Patent Application
20070017276
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.
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.
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.
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.
Other features and advantages of the present invention will be apparent from the detailed description that follows.
The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:
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.
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.
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.
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.
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.
The metal structures 20 and 24 may be aluminum. The membrane structure 22 may be aluminum-nitride.
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.
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.
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.
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.
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.
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.
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.
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.
