Tunable humidity sensor with integrated heater

A capacitive humidity sensor includes a first electrode, a humidity sensitive dielectric layer, and a second electrode. The humidity sensitive dielectric layer is between the first and the second electrodes. The humidity sensitive dielectric layer is etched at selected regions to form hollow regions between the first and second electrodes.

BACKGROUND

1. Technical Field

The present disclosure relates to the field of humidity sensors. The present disclosure relates in particular to a capacitive humidity sensor.

2. Description of the Related Art

Humidity sensing is important in many fields. In many applications it is beneficial to control the humidity level. In other applications it is beneficial simply to know what the humidity level is. In many manufacturing settings it is important that the relative humidity not rise above a certain level or the products being manufactured may be adversely affected. In many scientific settings the relative humidity is taken into account while performing experiments. When processing certain types of integrated circuits the humidity level may be closely monitored or controlled both in the clean room settings as well as in the various deposition chambers and processing equipment.

Humidity sensors come in a variety of forms. Humidity sensors can include resistive humidity sensors, thermal conduction humidity sensors, capacitive humidity sensors, and others. Humidity sensors can also be manufactured using thin film techniques. Thin film technology has reduced the size and cost of humidity sensors. However, humidity sensors with specific design requirements for particular applications may yet be very expensive.

BRIEF SUMMARY

One embodiment is a capacitive humidity sensor including a first electrode, a second electrode, and a humidity sensitive dielectric layer between the first and second electrodes. The humidity sensitive dielectric layer has uneven thickness between the first and second dielectric layers. The uneven thickness of the humidity sensitive dielectric layer leaves hollow portions between the first and second electrodes. An opening in the second electrode exposes the humidity sensitive dielectric layer to the environment surrounding the capacitive humidity sensor. Humid air in the environment enters into the humidity sensitive dielectric layer. The dielectric constant of the humidity sensitive dielectric layer changes as water vapor enters the humidity sensitive dielectric layer. As the dielectric constant of the humidity sensitive dielectric layer changes, the capacitance between the first and second electrodes changes. The capacitance of the first and second electrodes is therefore indicative of the humidity in the environment surrounding the capacitive humidity sensor.

In one embodiment the first electrode includes contact regions connected thereto. The contact regions are configured to pass a current through the first electrode. The current generates heat in the first electrode and heats the humidity sensitive dielectric layer to release humidity from the humidity sensitive dielectric layer.

One embodiment is a method for forming a capacitive humidity sensor. The method includes forming a first electrode on a substrate, forming a humidity sensitive dielectric layer on the first electrode, and forming a second electrode on the humidity sensitive dielectric layer. The method further includes forming openings in the second electrode and isotropically etching the humidity sensitive dielectric layer for a selected length of time. The isotropic etch removes portions of the humidity sensitive dielectric layer between the first and second electrodes leaving hollow spaces between the first and second electrodes. The longer the duration of the isotropic etch, the larger the hollow spaces between the first and second electrodes. The base capacitance between the first and second electrodes varies according to the ratio of the volume of the humidity sensitive dielectric layer to the volume of the hollow spaces between the first and second electrodes. The base capacitance of the capacitive humidity sensor can be selected by selecting a duration of the isotropic etch.

Humid air in the environment enters into the humidity sensitive dielectric layer through the openings in the second electrode. The dielectric constant of the humidity sensitive dielectric layer changes as water vapor enters the humidity sensitive dielectric layer. As the dielectric constant of the humidity sensitive dielectric layer changes, the capacitance between the first and second electrodes changes. The capacitance of the first and second electrodes is therefore indicative of the humidity in the environment surrounding the capacitive humidity sensor.

DETAILED DESCRIPTION

FIG. 1illustrates a capacitive humidity center20according to one embodiment. The capacitive humidity sensor20is a capacitor including a bottom electrode22and a top electrode24. A humidity sensitive dielectric layer26is positioned between the bottom electrode22and the top electrode24. Hollow spaces28are formed in the humidity sensitive dielectric layer26. Portions of the hollow spaces28are positioned directly between the bottom electrode22and the top electrode24. Openings29are formed in the top electrode24.

The capacitive humidity sensor20outputs a capacitance signal indicative of the humidity in the surrounding environment. The capacitance of a capacitor varies according to the following relationship:
C˜∈×A/d
where C is the capacitance, ∈ is the dielectric constant of the dielectric material between the electrodes of the capacitor, A is the area of overlap of the electrodes of the capacitor, and d is the distance between the electrodes of the capacitor. Thus, the capacitance of the capacitor is proportional to the dielectric constant ∈ of the dielectric material between the plates of the capacitor. The higher the dielectric constant of the dielectric material between the plates of the capacitor, the higher the capacitance of the capacitor will be.

Each dielectric material has a dielectric constant particular to that material. The dielectric constant of some materials changes according to environmental conditions. For example, some dielectric materials have a dielectric constant that changes with fluctuations in temperature, pressure, frequency of electric field, or humidity. Some capacitors have multiple dielectric materials between the two electrodes. The capacitance of such a capacitor will include components of capacitance from each dielectric material.

The capacitive humidity sensor20ofFIG. 1is a capacitor having a bottom plate22separated from top plate24by a combination of a humidity sensitive dielectric material26and air in the hollow pockets28. The capacitor20has a lower capacitance than it would if the space between the bottom electrode22and top electrode24were filled completely by the dielectric material26. This is because the dielectric constant of air is lower than the dielectric constant of the dielectric material26.

The openings29in the top electrode24allow air from the environment surrounding the capacitive humidity sensor20to enter into the hollow regions28and to come into contact with the humidity sensitive dielectric layer26. The humidity sensitive dielectric layer26absorbs water from the air. The dielectric constant of the humidity sensitive dielectric layer26increases as the humidity sensitive dielectric layer26absorbs humidity from the air. As the dielectric constant of the humidity sensitive dielectric layer26increases, the capacitance of the humidity sensitive capacitor20also increases. As the humidity of the air increases, the humidity sensitive dielectric material absorbs more moisture. Thus, by measuring the capacitance of the capacitive humidity sensor20, a measurement of the humidity in the air surrounding the capacitive humidity sensor20can be obtained.

Because the total capacitance of the capacitive humidity sensor20depends on both the portion of capacitance contributed by the humidity sensitive dielectric layer26and the portion contributed by the air pockets28, the range of capacitance of the capacitive humidity sensor20can be tuned by increasing or decreasing the size of the hollow pockets28between the top plate24and the bottom plate22. If the volume of the hollow space28between the top plate24and the bottom plate22is greater, the capacitance of the capacitive humidity sensor20lowers. In some applications, it is desired to select a particular base capacitance value (capacitance value at 0% humidity) for the capacitive humidity sensor20. The base capacitance of the capacitive humidity sensor20can be easily adjusted during manufacture of the capacitive humidity sensor20by forming a larger or a smaller hollow portion28between the bottom electrode22and the top electrode24.

In one embodiment, the capacitive humidity sensor20is formed using thin film processes, such as those used in the formation of integrated circuits. In a thin film process for forming a capacitive humidity sensor20, a first thin film conductive layer22is formed on an insulating substrate and the humidity sensitive dielectric layer26is deposited on the first thin film conductive layer. A second thin film conductive layer24is then formed on top of the humidity sensitive dielectric layer26. Openings29are formed in the second conductive thin film layer24.

The humidity sensitive dielectric layer26is then etched through the holes29. If the etch is isotropic, it removes the humidity sensitive dielectric layer26both in the vertical direction below the holes29and laterally between the thin film metal layers22,24. If the etch is anisotropic, the etch will be vertical and have the same width as the openings29. Usually, an isotropic etch is preferred since this will have a much greater effect on changes in the dielectric constant of the material26. The amount of material removed from the humidity sensitive dielectric layer26directly between the first and second conductive films22,24will affect the capacitance of the capacitive humidity sensor20. Thus, after forming openings29in the second conductive thin film24, the capacitance of the capacitive humidity sensor20can be selected by selecting a particular amount of material to etch of the humidity sensitive dielectric layer26. For a particular etch chemistry, a longer duration of the etch will etch more of the humidity sensitive dielectric layer26between the top electrode24and the bottom electrode22. A shorter etch will etch less of the humidity sensitive dielectric layer26between the top electrode24and the bottom electrode22. Alternatively, etch parameters other than etch duration may be varied in order to select a particular capacitance value of the capacitor. For example the pressure in the deposition chamber, flow rates of gasses in the deposition chamber, temperature in the deposition chamber, etch chemistry, or other parameters can each be varied to select a particular target value of the capacitance of the capacitive humidity sensor20.

In one embodiment, the humidity sensitive dielectric layer26is polyimide. The dielectric constant of polyimide varies from about 2.9 at 0% humidity to about 3.4 at 100% humidity. Other suitable humidity sensitive dielectric layers may be used in forming the capacitive humidity sensor20. A humidity sensitive material with respect to this application refers to a material whose dielectric constant varies with variations in the humidity.

When making successive humidity measurements, if the humidity decreases there is a time lag between when the humidity goes down and the water leaves the dielectric26. In some conditions, it may take some time for the water content absorbed by the humidity sensitive dielectric layer26to leave the humidity sensitive dielectric layer26. Thus, it is possible that a measurement of the humidity may be erroneously high because humidity has been previously absorbed by the humidity sensitive dielectric layer26and has not yet left. The humidity in the air surrounding the capacitive humidity sensor20may have dropped while the dielectric constant of the humidity sensitive dielectric layer26has not yet dropped correspondingly because water content that was previously absorbed has not yet been expelled. In addition to this, hysteretic effects may occur in measurements of the capacitance of the capacitive humidity sensor20. In other words, as the humidity sensitive dielectric layer26absorbs moisture from the air, the dielectric constant will increase along a first curve. But as the humidity decreases, the dielectric constant of the humidity sensitive dielectric layer26does not decrease along the same curve. Therefore, it is possible for the capacitive humidity sensor20to give two different readings for the same actual humidity level in the environment surrounding the capacitive humidity sensor20. For this reason, it is desirable to be able to quickly expel moisture content from the humidity sensitive dielectric layer26after each reading of the capacitance of the capacitive humidity sensor20.

Therefore, in one embodiment, the bottom electrode22is also a heating element. The bottom electrode22is configured to generate heat and to heat up the humidity sensitive dielectric layer26. When the humidity sensitive dielectric layer26is heated up, moisture content is expelled from the humidity sensitive dielectric layer26. A subsequent measurement of humidity will not be erroneously affected by either hysteresis or by an unduly high portion of moisture remaining in the humidity sensitive dielectric layer26. In one embodiment, a current is passed through the bottom electrode22. The current causes the bottom electrode22to generate heat and to heat up the humidity sensitive dielectric layer26and to expel moisture content from the humidity sensitive dielectric layer26.

While particular materials, heating elements, structures and processes have been described for forming a capacitive humidity sensor20, many other particular structures, processes and materials can be used in accordance with principles of the present disclosure. All such materials, processes and structures fall within the scope of the present disclosure.

FIG. 2illustrates an electronic device30according to one embodiment. The electronic device30includes a sensor capacitor20, which is a capacitive humidity sensor20as described in relation toFIG. 1; a reference capacitor32and electrical contacts34. The sensor capacitor20senses humidity in the air surrounding the electronic device30and provides a capacitance signal indicative of the humidity in the air surrounding the electronic device30. The reference capacitor32is a capacitor similar to the sensor capacitor20except that the dielectric layer is not exposed to open air. The capacitance is therefore a constant and does not change with humidity. In one embodiment, hollow portions28are not formed in the humidity sensitive dielectric layer26. Openings29are also not formed in the humidity sensitive dielectric layer26of the reference capacitor32.

The capacitance of the reference capacitor32is independent of humidity. The control capacitive signal is independent of the humidity because openings29have not been formed in the reference capacitor32, and is otherwise sealed to block ambient air from coming in contact with the humidity sensitive dielectric material26. The reference capacitor32therefore gives a constant control capacitance signal.

Electrical contacts34are connected to the sensor capacitor20and the reference capacitor32. The electrical contacts34receive the capacitive signals from the sensor capacitor20and the reference capacitor32and outputs them to processing circuitry which can be connected to the electrical device30. The processing circuitry is not shown inFIG. 2for simplicity.

In one embodiment, the electronic device30is a standalone integrated circuit package. The sensor capacitor20, the reference capacitor32, and the electrical contacts34are formed on an integrated circuit die. The electrical contacts34can include contact pads on top of an integrated circuit die, solder balls connected to the integrated circuit package30, leads of a lead frame, pins of a pin grid array, or any other suitable electrical contacts. The integrated circuit package30, including the sensor capacitor20, can be conveniently installed in an electrical system which can process the capacitance signals generated by the sensor capacitor20and the reference capacitor32and output a measurement of the humidity of the air surrounding the integrated circuit package30.

FIG. 3is a block diagram of an electronic device according to one embodiment. The electronic device30includes a sensor capacitor20, a reference capacitor32, an analog-to-digital converter36, and a microcontroller38. The sensor capacitor20and the reference capacitor32each are coupled to the analog-to-digital converter36. The sensor capacitor20outputs an analog humidity signal indicative of the humidity in the environment surrounding the electronic device30. The analog-to-digital converter36receives the analog humidity signal from the sensor capacitor20and converts the analog humidity signal to a digital humidity signal. The analog-to-digital converter36outputs the digital humidity signal to the microcontroller38. The microcontroller38receives the digital humidity signal from the analog-to-digital converter36and computes a value of the humidity in the environment surrounding the electronic device30. The microcontroller38references a calibration table stored in memory in the microcontroller38when computing the value of the humidity. The microcontroller38may compare the digital humidity to values stored in the calibration table and compute, calculate, or estimate a value of the humidity signal based on the digital humidity signal and the value stored in the calibration table.

The reference capacitor32also outputs an analog reference signal to the analog-to-digital converter36. The analog reference signal is a reference capacitance signal from the reference capacitor32. The analog-to-digital converter36converts the analog reference capacitor signal to a digital reference capacitor signal and outputs the digital reference capacitor signal to the microcontroller38.

The capacitance of the capacitive humidity sensor20can be affected by factors other than humidity. The temperature, pressure, or other factors can each affect the capacitance of the capacitive humidity sensor20. The reference capacitor32helps to maintain accuracy in estimating the humidity based on the capacitive humidity sensor20. The capacitance of the reference capacitor is also affected by temperature, pressure, and factors other than humidity. Therefore if the reference capacitor has a higher or lower value than expected, the final humidity measurement can take into account that the capacitance of the capacitive humidity sensor20may be higher or lower than can be accounted for based on humidity alone.

The microcontroller38references the digital control signal when computing the value of the humidity from the digital humidity signal. In one embodiment, the digital control capacitor signal acts as a base reference signal for the microcontroller38. The microcontroller38can store in the calibration table values of the digital control capacitor signal. The microcontroller38can then take into account the value of digital control capacitor signal or fluctuations in that signal when calculating the value of the humidity from the digital humidity signal. The microcontroller38can then output the value of the humidity through the electrical contacts34to a display device or any other peripheral device coupled to the electronic device30. The microcontroller38can also receive input commands and requests as well as supply voltages through the contacts34. In some embodiments, the value of the humidity is used by other circuits on the same electronic device30. In those cases, the signal is sent to other locations on the device30and does not need to leave through contacts34.

In one embodiment, the electronic device30is an integrated circuit package. The integrated circuit package includes an integrated circuit die in which is formed the sensor capacitor20, the reference capacitor32, analog-to-digital converter36, and the microcontroller38. The analog-to-digital converter36and the microcontroller38can be formed from transistors formed in the integrated circuit die. The transistors can be formed from a monocrystalline semiconductor substrate in the integrated circuit die as well as through metal interconnection layers and dielectric layers in the integrated circuit die. The sensor capacitor20and the reference capacitor32can be formed in an upper portion of the integrated circuit die above the semiconductor substrate. Openings can be made in the integrated circuit die to expose the sensor capacitor20to the air surrounding the integrated circuit package30. The electrical contacts34can be contact pads, leads of a lead frame, solder balls, pins of a pin grid array, or any other suitable electrical contacts. Further details regarding the formation of an integrated circuit package including a capacitive humidity sensor, microcontroller, and analog to digital converter can be found in U.S. patent application Ser. Nos. 13/285,911, 13/285,894, 13/285,867, all of which are incorporated by reference herein in their entirety. While specific components and connections have been described in relation toFIG. 3, many other specific configurations including computation schemes, conversion schemes, analog signals, and digital signals can be used without departing from the scope of the present disclosure. Many circuits for comparing the value of two capacitors to each other and outputting a signal based on this difference are well known in the art today and any of these are acceptable for use in the sensing circuit herein. For example, in one embodiment, the sensor capacitor20and the reference capacitor22are coupled to a comparator circuit that outputs a single signal indicating the difference in capacitor values between the two. This single signal can then be used as corresponding to the humidity value. In this embodiment, the comparator circuit is directly coupled to the capacitors20and23prior to the A to D converter.

FIG. 4Aillustrates a capacitive humidity sensor20at an intermediate stage of processing. A thin film layer22of a conductive material is formed on the dielectric substrate40. The thin film layer22is the bottom electrode of the capacitive humidity sensor20. The bottom electrode22is patterned and etched to form openings41exposing portions of the dielectric substrate40.

The dielectric substrate40is, in one example, a layer of silicon dioxide about 1 μm thick. The bottom electrode22is, in one example, a tantalum aluminum layer about 800 Å thick. The tantalum aluminum layer22can be deposited by a physical vapor deposition process, such as sputtering, or by any other suitable process for deposing a thin film of conductive material. The dielectric substrate40may be formed on a semiconductor substrate (not shown). The semiconductor substrate may include transistors. The dielectric substrate40may include metal interconnections connected to the transistors that are part of analog to digital converter36, and microcontroller38.

InFIG. 4B, a humidity sensitive dielectric layer26is formed on the bottom electrode22and contacts the exposed portions of the dielectric substrate40. The humidity sensitive dielectric layer26has a dielectric constant which changes according to moisture content absorbed by the humidity sensitive dielectric layer26. In one example, the humidity sensitive dielectric layer26is a layer of polyimide about 8 μm thick. Polyimide adheres poorly to the tantalum aluminum of the bottom electrode22. This is one reason openings41are formed in the bottom electrode22, to expose the dielectric substrate40. The polyimide26contacts the silicon dioxide of the dielectric substrate40and adheres relatively strongly with the dielectric substrate40. The openings41and the bottom electrode22are between 5 and 10 μm across and may be round, square or other shape. The polyimide layer26is deposited by a chemical vapor deposition process. The polyimide layer26can also be formed by any other suitable process. Such processes for forming polyimide layers are well known to those of skill in the art and all such suitable processes fall within the scope of the present disclosure. In one embodiment the etched polyimide layer26has a dielectric constant that is about 2.9 at 0% humidity and about 3.4 at 100% humidity. Different types of polyimide may be used having lower or higher dielectric constants than described herein.

Alternatively, the humidity sensitive dielectric layer26may be a material other than polyimide. Any suitable humidity sensitive dielectric material may be used. All such suitable humidity sensitive dielectric materials fall within the scope of the present disclosure. In some cases, the adherence openings41are not necessary if the dielectric26has sufficient adherence to the electrode22.

As shown inFIG. 4C, a dielectric layer42is deposited on the humidity sensitive dielectric layer26. The dielectric layer42is, for example, a layer of phosphosilicate glass about 5,000 Å thick. Alternatively, the dielectric layer42can be any other suitable dielectric layer, such as silicon dioxide, silicon nitride, or any other suitable dielectric material.

As shown inFIG. 4D, a thin film conducting layer24is deposited on the dielectric layer42. The thin film conducting layer24is the top electrode of capacitive humidity sensor20. The top electrode24is, in one example, an aluminum layer about 5,000 Å thick on a barrier layer of titanium tungsten about 1,000 Å thick. Alternatively, other suitable conductive materials and barrier layers may be used in place of aluminum or titanium tungsten. All such suitable conductive materials fall within the scope of the present disclosure.

As shown inFIG. 4E, the top electrode24is patterned and etched to form openings29exposing the dielectric layer42. The openings29are formed directly above the openings41in the bottom electrode22in one embodiment. The area A of the capacitor20formed by the top plate24, the bottom plate22, and the dielectric layers42,26between the top and bottom plates24,22is determined in part by the overlapping area of the top electrode24and the bottom electrode22. The openings29,41which have been etched in the top electrode24and the bottom electrode22do not contribute to the capacitance of the capacitor20. Therefore, it is advantageous that holes29formed in the top electrode24be aligned with holes41formed in the bottom electrode22. The openings29in the top electrode24are approximately the same size as the openings41in the bottom electrode22. The openings29are, therefore, about 5 to 10 μm across in one embodiment. Alternatively, it may be advantageous to form the openings41slightly larger than the openings29to allow for some misalignment between the masks used to form the openings41and the openings29.

In one embodiment, few if any of the openings29will actually be in alignment with the apertures41. Rather, in one embodiment, the apertures29will be over to the electrode surface as shown inFIG. 1and not over the openings41.

As shown inFIG. 4G, a dielectric layer46is formed on the top electrode24and on exposed portions of the dielectric42. The dielectric46is, in one example, a layer of phosphosilicate glass about 1 μm thick. The phosphosilicate glass46acts as a passivation layer for the capacitive humidity sensor20. The dielectric layer46and the dielectric layer42are anisotropically etched to expose portions of the humidity sensitive dielectric layer26. The dielectric46and the dielectric42can be etched by any suitable process, including dry etches or wet etches performed for suitable lengths of time.

As shown inFIG. 4H, the humidity sensitive dielectric layer26is isotropically etched through the openings29. Because the humidity sensitive dielectric layer26is isotropically etched, hollow portions28are formed between the top electrode24and the bottom electrode22. The isotropic etch leaves a substantially semicircular cross-section of the hollow portions28. The capacitance of the capacitor20inFIG. 4His lower than the capacitance of the capacitor20inFIG. 4G. This is because the humidity sensitive dielectric layer26has been replaced by air to some extent between the bottom electrode22and the top electrode24. The dielectric constant of air is 1 and is lower than the dielectric constant of humidity sensitive dielectric layer26. Therefore, the larger the volume of air between the top electrode24and the bottom electrode22the smaller the capacitance of the capacitor20.

The humidity sensitive dielectric layer26can be isotropically etched by a dry plasma etch. The dry plasma etch can be supplemented by a reactive ion etch as well. The duration of the plasma etch, the pressure of the plasma etch, and the temperature at which the plasma etch is performed all affect the extent to which the polyimide26is removed. The value of the capacitance of the capacitor20can be selected by simply varying the time, pressure, or temperature of the plasma etch which isotropically etches the polyimide26. A manufacturer of the capacitive humidity sensor20can, therefore, provide a capacitive humidity sensor20which has a capacitance custom selected by a customer with little to no extra cost because no new masks or layers have been deposited, only one or more etch parameters have been altered. In one embodiment, the etch which anisotropically etches the dielectric layers46,42, isotropically etches the humidity sensitive dielectric layer26. Alternatively, the humidity sensitive dielectric layer26is etched in a separate etch from the etch which anisotropically etches the dielectric layers46,42.

Because the humidity sensitive dielectric layer26is exposed to air through openings29, the humidity sensitive dielectric layer26can absorb moisture from the air. The dielectric constant of the humidity sensitive dielectric layer26changes according to an amount of moisture absorbed from the air. A measurement of the capacitance of the capacitor20gives an indication of the humidity content in the air. Pure polyimide has a dielectric constant in the range of about 3.2. Pure water has a dielectric constant in the range of 80. Therefore, when even small amounts of water are absorbed by the dielectric polyimide, the change is measurable.

A capacitive humidity sensor20which includes both air and polyimide as the dielectric between electrodes provides better precision in the spectrum of 70% to 100% relatively humidity. A capacitive humidity sensor20which includes only polyimide between the top electrode24and the top electrode22has a capacitance which varies with a relatively small slope throughout the range from 0% to 100% relatively humidity. However, a capacitive humidity sensor20which includes a combination of air and a humidity sensitive dielectric layer26has an increased, almost exponential slope in the range between 70% and 100% relative humidity.

One preferred method to tune the sensitive region of the polyimide is to control the size of the hollow regions28and the amount of undercut below electrode42. With little to no undercut, the sensor has more sensitivity in the humidity range of 20% to 60%, while with a large undercut, remaining over 50% of the polyimide, the greatest sensitivity is in the range of 70% to 100% humidity. The sweet spot for sensitivity can therefore be tuned based on the depth of the etch and the amount of material removed. The present humidity sensor is therefore beneficial since the sensitivity can be tuned easily and at low cost, with no additional masks.

The capacitive humidity sensor20ofFIG. 4His also highly sensitive to charge variations between the two plates. This allows sub-pF capacitance changes to be measured accurately. Because sub-pF capacitances can be measured accurately, the size of the capacitive humidity sensor20can be very small. The capacitor20can be small enough to have a capacitance about 2 pF. While particular materials for the bottom and top electrodes22,24, the humidity sensitive dielectric layer26, and the dielectric layers42,46have been described herein, any suitable materials can be used in accordance with principles of the present disclosure. While specific thicknesses of the various layers and widths of the openings have been described, any suitable thicknesses and widths can be used according to principles of the present disclosure. While particular structures and sequences of etching have been described, any suitable structures and etches can be used in accordance with principles of the present disclosure.

In one embodiment, the bottom electrode22is selected for its electrical properties and ease of manufacture, such as polysilicon, or an alloy with small amounts of Cu and Si. A separate material, selected for its heating properties, is under the electrode22. In this embodiment, the electrode22acts only as a plate of capacitor and a heater under the plate22. The heater is coupled to a high current source and can rapidly be heated to a high temperature. Since the capacitor plate22is made of thermally conductive material, such as aluminum or polysilicon, and the heater is under it. The heat transfers easily to the polyimide26.

By providing a separate heater layer under the electrode22, each of these can be constructed to perform its desired function, heating and as an electrode, respectively, thus improving the properties of each and the overall operation of the circuit. For example, if a separate, specialized heater is used, it can achieve a higher temperature, with less current, more rapidly than if use of the electrode22as both the heater and as the electrode.

In some embodiments, there is a thin, dielectric insulator between the heater and the server electrode to ensure they are fully electrically isolated, while in other, they are in direct electrical contact with each other and the electrode22is disabled for use as a capacitor plate while the heater is functioning.

Contact region50cis formed of the same metal layer as the bottom electrodes22a,22b, but is electrically isolated from the electrodes22a,22b. The contact region50cis in electrical contact with the top layer24described layer herein. The bottom electrode22bof the reference capacitor32includes contact regions50d,50e. Contact region50fis formed of the same metal layer as the bottom electrodes22a,22b, but is electrically isolated from the electrodes22a,22b. As the humidity sensitive dielectric layer26in the reference capacitive humidity sensor20will absorb no moisture from the air, there is no need to pass a current between contacts50d,50e.

FIG. 6is a top view of the top electrode24aof the sensor capacitor20and the top electrode24bof the reference capacitor32. The top electrodes24a,24bare formed of the same metal layer. The metal layer is deposited on the dielectric layer42, is patterned and etched to leave top electrodes24a,24bas shown inFIG. 6. Top electrode24aof the sensor capacitor20includes contact region52a. Contact region52ais in electrical contact with metal vias which allow for signals to be read from the top electrode24a. Contact region52bis in contact with a metal via and allows signals to be read from the top electrode24bof the reference capacitor32.

FIG. 7is a cross-section of the sensor capacitor20taken along cross-section lines7ofFIG. 5. InFIG. 7, the bottom electrode22aof the sensor capacitor20overlies the dielectric layer40, as described previously (or in the alternative embodiment, overlies a separate heater). The bottom electrode22ahas been patterned and etched to form openings41which expose portions of the dielectric substrate40. Contact region50ais a region at which the bottom electrode22aelectrically contacts metal track54a. Metal track54ais electrically connected to via56a. At contact region50b, metal from the metal layer that was used to form the bottom electrode22acontacts the metal track54b. At contact region50a, metal track54bcontacts the top electrode24aof the sensor capacitor20. The top electrode24ais in electrical contact with via56b. Metal tracks54a,54bare, for example, aluminum copper alloy metal tracks about 5,000 Å thick. The metal tracks54a,54bcomprise about 98% aluminum and about 2% copper. Other alloys and compositions can be used to form metal tracks54a,54bin accordance with principles of the present invention. Vias56a,56bare also formed of aluminum copper alloy. The vias56a,56ballow electrical contact to be made to the bottom plate22aand the top plate24afrom the top of the capacitive humidity sensor20. Contact pads34may be formed on top of the metal vias56a,56b.

As described previously, air may enter through openings29to contact the humidity sensitive dielectric layer26. The humidity sensitive dielectric layer26absorbs moisture from the air and the dielectric constant thereof changes. The capacitance of the sensor capacitor20changes as the dielectric constant of the humidity sensitive dielectric layer26changes. Also, the capacitance between the bottom electrode22aand the top electrode24ais based, in part, by the amount of air between the electrodes24a,22a. Other shapes, structures, and materials can be used in accordance with principles of the present disclosure.

FIG. 8is a cross-section of the reference capacitor32taken along lines8-8ofFIG. 5. The reference capacitor32is substantially identical to the sensor capacitor20ofFIG. 7. The bottom electrode22bof the reference capacitor32contacts metal track54cat contact area50d. The metal track54cis in electrical contact with metal via56c. The top electrode24bof the reference capacitor32contacts metal track54dand the metal track56dat contact region52b. The capacitance of the reference capacitor32can be measured through vias56c,56ewhich are electrically connected to the top electrode24band the bottom electrode22bof the reference capacitor32. The primary difference between the reference capacitor32and the sensor capacitor20is that the dielectric layers46,42have not been etched to form openings29therein. Openings29are only formed in the top electrode24bof the reference capacitor32. The humidity sensitive dielectric layer26is not exposed to the surrounding environment and, therefore, the capacitance of the reference capacitor32will not change with a change in humidity.

FIG. 9is a top view of a sensor capacitor20which is a capacitive humidity sensor and a reference capacitor32. Only the metal layers used to form the bottom electrode22and the top electrodes24a,24bof the capacitors20,32are shown. InFIG. 9, a single bottom electrode22serves as the bottom electrode for the reference capacitor32and the sensor capacitor20. Top electrode24ais formed over a portion of the bottom electrode22. Top electrode24bof the reference capacitor32is formed over a separate portion of the bottom electrode22. Openings41in the bottom electrode22are formed as described previously to enable adhesion of the humidity sensitive dielectric layer26(not shown) to the dielectric substrate40below the bottom electrode22. Openings29have been formed in the top electrode24aof the sensor capacitor20and in the top electrode24bof the reference capacitor32. Openings29in the sensor capacitor20expose the humidity sensitive dielectric layer26to the surrounding environment. The humidity sensitive dielectric layer26has only been shown in the openings29in the top electrode24aof the sensor capacitor20. In practice, the humidity sensitive dielectric layer26would cover the entirety of the bottom electrode22. The humidity sensitive dielectric layer26is not illustrated in the holes29of the top electrode24bof the reference capacitor32because the openings29in the top plate of the top electrode24bdo not expose the humidity sensitive dielectric layer26. Though not illustrated inFIG. 9, holes41are also formed directly below the holes29of both the top electrode24aand the top electrode24b.

In one embodiment, the openings29in the top electrodes24a,24bare about 9 μm wide. The holes29in the top electrode24aand the top electrode24bare spaced apart by about 16 μm edge to edge. The openings41in the bottom electrode22are the same size as the opening29in the top electrodes24a,24b. Alternatively, the openings41in the bottom electrode22may be slightly larger than the openings29in the top electrodes24a,24b. In one example, the openings41in the bottom electrode22are about 15 μm wide in order to allow for some mismatch in the alignments of the masks used to form the openings29,41. The openings29,41are shown having square cross-sections. However, the cross-sections may be circular, elliptical, or any other suitable cross-section. A current may be passed through the bottom electrode22through contacts50a,50b. The top electrode24aof the sensor capacitor20can be electrically contacted at contact region50c. The top electrode24bof the reference capacitor32can be electrically contacted through contact50d. Variations between the reference capacitor32and the sensor capacitor20can be reduced by forming the bottom electrode22of the sensor and reference capacitors20,32from single metal plate. When the microcontroller38computes a value of the humidity based on the digital humidity signal and the digital control signal, as described previously, it is very beneficial to have a digital control signal that has as little variation as possible from the digital humidity signal except in a component of the digital humidity signal contributed by the change in dielectric constant of the humidity sensitive dielectric layer26.

FIG. 10is a top view of a sensor capacitor20and a reference capacitor32, according to one embodiment. InFIG. 10, the sensor capacitor20and the reference capacitor32have a common bottom electrode22. The sensor capacitor20has two adjacent top plates24aconnected by conductive tracks64a,64b. The reference capacitor32includes two adjacent top plates24b. The two adjacent top plate portions24bof the reference capacitor32are connected to each other by conductive tracks64c,64d. Having two top plate portions24a,24bfor the sensor capacitor20and the reference capacitor32helps to compensate for capacitance variations in individual sensor and reference capacitor20,32. The common bottom plate22can be electrically contacted by contact50b. The top plates24aof the sensor capacitor20can be contacted through contact50a. The top plates24bof reference capacitor32can be contacted through contact50c. InFIG. 10, the conductive tracks64a-64aare shown as being formed in the same metal layer as the top electrodes24a,24b. However, in one embodiment, the metal tracks64a-64dare formed of a higher metal layer than the top electrodes24a,24band contact the top electrodes24a,24bat selected contact regions.

FIG. 11illustrates a sensor capacitor20and a reference capacitor32according to one embodiment. The sensor capacitor20includes two top plates24a. The reference capacitor32includes two top plates24b. The two top plates24aof the sensor capacitor20are separated by one of the top plates24bof the reference capacitor32. The two top plates24aof the sensor capacitor20are connected to each other by a conductive track64a. The two top plates24bof the reference capacitor32are separated from each other by one of the top plates24aof the sensor capacitor20. The two top plates24bof the reference capacitor32are connected to each other by conductive tracks64b. The configuration of the reference capacitor32and the sensor capacitor20inFIG. 11further helps to compensate for capacitance variations in individual sensor and reference capacitors. This helps to enable computation of a more accurate value of the humidity in the surrounding environment. Contacts50a-50callow for electrical connection to the common bottom plate22and to plates24a,24bof the reference capacitor32and the sensor capacitor20.

FIG. 12illustrates an integrated circuit package30according to one embodiment. The integrated circuit package30includes a capacitive humidity sensor20as described previously. The capacitive humidity sensor20is formed on an integrated circuit die which is encapsulated in the integrated circuit package30. The humidity sensitive dielectric layer26is exposed to the environment surrounding the integrated circuit package30by openings29formed in the passivation layer46encapsulating the integrated circuit package30. Electrical contacts34, shown as leads of lead frame inFIG. 12, allow for electrical connection to the capacitive humidity sensor20. Electrical contacts24also allow electrical connection to a microcontroller38formed in an integrated circuit die within the integrated circuit package30. A reference capacitor32is also formed in the integrated circuit package30but is not exposed to the surrounding environment as described previously. The integrated circuit package30shown as a lead frame package inFIG. 12, alternatively may be a ball grid array package, a pin grid array package, an embedded wafer level ball grid array package or any other suitable integrated circuit package.

FIG. 13illustrates a method for forming a capacitive humidity sensor20according to one embodiment. At100, a first electrode22is formed on a dielectric substrate40. The first electrode is patterned to expose portions of the dielectric substrate40. The first electrode is, for example, tantalum aluminum. The first electrode22may also be a heating element as described previously. At102, a humidity sensitive dielectric layer26is formed on the first electrode. The humidity sensitive dielectric layer26contacts the dielectric substrate through the openings41formed in the first electrode22. The humidity sensitive dielectric layer26has a dielectric content which changes as the humidity sensitive dielectric layer26absorbs moisture. The humidity sensitive dielectric layer26is, for example, polyimide as described previously. At104, a second electrode24is formed on the humidity sensitive dielectric layer26. The second electrode24is, for example, aluminum. At106, an opening is formed in the second electrode24. The opening exposes a portion of the humidity sensitive dielectric layer26. At108, the humidity sensitive dielectric layer26is isotropically etched through the opening29in the second electrode24. The isotropic etch of the humidity sensitive dielectric layer26causes hollow portions to be formed between the first electrode22and the second electrode24. The capacitance of the capacitive humidity sensor20can be selected by selecting the duration of the isotropic etch of the humidity sensitive dielectric layer26. By selecting a longer duration of the isotropic etch, the capacitance of the capacitive humidity sensor20can be made comparatively small. By selecting a smaller duration of the isotropic of the humidity sensitive dielectric layer26, the capacitance of the capacitive humidity sensor20can be made comparatively large.

FIG. 14is a graph of the capacitance of a capacitive humidity sensor20vs. relative humidity (RH). The solid black line illustrates the relative humidity of the capacitive humidity sensor20while the relative humidity is increasing. The capacitance follows a nearly linear curve from 0% RH to about 70% RH. Between about 70% and 100% RH the capacitance follows a somewhat exponential curve. The capacitance of the capacitive humidity sensor20varies from about 2.2 pF at 0% humidity to about 2.5 pF at 100% humidity.

The graph ofFIG. 14illustrates that the capacitive humidity sensor20exhibits hysteretic effects. The capacitance of the capacitive humidity sensor20follows a different curve, shown in dashed lines, when decreasing from 100% RH. For this reason it is beneficial to utilize a heating element to expel humidity from the humidity sensitive dielectric layer26prior to making a new measurement of the humidity. Prior to taking a new measurement of humidity, a current is passed through the bottom electrode22of the capacitive humidity sensor20to expel moisture from the humidity sensitive dielectric layer26. After passing the current through the bottom electrode22, a brief time can be allowed for the humidity sensitive dielectric layer26to absorb moisture from the air before taking the new humidity measurement.

As can be seen in the graph ofFIG. 14, the sensitivity of the sensor for polyimide can be easily measured over a range from 20% to 100%, with the greatest sensitivity being over 60% when large recesses28are formed. While not shown on the graph, reliable measurements can also be made with polyimide over the range of a humidity from 0% to 20% and the controlled size of hollow portions28increases the sensitivity selection in this range more than is possible with only polyimide as the dielectric. Of course, other humidity sensitive dielectrics can be used in place of polyimide that provide improved sensitivity over the range of 0% to 50% humidity. Such materials include undoped silicon, nanoporous silicon, ceramics, porous ceramics, glass, aerogels, nanoporous silicon dioxide compositions, and the like.

According to one embodiment, two humidity sensitive capacitors having different ranges for peak sensitivity are placed adjacent to each other on the same integrated circuit. One capacitor has a dielectric with a very sensitive response in the 0% to 60% range and adjacent to it is another capacitor having a very sensitive response in the 50% to 100% range. The circuit38can use data from just one of the sensors to determine the humidity, selecting the one with the most sensitivity for the measured humidity, or alternatively, it can use data from both to aid in arriving at the most reliable and accurate humidity measurement.