Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
1999-09-09
2001-10-09
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S215000, C136S224000, C136S213000
Reexamination Certificate
active
06300554
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a thermoelectric sensor. More particularly, the present invention relates to increasing the length of the thermocouple element line and decreasing the heat conductivity by using a zigzag structure, or a meander structure, of the thermocouple element line, as well as etching the silicon substrate by using front side Si bulk set etching. This invention will not only increase the gross die throughput of the silicon wafer, but also make a resister to be treated as a heater on the sensor membrane structure for electrical calibration of the device.
2. Description of the Prior Art
Infrared radiation detectors measure an object temperature by receiving and counting the infrared ray intensity generated from the object. It is a so-called non-contact temperature measurement. The typical applications include in-situ monitoring of semiconductor process steps, infrared spectroscopy, detection of various gases and liquids in industrial process environments, and measurement of temperature distribution in house for air conditioner. These applications also include medical thermography and clinical tympanic thermometers, and security applications for a passive intrusion detector. The operation principle basically is that the infrared rays radiate onto the detector, causing the temperature of a radiation absorber area to rise. Then this temperature increase will cause a change in physical nature of the device. This change can be converted into a change of electrical output. The common sensors are known as pyroelectric sensor, bolometer, and thermopile.
The pyroelectric sensor is made of ceramics of polymer, and it needs to be carefully treated during assembly and packaging. The manufacturing cost of pyroelectric sensors is comparatively high than thermoelectric sensors, since a thermoelectric sensor can be batch fabricated by using semiconductor mass production line. On the other hand, the bolometer sensor needs bias to measure the variation of its resistance and 1/f noise will occur due to such bias reading. The thermopile, i.e., the thermoelectric sensor, is a group of thermocouples connected in series. A thermocouple consists of a pair of dissimilar conductors so joined at two points that an electromotive force is developed by the thermoelectric effect when these two points, i.e., junctions, are at different temperatures. The thermoelectric sensor requires no additional bias, and is useful over a wide range of ambient temperatures. Because the thermoelectric sensor can be fabricated by semiconductor process technology, it is easily interlaced with monolithic integrated circuits. As a result, the thermoelectric sensor shows strong market potential and cost competitiveness.
From the view point of sensor function, if the device can include a thermoelectric sensing element and amplifying circuit, then the function and performance of the device will be increased, and the noise of signal interface will be decreased as well. It points out that the advantage of developing a compatible thermoelectric sensing element with the standard CMOS process. This arrangement provides the possibility of integrating a thermoelectric sensing element and signal processing circuit into a monolithic integrated circuit (IC) sensor.
According to the post-process that thermoelectric sensors in the prior art were usually possessed of the freestanding membrane structure made by back side silicon wet etching, but this way exhibits shortcomings as below:
Referring to
FIG. 1A
, a cross-sectional view of the rim of a thermoelectric sensor is shown. A silicon substrate
1
, a close membrane
2
, a plurality of first thermocouple element lines
3
, isolation layer
4
, a plurality of second thermocouple element lines
5
, and radiation absorber layer
6
are provided. The radiation absorber layer
6
that is isolated to the isolation layer
4
′ couples with the first thermoelectric element line
3
and second thermocouple element line
5
. The close membrane
2
, first thermocouple element line
3
, isolation layer
4
, and second thermocouple element line
5
are symmetrical as a result of the rim of thermoelectric sensor.
FIB.
1
B shows the connection of interconnects according to the thermoelectric sensor of FIG.
1
A. The opening of first thermocouple element line
3
couples the extremity of second thermocouple element line
5
with heated junction H nearby radiation absorber layer
6
by way of series connection, and cold junction C is distant from radiation absorber layer
6
. The first cold junction C of first thermocouple element line
3
couples electrically to the first metal pad
7
, and the last cold junction C of second thermocouple element line
5
couples electrically to the second metal pad
8
.
The heated junction H is located above close membrane
2
and located beneath radiation absorber layer
6
, and its temperature will be increased due to the heat coming from radiation absorber layer
6
. This radiation absorber layer
6
will be heated when it received infrared radiation. The cold junction C is located on silicon substrate
1
. Its temperature will be the same as the ambient temperature, because silicon substrate has high solid conductance and heat from cold junction dissipates through silicon substrate
1
. In order to form the close membrane
2
, the silicon anisotropic wet etching process is applied to remove the underneath silicon substrate from close membrane
2
. If the etching window is formed on the backside of silicon substrate, then the etching solution will attack silicon from the backside. A larger etching window area is necessary to define the area of close membrane
2
, due to the different etching rate of silicon crystal facet. It means the overall device size will become much larger than the defined close membrane
2
area, since the backside etching window is larger than the defined close membrane
2
area, and the device must be larger than the backside etching window. However, if the etching window is formed on the front side of silicon substrate, then the induced cavity will be like the one shown in FIG.
2
A. It means the overall device size will become almost the same as the defined close membrane
2
area. In such case, a cantilever beam of a 4-arm bridge will be generally used to support the structure of radiation absorber layer and the group of thermocouples. Ascribing to the different sizes of device made by front side or backside silicon anisotropic wet etching, the gross die of thermoelectric sensor from the wafer made by front side wet etching will be larger than the one made by backside wet etching.
In general, the characteristics of thermoelectric sensor express with responsivity in volts per watt (Rv), Johnson noise (V
j
), Noise Equivalent Power (NEP), and specific detectivity (D*). The corresponding formula is according to the following equation:
R
v
=
N
α
G
s
+
G
g
+
G
r
(
1
)
V
J
=
4
k
T
R
Δ
f
(
2
)
NEP
=
10
V
J
R
v
(
3
)
D
*
=
A
Δ
f
NEP
(
4
)
Where N is the number of thermocouples, and a is the seebeck coefficient (V/° C.). The G
s
, G
g
, G
r
are separately the thermal conductivity of solid, gas, and radiation, respectively. The k is Boltzmann's constant, T is the absolute temperature (° K.), R is the electrical resistance, &Dgr;f is the bandwidth of the amplifier, and A is the area of radiation absorber layer.
According to the aforementioned illustration, thermoelectric sensor measures the temperature of an object, the sensor performance depends on the quantity of output signal and the sensitivity of temperature variation of a measured object. The responsivity Rv is the output voltage for unit input radiation power. It represents the output efficiency of a sensor. Noise equivalent power NEP is the input power when the output voltage of thermopile is equal to the noise level.
Chou Bruce C. S.
Du Chen-Hsun
Lee Chengkuo
Gorgos Kathryn
Metrodyne Microsystem Corp.
Parsons Thomas H
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