Gas sensors including microhotplates with resistive heaters, and related methods

A microhotplate comprising a membrane suspended over a substrate by a plurality of tethers connected between the substrate and the membrane. The membrane comprises a resistive heater comprising an electrically conductive material having a varying width from a peripheral portion of the membrane to a center of the membrane. The electrically conductive material comprises a first portion spiraling in a first direction and a second portion spiraling in a second direction and in electrical communication with the first portion at the center of the membrane. The microhotplate further comprises a first electrically conductive trace extending over a first tether and in electrical contact with a bond pad on the substrate and the first portion and a second electrically conductive trace extending over another tether and in electrical contact with another bond pad on the substrate and the second portion. Related chemical sensors and related methods of detecting at least one analyte are also disclosed.

TECHNICAL FIELD

Embodiments of the disclosure relate generally to micro-electro-mechanical systems (MEMS), such as microhotplate devices, gas sensors including the microhotplate devices, and to related methods of forming and operating the microhotplate devices and gas sensors. More particularly, embodiments of the disclosure relate to microhotplates having resistive heaters configured to uniformly heat a membrane of the microhotplate while reducing power consumption and localized overheating in the structure of the microhotplate, to related methods of operating the microhotplates, and to related methods of fabricating the microhotplates.

BACKGROUND

Microhotplates may be useful in sensors used for chemical detection applications. Microhotplates may include a chemical sensitive coating for detecting one or more properties of one or more gases or analytes. The microhotplates may be sized to have dimensions on the order of tens to hundreds of microns to consume less power and be more easily integrated into smaller packages than existing microhotplate devices measuring many hundreds of microns. Microhotplates may be formed using MEMS-based or CMOS-based silicon processes.

Conventional heating elements associated with a microhotplate may provide a heat source for heating a suspended membrane of the microhotplate or a material disposed over the heating element (e.g., the chemical sensitive coating). However, due to differences in thermal losses to portions of the microhotplate away from the membrane, the heating element and any chemical sensitive coating materials associated with the heating element often exhibit different temperatures, resulting in a non-uniform temperature profile across the microhotplate. By way of non-limiting example, peripheral portions of the membrane supporting the heating element may lose relatively more heat than central portions thereof, resulting in a non-uniform temperature profile across the heating element and the membrane. For example, in some instances, the peripheral portions often exhibit greater convection than central portions of the membrane, resulting in more heat loss from the peripheral portions to than from the central portions of the membrane. In addition, heat losses from the suspended microhotplate to its supporting substrate by conduction through supporting tethers may further exacerbate the non-uniformity of the temperature profile of the microhotplate. Furthermore, chemical reactions on the chemical sensitive coating material, as well as physical measurements made on uncoated plate surfaces of a reference microhotplate (such as, for example, for thermal conductivity measurements) may be sensitive to temperature changes, and performance of an associated sensor may be negatively impacted by a non-uniform operating temperature of the microhotplate.

In order to compensate for non-uniform heat losses across the microhotplate, some microhotplates incorporate a heat spreader plate to facilitate somewhat uniform heat transfer through the membrane. Other devices include heating elements having unique shapes including sharp corners and abrupt changes in direction. However, such sharp corners and abrupt changes in direction may adversely affect the operation and lifetime of the resistive heater.

BRIEF SUMMARY

Embodiments disclosed herein include microhotplates, chemical sensors including at least one microhotplate, and methods of detecting at least one analyte. For example, in accordance with one embodiment, a microhotplate comprises a membrane suspended over a substrate by a plurality of tethers connected between the substrate and the membrane. The membrane comprises a resistive heater comprising an electrically conductive material having a varying width from a peripheral portion of the membrane to a center of the membrane. The electrically conductive material comprises a first portion spiraling in a first direction, and a second portion spiraling in a second direction and in electrical contact with the first portion at the center of the membrane. The microhotplate further comprises a first electrically conductive trace extending over a first tether and in electrical contact with a bond pad on the substrate and the first portion and a second electrically conductive trace extending over another tether and in electrical contact with another bond pad on the substrate and the second portion.

In additional embodiments, a chemical sensor comprises at least one microhotplate. The at least one microhotplate comprises a plurality of tethers extending over a void formed in a substrate, the plurality of tethers supporting the membrane over the substrate and comprising a plurality of dielectric layers. The membrane comprises a resistive heater between two dielectric layers of the plurality of dielectric layers, the resistive heater comprising an electrically conductive material having a first portion spiraling in a first direction and a second portion spiraling in a second, opposite direction, the electrically conductive material having a varying width from an outer portion of the resistive heater to a central portion thereof. The microhotplate further comprises electrically conductive heater traces configured to provide power to the resistive heater, the electrically conductive heater traces overlying at least one of the tethers.

In further embodiments, a method of measuring at least one of a thermal conductivity, an exothermic event, and an endothermic event comprises providing a current to a resistive heater of at least one microhotplate, the resistive heater comprising a varying width from a peripheral portion thereof toward a center thereof, the resistive heater comprising a first portion extending from the peripheral portion toward the center thereof and spiraling in a clockwise direction and a second portion in contact with the first portion at the center of the resistive heater and extending from the center of the resistive heater toward the peripheral portion thereof and spiraling in a counterclockwise direction. The method further comprises measuring a voltage and current across the resistive heater and calculating a resistance of the resistive heater to determine an average temperature of the resistive heater.

In yet other embodiments, a sensor for providing orthogonal analysis of a sample comprises an array of microhotplates. Each microhotplate comprises a resistive heater comprising an electrically conductive material having a varying width from a peripheral portion of the membrane to a center of the membrane. The electrically conductive material comprises a first portion spiraling in a first direction, and a second portion spiraling in a second direction and in electrical contact with the first portion proximate the center of the membrane. The sensor further comprises a controller configured to determine one or more of at least one property of the resistive heater of at least one microhotplate of the array of microhotplates and a resistance between interdigitated electrodes of at least one microhotplate of the array of microhotplates.

In yet further embodiments, a method of measuring a response from a sensor comprising an array of microhotplates comprises providing a current to a resistive heater of each microhotplate of an array of microhotplates, the resistive heater of each microhotplate having a varying width from a peripheral portion of the membrane to a center of the membrane. The electrically conductive material comprises a first portion spiraling in a first direction, and a second portion spiraling in a second direction and in electrical contact with the first portion proximate the center of the membrane. The method further comprises measuring a response from each microhotplate of the array of microhotplates, wherein measuring a response from each microhotplate of the array of microhotplates comprises analyzing a response from at least one reference microhotplate free of a coating material or comprising an inert material overlying a dielectric material over its resistive heater, analyzing a response from at least one microhotplate comprising a catalytic material overlying a dielectric material over its resistive heater, and analyzing a response from at least one microhotplate comprising a chemical sensing material selected from the group consisting of a p-type semiconductor, an n-type semiconductor, and an ionic conductor overlying a dielectric material over its resistive heater.

DETAILED DESCRIPTION

Illustrations presented herein are not meant to be actual views of any particular material, component, or system, but are merely idealized representations that are employed to describe embodiments of the disclosure.

The following description provides specific details, such as material types, material thicknesses, and processing techniques in order to provide a thorough description of embodiments described herein. However, a person of ordinary skill in the art will understand that the embodiments disclosed herein may be practiced without employing these specific details. Indeed, the embodiments may be practiced in conjunction with conventional fabrication techniques employed in the industry.

As used herein, the term “tether” means and includes a structure that supports a portion of a membrane of a device over a substrate. A tether may extend from a peripheral portion of a device to a membrane of a microhotplate and may suspend the membrane over the substrate of the device. The tether may be suspended over a void formed in the substrate.

As used herein, the term “membrane” means and includes a central portion of a microhotplate, which may be suspended over a substrate by one or more tethers. The one or more tethers may extend from a peripheral portion of a sensor to the membrane, wherein the membrane is suspended over a cavity in a substrate by the tethers.

According to embodiments described herein, a microhotplate includes a membrane suspended over a substrate by a plurality of tethers extending from the membrane to a portion of the substrate. The membrane may be separated from the substrate by a void (e.g., a cavity) formed in a portion of the substrate. The membrane may be supported over the void in the substrate by the plurality of tethers extending from a periphery of the void in the substrate to the membrane. The depth of the cavity may be precisely controlled to modify the sensitivity and power consumption of the sensor. A more shallow depth may increase the conductive heat losses to the substrate, which may increase the sensitivity of the sensor to surrounding gas thermal conductivity. An increased depth may reduce losses to the substrate and may be desirable for increased efficiency and decreased sensitivity to the surrounding environment. In other words, the depth of the substrate relative to the tethers may be decreased to increase conduction heat losses to the substrate, which may improve determination of at least one property (e.g., a thermal conductivity) of a gas proximate the microhotplate.

The membrane may comprise a resistive heater shaped and configured to provide a substantially uniform temperature profile (e.g., an isothermal temperature profile) across the microhotplate. In some embodiments, the tethers and the membrane may be formed of and include the same materials. The resistive heater may comprise an electrically conductive material having a spiral shape and extending from a pair of bond pads located on the substrate and configured to provide a current through the resistive heater. The electrically conductive material may comprise a varying (e.g., an increasing, a continuously increasing) width along a length thereof as the electrically conductive material spirals from an outer portion (e.g., a periphery) of the membrane toward an inner portion (e.g., the center) of the membrane. As used herein, the term “varying” when used to describe a width of a structure means that the width changes along a length of the structure. The width may change in stepped increments, may change substantially continuously, may be tapered, may change substantially continuously over some portions and may change in stepped portions in other portions, etc. The varying width of the electrically conductive material may change a localized resistance (and hence a local heat output) of the electrically conductive material, thereby providing a substantially uniform temperature profile across the membrane and associated materials (e.g., a chemical sensing material, a catalyst coating, an inert coating, etc., of the membrane) disposed on the microhotplate. The electrically conductive material may comprise a first portion extending from an intersection between a first tether and the membrane and spiraling from an outer portion of the membrane toward the center of the membrane, the first portion spiraling in a first direction (e.g., a clockwise direction). The electrically conductive material may further comprise a second portion extending from an intersection between a second tether and the membrane and spiraling from an outer portion of the membrane toward the center of the membrane. The second portion may spiral in a second direction opposite the first direction (e.g., the second portion may spiral in a counterclockwise direction). The first portion and the second portion may be in electrical communication at the central portion of the membrane. The electrically conductive material may reverse direction of rotation at the center of the membrane, at a location where the first portion contacts the second portion.

Portions of the resistive heater having a relatively smaller width may exhibit a relatively greater electrical resistance than portions of the resistive heater with a relatively greater width. The portions of the resistive heater exhibiting the greater electrical resistance may generate more heat than portions having a relatively smaller electrical resistance. Portions of the resistive heater with a relatively smaller width may be located at locations of the resistive heater that are subject to relatively greater heat loss, such as at peripheral portions of the membrane, whereas portions of the resistive heater with a relatively greater width may be located at locations of the resistive heater that are not subject to as great a heat loss (e.g., at central portions of the membrane). The resistive heater may be sized and shaped such that the membrane exhibits a substantially uniform temperature profile, even though peripheral portions of the membrane may be subject to greater heat losses than central portions thereof.

The resistive heater may be free of sharp corners and abrupt changes in direction, that can lead to current crowding and high current densities, which in turn, may cause an undesired phenomenon known as “electromigration” wherein atoms of the resistive heater redistribute, leading to an effective thinning of the resistive heater in certain regions as atoms migrate to other regions of the resistive heater and ultimately leading to a non-uniform temperature profile and failure of the resistive heater. Forming the resistive heater with the varying (e.g., continuously varying) width may substantially reduce negative effects, such as electromigration, exacerbated by regions of high current density, high temperature, or both. In addition, the resistive heaters described herein may reduce the power required to heat the associated microhotplates to a desired temperature. In some embodiments, such as where the resistive heaters are used in microhotplate devices configured to measure a thermal conductivity, the heat transfer from the microhotplate to the environment proximate the microhotplate may be increased relative to conventional microhotplates. In addition, heat losses from the tethers to the extending substrate may be reduced compared to conventional microhotplates. The microhotplates including the resistive heaters with a varying width may be operated at temperatures between about 200° C. and about 1,200° C., such as between about 300° C. and about 800° C., or between about 800° C. and about 1,200° C. In some embodiments, the microhotplates are operated at temperatures up to about 1,200° C. without damaging the microhotplate (e.g., causing failure of the resistive heater or the membrane). By way of comparison, prior art microhotplates may fail at temperatures of greater than about 500° C., at least partially due to areas that become depleted in atoms due to electromigration.

A sensor may include a plurality of microhotplates. As described herein, at least some of the microhotplates may be configured to measure a thermal conductivity of a sample (e.g., a gaseous analyte, a concentration of an analyte in a sample, etc.), at least some of the microhotplates may be configured to determine a temperature at which the sample exhibits one or more reactions, and at least some of the microhotplates may be configured to include one or more coatings configured to interact with particular components that may be contained in the sample (e.g., may include one or more metal oxide semiconductor coatings formulated and configured to interact with particular species that may be contained within the sample). The use of a plurality of microhotplates in the sensor may increase a range of analytes that may be detected using the sensor and may increase the sensitivity and selectivity of the sensor. By way of non-limiting example, the plurality of microhotplates may be used to perform orthogonal analysis of the sample and determine one or more of a composition of the sample, a concentration of one or more gases in the sample, or another property of the sample.

FIG. 1Ais a top view of a device100comprising a microhotplate according to one embodiment of the disclosure. The device100comprises a membrane101formed over a substrate102(FIG. 1C). The substrate102may be a conventional silicon substrate or other bulk substrate including semiconductor material. As used herein, the term “substrate” means and includes not only silicon wafers, but also silicon-on-insulator (“SOI”) substrates, such as silicon-on-sapphire (“SOS”) substrates or silicon-on-glass (“SOG”) substrates, epitaxial layers of silicon on a base semiconductor foundation, or other semiconductor materials. In some embodiments, the substrate102comprises silicon. In other embodiments, the substrate102, or at least a portion thereof, may be oxidized and comprise, for example, a silicon oxide (e.g., SiO2).

The membrane101may have a circular shape, a square shape, a rectangular shape, a polygonal shape (e.g., a pentagonal shape, a hexagonal shape, an octagonal shape, etc.), or another shape. The membrane101may be supported over the substrate102by a plurality of tethers105,105a,105b. In some embodiments, the device100may comprise six tethers105, although the disclosure is not so limited. In other embodiments, the device100includes any number of tethers105, such as three tethers105, four tethers105, five tethers105, six tethers105, seven tethers105, eight tethers105, or any other number of tethers105. In some embodiments, the device100comprises an odd number of tethers105. Where the membrane101comprises a polygonal shape, corners (e.g., points) of the polygonal shape may be centered over a respective tether105. For example, and with reference toFIG. 2, a device200may comprise a hexagonally-shaped membrane201. Points of the membrane201may be disposed over a portion of a respective tether105. In some embodiments, the points of the polygonal shape are disposed over a central portion of the respective tether105.

The tethers105may extend from a peripheral portion of the device100(e.g., from the substrate102) to the membrane101over a void104formed in the substrate102. In other words, the tethers105may extend from the substrate102to the membrane101. The tethers105may support the membrane101over the substrate102. The membrane101may be separated from the substrate by the void104, as shown in, for example,FIG. 1C. The tethers105may be separated from a portion of (e.g., a central portion of) the substrate102by the void104.

The tethers105may have a width selected to reduce (e.g., minimize) a net heat flux along a length thereof and to reduce heat losses to the substrate102. By way of non-limiting example, a width of the tethers105may be minimized to reduce heat loss from the periphery of the membrane101via conduction. However, the width of the tethers105may be large enough to provide sufficient mechanical support to the membrane101, such as during heating thereof. Accordingly, the tethers105,105a,105bmay reduce heat losses from the membrane101through the tethers105,105a,105bto the underlying substrate102compared to conventional microhotplate devices. In some embodiments, the tethers105may include a widened portion proximate the intersection of the tethers105and the membrane101, the widened portion having a relatively greater width than other portions of the tether105. In some embodiments, the tethers105may also include a widened (e.g., a filleted) portion proximate an intersection of the tethers105and substrate102. Such filleting may reduce a corner stress concentration of the tethers105. In some embodiments, a width of the tethers105may be between about 3 μm and about 20 μm, such as between about 3 μm and about 15 μm, or between about 5 μm and about 10 μm. As used herein, the term “fillet” means and includes a rounding of an interior or exterior corner, such as where the tethers105intersect the membrane101. By way of non-limiting example, a filleted shape, as used herein, may have a shape similar to a shape of a fillet weld or a double tangent arc.

With continued reference toFIG. 1A, the device100may include a resistive heater110comprising an electrically conductive trace115electrically coupled to a pair of bond pads112a,112b. The resistive heater110may be powered by application of a current between the bond pads112a,112b. The resistive heater110may have a substantially circular shape. In other embodiments, the resistive heater110may exhibit an oval shape, a circular shape, or an elliptical shape. A resistive heater110with a circular shape may provide a substantially uniform temperature profile across the resistive heater110.

The electrically conductive trace115may extend from a first bond pad112aover a surface of a first tether105ato the resistive heater110at the membrane101. Another electrically conductive trace115may extend from a second bond pad112bover a surface of a second tether105bto the resistive heater110at the membrane101. In some embodiments, the first tether105aand the second tether105bmay be located substantially opposite one another and the first bond pad112aand the second bond pad112bmay be located substantially opposite one another. The electrically conductive traces115may also be referred to herein as “electrically conductive heater traces.”

Portions of the electrically conductive trace115over the tethers105may be substantially linear. At an intersection of the first tether105aand the membrane101, the electrically conductive trace115may transition from a linear shape to a curved (e.g., spiral, winding, rotating, etc.) shape. Similarly, at an intersection of the second tether105band the membrane101, the electrically conductive trace115may transition from a linear shape to a curved (e.g., spiral, winding, rotating, etc.) shape.

The resistive heater110may comprise a first portion114extending from an outer portion (e.g., a periphery) of the membrane101proximate the first tether105ato a location proximate the center of the membrane101and a second portion116extending from the periphery of the membrane101proximate the second tether105bto a location proximate the center of the membrane101. The first portion114and the second portion116may be in electrical contact with each other at a central portion of the membrane101.

In some embodiments, a first surface (e.g., a radially varying outer surface)117and a second surface (e.g., a radially varying inward surface)119opposing the first surface117of portions of the resistive heater110may not be substantially parallel. Stated another way, opposing portions of the first surface117and the second surface119may not be parallel. In some embodiments, the first portion114and the second portion116each comprise a spiral shape and the first surface117and the second surface119comprise curved (e.g., arcuate) surfaces. Accordingly, outer surfaces of the resistive heater110may comprise arcuate surfaces.

The first portion114may comprise a spiral shape and may spiral (e.g., wind, rotate, coil, curl, twist, etc.) in a first direction (e.g., one of a clockwise direction or a counterclockwise direction). The second portion116may comprise a spiral shape and may spiral (e.g., wind, rotate, coil, curl, twist, etc.) in a second direction opposite the first direction (e.g., another of the clockwise direction and the counterclockwise direction). In some embodiments, the direction of the spiral may change proximate the center of the membrane101where the first portion114and the second portion116contact each other. Stated another way, at the center of the membrane101, a direction of rotation of the resistive heater110may change from a first direction to a second, opposite direction.

Radially adjacent regions of the first portion114may be isolated from each other by a same distance. Similarly, radially adjacent regions of the second portion116may be isolated from each other by a same distance. In some embodiments, a region of the second portion116may be disposed between radially adjacent regions of the first portion114and a region of the first portion114may spiral and be disposed between radially adjacent portions of the second portion116. In other words, coils (e.g., spirals) of the first portion114may be radially surrounded by coils (e.g., spirals) of the second portion116and coils of the second portion116may be radially surrounded by coils of the first portion114. Stated another way, coils of the first portion114and coils of the second portion116may intertwine with each other and may be separated from each other by a distance.

In some embodiments, a gap120between adjacent portions of the first portion114and the second portion116may be substantially constant. The gap120may exhibit a spiral shape, similar to the spiral shape of the first portion114and the second portion116. The gap120may have a substantially constant width. In some embodiments, the width of the gap120may be less than a width W of the resistive heater110(e.g., less than the narrowest width W). In some embodiments, the width of the gap120may be minimized to facilitate a uniform temperature profile across the microhotplate or the membrane101, such as by using the resistive heater110as a heat spreader. By way of non-limiting example, a width of the gap120may be minimized such that a distance between adjacent portions of the resistive heater110is reduced. Since the resistive heater110exhibits a greater thermal conductivity than, for example, a dielectric material disposed over the resistive heater110and in the gaps120, the resistive heater110may approximate a heat spreader when the gaps120comprise a substantially reduced width. In some embodiments, the gap120may have a width between about 0.5 μm and about 5.0 μm, such as between about 1.0 μm and about 4.0 μm, between about 1.5 μm and about 3.5 μm, or between about 2.0 μm and about 3.0 μm. In some embodiments, the width of the gap120may be about 3.0 μm. In other embodiments, the width of the gap120may vary from portions proximate the periphery of the membrane101to portions proximate the center of the membrane101. In some embodiments, a width of the gap120may be greater at radially inward portions that at radially outward portions of the resistive heater110.

The electrically conductive material of the resistive heater110may comprise a metallization layer such as, for example, tungsten, molybdenum, tantalum, platinum, palladium, aluminum, titanium, titanium tungsten (TiW), copper, gold, doped silicon, doped polysilicon, other conductive metals or alloys, combinations thereof, or a layered structure comprising one or more of the aforementioned materials. The layered structure may include, for example, a first layer comprising one or more of tungsten, molybdenum, tantalum, platinum, palladium, aluminum, titanium, titanium tungsten (TiW), copper, gold, doped silicon, doped polysilicon, other conductive metals or alloys and at least a second layer comprising another of tungsten, molybdenum, tantalum, platinum, palladium, aluminum, titanium, titanium tungsten (TiW), copper, gold, doped silicon, doped polysilicon, other conductive metals or alloys over the first layer. In some embodiments, the resistive heater110may include one or more adhesion layers (e.g., titanium, tungsten, a combination thereof, etc.) configured to improve adhesion to one or more insulating materials underlying or overlying the resistive heater110, to vary the composite resistive properties of the resistive heater110, or both. In some embodiments, and at least another metallization layer and/or at least one passivation layer may overlie the one or more adhesion layers. In some embodiments, the electrically conductive material comprises tungsten. In some such embodiments, the resistive heater110may be operated at temperatures up to 1,200° C. without damaging the device100.

The first portion114may exhibit a varying (e.g., an increasing, a continuously increasing, a tapered) width W from a location proximate a periphery of the membrane101to a location proximate the center of the membrane101. The first portion114may exhibit an increasing width W from outer portions of the membrane101toward the center of the membrane101. In some embodiments, the width W increases from a minimum width proximate the periphery of the membrane101(e.g., proximate a widened portion150of the first portion114) to a maximum width proximate the center of the membrane101. The width W may increase substantially continuously from the outer portion of the membrane101to the inner portion thereof. Similarly, the second portion116may exhibit a varying (e.g., an increasing, a continuously increasing, a tapered) width from the outer portion of the membrane101(e.g., proximate an intersection between the second tether105band the membrane101) toward the center of the membrane101.

In some embodiments, the width of each location of the resistive heater110may be related to a distance of each location of the resistive heater110from a center of the membrane101. By way of non-limiting example, where the membrane101comprises a circular shape, the resistive heater110may have a decreasing width as a radial distance from the center of the membrane101increases. Since the resistive heater110comprises a spiral shape with a changing (e.g., continuously changing) distance from the center of the membrane101, the width of the resistive heater110may change (e.g., continuously change) along a length thereof.

In some embodiments, a maximum width of the resistive heater110(e.g., a width at the center thereof) may be at least about 2.5 times a minimum width of the resistive heater110(e.g., a width proximate the widened portion150). In some embodiments, the maximum width is greater than about 3.0 times the minimum width, greater than about 3.5 times the minimum width, greater than about 4.0 times the minimum width, or even greater than about 5.0 times the minimum width of the resistive heater110.

In some embodiments, for a predetermined first distance from the center of the membrane101, the width of the resistive heater110may be greater at substantially all distances (e.g., radial distances) from the center that are less than the first distance and may be smaller at substantially all distances from the center that are more than the first radial distance. By way of non-limiting example, the width of the resistive heater110as a function of distance from the center of the membrane may be approximated by, for example, Equation (1) below:
Wr=r×A(1)
wherein Wris the width of the resistive heater110for a predetermined distance from the center of the membrane101, r is the distance from the center of the membrane101, and A is a constant.

In other embodiments, the width of the resistive heater110may increase as the resistive heater110approaches the center of the membrane101according to a continuously differentiable formula. In some such embodiments, the resistive heater110may exhibit a shape such that its derivative exists at each point along the resistive heater110, thereby reducing and, in some embodiments, eliminating any sharp corners in the resistive heater110. In some such embodiments, the resistive heater110may not include any sharp corners (e.g., such as a 90° corner, a vertex, an angular point of a polygon, etc.) or abrupt changes in direction. Stated another way, the resistive heater110may not include abrupt changes in direction or sharp corners, such as a square corner. In other words, the resistive heater110may not comprise a corner converging at a single point (e.g., such as at a vertex). Rather, the resistive heater110may comprise arcuate (e.g., curved) surfaces, such as the first surface117and the second surface119. Accordingly, the resistive heater110may be substantially free of corners. Stated another way, the side surfaces (e.g., the first surface117and the second surface119) may be substantially free of corners and may comprise arcuate surfaces.

In some embodiments, the resistive heater110exhibits an Archimedean Spiral shape that may be offset to create the gap120between the first portion114and the second portion116.

The width W of the resistive heater110may increase from about 3 μm at a location proximate the periphery of the membrane101to about 20 μm at a location proximate the center of the membrane101.

In some embodiments, the resistive heater110may include a widened portion150at a location where the electrically conductive trace115transitions into the resistive heater110at a peripheral portion of the membrane101(which region may be referred to herein as a “transition region”). In some embodiments, the widened portion150may facilitate a transition from the substantially linear shape of the electrically conductive trace115on the tether105to the spiral shape of the resistive heater110. In some such embodiments, the resistive heater110may have a localized wide area at the widened portion150, a relatively smaller width radially inward from the widened portion150and an increasing (e.g., continuously increasing) width as the resistive heater110spirals toward the center of the membrane101.

The widened portion150may substantially reduce the current density at the transition region and reduces electromigration, enhancing the lifetime and overall operation of the resistive heater110. In other words, the widened portion150may facilitate an improved current density and a reduction in electromigration proximate the region where the electrically conductive material of the resistive heater110transitions from the linear portion over the tethers105a,105b(e.g., the electrically conductive traces115) to the resistive heater110.

The resistive heater110may be formed by a lithographic process. By way of non-limiting example, a reticle having a pattern of the resistive heater110may be used to form (e.g., deposit and pattern) the material of the resistive heater110.

In some embodiments, an electrical resistance of the resistive heater110may be related to the width W thereof. The electrical resistance of the resistive heater110may be relatively greater in magnitude at portions of the resistive heater110that have a smaller width W than at portions of the resistive heater110having a relatively larger width W (e.g., at portion proximate the center of the membrane101(i.e., radially inward portions)).

Increasing the width of the resistive heater110from the outer portions of the membrane101to the central portion of the membrane101may facilitate a substantially uniform temperature profile across the resistive heater110and the associated membrane101. At the peripheral portions, the membrane101and the resistive heater110may exhibit a greater heat loss than at central portions thereof. Accordingly, the electrical resistance of the resistive heater110may be greater at locations having a relatively smaller width (e.g., at locations proximate the periphery of the membrane101) than at locations having a relatively greater width (e.g., at locations proximate the center of the membrane101). Thus, the resistive heater110(and the membrane101) may exhibit a substantially uniform temperature profile since outer portions of the membrane101that are subject to greater heat losses are heated more by the resistive heater110than the central portions thereof. In other words, the tapered width of the resistive heater110may create a substantially isothermal temperature profile across the resistive heater110and the membrane101. In addition, since the resistive heater110provides a substantially uniform temperature profile of the membrane101, compared to conventional microhotplates, the device100may use less power to heat the membrane101. Further, the reduced width of the tethers105,105a,105bmay reduce conductive heat losses from the membrane101to the substrate102.

With continued reference toFIG. 1A, the device100may further include sense lines124configured to measure a voltage drop across the resistive heater110. The sense lines124may be located at a location such that the average temperature of an active area of the resistive heater110may be determined by measuring the voltage drop across the resistive heater110with the sense lines124. The sense lines124may comprise a high impedance voltage measurement system such that there is substantially no voltage drop through the sense lines124. The sense lines124may also be referred to herein as “electrically conductive sense line traces.”

The sense lines124may be coupled to respective sense line bond pads126. The sense line bond pads126may be located on the substrate102at a periphery of the device100. The sense lines124may extend from the sense line bond pads126to the resistive heater110. The sense lines124may extend over opposing tethers105, which may be different tethers105than the tethers105a,105bover which the electrically conductive traces115extend.

The device100may further include another pair of bond pads130. With reference toFIG. 1B, each bond pad130may be operably coupled to an electrode trace132that may extend over a tether105to the center of the membrane101. A first electrode134may be coupled to an electrode trace132and a second electrode136may be coupled to another electrode trace132. The first electrode134and the second electrode136may comprise interdigitated electrodes135. The electrode traces132may also be referred to herein as “chemical sensing electrode traces.”

The first electrode134and the second electrode136may form one or more patterns and may be referred to herein as “interdigitated electrodes.” As illustrated inFIG. 1B, the first electrode134may be in electrical contact with one of the bond pads130of the pair of bond pads130and the second electrode136may be in electrical contact with the other bond pad130of the pair of bond pads130. The first electrode134may include protrusions138extending from a base thereof and may be received by gaps (e.g., spaces) between adjacent protrusions140extending from a base of the second electrode136. The second electrode136may include protrusions140extending from the base thereof and may be received by gaps between adjacent protrusions138extending from the second electrode136. In other embodiments, the first electrode134and the second electrode136may not include the protrusions138,140, respectively.

Accordingly, with reference toFIG. 1AandFIG. 1B, each tether105of the device100may include a conductive trace thereon. For example, two of the tethers105(e.g., the first tether105aand the second tether105b) may include the electrically conductive traces115thereon, two of the tethers105may include the sense lines124thereon, and two of the tethers105may include the electrode traces132thereon. In other embodiments, it is contemplated that at least some of the tethers105may not include a conductive trace thereon. By way of non-limiting example, in some embodiments, the device100may not include the sense lines124, but may include the electrically conductive traces115and the electrode traces132.

FIG. 1Cis a side cross-sectional view of the device100taken along section line C-C (FIG. 1A). The cross-sectional view of the device100inFIG. 1Cdoes not transverse any of the tethers105. As illustrated, the void104may extend under the membrane101and separate the membrane101from the substrate102.

In some embodiments, a thickness of the resistive heater110(e.g., a thickness in the vertical direction illustrated inFIG. 1C) may be between about 1,000 Å and about 4,000 Å, such as between about 1,500 Å and about 3,500 Å, or between about 2,000 Å and about 3,000 Å. However, the disclosure is not so limited and the thickness of the resistive heater110may be greater than or less than the thicknesses described above.

The membrane101and the tethers105may comprise a plurality of dielectric materials. A first dielectric material (e.g., an electrically insulating material)160may be disposed over and in contact with the substrate102at peripheral portions of the device100and extend over the void104to the membrane101. The first dielectric material160may include silicon, a silicon oxide (e.g., silicon dioxide (SiO2)), a nitride material (e.g., silicon nitride (e.g., Si3N4), hafnium nitride (e.g., Hf3N4), zirconium oxide (e.g., Zr3O4), or another insulating nitride material), a silicon carbide material, an oxynitride (e.g., silicon oxynitride (e.g., Si2N2O)), or combinations thereof. In some embodiments, the first dielectric material160comprises silicon dioxide. A thickness of the dielectric material160may be between about 100 Å and about 1,000 Å, such as between about 200 Å and about 800 Å, or between about 400 Å and about 600 Å. In some embodiments, the thickness of the dielectric material160is about 500 Å. However, the disclosure is not so limited and the thickness of the dielectric material160may be greater than or less than the thicknesses described above.

A second dielectric material (e.g., another electrically insulating material)162may overlie the first dielectric material160. The second dielectric material162may directly overlie and contact the first dielectric material160. The second dielectric material162may include silicon, a silicon oxide, a nitride material, a silicon carbide material, an oxynitride, or combinations thereof. In some embodiments, the second dielectric material162comprises a nitride material, such as a silicon nitride material. A thickness of the second dielectric material162may be between about 1,000 Å and about 6,000 Å, such as between about 2,000 Å and about 5,000 Å, or between about 3,000 Å and about 4,000 Å. However, the disclosure is not so limited and the thickness of the second dielectric material162may be greater than or less than the thicknesses described above. The second dielectric material162may be formed by one or more of atomic layer deposition (ALD), chemical vapor deposition (CVD), low pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), or other deposition process. In some embodiments, the second dielectric material162is formed by LPCVD. Accordingly, in some such embodiments, the second dielectric material162may comprise a LPCVD nitride material.

With continued reference toFIG. 1C, the resistive heater110may overlie portions of the second dielectric material162. A third dielectric material164may overlie and surround side surfaces of the resistive heater110at the membrane101. The third dielectric material164may include one or more layers of silicon, silicon oxide (e.g., silicon dioxide), a silicon nitride (e.g., SixNy) material, a silicon carbide material, an oxynitride, or combinations thereof. In some embodiments, the third dielectric material164comprises a silicon oxide (e.g., silicon dioxide) over a PECVD silicon nitride layer. In some embodiments, the third dielectric material164may be disposed in the gaps120(FIG. 1A) between adjacent portions (e.g., adjacent portions of the spiral) of the resistive heater110. At the peripheral portions of the device100, the third dielectric material164may directly overlie and contact the second dielectric material162. A thickness of the third dielectric material164may be between about 1,000 Å and about 6,000 Å, such as between about 2,000 Å and about 5,000 Å, or between about 3,000 Å and about 4,000 Å. However, the disclosure is not so limited and the thickness of the third dielectric material164may be greater than or less than the thicknesses described above. The third dielectric material164may be formed by one or more of ALD, CVD, LPCVD, PECVD, or other deposition process. In some embodiments, the third dielectric material164is formed by PECVD. In some such embodiments, the third dielectric material164comprise a PECVD silicon oxide material.

Each of the first dielectric material160, the second dielectric material162, and the third dielectric material164may be selected to exhibit at least one of a different tensile stress or a compressive stress, which may be selected to have a magnitude between about 200 MPa and about 2 GPa at room temperature (e.g., about 25° C.). In some embodiments, the tethers105,105a,105b, which may include one or more of the first dielectric material160, the second dielectric material162, and the third dielectric material164may exhibit a composite stress such that the membrane101is held in tension and remains substantially planar suspended over the void104. The different stress values may be selected to balance a stress of the membrane101or the device100about a neutral axis of the stack of materials (e.g., the stack of materials comprising the membrane101(i.e., the first dielectric material160, the second dielectric material162, and the third dielectric material164)), such as at operating temperatures (e.g., at temperatures between about 600° C. and about 1,200° C., such as between about 600° C. and about 800° C., between about 800° C. and about 1,000° C., or between about 1,000° C. and about 1,200° C.) of the device100. Accordingly, the different materials and thickness of the tethers105,105a,105bmay be selected to achieve the desired stress (e.g., composite stress, compressive stress, tensile stress, etc.) and exhibit a desired tension on the membrane101.

A thickness of each of the first dielectric material160, the second dielectric material162, and the third dielectric material164may be selected and tuned to exhibit an optimal residual tensile stress, which may result in a reduced mechanical deflection of the membrane101at operating temperatures thereof. In some embodiments, a formation temperature (e.g., a deposition temperature), a formation pressure (e.g., a deposition pressure), or both of one or more of the first dielectric material160, the second dielectric material162, and the third dielectric material164may be selected to tune a residual stress (e.g., a residual tensile stress) of one or more of the first dielectric material160, the second dielectric material162, and the third dielectric material164. By way of non-limiting example, one or more of the first dielectric material160, the second dielectric material162, and the third dielectric material164may be formed (e.g., deposited) at a temperature between about 300° C. and about 700° C. In some embodiments, a residual stress of the membrane101may be tuned by controlling a deposition power at which one or more of the first dielectric material160, the second dielectric material162, and the third dielectric material164is deposited. A residual stress between layers of the membrane101may change responsive to heating and expansion of the materials (e.g., such as during operation of the resistive heater110). Responsive to heating, the materials of the membrane101may exhibit a change in overall stress tensor (e.g., a reduction in the overall stress tensor). In some embodiments, the materials of the membrane101may be formed, formulated, and configured such that the stress does not become compressive during operation (e.g., at operating temperatures of the resistive heater110). In some such embodiments, buckling or substantial out of plane motions (i.e., up and down in the view ofFIG. 1C) of the membrane101may be reduced or even eliminated. Accordingly, tuning the residual tensile stress may reduce a likelihood of the membrane101from separating from the tethers105.

In some embodiments, after the first dielectric material160, the second dielectric material162, and the third dielectric material164are formed, the stack of materials may be annealed. In some embodiments, the annealing may be performed at a temperature between about 400° C. and about 800° C., such as between about 500° C. and about 700° C. In some embodiments, the annealing is performed at a temperature of about 600° C. Annealing the materials may form the materials having a desired stress. After annealing, the materials may be patterned to form the void104in the substrate102, such as by wet etching using, for example, a wet etchant. In some embodiments, the wet etchant may comprise potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), calcium hydroxide (Ca(OH)2), or other suitable caustic material. In some embodiments, the etching comprises isotropic etching. The etching may be performed from a back side of the substrate102(e.g., from a lower side of the substrate102illustrated inFIG. 1C), a front side of the substrate102, of from both sides of the substrate102. Etching may form the membrane101suspended above the substrate102by the tethers105. In some embodiments, etching from the back side may facilitate forming the tethers105extending from the substrate102at a peripheral portion of the device100over the void104to the membrane101.

As shown inFIG. 1C, taken by cross-section, an outermost portion of the resistive heater110may have a relatively smaller width (e.g., a distance from left to right in the cross-section illustrated inFIG. 1C) than portions of the resistive heater110proximate to the center of the membrane101.

With continued reference toFIG. 1C, the interdigitated electrodes135(e.g., each of the first electrode134and the second electrode136) may directly overlie and contact the third dielectric material164at the membrane101. In some embodiments, a chemical sensing material166may directly overlie and contact the interdigitated electrodes135. The chemical sensing material166may overlie and be disposed in between gaps of the interdigitated electrodes135. The chemical sensing material166may be in electrical contact with the interdigitated electrodes135such that the electrical characteristics of the chemical sensing material166(e.g., a resistivity between the interdigitated electrodes due to the chemical sensing material166) may be determined through the bond pads130(FIG. 1B,FIG. 1D). As used herein, the terms “resistivity” and “electrical resistance” are used interchangeably. The chemical sensing material166may comprise a material formulated and configured to exhibit a change in electrical resistance responsive to interaction with (e.g., reaction with, adsorption of, absorption of, oxidation by, reduction by, etc.) one or more chemicals (e.g., analytes) of interest, such as when an analyte is present thereon. In some embodiments, the chemical sensing material166may be formulated and configured to adsorb, absorb, or chemically react with at least one analyte of interest. The chemical sensing material166may also be referred to herein as a metal oxide semiconductor (MOS) coating (“MOS coating”) and the device100may also be referred to herein as a “MOS microhotplate.”

The chemical sensing material166may comprise a metal oxide (e.g., tin oxide, zinc oxide, tungsten oxide (e.g., WO3), a manganese oxide (e.g., MnO, MnO2, Mn2O3), LaCoO3, LaNiO3, vanadium oxide (e.g., V2O5), phosphorous pentoxide (e.g., P2O5), molybdenum oxide (MoO2), cesium oxide (e.g., Cs2O), etc.), a doped metal oxide (e.g., platinum-doped tin oxide), a polymer material (e.g., an electrically conductive polymer material), an ionic conductor (e.g., an electrochemical coating (also referred to as an e-chem coating)) material, an n-type semiconductor material, a p-type semiconductor material, a thermoelectric material, another material, or combinations thereof. In other embodiments, the chemical sensing material166comprises a semistor material formulated and configured to exhibit a change in one or more electrical properties responsive to reacting with an analyte. The semistor material may comprise, for example, tin oxide (e.g., SnO2), titanium oxide (e.g., TiO2), tungsten oxide (e.g., WO3), yttria-stabilized zirconia (YSZ), or combinations thereof.

With continued reference toFIG. 1C, the membrane101may include the first dielectric material160suspended over the void104, the second dielectric material162over the first dielectric material160, and the resistive heater110over the second dielectric material162. The third dielectric material164may overlie the second dielectric material162and the resistive heater110and may be disposed in the gaps120(FIG. 1A) of the resistive heater110. The interdigitated electrodes135may overlie the third dielectric material164and the chemical sensing material166may overlie the interdigitated electrodes135. At the periphery of the device100, the first dielectric material160may overlie the substrate102, the second dielectric material162may overlie the first dielectric material160, and the third dielectric material164may overlie the second dielectric material162. Accordingly, in some embodiments, the tethers105,105a,105bmay be formed of and comprise the same materials as the membrane101.

FIG. 1Dis a side cross-sectional view of the device100taken along the electrode trace132(FIG. 1B) and the tethers105(FIG. 1A) supporting the electrode trace132. The interdigitated electrode135and the electrode trace132may be disposed over the third dielectric material164and above the resistive heater110. The electrode trace132may be in electrical contact with the chemical sensing material166and may be configured to detect electrical properties (e.g., an electrical resistance) of the chemical sensing material166.

FIG. 1Eis a side cross-sectional view of the device100taken along the electrically conductive trace115(FIG. 1A) andFIG. 1Fis a side cross-sectional view of the device100taken along the sense lines124(FIG. 1A). As illustrated, the electrically conductive trace115and the sense lines124may be coplanar and may be in electrical contact with the resistive heater110.

In some embodiments, forming one or more of the electrically conductive traces115, the sense lines124, and the electrode traces132to comprise a material exhibiting a low thermal conductivity (e.g., tungsten) relative to other conductive materials may reduce conductive thermal transfer and heat loss from the one or more of the electrically conductive traces115, the sense lines124, and the electrode traces132to the substrate102through the tethers105,105a,105b. In addition, forming the tethers105,105a,105bfrom one or more materials exhibiting a relatively low thermal conductivity (such as the first dielectric material160, the second dielectric material162, and the third dielectric material164, each of which may comprise, for example, one or more of a nitride, an oxide, a low thermal conductivity ceramic material) may reduce conductive heat losses from the one or more of the electrically conductive traces115, the sense lines124, and the electrode traces132to the substrate102through the tethers105,105a,105b. Forming the tethers105,105a,105bto have a relatively thin width may further reduce conductive heat losses from the electrically conductive traces115, the sense lines124, and the electrode traces132to the substrate102.

Although the device100has been described and illustrated as including the resistive heater110, the disclosure is not so limited.FIG. 1Gis a top view of a device100′ comprising a microhotplate, in accordance with embodiments of the disclosure. The device100′ may be substantially similar to the device100described above with reference toFIG. 1AthroughFIG. 1F, except that the device100′ may include a resistive heater110′ different than the resistive heater110ofFIG. 1A. The resistive heater110′ may be heated by application of a current between the first portion114and the second portion116applied through the electrically conductive traces115, as described above with reference toFIG. 1A. The first portion114of the resistive heater110′ may include an extended portion170extending between the sense line124and the intersection of the tether105aand the first portion114. Similarly, the second portion116of the resistive heater110′ may include an extended portion170extending between the sense line124and the intersection of the tether105band the second portion116. The first portion114and the second portion116may include a protrusion172extending beyond the intersection of the sense line124and the respective first portion114and second portion116.

In some embodiments, electrically connecting the sense lines124to the resistive heater110′ at a location of the resistive heater110′ that is not in a current path between the bond pads112a,112bmay facilitate improved sensitivity of the sense lines124. It is believed that because substantially no current flows in the extended portions170(since the extended portions170are not located in a current path of the resistive heater110′ between the bond pads112a,112b), there is substantially no voltage drop in the extended portions170. Accordingly, improved voltage measurements may be obtained with the sense lines124electrically coupled to the extended portions170of the resistive heater110′.

In some embodiments, the protrusions172may reduce radiative and convective heat losses from the membrane101and may improve a temperature uniformity of the membrane101. A width of the protrusions172may decrease with a distance from the widened portions150.

Although the device100has been described and illustrated as comprising a circular membrane101, the disclosure is not so limited. In other embodiments, the membrane101may have a polygonal shape, such as a pentagonal shape, a hexagonal shape, a heptagonal shape, an octagonal shape, or other polygonal shape having a plurality of sides. In some embodiments, and as illustrated inFIG. 2, a device200may comprise a hexagonal membrane201, as shown in the dotted lines.

Although the device100has been described as comprising interdigitated electrodes135(FIG. 1B) having a particular shape, the disclosure is not so limited.FIG. 3A, is a cross-sectional view of another device300including interdigitated electrodes135′ comprising a first electrode134′ and a second electrode136′ having a different pattern than the interdigitated electrodes135ofFIG. 1B. The first electrode134′ and the second electrode136′ may be shaped and configured to optimize a distance between the electrodes. In some embodiments, the distance between the first electrode134′ and the second electrode136′ may be selected depending on electrical characteristics of the material (e.g., the chemical sensing material166) disposed over the interdigitated electrodes135′.

The first electrode134′ may be in electrical contact with one of the bond pads130of the pair of bond pads130and the second electrode136′ may be in electrical contact with the other bond pad130of the pair of bond pads130. The first electrode134′ and the second electrode136′ may comprise alternating concentric regions. By way of non-limiting example, an outermost portion of the first electrode134′ may be located further from the center of the membrane101than an outer portion of the second electrode136′. The outer portion of the second electrode136′ may be adjacent the outer portion of the first electrode134′ and another portion of the first electrode134′.

FIG. 3Bis a cross-sectional view of another device300′ including interdigitated electrodes135″ comprising a first electrode134″ and a second electrode136″ having a different pattern than those illustrated inFIG. 1BandFIG. 3A. The first electrode134″ and the second electrode136″ may each have a spiral shape. The first electrode134″ may be in electrical contact with one bond pad130of the pair of bond pads130and the second electrode136″ may be in electrical contact with the other bond pad130of the pair of bond pads130. The first electrode134″ may spiral in a first direction (e.g., one of a clockwise direction and a counterclockwise direction) and the second electrode136″ may spiral in a second direction (e.g., the other of the clockwise direction and the counterclockwise direction). In some embodiments, the spiral shape is substantially the same as the spiral shape of the resistive heater110(FIG. 1A), but the first electrode134″ and the second electrode136″ may have a substantially constant width along a length thereof. Accordingly, in some embodiments, the device300′ may include interdigitated electrodes135″ and a resistive heater exhibiting a spiral shape.

In some embodiments, the devices100,200,300,300′ may include a heat spreader. The heat spreader may be disposed above or below one or more of the resistive heater110and may be isolated from the resistive heater110by one or more dielectric materials (e.g., one or more of the first dielectric material160, the second dielectric material162, or the third dielectric material164). In some embodiments, the heat spreader may be disposed above the resistive heater110and below the interdigitated electrodes135(i.e., between the resistive heater110and the interdigitated electrodes135). The heat spreader may improve the heat distribution (i.e., heat transfer) and temperature uniformity of the resistive heater110and the membrane101.

In use and operation, a sensor including one or more of the devices100,200,300,300′ may be used to determine one or more properties (e.g., a composition, a presence of at least one species, etc.) of an analyte. The resistive heater110may be heated to a predetermined temperature by applying a current to the resistive heater110through the electrically conductive traces115. A temperature of the resistive heater110may be determined by measuring the voltage drop across the resistive heater110and determining the resistance according to Equation (2) below:
R=V/I(2),
wherein R is the resistance of the resistive heater110, V is the voltage drop measured across the resistive heater, and I is the current provided to the resistive heater110through the electrically conductive traces115. A temperature of the resistive heater110may be determined based on the resistance of the resistive heater110, since a temperature of the resistive heater110may be proportional to the resistance thereof. In some embodiments, the voltage drop across the resistive heater110may be measured with the sense lines124. In other embodiments, the voltage drop may be measured at a printed circuit board from which the current to the resistive heater110is provided. In some embodiments, measuring the voltage drop with the sense lines124may increase the accuracy of such measurements since measurement with the sense lines124reduces resistance losses due to wiring and other circuitry prior to obtaining a differential voltage measurement across the resistive heater110. If sense lines124are not used, the accuracy of the measured resistance (which is proportional to the temperature) can be improved by compensating the measured resistance for the resistance of the interconnect wiring (i.e., the resistance from, for example, the printed circuit board to the bond pads112a,112b) and tether connection to the heater (i.e., the electrically conductive traces115). In one embodiment, the compensation may be accomplished by measuring a total resistance from the current source and across the resistive heater110(which total resistance may account for the resistance of the bond pads112a,112b, the resistance of the electrically conductive traces115, the resistance of the resistive heater110, and the resistance of any bonding wires and/or interconnect structures between the current source and the bond pads112a,112b). In other embodiments, the compensation may be accomplished by applying a mathematical formula. An example of such a compensation formula is given below in Equation (3):
Rcomp=Rtarget(1+B(Tamb−To))  (3),
wherein Rcompis the compensated resistance value, Rtargetis the measured resistance from the total power applied to the resistive heater110, B is the compensation factor (° C./° C.) that may be unique for a given device100and membrane101of particular dimension and materials (i.e., the value of B may be constant and unique for particular dimensions and materials of the microhotplate), Tambis the current ambient temperature as measured from an environmental sensor (or measurement of the ambient resistance of the microhotplate), and Tois a calibration temperature.

In other embodiments, the resistance measured across the resistive heater110may be compensated using a reference microhotplate. In some such embodiments, the reference microhotplate may include sense lines (e.g., sense lines124). The sense lines of the reference microhotplate may be used to set a temperature of the device100to a desired temperature, such as by adjusting the current through the resistive heater until the resistance of the resistive heater is at a value that corresponds to the desired temperature. The current and voltage to the microhotplate device100without sense lines124may be set to the same values as the current and voltage applied to the resistive heater of the reference microhotplate, thus achieving the same temperature of the resistive heater110of the device100as the temperature of the reference microhotplate with the sense lines.

A resistance (i.e., an electrical conductivity) of the chemical sensing material166may be measured at the bond pads130. The resistance of the chemical sensing material166may be a function of interactions of an analyte with the chemical sensing material166. Stated another way, an electrical resistance of the chemical sensing material166may change when it interacts with one or more species in an analyte. In some embodiments, the chemical sensing material166may be formulated and configured to interact with particular species (e.g., gases).

In some embodiments, the electrical conductivity of the chemical sensing material166may be measured at a plurality of temperatures to determine a presence of a particular species (e.g., a gas) in an analyte. The response (e.g., the electrical conductivity) of the chemical sensing material166may vary with temperature and the temperature profile may be used to determine a composition of an analyte or the presence of one or more gases in the analyte. In some embodiments, a sensor may include a plurality of devices100,200,300,300′, each including a chemical sensing material166comprising a different composition and formulated and configured to interact with different species. The sensor may be used to determine the presence of one or more species in the analyte to which the devices100,200,300,300′ are exposed.

In some embodiments, the device100may not include the interdigitated electrodes135.FIG. 4AandFIG. 4Bare cross-sectional views of a device400according to other embodiments of the disclosure. The device400may be substantially the same as the devices100,200,300,300′ described above, but may not include the chemical sensing material166(FIG. 1D). Accordingly, the device400may include a membrane101suspended over a substrate102, which may comprise silicon. The membrane101may be suspended over the substrate102by a plurality of tethers105, as described above with reference toFIG. 1A. The electrically conductive traces115may electrically connect the resistive heater110to the bond pads112a,112band may extend over the tethers105, as described above with reference toFIG. 1AandFIG. 1E. Accordingly, the resistive heater110may be powered by application of a current between the bond pads112a,112b.

Although not shown inFIG. 4AandFIG. 4B, the device400may include sense lines124in communication with the resistive heater110, as described above with reference toFIG. 1AandFIG. 1G, for example. A voltage drop across the resistive heater110may be measured with sense lines124in electrical communication with the resistive heater110and with sense line bond pads126, as described above with reference toFIG. 1AandFIG. 1F. Use of the sense lines124and sense line bond pads126may increase the sensitivity and accuracy with which the voltage drop across the resistive heater110may be measured. In other embodiments, the voltage drop across the resistive heater110may be measured without the sense lines or with sense lines and bond pads located elsewhere in the device400.

The device400may include a coating material402and may comprise a catalytic microhotplate or a reference microhotplate, depending on a composition of the coating material402. As used herein, the term “catalytic microhotplate” means and includes a device including a resistive heater, sense lines (e.g., sense lines124) in electrical communication with the resistive heater, and a coating material402comprising a catalytically active material over a dielectric material overlying the resistive heater110. As used herein, the term “reference microhotplate” means and includes a device including a resistive heater, sense lines (e.g., sense lines124) in electrical communication with the resistive heater110, and either no coating material or a chemically inert coating material over a dielectric material overlying the resistive heater. Accordingly, the reference microhotplate may be free of a coating material over the resistive heater110or may include a coating material402comprising an inert material over a dielectric material overlying the resistive heater.

The coating material402may be electrically isolated from the resistive heater110by one or more dielectric materials. By way of non-limiting example, the coating material402may directly overlie and contact the third dielectric material164. In other embodiments, the coating material402may directly overlie and contact another dielectric material (e.g., the first dielectric material160or the second dielectric material162). The coating material402may comprise an inert material (e.g., a reference material) or a catalyst material formulated and configured to catalyze, for example, an oxidation reaction and produce heat in the presence of predetermined analytes. The inert coating material may be configured and formulated to exhibit at least one of substantially a same mass (e.g., thermal mass), emissivity, convective heat loss, thermal conductivity, and surface area of a coating material comprising the catalyst material.

In some embodiments, the inert coating material comprises aluminum oxide (e.g., Al2O3). In some embodiments, such as where the coating material comprises a catalytic coating material, the catalyst material comprises palladium, platinum, ruthenium, silver, iridium, another catalyst metal, or combinations thereof. In some embodiments, the catalyst material may exhibit a relatively high porosity and may exhibit a high surface roughness, which may increase a total surface area of the catalyst material.

In some embodiments, a sensor system may comprise a device400having a coating material402comprising a catalyst material as a catalytic microhotplate and another device400comprising an inert coating material as a reference microhotplate. In some embodiments, the devices400may be formed in the same substrate. Stated another way, a sensor system may include at least one catalytic microhotplate and at least one reference microhotplate fabricated on the same substrate and may have identical features, except that the coating material402of the catalytic microhotplate may comprise a catalyst material and the reference microhotplate may comprise an inert coating material or may not include a coating material. As will be described herein, the system may be useful for measuring a catalytic heat of combustion or oxidation (such as with the catalytic microhotplate), or for directly measuring a thermal conductivity of a material (such as with the reference microhotplate), or both. In some embodiments, the device400comprising the inert coating material or not including the coating material may comprise a reference microhotplate. In some embodiments, the sensor system may further comprise a microhotplate comprising a chemical sensing material166(e.g., device100(FIG. 1A), and may comprise a MOS microhotplate).

In embodiments where the device400comprises a catalytic microhotplate (e.g., where the coating material402comprises a catalytic coating), the device400may be used to determine at least one of an exothermic event, an endothermic event, an onset of such events, or an ignition temperature of an analyte. In some embodiments, an exothermic event or an endothermic event may be detected by measuring a power required to achieve a given temperature. By way of non-limiting example, a temperature of the resistive heater110of the catalytic microhotplate may be ramped according to predetermined temperature steps by changing (e.g., ramping) a current provided to the resistive heater110through the electrically conductive traces115. A voltage drop across the resistive heater110may be measured at each temperature while the temperature is changed (e.g., during the temperature ramp), such as by using the sense lines in electrical communication with the resistive heater110. A power to achieve each temperature may be determined from the measured voltage drop at each temperature and the current provided at each temperature, according to Equation (4) below:
P=I*V(4),
wherein P is the power, I is the current provided to the resistive heater110, and V is the measured voltage drop across the sense lines. At a temperature between about 150° C. and about 250° C., any physiosorbed species (e.g., species that have been physically adsorbed to the catalytic coating) may be desorbed from the surface of the catalytic coating material402prior to ramping a temperature of the resistive heater110to greater temperatures where poisoning of the catalytic coating material402by undesired chemical reactions may occur. Accordingly, the catalytic coating material402may be preserved by ramping the temperature of the device400to a first lower temperature, followed by ramping the temperature to at least a second, higher temperature.

Baseline data (e.g., a current, a resistance, and a power required to maintain each temperature) may be stored in a memory associated with the catalytic microhotplate. The baseline data may include historical power versus temperature data from previous catalytic sensor temperature ramps. The baseline data may be subtracted from the current data to obtain a signal representative of changes in the catalytic microhotplate thermal response, according to Equation (5) below:
Delta Cat=Cat(n)−Cat(baseline)  (5),
wherein Delta Cat is the relative change in the catalytic microhotplate thermal response, Cat(n) is the thermal response of the current temperature ramp (e.g., the power required to maintain a predetermined temperature), and Cat(baseline) is the baseline data. The Cat(baseline) may comprise a historic average value of the power required to maintain each temperature of the resistive heater110and may be continuously updated during each temperature ramp. The Delta Cat value may be determined at each temperature during the temperature ramp. Accordingly, Delta Cat may correspond to a difference in power required to maintain a given temperature of the catalytic microhotplate compared to previous temperature ramps. In some embodiments, a Delta Cat value that deviates from zero may be an indication of a reaction on the catalytic microhotplate, an ignition temperature of an analyte in contact with the catalytic microhotplate, or both.

With continued reference toFIG. 4AandFIG. 4B, a reference microhotplate may comprise a device wherein the coating material402comprises an inert coating material. In other embodiments, the reference microhotplate may not include a coating material, as described above. The reference microhotplate may be fabricated on the same wafer (e.g., the same silicon wafer) as the catalytic microhotplate sensor. A temperature of the reference microhotplate may be changed (e.g., ramped) according to a same temperature changes (e.g., ramp) as the catalytic microhotplate. In some embodiments, the reference microhotplate and the catalytic microhotplate are exposed to a temperature ramp simultaneously. In some such embodiments, measurements from the catalytic microhotplate and measurements from the reference microhotplate may be correlated in time, may be exposed to substantially the same analyte, and may exhibit improved sensor accuracy.

Baseline data (e.g., a current, a resistance, and a power required to maintain each temperature) may be stored in a memory associated with the reference microhotplate. The baseline data may include historical power versus temperature data from previous reference microhotplate temperature changes (e.g., ramps). The baseline data may be subtracted from the current data to obtain a signal representative of changes in the reference microhotplate thermal response, according to Equation (6) below:
Delta Ref=Ref(n)−Ref(baseline)  (6),
wherein Delta Ref is the relative change in the reference microhotplate thermal response, Ref(n) is the thermal response of the current temperature (e.g., current temperature ramp), and Ref(baseline) is the baseline data (e.g., an average of Ref(n) data from previous temperature changes (ramps)). The Delta Ref value may be determined at each temperature (such as during the temperature ramp). The Delta Ref value may be an indication of the thermal conductivity of an analyte in contact with or proximate to the reference microhotplate. For example, a Delta Ref value that is greater than zero may be an indication that the thermal conductivity of the analyte is greater than a thermal conductivity of gases to which the reference microhotplate was exposed (e.g., air) during calibration or in previous ramps. Similarly, a Delta Ref value that is less than zero may be an indication that the thermal conductivity of the analyte is less than a thermal conductivity of gases to which the reference microhotplate was exposed in previous ramps. In some such embodiments, a thermal conductivity of the analyte may be determined according to differential thermal analysis (DTA) or differential scanning calorimetry (DCS) techniques. The thermal conductivity of a species or an analyte may be a function of temperature. Accordingly, in some embodiments, the thermal conductivity of an analyte may be determined at more than one temperature.

In other embodiments, a current provided to the resistive heater may be maintained and a resistance of the resistive heater may be measured with the sense lines to determine a temperature of the resistive heater. A thermal conductivity of the analyte may be determined based on the determined temperature for the power provided to the resistive heater. In some such embodiments, the thermal conductivity may be determined according to differential scanning calorimetry (DSC).

In some embodiments, the thermal conductivity may be measured at two or more temperatures. By way of non-limiting example, the thermal conductivity may be measured at relatively low temperatures (e.g., between about 50° C. and about 250° C.) and at relatively high temperatures (e.g., between about 400° C. and about 1,000° C.). Thermal conductivity generally increases with increasing temperatures. Therefore, Delta Ref measurements made at higher temperatures may exhibit a larger sensor response from the reference microhotplate, and may, therefore, increase the sensitivity of the reference microhotplate.

The Delta Ref signal may be subtracted from the Delta Cat signal to produce a signal response that is proportional to the heat generated on or removed from the catalytic sensor, according to Equation (7) below:
Exo(new)=Delta Cat−Delta Ref  (7),
wherein Exo(new) is the signal response that is proportional to the heat generated on or removed from the catalytic microhotplate and Delta Cat and Delta Ref are as previously described. Subtracting the Delta Ref signal from the Delta Cat signal may compensate the Delta Cat signal for the effects of thermal conductivity, thermal diffusivity, density, viscosity, temperature, pressure, relative humidity, flow variations, and other noise in the system and in the analyte being detected.

If the value of Exo(new) deviates from its nominal value, one or more of an exothermic reaction, an endothermic reaction, or an ignition of such reactions, which may also be referred to herein as a “light-off” event, may be detected. By way of example only, an Exo(new) value that is less than zero may be an indication of less power required to maintain a temperature of the catalytic microhotplate, which may be an indication of an exothermic reaction. Similarly, an Exo(new) value that is greater than zero may be an indication of more power required to maintain the temperature of the catalytic microhotplate, which may be an indication of an endothermic reaction.

The temperature of the light-off event may be an indication of a presence of a gas in the sample being detected. Since different gases catalytically oxidize at different temperatures, the light-off temperature of an analyte may be an indication of a presence of one or more gases in the analyte. Multiple light-off events at different temperatures may be an indication of multiple flammable gases present in the sample. A database may store the sensor responses, training data, and calibration data used in the analysis.

Although the devices100,200,300,300′,400include a conductive trace (e.g., the electrically conductive trace115, the sense lines124, and the electrode trace132), over each tether105, in some embodiments, at least some of the tethers105may not include a conductive trace extending thereon.

In use and operation, the devices100,200,300,300′,400may be used for sensing one or more gases, one or more properties of one or more gases, or a combination thereof.FIG. 5is a flowchart illustrating a method500of operating a device including at least one microhotplate according to embodiments of the disclosure. The device may include one of at least one device100,200,300,300′,400described above. By way of non-limiting example, the device may include at least one MOS microhotplate (e.g., at least one device100,200,300,300′ as described above with reference toFIG. 1AthroughFIG. 1G,FIG. 2,FIG. 3A, andFIG. 3B), at least one reference microhotplate device (e.g., at least one device400including an inert coating material402, or no coating material, as described above with reference toFIG. 4AandFIG. 4B), and at least catalytic microhotplate (e.g., at least another device400including a catalytic coating material402as described above with reference toFIG. 4AandFIG. 4B).

The method500may include act502including providing a known current to a resistive heater of at least one microhotplate through electrically conductive traces; act504including measuring a voltage drop across the resistive heater with voltage sense lines; act506including determining a resistance of the resistive heater and optionally determining a resistivity between interdigitated electrodes; act508including determining a temperature of the resistive heater; and act510including adjusting the current to maintain a desired temperature of the resistive heater or to change a temperature of the resistive heater.

Act502includes providing a known current to a resistive heater of at least one microhotplate through conductive traces in electrical contact with the resistive heater. The current may raise a temperature of the resistive heater to a desired temperature. In some embodiments, the current is supplied in a stepped manner to raise the temperature of the resistive heater in a stepped manner. In some embodiments, the current is provided to the resistive heater to maintain a predetermined temperature of the resistive heater. In some embodiments, act502includes providing a current to a resistive heater of at least one MOS microhotplate, a resistive heater of at least one reference microhotplate, and a resistive heater of at least one catalytic microhotplate.

Act504includes measuring a voltage (e.g., a voltage drop) across the resistive heater. In some embodiments, the voltage drop across the resistive heater may be measured with voltage sense lines (e.g., sense lines124(FIG. 1A)) in electrical contact with the resistive heater. In other embodiments, the voltage drop may be measured without the sense lines. In other words, act504may include at least one of measuring a voltage drop across the resistive heater of at least one reference microhotplate, measuring a voltage drop across the resistive heater of at least one catalytic microhotplate, and measuring a voltage drop across a resistive heater of at least one MOS microhotplate. By way of non-limiting example, where the microhotplate device comprises a catalytic microhotplate or a reference microhotplate, act504may include measuring a voltage drop across sense lines in electrical contact with the resistive heater. Where the microhotplate device comprises a MOS microhotplate, act504may include measuring the voltage drop across the resistive heater at a location proximate where the current to the resistive heater is supplied (e.g., such as at a printed circuit board). In some embodiments, act504includes measuring a voltage drop across the resistive heater of at least one reference microhotplate, measuring a voltage drop across the resistive heater of at least one catalytic microhotplate, and measuring a voltage drop across the resistive heater of at least one MOS microhotplate.

Act506includes determining a resistance of the resistive heater and optionally determining a resistivity between the interdigitated electrodes. The resistivity between the interdigitated electrodes may correspond to the resistivity of the MOS coating of a MOS microhotplate. In some embodiments, act506includes determining a resistance of the resistive heater of one or more of at least one catalytic microhotplate, at least one reference microhotplate, and the resistive heater of at least one MOS microhotplate. The resistance of the resistive heater may be determined, based at least in part, on the current provided to the resistive heater and the voltage measured by the sense lines or elsewhere in the device. The resistance may be proportional to the measured voltage divided by the provided current (i.e., Ohms law, R=V/I, wherein R is the resistance, V is the measured voltage, and I is the provided current). In embodiments, where act506includes determining a resistivity between the interdigitated electrodes, the resistivity may be determined based on the measured voltage drop across interdigitated electrodes (e.g., across the electrode traces132(FIG. 1B)). The resistance of the interdigitated electrodes may be determined based on Ohms law, as described above with reference to the resistance of the resistive heater. In some embodiments, the resistivity between the interdigitated electrodes may be determined by providing a current to the interdigitated electrodes and measuring a voltage drop across the interdigitated electrodes. In other embodiments, the resistivity of the interdigitated electrodes may be determined by providing a voltage to the interdigitated electrodes and measuring a current through the interdigitated electrodes.

Act508includes determining a temperature of the resistive heater based, at least in part, on the determined resistance of the resistive heater. For example, the temperature of the resistive heater may be proportional to the resistance thereof according to, for example, the temperature coefficient of resistance of the resistive heater (e.g., dR/R=α dT/dR, wherein R is the resistance of the resistive heater, dR is the change in resistance between a baseline resistance and a resistance of the resistive heater to exposure to a sample, dT is a difference in temperature between a baseline temperature and a temperature of the resistive heater to exposure to the sample, and α is a coefficient of thermal resistance of the resistive heater). Accordingly, the resistance may be used to determine the temperature of the resistive heater.

Act510includes adjusting the current to maintain a desired temperature of the resistive heater or to change (e.g., ramp) a temperature of the resistive heater. In some embodiments, the current may be adjusted in a stepped manner to change the temperature of the resistive heater in a corresponding stepped manner. A power required to maintain the temperature at each step may be determined based on the measured voltage drop across the resistive heater in act506and the current provided to the resistive heater in act502for each particular temperature. In other embodiments, the current provided to the resistive heater may be maintained at a substantially constant value to facilitate maintaining the temperature of the resistive heater at a substantially constant temperature. A power required to maintain the substantially constant temperature may be determined based on the measured voltage drop across the resistive heater in act506and the current provided to the resistive heater in act502.

AlthoughFIG. 5has been described as including determining a resistance of the resistive heater by applying a current to the resistive heater and measuring a voltage drop across the resistive heater, the disclosure is not so limited. In other embodiments, a voltage may be applied to the resistive heater and a current through the resistive heater may be measured. The provided voltage and measured current may be used to determine the resistance.

In some embodiments, a power required to maintain a temperature of the resistive heater may be determined. The power may be related to the provided current and the measured voltage (e.g., P=IV, as described above with reference to Equation (4)). In other embodiments, the power may be related to the provided current and the resistance (e.g., P=I2R). In some embodiments, the power required to maintain a predetermined temperature may be a function of the composition of the vapor being analyzed. In some embodiments, and as described above, a lower power required to maintain a desired temperature of the catalytic microhotplate may be an indication of an exothermic reaction occurring at the catalytic microhotplate. Similarly, a greater power required to maintain a desired temperature of the catalytic microhotplate may be an indication of an endothermic reaction occurring at the catalytic microhotplate. In some embodiments, use of the sense lines124(FIG. 1A) may facilitate an increased system resolution and accuracy of reference microhotplates and catalytic microhotplates. By way of non-limiting example, the sense lines124may facilitate determining powers as low as about 10 microWatts, or even as low as 1.0 microWatt, such as a power between about 1.0 microWatt and about 10 microWatts. Where the microhotplate comprises a reference microhotplate, a greater power required to achieve a given temperature may correlate to a gas having a higher thermal conductivity proximate the reference microhotplate. Similarly, a lower power required to achieve a given temperature may correlate to a gas having a lower thermal conductivity proximate the reference microhotplate. The thermal conductivity of the analyte may be determined based on the power required to maintain one or more temperatures.

FIG. 6is a simplified block diagram of a system600configured for carrying out one or more embodiments of the present disclosure. The system600is configured for executing programs containing computing instructions and may include one or more processors602, one or more memory devices604, one or more driver circuits606for driving one or more resistive heaters of one or more devices, one or more sensors608for sensing one or more outputs of the one or more microhotplates (e.g., an output from a resistive heater, such as may be measured with sense lines, an output from an interdigitated electrode, such as may be measured with electrode traces, etc.), and an element610, which may comprise a resistive heater for heating a microhotplate associated with the resistive heater, interdigitated electrodes, or both.

The one or more processors602may be configured for executing a wide variety of operating systems and applications including the computing instructions for carrying out embodiments of the present disclosure. The one or more processors602may be in communication with each of the one or more memory devices604, the one or more driver circuits606, and the one or more sensors608. The one or more processors602may be configured to transmit operating instructions to the one or more driver circuits606. By way of non-limiting example, the one or more processors602may be configured to provide operating instructions, such as instructions to provide a current, to the one or more driver circuits606, which may drive a resistive heater associated with each device associated with the sensors608. As only one example, the one or more processors602may be configured to provide operating instructions to the one or more driver circuits606to maintain a desired temperature of resistive heaters or to ramp a temperature of one or more resistive heaters associated with the one or more driver circuits606.

The one or more driver circuits606may be operably coupled to the element610and configured to provide a current to the element610responsive to receiving operating instructions from the one or more processors602. In some embodiments, the one or more driver circuits606comprise a closed-loop controller configured for modulating a temperature of the element610. The closed-loop controller may be configured to control a temperature of a resistive heater of the microhotplate, such as by driving the element610. In other embodiments, the closed-loop controller may be configured to control a temperature of interdigitated electrodes of the microhotplate, such as by driving the element610. In some embodiments, the one or more driver circuits606may comprise a digital-to-analog converter.

The element610may be configured to receive a current from the one or more driver circuits606. Wherein the element610comprises a resistive heater, the resistive heater may be substantially similar to the resistive heaters described above with respect to the devices100,200,300,300′,400. The one or more sensors608may be configured to sense at least one property of the element610. By way of non-limiting example, the one or more sensors608may be configured to sense a voltage drop across the element610. In other embodiments, the one or more sensors608may comprise an interdigitated electrode, such as the interdigitated electrodes described above with respect to the devices100,200,300,300′,400and may be configured to determine a resistivity between the interdigitated electrodes. The one or more sensors608may comprise an analog-to-digital converter.

The one or more processors602may be configured to receive data from one or more sensors608and determine one or more properties of a material (e.g., gas, vapor, liquid, or solid) being analyzed. The one or more processors602may comprise a circuit, a controller, or both and may be configured to receive a sensed voltage from the one or more sensors608and may be configured to determine one or more of a power, a resistance, and a temperature of one or more microhotplates to determine a composition of an analyte based, at least in part, on a provided current from the one or more driver circuits606and as described above with reference to Equations (1) through (7). In some embodiments, the one or more processors602may be configured to determine an analyte (e.g., a presence thereof, a concentration thereof, etc.) in contact with a coating material of a microhotplate based, at least in part, on a voltage measured across sense lines, as described above with reference toFIG. 4AandFIG. 4B. In some embodiments, the one or more processors602may be configured to determine an analyte in contact with a chemical sensing material by measuring a resistance of interdigitated electrodes, as described above with reference toFIG. 1B,FIG. 3A, andFIG. 3B, for example.

The memory device604may be used to hold computing instructions, data, and other information for performing a wide variety of tasks including performing embodiments of the present disclosure. In some embodiments, the baseline data from previous temperature ramps (e.g., power versus temperature data), as described above with reference to Equations (4) through (7), may be stored in the memory. The processor602may be configured to subtract current data from previous data to produce a signal representing changes in microhotplate catalytic thermal response (Delta Cat) with respect to temperature. By way of example, and not limitation, the memory device604may include Synchronous Random Access Memory (SRAM), Dynamic RAM (DRAM), Read-Only Memory (ROM), Flash memory, phase change memory, and other suitable information storage devices. The memory device604may include data related to the device, including operating parameters (e.g., a temperature of the microhotplate, a composition of the microhotplate, etc.). The memory device604may include data relating to the microhotplate, the composition of the microhotplate, the temperature of the microhotplate, the resistance of the microhotplate, and a voltage of a resistive heater associated with the microhotplate.

FIG. 7is a plan view of a system700(e.g., a sensor) showing the layout of a plurality of devices, each including a microhotplate. The system700may comprise an array of microhotplate devices and may include at least some devices701that do not include interdigitated electrodes and at least some devices702that include interdigitated electrodes735over a resistive heater710. The array may be referred to herein as a multi-sensor array, since it includes a plurality of sensors. In some embodiments, the devices701that do not include interdigitated electrodes may comprise a reference microhotplate or a catalytic microhotplate and the devices that include the interdigitated electrodes735may comprise a MOS microhotplate. In some embodiments, the system700includes at least one reference microhotplate device, at least one catalytic microhotplate device, and at least one MOS microhotplate device.

The devices701,702may each include a void704formed in a substrate750. The devices701,702may further include a plurality of tethers705extending from the substrate750at from a periphery of the devices701,702to a center portion thereof, as described inFIG. 1AthroughFIG. 4B.

The devices701that do not include the interdigitated electrodes735may have a diameter of about 100 μm, which may be about twice as large as a diameter of the devices702that include the interdigitated electrodes735. In some embodiments, the devices702have a diameter of about 50 μm. In some embodiments, the devices701may include a catalyst material (e.g., such as the coating material402described above with reference toFIG. 4AandFIG. 4B) and may be relatively larger than the devices702. In some embodiments, the larger size of the devices701may facilitate a more sensitive sensor device, since changes in power to maintain a predetermined temperature may be measured with a greater sensitivity. In some embodiments, at least one of the devices701comprises a catalyst material and at least another of the devices701comprises an inert material having at least one of a similar mass, thermal mass, or other property as the catalyst material of the other at least one device701. In some embodiments, at least one of the devices701comprises a catalyst material and at least another of the devices701comprises no coating material.

The devices701may include sense lines724that are electrically coupled to each of the resistive heater710and sense line bond pads726. The sense lines724may be configured to measure a voltage drop across the resistive heaters710, as described above with reference toFIG. 1AthroughFIG. 1F. In some embodiments, the sense lines724may comprise Kelvin sense lines (i.e., four-terminal sensing, with two terminals comprising a bond pad712, a common power source740, and the two sense line bond pads726).

Each device701,702may include bond pads712coupled to electrically conductive traces715, which, in turn, may be coupled to a resistive heater710. In some embodiments, the devices702are substantially similar to the devices100,200,300,300′ described above with reference toFIG. 1AthroughFIG. 3B. The devices702may further include interdigitated electrodes735at the center thereof and overlying respective resistive heaters710thereof, as described above with reference to, for example,FIG. 1B. A chemical sensing material may overlie and be in direct contact with the interdigitated electrodes735, as described above with reference to, for example,FIG. 1C,FIG. 1D, andFIG. 1E. The interdigitated electrodes735may be in electrical contact with electrode traces732that, in turn, may be in electrical contact with bond pads726.

The resistive heater710of each device701,702may be in electrical contact with the common power source740. By way of non-limiting example, bond pads745may be in electrical contact with conductive lines746, which may be in electrical contact with one conductive trace715of each device701,702. In some such embodiments, each device701,702may be electrically coupled to a common power source. The common power source740may include a metallization layer (e.g., a bond pad) configured to electrically couple each of the bond pads745.

In some embodiments, each of the common power source740, the bond pads745, the bond pads712, the sense line bond pads726, the bond pads730, and the conductive lines746may comprise the same material, such as, for example, gold. In some embodiments, each of the common power source740, the bond pads745, the bond pads712, the sense line bond pads726, the bond pads730, and the conductive lines746may comprise a different material (e.g., such as a material exhibiting a relatively lower electrical resistance) than the electrically conductive traces715, the sense lines724, the resistive heaters710, or the interdigitated electrodes735, which may comprise, for example, tungsten, palladium, or other materials, as described above.

Accordingly, an array may comprise any combination of catalytic microhotplates, reference microhotplates, and metal oxide semiconductor (MOS) microhotplates of varying sizes and coatings. One or more reference microhotplates may be used to determine a thermal conductivity of an analyte, one or more catalytic microhotplates may be used to determine one or more ignition temperatures of one or more species in an analyte, an exothermic event, and an endothermic event of an analyte, and one or more MOS microhotplates may be used to determine an electrical response of a chemical sensing material at different temperatures to determine a presence of one or more species (e.g., analytes) in the sample. In some embodiments, the use of each microhotplate device may provide for orthogonal detection of one or more species in the analyte and may be used to analyze, differentiate, and quantify a plurality of chemical species. In some embodiments, flame arrestors or filters may be included over some or all of the microhotplate devices701,702.

In some embodiments, the interdigitated electrodes735of at least some of the devices702may be different (have a different size, shape, different composition, etc.) than the interdigitated electrodes735of other of the devices702. In some embodiments, gaps between interdigitated electrodes735(such as between a first electrode and a second electrode comprising the interdigitated electrodes735) may be different.

In some embodiments, the system700may include a plurality of devices701and devices702to facilitate additional analysis of a sample and/or analyte. For example, the system700may include a plurality of devices701including, for example, devices701comprising an inert coating material and devices701comprising a catalytic coating material, as described above with reference toFIG. 4AandFIG. 4B. The devices701may be used to determine, for example, a thermal conductivity of a sample and/or analyte, an exothermic reaction, an endothermic reaction, a temperature of an exothermic reaction, a temperature of an endothermic reaction, another property, or combinations thereof. The system700may further include devices702including interdigitated electrodes735and configured to measure a temperature of interaction between at least one analyte and a metal oxide semiconductor coating material of the devices702.

The devices100,200,300,300′ described above may be configured to reduce heat losses from the membrane101to the underlying substrate102through the tethers105,105a,105b. In addition, the resistive heater110may be shaped and configured to reduce radiative heat losses therefrom to the surrounding environment. In some embodiments, heat transferred from the membrane101to the environment proximate the membrane101may be increased, which may facilitate determining one or more properties of an analyte proximate the membrane101.

Additional non-limiting example embodiments of the disclosure are set forth below:

Embodiment 1: A microhotplate, comprising: a membrane suspended over a substrate by a plurality of tethers connected between the substrate and the membrane, the membrane comprising: a resistive heater comprising an electrically conductive material having a varying width from a peripheral portion of the membrane to a center of the membrane, the electrically conductive material comprising: a first portion spiraling in a first direction; and a second portion spiraling in a second direction and in electrical contact with the first portion proximate the center of the membrane; and a first electrically conductive trace extending over a first tether and in electrical contact with a bond pad on the substrate and the first portion and a second electrically conductive trace extending over another tether and in electrical contact with another bond pad on the substrate and the second portion.

Embodiment 2: The microhotplate of Embodiment 1, wherein the membrane comprises two or more dielectric materials, the resistive heater disposed between the two or more dielectric materials; and each tether of the plurality of tethers comprises the two or more dielectric materials, at least one tether of the plurality of tethers comprising an electrically conductive trace.

Embodiment 3: The microhotplate of Embodiment 1 or Embodiment 2, wherein the membrane further comprises at least one of a chemical sensing material, a catalytic coating material, and an inert coating material.

Embodiment 4: The microhotplate of any one of Embodiments 1 through 3, wherein the membrane comprises at least one material selected from the group consisting of silicon, a silicon oxide, a silicon nitride material, a silicon carbide, or a silicon oxynitride.

Embodiment 5: The microhotplate of any one of Embodiments 1 through 4, wherein the resistive heater is disposed between two or more dielectric materials, each dielectric material of the two or more dielectric materials comprising silicon nitrides, silicon oxides, silicon carbides, oxynitrides, or combinations thereof.

Embodiment 6: The microhotplate of Embodiment 5, wherein the two or more dielectric materials exhibit different residual stresses.

Embodiment 7: The microhotplate of Embodiment 5 or Embodiment 6, wherein at least one dielectric material of the two or more dielectric materials exhibits a residual tensile stress of between about 200 MPa and about 2.0 GPa at about 20° C.

Embodiment 8: The microhotplate of any one of Embodiments 5 through 7, wherein the two or more dielectric materials are selected to exhibit a reduced residual tensile stress at operating temperatures of the microhotplate.

Embodiment 9: The microhotplate of any one of Embodiments 5 through 8, wherein the two or more dielectric materials s are in tension between a temperature between 600° C. and about 1,200° C.

Embodiment 10: The microhotplate of any one of Embodiments 1 through 9, wherein the second portion is disposed at least between adjacent spirals of the first portion.

Embodiment 11: The microhotplate of any one of Embodiments 1 through 10, wherein a gap between the first portion and the second portion is substantially constant and smaller than a minimum width of the electrically conductive material.

Embodiment 12: The microhotplate of any one of Embodiments 1 through 11, wherein the membrane has a polygonal shape.

Embodiment 13: The microhotplate of any one of Embodiments 1 through 12, further comprising an electrically conductive sense line trace in electrical contact with the first portion and an electrically conductive sense line trace in electrical contact with the second portion, the electrically conductive sense line traces configured to measure a voltage across the resistive heater.

Embodiment 14: The microhotplate of any one of Embodiments 1 through 13, wherein the resistive heater comprises a widened curved portion at an intersection of at least one tether and the membrane.

Embodiment 15: The microhotplate of any one of Embodiments 1 through 14, wherein the resistive heater comprises an increasing width from the widened portion to the center of the membrane.

Embodiment 16: The microhotplate of any one of Embodiments 1 through 15, wherein each tether of the plurality of tethers has a greater width proximate the membrane and the substrate than at portions distal from the membrane and the substrate.

Embodiment 17: The microhotplate of any one of Embodiments 1 through 16, wherein each tether of the plurality of tethers comprises a fillet shape or a double tangent arc shape proximate the membrane and the substrate.

Embodiment 18: The microhotplate of any one of Embodiments 1 through 17, wherein outer edges of the electrically conductive material are substantially free of corners and comprise arcuate surfaces.

Embodiment 19: The microhotplate of any one of Embodiments 1 through 18, further comprising a chemical sensitive material over a dielectric material overlying the resistive heater.

Embodiment 20: The microhotplate of Embodiment 19, further comprising a plurality of electrodes in electrical contact with the chemical sensitive coating material and configured to measure a resistivity of the chemical sensitive coating material.

Embodiment 21: The microhotplate of Embodiment 20, wherein the plurality of electrodes comprises a plurality of interdigitated electrodes, a plurality of interdigitated spiral electrodes, or a plurality of interdigitated concentric electrodes.

Embodiment 22: The microhotplate of any one of Embodiments 1 through 21, further comprising one of a catalytic coating material and one of an inert coating material or no coating material over resistive heater.

Embodiment 23: A chemical sensor comprising at least one microhotplate, the at least one microhotplate comprising: a plurality of tethers extending over a void formed in a substrate, the plurality of tethers supporting a membrane over the substrate and comprising a plurality of dielectric layers, the membrane comprising: a resistive heater between two dielectric layers of the plurality of dielectric layers, the resistive heater comprising an electrically conductive material having a first portion spiraling in a first direction and a second portion spiraling in a second, opposite direction, the electrically conductive material having a varying width from an outer portion of the resistive heater to a central portion thereof; and electrically conductive heater traces configured to provide power to the resistive heater, the electrically conductive heater traces overlying at least one of the tethers.

Embodiment 24: The chemical sensor of Embodiment 23, wherein the plurality of tethers comprise six tethers, the electrically conductive heater traces overlying two of the tethers, electrically conductive sense line traces overlying two of the tethers in electrical communication with the resistive heater, and chemical sensing electrode traces overlying two of the tethers and in electrical communication with interdigitated electrodes overlying the resistive heater.

Embodiment 25: The chemical sensor of Embodiment 23 or Embodiment 24, further comprising electrically conductive sense line traces in electrical contact with the resistive heater and configured to measure a voltage across the resistive heater.

Embodiment 26: The chemical sensor of Embodiment 25, further comprising a controller configured to determine a temperature of the resistive heater at least by dividing a voltage measured by the electrically conductive sense line traces by a current provided to the resistive heater.

Embodiment 27: The chemical sensor of any one of Embodiments 23 through 26, further comprising a controller configured to control a temperature of the resistive heater.

Embodiment 28: The chemical sensor of any one of Embodiments 23 through 27, wherein the resistive heater comprises tungsten, platinum, molybdenum, tantalum, titanium tungsten, alloys thereof, and multilayer structures thereof.

Embodiment 29: The chemical sensor of any one of Embodiments 23 through 28, further comprising a controller configured to determine a temperature of the resistive heater based, at least in part, on a current supplied to the resistive heater.

Embodiment 30: The chemical sensor of any one of Embodiments 23 through 29, wherein each tether of the at least five tethers is wider proximate the membrane and proximate the substrate than at other portions of the tether.

Embodiment 31: The chemical sensor of any one of Embodiments 23 through 30, wherein the substrate comprises at least one of silicon, silicon dioxide, and silicon nitride.

Embodiment 32: A method of measuring at least one of a thermal conductivity, an exothermic event, and an endothermic event, the method comprising: providing a current to a resistive heater of at least one microhotplate, the resistive heater comprising a varying width from a peripheral portion thereof toward a center thereof, the resistive heater comprising a first portion extending from the peripheral portion toward the center thereof and spiraling in a clockwise direction and a second portion in contact with the first portion at the center of the resistive heater and extending from the center of the resistive heater toward the peripheral portion thereof and spiraling in a counterclockwise direction; and measuring a voltage across the resistive heater; and calculating a resistance of the resistive heater to determine an average temperature of the resistive heater.

Embodiment 33: The method of Embodiment 32, further comprising determining a resistivity of a chemical sensing material disposed over the resistive heater.

Embodiment 34: The method of Embodiment 33, wherein determining a resistivity of a chemical sensing material comprises measuring the resistivity between interdigitated electrodes in contact with the chemical sensing material.

Embodiment 35: The method of any one of Embodiments 32 through 34, wherein measuring a voltage across the resistive heater comprises measuring the voltage across the resistive heater with sense lines coupled to the resistive heater.

Embodiment 36: The method of any one of Embodiments 32 through 35, wherein calculating a resistance of the resistive heater comprises calculating the resistance of the resistive heater based, at least in part, on the voltage measured across the resistive heater.

Embodiment 37: The method of any one of Embodiments 32 through 36, wherein determining an average temperature of the resistive heater comprises determining the average temperature of the resistive heater based, at least in part, on the resistance of the resistive heater and a temperature coefficient of resistance of the resistive heater.

Embodiment 38: The method of any one of Embodiments 32 through 37, further comprising determining a power supplied to the resistive heater based, at least in part, on the voltage measured across the resistive heater and the provided current.

Embodiment 39: The method of any one of Embodiments 32 through 38, further comprising determining a power required to maintain a temperature of a catalytic material over the resistive heater.

Embodiment 40: A sensor for providing orthogonal analysis of a sample, the sensor comprising: an array of microhotplates, at least one microhotplate of the array of microhotplates comprising a resistive heater comprising an electrically conductive material having a varying width from a peripheral portion of the membrane to a center of the membrane, the electrically conductive material comprising: a first portion spiraling in a first direction; and a second portion spiraling in a second direction and in electrical contact with the first portion proximate the center of the membrane; and a controller configured to determine one or more of at least one property of the resistive heater of at least one microhotplate of the array of microhotplates and a resistance between interdigitated electrodes of at least one microhotplate of the array of microhotplates.

Embodiment 41: The sensor of Embodiment 40, wherein at least one microhotplate of the array of microhotplates comprises a catalytic coating over a dielectric material overlying the resistive heater.

Embodiment 42: The sensor of Embodiment 40 or Embodiment 41, wherein at least one microhotplate of the array of microhotplates comprises an inert coating material or no coating material over a dielectric material overlying the resistive heater.

Embodiment 43: The sensor of any one of Embodiments 40 through 42, wherein at least one microhotplate of the array of microhotplates comprises an n-type semiconductor material.

Embodiment 44: The sensor of any one of Embodiments 40 through 43, wherein at least one microhotplate of the array of microhotplates comprises a p-type semiconductor material.

Embodiment 45: The sensor of any one of Embodiments 40 through 44, wherein at least one microhotplate of the array of microhotplates comprises an ionic conductor.

Embodiment 46: The sensor of any one of Embodiments 40 through 45, wherein the array of microhotplates comprises: at least one reference microhotplate comprising an inert material overlying a dielectric material over its resistive heater or free of a coating material; at least one microhotplate comprising a catalytic coating over a dielectric material of its resistive heater; and at least one microhotplate comprising a chemical sensing material selected from the group consisting of a p-type semiconductor, an n-type semiconductor, and an ionic conductor overlying a dielectric material over its resistive heater.

Embodiment 47: The sensor of any one of Embodiments 40 through 46, further comprising at least one filter configured to filter one or more materials from the sample.

Embodiment 48: The sensor of any one of Embodiments 40 through 47, wherein the resistive heater is configured to operate at a temperature between about 200° C. and about 1,200° C.

Embodiment 49: A method of measuring a response from a sensor comprising an array of microhotplates, the method comprising: providing a current to a resistive heater of each microhotplate of an array of microhotplates, the resistive heater of each microhotplate having a varying width from a peripheral portion of the membrane to a center of the membrane, the electrically conductive material comprising: a first portion spiraling in a first direction; and a second portion spiraling in a second direction and in electrical contact with the first portion proximate the center of the membrane; and measuring a response from each microhotplate of the array of microhotplates, wherein measuring a response from each microhotplate of the array of microhotplates comprises: analyzing a response from at least one reference microhotplate free of a coating material or comprising an inert material overlying a dielectric material over its resistive heater; analyzing a response from at least one microhotplate comprising a catalytic material overlying a dielectric material over its resistive heater; and analyzing a response from at least one microhotplate comprising a chemical sensing material selected from the group consisting of a p-type semiconductor, an n-type semiconductor, and an ionic conductor overlying a dielectric material over its resistive heater.

Embodiment 50: The method of Embodiment 49, wherein analyzing a response from at least one microhotplate comprising a catalytic material overlying a dielectric material over its resistive heater comprises determining a difference between the response from the at least one microhotplate comprising the catalytic material and the response from the at least one reference microhotplate.

Embodiment 51: The method of Embodiment 49 or Embodiment 50, wherein analyzing a response from at least one reference microhotplate comprises maintaining a temperature of the at least one reference microhotplate and determining a power required to maintain the temperature of the at least one reference microhotplate.

Embodiment 52: The method of any one of Embodiments 49 through 51, wherein analyzing a response from at least one reference microhotplate comprises maintaining a current provided to the resistive heater of the at least one reference microhotplate and measuring a change in temperature of the resistive heater of the at least one reference microhotplate.

Embodiment 53: A sensor for analyzing a sample, the sensor comprising: a microhotplate comprising a membrane suspended over a substrate by a plurality of tethers connected between the substrate and the membrane, the membrane comprising: a resistive heater comprising an electrically conductive material having a varying width from a peripheral portion of the membrane to a center of the membrane, the electrically conductive material comprising: a first portion spiraling in a first direction; and a second portion spiraling in a second direction and in electrical contact with the first portion proximate the center of the membrane; and a first electrically conductive trace extending over a first tether and in electrical contact with a bond pad on the substrate and the first portion and a second electrically conductive trace extending over another tether and in electrical contact with another bond pad on the substrate and the second portion.

Embodiment 54: The sensor of Embodiment 53, wherein: the membrane comprises two or more dielectric materials, the resistive heater disposed between the two or more dielectric materials; and each tether of the plurality of tethers comprises the two or more dielectric materials, at least one tether of the plurality of tethers comprising an electrically conductive trace.

Embodiment 55: The sensor of Embodiment 53 or Embodiment 54, wherein the membrane further comprises at least one of a chemical sensing material, a catalytic coating material, and an inert coating material.

Embodiment 56: The sensor of any one of Embodiments 53 through 55, wherein the second portion is disposed at least between adjacent spirals of the first portion.

Embodiment 57: The sensor of any one of Embodiments 53 through 56, wherein a gap between the first portion and the second portion is substantially constant and smaller than a minimum width of the electrically conductive material.

Embodiment 58: The sensor of any one of Embodiments 53 through 57, wherein the membrane has a polygonal shape.

Embodiment 59: The sensor of any one of Embodiments 53 through 58, further comprising a first electrically conductive sense line trace in electrical contact with the first portion and a second electrically conductive sense line trace in electrical contact with the second portion, the first electrically conductive sense line trace and the second electrically conductive sense line trace configured to measure a voltage across the resistive heater.

Embodiment 60: The sensor of Embodiment 59, wherein the first electrically conductive sense line trace is in electrical contact with the first portion at a location that is not located between the first portion and the second portion.

Embodiment 61: The sensor of any one of Embodiments 53 through 60, wherein the resistive heater comprises a widened curved portion at an intersection of at least one tether and the membrane.

Embodiment 62: The sensor of any one of Embodiments 53 through 61, wherein the resistive heater comprises a continuously increasing width from the widened portion to the center of the membrane.

Embodiment 63: The sensor of any one of Embodiments 53 through 62, wherein each tether of the plurality of tethers has a greater width proximate the membrane and the substrate than at portions distal from the membrane and the substrate.

Embodiment 64: The sensor of any one of Embodiments 53 through 63, further comprising a plurality of electrodes in electrical contact with a chemical sensitive coating material overlying the resistive heater, the plurality of electrodes comprising interdigitated electrodes and configured to measure a resistivity of the chemical sensitive coating material.

Embodiment 65: The sensor of any one of Embodiments 53 through 64, wherein the plurality of tethers connected between the substrate and the membrane comprises at least two tethers.

Embodiment 66: The sensor of any one of Embodiments 53 through 65, wherein the plurality of tethers comprises six tethers, the first electrically conductive trace and the second electrically conductive trace overlying two of the tethers, electrically conductive sense line traces overlying two of the tethers in electrical communication with the resistive heater, and chemical sensing electrode traces overlying two of the tethers and in electrical communication with interdigitated electrodes overlying the resistive heater.

Embodiment 67: The sensor of any one of Embodiments 53 through 66, further comprising a controller configured to determine a resistance, which is proportional to temperature, of the resistive heater at least by dividing a voltage measured by the first electrically conductive sense line trace and the second electrically conductive sense line trace by a current provided to the resistive heater.

Embodiment 68: The sensor of any one of Embodiments 53 through 67, further comprising: an array of microhotplates; and a controller configured to determine one or more of at least one property of the resistive heater of at least one microhotplate of the array of microhotplates and a resistance between interdigitated electrodes of at least one microhotplate of the array of microhotplates.

Embodiment 69: The sensor of Embodiment 68, wherein the array of microhotplates comprises: at least one reference microhotplate comprising an inert material overlying a dielectric material over its resistive heater or free of a coating material; at least one microhotplate comprising a catalytic coating over a dielectric material of its resistive heater; and at least one microhotplate comprising a chemical sensing material selected from the group consisting of a p-type semiconductor, an n-type semiconductor, and an ionic conductor overlying a dielectric material over its resistive heater.

Embodiment 70: A method of measuring at least one of a thermal conductivity, an exothermic event, an endothermic event, and a presence of one or more chemicals in a sample, the method comprising: providing a current to a resistive heater of at least one microhotplate of a multi-sensor array, the resistive heater comprising a varying width from a peripheral portion thereof toward a center thereof, the resistive heater comprising a first portion extending from the peripheral portion toward the center thereof and spiraling in a clockwise direction and a second portion in contact with the first portion at the center of the resistive heater and extending from the center of the resistive heater toward the peripheral portion thereof and spiraling in a counterclockwise direction; measuring a voltage across the resistive heater; calculating a resistance of the resistive heater based at least on the measured voltage across the resistive heater and the current provided to the resistive heater; determining a temperature of the resistive heater based on the resistance of the resistive heater; and determining a power required to maintain a given temperature to determine a thermal conductivity, an endothermic event, an exothermic event, or a presence of one or more chemicals of the sample.

Embodiment 71: The method of Embodiment 70, wherein measuring a voltage across the resistive heater comprises measuring the voltage across the resistive heater with sense lines coupled to the resistive heater.

Embodiment 72: The method of Embodiment 70 or Embodiment 71, further comprising determining a power supplied to the resistive heater based, at least in part, on the voltage measured across the resistive heater and the provided current.

Embodiment 73: The method of any one of Embodiments 70 through 72, further comprising: providing an electrical current to a resistive heater of at least one metal oxide semiconductor microhotplate comprising a chemical sensing material selected from the group consisting of a p-type semiconductor, an n-type semiconductor, and an ionic conductor overlying sense electrodes overlying a dielectric material over the resistive heater of the at least one metal oxide semiconductor microhotplate; and measuring a response of the at least one metal oxide semiconductor microhotplate.

Embodiment 74: The method of Embodiment 73, further comprising determining a resistivity of a chemical sensing material disposed over an interdigitated electrode disposed over the resistive heater of the at least one metal oxide semiconductor microhotplate.

Embodiment 75: The method of any one of Embodiments 70 through 74, wherein measuring a response of the at least one microhotplate comprises: measuring a response of at least one microhotplate comprising a catalytic material overlying a dielectric material over its resistive heater; measuring a response of a reference microhotplate; and determining a difference between the response of the at least one microhotplate comprising the catalytic material and the response of the reference microhotplate.

Embodiment 76: The method of any one of Embodiments 70 through 75, wherein calculating a resistance of the resistive heater comprises compensating the measured voltage by subtracting a voltage drop across electrically conductive traces and a heater interconnect structure in electrical communication with the resistive heater.