Abstract:
A method and system for adjusting characteristics of a relative humidity sensor in order to achieve a desired value of accuracy is presented. A relative humidity sensor charge balance circuit includes a series of sensing capacitors Cx 1 , Cx 2  including a thin porous platinum top plate, a humidity sensitive polyimide dielectric, and two metal bottom plates on a semiconductor substrate; and two fixed oxide capacitances Cref, and C 0 . Changes in humidity affect the humidity sensitive dielectric thereby causing changes in the sensing capacitive value of the capacitive circuit. The charge in the sensing capacitor and the fixed capacitor C 0  can be controlled separately by adjusting and/or trimming the supply voltage using a voltage trimmer; thereby the slope and offset of the relative humidity sensor circuit can be modified and controlled to particular desired values.

Description:
TECHNICAL FIELD 
     Embodiments are related to semiconductor wafer-based devices. Embodiments are also related to relative humidity sensors. Embodiments are additionally related to methods and systems for adjusting characteristics of relative humidity sensors. 
     BACKGROUND OF THE INVENTION 
     Humidity plays a very major role in various industrial and commercial applications. Monitoring and controlling humidity is of great importance for the reliable operation of various systems. For example, solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors are fabricated utilizing semiconductor processes. Humidity sensors represent but one class of semiconductor-based sensors finding a useful industrial application. Modern manufacturing processes, for example, generally require measurement of moisture contents corresponding to dew points between −40° C. and 180° C., or a relative humidity between 1% and 100%. There is also a need for a durable, compact, efficient moisture detector that can be used effectively in these processes to measure very small moisture content in gaseous atmospheres. 
     Humidity can be measured by a number of techniques. In a semiconductor-based system, for example, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Two of the most common physical changes are variations in resistance and the change in dielectric constant, which can be respectively translated into a resistance change and a capacitance change. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result includes erroneous readings, among other problems. 
     Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. It is important in the construction of capacitive elements, however, to avoid problems that can arise with certain constructions for such elements. In addition, there can also be inaccuracy incurred at high relative humidity values where high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element. By making the component parts of the element thin, it has been found that the above-mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables. 
     A conventional capacitive humidity sensor, in general, can include a semiconductor substrate, and a pair of electrodes, which are formed on a surface of the semiconductor substrate and face each other across a particular distance. A humidity-sensitive film may also be placed between the electrodes and formed on a surface of the semiconductor substrate. The capacitance of the film changes in response to humidity. The sensor detects humidity by detecting changes in capacitance between the pair of electrodes in response to variations in the surrounding humidity. Humidity sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure, along with a layer or coating of a dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and attaining equilibrium in a short period of time. The response offset and slope for the integrated relative humidity sensor can be set to particular values in order to achieve a desired value of accuracy for the sensor. 
       FIG. 1  illustrates a “prior art” charge balancing circuit  100  of a humidity sensor for transforming measurements of relative humidity into a linear voltage. The high impedance capacitive nature of the humidity sensor can be readily handled by control of charge.  FIG. 1  includes fixed capacitors C 0 , C 1 , C 2 , C 3 , and Cref that are designed to be insensitive to humidity and that can be fabricated at the same time and from the same materials. A humidity sensitive capacitor Cx can be designed to be sensitive to humidity and is fabricated at a different time and from different materials than the aforementioned capacitors. A switching matrix  120  varies the wiring scheme for capacitors: Cx, C 0 , and Cref utilizing two-phase, non-overlapping, dual polarity clocks, as can be provided by clock generator  110 . Inverters A 1 , A 2 , and A 3 , and capacitor C 1 , and a pair of associated transmission gates  130  and  140  form a high gain comparator. The capacitor C 2  and its pair of associated transmission gates  150  and  160  are the switched capacitor equivalent of a resistor which can be coupled with amplifier A 4  and feedback capacitor C 3  from an integrator. The capacitive values of the sensing capacitor Cx and the fixed capacitor C 0  can be varied by laser trimming or by etching the sensing capacitor Cx to create voids in order to keep their values substantially equal. 
       FIGS. 2A and 2B  illustrate “prior art” charge balancing circuit  200  and  250  during “Phase 1” and “Phase 2” operation respectively. In Phase 1 C 0  is pulled up to Vcc and Cx is pulled down to GND and vice versa during Phase 2. Thus a periodic differential voltage can be created which is a function of the difference in capacitance values. The following equations mathematically describe the operation of the circuit  200  and  250 . The charge at the summing node during Phase 1 and 2, can be calculated utilizing equations (1) and (2) respectively. The negative feedback results in Qs 1  and Vs 1  being substantially equal to Qs 2  and Vs 2 . equation (3) mathematically describes the resulting transfer function for the complete circuit operation.
 
 Qs 1 =Cx*Vs 1 +C 0*( Vs 1 −Vcc )+ C ref*( Vs 1 −V out)  (1)
 
 Qs 2 =Cx *( Vs 2 −Vcc )+ C 0 *Vs 2 +C ref* Vs 2  (2)
 
 V out= Vcc *( Cx *(1+α*RH)/ C ref)− Vcc *( C 0 /C ref)  (3)
 
     In a majority of prior art humidity sensors the humidity sensitive capacitor Cx can be laser trimmed for offset adjustment and a photo mask layer of the reference capacitor C 0  can be varied for slope adjustment. The laser trimming of humidity sensitive capacitor Cx for offset adjustment can introduce a reliability issue, due to exposure of the trimming site of the humidity sensitive capacitor to various application conditions. Also, the slope adjustment by variation of photo mask layer is costly and time consuming. 
     Based on the foregoing it is believed that a need exists for an improved methods and systems for adjusting characteristics of the relative humidity sensors in order to provide a more accurate measurement of humidity as will be disclosed in further detail herein. 
     BRIEF SUMMARY 
     The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
     It is, therefore, one aspect of the present invention to provide for improved sensor methods and systems. 
     It is another aspect of the present invention to provide for an improved method and system for capacitive balancing of relative humidity sensors. 
     It is another aspect of the present invention to provide for an improved method and system for adjusting characteristics of relative humidity sensors. 
     The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A method and system for adjusting the characteristics of a relative humidity sensor in order to achieve a desired value of accuracy is disclosed. The sensor includes a pair of electrodes with a gap-interposed therebetween to form a sensing capacitor on a silicon substrate with a silicon oxide film formed on a surface thereof. The charge in the sensing capacitor and a fixed capacitor can be controlled separately by adjusting and/or trimming a supply voltage utilizing a voltage trimmer to achieve capacitive balance in a charge balance circuit. The slope and offset of the relative humidity sensor can also be modified and controlled to particular desired values by adjusting the voltage for the sensing capacitor and the fixed capacitor respectively. 
     The pair of electrodes can be connected to a signal processing circuit for detecting the variation of the electrostatic capacitance between the pair of electrodes. 
     The relative humidity sensor can be formed on the semiconductor substrate, and thus the signal processing circuit for detecting the variation of the capacitance type humidity sensor can be formed on the principal surface of the semiconductor substrate. 
     The capacitance formed between the pair of electrodes changes in accordance with ambient humidity. 
     The capacitive values of the sensing capacitor and the fixed capacitor can be adjusted while keeping their values substantially equal. The ability of adjusting the charge level of the sensing capacitor Cx and the fixed capacitor C 0  disclosed herein can therefore provide for better control of sensor output accuracy and thereby enhance the reliability of the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein. 
         FIG. 1  illustrates a “prior art” charge balancing circuit for transducing relative humidity to a linear voltage; 
         FIG. 2A  illustrates the “prior art” charge balancing circuit during Phase 1 operation; 
         FIG. 2B  illustrates the “prior art” charge balancing circuit during Phase 2 operation; 
         FIG. 3  illustrates a cut-away side view of a relative humidity sensor, in accordance with a preferred embodiment; 
         FIG. 4  illustrates an improved charge balancing circuit for transducing relative humidity to a linear voltage, in accordance with a preferred embodiment; 
         FIG. 5A  illustrates the improved charge balancing circuit during a Phase 1 operation, in accordance with a preferred embodiment; 
         FIG. 5B  illustrates the improved charge balancing circuit during a Phase 2 operation, in accordance with a preferred embodiment; and 
         FIG. 6  illustrates a high-level logical flowchart of operations illustrating logical operational steps of a method for adjusting characteristics of the relative humidity sensor, in accordance with a preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. 
     Referring to  FIG. 3 , a cut-away side view of a relative humidity sensor  300  is illustrated, in accordance with a preferred embodiment. The humidity sensor  300  depicted in  FIG. 3  can be used for humidity control in, for example, an air conditioner or to detect humidity for weather observation purposes. It is understood, however, that a wide variety of other applications for humidity sensor  300  can also be implemented, depending upon design goals and considerations. As depicted in  FIG. 3 , an N-type silicon substrate  310  can be employed whereon a silicon oxide film  320  can be formed on the semiconductor substrate  310  as a first insulation film. First and second electrodes  330  and  335  are configured on an identical plane of the silicon oxide film  320  so as to oppose each other with a gap  365  interposed between them. 
     A material capable of being utilized in a normal semiconductor producing line can be employed to form the first and second electrodes  330  and  335 . Such material can be, for example Al, Ti, Au, Cu, poly-Si, and the like. In one particular embodiment, a silicon nitride film  336  can be formed on the electrodes  330  and  335  as a second insulation film. It can be appreciated, however, that in other embodiments, materials other than silicon nitride may be utilized to implement film  336 . The silicon nitride film  336  can be utilized as a protection film to cover the pair of electrodes  330  and  335 . The silicon nitride film  336  can be formed by plasma CVD method or the like, so as to have the same thickness over the whole area on the semiconductor substrate  310 . 
     As shown in  FIG. 3 , the pair of electrodes  330  and  335  can be equipped with a first electrical contact  370  and a second electrical contact  375  through which the electrodes  330  and  335  are connected to a signal processing circuit (not shown in  FIG. 3 ) for detecting the variation of the electrostatic capacitance between the pair of electrodes  330  and  335 , respectively. The electrical contacts  370  and  375  are required to be exposed so that they are connected to the signal processing circuit, and thus are not covered by the silicon nitride film  336 . Furthermore, according to such an embodiment, the capacitance type humidity sensor  300  can be formed on the semiconductor substrate  310 , and thus the signal processing circuit for detecting the variation of the capacitance type humidity sensor  300  can be formed on the principal surface of the semiconductor substrate  310 . 
     A sensing medium  360  having a permittivity that changes according to humidity can be formed over the silicon nitride film  320  so as to cover the electrodes  330  and  335 . A porous platinum top plate  350  having moisture-permeability through which moisture (e.g., water) is allowed to permeate can be formed so as to cover the humidity sensing medium  360 . The top plate  350  possesses a higher dielectric constant than that of the sensing medium  360 . When water infiltrates into the humidity sensing medium  360 , the dielectric constant of the humidity sensing medium  360  is varied in accordance with the amount of water, thereby infiltrating because the dielectric constant of water is large. 
     As a result, the electrostatic capacitance of the capacitor as indicated by Cx 1  and Cx 2  constructed by the pair of electrodes  330  and  335  with the humidity sensing medium  360  as a part of the dielectric material. Humidity can be detected on the basis of the electrostatic capacitance between the pair of electrodes  330  and  335 , because the amount of water contained in the humidity sensing medium  360  corresponds to the ambient humidity around the capacitance type humidity sensor  300 . 
     As described above, the variation of the electrostatic capacitance between the pair of electrodes  330  and  335  in accordance with the humidity variation of the humidity sensing medium  360  can be increased by forming the top plate  350  having a higher dielectric constant than the sensing medium  360  on the sensing medium  360 . Furthermore, as the dielectric constant of the moisture-affected top plate  350  is higher, the variation of the electrostatic capacitance between the pair of electrodes  330  and  335  in accordance with the humidity variation is increased. 
     The relative humidity sensing capacitor Cx 1  and Cx 2  can be fabricated utilizing standard silicon wafer processing techniques commonly used to configure existing relative humidity sensors. An area where the moisture-sensitive film  360  can be located on the semiconductor substrate  310  constitutes a humidity-sensing portion  360 . Namely, ambient humidity can be detected via the humidity-sensing portion  360  based on the capacitance formed between the detection electrodes  330  and  335  and the capacitive path to the sensing capacitor Cx 1  that varies according to a change in humidity around the sensor  300 . 
     Referring to  FIG. 4  illustrated is an improved charge balancing circuit  400  for transducing relative humidity to a linear voltage, in accordance with a preferred embodiment. The charge balancing circuit  400  can be utilized to transduce relative humidity to a linear voltage. The high impedance capacitive nature of the relative humidity sensor  300  can be more readily handled by control of charge.  FIG. 4  shows the essential components that include the relative humidity-to-voltage transfer function in terms of a circuit diagram  400 . The capacitors C 0 , C 1 , C 2 , C 3 , and Cref can be designed to be insensitive to humidity and can be fabricated at the same time and from the same materials. Thus, while their absolute values of capacitance will vary, the ratios can be tracked very closely. The relative humidity sensing capacitor Cx can be designed to be sensitive to humidity and can be fabricated at a different time and from different materials than the fixed capacitors C 0 , C 1 , C 2 , C 3 , and Cref. 
     A charge switching matrix  420  can be utilized to vary the wiring scheme for capacitors: Cx, C 0 , and Cref utilizing two-phase, non-overlapping, dual polarity clocks, as can be provided by clock generator  410 . Note that one end of all three capacitors Cx, C 0 , and Cref can be always connected in common, thus providing a charge summing node Qs. Inverters A 1 , A 2 , and A 3 , and capacitor C 1 , and the pair of associated transmission gates  430  and  440  form a high gain comparator. C 2  and its pair of associated transmission gates  450  and  460  illustrates a switched capacitor equivalent of a resistor which can be coupled with amplifier A 4  and feedback capacitor C 3  in order to form an integrator. The circuit  400  can include an adjustable voltage trimmer  470  for modification of the supply voltage Vcc to VCx for the sensing capacitor Cx and VC 0  for the fixed capacitor C 0 . 
       FIGS. 5A and 5B  illustrate the circuit connectivity  500  and  550  of the improved charge balancing circuit  400  during “Phase 1” and “Phase 2” respectively. Neglecting Cref for the moment and concentrating on C 0  and Cx, note that they effectively form a voltage divider. The charge in the sensing capacitor Cx and the fixed capacitor C 0  can be controlled separately by adjusting and/or trimming the supply voltage Vcc utilizing a voltage trimmer  470  in order to achieve capacitive balance. The slope and offset of the relative humidity sensor  300  can also be modified and controlled to particular desired values by adjusting the voltage for the sensing capacitor Cx and the fixed capacitor C 0  respectively. 
     Hence, in Phase 1 C 0  can be pulled up to VC 0  by adjusting or trimming the supply voltage Vcc utilizing a voltage trimmer  470  and Cx can be pulled down to GND. Similarly, in Phase 2 Cx can be pulled up to VCx and C 0  can be pulled down to GND. Thus a periodic differential voltage can be created which is a function of difference in capacitance values. Those skilled in the art will recognize this as a half bridge sensor configuration. During Phase 1, the inverters A 1  and A 2  short the input node to the output node, which, when implemented with complementary FETs, forms a voltage divider. 
       FIG. 5A  indicates that the three inverters A 1 , A 2  and A 3  can be designed to produce a half supply transfer function in this configuration, thus driving both the charge summing node and the output of A 3  to Vcc/2 during Phase 1. During Phase 2 these transmission gate shorts can be opened up in order to create a high gain inverting comparator, which allows small movement in the charge summing node voltage relative to Vcc/2 to drive the output of A 3  to Vcc or GND. Thus A 3 , the output of the comparator, controls the integrator. During Phase 1, the output of A 3  and the non-inverting input of A 4  are both at Vcc/2, which puts the integrator into a “Hold” state. So Phase 1 can be considered as a measurement or sampling phase during which Cref can be charged. 
     During Phase 2Cref can be disconnected from the integrator output and reconnected to GND and the comparator responds to the charge-summing node. If the comparator output goes to GND, then the output of the integrator increases linearly. If the comparator output goes to Vcc, then the output of the integrator decreases linearly. If the charge-summing node effectively remains at Vcc/2 during Phase 2, then the integrator remains in the “Hold” state. So Phase 2 can be thought of as the negative feedback adjustment phase. The following equations mathematically describe the operation of the circuit  500  and  550 . Equations (1) and (2) calculate the charge at the summing node during Phase 1 and 2, respectively. The negative feedback results in Qs 1  and Vs 1  being substantially equal to Qs 2  and Vs 2 . Equation (3) mathematically describes the resulting transfer function for the complete circuit operation.
 
 Qs 1 =Cx*Vs 1 +C 0*( Vs 1 −VC 0)+ C ref*( Vs 1 −V out)  (1)
 
 Qs 2 =Cx *( Vs 2 −VCx )+ C 0 *Vs 2 +C ref* Vs 2  (2)
 
 V out= VCx *( Cx/C ref)− VC 0*( C 0/ C ref)  (3)
 
     As described and shown with respect to  FIG. 3 , the adjustments made to Cx can be divided substantially equally between Cx 1  and Cx 2  to minimize the sensitivity reduction due to mismatch error. The supply voltage Vcc can be trimmed or adjustable to VCx for RH (Relative Humidity) sensitive capacitor Cx and/or VC 0  for fixed Capacitor C 0  in order to control the charge in the capacitors Cx and C 0 . Hence, the offset i.e., the output value at 0RH, and the sensitivity over entire RH. Span of the Relative humidity sensor circuit  400  can be modified and controlled to particular desired values. The ability of adjusting the charge level both in Cx and C 0  provides for better control of the sensor output accuracy. Also by replacing the trimming site from the exposed Cx to a network in the circuit the reliability of the sensor can be enhanced considerably. 
     Referring to  FIG. 6 , a high-level logical flowchart of operations illustrating logical operational steps of a method  600  for adjusting characteristics of the relative humidity sensor  300  is illustrated, in accordance with a preferred embodiment. A relative humidity sensor such as a sensor  300  as depicted in  FIG. 3  can be provided, as illustrated at block  610 . Thereafter, as indicated at block  620 , capacitive measurement of sensing capacitor Cx can be conducted by sampling signals rendered by the sensing capacitor Cx. 
     The sensing capacitor Cx and a fixed capacitor C 0  can be connected in a charge balance circuit in order to determine capacitive values, as shown at block  630 . Next, as described at block  640 , the capacitive values of sensing capacitor Cx and fixed capacitor C 0  can be adjusted by trimming supply voltage Vcc to VCx and VC 0  respectively. The charge balance circuit can be monitored to detect changes in sensing capacitor Cx and fixed capacitor C 0 , as depicted at block  650 . The resulting transfer function for the complete circuit operation is described in equations (7) and (8).
 
 V out= VCx*[Cx *(1+α*RH)/ C ref]− VC 0*( C 0/ C ref)  (7)
 
 V out=( VCx/C ref)*[ Cx *(1+α*RH)]− VC 0*( C 0 /C ref),  (8)
 
where ‘α’ represents the property of polyimide coefficient and RH represents relative humidity. The present device is used to sense the relative humidity in the ambient environment around the sensor. During operation, a relative humidity level is sensed and then the sensor  300  generates a voltage output proportional to the relative humidity. This voltage can then be used by other circuits to implement functions such as relative humidity control, enthalpy control for building HVAC, weather sensing instruments, process controls for drying, process controls for batch or continuous production where relative humidity is a parameter that controls the output of a process or is related to some process variable to be controlled, length or end of cycle in drying applications, and other applications.
 
     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.