Method for trimming a planar capacitor
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for trimming a planar capacitor. More particularly, the present invention relates to a method for trimming a planar capacitor in which method a laser emission is focussed to an electrode of a capacitor formed on a substrate transparent to laser emission and including an upper electrode, an insulation layer, and a lower electrode in order to achieve a passivation effect on the electrode. 
     2. Description of Related Art 
     Known from U.S. Pat. No. 3 597 579 is a trimming method for capacitors, in which method the area of the capacitor electrode is altered by the use of radiation energy and, in the interest of avoiding the occurrence of short circuits, a voltage is applied between the capacitor electrodes. 
     A disadvantage of conventional technology is that the above described method is applicable only to a restricted selection of materials. The trimming operation is performed on the upper capacitor electrode thus leaving the trimmed area subject to contamination and degradation particularly when the sensor is used in a humid environment. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to overcome the drawbacks of the aforementioned prior art technology and to provide a completely novel method for trimming a planar capacitor. 
     The invention is based on trimming the capacitor by oxidizing the lower electrode by heating the electrode through the substrate using, for example, a laser. 
     More specifically, the method in accordance with the invention is characterized by a laser emission being focussed locally onto the lower electrode through the substrate in order to achieve a heating effect on the lower electrode, thereby oxidizing the electrode material so that the lower electrode is locally converted into an electrically non-conductive state while being protected by the substrate and the insulation layer. 
     The invention is significantly beneficial because the capacitor can be accurately trimmed to a desired capacitance value, and this value will stay stable, since the lower electrode remains protected under the insulation layer. 
     In the following, the invention will be examined in more detail by means of the exemplifying embodiments in accordance with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a longitudinally sectional side view of a typical capacitor construction, which is processed using the method according to the present invention; and 
     FIG. 2 shows a bottom view of another typical capacitor construction, which is compatible with the use of the method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to FIG. 1, the capacitor comprises a substrate 1, which generally is of glass. Alternatively, other substrate materials transparent to laser emission may be used. Fabricated on the substrate 1 is a lower electrode 2, whose material can be of, for example, aluminium. Next covering the lower electrode is an insulation layer 3, which in a humidity sensor construction, is made of a suitable water absorbing polymer. Deposited on the insulation layer 3 is an upper electrode 4, which in a humidity sensor construction is fabricated from a humidity-permeable material. Capacitor trimming is achieved by focusing a beam 6 of a laser 5 through the substrate 1 onto the lower electrode 2, thereby heating the lower electrode locally by the beam 6, which leads to an oxidation of the lower electrode 2 into an electrically nonconductive state. In this method the active area of the capacitor is reduced and the capacitance decreased to a desired value. The laser 5 may be, for example, a Q-switched Nd:YAG laser, whereby a proper drive current is, for example, 20 A. 
     Alternatively, the lower electrode 2 may be fabricated from tantalum. 
     A capacitor according to FIG. 2 has lower electrodes 8, covered by an insulation layer (not shown) vapor deposited on the glass substrate 7. A surface electrode 9 is further plated onto the insulation layer. The oxidation process described above in conjunction with FIG. 1 is carried out over the lower electrode area which, according to FIG. 2 remains below the surface electrode 9. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.