PATENT ABSTRACT
Apparatuses, systems, and methods utilizing capillary action and to control the movement or placement of liquids or other materials in micro-devices and nano-devices. In some embodiments, the present invention may be used to control polymer addition to micro-cantilevers and nano-cantilevers for biological sensing, chemical sensing, and other sensing. In other embodiments, the present invention may be used to deliver adhesives, dielectrics, chemo resistor materials, and other materials to micro-devices and nano-devices.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/810,995, filed Jun. 5, 2006, which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY-SPONSORED  RESEARCH AND DEVELOPMENT 
     This invention was made, in part, with government support under Grant Number F49620-02-0359 awarded by AFOSR MURI and Grant Number 200-2002-00528 awarded by NIOSH/CDCP. The United States government may have certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed generally to apparatuses, systems, and methods utilizing capillary action and, more specifically, to methods and apparatuses to control the movement or placement of liquids or other materials in micro-devices and nano-devices. 
     BACKGROUND OF THE INVENTION 
     Single-chip electronic noses, enabled by full on-chip integration of gas chemical microsensors with signal-conditioning electronics have tremendous medical, environmental and safety applications. Gravimetric detection is an important sensing modality for these microsystems. 
     Commercially available mass-sensitive devices for volatile organic compound detection use piezoelectric quartz substrates. Thickness shear mode resonators (TSMR), also known as quartz micro-balances (QMB) (Patel, R., Zhou, R. Zinszer, K., and F. Josse, “Real-time detection of organic compounds in liquid environments using polymer-coated thickness shear mode quartz resonators”, Analytical Chemistry, vol. 72, no. 20, p. 4888-4898, 2000) (Schierbaum, K. D., Gerlach, A., Haug, M., and W. Gopel, “Selective detection of organic molecules with polymers and supramolecular compounds: application of capacitance, quartz microbalance, and calorimetric transducers”, Sensors and Actuators A, 31, p. 130-137, 1992), and Rayleigh surface acoustic wave (SAW) devices (Ricco, A. J., Kepley, L. J., Thomas, R. C., Sun, L., and R. M. Crooks, “Self-assembling monolayers on SAW devices for selective chemical detection”, IEEE Solid-State Sensor &amp; Actuator Workshop, Hilton Head, S.C. June 22-25, p. 114-117, 1992) are examples of such devices. However, these piezoelectric devices have not been fully integrated with on chip electronics. In contrast, resonant cantilever chemical microsensors integrated with CMOS have been demonstrated (A. Hierlemann and H. Baltes, “CMOS-based chemical microsensors”, Analyst, 128, p. 15-28, 2003). Prior work on cantilever mass sensors includes detection of humidity, mercury vapor, and volatile organic compounds (Lange, D., Hagleitner, C., Hierlemann, A., Brand, O., and H. Baltes, “Complementary Metal Oxide Semiconductor Cantilever Arrays on a Single Chip: Mass-Sensitive Detection of Volatile Organic Compounds”, Analytical Chemistry, vol. 74., no. 13, p. 3084-3095, 2002) as well as biomolecular recognition in a liquid media (Fritz, J., Bailer, M. K., Lang, H. P., Rothuizen, H., Vettiger, P., Meyer, E., Guntherodt, H. J., Gerber, Ch., and J. K. Gimzewski, “Translating biomolecular recognition into nanomechanics”, Science, 288, p. 316-318, 2000.). Post-CMOS micromachining has been used to make fully integrated mass sensitive oscillators with pico-gram resolution (H. Baltes, D. Lange, A. Koll, “The electronic nose in Lilliput,” IEEE Spectrum, 9, 35, (1998)). These devices were formed through deposition of precise amounts of a chemically sensitive layer onto relatively wide cantilevers. 
     Another example of a CMOS-MEMS resonant gas sensor used electrostatic actuation and detection to form a free-running oscillator (S. S. Bedair and G. K. Fedder, “CMOS MEMS Oscillator for Gas Chemical Detection,” Proceedings of IEEE Sensors, Vienna, Austria, Oct. 24-27, 2004). A cantilever beam suspended a plate made large enough to accommodate drops of chemically sensitive polymer placed directly onto the plate using drop-on-demand ink jet deposition. Ink jet deposition can functionalize each cantilever in an arrayed structure with a separate polymer. This non-contact technology is scalable for large arrays, easy to use, versatile, and faster than other means of coating such as from micro-capillaries and drop casting from pipettes (A. Bietsch, J. Zhang, M. Hegner, H. P. Lang, and C. Gerber, “Rapid functionalization of cantilever array sensors by inkjet printing”, 2004 Nanotechnology 15 873-880). Other thin film application methods include dip pen and shadow mask processing which are both time consuming processes. 
     Another prior microfluidic system is described in U.S. patent application, 20050064581 and in a corresponding paper (T. P. Burg, A. R. Mirza, N. Milovic, C. H. Tsau, G. A. Popescu, J. S. Foster and S. R. Manalis, “Vacuum-packaged suspended microchannel resonant maass sensor for biomolecular detection,” J. Microelectromechanical Systems, December 2006). These prior art documents describe an enclosed microchannel. Material is flowed into the channel to functionalize sidewalls of the channel to capture biomolecules on the sidewalls. However, the channel in these works is not used or taught as a wicking structure for deposition of a non-liquid material, such as polymers, that fills or partly fills the channel. Specifically, the patent application describes a microfluidic channel to detect analyte that may have a liquid or gel in the channel. The analyte is flowed into the microchannel. The gel may be delivered by pressure flow or electrophoresis, but no description or teaching of gel deposition through wicking is provided. The invention requires an enclosed microchannel for analyte delivery through flow, and in order to package in vacuum. 
     It is beneficial to further scale down the size of the resonant microstructure to achieve an increased mass sensitivity and reduced cost. Scaling cantilevers down to micro- and nano-scale dimensions is achievable with optical or piezoresistive resonant detection. However, microstructures with low-noise electrostatic actuation and detection require narrow air gaps that are generally incompatible with existing polymer deposition techniques. 
     Accordingly, there is a need for improved apparatuses and methods to control polymer addition to micro-cantilevers and nano-cantilevers for biological and chemical sensing. Those and other advantages of the present invention will be described in more detail hereinbelow. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed generally to apparatuses, systems, and methods utilizing capillary action. The present invention has many applications and many variations. For example, the present invention may be used in the operation of sensors. The present invention may be used to fill a space between two or more parts. In some embodiments, the present invention may be used to carry an adhesive to a desired location to fix two or more parts together. In other embodiments the present invention may be used to provide a dielectric between two or more electrodes or contacts. In another embodiment, the present invention may be used in a chemo resistor device in which electrically resistive material is positioned between two or more electrodes or contacts. The present invention may also be used with a mass sensor for gas chemical sensing applications. The present invention may also be used with many different fluids and materials, and in many specific applications such as to control material addition to micro-cantilevers and nano-cantilevers for biological sensing, chemical sensing, and other sensing. 
     In one specific embodiment, the present invention will be described in terms of apparatuses and methods to mass load a microstructure with polymer without affecting nearby gaps. Precise amounts of polymer or other materials, which may be suspended in solution, are wicked onto the microstructure through capillary action of micro-grooves formed along the length of the beam. The polymer or other material is left on the microstructure after drying of the solvent. Scaling down the mass of the mass sensitive cantilever leads to a higher mass sensitivity which leads to highly sensitive gas chemical sensing applications. The technique enables design of low-mass polymer-loaded cantilevers with electrostatic actuation and capacitive sensing for integrated gas chemical detector arrays. 
     The present invention may also include two or more devices formed in a single apparatus or on a single substrate. The single apparatus or substrate may contain several devices of the same type, such as to perform same test or operation many times. Alternatively, the single apparatus or substrate may contain several devices of different types, such as to perform a variety of different tests. In some embodiments, the devices receive different materials in their respective target areas, and in some embodiments they receive the same materials. As a result, redundant or different testing, sensing, or other functions may be performed on a single structure. 
     Although the present invention will generally be described in terms of specific embodiments, many variations, modifications, and other applications are possible with the present invention. These and other teachings, variations, and advantages of the present invention will become apparent from the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings for the purpose of illustrating the embodiments, and not for purposes of limiting the invention, wherein: 
         FIG. 1  illustrates one embodiment of an apparatus according to the present invention. 
         FIGS. 2   a - 2   h  illustrate embodiments of wicking devices according to the present invention. 
         FIG. 3   a  illustrates one embodiment of a gravimetric micro-cantilever resonant sensor according to the present invention. 
         FIGS. 3   b  and  3   c  illustrate cross-sectional views along lines IIIb-IIIb and IIIc-IIIc, respectively, in  FIG. 3   a.    
         FIGS. 4   a  and  4   b  illustrate an embodiment of an electrostatically actuated resonator with differential comb drive and sensing electrodes. 
         FIG. 5  illustrates one embodiment of a solution delivery to the resonator. 
         FIG. 6  illustrates one embodiment of an oscillator gas sensor schematic. The electrostatically actuated resonator is placed in a feedback loop with off-chip electronics for oscillation. 
         FIG. 7  illustrates one embodiment of post CMOS processing steps: (a) CMOS chip from foundry, (b) Reactive-ion etch of dielectric layers, (c) DRIE of silicon substrate, (d) isotropic etch of silicon substrate, (e) Ink jet deposition of polymer solution. 
         FIG. 8  illustrates an example of a beam pinned under stator electrodes after direct ink jet deposition onto cantilever. 
         FIG. 9   a - 9   d  illustrate one embodiment of a device before and after solution deposition in the well:(a) entrance from well to micro-channel which extends along the length of the resonator, (b) resonator at the base with polystyrene, (c) tip of resonator without polystyrene, (d) tip of resonator with polystyrene. 
         FIG. 9   e  illustrates frequency response of one embodiment of a device before and after polystyrene deposition into a 2 μm channel in the device. 
         FIGS. 10   a - 10   g  are scanning electron micrographs of several embodiments of the present invention. 
         FIG. 11  illustrates one embodiment of a gas test setup according to the present invention. 
         FIG. 12  illustrates spectrum analyzer output of resonant frequency shifts due to ethanol, IPA, and acetone gas flows according to one embodiment of the present invention. 
         FIGS. 13   a  and  13   b  illustrate another embodiment of the present invention in which one or more channels are used to provide an adhesive to secure two objects together. 
         FIGS. 13   c  and  13   d  illustrate another embodiment of the present invention in which a suspended beam is used for form a space or gap. 
         FIGS. 14 and 15  illustrate another embodiment of the present invention in which parts of an object or layer are joined with a material according to the present invention. 
         FIG. 16  illustrates one embodiment of a system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be generally described in terms of specific examples and embodiments, although many variations and modifications are possible. For example, although the present invention is sometimes described in connection with the use of polystyrene, many other materials may be used with the present invention. For example, the present invention may be used with materials that can be delivered with solvents, and other materials in solution and fluids. For example, these materials can be active polymers, nano particles, polymer composites, biomolecules, solgel, electro- and magneto-active polymers, adhesives, and sealants. Furthermore, the present invention may be constructed in scales other than those specifically defined herein. For example, specific dimensions are provided in some examples, although smaller devices may be desirable to provide additional sensitivity in some applications, and larger devices may be desirable in other applications. Similarly, the use of the term “micro”, such as in “micro-cantilever”, “microstructure”, “micro-capillary”, and others, is illustrative and is not a limitation of the present invention. For example, the present invention may also be used at nano-scales or at smaller or larger scales. 
       FIG. 1  illustrates one embodiment of an apparatus according to the present invention. In this embodiment the apparatus  10  is a sensor, although the apparatus of the present invention may take other forms, such as in assembling parts for microelectromechanical systems, providing adhesive, sealant, or other material between two or more parts, and in other applications. 
     In  FIG. 1  the sensor  10  includes wicking device  12 , a fluid well  14 , motion sensors  16 , actuators  18 , and a fluid dispenser  19 . 
     The wicking device  12  carries a fluid along a channel (not shown) in the wicking device  12  via capillary action. The wicking device  12  may be an elongated structure, such as a straight beam, or it may be a curved structure, or it may have other shapes. In some embodiments the wicking device is cantilevered, although it is not required to be cantilevered. The channel will be described in more detail hereinbelow and may take different forms, such as a gap between two surfaces, a groove or recess formed in a surface, or a passage through an object. In some embodiments, the material is deposited in the fluid well  14  and is wicked into the wicking device  12 , so that the deposited material does not interfere or contact any other parts of the devices, such as the motion sensors  16  or the actuators  18 . 
     In some embodiments the wicking device  12  will be suspended. As used herein, a “suspended” wicking device  12  means a significant portion of the wicking device  12  is surrounded by air, or void, or ambient conditions other than structural elements used to support the wicking device  12 . For example, in some embodiments the wicking device  12  is in the form of a cantilevered beam supported at one end and suspended in air (or in other conditions) for most of its length. In other embodiments, the wicking device  12  is in the form of an object or layer formed over a recess, in which at least a portion of the object or layer is suspended over the recess. In some embodiments, the wicking device  12  can be formed from two or more parts or pieces, and in some cases all parts or pieces are suspended, and in other cases some parts or pieces are suspended and other parts or pieces are not suspended. Although several embodiments of the present invention will be described in terms of a suspended wicking device, advantages of the present invention may also be realized with wicking devices  12  that are not suspended in any way. 
     The fluid well  14  is the source of the fluid that is carried along the channel in the wicking device  12  via capillary action. The fluid well  14  is significantly larger than the channel in the wicking device  12  and directly receives the fluid, such as through an ink jet deposition or through other means such as micro capillaries and pipettes, dip pen, and shadow mask processing. The fluid well  14  will sometimes be referred to as a “target area”, “target well area”, and other names. These terms are interchangeable. 
     The motion sensors  16  detect motion of the wicking device  12 . The motion sensors  16  are not required in the present invention, and some embodiments illustrated herein do not use the motion sensors  16 . 
     The actuators  18  cause the wicking device  12  to move. The actuators  18  may, for example, cause motion through the application of electrostatic forces, or through other means. The actuators  18  are not required in the present invention, and some embodiments illustrated herein do not use the actuators  18 . 
     The fluid dispenser  19  is oriented to dispense fluid into the fluid well. The fluid dispenser  19  may be, for example, drop-on-demand ink jet device, or a micro-pipette device, or a dip pen device, or it may be other forms of fluid dispensers  19 . Unlike the prior art, the present invention dispenses or deposits the fluid into a fluid well  14 , from which it is wicked onto the channel  20  or channels of the wicking device  12 . Many variations are possible with the fluid dispenser  19 . For example; there may be a dedicated fluid dispenser  19  for each fluid well, or one fluid dispenser  19  may be used with more than one fluid well  14 , such as by moving the fluid well  14 , moving the fluid dispenser  19 , or otherwise changing the orientation of the fluid well  14  and the fluid dispenser  19 . In some embodiments of the present invention, the fluid dispenser  19  is integrated into the device including the fluid well  14 , and in other embodiments the fluid dispenser  19  is separate from the rest of the device and is engaged with the apparatus  10  when it is needed. 
     The sensor  10  illustrated in  FIG. 1  is a gravimetric sensor in which the actuators  18  cause the wicking device to move, and the motion sensors  16  measure the movement. The frequency response of the wicking device is indicative of the mass, and the distribution of mass, of the wicking device  12  and any material deposited or absorbed on the wicking device  12 . The frequency response of an empty wicking device  12  can be established, so that any change in the response is indicate of the additional material added to the wicking device  12 . In this way, the mass of material deposited on the wicking device can be determined. A mass sensitive material can be deposited onto the wicking device. In this way, the mass of an additional material absorbed into the mass sensitive material on the wicking device can be determined. For example, the additional material to be determined can be a gas chemical analyte absorbed into a mass sensitive polymer. The present invention is not limited to use in gravimetric sensors, and may be used in other types of sensors  10 , such as chemo-resist sensors, capacitive sensors, or other types of sensors. The present invention can also be used in apparatuses other than sensors, as will be described in more detail hereinbelow. 
     Many variations are possible with the present invention. For example, the sensor  10  may or may not include motion sensors  16  and actuators  18 , or may contain a more or fewer motion sensors  16  and actuators  18  than shown herein. For example, a sensor  10  may include only one motion sensor  16  or actuator  18 , or it may include more than two motion sensors  16  and actuators  18 . Similarly, more than one fluid well  14  may be used, and more than one wicking device  12  may be used in the sensor  10 . More than one wicking device  12  may be used for each fluid well  14 . The sensor  10  may also include devices not shown in this figure, such as devices for applying and measuring electrical current and voltage, and other devices. In some embodiments, the sensor  10  includes sources of controlled electrical voltage or current, and devices for measuring one or more electrical characteristics, such as voltage, resistance, current, capacitance, and electro-magnetic fields. These embodiments may also include motion sensors  16  and actuators  18 , or the embodiments may exclude one or both of motion sensors  16  and actuators  18 . 
       FIG. 2   a  illustrates one embodiment of a wicking device  12  according to the present invention. The wicking device  12  includes a channel  20  formed in the wicking device  12 . The channel is in the form of a groove  20  and is defined by three surfaces of the wicking device  12 . 
       FIG. 2   b  illustrates another embodiment of the wicking device  12  in which the wicking device  12  is formed from two parallel plates and the channel  20  is defined by the space between the two plates. Although the plates are illustrated as being parallel, they may also be non-parallel. 
       FIG. 2   c  illustrates another embodiment of the wicking device  12  in which two plates are oriented vertically and the channel  20  is formed between the vertical plates. In other embodiments the plates may have other orientations, such as 45 degrees and others. 
       FIG. 2   d  illustrates another embodiment of the wicking device  12  in which one or more supports  22  are provided between the plates of the wicking device  12 . The supports  22  resist the forces applied to the plates from the capillary action of fluids between the plates. As a result, the supports prevent the plates of the wicking device  12  from bending inward and touching each other. 
     Supports may also be included in other orientations of channels, such as in  FIG. 2   c . Supports  22  are shown in  FIG. 2   d  as cylindrical and in the center of the channel, but supports can also be rectangular or other shapes and can be located at other locations in the channel. 
       FIG. 2   e  illustrates another embodiment of the wicking device in which at least one plate includes an opening  24 . One or more plates or other parts of the wicking device  12  may include one or more openings. The openings may be of any shape, spacing, and orientation. The opening  24  reduces the mass of the wicking device and, in some applications, allows for increased sensitivity. 
       FIG. 2   f  illustrates another embodiment of the wicking device  12  in which more than two plates are used. In that embodiment, two of the plates  26  are electrical conductors, and the other two plates  28  are electrical insulators. This embodiment may be used, for example, when the sensor is measuring capacitance with the fluid in the channel  20  of the wicking device  12 . 
       FIG. 2   g  illustrates another embodiment of the wicking device  12  in which the structure of the electrical conductor  26  and insulator  28  vary from the previous embodiment. In this embodiment, one of the electrical conductors  26  is embedded with one of the electrical insulators  28 . These and other variations are possible. 
       FIG. 2   h  illustrates another embodiment of the wicking device. In this embodiment, the channel  20  is formed in a more complex cross-sectional shape than in the previous embodiments. Also, there are several electrical conductors  26  and electrical insulators  28  on the walls of the channel  12 . This embodiment may be used, for example, to measure electrical resistance of the material in the channel  20 . Many combinations of measurements may be taken from the various electrical conductors  26 . In other embodiments, for example, different numbers of electrical conductors  26 , in different orientations, may be used. The electrical conductors  26  may be exposed to the channel  20  along the entire length of the channel, or the electrical conductors  26  may be exposed only in selected portions of the channel  20 , such as at the end or at other locations. 
     Many other variations of the wicking device  12  are also possible. For example, although the wicking devices  12  are shown as being open at their ends, they may also be capped or closed at the ends, and the wicking devices  12  may include different numbers, orientations, and structures of plates and other components forming the wicking device  12  and defining the channel  20 . In some embodiments, the channel  20  is tapered at the free end so as to wick more liquid to that end and, thereby, deposit addition material there. This results in non-uniform distribution of material, with more material at the free end where the device is more sensitive to mass. This embodiment, for example, may allow for the use of less liquid and less material while achieving greater sensitivity. 
     In another embodiment, a wider channel is used, thereby resulting in a larger volume that can be carried on wicking device  12 . However, if the same volume of liquid is used, the liquid and the material carried in the liquid will be driven to the free end of the wicking device  12 , resulting in all or most of the liquid and material at or near the free end of the wicking device  12 . As a result, there will be a non-uniform distribution of liquid and material along the wicking device  12 . 
     In another embodiment, the channel  20  may be non-linear. For example, the channel  20  may branch into several side channels, the channel  20  may include one or more “t” junctions, the channel  20  may include circular path components, the channel  20  may include a square spiral path component, and the channel  20  may include other path features and combinations of features. Similarly, the wicking device  12  may also have a shape other than a uniform, linear shape along its length. 
     Several embodiments of the present invention will now be described to illustrate the present invention. Those embodiments are illustrative, and not limiting. Other variations and embodiments of the present invention are also possible. 
     Micro-Cantilever Design 
       FIG. 3   a  is an image of an actual gravimetric micro-cantilever sensor  10  constructed according to one embodiment of the present invention.  FIGS. 3   b  and  3   c  illustrate cross-sectional views along lines and respectively, in  FIG. 3   a .  FIGS. 4   a  and  4   b  illustrate close up views of the beam microstructures forming the wicking device  12 , the motion sensors  16 , and the actuators  18 . The reservoir or target well area  14  for solution delivery is placed at the base of the cantilevered beam  12 , which is a movable structure in this embodiment. 
       FIG. 3   b  illustrates a cross-sectional view along line IIIb-IIIb of the invention illustrated in  FIG. 3   a .  FIG. 3   b  shows the wicking device  12  suspended above a substrate  36 . The substrate may be silicon, such as when semiconductor fabrication techniques are used to make the present invention. However, other materials and other fabrication techniques may also be used. 
       FIG. 3   c  illustrates a cross-sectional view along line IIIc-IIIc of the invention illustrated in  FIG. 3   a .  FIG. 3   c  shows the fluid well  14  built on a substrate  36  and with walls made from conductive  26  and insulating  28  materials such as those used to make the wicking device  12  and described hereinabove. In other embodiments, different materials and structures may be used to form the well  14  and its associated parts. 
       FIGS. 4   a  and  4   b  illustrate the structure of the device including the cantilevered beam, a differential comb drive  18 , and motion sensing electrodes  16 . The wicking device  12  is anchored  46  at one end (see  FIG. 4   b ), such as to a substrate, and is free to move or resonate at the other end. Ink jetting technology is used to deposit solution into the fluid well  14  which is sized to accommodate the volume of the drop emitted from the ink jet. In the illustrated embodiment the target area  14  includes grooves or ridges  40  to facilitate capillary motion of the fluid towards the groove  20  of the wicking device  12 . The ridges  40  are omitted from the cross-section in  FIG. 3C  for simplicity. 
     This non-contact technology is scalable for large arrays, easy to use, faster than other means of coating such as micro capillaries and pipettes, and versatile. Other thin film application methods include dip pen and shadow mask processing which are both time consuming processes. Ink jetting is an excellent technology for functionalizing each individual cantilever separately in an arrayed structure. Although the present invention will be discussed in terms of using ink jetting, other application technologies may also be used. 
     In one embodiment of the present invention, the solution is deposited into the reservoir  14  and the solution is wicked into the 2 μm wide groove  20  running the length of the micro-cantilever  12 , which is 4 μm wide. Once the solvent from the solution evaporates and dries, the polymer which was cast in solution is left in the 2 μm groove  20 . This process of depositing polymer onto a micro-cantilever  12  is done without destruction of the device. Such a delivery system using ink-jet printing technology is significant because the approximate volume of the drop, 3×10 −14  m 3 , is greater than the volume of the micro-cantilever 12, 1.4×10 −15  m 3 , by more than an order of magnitude. The present invention teaches a method for depositing polymer onto a micro-cantilever  12  without destruction of the device. Polymer delivery to the cantilever  12  leads to gas chemical sensing and other applications by using a mass sensitive polymer. The combination of a sensitive layer with an electrostatically actuated cantilever  12  yields a mass sensitive detector. On chip electronics can be integrated with this mass sensor for motion detection using capacitive detection. Applications of this fabrication method may include mass sensing and chemo-resistive sensing for gas chemical detection. 
     In the illustrated embodiments the resonant structure  12  is a simple 120 μm-long, 4 μm-wide beam with a 2 μm-wide micro-groove  20  running along the length of the beam, although other dimensions are possible with the present invention. Motion is parallel to the surface of the silicon substrate. Differential comb drives  18  with seven rotor fingers are located near the end of the beam  12  for lateral electrostatic actuation. The stator fingers are suspended by three cantilever beams connected in parallel and sized identically to the resonator cantilever beam  12 . Any curl from vertical stress gradient is matched to ensure the stator  18  and movable comb fingers are aligned in the same plane. Motion sensing of the plate is implemented using capacitive comb electrodes  16  placed on both sides of the main beam  12  and located further toward the base from the actuation combs. A limit stop  42  is located between the actuator  18  and sense combs of the motion sensor  16 . The beam  12 , sensors  16 , actuators  18 , limit stops  42 , and their associated structures are suspended over a silicon etch pit  44 . 
     A target well  14  area is located at the base of the cantilever  12  to collect the jetted drops. The well  14  has an approximate depth of 9 μm and a maximum width of 165 μm. The well  14  narrows in width toward the base of the cantilever  12  at a 45° angle on each side. Other sizes and shapes are also possible for the target well  14  area. 
     Although  FIGS. 3   a ,  3   b ,  3   c ,  4   a , and  4   b  illustrate a single apparatus  10 , the present invention may include multiple apparatuses  10  in a single device or on a single substrate. For example, multiple apparatuses  10  may be used to perform redundant tests to ensure accuracy. Alternatively, different apparatuses  10  may perform different tests to provide a wide variety of information. As a result, a single device may contain a single apparatus  10 , or it may contain multiple apparatuses in which there are several versions of the same apparatus  10 , or in which there are several different apparatuses  10 . The apparatuses  10  may, for example, receive the same material in their respective target wells  14 , or they may receive different materials. For example, these different materials can be different polymers each with different mass sensitivity to various gas chemical analytes. 
       FIG. 5  illustrates a schematic of the transition region between the well  14  and the groove  20 . A pressure difference exists between the two surfaces of the liquid/gas interface (R. Aoyama, M. Seki, J. W. Hong, T. Fujii, and I. Endo, “Novel Liquid Injection Method with Wedge-Shaped Microchannel on a PDMS Microchip System for Diagnostic Analyses,” Journal of MEMS, p. 1232, (2001)). This differential pressure is: 
                     Δ   ⁢           ⁢     P   XC       =     2   ⁢   γ   ⁢           ⁢   cos   ⁢           ⁢       θ   C     ⁡     (       1     W   X       -     1     W   C         )                 (   1   )               
where γ is the surface tension, θ C  is the contact angle, W X  is the width of the well and W C  is the width of the micro-groove  20  running along the length of the cantilever  12 . This causes a flow of the solution by capillary action from the well  14  to the resonator  12 . Once the solvent dries polymer is left in the micro-channel in the resonator  12 .
 
     Oscillator Design 
       FIG. 6  illustrates a block diagram of the closed-loop feedback system  50  to sustain the resonant oscillation. A dc polarizing voltage, V dc    52 , is applied to the movable beam  12 . The resonator velocity is detected by measuring the motional displacement current, V dc  dC/dt, through the comb finger capacitors. An on-chip preamplifier  54  produces a voltage, V s , that is proportional to the difference of the current through the differential capacitors formed by the sense comb electrodes. 
     An external amplifier  56  placed in series with the on-chip pre-amplifier  54  provides 40 dB of gain and −90° of phase shift at the mechanical resonance. This phase compensation is needed for free running oscillation. In this implementation, only one side of the differential actuator is used. During free oscillation, the actuator voltage amplitude is 0.2 V with a dc polarizing voltage of 23.0 V. A spectrum analyzer  58  is used to monitor the output of the amplifier  56  and determine differences in the resonance frequency of the device movable beam  12 . 
     The calculated resonant frequency from layout dimensions and prior to polymer deposition is 250 kHz. With analyte addition, a change in the mass of the cantilever  12  changes the resonance frequency. The mass sensitivity (gm/Hz) is: 
                       Δ   ⁢           ⁢   m       Δ   ⁢           ⁢   f       =       -       4   ⁢   π   ⁢       (       m   b     +     m   poly       )       3   2           k         =     -       2   ⁢     (       m   b     +     m   poly       )         f   o                   (   2   )               
where m b  is the mass of the beam, m poly  is the mass of the polystyrene or other material being measured, k is the spring constant of the cantilever, and f o  is the resonance frequency of the cantilever  12 . The calculated mass sensitivity for this device is 76 fg/Hz. The sensitivity (Hz/ppm) of the micro-balance due to analyte concentration is calculated as follows (see, S. S. Bedair and G. K. Fedder, “CMOS MEMS Oscillator for Gas Chemical Detection,” Proceedings of IEEE Sensors, Vienna, Austria, Oct. 24-27, 2004):
 
                       Δ   ⁢           ⁢   f       Δ   ⁢           ⁢     C   air         =           Δ   ⁢           ⁢   f       Δ   ⁢           ⁢   m       ⁢       Δ   ⁢           ⁢   m       Δ   ⁢           ⁢     C   air           =       -       f   o       2   ⁢     (       m   b     +     m   poly       )           ⁢     K   PG     ⁢     V   poly                 (   3   )               
where C air  is the concentration of the analyte in air, K PG  is the partition coefficient associated with the particular polymer/analyte combination, and V poly  is the volume of the polymer on the beam  12 . The volume of polymer to fill the micro-groove  20  is 7.2×10-16 m 3 . Assuming that the micro-groove  20  is filled with polystyrene the concentration sensitivity to ethanol, 2-propanol, and acetone is calculated to be 0.006 Hz/ppm, 0.005 Hz/ppm, and 0.01 Hz/ppm, respectively. Other materials will result in a different sensitivity of the device  10 .
 
     Fabrication 
       FIG. 7  illustrates one method of fabricating a sensor  10  according to the present invention. The sensor  10  was fabricated in the SiGe 0.35 μm BiCMOS technology from Jazz Semiconductor (Newport Beach, CA) followed by post-CMOS micromachining (G. K. Fedder, S. Santhanum, M. L. Reed, S. C. Eagle, D. F. Guillou, M. S. C. Lu, and L. R. Carley, “Laminated high-aspect-ratio microstructures in a conventional CMOS process,” Proceedings of the  9 th IEEE International Workshop on Micro Electro Mechanical Systems (MEMS &#39;96), San Diego, Calif., Feb. 15-17, 1996, pp. 13-18). As illustrated in  FIG. 7(   a ), the structures  60  that will form the sensor are embedded in silicon oxide-based layers  62  or other material used in the fabrication process. Metal interconnect layers  64  are also present. 
     After foundry CMOS fabrication, three dry etch steps are used for definition and release of the structures  60 , as illustrated in  FIG. 7(   b ). The intermetal dielectric layers are etched using an anisotropic CHF 3 /O 2  reactive-ion etch (RIE) ( FIG. 7(   b )) where the top metal layer  66  acts as a mask defining the pattern of the structure. A subsequent undercutting of the structures  60  by a Si etch is performed using an anisotropic deep reactive-ion etch (DRIE) to form a recess  70  in the bulk silicon  72  ( FIG.7(   c )) followed by an SF 6 /O 2  isotropic etch ( FIG. 7(   d )) of the bulk silicon  72  to form the undercut  74  of the structures  60  for structural release of the metal and dielectric stack  60 . In the illustrated embodiment, the sidewalls and bottom of the micro-grooves are defined by the metal- 3  and metal- 1  layer, respectively, in the CMOS technology. 
     In  FIG. 7(   e ) the chemically sensitive polymer dissolved in solvent  76  is deposited using a piezoelectric drop-on-demand ink jet purchased from MicroFab Technologies (Plano, Tex.). The orifice of the ink jet is 30 μm in diameter and the average drop size is 31 μm in diameter. An x-y stage (Aerotech Inc.) that moves the device under the ink jet provides positional accuracy of 0.2 μm. 
     Although the present invention has been described in terms of one embodiment with regard to  FIGS. 7(   a )- 7 ( e ), other variations and modifications are also possible with the present invention. For example, different devices, such as different types of sensors as well as devices other than sensors, may be used with the present invention. Similarly, other structures and other materials may be used with the present invention. In addition, other processes and technologies, such as other micromachining and nanomachining processes and technologies may also be used to manufacture apparatuses according to the present invention. 
     Polymer Delivery Results 
       FIG. 8  illustrates an attempt to directly deposit material onto a cantilever wicking device  12 . Polymer deposition tests used two mg/mL polystyrene mixed in a 1:1 mixture of HPLC grade toluene and xylene at room temperature. The solution was then sonicated for ten minutes. 
     As expected, attempts to directly deposit onto the cantilever  12  result in the destruction of the device  10 . This occurs because the ink-jetted drop volume (˜33 pL) greatly exceeds the target cantilever  12  size. The wicking device  12  is pinned under the actuating electrodes  18  due to surface tension effects rendering it inoperable. In addition, the deposited material covers both the cantilevered wicking device  12  and surrounding structures, such as actuators  18  and motion sensors  16 . As a result, even if the wicking device  12  were to remain in, or be returned to, a functioning state, the apparatus  10  would not function because other parts of the apparatus  10  are covered in the material that was supposed to be deposited only on the wicking device  12 . 
       FIGS. 9   a - 9   d  illustrate loading material onto the cantilever  12  according to the present invention. In that example, six drops of solution (two mg/mL polystyrene in 1 toluene:1 xylene) were deposited onto the target area at the base of the cantilever beam  12 . The polymer wicks onto the cantilever beam  12  and the solvent then evaporates. A view at the base of the cantilever micro-channel  20  is shown in  FIG. 9(   a ) and  FIG. 9(   b ) before and after polystyrene deposition, respectively. The tip of the micro-channel resonator  12  with and without polystyrene is shown in  FIG. 9(   c ) and  FIG. 9(   d ), respectively. The device was successfully operated with electrostatic actuation. 
       FIG. 9   e  illustrates the frequency response of the cantilever  12  before and after polystyrene loading. The resonance frequency shifted down by 5400 Hz. This corresponds to an added polymer mass of 410pg. The calculated mass of polystyrene in six drops of solution is 396 pg. 
       FIG. 10   a - 10   g  are scanning electron micrographs (“SEM”) illustrating several embodiments of the present invention after material deposition. In these embodiments, the material deposited is a polymer, although other materials may also be used with the present invention. 
       FIG. 10   a  is a top view SEM after polymer deposition.  FIG. 10   b  is a cross-sectional view along line Xb-Xb in  FIG. 10   a .  FIG. 10   b  shows material at the tip of the wicking device  12  after polymer deposition.  FIGS. 10   a  and  10   b  illustrate an embodiment of the present invention in which the channel  20  is a vertical slot type channel, having a channel width and height of 1.5 μm and 3.2 μm, respectively. 
       FIG. 10   c  is a top view SEM after polymer deposition of another embodiment of the present invention.  FIG. 10   d  is a cross-sectional view along line Xd-Xd in  FIG. 10   c .  FIG. 10   d  shows material at the tip of the wicking device  12  after polymer deposition.  FIGS. 10   c  and  10   d  illustrate an embodiment of the present invention in which the channel  20  is a vertical slot type channel, having a channel width and height of 2.4 μm and 4.8 μm, respectively. 
       FIG. 10   e  is a top view SEM after polymer deposition of another embodiment of the present invention.  FIG. 10   f  is a cross-sectional view along line Xf-Xf in  FIG. 10   e .  FIG. 10   f  shows material at the tip of the wicking device  12  after polymer deposition.  FIGS. 10   e  and  10   f  illustrate an embodiment of the present invention in which the channel  20  is a vertical slot type channel, having a channel width and height of 2.8 μm and 4.8 μm, respectively. 
       FIG. 10   g  is a cross-section view of the tip of a wicking device  12  after polymer deposition. The channel  20  in the wicking device  12  is a vertical groove type. The channel width and height are 1.5 μm and 3.5 μm, respectively. 
     Gas Test Measurements  
       FIG. 11  illustrates a gas test system  80  according to one embodiment of the present invention. Gas analytes are introduced with nitrogen as the carrier gas. The nitrogen supply  82  is connected through an adjustable flow-meter  84  and a 2-way ball valve  86 . The flow-meter  84  has a minimum flow rate of 0.21 liters per minute (Lpm) and a maximum rate of 1.21 Lpm. One outlet of the ball valve  86  connects to a T-connector  88  for direct connection of the carrier gas and analyte vapor to the test chamber  90 . The other outlet of the ball valve  86  is connected to the inlet of a bubbler  92  which is submersed in the liquid form of the analyte of interest. The outlet of the bubbler  92  is connected to the through a valve  94  and the T-connector  88  to the test chamber  90 . 
     Tests were performed with ethanol, 2-propanol, and acetone. N 2  was flowed at 1 Lpm through the chamber using an external bubbler until an equilibrium concentration is reached. In these initial tests, the equilibrium concentration was not measured but is assumed to be at or close to the saturation concentration of the corresponding vapor at a temperature of 300 K and a pressure of 1 atm. 
       FIG. 12  illustrates the free-running oscillator responses to ethanol, 2-propanol, and acetone flows. The mechanical resonance frequency with no exposure to analyte is 204.499 kHz. The oscillator signal has a 65 dB SNR and a 3 dB width of 3 Hz limited by the 3 Hz resolution bandwidth of the spectrum analyzer. From the frequency shifts in  FIG. 12 , the amount in grams of ethanol, 2-propanol, and acetone loaded into the polystyrene is calculated to be approximately 1.5 pg, 2.6 pg, and 9.9 pg, respectively. 
     Conclusions 
     The gas tests successfully demonstrate an organic vapor detector using the CMOS-MEMS self-excited resonator oscillator. The polymer loading method that exploits capillary action in the micro-groove enables design of narrow-gap electrostatic combs alongside the micro-cantilever. Compatibility with ink jet polymer delivery enables loading of different polymers to individual cantilever sensors. The precise amount of polymer loading with this method should lead to repeatable results from device to device. 
     Scaling down the cantilever size led to a high mass sensitivity of 76 fg/Hz for the 4 μm-wide cantilever design. With further design maturation, further device scaling and incorporation of further materials onto the wicking devices on the cantlivers, the technology should lead to highly sensitive gas chemical gravimetric sensor arrays on chip. 
     Other Embodiments 
       FIGS. 13   a  and  13   b  illustrate another application of the present invention in which one or more channels  20  are used to provide an adhesive to secure a first object  100  to a second object  102 . In  FIG. 13   a , the first  100  and second  102  objects are apart and a force  104  presses them together. In  FIG. 13   b , the first  100  and second  102  objects are together, and one or more channels  20  in the second object  102  are used to carry adhesive to an interface between the first  100  and second  102  objects. After the adhesive dries the first  100  and second  102  objects are bonded together. 
     This application of the present invention may be used, for example, to assemble parts, such as parts used to create microelectromechanical systems, or other parts. In the illustrated embodiment, the first object  100  includes offsets or stops  106  which engage the second object  102  and provide for a predetermined spacing or gap  108  between the first  100  and second  102  objects. This may allow, for example for very small and/or very precise gaps or spaces to be formed. 
     Narrow gaps, for example, smaller than possible with conventional photolithographic techniques, with electrodes at either side of the gap are of interest for providing high electrostatic forces and high capacitance sensitivity. In other embodiments, stops  106  may be omitted, and two or more parts may be assembled in different orientations. Many other variations and modification are possible with this application of the present invention. 
       FIGS. 13   c  and  13   d  illustrate another embodiment of the present invention in which material, such as an adhesive, is deposited in the well  14  and wicked through the channel  20 . Part of the channel is defined by a moveable beam  109 . When the material flows through the portion of the channel  20  formed by the moveable beam  109 , the surface tension of the material causes the beam  109  to bend inward. If the material is an adhesive, it will dry and fix the beam  109  in that position. In the illustrated embodiment, the bent beam  109  engages stops  106  limiting the motion of the beam and forming a space or gap  108 . 
       FIG. 14  illustrates another application of the present invention in which the channel  12  is an opening or void in an object or layer  110 . In the illustrated embodiment, the channel  20  . defines a circular portion  112  within the larger object  110 . In this embodiment, an adhesive or other material is provided in the fluid well  14 , from which the material flows into the channel  20  and fills the channel  20 . The material filling the channel  20  may be flexible and allow for relative movement between the circular portion  112  and the larger object, or it may have some other function. Many different shapes  114  and other variations of this embodiment may be practiced with the present invention. 
       FIG. 15  illustrates a cross-section view along line XV-XV of the apparatus illustrated in  FIG. 14 . The two portions  110 ,  112  of the top layer are joined by the material in the channel  20 . 
     In this embodiment, an opening  114  exists below the two portions  110 ,  112  of the top layer and a lower layer  116 , although it is not required for an opening  114  to exist below the two portions  110 ,  112  of the top layer. The material filling the channel can be used to seal the opening  114  from the outside ambient. For example, this sealing can be used to keep liquids from entering area  114 , or can be used to seal liquids inside area  114 . 
       FIG. 16  illustrates a system  120  according to the present invention. In that embodiment, several apparatuses  10 , such as that illustrated in  FIG. 1 , are on a single substrate  122  or material.  FIG. 16  illustrates the apparatuses  10  as being “sensors”, although any apparatus  10 , or any combination of different types of apparatuses, may be formed in this manner. Accordingly, the present invention allows for a large number of apparatuses to be made or used as part of a single system  120  or unit. In some embodiments, the apparatuses  10  may all be the same, such as to perform the same test multiple times on the same or different samples. In other embodiments, the apparatuses may be different, such as to provide for a variety of functions from a single system  120 . 
     Many variations and modifications are possible with the present invention. For example, the present invention may be used in the operation of sensors. The present invention may be used to fill a space between, or to connect, two or more parts. In some embodiments, the present invention may be used to carry an adhesive to a desired location to fix two or more parts together. In other embodiments the present invention may be used to provide a dielectric between two or more electrodes or contacts. In another embodiment, the present invention may be used in a chemo resistor device in which electrically resistive material is positioned between two or more electrodes or contacts. In another embodiment, the present invention may be used as an electrostatic actuator. The present invention may also be used with a mass sensor for gas chemical sensing applications. The present invention may also be used with other fluids and materials and in other applications, such as chemo-resistive fabrication and devices, chemo-capacitive fabrication and devices, applying adhesives for capping or otherwise connecting devices, and other applications. In addition, different materials and structures may be used with the present invention. For example, some embodiments are described in terms of particular materials, although different materials may also be used. Similarly, some of the embodiments herein show a particular number and orientation of material layers used to create the various parts of the present invention. Those examples are illustrative and not limiting, and different numbers and orientations of layers may be used with the present invention. Those and other variations of the present invention are possible. 
     The present invention may also include two or more devices formed in a single apparatus or on a single substrate. The single apparatus or substrate may contain several devices of the same type, or it may contain different types of devices. In some embodiments, the devices receive different materials in their respective target areas, and in some embodiments they receive the same materials. As a result, different testing, sensing, or other functions may be performed on a single structure. 
     These and other variations and modifications of the present invention are possible and contemplated, and it is intended that the foregoing specification and the following claims cover such modifications and variations.