Abstract:
A humidity sensor that includes a resonant structure and a structure for altering a resonant frequency of the resonant structure in response to a change in humidity. The structures of a humidity sensor according to the present teachings may be formed in relatively small form factors and are well suited to remote applications and providing mechanisms for compensating for temperature drift.

Description:
BACKGROUND  
       [0001]     Humidity sensors may be employed in a wide variety of applications. Example applications for humidity sensors include heating and air conditioning systems. In addition, humidity sensors may be used in process control systems, weather stations, agricultural environments, etc.  
         [0002]     A humidity sensor may include a humidity sensitive capacitor that changes its capacitance in response to changes in humidity. For example, a humidity sensitive capacitor may include a water permeable dielectric material sandwiched between two metal plates. The metal plates may have holes that allow water to reach the dielectric material. An increase in humidity may cause the dielectric material to absorb water. The water absorbed by the dielectric material increases the dielectric constant of the dielectric material which increases the capacitance of the capacitor.  
         [0003]     Unfortunately, a humidity sensor that employs a humidity sensitive capacitor may not be suitable for many applications. For example, humidity sensitive capacitors and associated circuitry may be too bulky for many applications. In addition, prior humidity sensors may be subject to temperature drift.  
       SUMMARY OF THE INVENTION  
       [0004]     A humidity sensor is disclosed that includes a resonant structure and a structure for altering a resonant frequency of the resonant structure in response to a change in humidity. The structures of a humidity sensor according to the present teachings may be formed in relatively small form factors and are well suited to remote applications and providing mechanisms for compensating for temperature drift.  
         [0005]     Other features and advantages of the present invention will be apparent from the detailed description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:  
         [0007]      FIG. 1  shows a humidity sensor according to the present teachings;  
         [0008]      FIG. 2  shows a resonant structure in one embodiment;  
         [0009]      FIG. 3  shows a humidity sensor including circuitry for measuring a resonant frequency of a resonant structure;  
         [0010]      FIG. 4  shows a humidity sensor having a temperature compensation circuit according to the present teachings.  
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  shows a humidity sensor  10  according to the present teachings. The humidity sensor  10  includes a resonant structure  12  and a structure  14  for altering a resonant frequency of the resonant structure  12  in response to a change in humidity. The resonant structure  12  and the structure  14  in one embodiment are disposed on a substrate  16 .  
         [0012]     The mass of the structure  14  is responsive to changes in humidity. The mass of the structure  14  provides a mass loading onto the resonant structure  12  that influences the resonant frequency of the resonant structure  12 . An increase in the mass of the structure  14  decreases the resonant frequency of the resonant structure  12  whereas a decrease in the mass of the structure  14  increases the resonant frequency of the resonant structure  12 . As a consequence, the resonant frequency of the resonant structure  12  provides an indication of humidity.  
         [0013]     In one embodiment, the structure  14  includes a material that is permeable to water. An increase in humidity causes the structure  14  to absorb more water and increase its mass whereas a decrease in humidity causes the structure  14  to release water and decrease its mass. As a consequence, an increase in humidity is reflected in a decrease in the resonant frequency of the resonant structure  12  whereas a decrease in humidity is reflected as an increase in the resonant frequency of the resonant structure  12 .  
         [0014]     The structure  14  may be a water absorbing polymer material. One example of a water absorbing polymer material is dimethyl siloxane. Other example materials for the structure  14  include the following water sensitive polymers—4-vinyl phenol, N-vinyl pyrrolidone, ethylene oxide, and caprolactone.  
         [0015]     The structure  14  may be disposed onto the resonant structure  12  in a solution, e.g. by paint, by spin coating, by dipping, or by photolithographic patterning, to name a few examples. The resonant structure  12  may be formed using photolithographic patterning.  
         [0016]      FIG. 2  shows the resonant structure  12  in one embodiment. The resonant structure  12  in this example is a thin film bulk acoustic resonator (FBAR) structure. The FBAR structure includes a pair of metal structures  20  and  24  and an intervening membrane structure  22 .  
         [0017]     The membrane structure  22  resonates in response to an acoustic wave having a wavelength of approximately one-half the thickness of the membrane structure  22 . The resonant frequency of the membrane structure  22  may be in the range of 0.6 to 8 Ghz depending on the thickness of the membrane structure  22 . The mass of the structure  14  alters the resonant frequency of the membrane structure  22  in response to changes in humidity.  
         [0018]     The metal structures  20  and  24  may be aluminum. The membrane structure  22  may be aluminum-nitride.  
         [0019]     The FBAR structure in one embodiment is approximately 200 microns in diameter. The thickness of the FBAR structure may be between 2 and 3 microns.  
         [0020]      FIG. 3  shows an embodiment of the humidity sensor  10  including circuitry for measuring humidity by measuring the resonant frequency of the resonant structure  12 . The circuit for measuring the resonant frequency of the resonant structure  12  uses the resonant structure  12  as a filter element in an oscillator. The resonant structure  12  is placed in a feedback loop of an amplifier  30 . The piezoelectric effect from resonant vibration of the resonant structure  12  causes oscillation at an output  32  of the amplifier  30 . The electrical signal at the output  32  has a frequency that depends on the resonant frequency of the resonant structure  12 . As a consequence, the frequency of the electrical signal at the output  32  indicates the changes to the mechanical loading of the structure  14  on the resonant structure  12  in response to changes in humidity.  
         [0021]     In the embodiment shown, the electrical signal at the output  32  drives an antenna  40 . The frequency of an over the air signal from the antenna  40  indicates the humidity sensed in the humidity sensor  10 . The signal from the antenna  40  may be received at a remote site for remote humidity sensing applications. The RF resonant frequencies associated with an FBAR structure are particularly well suited to over the air remote sensing.  
         [0022]     Alternatively, the electrical signal at the output  32  may be provided to a signal processing circuit (not shown). The signal processing circuit may compute a humidity figure in response to the frequency of the electrical signal at the output  32 .  
         [0023]      FIG. 4  shows an embodiment of the humidity sensor  10  having a temperature compensation circuit. The temperature compensation circuit includes a resonant structure  60 , an amplifier  62 , and a mixer  64 . The temperature compensation circuit subtracts out the common mode temperature drift in the resonant structures  12  and  60 .  
         [0024]     The resonant frequency of the resonant structure  60  tracks the resonant frequency of the resonant structure  12  with temperature changes. In one embodiment, the resonant structure  60  is an FBAR structure that is substantially similar to an FBAR structure of the resonant structure  12 . For example, the FBAR structures may have substantially similar metal structures and membrane structures, i.e. same materials and dimensions, and may be formed on the same substrate and be subject to the same changes in temperature.  
         [0025]     The resonant structure  60  is placed in a feedback loop of the amplifier  62  and the electrical signal at an output  66  of the amplifier  62  has a frequency that depends on the resonant frequency of the resonant structure  62 . The mixer  64  generates a difference signal  70  that indicates a difference in the frequencies of the electrical signals at the outputs  32  and  66  of the amplifiers  30  and  62 , i.e. a difference in the in the resonant frequencies of the resonant structures  12  and  62 . The difference signal  70  may drive an antenna or may be provided to a signal processing circuit as previously described.  
         [0026]     Alternatively, the output signals  32  and  60  may be transmitted via an antenna to a remote site and the difference in the frequencies may be determined at the remote site.  
         [0027]     In one embodiment, the FBAR structure of the resonant structure  60  and the FBAR structure of the resonant structure  12  are each approximately 200 microns in diameter with a thickness between 2 and 3 microns. The two FBAR structures with bonding pads may be placed on a die about 0.5 mm by 0.5 mm.  
         [0028]     The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.