Capacitive humidity sensor and method for the manufacture of same

The present publication describes a capacitive humidity sensor and a method for the manufacture of same. The sensor comprises a substrate (1), first (5) and second bottom electrodes (4), which have been fitted close to each other on the substrate (1), an active layer (7), which fills the area between the first (5) and the second bottom electrodes (4) on the substrate (1) and extends to above the top face of the second electrode (4). According to the invention, the sensor further consists of structures (6) connected to the top face of the first electrode (5), made of a conductive material, and being beam-shaped with an upwardly widening section and being placed side by side, the said structures (6) limiting portions (7) of the active layer between themselves from two sides, and a narrow slot (26) being provided between the top faces of the said structures. Owing to its construction, the sensor is not sensitive to touching.

The present invention is concerned with a capacitive humidity sensor. 
BACKGROUND OF THE INVENTION 
The invention is also concerned with a method for the manufacture of such a 
humidity sensor. 
FIGS. 1 and 2 show the cross-sections of two capacitive humidity sensors of 
types known in the prior art. In the construction in accordance with FIG. 
1, there are two bottom electrodes 2 between the substrate 1 and the 
isolation layer 15. In addition to the parts of the above construction, 
the construction of FIG. 2 also includes a surface electrode 3 placed on 
top of the isolation layer 15. 
As a rule, the electrode structure of a capacitive humidity sensor must 
meet, e.g., the following requirements: 
(a) humidity must have unhindered access of penetration into the isolation 
material, 
(b) the electrodes must be electrically well conductive and mechanically 
durable, and 
(c) the electric field between the electrodes must not penetrate onto the 
sensor surface, where there may be electrically conductive impurities. 
The construction of FIG. 1 meets the requirements a and b, but not the 
requirement c. The construction of FIG. 2, on the other hand, meets the 
requirement c, but the requirements a and b substantially exclude each 
other. 
As regards the state of prior-art technology, reference should be made in 
particular to the following publications: 
[1] K. E. Bean, "Anisotropic Etching of Silicon" IEEE Transactions on 
Electron Devices UED-25 (1978) No. 10, pp. 1185-93 
[2] U.S. Pat. No. 3,397,278 (Pomerantz) 
[3] FI Pat. No. 48,229 (Suntola). 
OBJECT AND SUMMARY OF THE INVENTION 
The object of the present invention is to provide a capacitive humidity 
sensor which meets all of the above requirements a, b and c, as well as a 
method for the manufacture of such a sensor. 
The invention is based on the idea that the first bottom electrode is 
connected to the sensor surface by means of conductive structures 
expanding from the support base outwards so that only small slots remain 
between the top edges of these structures and that the active layer 
remains in the hollow space formed by adjoining structures. 
More specifically, the capacitive humidity sensor in accordance with the 
invention is characterized in what is stated in the characterizing part of 
claim 1. 
On the other hand, the method for the manufacture of the humidity sensor is 
characterized in what is stated in the characterizing part of claim 6. 
By means of the invention, remarkable advantages are obtained. Thus, the 
sensor in accordance with the invention meets all of the requirements a, b 
and c mentioned above. Nor is it sensitive to touching. Moreover, its 
handling is easy.

DETAILED DESCRIPTION OF THE INVENTION 
The capacitive humidity sensor shown in FIGS. 3 and 4 has electrode 
patterns 4 and 5 on a glass substrate 1. The electrodes are of a 
chemically resistant metal (e.g., Pt, Pd, Ta, Au), and they are protected 
by a thin isolator film 25 (e.g., Ta.sub.2 O.sub.5, Si.sub.3 N.sub.4, 
SiO.sub.2, etc.). The metal pattern 5 creates an electrical contact with 
the beams 6, which are of trapezoidal section. They are made of silicon, 
metal, or of some other conductive material and are fixed to the substrate 
1. Between the top portions of the beams 6, there is a narrow (e.g., about 
1 .mu.m) slot 26. Between the beams 6, there is an insulator material 
sensitive to humidity, e.g. an appropriately selected polymer 7. The 
capacitance sensitive to humidity is formed between the metal areas 4 and 
the side walls 8 of the beams 6. 
The construction described can be manufactured, e.g., as follows (FIGS. 6 
to 12): 
An epitactic layer 10 weakly doped with boron (N lower than 10.sup.17 
1/cm.sup.3) is deposited on a (100)-directional silicon disc 9 strongly 
doped with boron (N higher than 10.sup.17 1/cm .sup.3). The surface of the 
layer 10 is doped strongly with boron (N higher than 10.sup.20 
1/cm.sup.3). The thickness of the doped surface layer 10b is about 1 
.mu.m. Into the layer 10b, areas 11 are patterned photolithographically in 
which the surface layer is retained (from the remaining areas, it is 
etched off by means of an etching agent which is selective in respect of 
strongly doped silicon). When the layer 10 is etched in an appropriate 
etching agent 1 (e.g., KOH), beams 12 with diagonal edges and of a 
thickness equal to the thickness of the layer 10 are obtained at the areas 
11. The sides 13 of the beams 12 are parallel to the crystal plane (111) 
and form an angle of 54.74.degree. with the face of the disc. 
Onto the glass substrate 14, a thin metal film was deposited by means of 
prior art methods, which metal film was photolithographically patterned to 
form areas 16 and 17. Onto the areas 16 and 17, it was possible to form an 
isolator film 18, e.g. by anodically oxidizing the metal film, as well as 
contact areas 19 and 20 of a different metal, which areas can be omitted 
if the soldering quality of the metal film is good and it makes a good 
contact with p.sup.+ -Si (silicon strongly doped with boron). 
The substrate 14 and the silicon disc 9 are placed against each other so 
that the fingers 21 on the metal area 16 (FIGS. 9 and 10) reach contact 
with the beams 12 on the silicon disc and the fingers 22 on the area 17 
become placed between the beams 12 without reaching contact with them. In 
this position, the glass substrate and the silicon disc 9 are joined 
together by means of the method described in the cited publication No. 2. 
The faces of the beams 12 form a chemical bond with the substrate 14. 
Hereinafter, the silicon disc 9 is etched off by means of an etching agent 
which etches the strongly doped area but not the weakly doped beams 12. 
The construction in accordance with FIGS. 11 and 12, wherein the beams 12 
adhere firmly to the substrate glass 14 and form an electrical contact 
with the fingers 21 on the metal area 16, is submerged into a solution of 
a polymer sensitive to humidity. The solution penetrates into the space 24 
between the beams 12. The construction is lifted off out of the solution 
and the solvent of the polymer is allowed to evaporate, whereby a solid 
polymer remains in the space 24. 
Within the scope of the invention, it is also possible to conceive 
solutions differing from the exemplifying embodiment described above. 
Thus, the beam structures 6 may also be, e.g., of T-section in the way 
shown in FIG. 4. The essential feature is that a narrow slot remains 
between the top faces of adjoining beam structures 6.