Abstract:
An integrated humidity sensor includes at least one measuring capacitor and one humidity-sensitive polymer as a dielectric that is also suited for use in a dirty, i.e., particle-laden measurement environment. The measuring capacitor of the humidity sensor is in the form of a plate capacitor in the layered structure of the sensor element, the outer of two electrodes being located at the surface of the layered structure. Disposed between the two electrodes of the measuring capacitor is a humidity-sensitive polymer layer that is in direct contact with the measurement environment via humidity-permeable paths in the outer electrode of the measuring capacitor. These humidity-permeable paths extend from the surface of sensor element to the polymer layer, and are so small in lateral extent that they do not significantly affect the electrical conductivity within the outer electrode.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an integrated humidity sensor that includes at least one measuring capacitor and one humidity-sensitive polymer as a dielectric that is in direct contact with the measurement environment. The present invention also relates to a method for manufacturing an especially advantageous variant of such a humidity sensor. 
       BACKGROUND 
       [0002]    Humidity sensors are used in air-conditioning systems, for example, which, besides room temperature, also monitor and control air humidity. Improving thermal comfort is not the only purpose of this control. The relative air humidity is also controlled for safety reasons in the passenger compartment of a motor vehicle, for example, namely to prevent or reduce as quickly as possible any fogging of the windows and thereby provide the driver with optimal visibility conditions. 
         [0003]    An integrated humidity sensor is known from the field, where measured value acquisition is performed capacitively. The measuring capacitor is realized here in the form of an interdigital capacitor, whose comb-like intermeshing electrodes are configured on the surface of a substrate. A humidity-sensitive polymer layer, which is located above the electrodes on the substrate surface, functions as a dielectric of the measuring capacitor, so that the electrodes of the measuring capacitor are embedded in the humidity-sensitive polymer. The substrate surface, which includes the polymer layer, is exposed to the measurement environment. Since the dielectric properties of the polymer are dependent on the humidity, the humidity of the measurement environment influences the capacitance of the measuring capacitor, allowing inferences to be made about the humidity of the measurement environment based on a change in the capacitance of the measuring capacitor. 
       SUMMARY 
       [0004]    However, acquiring measured values using the known humidity sensor is a relatively error-prone process. Since the polymer layer is in direct contact with the measurement environment, the deposition of particles, dirt, or liquid droplets from the measurement environment onto the polymer layer cannot be prevented in many applications. Such substances on the polymer layer, regardless of whether they are electrically conductive or dielectric, also influence the electrical field of the measuring capacitor due to the form and configuration of the electrodes and the embedding of the electrodes in the polymer layer. This inevitably leads to a falsification of the measurement signal. 
         [0005]    The present invention provides a humidity sensor that is also suited for use in a dirty, i.e., particle-laden, measurement environment. 
         [0006]    To this end, the measuring capacitor of the humidity sensor according to the present invention is realized in the form of a plate capacitor in the layered structure of the sensor element, the outer of the two electrodes being located in the surface of the layered structure. A humidity-sensitive polymer layer is disposed between the two electrodes of the plate capacitor. Humidity-permeable paths, which extend from the surface of the sensor element to the polymer layer, are formed in accordance with the present invention in the outer electrode of the measuring capacitor. The humidity-permeable paths are so small in lateral extent, that they do not significantly affect the electrical conductivity within the outer electrode. Thus, the humidity-permeable paths in the outer electrode provide a direct contact for the humidity-sensitive polymer layer of the measuring capacitor with the measurement environment. 
         [0007]    In the case of the inventive structure of the sensor element, the outer electrode of the plate capacitor not only functions as a component of the measuring capacitor, but also as a mechanical shield for the humidity-sensitive dielectric against relatively large particles, dirt, and liquid droplets. The present invention recognized, namely, that these types of substances on the outer electrode do not influence the capacitance of the measuring capacitor. However, since the present invention provides that the outer electrode of the measuring capacitor be permeable for the humidity of the measuring environment, and that the dielectric properties of the polymer layer between the two electrodes of the measuring capacitor be dependent on humidity, the measurement signal of the humidity sensor according to the present invention is substantially dependent on the humidity of the measurement environment. At any rate, any possible contamination of the measurement environment does not influence the measurement signal. 
         [0008]    In principle, there are different ways to realize the humidity-permeable paths in the outer electrode of the measuring capacitor, as long as the lateral extent thereof is small enough. 
         [0009]    Depending on the material and manufacturing process, the humidity-permeable paths may be realized in the form of a porosity, in the form of randomly distributed cracks, or also in the form of a defined patterning of the outer electrode. 
         [0010]    The outer electrode is preferably formed in a thin metal layer since there are processes for producing a suitable porosity or also a defined patterning in a metal layer of this kind. Thus, a thin metal layer may be photolithographically patterned, for example. This method is suited, in particular, for producing a defined grid structure in the electrode region. 
         [0011]    It is intended that the grid structure preferably extend over the entire surface area of the polymer layer, allowing the humidity, depending on the moisture content of the measurement environment, to penetrate, and be released via the grid openings uniformly and over the entire surface into the polymer layer. Moreover, to achieve the smallest possible diffusion lengths, the width of the grid bars should be smaller or equal to the thickness of the polymer layer. A layout of this kind contributes to the shortening of the response time of the measuring capacitor. 
         [0012]    In a manufacturing method according to an example embodiment of the present invention, the humidity-permeable paths in the outer electrode of a humidity sensor, that are formed in a metal layer, are realized in the form of cracks. This merely requires implementation of an annealing step following deposition of the metal layer over the polymer layer. In this process, the polymer layer expands to a significantly greater degree than the superjacent metal layer, causing it to pull apart. The cracks are thereby randomly formed, but are uniformly distributed over the electrode surface. Following cooling, the cracks in the metal layer close up again, humidity-permeable paths remaining in the metal layer, however. Under a suitable annealing temperature, a cohesive metal layer is formed in this manner as an electrode that is electrically conductive and, nevertheless, humidity-permeable. Thus, the method according to the present invention exclusively employs standard processes that may be readily integrated in the overall chip production process. 
         [0013]    In any case, the outer electrode of the humidity sensor according to the present invention should be of a most media-resistant possible design since it is configured in the surface of the sensor element and is in direct contact with the measurement environment. To that end, the outer electrode may be provided with a suitable coating, for example. In one example embodiment of the humidity sensor according to the present invention, the outer electrode of the measuring capacitor is realized in a corrosion-resistant metal layer, such as in an Au or Pt layer, for example. In this case, the need for such a coating may be eliminated. 
         [0014]    It is noted here that, in principle, the material of the lower electrode may be selected to be independent of the material of the upper electrode. However, it is advantageous that the same material be selected for the upper and lower electrodes to prevent electrolysis-induced corrosion since, upon readout, the two electrodes are at different electric potentials. 
         [0015]    In one especially advantageous example embodiment of the present invention, the lower electrode of the measuring capacitor is of a meander-shaped design. Moreover, an arrangement is provided for optionally energizing the lower electrode. In this example embodiment, a current for heating the polymer layer may be conducted through the meander-shaped electrode in order to accelerate the release of moisture from the polymer. The humidity sensor&#39;s response time may be significantly reduced in this manner. 
         [0016]    In any case, according to example embodiments of the present invention, the outer electrode of the measuring capacitor is preferably connected to ground potential since, in this manner, an electrolytic destruction of the measuring capacitor in an aggressive measurement environment may be prevented. 
         [0017]    One advantageous example embodiment of the humidity sensor according to the present invention provides that, in addition to the measuring capacitor, in the layered structure of the sensor element, at least one reference capacitor is provided whose design essentially corresponds to that of the measuring capacitor. However, in contrast to the measuring capacitor, the outer electrode of the reference capacitor does not include any humidity-permeable paths, so that humidity is not able to penetrate here into the humidity-sensitive polymer layer. Accordingly, the capacitance of the reference capacitor is humidity-independent. An appropriate evaluation circuit may be used to analyze the difference between the signals of the measuring capacitor and the reference capacitor. This makes it possible to not only reduce the influence of material parameters of the capacitors on the sensor signal, but also the influence of disturbance parameters, such as the temperature effects arising from the thermal expansion of the polymer, or the influence of long-term drifts which occur simultaneously in both capacitors. 
         [0018]    To prevent an electrolytic destruction of the electrodes in this case as well, according to an example embodiment, the outer electrodes of the measuring capacitor and of the reference capacitor are connected to ground potential, since both certainly come in contact with the measuring medium. 
         [0019]    In one especially space-saving example embodiment of the humidity sensor according to the present invention, at least parts of an evaluation circuit for the measuring capacitor are integrated in the layered structure underneath the measuring capacitor. Since the measuring capacitor is realized as a plate capacitor in accordance with the present invention, the electrical field of the measuring capacitor is not thereby influenced. 
         [0020]    The teaching of the present invention may be advantageously embodied and further refined in various ways. The following is a description of two example embodiments of the present invention, with reference to the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIGS. 1   a - 1   c  each shows a section through the layered structure of a humidity sensor  10  according to an example embodiment of the present invention, in successive stages of the manufacture thereof. 
           [0022]      FIG. 2  shows a section through humidity sensor  10  in accordance with an optional manufacturing step for shortening the diffusion paths in the polymer layer, according to an example embodiment of the present invention. 
           [0023]      FIG. 3  shows a section through humidity sensor  10  including the molded housing, according to an example embodiment of the present invention. 
           [0024]      FIG. 4  shows a section through a humidity sensor  40  including a measuring capacitor and a reference capacitor. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The design of humidity sensor  10  shown in  FIG. 1   c  is the result of a manufacturing process that is explained in the following with reference to  FIGS. 1   a - 1   c.  The starting point of this manufacturing process is a semiconductor substrate  1  that has been equipped with a microelectromechanical systems (MEMS) functionality in the course of a preprocessing. The MEMS functionality is merely shown schematically here and is denoted by  20 , and may include parts of an evaluation circuit, for example, that are integrated in the substrate surface. 
         [0026]    The sensor function of humidity sensor  10  is realized here in a layered structure on the substrate surface and as a function of MEMS functionality  20 . To this end, the substrate surface is first provided with an electrically insulating oxide layer  2 , which, in the course of a patterning process, is merely open in connecting regions  21  and  22  for electrically contacting evaluation circuit  20 . Deposited onto patterned oxide layer  2  is a metal layer  3  that functions as first electrode layer. The metal layer may be formed of Al, AlSiCu, AlCu, Au, Pt, or a similar material, for example. Patterned from this metal layer  3  is first, lower electrode  31  of a measuring capacitor, as well as a connecting lead  32  to connecting region  21 , where electrode  31  is connected to evaluation circuit  20 . A passivation layer  4  is then deposited on the layered structure. It may be a nitride or an oxynitride layer, for example. This passivation layer  4  is also patterned. Passivation layer  4  is open in electrode region  31  and in connecting region  22 . This situation is illustrated in  FIG. 1   a.    
         [0027]    Referring to  FIG. 1   b,  a humidity-sensitive polymer layer  5  is deposited onto the layered structure and patterned in such a way that polymer layer  5  essentially remains only in electrode region  31 , but completely covers the same. Deposited thereover is a second electrode layer in the form of a thin metal layer  6 . Second, outer electrode  61  of the measuring capacitor, as well as a connecting lead  62  are patterned from this metal layer  6 . Since outer electrode  61  is exposed to the humid measurement environment, the use of a corrosion-resistant metal, such as Au or Pt, for example, is recommended. The patterning of a metal layer of this kind may be simply carried out in an etching process with the aid of a photolithographically produced masking. Via connecting region  22 , connecting lead  62  establishes an electrical connection between outer electrode  61  and evaluation circuit  20 .  FIG. 1   b  shows that outer electrode  6  extends to beyond the edge of, and therefore completely covers, polymer layer  5 . In this edge region, outer electrode  6  is electrically insulated by passivation layer  4  from lower electrode  3 . 
         [0028]    In accordance with the claimed manufacturing method, substrate  1  with the layered structure undergoes an annealing step. In the process, polymer layer  5  expands to a significantly greater degree than superjacent metal layer  6  of outer electrode  61 , so that cracks  7  form in entire electrode region  61  over polymer layer  5 , as illustrated in  FIG. 1   c.  However, due to the lower thermal expansion of passivation layer  4 , metal layer  6  does not tears apart in the edge region of electrode  61  and does not tear apart in the region of connecting lead  62 , thereby ensuring a reliable electrical connection of outer electrode  61  to evaluation circuit  20 . Following cooling, cracks  7  close up substantially again. All that remain are humidity-permeable paths  7  in outer electrode  61 , so that these are continuous and electrically conductive, but, nevertheless, humidity-permeable. 
         [0029]    The manufacturing method described above may also be supplemented by a polymer etching step, where the material of polymer layer  5  is easily ablated through open cracks  7 . This is achieved, for example, by the short-term addition of oxygen plasma during the annealing process. The result of such a polymer etching step is shown in  FIG. 2 . Hollow spaces, respectively depressions  71 , formed in polymer layer  5  during the process, shorten the diffusion paths within polymer layer  5 . Employing this measure makes it possible to shorten the response time of humidity sensor  10  according to the present invention. 
         [0030]    Prior to installation on site, humidity sensor  10  is also provided with a packaging. This may be a molded housing  30 , for example, as shown in  FIG. 3 . To this end, humidity sensor  10  is first mounted on a leadframe  31  and electrically contacted via a bonding connection  32  by bonding wires  33 . Entire sensor element  10 , together with leadframe  31  and bonding connection  32 ,  33 , is then embedded in a molding compound  34 . As a connection to the measurement environment, molded housing  30  features an access opening  35  merely in the area of outer electrode  61 . This type of packaging is very cost-effective and may be manufactured using standard molding tools. 
         [0031]    Humidity sensor  40  illustrated in  FIG. 4  includes a measuring capacitor  41  and a reference capacitor  42 . The two capacitors  41  and  42  are configured side-by-side and over an evaluation circuit  20  that is integrated in substrate  1  of humidity sensor  40 . The layered structure of humidity sensor  40  essentially corresponds to that of humidity sensor  10  illustrated in  FIGS. 1 and 2  and includes a patterned oxide layer  2  on the substrate surface as an electrical insulation between substrate  1 , including evaluation circuit  20 , and capacitors  41  and  42 . Disposed above layer  2  is a patterned metal layer  3  as a first electrode layer in which both lower electrode  311  of measuring capacitor  41 , as well as lower electrode  312  of reference capacitor  42 , along with corresponding connecting leads  32 , are configured. These two lower electrodes  311  and  312  are designed to be mutually congruent and are connected in the example embodiment shown here via a common middle connection  21  to evaluation circuit  20 . Located above first electrode layer  3  is a patterned passivation layer  4  that is open over two bottom electrodes  311  and  312 . A humidity-sensitive polymer layer  51 ,  52 , respectively, completely covers these two electrode regions  311  and  312 , but is limited to approximately these two respective regions  311  and  312 . Disposed above layer  51 ,  52  is a second patterned metal layer  6  as a second electrode layer. Configured in this metal layer  6  are two outer electrodes  611  and  612  of measuring capacitor  41  and of reference capacitor  42 , along with corresponding connecting leads  62 . Similar to the case of two lower electrodes  311  and  312 , the electrode surfaces are essentially identical in the case of the two outer electrodes  611  and  612  as well, so that the design of measuring capacitor  41  and reference capacitor  42  is the same, except for the sole distinction between the two capacitors  41  and  42  that humidity-permeable paths  8  are realized in outer electrode  611  of the measuring capacitor, while outer electrode  612  of reference capacitor  42  is unpatterned, thus forming a closed, humidity-impermeable surface. 
         [0032]    As already mentioned, in the variant illustrated here, two lower electrodes  311  and  312  are interconnected via common middle connection  21  and, accordingly, are connected to ground potential. However, particularly when such a humidity sensor is used in an aggressive measurement environment, it proves to be advantageous when the measuring capacitor and the reference capacitor include a common cover electrode, i.e., the outer electrodes are connected by the measuring and reference capacitor and are connected to ground potential. As a result, no electrolysis takes place, even in the case of condensation of the electrodes on the sensor surface. Moreover, a configuration of this kind is also shielded from external radiation (EMC). To realize humidity-permeable paths  8 , outer electrode  611  of measuring capacitor  41  is provided with a grid structure in the course of pattering metal layer  6 . In the process, small, grid-arrayed openings  8  are etched into metal layer  6  with the aid of a photolithographically patterned mask. To this end, a hard mask may also be used that is composed of an oxide or nitride layer, for example, and subsequently remains as a passivation layer on the surface of sensor element  40 . The width of grid bars  81  is selected here to be smaller than the thickness of polymer layer  51 .  FIG. 4  illustrates that the grid structure extends to the edge of polymer layer  51 , thereby allowing the humidity of the measuring environment to act uniformly on the entire surface of polymer layer  51  of measuring capacitor  41 . 
         [0033]    While the humidity of the measurement environment penetrates through the grid structure of outer electrode  611  to humidity-sensitive polymer layer  51  of measuring capacitor  41 , polymer layer  52  of reference capacitor  42 , including outer electrode  612 , remains unaffected therefrom. Accordingly, the capacitance of reference capacitor  42  is independent of the humidity of the measurement environment. By performing difference, quotient operations on the signals of measuring capacitor  41  and of reference capacitor  42 , it is now possible to generate a sensor signal that is purged of interference effects that occur in equal measure on both capacitors  41  and  42 , such as by the influence of material parameters, temperature effects arising from the thermal expansion of the polymer, and long-term drifts, for example.