higher
performance and more sophisticated functions yet at a
lower cost. We also plan to offer small thermopile detectors
with a lens, which come in a wafer level package.
1-6
Bolometers
Bolometers are small infrared sensors that do not require
cooling. When infrared light enters a bolometer, the
bolometer resistance heats up, causing a change in its
resistance. This change is converted into a voltage for
readout. Bolometers include a readout circuit to minimize
intrusion of external noise.
0.8
1-5
2.
(If=10 μA)
1.0
Applications
Operating principle and structure
Since bolometers are thermal detectors, the membrane
(thin film) that absorbs infrared light must be thermally
isolated from the substrate, so the structure has two long,
thin legs called “beams” to support the membrane. The
membrane is formed by sacrificial layer etching and floats
about 2 µm from the substrate. Infrared light radiated
from an object is absorbed by the infrared absorber on
the membrane, causing the membrane temperature to
increase and the bolometer resistance to decrease. The
incident light level can be read out as a voltage signal by
applying an electric current to the bolometer resistance. As
the bolometer resistance material, we use a-Si (amorphous
silicon) whose resistance greatly varies with temperature. A
CMOS readout circuit (ROIC: readout integrated circuit) is
fabricated on the substrate.
[Figure 2-1] Cross-sectional view (one pixel)
CO2 sensors
Thermopile detectors are used for non-dispersive infrared
(NDIR) detection type CO2 sensors. These CO2 sensors allow
precision measurements with high accuracy (minimal error
deviation from the true value).
Bolometer resistance
Infrared absorber
Infrared
light
Isolation film
a-silicon
Temperature and human body sensing in specific areas
Electrode
Substrate (CMOS readout circuit)
Thermopile linear and area arrays are used for temperature
and human body sensing in specific areas such as for air
conditioner operation control. These can detect locations
where persons are present and the direction that a person
moves.
8
Beam
KIRDC0115EA
[Figure 2-2] Enlarged photograph of photosensitive
area (bolometer)
If there are gas molecules such as air around the membrane,
the heat absorbed by the membrane is conducted to the
gas molecules, causing the bolometer sensitivity to drop.
To prevent this, bolometers are sealed inside a vacuum
package.
Figure 2-3 shows a block diagram of a bolometer. The
vertical shift register selects each pixel, and the amplifier
array converts current changes to voltage changes, which
are sampled and held, and are finally output from one
line of the horizontal shift register. Figure 2-4 shows the
readout circuit (one pixel) from the photosensitive area to
the amplifier. This circuit has a reference resistance that
is equivalent to the bolometer resistance, and both are
connected in series so that changes in current equivalent
to changes in the bolometer resistance are converted to
voltage signals by a later stage amplifier.
Vertical shift register
Amplifier array
Sample-and-hold circuit
Output
Horizontal shift register
α :
η :
Vb :
G :
t :
τ :
( )
1 - exp -
t
τ
……… (1)
temperature coefficient of resistance
infrared absorptance
supply voltage
thermal conductance
readout time
thermal time constant
If t >> τ in equation (1), the voltage sensitivity is expressed
by equation (2).
Rv =
α η Vb
……… (2)
G
Noise
Bolometer noise comes from several sources including
thermal noise caused by temperature fluctuations in the
bolometer resistance, 1/f noise resulting from factors
such as bolometer materials and electrical conductivity
of contact, temperature fluctuation noise caused by
temperature changes in the membrane, photon noise
caused by photons in the environment, and readout
circuit noise caused by the amplifier and other devices.
Temperature fluctuation noise and photon noise are low
compared to other sources of noise, so thermal noise and
1/f noise are usually predominant.
The total noise (VN) of a bolometer is given by equation
(3).
[Figure 2-3] Block diagram (bolometer)
Photosensitive area
α η Vb
G
Rv =
KIRDC0116EA
[Figure 2-4] Readout circuit (one element)
Vref2
V J2 + V 1/f 2 + V TH2 + V PN2 + V ROIC2 ……… (3)
VN =
VJ :
V1/f :
VTH :
VPN :
VROIC :
thermal noise
1/f noise
temperature fluctuation noise
photon noise
readout circuit noise
(1) Thermal noise
Reference resistance
4k T Rbol (f 2 - f 1) ……… (4)
VJ =
Vdd
C
−
+
SR
Bolometer resistance
To sample-and-hold
circuit
k
:
T :
Rbol :
f1 :
f2 :
Boltzmann’s constant
membrane temperature
bolometer resistance
lower limit of frequency bandwidth
upper limit of frequency bandwidth
(2) 1/f noise
Vref
K Vb 2 ln
V 1/f =
KIRDC0117EA
( )
f2
f1
……… (5)
K: 1/f coefficient
2-2
Characteristics
Voltage sensitivity
The voltage sensitivity of a bolometer is defined as the
output voltage divided by the infrared light level incident
on the photosensitive area. The voltage sensitivity (Rv) is
expressed by equation (1).
f1 and f2 are respectively defined by equations (6) and (7).
f1 ≈
1
4tstare
……… (6)
t stare: correction period of reference output
f2 =
1
2t
……… (7)
9
(3) Temperature fluctuation noise
RV
η
V TH =
4k T 2 G f TF ……… (8)
f TF : thermal equivalent noise bandwidth
fTF is defined by equation (9).
f TF =
1
4τ
……… (9)
(4) Photon noise
V PN = R V
8A σ k (T 5 + T BKG5) f TF ………
(10)
η
A : photosensitive area
σ : Stefan-Boltzmann constant (5.67 × 10-8 W/m2 K4)
TBKG : background temperature
Noise equivalent power
Noise equivalent power (NEP), which corresponds to the
S/N, is used to indicate the relation between the bolometer
signal output and noise. The NEP signifies the incident
light level required to obtain a signal output equivalent to
the total noise level and is given by equation (11).
V N ………
(11)
RV
NEP =
Noise equivalent temperature difference
Noise equivalent temperature difference (NETD) is used
to express the bolometer performance including the
readout circuit. NETD indicates the temperature change
that occurs when infrared light with a power equivalent to
NEP enters the bolometer, and is given by equation (12).
NETD =
Fno :
ɸλ1-λ2 :
L λ1-λ2 :
λ
:
10
4Fno 2 V N
RV A ɸλ1-λ2 π Lλ1-λ2
……… (12)
F number of optical system
transmittance of optical system
temperature contrast in wavelength interval
wavelength
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