Abstract:
A conductivity type thin film humidity sensor on a silicon chip which sensor includes structure which electrically shields the sensing area from highly dissociative contaminants.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The field of the invention is that of humidity sensors which are compatible with silicon technology and include structure which electrically shields the sensing area from highly dissociative contaminants. There are other known types of humidity sensors having contaminant protection devices such as shown in U.S. Pat. Nos. 3,961,301 and 4,080,564. 
     The present invention relates to a porous SiO 2  or FE 2  O 3  conductivity type humidity sensor. It has electrode layers incorporated in the sensor structure which are energized to prevent contaminants such as H 2  SO 4 , HNO 3 , NaOH, KOH, NaCl and CuCl 2  from reaching the active area of the sensor structure. Applied bias voltages minimize baseline drift by preventing contaminants from entering the sensing region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE of the drawing is a cross sectional view of the humidity sensor according to the invention. 
    
    
     DESCRIPTION 
     The present invention is directed to a humidity sensor structure which minimizes the effect of air borne contaminants such as sulfates, nitrates and other contaminants on the stability of the sensor. Referring now the FIGURE there is shown as a supporting substrate for the sensor a silicon chip 10 which has sputtered thereon about 1000 A° of Ni-Fe to form one electrode 11 of the sensor. Over the electrode is sputtered porous SiO 2  or Fe 2  O 3  in a layer 12 about 2 μm thick. Then a porous electrode 13 of &#34;clumped&#34; gold or platinum consisting of interconnected clumps having an average thickness of about 100 A° is deposited by sputtering. Elements 11, 12 and 13 form the basic conductivity-type sensor. Sputtering a second portion of porous SiO 2  or Fe 2  O 3  continues at 14 for a thickness about the same as layer 12 or about 2 μm. A second porous electrode 15 like electrode 13 is then sputtered. Finally a porous overlayer 16 of SiO 2  or Fe 2  O 3  is sputtered comparable to layers 12 and 14. Electrical contact terminals 20, 21 and 22 make contact with electrodes 11, 13 and 15 respectively. These may be integrated on the silicon chip. 
     In the sputtering of the porous SiO 2  or Fe 2  O 3  layers described above, these layers are preferably originally sputtered down as an alkali glass, or as as a SiO 2  -B 2  O 3  mixture, or as a Fe 2  O 3  -B 2  O 3  mixture. When the sputtering operations are completed the structure may be annealed to permit the proper amount of phase separation. The structure is then boiled in water to leach out the interconnected phase regions containing the alkali oxides, or the B 2  O 3  molecules leaving the porous SiO 2  or Fe 2  O 3  structure. Variations of the mixture of the sputtering oxides and also the conditions of the anneal are effective in adjusting to a preferred value the final porosity of the finished sensors. In use, atmospheric water concentrations are sensed when the water vapor in the ambient air equilibrates with adsorbed water on the internal surfaces of the porous structure by surface diffusion through the pores, and thus alters the AC conductivity of the lower layer 12. 
     In certain applications it may be desirable to modify the completed sensor by an additional step, and convert the porous SiO 2  to porous silicon nitride (Si 3  N 4 ). An advantage of a nitride is that the silicon nitride material, per se, is not permeated with or penetrated by water vapor to the extent that the silicon oxide material is. The conversion is accomplished by heating the completed silicon oxide structure in an ammonia (NH 3 ) atmosphere at about 800-900 C. so that a reaction takes place in which the nitrogen replaces the oxygen thereby converting the SiO 2  to Si 3  N 4 . One reason for converting is that under certain operating conditions there may be a tendency for an undesirable limited hydrolyzing to occur with the SiO 2  which will not occur with the Si 3  N 4 . 
     In operation, an AC potential is applied to the sensor terminals 20 and 21 so that the proper sensing level is applied across sensing element 12 and the conductivity of the circuit is measured. The humidity diffuses through the pores of layers 16 and 14 and into sensing layer 12 where changes in humidity result in changes in conductivity measured. The layers of porous SiO 2  16 and 14 together with electrodes 15 and 13 may be referred to as contaminant capture layers. A DC potential is applied across terminals 21 and 22 and the electric field capture layer repels the ionized atoms of the strong electrolytes which ionize easily, and prevent them from penetrating to the sensing layer 12. The polarity of the DC bias voltage on the conductive layer 22 prevents the negative ions of the contaminants (i.e. SO 4 , NO 3 , etc.) from diffusing across to the sensing region 12. While two porous gold electrode layers 13 and 15 are shown in the drawing, a modification would include another porous electrode layer spaced above layer 15 and electrically biased with the opposite polarity where positive contaminant ions prove to be a problem. As examples, such airborne contaminants which may diffuse into the sensor and cause erroneous readings include acids such as H 2  SO 4  and HNO 3 , such gases as NaOH and KOH, and such salts as NaCl and CuCl. The important result of the capture layer as shown in the FIGURE is that the negative ions of highly dissociative compounds will not get to the sensing layer. The second layer and electrode spaced above 15 and oppositely biased prevent the corresponding positive ions from reaching the sensing layer. The second layer and electrode are directed particularly toward preventing positive ions other than hydrogen ions from reaching the sensing region. Hydrogen ions that might tend to collect near a negative electrode would combine to form H 2  and thus would diffuse out of the structure harmlessly as a gas. Other positive ions would not diffuse out as a gas.