Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
2000-03-24
2002-02-19
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S224000, C136S208000
Reexamination Certificate
active
06348650
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a thermopile-type infrared sensor and a process for producing the same and, more specifically, to a thermopile-type infrared sensor that increases a S/N ratio by improving a thermopile pattern structure thereof and a process for producing the same, which enhances production yield thereof.
(2) Description of the Related Art
In general, a thermal equilibrium equation for a thermal infrared sensor is expressed as follows:
C·&dgr;T/dt+G·&dgr;T=W
(1)
where, C is a heat capacity, &dgr;T is a temperature change of a light-receiving part, G is a thermal conductance between the light-receiving part and the surroundings, and W is light-receiving power. When the light-receiving power W varies according to an equation W=W
0
exp(j&ohgr;t), &dgr;T is expressed as follows:
|&dgr;
T|=W
0
/G
(1+&tgr;
2
&ohgr;
2
)
½
(2)
wherein a thermal time constant &tgr; is expressed as follows:
&tgr;≡
C/G
(3)
Accordingly, in order to increase a response rate of a thermal infrared sensor (i.e. to decrease thermal time constant &tgr;), heat capacity C is needed to be decreased and thermal conductance G is needed to be increased. However, an increase in G causes temperature change of light-receiving part &dgr;T to decrease with respect to the same light-receiving power and sensitivity to deteriorate. Therefore, in order to improve sensitivity and response rate of a thermal infrared sensor, G is needed to be small and C is needed to be decreased. From this point of view, a heat-sensitive part, in which a hot junction is placed, is constructed to have a membrane structure of several microns thickness so that the heat capacity C and thermal conductance G in relation to a substrate decrease, resulting in improvement in sensitivity and response rate of a thermopile-type thermal infrared sensor.
A figure of merit Z for a thermoelectric element such as a thermopile is expressed as follows:
Z=&agr;
2
·&sgr;/&lgr;∝m*
{fraction (3/2)}
(&mgr;/&lgr;
L
) (4)
where, &agr; is the Seebeck coefficient, a electrical conductivity, &lgr;
L
is thermal conductivity, m* is effective mass, &mgr; is mobility of carrier, and &lgr;
L
is thermal conductivity of lattice.
Accordingly, in order to improve electrical characteristics of a thermoelectric element, Seebeck coefficient &agr; and electrical conductivity a are needed to be increased while thermal conductivity &lgr; is needed to be decreased. Therefore, a semiconductive material having a large figure of merit Z compared to metal is employed as a thermoelectric material.
On the other hand, a Seebeck coefficient &agr; of n-type silicon as a thermoelectric material is expressed as follows:
&agr;=(
V
F
/T+
2
k/q
) (5)
wherein V
F
is energy difference between a bottom of conduction band and Fermi level, T is absolute temperature, k is the Boltzmann's constant, and q
is electron charge.
The relation between V
F
and electrical conductivity &sgr; is expressed as follows:
&sgr;=
q·n
(6)
where, n is number of carriers and &mgr; mobility of carrier.
n=N
c
/exp(
V
F
/kT
) (7)
wherein N
c
is effective density of states (quoted from “semiconductor device” written by S. M. Sze).
Consequently, electrical conductivity is expressed as follows:
&sgr;=
q·N
c
·&mgr;/exp(
V
F
/kT
) (8)
According to equation (8), there is a trade-off relation between a and &sgr;. That is, when V
F
is increased for attempting to increase Seebeck coefficient &agr;, the number of carriers n is decreased, causing &sgr; to decrease. Thermal conductivity &lgr; of single crystalline silicon markedly depends upon thermal conductivity of lattice, which is very high because silicon atoms are bonded by covalent bonding with each other. Thermal conductivity of lattice decreases with decreasing crystallinity of a crystal such as polycrystalline silicone, in which &mgr; also decreases, causing a to decrease. Consequently, performance parameters of thermoelectric material are parameters such as impurity concentration, crystallinity, size of thermoelectric element, and number of thermopiles, for which the most suitable design is required.
As a conventional example No. 1 of a thermopile, a thermopile attempted to be highly sensitive, produced by a process containing: laminating thin films of SiO
2
and SiN on a single crystalline silicon substrate by using semiconductor photolithography processing technology; employing a combination of p-type polycrystalline silicon-Au/Cr and n-type polycrystalline silicon-Au/Cr as a thermoelectric material; and forming hollow portions by anisotropic etching of the substrate using Ethylene Diamine Pyrocatechol (EDP) so as to allow a heat-sensitive part to have a membrane structure, is reported (“A Silicon-Thermopile-Based Infrared Sensing Array for Use in Automated Manufacturing”, IEEE Trans. Electron Devices, vol. ED-33 No. 1, pp 72-79, 1986).
In
FIG. 21
, there is shown a conventional example No. 2 of a thermopile attempted to be highly sensitive, which is disclosed in Japanese Patent Application Laid-Open No. H3-191834. This thermopile is produced by a process containing the steps of: forming an epitaxial layer
21
on a single crystalline silicon substrate
20
; forming a thermopile material of p-type diffusion layers
22
in the epitaxial layer
21
; forming a thermopile material of n-type polycrystalline silicon layers
24
thereon through an insertion of insulator
23
; and connecting with the polycrystalline silicon layers
24
by forming an aluminum layer
25
, thereby constructing a multiple layers-thermopile structure consisting of single crystalline silicon-aluminum-polycrystalline silicon. This is a thermopile, in which multiple layers of thermopile material are constructed so as to increase the number of thermopiles per unit area, thereby attempting to achieve high output and chip size miniaturization.
However, the above conventional thermopiles have problems as explained in the following. In the conventional example No. 1 of thermopile, thin films of SiO
2
and SiN are laminated on a single crystalline silicon substrate, and Au/Cr and polycrystalline silicon having a large Seebeck coefficient are employed as thermoelectric material. Since Au has high thermal conductivity, dissipating of absorbed heat is large, resulting in an insufficient ratio of temperature rise at the light-receiving part. Accordingly, the problem is that sufficient temperature rise cannot be obtained.
Further, a p-type polycrystalline silicon is used as a thermoelectric material, in which the majority carrier, i.e. hole has low mobility, causing high electrical resistivity. Consequently, S/N ratio, i.e. ratio of output voltage to Johnson noise becomes low, causing the thermopile to have inferior accuracy when used as a noncontacting temperature sensor. Further, since a pattern of polycrystalline silicon thermoelectric material is not arranged over a whole thin membrane part, bends and cracks are easily occurred in the film upon an anisotropic etching, causing a problem of deterioration in the yield.
On the other hand, the conventional example No. 2 of thermopile has tried to solve the above problem of the conventional example No. 1 and also to achieve higher sensitivity. Single crystalline and polycrystalline silicon are employed as the thermoelectric material, each of which is formed as a p-type and n-type semiconductor, respectively, and then, electromotive force polarity of which is set up to be reverse with each other, attempting to achieve higher sensitivity. However, since the thermopile employs a p-type diffusion layer formed on a surface of single crystalline silicon substrate as one of the thermoelectric materials thereof, it has been needed to leave a single crystalline silicon and an epitaxial layer having thickness of 5 &mgr;m or over including a thickness of the p-type diffusion layer.
Seebeck coefficient of single crystal
Endo Haruyuki
Fuse Takeshi
Matsudate Tadashi
Okada Toshikazu
Tanaka Yasutaka
Ishizuka Electronics Corporation
McDermott & Will & Emery
Parsons Thomas H
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