Abstract:
A microelectromechanical system (MEMS) device and method for operating the device to determine a property of a fluid. The device has a tube that extends from a base and is spaced apart from a substrate surface for vibrational movement in a plane normal to the surface. The tube defines a continuous internal passage having a fluid inlet and fluid outlet fluidically connected to the base. A cantilevered member attached to a distal portion of the tube opposite the base is configured for vibrational movement relative to the distal portion. A drive electrode operable to induce vibrational movements in the tube and cantilevered member is disposed on the substrate surface. Sensing electrodes are disposed on the substrate surface for sensing Coriolis-induced deflections of the tube when vibrated, generating outputs from which a property of a fluid flowing through the tube can be determined.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/065,293 filed Feb. 11, 2008, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to devices and methods for measuring properties of fluids. More particularly, this invention relates to a microfluidic device equipped with a resonating structure, a microchannel within the resonating structure through which a fluid flows, and means for ascertaining properties of the fluid while flowing through the microchannel. The performance of the device is improved with the addition of means capable of minimizing mechanical losses resulting from the mechanical energy of the resonating structure being dissipated to a supporting substrate. 
         [0003]    Fluid delivery devices capable of precise measurements find use in a variety of industries, nonlimiting examples of which include medical treatment systems such as drug infusion and anesthesia, energy and fuel systems including fuel delivery systems and fuel cells such as direct methanol fuel cells (DMFC), and consumer goods. Various types of flow rate and concentration sensors have been proposed, including electrolytic, refractometer, ultrasonic, electrochemical, electromagnetic, and electromechanical sensors. An example of the latter is a Coriolis-based microfluidic device disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., whose contents relating to the fabrication and operation of a Coriolis-based sensor are incorporated herein by reference. 
         [0004]    Coriolis-based microfluidic devices of the type disclosed by Tadigadapa et al. include a micromachined tube supported above a substrate to have a freestanding portion. Drive and sensing electrodes are located on the substrate surface beneath the freestanding portion of the tube. The drive electrode can be, for example, capacitively coupled to the freestanding portion of the tube for capacitively (electrostatically) driving the freestanding portion at or near resonance, while the sensing electrodes sense (e.g., capacitively, optically, etc.) the deflection of the resonating tube relative to the substrate and provide feedback to enable the vibration frequency induced by the drive electrode to be controlled with appropriate circuitry. In use, while a fluid flows through an internal passage within the tube, the freestanding portion is vibrated at or near resonance by the drive electrode to ascertain certain properties of the fluid, such as flow rate and density, using Coriolis force principles. In particular, as the freestanding portion is driven at or near resonance by the drive electrode, the sensing electrodes sense a twisting motion of the freestanding portion, referred to as the Coriolis effect, about the axis of symmetry of the freestanding portion. The degree to which the freestanding portion twists (deflects) during a vibration cycle as a result of the Coriolis effect can be correlated to the mass flow rate of the fluid flowing through the tube, while the density of the fluid is proportional to the frequency of vibration at resonance. 
         [0005]    Notable advantages of Coriolis-based microfluidic devices include the miniaturized scale to which they can be fabricated using semiconductor technology. As taught by Tadigadapa et al., the structural components of the device can be combined with electronics on a single chip by micromachining techniques, such as bulk etching and surface thin-film etching, to yield a microelectromechanical system (MEMS) capable of precisely analyzing very small quantities of fluids. When suitable miniaturized, a Coriolis-based microfluidic device can be enclosed by a capping wafer to allow for vacuum packaging that further improves the performance of the device by reducing air damping effects. 
         [0006]    The microfluidic device disclosed in Tadigadapa et al. can be used in a wide variety of applications, as evident from commonly-assigned U.S. Pat. Nos. 6,637,257, 6,647,778, 6,932,114, 7,059,176, 7,228,735, 7,263,882, 7,354,429 and 7,437,912, U.S. Published Patent Application Nos. 2004/0171983, 2005/0126304, 2005/0284815, 2005/0235759, 2006/0211981, 2007/0151335, 2007/0157739, 2008/0154535, and pending U.S. patent application Ser. Nos. 12/031,839, 12/031,860, 12/106,642 and 12/143,942. As particular examples, U.S. Pat. No. 7,263,882 teaches that chemical concentrations, including those of fuel cell solutions, can be measured by sensing changes in fluid density as a fluid sample flows through a microchannel within a resonating tube of a MEMS-based Coriolis microfluidic device, and U.S. Published Patent Application No. 2007/0157739 teaches the capability of detecting potential measurement errors attributable to second phases such as gas bubbles in a fluid being evaluated by a resonating tube of a MEMS-based Coriolis microfluidic device. 
         [0007]    While exhibiting very high sensitivity to mass flow rate, density and various other properties of a fluid, the performance of MEMS-based Coriolis microfluidic devices of the type taught by Tadigadapa et al. is subject to mechanical losses resulting from the attachment of the resonating tube to a substrate. In particular, clamping losses occur as a result of the tube&#39;s substrate anchor (attachment) to the MEMS substrate being stressed by tube displacement. A fraction of the vibration energy is lost from the tube though wave propagation into the MEMS substrate. While accounting for only a fraction of the vibration energy, clamping losses are sufficient that optimum performance requires a relatively large packaging mass to dissipate the mechanical energy loss and isolate the resonating tube from external mechanical stress and vibration. As such, further improvements in the sensitivities of MEMS-based Coriolis microfluidic devices are desired to fully realize the capabilities of these devices. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a microelectromechanical system (MEMS) device and a method for operating the device to determine at least one property of a fluid. The invention provides the capability of improving the performance of the device by minimizing clamping losses attributable to the attachment of a resonating tube to a substrate. 
         [0009]    According to a first aspect of the invention, the MEMS device comprises a structure on a substrate. The structure comprises a base and a tube extending from the base and spaced apart from a surface of the substrate so as to be capable of vibrational movement in a plane normal to the surface of the substrate. The tube comprises a continuous internal passage, a fluid inlet and a fluid outlet of the internal passage fluidically connected to the base, and a distal portion relative to the base. A cantilevered member is attached to the distal portion of the tube and configured for vibrational movement relative to the distal portion and in a plane normal to the surface of the substrate. At least one drive electrode is disposed on the surface of the substrate adjacent the cantilevered member and/or the distal portion of the tube, and is operable to induce the vibrational movements of the tube and the cantilevered member. Sensing electrodes are disposed on the surface of the substrate and adapted to sense deflections of the tube when vibrated with the drive electrode and produce outputs corresponding to the sensed deflections. Finally, means is provided for determining from the outputs at least one property of the fluid flowing through the internal passage. 
         [0010]    According to a second aspect of the invention, the method entails operating the MEMS device to sense at least one property of the fluid. The method comprises operating the at least one drive electrode to induce the vibrational movements of the tube and the cantilevered member as the fluid flows through the internal passage within the tube so that the vibrational motion of the cantilevered member is not in phase with the vibrational motion of the tube. The sensing electrodes are operated to sensing the deflections of the tube relative to the substrate, and outputs are produced that correspond to the sensed deflections and from which the at least one property of the fluid is determined. 
         [0011]    According to a preferred aspect of the invention, the location of the cantilevered member is preferably chosen to enhance the performance of the MEMS device. More particularly, the cantilevered member is preferably configured and used as a counterbalance to the mass of the tube and the fluid within the tube, and the vibrational movement of the cantilevered member is preferably about 180 degrees out of phase with the vibrational movement of the tube, thereby minimizing the mechanical (clamping) losses that are dissipated to the substrate. This aspect of the invention can be utilized to promote the sensitivity of the MEMS device, and/or allow for the use of packaging processes and materials that are less expensive that conventional MEMS devices. In addition, the presence and operation of the cantilevered member can potentially allow mechanical stresses applied to the package from adversely impacting the performance of the device. 
         [0012]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a plan view of a microfluidic device with a resonating micromachined tube, a cantilevered member, and a drive electrode beneath the cantilevered member in accordance with a first embodiment of this invention. 
           [0014]      FIG. 2  is a plan view of a microfluidic device similar to the device of  FIG. 1 , but with the drive electrode beneath the tube in accordance with a second embodiment of this invention. 
           [0015]      FIG. 3  is a plan view of the micromachine level of a microfluidic device similar to the device of  FIGS. 1 and 2 , but with the tube having a different shape in accordance with a third embodiment of this invention. 
           [0016]      FIG. 4  is a plan view of the micromachine level of a microfluidic device similar to the device of  FIG. 3 , but with the cantilevered member having an internal chamber fluidically coupled to the tube in accordance with a fourth embodiment of this invention. 
           [0017]      FIG. 5  is a side view showing the tube and cantilevered member of the devices of  FIGS. 1 through 4  vibrating out of phase with each other. 
           [0018]      FIG. 6  is a plan view of a microfluidic device similar to the devices of  FIGS. 1 through 5 , but with the tube having a linear shape in accordance with a fifth embodiment of this invention. 
           [0019]      FIG. 7  is a side view showing the tube and cantilevered member of the device of  FIG. 6  vibrating out of phase with each other. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIGS. 1 through 7  represent MEMS microfluidic devices  10  similar in construction and operation to the MEMS microfluidic device disclosed by Tadigadapa et al., but modified to exhibit improved operating characteristics. While the invention will be discussed in reference to the microfluidic devices  10 , aspects of the invention are also applicable to other MEMS devices, including motion sensors and RF-MEMS. 
         [0021]    In  FIGS. 1 through 7 , consistent reference numbers are used to identify functionally equivalent structures. Each device  10  is represented as being fabricated on a substrate  12 , which can be formed of silicon, doped silicon and other semiconductor materials, quartz, glass materials, ceramic materials, metallic materials including titanium, stainless steels and KOVAR® (a nickel-cobalt ferrous alloy commercially available from Carpenter Technology Corporation), composite materials, and other materials capable of being micromachined. A tube  14  is cantilevered from a base  16  bonded to the substrate  12 , such that the tube  14  is suspended above a surface  18  of the substrate  12 . The substrate surface  18  beneath the tube  14  is shown as defined by a single recess in the substrate  12  underlying the entire tube  14 , though the surface  18  may be defined in any suitable manner to define a gap between the tube  14  and substrate  12 . The tube  14  defines a continuous internal passage  20  through which a fluid can flow. In the embodiments shown in the Figures, fluid enters and exits the tube  14  via an inlet  22  and outlet  24  located in the base  16 . According to Tadigadapa et al., the tube  14  can be vibrated at or near resonance to determine the mass flow rate and density of the fluid flowing through the tube  14  using Coriolis force principles. The shape and size of the tube  14  can be chosen to provide an adequate flow capacity for the fluid and to have suitable vibration parameters for the intended fluids to be evaluated with the device  10 . 
         [0022]    The tube  14  may have a variety of shapes, including but not limited to a generally C-shaped configuration as shown in  FIGS. 1 and 2 , a generally U-shaped configuration as shown in  FIGS. 3 and 4 , or a linear or straight shape as shown in  FIG. 6 . In each case, the tube  14  has proximal portions  14 A and  14 B attached to the base  16  and a distal portion  14 C from the base  16  and midway between the proximal portions  14 A and  14 B. In  FIGS. 1 and 2 , the tube  14  is cantilevered from the base  16 , the base  16  is between the proximal portions  14 A and  14 B of the tube  14 , the proximal portions  14 A and  14 B are coaxial, and the distal portion  14 C is parallel to the proximal portions  14 A and  14 B. In  FIGS. 3 and 4 , the tube  14  is again cantilevered from the base  16 , but the proximal portions  14 A and  14 B of the tube  14  extend in parallel from the base  16  and the distal portion  14 C is perpendicular to the proximal portions  14 A and  14 B. In  FIG. 6 , the tube  14  is not cantilevered but instead is located between spaced-apart portions  16 A and  16 B of the base  16 , and the proximal and distal portions  14 A,  14 B and  14 C of the tube  14  are coaxial as a result of the linear shape of the tube  14 . Other tube shapes—both simpler and more complex—are also within the scope of the invention. 
         [0023]    The tube  14 , base  16  and internal passage  20  are preferably formed by micromachining, which is known and used herein to refer to techniques for forming very small elements by bulk etching a substrate (e.g., a silicon wafer) or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. The tube  14  and base  16  can either be fabricated entirely from layers of the chosen materials deposited on the substrate  12 , or fabricated in part by etching the substrate  12 . Because micromachining technologies are employed to fabricate the tube  14 , the size of the tube  14  can be extremely small, such as lengths of about 0.5 mm and cross-sectional areas of about 250 μm 2 , with smaller and larger tubes also being within the scope of this invention. Particularly suitable configurations and processes for fabricating resonant mass flow and density sensors using micromachining techniques are disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., commonly-assigned U.S. Pat. No. 6,647,778 to Sparks, and commonly assigned U.S. Pat. No. 7,381,628 to Sparks et al., whose disclosures relating to micromachining processes are incorporated herein by reference. Because of their miniature size, the micromachined tubes  14  shown in  FIGS. 1 through 7  can be used to very accurately determine the mass flow rate, density, and/or specific gravity of a fluid flowing through the tube  14 . As such, the devices  10  are suitable for use in a wide variety of applications where accuracy and precision are important, such as chemical concentration applications including but not limited to drug infusion systems, fuel cell systems, and drug and chemical mixing systems. Coriolis force principles can also be used to ascertain the volumetric flow rate, viscosity, lubricity, and other properties of a fluid flowing through the tube  14 . 
         [0024]    As in Tadigadapa et al., the tube  14  is vibrated in a direction perpendicular to the surface  18  of the substrate  12 , preferably at or near its resonant frequency. During half of the vibration cycle in which the tube  14  travels upward, the tube  14  has upward momentum as the fluid travels therethrough, the fluid entering the tube  14  through the proximal portion  14 A resists the vertical upward motion of the tube  14  by pushing downward on the leg of the tube  14  nearest the fluid inlet  22 , and the fluid exiting the tube  14  through the proximal portion  14 B resists having its upward vertical motion (acquired from the tube  14 ) decreased by pushing upward on the leg of the tube  14  nearest the fluid outlet  24 . The resulting forces cause the tube  14  to twist about its axis of symmetry  36 . In  FIGS. 1 through 4 , the axis of symmetry  36  extends from the base  16  to the distal portion  14 C of the tube  14 , whereas in  FIG. 6  the axis of symmetry  36  is along the axis of the tube  14  between the portions  16 A and  16 B of the base  16 . As the tube  14  moves downward during the second half of its vibration cycle, the tube  14  twists in the opposite direction. This twisting characteristic is referred to as the Coriolis effect, and the degree to which the tube  14  deflects during a vibration cycle as a result of the Coriolis effect can be correlated to the mass flow rate of the fluid flowing through the tube  14 , while the density of the fluid is proportional to the frequency of vibration at resonance. 
         [0025]    Though necessary to the operation and sensing technique used by the MEMS devices  10 , the twisting motion of the tube  14  applies mechanical stresses to the attachment between the tube  14  and base  16 , resulting in clamping losses that must be dissipated to the substrate  12  and any additional packaging in which the device  10  is enclosed. A desired aspect of the invention is intended to reduce these losses by the inclusion of additional mass attached to the tube  14  by a flexible attachment to enable the mass to vibrate out of phase with the tube  14 . In the Figures, such a mass is represented by a cantilevered member  30  that projects roughly perpendicularly from the distal portion  14 C of the tube  14 , and more particularly at the axis of symmetry  36  of the tube  14  about which the tube  14  twists due to the Coriolis effect. The member  30  is shown in  FIGS. 1 through 4  as disposed within an opening defined and surrounded by the tube  14  and base  16 , though it is also within the scope of the invention that the member  30  could project outward the tube  14 . The member  30  defines a pivot axis  32  about which the member  30  pivots relative to the distal portion  14 C of the tube  14 . The member  30  is effectively a counterbalance to the combined mass of the tube  14  and any fluid flowing through the tube  14 . By configuring and attaching the member  30  to that its vibration is opposite the tube  14 , in other words, the member  30  vibrates approximately 180 degrees out of phase with the tube  14 , the vibrational movement of the member  30  sufficiently counteracts the vibrational movement of the tube  14  to reduce mechanical (clamping) losses dissipated to the substrate  12 . 
         [0026]    In each of  FIGS. 1 through 4  and  6 , the cantilevered member  30  is configured to have a rectangular shape when viewed from above, though other shapes are also within the scope of the invention. The Figures also show the member  30  as being attached to the tube  14  via a pivot arm  34  having a reduced cross-section relative to the member  30  so as to concentrate flexure of the member  30  adjacent the tube  14 . However, it is foreseeable that the member  30  could be directly attached to the tube  14  if the attachment point and/or the member  30  are constructed to be adequately flexible. As evident from  FIG. 5 , which depict the half of the vibration cycle in which the tubes  14  of  FIGS. 1 through 4  travel upward, as the distal portion  14 C of the tube  14  pivots upward relative to the base  16 , the member  30  pivots downward relative to the distal portion  14 C. The opposite motions occur during the second half of the vibration cycle of the tube  14 . In  FIG. 7 , which depicts the half of the vibration cycle in which the tube  14  travels upward, the distal portion  14 C of the tube  14  is deflected upward relative to the proximal portions  14 A and  14 B. The opposite motions occur during the second half of the vibration cycle of the tube  14 . 
         [0027]    Each of the above out-of-phase vibrational modes has the ability to reduce the mechanical losses that must be dissipated to the substrate  12 . The ability to minimize mechanical losses improves as the masses of the tube  14  and member  30  become closer, with optimal results believed to occur when the masses of the tube  14  and member  30  are approximately equal. Consequently, the size and shape of the member  30  will depend in part on the tube configuration, as well as the density of particular fluid flowing through the device  10 . The resonant frequencies of the tube  14  and member  30  are influenced by their mechanical design (shape, size, mass, construction and materials), which can be optimized for a particular application using, for example, known finite element modeling. For many applications, suitable resonant frequencies will generally be in a range of about 1 kHz to about 150 kHz. 
         [0028]    The relative amplitudes of vibration for the tube  14  and member  30  will also be determined by their respective mechanical designs, while amplitude as a whole can be adjusted through the means used to vibrate the tube  14  and member  30 . For this purpose,  FIGS. 1 through 4  and  6  show a drive electrode  26  located on the surface  18  of the substrate  12  beneath either the cantilevered member  30  or the distal portion  14 C of the tube  14 . In  FIGS. 1 ,  3 ,  4  and  6 , the drive electrode  26  is located directly beneath the member  30 , while in  FIG. 2  the drive electrode  26  is located directly beneath the distal portion  14 C of the tube  14 . In the former case, direct inducement of vibration in the member  30  serves to indirectly induce vibration in the tube  14 , and in the latter case direct inducement of vibration in the tube  14  indirectly induces vibration in the member  30 . It is also within the scope of this invention to place drive electrodes  26  beneath the cantilevered member  30  and the distal portion  14 C of the tube  14 . 
         [0029]    If formed of an electrically-conductive material, such as doped silicon, the tube  14  can serve as an electrode that can be capacitively coupled to the drive electrode  26 , enabling the electrode  26  to electrostatically drive the tube  14 . However, it is foreseeable that the tube  14  could be formed of a nonconductive material, and a separate electrode formed on the tube  14  facing the electrode  26  for vibrating the tube  14  electrostatically. An alternative driving technique shown in  FIG. 3  is to provide a film  40  on the upper surface of the tube  14  for vibrating the tube  14  electromagnetically or piezoelectrically (for convenience,  FIG. 3  represents a plan view of only the micromachine level of the device  10 , and omits the substrate  12  and metallized level of the device  10 ). For example, forming the film  40  of a magnetic material enables the tube  14  to be driven electromagnetically with an electromagnet positioned above the tube  14  (not shown). Alternatively, the film  40  can be formed as a piezoelectric element to generate alternating forces in the plane of the tube  14  that flex the tube  14  in directions normal to the plane of the tube  14 . Other alternative driving techniques include thermal, piezoresistive, optical, and other actuation technique. 
         [0030]    The Figures further show sensing electrodes  28  arranged in at least two pairs  28 A-D to sense the deflection of the tube  14  relative to the substrate  12 , as well as provide feedback to the drive electrode  26  to enable the vibration frequency to be controlled with any suitable on-chip or remote microprocessor or microcontroller  42 . The sensing electrodes  28  can sense the proximity or motion of the tube  14  capacitively, electrostatically, electromagnetically, piezoelectrically, piezoresistively, thermally, optically, or in any other suitable manner capable of sensing the proximity or motion of the tube  14 . Furthermore, the degree to which the tube  14  twists during a vibration cycle as a result of the Coriolis effect can be detected by the sensing electrodes  28  on the basis of the amplitude of the deflection and/or the phase difference between the respective sides (legs) of the tube  14  nearest each electrode  28 A,  28 B,  28 C and  28 D. Input and output signals to the electrodes  26  and  28  (and the magnetic/piezoelectric film  40  of  FIG. 3 ) can be made through bond pads  44  along an edge of the substrate  12 , and are transmitted to the microcontroller  42  with appropriate signal conditioning circuitry  46 , as schematically represented in  FIGS. 1 through 4  and  6 . Ground contacts  48  are shown as being formed in the same metal layer as that used to form the electrodes  26  and  28  and bond pads  44 , and by which an electrical ground to the tube base  16  is provided to enable the tube  14  or an electrode formed on the tube  14  to be capacitively coupled to the drive electrode  26 . 
         [0031]    Though represented as solid in  FIGS. 1 through 3  and  5  through  7 , the member  30  can be hollow to contain a sealed gas or vacuum chamber. The inclusion of a hollow chamber enables the use of a larger member  30  to increase the electrostatic force that can be applied by the drive electrode  26  to vibrate the tube  14  into resonance. Alternatively,  FIG. 4  represents an embodiment in which the member  30  is fabricated to have an internal chamber  38  into which fluid within the tube  14  can enter the member  30 , with the result that the mass of the member  30  is influenced by the fluid being evaluated. (Similar to  FIG. 3 ,  FIG. 4  represents a plan view of only the micromachine level of the device  10  and omits the substrate  12  and metallized level of the device  10 .) In this manner, the vibrational mass of the member  30  is less when evaluating a relatively low-density fluid, including gases, and is greater when evaluating a relatively denser fluid. Various fluid paths through the member  30  are also possible, including flow paths with no stagnant sections. 
         [0032]    The accuracy of measurements made with the devices  10  can be improved by monitoring the temperature of the fluid. For this purpose, the devices  10  are represented as equipped with a temperature sensing element  50 . A suitable construction for the sensing element  50  can make use of one or more metal layers of the type employed to form the electrodes  26  and  28  and their associated conductive runners. For example, a resistive-based temperature sensing element  50  can be formed by a thin-film metal layer of gold, platinum, palladium, chromium, nickel, or another metal or alloy, in accordance with known practices. With the temperature sensing element  50 , changes in mechanical properties of the tube  14  and properties of the fluid therein attributable to temperature changes can be compensated for with the signal conditioning circuitry  46 . 
         [0033]    The MEMS devices  10  of  FIGS. 1 through 7  can be enclosed by a capping wafer (not shown) to form a sensing package. The use of a capping wafer allows for vacuum packaging that reduces air damping of the tube vibration. A variety of package and wafer-level methods exist to vacuum package devices. These include solder or weld hermetic packages, and wafer bonding using glass frit, solder, eutectic alloy, adhesive, and anodic bonding. Silicon is a particular example of a suitable material for the capping wafer, which has the advantage of allowing silicon-to-silicon bonding techniques to be used, though it is foreseeable that a variety of other materials could be used, including metals and glass materials, the latter including borosilicate glass (e.g., Pyrex). Notably, the reduced mechanical losses made possible with this invention may enable the devices  10  to be packaged in less expensive plastic packages and/or over molded. Reduced mechanical losses also offer the possibility of the sensor package being able to withstand greater mechanical stress without adversely impacting the performance of the device  10 . 
         [0034]    In preferred embodiments of the invention, the bond between the capping wafer and substrate  12  is hermetic, and the resulting enclosure is evacuated to enable the tube  14  to be driven efficiently at high quality (Q) factor values without damping. In such an embodiment, a getter material is preferably placed in the enclosure to assist in reducing and maintaining a low cavity pressure. As an alternative to a hermetically sealed package, the tube  14  could be enclosed such that a vacuum can be drawn when desired through the use of a pump. 
         [0035]    If a magnetic or piezoelectric actuation scheme is employed to drive the tube  14  as represented in  FIG. 3 , the device  10  can operate with larger gaps between the tube  14  and substrate  12 , with the potential for sufficiently reducing squeeze film damping of the tube  14  to eliminate the need for vacuum packaging of the device  10 . 
         [0036]    While the invention has been described in terms of certain embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.