Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
2000-07-28
2002-01-01
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S225000
Reexamination Certificate
active
06335478
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a thermopile infrared sensor, thermopile infrared sensors array, and a method of manufacturing the same, in particular, to a thermopile infrared sensor having hidden thermocouple cantilever beams with low solid thermal conductance, and to a thermopile infrared sensors array having a high fill factor and a low noise equivalent temperature difference (NETD) and method of manufacturing the same.
2. Description of the Related Art
Thermocouples have been widely used for temperature measurement. By heating a junction between two conductors, a temperature difference between the junction and a portion located away from the junction is produced, thereby generating a diffusion current. A reverse electromotive force (also called the Seebeck voltage) is, therefore, corresponding to compensate the diffusion current. By measuring the Seebeck voltage, the temperature difference between the two ends of the thermocouple can be obtained. The value of the Seebeck voltage is determined from the product of the temperature difference between the two ends of the thermocouple and the Seebeck coefficient of the two conductors forming the thermocouple. Furthermore, the thermally generated voltage can be amplified by connecting plural pairs of thermocouples in series to form a thermopile. Therefore, the Seebeck voltage of the thermopile is equal to a value that is obtained from the product of the Seebeck voltage of a single thermocouple and the number of the thermocouples in series.
From 1980, with the development of silicon micromachining technologies, both the responsivity (V/W) and the response speed of the thermopile sensor is greatly increased by a suspending membrane structure that is capable of lowering its thermal capacitance and has a high thermal isolation effect (low thermal conductance). As a result, various high performance thermopile sensors are well developed.
In addition to a single sensor fabrication, it is also possible to manufacture a monolithic sensors array, mainly a infrared focal plane array (IRFPA) for thermal imaging in the fields of military detection systems, the automotive industry medicine, industrial automation and home security monitoring.
An advantage of a thermopile sensor is that it does not need a power supply. Thus, it rejects the noise voltage against the power source, which other thermal sensors such as a bolometer suffers from. Moreover, because the current flowing through the thermopile sensor is very small or even zero, a low frequency noise (1/f noise) caused by the driving current can also be ignored. Because thermopiles detect the temperature difference between the hot and the cold junctions, and because the cold junction is located on the heat reservoir, the cold junction plays the important role as the temperature reference. Therefore, the thermopile does not need temperature stabilization, whereas bolometers generally do. At the same time, the thermopile sensor does not need a chopper, whereas pyroelectric sensors do.
FIG. 1
is a locally enlarged schematic illustration showing a conventional thermopile infrared sensors array as disclosed by Kanno et al. in “Uncooled infrared focal plane array having 128×128 thermopile detector elements.” It should be noted that only one complete thermopile sensor
10
is shown in FIG.
1
.
Referring to
FIG. 1
, the thermopile sensor
10
includes a substrate
100
and a suspending membrane
101
that is formed on the substrate
100
and has a plurality of thermocouples
102
. A hot junction
103
is located at the central portion of the suspending membrane
101
, while a cold junction
104
is located on the peripheral portion of the suspending membrane
101
attached to the substrate
100
that acts as a heat reservoir. A plurality of etching windows
105
are formed on the suspending membrane
101
. A polysilicon sacrificial layer (not shown) under the suspending membrane
101
can be etched via the etching windows
105
to construct a suspending structure. The structure of the thermopile sensor
10
will be better understood with reference to the cross-section view taken along a line
2
—
2
.
FIG. 2
is a cross-section view taken along the line
2
—
2
as shown in FIG.
1
. Referring to
FIG. 2
, the thermopile sensor
10
includes a substrate
100
and a suspending membrane
101
. The substrate
100
includes an integrated circuit
107
. Furthermore, a concavity
106
is formed between the suspending membrane
101
and the substrate
100
. The suspending membrane
101
includes a first dielectric layer
108
, a P-type polysilicon
102
a
, a second dielectric layer
109
, an N-type polysilicon
102
b
, a third dielectric layer
110
, and a metallic wiring
102
c
. The metallic wiring
102
c
connects the P-type polysilicon
102
a
with the N-type polysilicon
102
b
. A thermocouple
102
is composed of the P-type polysilicon
102
a
, the N-type polysilicon
102
b
, and the metallic wiring
102
c
. It should be noted that the regions of both the hot junction
103
and the cold junction
104
are also shown in this figure.
The thermopile infrared sensors array disclosed in Reference
1
is composed of 128×128 thermopile sensors
10
arranging in an array form, the specifications of which are as follows:
pixel size: (100×100) &mgr;m
2
;
suspending membrane area: (80×84) &mgr;m
2
;
fill factor: 67%
thermocouple: 32 pairs;
responsivity: 1550 V/W (in a vacuum environment); and
NETD (noise equivalent temperature difference): 0.5° C.
The high performance of this cited example is due to the manufacturing processes of the sensors array are compatible with standard IC processing. Furthermore, the signals produced by these sensors are amplified through the corresponding integrated circuit
107
. Therefore, a relatively high responsivity (1550 V/W) can be obtained. In addition, a sacrificial layer technology has been adopted to reach a high fill factor of 67%. Nevertheless, compared to the bolometric FPA (focal plane array) that has reported a low NETD of 0.039° C., the sensitivity of this thermopile FPA is still not enough.
In order to improve the thermopile performance, the basic physics of this sensor should be analyzed first.
FIG. 3
is a schematic illustration showing a simplified infrared imaging system, in which thermal radiation irradiated from an object
120
is absorbed by a thermopile sensor
122
via an optical system
121
.
In an idealized situation, that is, assuming that there is no optical absorption by the delivery medium and the optical system
121
, NETD is given by Equation (1):
NETD
=
4
F
2
Vn
RvAsL
=
4
F
2
AsL
NEP
(
1
)
wherein “As” is the effective absorbing area of thermal radiation on the thermopile sensor
122
; “Vn” is the total noise voltage within the system bandwidth; “Rv” is the responsivity; “F” is the focal ratio of the optical system
121
; “NEP” is the noise equivalent power, the value of which is Vn/Rv; and “L” is the change in power per unit area radiated by the object
120
within a spectral band.
NETD which is defined as the change in temperature of the object
120
that will cause the signal-to-noise ratio at the output of the thermopile sensor
122
and its read-out electronic s to change by unity. Usually, NETD is used to judge the figure of merit of an infrared imaging system, and its value is lower to better
The responsivity Rv of the above thermopile sensor is represented by Equation (2):
R
v
=
η
N
α
Gs
+
Gg
+
Gr
(
2
)
wherein “&eegr;” is the absorption of the incident radiation , “N” is the number of thermocouples connected in series, “&agr;” is the Seebeck coefficient (V/° C.) of each thermocouple, and “Gs”, “Gg”, and “Gr” are the solid, gas, and radiation thermal conductance, respectively, of the suspending membrane structure of the thermopile sensor.
For a thermopile, “Vn” is mainly dominated by the Johnson noise; it is represented by Equation (3):
Vn={square root over (4+L kTsR&Dgr;f)} &ems
Chou Bruce C. S.
Ou-Yang Mang
Chou Bruce C. S.
Gorgos Kathryn
Parsons Thomas H
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