Sensing devices, sensors, and methods for monitoring environmental conditions

Sensors, systems, and methods for monitoring environmental conditions, such as physical, electromagnetic, thermal, and/or chemical parameters within an environment, over extended periods of time with the use of one or more electromechanical sensing devices and electronic circuitry for processing an output of the sensing devices. The sensing devices each include a cantilevered structure and at least one contact configured for contact-mode operation with the cantilevered structure in response to the cantilevered structure deflecting toward or away from the contact when exposed to the parameter of interest. The cantilevered structure has at least first and second beams of dissimilar materials, at least one of which has at least one property that changes as a result of exposure to the parameter.

BACKGROUND OF THE INVENTION

The present invention generally relates to electromechanical devices. More particularly, this invention relates to electromechanical devices and electromechanical device-based sensors, systems, and methods capable of monitoring environmental conditions, such as physical, electromagnetic, thermal, and/or chemical parameters within an environment.

Wireless sensors are capable of high reliability, efficiency, and performance and enable ambient intelligence, total visibility, and smart adaptive systems. As such, wireless sensors have found uses in a wide range of applications including supply chain and logistics, industrial and structural monitoring, healthcare, homeland security, and defense. Wireless sensors also find use as nodes of wireless networks, including the Internet of Things (IoT) that connects objects together and to people. Wireless sensors typically include a battery or other energy source.

Generally, it is desired to minimize the power dissipation, size, and cost of wireless sensors by minimizing their power requirements. Wireless sensors can be equipped with integrated miniature batteries or capacitors as a dedicated on-board power source, as well as configured for operation without a power storage device. In many applications, battery less operation may be preferred or required due to lack of battery replacement feasibility or stringent cost, form factor, and lifetime requirements. One approach to address this need is scavenging energy from environmental sources such as ambient heat, radio and magnetic waves, vibrations, and light, provided that at least one of these parameters is adequately available. Another approach is to remotely power a sensor by inductive or electromagnetic coupling, in which case energy may be optionally stored on an integrated capacitor so that sensor operation may occur over a short period of time prior to the capacitor becoming completely discharged. Finally, there are sensors that do not need any external energy source for sensing because they operate based on chemical reactions or mechanical events resulting in a color change or another change in their properties that can be detected by visual inspection or with use of an electronic detection system. The latter types of wireless sensors are generally in the form of sensor labels and do not readily lend themselves to automation.

U.S. Pat. Nos. 7,495,368, 7,619,346, 7,827,660, 7,907,037, 8,487,508, and 8,677,802 and U.S. Patent Application Publication No. 2017/0102249 disclose digital micro-electro-mechanical-system (MEMS) sensing devices that can be manufactured and packaged at wafer-level with integrated circuits to yield a sensing module. The sensing devices can be fabricated to form arrays configured to respond to different levels of an environmental condition or parameter to cumulatively measure the environmental condition.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides sensors, systems, and methods capable of monitoring environmental conditions, such as physical, electromagnetic, thermal, and/or chemical parameters within an environment, over extended periods of time using one or more electromechanical cantilevered structures that deflect to open or close a contact in response to the parameter.

According to one aspect of the invention, an electromechanical sensing device is provided that comprises a cantilevered structure and at least one contact configured for contact-mode operation with the cantilevered structure. The cantilevered structure is responsive to a parameter within an environment and to deflect toward or away from the contacts thereof in response to the parameter in the environment. The cantilevered structure comprises first and second beams containing dissimilar first and second materials, respectively. The first and second beams are side-by-side, spaced apart so as to define a gap there between along the lengths of the first and second beams, and lie in a plane that contains the cantilevered structure. The first material has at least one property that changes due to exposure to the parameter and a change in the property causes the cantilevered structure to deflect in a direction lying in the plane of the first and second beams as a result of the dissimilarity of the first and second materials. The cantilevered structure is configured to contact and close or break contact and open the contacts at a predetermined level of the parameter or from cumulative exposure to the parameter over time.

According to another aspect of the invention, a sensor is provided that includes at least one electromechanical sensing device of a type described above. The sensor further includes means for producing a digital output when the cantilevered structure contacts and closes at least one of the contacts thereof.

According to yet another aspect of the invention, a method is provided for sensing and optionally also monitoring a parameter within an environment. The method entails the use of at least one electromechanical sensing device of a type described above, and includes exposing the cantilevered structure to the parameter within the environment to cause the cantilevered structure to deflect in the direction lying in the plane of the first and second beams to contact and close one of the contacts or break contact and open one of the contacts. A digital output is produced when the cantilevered structure contacts and closes or breaks contact and opens the contacts thereof.

Sensing devices of the type described above may be self-powered electromechanical sensing elements that deflect and close or open an electrical contact based on various parameters that may exist within an environment. As such, the sensing devices do not require a dedicated power source to sense and monitor a parameter within an environment, but instead utilize the parameter to generate a digital output. Technical effects of sensors that utilize such sensing devices preferably include the ability to continuously monitor environmental conditions over extended periods of time. Such a sensor can be used in a network adapted to continuously monitor various environmental conditions, for example, exposure to heat, humidity, chemicals, or electromagnetic radiation, in a wide variety of applications including supply-chain management of perishable goods such as pharmaceuticals, chemicals, and fresh agriculture products, as well as environmental and industrial applications that benefit from detecting the presence of heat, chemicals, electromagnetic radiation, and chemicals.

Other aspects and advantages of this invention will be appreciated from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The following describes electromechanical devices, electromechanical device-based sensors and systems, and methods adapted to monitor environmental conditions, such as physical, electromagnetic, thermal, and/or chemical parameters within an environment. As used herein, the term “electromechanical device(s)” will be used to mean various types of miniaturized electromechanical systems, including micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS), that are generally on a scale of less than a millimeter, incorporate both electronic and mechanical functionalities, and are produced by micromachining techniques such as bulk etching and/or surface thin-film etching.

The electromechanical devices are configured as sensing devices that include at least one electrical contact and a sensing element capable of moving to open or close the electrical contact(s) in response to the presence or absence of an environmental parameter of interest. The sensing element comprises a cantilevered structure, preferably on a scale of less than a millimeter and formed by a micromachining technique. In preferred embodiments, the cantilevered structure comprises at least two beams that are arranged side-by-side but spaced apart from each other, resulting in a gap between the beams along their lengths. Additionally, the beams are formed to contain dissimilar materials. Because the cantilevered structures are represented as comprising beams in the embodiments shown in the drawings, the term “beam” will be used in the following discussion though the invention will be understood to encompass other cantilevered structures, including diaphragms. According to a preferred aspect, at least one of the dissimilar materials of the cantilevered structure is formed of a material that has at least one property that changes when exposed to the parameter of interest, and in so doing causes the cantilevered structure to deflect in a direction within a plane in which the cantilevered structure lies. The term bimorphic effect is commonly used to refer to such a bending effect in a single beam formed by two active layers, and this term will be used herein to more generally refer to a bending effect that a combination of dissimilar materials will cause in a cantilevered structure in response to exposure to a parameter of interest. In particular, the dissimilar materials are selected to have mismatched expansion/contraction (elongation/shrinkage) responses to a parameter of interest, resulting in the cantilevered structure bending toward the beam that shrinks more or does not elongate as much as the other beam. If the parameter is at a sufficiently high (threshold) level, the deflection of the cantilevered structure is sufficient to open or close (depending on the operating mode of the sensing device) an electrical contact associated therewith, which can serve to interrupt or allow, respectively, the transfer of a charge or electrical current, generation of an electrical voltage, or provide another form of output capable of corresponding to a digital signal. Contact between the cantilevered structure and an electrical contact is referred to herein as a contact-mode switching function or contact mode operation, and is preferably non-latching, in other words, other than as a result of its bending or deflection, the cantilevered structure is not mechanically latched or otherwise secured to the contact.

In certain embodiments, the property change in the beam responsive to the parameter (hereinafter, the “sensing beam”) is temporary and reversible, for example, if the result of moisture (moisture-induced expansion or contraction) or temperature (thermal expansion or contraction), and the resulting response (deflection) of the cantilevered structure is temporary. In certain other embodiments, the property change in the sensing beam alters the intrinsic stress of the sensing beam due to a curing or absorption process that occurs within the material of the sensing beam. For example, the sensing beam can be formed of an organic material, in which shrinkage or expansion or an increase in hardness of the material occurs that is caused by absorption of an environmental agent or by cross linking of polymer chains within the material resulting from a chemical reaction that increases the average length and/or degree of cross linking between constituent oligomers of the material. Such property changes in an organic material of the sensing beam can be induced by a number of different parameters that may exist or occur within an environment, nonlimiting examples of which include temperature, moisture (humidity), electromagnetic radiation (as nonlimiting examples, visible light, ultraviolet (UV) radiation, etc.), nuclear material radiation (gamma, beta, neutron), and chemicals (as nonlimiting examples, gases, biological agents, etc.). In most instances in which the property change in the sensing beam is the result of curing of an organic material in the sensing beam, the curing of the organic material will be irreversible and the response (deflection) of the cantilevered structure will tend to be permanent. Because curing and absorption are commonly time-dependent processes, the property change in the material and the resultant response of the cantilevered structure will also depend on the level of the parameter within the environment and the total exposure to the parameter over time, referred to herein as a cumulative time-parameter level combination, for example, a cumulative time temperature combination if heat is the parameter of interest. It is foreseeable that the response (deflection) of the cantilevered structure could be temporary or reversible. For example, another beam of the cantilevered structure could be formed of a material in which a property change can be induced to cause the cantilevered structure to deflect in a direction lying in the plane of the beams, but opposite the direction caused by the property change induced in the sensing beam by the property of interest. Alternatively, the curing of the material of the sensing beam could be reversible through a thermal or chemical treatment, causing the cantilevered structure to at least partially return to its original orientation relative to the contacts. Similarly, in some instances absorption of an environmental agent, for example, a chemical, particles, moisture, etc., may be reversed by an appropriate desorption process, causing the cantilevered structure to at least partially return to its original orientation relative to its contacts.

FIG. 1schematically represents an example of an electromechanical device-based sensor10in the form of a radio frequency identification (RFID) tag that may contain one or more arrays of electromechanical sensing devices comprising sensing elements of the types discussed above.FIG. 1represents components of the sensor10as including a substrate12that carries a sensor package14containing the sensing devices. The sensor10is also represented as including electronic circuitry, represented inFIG. 1as including, but not limited to interface electronics16, an RFID front-end transceiver20, and an antenna22. The interface electronics16may be adapted to digitally process the outputs generated by the sensing elements of the package14to produce a sensor output that can be wirelessly transmitted by the transceiver20. Alternatively, digital processing of the outputs of the sensing devices may not be necessary, in which case the interface electronics16may be adapted to simply receive the outputs of the sensing devices and relay these outputs to the transceiver20as the sensor output. Over any period of time during the operation of the sensor10, the sensor output may be reported as a cumulative output indicating the responses of the sensing devices to the parameter of interest over time.

The substrate12of the sensor10can be of any suitable construction and material, such as those currently used in RFID and/or electronics technologies, and therefore will not be discussed in any detail here. Other than as noted below, the transceiver20and antenna22can also be of known construction and design, and therefore will only be discussed to the extent necessary for those skilled in the art to understand and implement various embodiments of the invention. Wireless communication between the sensor10and a suitable wireless interrogator (reader unit) may be through a passive RFID communications protocol, though other wireless protocols are also foreseeable. RFID standards (nonlimiting examples of which include NFC, ISO-18000-3, ISO 18000-6, UHF Gen2, ISO-15693) support simultaneous data collection by a single RFID interrogator from multiple sensors having unique electronic ID codes, enabling more than one sensor10to be used in a monitoring system or network without requiring a battery. From the following discussion, it will become apparent that not all components depicted inFIG. 1are required by the invention, and additional components could be added. As a nonlimiting example,FIG. 1shows an optional battery18included in the sensor10to extend the wireless communication range as commonly known in the industry.

The sensor package14ofFIG. 1may contain one or more arrays of electromechanical sensing devices. The sensor package14and its sensing devices are preferably configured to provide certain advantages particular to the present invention. The sensing devices are preferably fabricated on a substrate and enclosed with a capping wafer that provides access to the environment as may be required by the sensing devices. Though not shown, the interface electronics16may also be enclosed within the package14.

A nonlimiting example of an electromechanical sensing device26capable of use with the sensor10ofFIG. 1is represented inFIGS. 2A, 2B, 3A, 3B, 4A, and 4B. The sensing device26is representative of one of any number of sensing devices26within the package14. The device26is represented as having a cantilevered structure30, which serves as a moving sensing element of the sensing device26. The cantilevered structure30is represented as comprising first and second beams44and46that are formed so that one of the beams44/46contains at least one material that is dissimilar from at least one material within the other beam44/46. The beams44and46of the nonlimiting embodiment ofFIGS. 2A-4Bare represented as being side-by-side, parallel, spaced apart so as to define a gap50there between along their lengths, and lying in a single plane that contains the entire cantilevered structure30. One end of the cantilevered structure30is affixed to or integrally formed with an anchor34, as a nonlimiting example, fabricated as a feature on a conventional CMOS circuit substrate in which the interface electronics16may also be fabricated. The opposite cantilevered (distal) end of the cantilevered structure30is shown as being suspended in proximity to a set of contacts40and42. The adjacent distal ends of the beams44and46are shown as being bridged or otherwise joined by a connector48, which in the embodiment ofFIGS. 2A-4Bis represented as an extension of the beam46that extends transverse to the parallel direction of the beams44and46. Alternatively, the connector48may be a separate structure and/or formed of a material that is different from the materials used to fabricate the beams44and46.

The device26and its cantilevered structure30may be fabricated directly on an integrated circuit substrate (e.g., CMOS wafer) in which electronic devices of the sensor10can also be formed. An alternative is to fabricate the device26and its cantilevered structure30on a separate substrate that is subsequently electrically coupled or bonded to a substrate. It can be readily appreciated that the cantilevered structure30of the sensing device26is simple and compatible with post-CMOS processing, and that very large, high-density arrays of the sensing device26can be fabricated in a very small area. It is foreseeable that structures other than cantilevered beams could be employed that are capable of responding to an environmental parameter of interest by closing and/or opening electrical contacts.

The dissimilar materials of the beams44and46are chosen to cause the cantilevered structure30to bend or deflect in response to an environmental parameter of interest. As noted above, the individual responses of the cantilevered structure30to an environmental parameter of interest may be referred to as bimorphic in the following discussion, though it should be understood that a strictly bimorphic cantilevered structure is not required, in other words, the cantilevered structure30do not require two active beams, and instead may have a single active beam or more than two active beams. The device26can be configured to sense a wide variety of different environmental parameters to which the sensor10might be subjected, nonlimiting examples of which include temperature, moisture/humidity, electromagnetic radiation, nuclear particle radiation, chemicals, biological agents, etc., as previously noted. Such capabilities can be achieved by using appropriate materials to form the cantilevered structure30of the sensing device26, as will be understood from the following discussion.

As previously noted, the dissimilar materials of the beams44and46are selected to have mismatched expansion/contraction (elongation/shrinkage) responses to a parameter of interest, resulting in the cantilevered structure30bending toward the beam44or46that shrinks more or does not elongate as much as the other beam46or44. In the nonlimiting embodiment ofFIGS. 2A-4B, in which the cantilevered (distal) end of the cantilevered structure30is suspended between a set of contacts40and42, the cantilevered structure30may deflect in either of two directions to contact and close (or break contact and open) either contact40and42. Due to the side-by-side arrangement of the beams44and46, the deflection of the cantilevered structure30is in a direction within a plane in which the cantilevered structure30and its beams44and46lie, as evident from comparingFIGS. 2A and 2BwithFIGS. 3A and 3BandFIGS. 4A and 4B. The gap50defined by and between the beams44and46is depicted as being uniform in its width along the lengths of the beams44and46as a result of the beams44and46being side-by-side and parallel. The presence of the gap50results in the beams44and46not being in direct contact with each other along their entire lengths, with only the distal ends being interconnected through the connector48. As such, the active length portions of the beams44and46that produce the bimorphic effect do not directly contact or interact with each other.

According to a preferred aspect of the invention, the sensing device26is an electromechanical structure that functions as a switch in response to one or more environmental parameters of the environment surrounding the sensor10. As such, the sensing device26is able to extract the energy needed for mechanical switching from the environment itself, thereby drastically reducing the power required to sense an environmental parameter. The mechanical switching operations of an array of the sensing device26are inherently digital and can be converted to an electrical signal using, for example, simple compact front-end circuitry. Such circuitry may make use of a minimal number of transistors and dissipate less than a few or tens of picowatts per sensing device26, resulting in a total electrical power dissipation from the sensor10on the order of ones or tens of nanowatts when the sensor10is placed in the electrical field of a wireless interrogator (for battery less operation) or powered (if needed or desired) by an on-sensor battery or other energy source (if included). As such, a sensor10utilizing one or more of the sensing device26is capable of operating in a manner that avoids the limitations of many existing IC-based sensors that are designed to operate in a battery less configuration in which the sensor10is powered and its sensor output is transmitted through a wireless link when in the electrical field of a wireless interrogator (e.g., an RFID interrogator). Even if designed for lower power consumption, existing IC-based sensors are incompatible for continuous monitoring of environmental parameters over a period of a few years if relying on the energy capacity of existing miniature batteries. In combination, these features significantly decrease the complexity of the sensor10and its electronics to attain reductions in size, cost, and power not attainable with current commercial embodiments of environmental sensors.

With reference again toFIGS. 2A-4B, the combined effect of the beams44and46and their dissimilar materials is to cause the cantilevered structure30to bend when subjected to the environmental parameter as a result of a property of the material of at least one of the beams44and46changing relative to the corresponding property of the other beam44or46within the cantilevered structure30. As noted above, a preferred aspect of the invention is that at least one of the beams44and46is a sensing beam that is entirely formed of or contains a dissimilar material having one or more properties that reversibly or irreversibly changes as a result of exposure to a parameter of interest in the environment to which the device26is subjected. For convenience, the beam44represented in the drawings will be referred to as the sensing beam44and the beam46will be referred to as a second beam46, though it should be understood that the locations and number of sensing and second beams44and46may be changed within the cantilevered structure30.

Various materials are capable of exhibiting reversible or irreversible changes in response to a parameter present in an environment, including metals (including metal alloys), ceramic materials, and organic materials, and therefore such materials may be candidates for use as the dissimilar material of the sensing beam44. If the dissimilar material is curable, nonlimiting examples of suitable organic materials include thermoplastic and thermoset materials that undergo curing when subjected to an environmental parameter that acts as a curing stimulus to the organic material, nonlimiting examples of which include electromagnetic radiation, chemicals, biological agents, temperature, moisture/humidity, nuclear particle radiation, etc. Particular organic materials believed to be suitable for use include epoxies, silicone compounds, etc.

Suitable materials for the second beam46(and, in some cases, additional beams) may depend on the composition of the dissimilar material of the sensing beam44and whether the response of the cantilevered structure30is intended to be reversible or irreversible. Materials for the second beam46may be referred to as inert, meaning that the material undergoes less change relative to the sensing beam44, and in some cases no change, when subjected to the environmental parameter, i.e., the stimulus that induces a property change in the material of the sensing beam44. For example, if the parameter is temperature the material for the second beam46would be chosen on the basis of having a different (e.g., lower) coefficient of thermal expansion (CTE) than the sensing beam44, and if the parameter is humidity the material for the second beam46would be chosen on the basis of having a different (e.g., lower) coefficient of moisture expansion (CME) than the sensing beam44. In other cases in which the change is to be irreversible, suitable inert materials for the second beam46undergo less and in some cases no curing or absorption relative to the sensing beam44when subjected to the environmental parameter. Notable but nonlimiting examples of inert materials include metals (including metal alloys), nonmetals (including silicon, silicon-germanium alloys, electrically non-conductive materials such as silicon dioxide and silicon nitride), and certain polymers.

Though curing is often considered to be irreversible, in situations in which the response (deflection) of the cantilevered structure30utilizing a curable sensing beam44is desired to be temporary or reversible, the property change in the sensing beam44may be reversible, for example, through a thermal or chemical treatment, causing the cantilevered structure30to at least partially return to its original orientation relative to its contacts40and42. Alternatively, the second beam46(or another layer within the cantilevered structure) of the cantilevered structure30could be formed of a material in which a property change can be induced to cause the beam to deflect in a direction normal to the beams44and46, but opposite the direction caused by the property change induced in the sensing beam44by the parameter of interest. Alternatively, it is foreseeable that the cantilevered structure30could be reset by using one or more sensing beams44and one or more second beams46that in combination are able to at least partially reverse the deflection of the cantilevered structure30by exposing the beams44and46to one or more different environmental parameters.

As understood by those skilled in the art, the cantilevered structure30could include additional layers/films, such as stress compensation layers to improve the distribution of any processing-induced strain within the cantilevered structure30. It is also within the scope of the invention to fabricate either or both beams44and46of the cantilevered structure30to comprise layers that can be patterned for the purpose of modifying the beams44and/or46, including their responses to the environmental parameter being sensed, such as temperature, humidity, chemicals, electromagnetic and particle radiations, UV light, and/or other environmental conditions.

While the beams44and46are shown as being positioned side-by-side, parallel to each other, spaced apart from each other so as to define a gap50of uniform width there between along their lengths, and lying in a single plane that contains the entire cantilevered structure30, other configurations are possible if the end result is the ability for the dissimilar materials of the beams44and46to induce deflection in the cantilevered structure30. Furthermore, while the side-by-side arrangement of the beams44and46yields a horizontal cantilevered structure (which as used herein means that the beams44and46are arranged side by side in a direction parallel to the surface of a substrate above which the cantilevered structure30is supported by the anchor34), it should be understood that the beams44and46could be arranged one above the other to yield a vertical cantilevered structure (again, relative to the surface of a supporting substrate).

In any case, the cantilevered structure30moves in response to the external environmental parameter (stimulus) relative to its contacts40and42, in one operating mode toward one of the contacts40or42if the cantilevered structure30is initially separated from both contacts40and42(each set initially constituting an open electrical path), or in a second operating mode away from one of the contacts40or42if the cantilevered structure30initially contacts that contact40or42(which therefore creates an initially closed electrical path). Depending on the operating mode, closure or opening of the contacts40or42results from the environmental parameter having been at or above a sufficient level for a sufficient amount of time to cause a sufficient change in a property of the sensing beam44that leads to bending of the cantilevered structure30and contact with the contacts40or42. The direction of deflection is determined by the location of the sensing beam44among the beams44and46within the cantilevered structure30and the response of the sensing beam44to the environmental parameter resulting from the particular property change in the sensing beam44.

WhereasFIGS. 2A and 2Brepresent a null position of the sensing device26in which the cantilevered structure30does not make contact with either contact40or42,FIGS. 3A and 3Brepresent the sensing device26at one extreme of its operating range andFIGS. 4A and 4Brepresent the sensing device26at an opposite extreme of its operating range, indicative of two threshold conditions. The condition of the cantilevered structure30as represented inFIGS. 3A and 3Bmay be the result of only the beam44contracting, only the beam46expanding, the beam44contracting and the beam46expanding, both beams44and46contracting but the beam44exhibiting greater contraction, or both beams44and46expanding but the beam46exhibiting greater expansion as a result of the structure30being exposed to an environmental parameter of interest. Similar circumstances can be ascribed to the condition of the cantilevered structure30as represented inFIGS. 4A and 4B, but with the conditions of the beams44and46reversed.

InFIGS. 3A and 3B, exposure to the parameter of interest has caused the cantilevered structure30, initially separated from both contact40and42as seen inFIGS. 2A and 2B, to contact the contact42to form a closed electrical path. Alternatively,FIGS. 3A and 3Bcould be described as depicting the second operating mode in which the cantilevered structure30is initially in contact with the contact42to form a closed electrical path, and the effect of the parameter would be to cause the cantilevered structure30to deflect to the left, breaking contact with the contact42and create an open electrical path as shown inFIGS. 2A and 2B. InFIGS. 4A and 4B, exposure to the same or different parameter of interest has caused the cantilevered structure30, initially separated from both contact40and42as seen inFIGS. 2A and 2B, to contact the left contact40to form a closed electrical path. Alternatively,FIGS. 4A and 4Bcould be described as depicting the second operating mode in which the cantilevered structure30is initially in contact with the contact40to form a closed electrical path, and the effect of the parameter causes the cantilevered structure30to deflect to the right, breaking contact with the contact40to create an open electrical path as shown inFIGS. 2A and 2B. As such, it should be understood that the null position depicted inFIGS. 2A and 2Bmay be the result of the presence or absence of a parameter in an environment.

Either the contacts40and42or the cantilevered structure30may be connected to a power source, for example, the battery18ofFIG. 1or a capacitor (which may be an integrated component of the interface electronics16), such that closure of either contact40or42serves to transfer a charge or electrical current, generate an electrical voltage, or provide another form of output capable of corresponding to a digital signal. Because the cantilevered end of the cantilevered structure30is desired to electrically connect with the contacts40and/or42, at least part of the cantilevered structure30is formed of an electrical conductor. As a nonlimiting example, the embodiment ofFIGS. 2A-4Bdepicts the second beam46and the connector48as making contact with the contacts40and42, and therefore the second beam46and connector48are both formed entirely or at least partially of an electrically conductive material. Alternatively, the embodiment ofFIGS. 5A-5Crepresents both beams44and46and the connector48as being formed of electrically non-conductive materials, in which case the cantilevered structure30includes a separate electrically conductive layer or component52for making contact with the contacts40and42.

The sensitivity of the cantilevered structure30, in other words, the extent to which the cantilevered ends of the cantilevered structure30(adjacent the contacts40and42) will deflect when subjected to a given level of the parameter, will depend on the compositions of the beams44and46(including any additional beams) that make up the cantilevered structure30and certain geometric characteristics of the cantilevered structure30. The sensitivity of the cantilevered structure30of any given device26can be analytically predicted and controlled based on structure geometries and material properties, including the thickness and the Young's modulus of elasticity of each beam44and46. In this manner, the device26can be configured to have a cantilevered structure30that performs a switching function at a different level (threshold) of the parameter relative to the cantilevered structure30of other device26of the sensor10. Furthermore, an array of sensing devices26can contain one or more individual devices26whose cantilevered structures30are intentionally of different lengths, with longer cantilevered structure30being more sensitive to the parameter and resulting in contact with one of the contacts40and42at progressively smaller parameter changes with increasing beam lengths. Scaling of the feature sizes of the cantilevered structure30improves the achievable measurement resolution in addition to the die size reduction. While the movement of the cantilevered end of a cantilevered structure30relative to its contacts40and42will depend in part on the length of the cantilevered structure30, sensitivity is independent of the beam thickness such that the thicknesses of the cantilevered structures30within an array of devices26can be minimized to reduce the size of the array to the extent that manufacturing reliability allows. If an array contains a large number of sensing devices26, the package14of the sensor10can be fabricated to have a large redundancy of sensing devices26that enhances yield without any noticeable cost penalty. For example, significant yield enhancements can be achieved by fabricating the sensing device26in large arrays, and then selecting only a subset of devices26from each array for actual use by the sensor10to perform the sensing function.

As previously noted, the closing or opening of the contacts40and42of a device26by its cantilevered structure30may provide a direct indication of a cumulative time-level combination based on the duration that the cantilevered structure30was subjected to the environmental parameter at or above a level that initiates a property change (e.g., as a result of curing, absorption, etc.) of an organic material. As such, the sensor10is able to process the digital outputs of its devices26to not only generate data corresponding to levels of a parameter (for example, temperature), but also data corresponding to the duration of exposure to a parameter. The sensor10or the system/network with which it communicates may also be operable to combine or integrate the level and duration data obtained from its different device26according to a mathematical model.

As a result of the responses of the cantilevered structure30to an environmental parameter resulting in an open or closed electric contact that subsists regardless of subsequent levels of the parameter within the environment, the sensing devices26of the sensor10are effectively store digital data generated by the devices26even if there is no external power supplied to the sensor10for extended periods of time. In this manner, the sensor10is particularly well suited for long-term tracking and recording of one or more environmental parameters.

In a particular nonlimiting embodiment, the beams44and46of the cantilevered structures30of an array of sensing devices26are constructed of metallic and/or silicon layers. In such an embodiment, the sensing devices26can be fabricated using MEMS processes.FIGS. 6, 7 and 8set forth steps that may be performed in three nonlimiting examples of MEMS processes.FIG. 6represents an example process flow for fabricating a cantilevered structure30when silicon and an electrically-conductive metallic structure are used to form the beams44and46, respectively.FIG. 7represents an example process flow for fabricating a cantilevered structure30when silicon and a non-electrically conductive material are used to form the beams44and46.FIG. 8represents an example process flow for fabricating a cantilevered structure30using a surface micromachined MEMS process. Those skilled in the art will appreciate that other MEMS fabrication processes and variations of these MEMS processes could be used to fabricate sensing devices26of types described above.

Under certain circumstances, sensing devices26fabricated with cantilevered structures30as described above may be exposed to levels of a parameter that far exceed the level required for the structures30to contact one of their contacts40or42. The resulting additional force to which a cantilevered structure30and its contacts40and42are subjected may in some cases result in irreversible (plastic) deformation of the cantilevered structure30that may negatively affect its mechanical switching properties or reliability over time. To address this possibility,FIGS. 9A, 9B, and 9Cschematically represent an embodiment of a sensing device26that employs one or more elastic contact structures140and142, in which the contact40and42are suspended between a pair of flexible support beams152, each connected to an anchor154to enable the contacts40and42to deflect with their corresponding cantilevered structures30in the event of a large contact force imposed by the structures30.FIG. 9Adepicts the cantilevered structure30of the sensing device26in a null position in which the structure30contacts neither contact40or42, andFIG. 9Bdepicts the cantilevered structure30in a contact position in which the structure30has made initial contact with the contact42.FIG. 9Crepresents the cantilevered structure30in an extreme position resulting from being subjected to an excessive level of the parameter being sensed, and evidences that the contact42is free to deflect with the cantilevered structure30within the plane containing the beams44and46to avert permanent damage to the structure30when subjected to potential extreme conditions over its operating range. This capability relieves the contact force and reduces if not avoids potential irreversible deformation or damage to the cantilevered structure30and contacts40and42. Other aspects of the sensing devices26represented inFIGS. 9A-9Cand not discussed in any detail here can be, in terms of structure, function, materials, etc., essentially as was described for the previously-described embodiments.

While the invention has been described in terms of particular embodiments, it should be apparent that alternatives could be adopted by one skilled in the art. For example, the sensing devices and their components could differ in appearance and construction from the embodiments described herein and shown in the drawings, functions of certain components of the devices could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, process parameters could be modified, and appropriate materials could be substituted for those noted. As such, it should be understood that the above detailed description is intended to describe the particular embodiments represented in the drawings and certain but not necessarily all features and aspects thereof, and to identify certain but not necessarily all alternatives to the represented embodiments and described features and aspects. As a nonlimiting example, the invention encompasses additional or alternative embodiments in which one or more features or aspects of a particular embodiment could be eliminated or two or more features or aspects of different embodiments could be combined. Accordingly, it should be understood that the invention is not necessarily limited to any embodiment described herein or illustrated in the drawings, and the phraseology and terminology employed above are for the purpose of describing the illustrated embodiments and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims.