Miniaturized multi-gas and vapor sensor devices and associated methods of fabrication

The invention provides a miniaturized sensor device including a thin film membrane having a first surface and a second surface, one or more resistive thin film heater/thermometer devices disposed directly or indirectly adjacent to the first surface of the thin film membrane, and a frame disposed directly or indirectly adjacent to the second surface of the thin film membrane, wherein one or more internal surfaces of the frame define at least one cell having at least one opening. The sensor device also includes a thin film layer disposed directly or indirectly adjacent to the frame. The sensor device further includes a sensing layer disposed directly or indirectly adjacent to the thin film membrane.

FIELD OF THE INVENTION

The invention relates generally to the field of miniaturized sensor devices and platforms and, more specifically, to the field of nano and pico-scale sensor devices and platforms. An aspect of the invention provides robust, high-sensitivity, high-selectivity, high-stability multi-gas and vapor sensor devices, among other sensor devices, and associated methods of fabrication. Variants of the multi-gas and vapor sensor devices of the invention may be used for the in situ measurement of soluble analytes in liquid media. Another aspect of the invention provides a thermally-isolated micro-platform for such microelectromechanical systems (MEMS). A further aspect of the invention provides a design for wide-dynamic range, micro-machined humidity sensor devices, among other sensor devices, that relieves the generated stresses in the associated sensing films caused by sensing film swelling due to the adsorption of water. A still further aspect of the invention provides a protocol for the deposition of self-assembled monolayers (SAMs) as multi-gas and vapor sensing films.

BACKGROUND OF THE INVENTION

The scientific and technological interest in miniaturized gas, humidity, chemical, temperature, and pressure sensor devices has grown in recent years. The need for such devices spans a wide range of industries and applications, such as the medical instrumentation, food and agriculture, paper, automotive, electric appliance, petrochemical, and semiconductor industries, as well as the military, in, for example, gas, humidity, chemical, temperature, and pressure sensing applications. The wide range of environments to which these devices may be exposed severely limits the candidate materials that may be used to build the devices. A number of gas, humidity, chemical, temperature, and pressure sensor devices have been developed and built for specific applications. However, none of these devices demonstrate a suitable combination of the desired robustness, sensitivity, selectivity, stability, size, simplicity, reproducibility, reliability, response time, resistance to contaminants, and longevity. Thus, what are still needed, in general, are multi-gas and vapor sensor devices, among other sensor devices, that exploit the high sensitivity of differential scanning nano and picocalorimetry microelectromechanical systems (MEMS) to heat flow and the unique properties of certain thin films and nano and picoparticles, including their high adsorption potential, high adsorption rate under optimized conditions, high desorption rate under optimized conditions, high chemical stability, and heat release associated with the physisorption of gas and vapor molecules.

Response time, mechanical strength, power consumption, and crosstalk between unit sensor devices are major areas of concern with respect to thermally-sensitive microelectromechanical systems (MEMS), such as gas, humidity, chemical, temperature, and pressure sensor devices, as well as calorimeter and microheater devices, in general. For example, faster response time provides higher sensitivity and greater mechanical strength provides higher reliability. Likewise, lower power consumption is desired for portable and wireless devices and less crosstalk between unit sensor devices provides greater accuracy. Response time and sensitivity are critical in many sensing applications, such as in sensing for warfare agents, measuring low dew points, detecting trace gases, etc., but are difficult to optimize with conventional multi-gas and vapor sensor devices without making sacrifices with respect to other performance parameters. Power consumption and crosstalk between unit sensor devices are both affected by thermal isolation. Typically, thermal isolation has been addressed by fabricating microelectromechanical systems (MEMS) on thin insulating membranes with low heat capacity. However, such thin membranes are fragile, resulting in low yield and reliability problems. Moreover, the peripheries of these thin membranes are typically bonded to a silicon substrate, introducing lateral heat conduction losses. Thus, what are needed are microelectromechanical systems (MEMS) that are built with, for example, low-thermal conductivity regions around the active thin membrane regions, resulting in more robust, high-performance, high-sensitivity microelectromechanical systems (MEMS).

Two additional areas of concern are raised with respect to miniaturized vapor (e.g., humidity) sensor devices, among other sensor devices. First, the polymeric sensing films associated with such vapor sensor devices often become significantly swollen while at relatively high humidity due to their high affinity for water vapor. The swelling of these sensing films generates lateral stresses that impinge upon the thin membranes, potentially breaking them. Second, sensing films having larger surface areas are desired in order to reduce the thickness of the sensing films at a given mass. Reducing the thickness of the sensing films and incorporating nanostructures (e.g., nano-spheres, nano-rods, nano-fibers, etc.) into the sensing materials decreases the diffusion time constant of the water adsorption/desorption, reducing the response time of the vapor sensor devices. Thus, what are needed are micro-machined vapor sensor devices, among other sensor devices, that utilize, for example, high-aspect ratio silicon microstructures etched adjacent to the thin membranes. These silicon microstructures would serve as stress relievers at varying vapor (e.g., humidity) levels and provide both large surface areas for the sensing films, increasing the sensitivity of the vapor sensor devices, and effective heat conduction paths to the microheaters also disposed adjacent to the thin membranes.

BRIEF SUMMARY OF THE INVENTION

In various embodiments, the invention provides robust, high-sensitivity, high-selectivity, high-stability multi-gas and vapor sensor devices and platforms, among other sensor devices and platforms, and associated methods of fabrication. The multi-gas and vapor sensor devices exploit the high sensitivity of differential scanning nano and picocalorimetry microelectromechanical systems (MEMS) to heat flow and the unique properties of certain thin films and nano and picoparticles, such as zeolite thin films and nano and picoparticles, as well as porous ceramics, crosslinked polyelectrolytes, aluminosilicates, and carbon nanotubes, including their high adsorption potential, high adsorption rate under optimized conditions, high desorption rate under optimized conditions, high chemical stability, and heat release associated with the physisorption of gas and vapor molecules. The origins of this heat release are the energy conversions associated directly with the adsorption of a sensed substance, as well as any secondary thermal transitions characteristic of the dry material.

In various embodiments, the invention also provides a thermally-isolated micro-platform for robust, high-performance, high-sensitivity microelectromechanical systems (MEMS). Using various micro-machining techniques, microstructures with low thermal conductivities are incorporated into the peripheries of active thin membrane areas, the thermally-sensitive microelectromechanical systems (MEMS) disposed on either side of the thin membranes. The resulting thermal isolation provides faster response time, greater mechanical strength, lower power consumption, and less crosstalk between unit sensor devices than is possible with a purely thin membrane-based design.

In various embodiments, the invention further provides micro-machined vapor (e.g., humidity) sensor devices, among other sensor devices, that utilize high-aspect ratio silicon microstructures etched adjacent to the thin membranes. These high-aspect ratio silicon microstructures serve as stress relievers due to the large Young's modulus coefficient of silicon. By varying the dimensions of the silicon microstructures, different spring constants may be achieved, accommodating the generated stresses caused by the swelling problems described above at varying vapor/humidity levels. The silicon microstructures provide both large surface areas for the sensing films, increasing the sensitivity of the vapor sensor devices, and effective heat conduction paths to the microheaters also disposed adjacent to the thin membranes. Another method for alleviating the stresses resulting from the adsorption of water vapor by intensely hydrophilic organic polymer materials is provided, involving the creation of a self-assembled monolayer (SAM) with polyelectrolyte functionality on a highly-reticulated substrate of silicon oxide which has been vapor deposited onto the thermally-conductive membrane of a hot plate.

In one specific embodiment of the invention, a miniaturized sensor device includes a thin film membrane having a first surface and a second surface, one or more resistive thin film heater/thermometer devices disposed directly or indirectly adjacent to the first surface of the thin film membrane, and a frame disposed directly or indirectly adjacent to the second surface of the thin film membrane, wherein one or more internal surfaces of the frame define at least one cell having at least one opening. The sensor device also includes a thin film layer disposed directly or indirectly adjacent to the frame. The sensor device further includes a sensing layer disposed directly or indirectly adjacent to the thin film membrane.

In another specific embodiment of the invention, a method for fabricating a miniaturized sensor device includes providing a silicon layer having a first surface and a second surface, depositing a first thin film layer having a first surface and a second surface on the first surface of the silicon layer, and depositing a second thin film layer on the second surface of the silicon layer. The method also includes masking the first surface of the first thin film layer and selectively depositing a sacrificial layer on the first surface of the first thin film layer, wherein the sacrificial layer defines one or more exposed regions of the first surface of the first thin film layer. The method further includes depositing a conductive layer on a surface of the sacrificial layer and the one or more exposed regions of the first surface of the first thin film layer defined by the sacrificial layer and removing the sacrificial layer and a portion of the conductive layer deposited on the surface of the sacrificial layer to form one or more resistive thin film heater/thermometer devices on the first surface of the first thin film layer. The method still further includes selectively removing a portion of the second thin film layer and selectively removing a portion of the silicon layer to form at least one cell, wherein the at least one cell is disposed directly or indirectly adjacent to the second surface of the first thin film layer, and wherein the cell is substantially aligned with the one or more resistive thin film heater/thermometer devices. The method still further includes disposing a sensing layer on the second surface of the first thin film layer.

In a further specific embodiment of the invention, a microelectromechanical system includes a thin film membrane having one or more active membrane areas and one or more inactive membrane areas. The microelectromechanical system also includes one or more resistive thin film heater/thermometer devices disposed directly or indirectly adjacent to the one or more active membrane areas of the thin film membrane. The microelectromechanical system further includes a frame disposed directly or indirectly adjacent to the one or more inactive membrane areas of the thin film membrane. The microelectromechanical system still further includes one or more low-thermal conductivity microstructures disposed between the one or more active membrane areas of the thin film membrane and the one or more inactive membrane areas of the thin film membrane.

In a still further specific embodiment of the invention, a miniaturized sensor device includes a thin film membrane having one or more active membrane areas and one or more inactive membrane areas, one or more resistive thin film heater/thermometer devices disposed directly or indirectly adjacent to the one or more active membrane areas of the thin film membrane, and a frame disposed directly or indirectly adjacent to the one or more inactive membrane areas of the thin film membrane. The sensor device also includes one or more low-thermal conductivity microstructures disposed between the one or more active membrane areas of the thin film membrane and the one or more inactive membrane areas of the thin film membrane. The sensor device further includes one or more stress relief structures disposed directly or indirectly adjacent to the one or more active membrane areas of the thin film membrane. The sensor device still further includes one or more sensing films disposed directly or indirectly adjacent to the one or more stress relief structures.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, the sensor device10of the invention, which may be a multi-gas or vapor (e.g., humidity) sensor device, among other sensor devices, consists of a multi-cell ultra high-sensitivity differential scanning calorimeter (UHSDSC), which is a microelectromechanical system (MEMS). The sensor device10is fabricated using standard silicon processing techniques, well known to those of ordinary skill in the art. The sensor device10includes a thin, thermally-insulating silicon oxinitride (SiONx) membrane12disposed directly adjacent to a silicon (Si) frame14. It should be noted that other suitable materials may replace the silicon oxinitride membrane12and/or the silicon frame14. For example, the silicon oxinitride membrane12may be replaced with a silicon, polysilicon, parylene, or polyimide membrane. Preferably, a thin silicon oxinitride layer16is also disposed directly adjacent to the silicon frame14opposite the silicon oxinitride membrane12. Again, other suitable materials may replace the silicon oxinitride layer16. Preferably, the sensor device10has an overall length of between about 0.5 mm and about 3 cm, and an overall width of between about 0.5 mm and about 3 cm, although other suitable dimensions may be used. Preferably, the silicon oxinitride membrane12has a thickness of between about 50 nm and about 1 micron, although other suitable dimensions may be used. Specifically, the thickness of the silicon oxinitride membrane12may be varied depending upon the material(s) used and/or the sensitivity desired. Preferably, the silicon frame14has a thickness of between about 50 microns and about 650 microns, although other suitable dimensions may be used. The silicon frame14divides the sensor device10into two or more cells18, which are ideally identically symmetric and identical. One of the cells18may be used as a reference cell during operation, while the other cells18may be used as sensing cells. A plurality of thin film heater/thermometers20are disposed directly adjacent to the silicon oxinitride membrane12opposite the silicon frame14. Preferably, the location of each of the plurality of thin film heater/thermometers20generally corresponds to each of the two or more cells18. The plurality of thin film heater/thermometers20may be made of platinum (Pt) and/or titanium (Ti), although other suitable materials may be used, such as gold (Au) and/or chromium (Cr), gold and/or nickel (Ni) and/or copper (Cu), aluminum (Al), etc., as well as polysilicon, heavily-doped silicon, silicon carbide, etc. Advantageously, the silicon oxinitride membrane12allows rapid heat propagation in the z-direction, to and from the plurality of thin film heater/thermometers20. It should be noted that the layout and configuration of the sensor device10illustrated inFIG. 1is exemplary only, and is not intended to be limiting. Alternative layouts and configurations may be implemented to fit different geometrical requirements for specific applications.

Referring toFIG. 2, a thin film or nanoparticle layer22is added to one or more cells18of the sensor device10, directly adjacent to the silicon oxinitride membrane12opposite the corresponding thin film heater/thermometer20. Preferably, the thin film or nanoparticle layer22has a thickness of between about 1 nm and about 5 microns, although other suitable dimensions may be used. The thin film or nanoparticle layer22consists of a zeolite thin film, a suitable cross-linked organic polyelectrolyte, a self-assembled monolayer of ionic character, or the like, generally comprising materials that generate heat upon the physisorption of gasses and/or vapors. Preferably, the thin film or nanoparticle layer22is nano-structured (consisting of spheres, rods, hollow fibers, etc.) such that heat propagates in the z-direction, to and from the plurality of thin film heater/thermometers20, and not into the surrounding environment. In general, because the thin film or nanoparticle layer22consists of a plurality of nanopores, molecules are allowed to travel in and out of the nanopores. Surface saturation would undesirably increase the response time of the sensor device10. The thin film or nanoparticle layer22acts as an interface between a substance to be detected, present in one or both of the cells18, and the sensor device10. Upon adsorption of a given amount of this substance onto the surface of the thin film or nanoparticle layer22, a corresponding amount of heat is released. This heat exchange is measured by the sensor device10(operated under power compensation conditions) and subsequently related to the amount of adsorbate in the environment based upon data collected during calibration of the sensor device10. The adsorbate is driven out of the porous structure of the thin film or nanoparticle layer22naturally as its partial pressure in the environment drops. It is possible to accelerate desorption of the adsorbate from the porous structure of the thin film or nanoparticle layer22by pulse heating the thin film or nanoparticle layer22without damaging its structure. Preferably, the microstructure of the thin film or nanoparticle layer22and its pore dimensions are customized to ensure the high selectivity of the sensor device10towards a specific adsorbate. In addition, active selectivity may be achieved by operating the sensor device10in a desorption mode. In this mode, heat is applied to the sensing material by flowing a direct or modulated current through the thin film heater/thermometers20, leading to the desorption of all adsorbed species at specific temperatures. The desorption temperature is used to discriminate against undesired adsorbates and contaminants. The sensor device10is operated under power compensation conditions as, under these conditions, the sensor device10is least prone to picking-up noise signals. In the power compensation mode, a reference power source compensates for any temperature changes that the reference thin film heater/thermometer20experiences relative to the sensing thin film heater/thermometer20due to heat exchange with the thin film or nanoparticle layer22.

Referring toFIG. 3, in an alternative embodiment of the invention, the sensor device10is equipped with a built-in protection mechanism designed to prevent the “locking” of the pores of the thin film or nanoparticle layer22. In general, the sensor device10described above is disposed directly adjacent to an additional silicon oxinitride membrane24, an additional silicon frame26, and an additional silicon oxinitride layer28via bonding in a controlled environment. As before, other suitable materials may replace the additional silicon oxinitride membrane24, the additional silicon frame26, and the additional silicon oxinitride layer28. Preferably, the additional silicon oxinitride membrane24, the additional silicon frame26, and the additional silicon oxinitride layer28collectively define an additional cell30. The opening of this additional cell30to the environment is guarded by a grid32operable for keeping particulates and/or contaminants away from the thin film or nanoparticle layer22. The grid32may be fabricated using standard silicon processing and lithography techniques, well known to those of ordinary skill in the art. Advantageously, the presence of the additional silicon oxinitride membrane24isolates one of the original cells18,34from the environment, which may then be maintained with an atmosphere of dry inert gas (e.g., air, nitrogen, etc.).

In general, the sensor devices10(FIGS. 1–3) of the invention require short heat transfer paths between the thin film or nanoparticle layer(s)22(FIGS. 2 and 3) and the thin film heater/thermometer(s)20(FIGS. 1–3), as well as minimal heat losses to the environment. While the former concern is addressed through the use of the thin silicon oxinitride membrane12(FIGS. 1–3), the latter concern must be addressed through the packaging of the sensor devices10. Referring toFIG. 4, an exemplary packaging assembly36includes a ceramic block38, such as a Maycor block or the like, having a recessed cavity40suitable for containing the sensor device10being used. The sensor device10is secured within the recessed cavity40such that at least one cell18(FIGS. 1–3) of the sensor device10is exposed to the environment. As described above, a grid32(see alsoFIG. 3) may be used to keep particulates and/or contaminants away from the thin film or nanoparticle layer22. Preferably, the volume42of the recessed cavity40surrounding the sensor device10is filled with dry air or an inert gas in order to keep certain components of the packaging assembly36from oxidizing. For example, a nitrogen (N2) atmosphere may be used. A plurality of copper-beryllium (Cu—Be) spring-loaded probes44or the like pass through the ceramic block38and come into electrical contact with the thin film heater/thermometer(s)20of the sensor device10. Advantageously, this and similar packaging assemblies ensure high thermal resistance between the thin film heater/thermometer(s)20and the environment. Thus, a high signal-to-noise ratio may be achieved.

Referring toFIG. 5, the first step in the fabrication of the sensor device10(FIGS. 1 and 2) of the invention includes the low stress deposition of a first thin film amorphous silicon oxinitride layer46(eventually becoming what is referred to above as the silicon oxinitride membrane12(FIGS. 1 and 2)) and a second thin film amorphous silicon oxinitride layer48(eventually becoming what is referred to above as the silicon oxinitride layer16(FIGS. 1 and 2)) on opposing sides of a silicon layer or wafer50(eventually becoming what is referred to above as the silicon frame14(FIGS. 1 and 2)). Preferably, the silicon layer50consists of single-crystal silicon oriented in the <100> or <110> direction. As described above, however, other suitable materials may replace the first silicon oxinitride layer46, the second silicon oxinitride layer48, and the silicon layer50. As used herein, “low stress deposition” refers to deposition wherein the stress level in the first silicon oxinitride layer46corresponds to tensile stresses and is adjusted to compensate for the compressive stresses applied by the sensing material to the membrane.

Referring toFIG. 6, the second step in the fabrication of the sensor device10includes depositing and baking a photoresist (PR) coating52on the surface of the second silicon oxinitride layer48. The photoresist (PR) coating52protects the second silicon oxinitride layer48from scratching during subsequent processing.

Referring toFIG. 7, the third step in the fabrication of the sensor device10is a lithography and image reversal step. A mask54is disposed adjacent to the surface of the first silicon oxinitride layer46and a photoresist (PR) layer56is selectively spun onto the surface of the first silicon oxinitride layer46. Preferably, the photoresist (PR) layer56has a thickness that is about three (3) times as thick as a metal layer that will subsequently be deposited (about 0.5 microns). The image is reversed using an ammonia diffusion bake, flood exposure, and development of the photoresist (PR).

Referring toFIG. 8, the fourth step in the fabrication of the sensor device10includes evaporating a metal layer58onto the surface of the photoresist (PR) layer56and the exposed portions of the first silicon oxinitride layer46. The metal layer58may include, for example, platinum, gold (Au), nickel (Ni), or aluminum (Al). Alternatively, the metal layer58may be replaced with a polysilicon layer, a heavily-doped silicon layer, or a layer of any other conductive material having a tunable resistance in order to modify its sensitivity. Optionally, the metal layer58consists of a titanium layer (about 4 nm thick, for example), which acts as a bonding layer, and a platinum layer (about 50 nm thick, for example).

Referring toFIG. 9, the fifth step in the fabrication of the sensor device10includes using acetone or the like and an ultrasound bath or the like to lift-off the photoresist (PR) layer56and selected portions of the metal layer58, forming the plurality of thin film heater/thermometers20(FIGS. 1 and 2) described above.

Referring toFIG. 10, the sixth step in the fabrication of the sensor device10includes performing backside optical lithography and a dielectric etch to selectively remove a portion of the photoresist (PR) coating52and the second silicon oxinitride layer48, exposing a portion of the silicon layer50.

Referring toFIG. 11, the seventh step in the fabrication of the sensor device10includes performing a potassium hydroxide (KOH), ethylene diamine pyrocatechol (EDP), or deep reactive ion (DRI) etch to selectively remove the remaining portions of the photoresist (PR) coating52and a portion of the silicon layer50, forming the silicon oxinitride membrane12, the silicon frame14, and one or more of the cells18(FIGS. 1 and 2) described above. At this point, the thin film or nanoparticle layer22(FIG. 2) may be deposited or grown directly on the surface of the silicon oxinitride membrane12within the one or more cells18.

Referring toFIGS. 12 and 18, the thermally-isolated micro-platforms60,62for microelectromechanical systems (MEMS) of the invention include a plurality of microstructures64,66with large thermal resistances built on the peripheries of active membrane areas68,70. These microstructures64,66are operable for reducing lateral heat conduction, reducing heat loss to the environment, and increase the mechanical strength of the microelectromechanical systems (MEMS) into which they are incorporated. Enhanced thermal isolation leads to enhanced sensitivity, faster response time, and decreased power consumption for the microelectromechanical systems (MEMS), which may include, for example, the multi-gas or vapor sensor devices10(FIGS. 1–3) described above, among other sensor devices. Two approaches are described for fabricating the thermally-isolated micro-platforms60,62for microelectromechanical systems (MEMS) of the invention: (1) a micro/nanostructure refill approach using a dielectric material with low thermal conductivity and high-aspect ratio micro/nanostructures, such as trenches, grids, posts, vias, or pores, and (2) a thick oxide approach using the thermal oxidation of high-aspect ratio micro/nanostructures, such as those described above.

Referring toFIG. 13, the first step of the trench refill approach using a dielectric material and high-aspect ratio trenches (HARTs) or grids includes the deposition of a first thin film dielectric layer72, such as a first thin film silicon oxinitride layer or the like, and a second thin film dielectric layer74, such as a second thin film silicon oxinitride layer or the like, on opposing sides of a silicon layer or wafer76or the like. As described above, any suitable materials may be used for the first thin film dielectric layer72, the second thin film dielectric layer74, and the silicon layer or wafer76. Preferably, the first thin film dielectric layer72has a thickness of between about 0.1 microns and about 5 microns, the second thin film dielectric layer74has a thickness of between about 0.1 microns and about 5 microns, and the silicon layer or wafer76has a thickness of between about 100 microns and about 1,000 microns. The first thin film dielectric layer72and the second thin film dielectric layer74may be deposited simultaneously and have the same thickness. The thickness of the first thin film dielectric layer72and the second thin film dielectric layer74is determined by the specifications of the given multi-gas or vapor sensor device10(FIGS. 1–3). For example, if the first thin film dielectric layer72is for building membrane, at a given area, a thicker membrane provides a higher natural frequency at resonance.

Referring toFIG. 14, the second step of the trench refill approach using a dielectric material and high-aspect ratio trenches (HARTs) or grids includes etching the high-aspect ratio trenches (HARTs) or grids78(also referred to generally as the high-aspect ratio micro/nanostructures78) in a portion of the first thin film dielectric layer72and the silicon layer or wafer76using a first mask (not shown). Preferably, each of the high-aspect ratio micro/nanostructures78has a width of between about 0.01 microns and about 10 microns, a depth of between about 1 micron and about 500 microns, and an aspect ratio of between about 1 and about 100. These high-aspect ratio micro/nanostructures78define and surround the active membrane area(s)68(FIG. 12). The high-aspect ratio micro/nanostructures78may be fabricated using either wet etching (e.g., KOH etching on <110> silicon (Si) or electrochemical etching) or dry etching (e.g., DRIE). The aspect ratio is limited by the etching technology and is preferably as high as possible. Refilled dielectric on the sidewalls may touch at the resulting opening and close the trench to form a void. This void may be vacuum-sealed if the dielectric deposition is performed in a vacuum.

Referring toFIG. 15, the third step of the trench refill approach using a dielectric material and high-aspect ratio trenches (HARTs) or grids includes the removal of the first thin film dielectric layer72outside of the active membrane area(s)68(FIG. 12) using a second mask (not shown) and the selective deposition of a dielectric layer80on the remaining portions of the first thin film dielectric layer72and the exposed portions of the silicon layer or wafer76using, for example, a low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), or spin-on coating technique, well known to those of ordinary skill in the art. Preferably, the dielectric layer80has a thickness of between about 0.5 microns and about 10 microns. The second mask may not be needed if the first thin film dielectric layer72is thin enough and does not alter the sensor device10(FIGS. 1–3) specifications, such as the natural frequency of the membrane at resonance. The dielectric layer80may include an oxide, a glass, a polyimide, a polymer, a nitride, any other suitable low-thermal conductivity material, or any suitable combination thereof. Advantageously, the spin-on coating technique provides a low-temperature process, thus reducing undesirable residual thermal stresses in the first thin film dielectric layer72. Additionally, an oxide/nitride/oxide or nitride/oxide/nitride may be deposited for stress compensation to reduce undesirable residual thermal stresses.

Referring toFIG. 16, the fourth step of the trench refill approach using a dielectric material and high-aspect ratio trenches (HARTs) or grids includes depositing and patterning the plurality of thin film heater/thermometers, as described above, on or adjacent to the surface of the dielectric layer80, adjacent to the surface of the first thin film dielectric layer72. This is done using a third mask (not shown). As described above, the plurality of thin film heater/thermometers20may include a metal, polysilicon, heavily-doped silicon, silicon carbide, or the like.

Referring toFIG. 17, the fifth step of the trench refill approach using a dielectric material and high-aspect ratio trenches (HARTs) or grids includes patterning and selectively etching the second thin film dielectric layer74and the silicon layer76to form one or more of cells18described above. This is done using a fourth mask (not shown). The etching process may comprise a wet and/or dry etching technique, such as potassium hydroxide (KOH) etching, tetramethylammonium hydroxide (TMAH) etching, ethylene diamine pyrocatechol (EDP) etching, and/or deep reactive ion (DRI) etching.

Referring toFIG. 19, the first step of the thick oxide approach using the thermal oxidation of high-aspect ratio trenches (HARTs) or grids includes the deposition of a first thin film dielectric layer72, such as a first thin film silicon oxinitride layer or the like, and a second thin film dielectric layer74, such as a second thin film silicon oxinitride layer or the like, on opposing sides of a silicon layer or wafer76or the like. As described above, any suitable materials may be used for the first thin film dielectric layer72, the second thin film dielectric layer74, and the silicon layer or wafer76. Preferably, the first thin film dielectric layer72has a thickness of between about 0.1 microns and about 5 microns, the second thin film dielectric layer74has a thickness of between about 0.1 microns and about 5 microns, and the silicon layer or wafer76has a thickness of between about 100 microns and about 1,000 microns. The first thin film dielectric layer72and the second thin film dielectric layer74may be deposited simultaneously and have the same thickness. The thickness of the first thin film dielectric layer72and the second thin film dielectric layer74is determined by the specifications of the given multi-gas or vapor sensor device10(FIGS. 1–3). For example, if the first thin film dielectric layer72is for building membrane, at a given area, a thicker membrane provides a higher natural frequency at resonance.

Referring toFIG. 20, the second step of the thick oxide approach using the thermal oxidation of high-aspect ratio trenches (HARTs) or grids includes etching the high-aspect ratio trenches (HARTs) or grids78in a portion of the first thin film dielectric layer72and the silicon layer or wafer76using a first mask (not shown). Preferably, each of the high-aspect ratio trenches (HARTs) or grids78has a width of between about 1 micron and about 10 microns, a depth of between about 1 micron and about 500 microns, and an aspect ratio of between about 1 and about 50. The spacing between the high-aspect ratio micro/nanostructures78is of importance and should be less than about 1.08 microns in order to seal the high-aspect ratio micro/nanostructures78as the final oxide thickness is approximately 54% above the original surface of the silicon and approximately 46% below the original surface. However, this thick oxide approach may be combined with the trench refill approach to seal the gap if small spacing cannot be achieved. The high-aspect ratio micro/nanostructures78define and surround the active membrane area(s)68(FIG. 18). The high-aspect ratio micro/nanostructures78may be fabricated using either wet etching (e.g., KOH etching on <110> silicon (Si) or electrochemical etching) or dry etching (e.g., DRIE). The aspect ratio is limited by the etching technology and is preferably as high as possible.

Referring toFIG. 21, the third step of the thick oxide approach using the thermal oxidation of high-aspect ratio trenches (HARTs) or grids includes the thermal oxidation of the high-aspect ratio trenches (HARTs) or grids78to form a thick oxide82within each of the high-aspect ratio trenches (HARTs) or grids78. The thickness of this thick oxide is determined by the etched depth and is between about 1 micron and about 1,000 microns, depending upon the etching technology used. The oxidation time is determined by the spacing between the high-aspect ratio micro/nanostructures78. For example, a 2-micon spacing between the high-aspect ratio micro/nanostructures78, a time of approximately 10 hrs is required to close the space.

Referring toFIG. 22, the fourth step of the thick oxide approach using the thermal oxidation of high-aspect ratio trenches (HARTs) or grids includes selectively depositing and patterning the plurality of thin film heater/thermometers, as described above, on or adjacent to the surface of the first thin film dielectric layer72. This is done using a second mask (not shown). As described above, the plurality of thin film heater/thermometers20may include a metal, polysilicon, heavily-doped silicon, silicon carbide, or the like.

Referring toFIG. 23, the fifth step of the thick oxide approach using the thermal oxidation of high-aspect ratio trenches (HARTs) or grids includes patterning and selectively etching the second thin film dielectric layer74and the silicon layer76to form one or more of cells18described above. This is done using a third mask (not shown). The etching process may comprise a wet a and/or dry etching technique, such as potassium hydroxide (KOH) etching, tetramethylammonium hydroxide (TMAH) etching, ethylene diamine pyrocatechol (EDP) etching, and/or deep reactive ion etching (DRIE).

Referring toFIG. 24, two related embodiments of the micro-machined humidity sensor device84of the invention that utilize high-aspect ratio silicon micro/nanostructures adjacent to the thin membranes include a thin film dielectric layer or multi-layer86, which may include, for example, a nitride, an oxide, polysilicon, heavily-doped silicon, silicon oxinitride, an oxide/silicon/oxide multi-layer, a nitride/oxide/nitride multi-layer, a nitride/silicon/nitride multi-layer, or the like. Preferably, the thin film dielectric layer or multi-layer86has a thickness of between about 0.1 microns and about 5 microns, although other suitable dimensions may be used. The thickness of each layer of the thin film dielectric layer or multi-layer86may be selected to achieve a stress-compensated membrane. The thin film dielectric layer or multi-layer86is physically divided into active membrane areas88and inactive membrane or supporting areas90. These active membrane areas88and inactive membrane or supporting areas90are separated by a plurality of microstructures92with large thermal resistances built on the peripheries of active membrane areas88, as described above. The microstructures92may be deposited as a layer using, for example, a low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), or spin-on coating technique, well known to those of ordinary skill in the art. The microstructures92may include an oxide, a glass, a polyimide, a polymer, a nitride, or any other suitable low-thermal conductivity material. Additionally, an oxide/nitride/oxide or nitride/oxide/nitride may be deposited for stress compensation to reduce undesirable residual thermal stresses. A plurality of metal, polysilicon, or heavily-doped silicon thin film heater/thermometers94are disposed adjacent to a first surface of the thin film dielectric layer or multi-layer86in locations corresponding to the active membrane areas88. Further, a silicon frame96is disposed adjacent to a second surface of the thin film dielectric layer86in locations corresponding to the inactive membrane or supporting areas90.

A thin silicon layer or self-assembled monolayer (SAM)98is disposed adjacent to a second surface of the thin film dielectric layer or multi-layer86in locations corresponding to the active membrane areas88and the microstructures92. Preferably, the silicon layer or self-assembled monolayer (SAM)98has a thickness of between about 1 nm and about 10 nm, although other suitable dimensions may be used. A conformal nitride or oxide layer100is then disposed adjacent to the exposed portions of the thin silicon layer98and the silicon frame96, in the case that a thin silicon layer98is used. Finally, a sensing film102, such as one of the sensing films described above, a polymer, or the like, is disposed adjacent to at least a portion of the thin silicon layer98or conformal nitride or oxide layer100. Preferably, the sensing film102has a thickness of between about 0.01 microns and about 5 microns prior to water adsorption, although other suitable dimensions may be used.

The self-assembled monolayer (SAM)98is disposed adjacent to the second surface of the thin film dielectric layer or multi-layer86, in part, by depositing a high-surface area layer of silicon oxide onto a nitride diaphragm. Optionally, for a dirty silicon oxide layer, the silicon oxide layer is exposed to a piranha solution at about 50 degrees C. for about 30 minutes, making sure that the metal serpentine heater used is not exposed to the piranha solution by exclusion or masking. In a dry box, a microelectromechanical systems (MEMS) die is dried at about −50 degrees C. dew point or less with dry nitrogen or dry air purge gas at about 100 degrees C. for about 5 hours or more. The die is then immersed in a 0.5–1.0% (w/v) solution of 2-(4-chlorosulfonylpheyl)ethyltrichlorosilane in anhydrous toluene and allowed to react for about 2 hours at about 70 degrees C. The die is then rinsed in anhydrous toluene, followed by anhydrous acetone. The die is then immersed in de-ionized water at room temperature with gentle stirring for about 5 hours. The die is then rinsed in de-ionized water. Finally, the die is dried with dry air purge gas at room temperature for about 3 hours before packaging.

Advantageously, the silicon layer or self-assembled monolayer (SAM)98described above serves as a stress reliever because of the large Young's modulus coefficient of silicon and no extra stresses are generated in this layer upon the adsorption/desorption of the sensed substance. The conformal nitride or oxide layer100is sometimes required because it does not react with water at elevated temperatures. When the self-assembled monolayer (SAM)98or sensing film102adsorbs water, it swells and generates stresses. If the sensing film102, for example, is deposited directly on the second surface of the thin film dielectric layer86in the active membrane areas88, the thin film dielectric layer86may be broken due to these generated stresses. Using the devices and methods of the invention, the swollen sensing film102, confined by the silicon layer98, swells towards the environment. This process is illustrated inFIG. 25. Further, the silicon layer98and conformal nitride or oxide layer100provide a large surface area for the deposition of the sensing film102and effective heat conducting paths to the plurality of thin film heater/thermometers94. Thus, the sensitivity and response time of the humidity sensor device84are significantly increased.

Referring toFIG. 26, another embodiment of the micro-machined humidity sensor device104of the invention that utilizes high-aspect ratio silicon microstructures adjacent to the thin membranes also includes a thin film dielectric layer86, which may include, for example, silicon oxinitride. Alternatively, the thin film dielectric layer86may include polysilicon or heavily-doped silicon. Preferably, the thin film dielectric layer86has a thickness of between about 0.1 microns and about 5 microns, although other suitable dimensions may be used. The thin film dielectric layer86is physically divided into active membrane areas88and inactive membrane or supporting areas90. These active membrane areas88and inactive membrane or supporting areas90are separated by a plurality of microstructures92with large thermal resistances built on the peripheries of active membrane areas88, as described above. The microstructures92may be deposited as a layer using, for example, a low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), or spin-on technique, well known to those of ordinary skill in the art. The microstructures92may include an oxide, a glass, a polyimide, a polymer, a nitride, or any other suitable low-thermal conductivity material. Additionally, an oxide/nitride/oxide or nitride/oxide/nitride may be deposited for stress compensation to reduce undesirable thermal stresses. A plurality of metal, polysilicon, or heavily-doped silicon thin film heater/thermometers94are disposed adjacent to a first surface of the thin film dielectric layer86in locations corresponding to the active membrane areas88. Further, a silicon frame96is disposed adjacent to a second surface of the thin film dielectric layer86in locations corresponding to the inactive membrane areas90.

A thin silicon layer98is disposed adjacent to a second surface of the thin film dielectric layer86in locations corresponding to the active membrane areas88. Preferably, the silicon layer98has a thickness of between about 1 nm and about 10 nm, although other suitable dimensions may be used. A plurality of substantially-parallel, high-aspect ratio silicon microstructures106are then disposed adjacent to the silicon layer98, in a substantially-perpendicular alignment with the silicon layer98. Preferably, each of the plurality of silicon microstructures106has a length of between about 0.01 microns and about 10 microns, a width of between about 0.01 microns and about 10 microns, and a depth of between about 0.01 microns and about 50 microns, although other suitable dimensions may be used. A conformal nitride or oxide layer100is then disposed adjacent to the exposed portions of the thin silicon layer98and the plurality of silicon microstructures106. Preferably, the conformal nitride or oxide layer100has a thickness of between about 0.01 microns and about 1 micron, although other suitable dimensions may be used. Finally, a sensing film102, such as one of the sensing films described above, a polymer, or the like, is disposed adjacent to at least a portion of the conformal nitride or oxide layer100, between the plurality of silicon microstructures106. Preferably, the sensing film102has a thickness of between about 0.01 microns and about 50 microns prior to water adsorption, although other suitable dimensions may be used.

Advantageously, the silicon layer98and the plurality of silicon microstructures106described above serve as stress relievers because of the large Young's modulus coefficient of silicon. The conformal nitride or oxide layer100is sometimes required because it does not react with water at elevated temperatures. When the sensing film102adsorbs water, it swells and generates stresses. If the sensing film102is deposited directly on the second surface of the thin film dielectric layer86in the active membrane areas88, the thin film dielectric layer86may be broken due to these generated stresses. Using the devices and methods of the invention, the swollen sensing film102, confined by the silicon layer98and the plurality of silicon microstructures106, swells towards the environment. This process is illustrated inFIG. 27. Further, the silicon layer98, plurality of silicon microstructures106, and conformal nitride or oxide layer100provide a large surface area for the deposition of the sensing film102and effective heat conducting paths to the plurality of thin film heater/thermometers94. Thus, the sensitivity and response time of the humidity sensor device104are significantly increased.

In general, the multi-gas and vapor sensor devices of the invention may be used in, but are not limited to, the following exemplary applications: humidity or toxic gas monitoring for the ventilation systems of structures, emissions monitoring for automotive engine control, environmental conditions monitoring for shipping containers, hazardous or bio-warfare agent monitoring for transportation security, humidity monitoring for appliances, fire detection and response systems, disposable weather monitoring and forecasting systems, measuring the alcohol content of a human's breath, minimally-invasive blood glucose monitoring systems, monitoring human airways gas for medical and disease diagnosis, food and agricultural packaging and shipping systems, monitoring on-chip humidity for electronic circuits, monitoring humidity or chemical leaks for pressure vessels and containers, immobilization and manipulation systems for cells and proteins, medical instrumentation systems, paper production systems, semiconductor process monitoring systems, natural resource exploration and development systems, and the like.

Although the invention has been illustrated and described with reference to preferred embodiments and examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve similar results. All such equivalent embodiments and examples are within the spirit and scope of the invention and are intended to be covered by the following claims.