Peltier effect device to detect in particular a condensation risk on a surface being in contact with a wet air volume

A Peltier effect device which detects in particular a condensation risk, includes a substrate and semiconducting bands disposed on the upper face of the substrate. The junctions connecting said bands which make up a series circuit are formed by semiconducting bands of N-type and P-type. Junctions of the same type, i.e., N-P type are situated on the central zone of the upper face of the substrate and defines a detection zone of the device. Semiconducting bands of one type are placed on one side of the upper face of the substrate and bands of the other type are placed on the other side of the upper face of the substrate. The substrate also includes at the peripheral zone of each band, except for a frontmost N-type band and a rearmost P-type band, a plated hole extending through the substrate to a lower face of the substrate and to a plating of the lower face such that a plated hole situated at an end of the P-type band is connected to a plated hole situated at an end of a next N-type band.

BACKGROUND OF THE INVENTION 
The present invention concerns a Peltier effect device aiming in particular 
at detecting a condensation risk on a surface being in contact with a wet 
air volume. 
Devices which detect the dew-point temperature and the ambient air relative 
humidity are already known. 
The U.S. Pat. No. 4,677,416 document describes a device with a substrate on 
the upper face of which two semiconducting PbTe bands, alternately of 
N-type and P-type, are mainly formed. The bands extend between a 
peripheral zone and a central zone of the upper face of the substrate, and 
are connected in series by N-P and P-N type junctions and constitutes a 
circuit alternately formed of N-type bands and P-type bands. All N-P type 
junctions are situated in the central zone of the substrate, and all P-N 
type junctions are situated in the peripheral zone of the substrate. 
In such a device, when current passes in the series circuit, the central 
zone of the substrate is cooled down by the Peltier effect. A quantity of 
heat is absorbed by the junctions present in the central zone of the 
substrate and released with an approximate amount equivalent to the Joule 
effect, by the junctions present in the peripheral zone of the substrate. 
However, the efficiency of this device may prove to be insufficient because 
the peripheral junctions are situated on the same face of the substrate as 
the detection zone, such that while the detection zone is cooled down by 
the Peltier effect, it may receive a part of the thermal energy released 
by the peripheral junctions. 
Besides, this thermal energy is mainly transmitted by conduction in the 
thickness of the substrate before being dissipated. 
Such inconveniences do not really matter in case of a dew-point temperature 
measurement, but when a condensation risk has to be detected, for instance 
by implementing the process described in the unpublished patent 
application No. 93 02099 of the requesting party, it is better to have a 
Peltier effect device which is able to quickly dissipate the thermal 
energy released by the junctions situated outside the detection zone, such 
that the detection zone is not subject to the influence of such 
dissipation. 
SUMMARY OF THE INVENTION 
The present invention aims at providing a Peltier effect device, 
particularly fit for the implementation of a process of detection of 
condensation risk on a surface in contact with a wet air volume, which has 
the advantages of quickly dissipating the thermal energy released by the 
junctions placed outside the detection zone and of thermally decoupling 
this detection zone from the junctions placed outside the detection zone. 
In addition, the device according to the invention includes a detection 
zone with a favorably weak thermal mass compared to that of the zone of 
the substrate where the dissipation of energy, released by the junctions 
situated outside the detection zone, occurs. 
Thus, the thermal inertia of the detection zone is weak enough to allow the 
zone to react promptly and with an optimal thermal efficiency, to any 
heating or cooling produced by the central junctions of the substrate when 
electric current is supplied to them, such that electricity consumption is 
reduced. 
Furthermore, the realization mode of the device according to the invention 
is particularly easy and economical. 
An object of the present invention is to provide a Peltier effect device, 
notably for the detection of a condensation risk on a surface in contact 
with a wet air volume, which includes substrate on which semiconducting 
bands of N and P types are formed and extends between a peripheral zone 
and a central zone of the upper face of the substrate. The semiconducting 
bands are connected by N-P and P-N type junctions in a series circuit 
formed alternately by N-type bands and P-type bands. All the N-P junctions 
are situated in the central zone of the upper face of the substrate and 
form a detection zone of the device, wherein the N-type semiconducting 
bands are placed on one side of the upper face of the substrate while the 
P-type semiconducting bands are placed on the other side of the upper face 
of the substrate. The substrate includes at an end of each band situated 
in the peripheral zone, a plated hole going through the substrate and 
emerging on the lower face of the substrate. A plating of its lower face 
forms junction bands which connect a crossing hole situated at an end of a 
P-type band to another crossing hole situated at an end of a next N-type 
band. 
It is understood that the platings going right through the substrate and 
extending on its lower face almost parallel to the semiconducting bands 
placed on its upper face, make up junctions of the same type (for instance 
P-N) which, if associated to the junctions (for instance of N-P type) 
present in the detection zone of the substrate, form a Peltier element and 
produce thermal exchanges between the central zone of the substrate and 
the outside of the said substrate. 
The junctions present in the central zone of the substrate upper face are 
preferably covered with an electrically insulating and thermally 
conductive material to form the detection zone of the device according to 
the invention. 
As explained before, the thermal mass of this material is weak, so that the 
detection zone can react properly to the thermal stresses of the central 
junctions. 
According to a preferred embodiment of the invention, the semiconducting 
bands are realized by thermal diffusion of impurities in silicon. For this 
purpose, arsenic and boron may be used as impurities. 
It is preferable to realize a high doping (of the order of 10.sup.19 
carriers per cm.sup.3), which has the following two advantages. 
First, since the electrical conductivity of the semiconducting bands is 
much higher than that of silicon (which has only approximately 10.sup.10 
carriers per cm.sup.3), there is practically no risk of electric leakage 
in the substrate. 
Second, the contact between the N-type bands and the P-type bands is of 
good quality and it is not necessary to overdope silicon at the junction 
level to get a ohmic-type contact. 
In order to thermally insulate the junctions present on the upper face of 
the substrate from the junctions present on the lower face, it is 
possible, according to a first variation, to place a thickness of an 
electrically and thermally insulating material, for instance a silicon 
oxide SiO.sub.2, in the central part of the substrate, this central part 
extending preferably not further than the holes within the substrate. 
It is indeed preferable to realize the crossing holes in the thermally 
conductive zone of the substrate, in order to make easier the transfer and 
dissipation of thermal energy released in the platings of the crossing 
hole, although this thermal energy is not dominant if compared to the 
energy released by the junctions placed on the lower face of the 
substrate. 
According to a second variation, the central part of the upper face of the 
substrate may be hollowed out to a given thickness, in order to separate 
the semiconducting bands from the bottom of the cavity by an air layer 
serving thus as a thermal insulator. 
The result is a better output of the device and an optimal decoupling 
between the detection zone and the junctions placed outside this detection 
zone. 
It is clear that, due to the surface of the junctions placed on the lower 
face of the substrate, the thermal energy released by these junctions may 
be directly transmitted by conduction to a radiator on which the substrate 
is placed, such that the radiator secures the dissipation of this energy 
by convection. 
In order to detect a condensation risk on a surface, the lower face of the 
device according to the invention is placed against the surface which acts 
as a radiator by realizing thermal exchanges with the junctions situated 
on the lower face of the substrate. 
In addition, due to the compactness of the device according to the 
invention, if there is no electric current circulating in the Peltier 
circuit, the whole substrate is of a homogeneous temperature which is that 
of the surface on which a condensation risk is detected. 
In this way, the process of detection may be implemented knowing that the 
initial temperature of the detection zone is essentially the same as that 
of the surface. 
Besides, due to the fact that all the N-type semiconducting bands are 
arranged on one side of the substrate whereas all P-type semiconducting 
bands are arranged on the other side of the substrate, the technical 
realization of the device according to the invention is easy. 
Indeed, one needs only to make a thermal diffusion of an impurity, for 
instance arsenic, to form a N-type layer sensibly rectangular on one side 
of the substrate upper face, and then form N-type bands by photo etching. 
The same process is used for the P-type bands, by diffusing, for instance, 
boron. 
In a preferred embodiment of the invention, the junctions present in the 
central zone of the substrate upper face are also realized by surface 
plating, by covering the nearby ends of the N-type and P-type 
semiconducting bands. 
Preferably, the substrate may also include other electronic circuits 
allowing the implementation of a process to determine a condensation risk, 
namely the one described by the requesting party in its unpublished patent 
application No. 93 02099. 
In particular, the substrate may integrate an amplification stage to 
provide amplified voltage signals to the terminals of the series circuit 
formed on the substrate, in order to analyze these signals and to conduct 
the process of detection. 
According to another embodiment of the invention, one of the junctions of 
the detection zone is reserved to the realization of a thermocouple, while 
the other junctions are connected, as described above, to form the Peltier 
circuit. 
The thermocouple thus realized allows the measurement of temperature of the 
detection zone during the execution of the detection process. 
This realization mode is advantageous because the thermocouple does not 
require a specific realization stage. Actually, the thermocouple results 
from the same thermal diffusion and photo etching operations as the other 
junctions. It is only necessary to make an additional surface plating 
connecting the terminals of this thermocouple to those of the electronic 
circuit, for instance, on the substrate. 
In a preferred embodiment of the invention, the substrate is covered with a 
protective layer, thermally and electrically insulating, made of, for 
instance, Silicon nitride Si.sub.3 N.sub.4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The device according to the invention is presented here as a rectangular 
plate 1 with an upper face 1a and a lower face 1b. 
The plate 1 constitutes a substrate made with an electrically insulating 
and thermally conductive material such as silicon. 
In its central part, the plate 1 includes a rectangular cavity 2 extending 
from its upper face 1a to its lower face 1b, and is filled up with a 
thermally and electrically insulating material such as silicon oxide 
SiO.sub.2. 
On the upper face 1a of the substrate, above the thermally and electrically 
insulating material, semiconducting bands of N-type 3 and P-type 4 have 
been formed by thermal diffusion of arsenic and boron. 
These bands extend from the median axis 5 of the substrate 1 to the 
thermally conductive peripheral zone of the substrate. 
The semiconducting bands of N-type 3 and P-type 4 are arranged 
symmetrically with respect to the median axis 5. All the N-type bands 3 
are on one side of axis 5, while all P-type bands 4 are on the other side 
of the median axis 5. 
The semiconducting bands have been formed by thermal diffusion of ions on a 
rectangular part extending on the corresponding side of the substrate 
upper face 1a, each layer having been then photo etched. In FIG. 1A, a 
vertical junction between the N-type band 3 and P-type band 4 is 
represented. 
In practice, it may be difficult to realize such a vertical junction. It is 
then preferred according to a variation of the invention, to space out the 
ends of the bands 3 and 4 and to form a junction by application of a 
surface plating partly covering the end of the said bands 3 and 4. 
Besides, the junctions between N-type bands 3 and P-type bands 4 are 
covered with a band 6 of an electrically insulating and thermally 
conductive material extending along the median axis 5. 
The material of the band 6 may advantageously be the same as that which 
forms the peripheral zone of the substrate 1. The band 6 will preferably 
have a weak thermal mass as already explained. 
The upper face of the band 6 constitutes the detection zone which is 
subject to heating and cooling phases when the process of detecting a 
condensation risk is implemented. 
At ends of the bands 3 and 4 adjacent to peripheral zone of the substrate 
1, crossing holes 7 are made, joining the upper face 1a of the substrate 
to its lower face 1b. 
The crossing holes 7 include a volume plating 8 to electrically connect the 
semiconducting bands 3 and 4. 
On the lower face 1b of the substrate, electrically conductive bands 9 are 
realized by plating and photo etching of the lower face 1b of the 
substrate. 
The bands 9 join the crossing holes 7 situated at an end of the N-type 
semiconducting band 3 to the crossing holes 7 situated at an end of the 
P-type semiconducting band 4, as it can be more clearly seen in FIG. 3. 
It is understood that the volume platings 8 and the surface platings 9 
constitute the junctions which, combined with the junctions formed on the 
upper face of the substrate, form the heating and cooling parts of a 
Peltier effect series circuit which includes sequentially a N-type band 3, 
a N-P junction, a P-type band 4, a P-N junction and so on. 
Due to the presence of the thermally insulating material in the cavity 2, 
the junctions of the upper and lower faces are thermally decoupled. 
One can clearly see that the junctions formed by the platings 9 which 
extend on the lower face 1b of the substrate can efficiently transmit 
their thermal energy to a radiator against which they are applied, or to a 
surface on which the condensation risk has to be detected. 
The terminals 10 and 11 of the Peltier circuit are connected, by means of 
platings 12, to an integrated circuit 13 also realized on the substrate. 
This circuit 13 may integrate notably an amplification stage to supply an 
amplified signal of the voltage present at the terminals 10, 11 of the 
Peltier effect series circuit. Power supply contacts 14 are formed on the 
upper face 1a of the substrate and allow to connect the device to an 
electric power source. 
As represented in FIGS. 2 and 2A, the device thus realized may include a 
protective layer 15, for instance, of silicon nitride Si.sub.3 N.sub.4, 
covering its lower face lb and its upper face 1a except for the detection 
band 6 and the power supply contacts 14 that have to stay in open air for 
the electric connection of the device according to the invention. 
In a non illustrated variation, the N-type 3 and the P-type 4 bands could 
be radially distributed on a circular shaped substrate, which would allow 
to realize a dot-shaped detection zone 6 situated at the center of the 
said substrate and having consequently a very limited surface, and thus 
allowing an even greater reduction of its thermal mass. 
The FIGS. 4, 4A, 4B and 4C represent a realization mode of the invention in 
which the central cavity 2' does not extend down to the whole thickness of 
the substrate 1' but includes a bottom 2'a. 
The cavity 2' has been hollowed on the upper face of the substrate 1' after 
N-type 3 and P-type 4 bands were formed. 
An air layer separates the semiconducting bands 3 and 4 from the substrate 
1' and constitutes a thermal insulator allowing to decouple the detection 
zone from the junctions 8 and 9. 
FIG. 4 also illustrates a particular realization mode, compatible with the 
description mode of FIGS. 1 and 2 according to which one of the central 
junctions is used outside the Peltier circuit as a thermocouple. 
To that purpose, the bands 3' and 4', the junction of which is also covered 
by the band 6, are directly connected to the integrated circuit 13 by 
platings 16. 
Thus, it is possible to conduct a detection process such as the one 
described in the patent application No. 93 02099 of the requesting party 
by controlling the exact values of temperature of the detection zone of 
the device. Due to the integrated circuit, it is possible to correct the 
possible temperature drifts of this detection zone with respect to the 
initial temperature of the surface on which a condensation risk is 
detected. 
It is obvious that the realization modes which have just been described, 
are not restrictive and that they may undergo any desirable modification 
without departing from the spirit or the scope of the invention.