Abstract:
A microfluidic device a micromachined freestanding member adapted to sense one or more properties of a fluid flowing through the freestanding member. The freestanding member is supported by a substrate and spaced apart and separated from the substrate to enable the freestanding member to move relative to the substrate under the influence of a vibration-inducing element. Movement of the freestanding member relative to the substrate is then sensed by a sensing element. The freestanding member has an inlet, an outlet, an internal passage that fluidically couples the inlet and outlet, and a wall that defines and separates first and second passage portions of the internal passage that are arranged in fluidic series so that a fluid flowing through the internal passage flows through the first and second passage portions in opposite directions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/756,202 filed Jan. 5, 2006, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention generally relates to micromachining devices and processes for their fabrication. More particularly, this invention relates to a microfluidic device having a compact micromachined freestanding member configured to sense one or more properties of a fluid flowing through an internal passage within the freestanding member.  
         [0003]      FIGS. 1 and 2  represent a Coriolis-based fluid sensing device  10  of a type 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. The fluid sensing device  10  is represented as including a substrate  12  that may be formed of silicon or another semiconductor material, quartz, glass, ceramic, metal, a polymeric material, a composite material, etc. A tube  14  is supported by the substrate  12  so as to have a base  28  attached to a surface  18  of the substrate  12  and a freestanding portion  16  suspended above the substrate  12 . As evident from  FIG. 1 , the freestanding portion  16  has a generally U or D-shaped configuration. Electrodes  22  and  24  are located on the substrate  12  beneath the freestanding portion  16  of the tube  14 , and bond pads  32  (only one of which is shown) are provided for transmitting input and output signals to and from the device  10 . The electrode  22  can be, for example, capacitively coupled to the tube  14  for capacitively (electrostatically) driving the freestanding portion  16  at or near resonance, while the remaining electrodes  24  sense (e.g., capacitively) the deflection of the tube  14  relative to the substrate  12  and provide feedback to enable the vibration frequency induced by the drive electrode  22  to be controlled with appropriate circuitry. With a fluid entering the device  10  through an inlet port  26  and flowing through an internal passage  20  within the tube  14 , the freestanding portion  16  can be vibrated at or near resonance to ascertain certain properties of the fluid, such as flow rate and density, using Coriolis force principles. Notable advantages of the device  10  include the extremely miniaturized scale to which it can be fabricated and its ability to precisely analyze very small quantities of fluids. In  FIG. 2 , the device  10  is schematically shown as enclosed by a cap  30  to allow for vacuum packaging that further improves the performance of the device  10  by reducing air damping effects.  
         [0004]     Tadigadapa et al., commonly-assigned U.S. Pat. No. 6,647,778 to Sparks, and commonly assigned U.S. Patent Application Publication No. 2006/0175303 to Sparks et al. disclose processes for fabricating flow sensing devices of the type shown in  FIGS. 1 and 2  using micromachining techniques. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), and/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. As disclosed by Tadigadapa et al., Sparks, and Sparks et al., wafer bonding and silicon etching techniques can be used to produce microelectromechanical systems (MEMS) comprising one or more flow sensing devices. Sensors of the type taught by Tadigadapa et al. have found use in a variety of applications, as evident from Sparks, Sparks et al., commonly-assigned U.S. Pat. Nos. 6,932,114, 6,942,169, and 7,059,176, and U.S. Patent Application Publication Nos. 2004/0171983, 2005/0126304, 2005/0235759, 2005/0284815, 2006/0010964, and 2006/0213552. As examples, the teachings of Tadigadapa et al. have been applied to mass flow sensors, density sensors, fuel cell concentration meters, chemical concentration sensors, specific gravity sensors, pressure sensors, temperature sensors, drug infusion devices, and other devices that can employ resonating and stationary microtubes. Nonetheless, further improvements would be desirable for use in the design and fabrication of devices such as Tadigadapa et al. that employ extremely miniaturized fluid channels, including the capability of further reducing the size of such devices.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention provides microfluidic devices, and particularly a microfluidic device with a micromachined freestanding member adapted to sense one or more properties of a fluid flowing through the freestanding member. The microfluidic device preferably operates in a manner similar to the microfluidic devices disclosed in U.S. Pat. Nos. 6,477,901 and 6,647,778, which sense the mass flow and/or density of a fluid flowing through a resonating tube, though other uses and operating techniques are also within the scope of this invention, including microfluidic devices that employ resonating and/or stationary microtubes for other purposes.  
         [0006]     According to a first aspect of the invention, the microfluidic device microfluidic device includes a micromachined freestanding member that is supported by a substrate and is spaced apart and separated from the substrate. As such, the freestanding member is able to move relative to the substrate under the influence of a vibration-inducing element associated with the freestanding member. Movement of the freestanding member relative to the substrate is then sensed by a sensing element also associated with the freestanding member. The freestanding member has an inlet, an outlet, an internal passage that fluidically couples the inlet and outlet, and a wall that defines and separates first and second passage portions of the internal passage arranged in fluidic series so that a fluid flowing through the internal passage flows through the first and second passage portions in opposite directions.  
         [0007]     With the above construction, the freestanding member can be fabricated, for example, with the wafer bonding and silicon etching techniques of Tadigadapa et al., Sparks, and Sparks et al., and operated as, for example, a resonating fluid passage capable of using Coriolis force principles to detect various properties of a fluid, including but not limited to mass flow and density. Because the wall of the freestanding member is shared by multiple portions of the internal passage, the freestanding member is more compact that previous tube configurations, such as the U-shaped resonating tubes of Tadigadapa et al., as well as omega and D-shaped resonating tubes proposed in the past.  
         [0008]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1 and 2  are perspective and cross-sectional views, respectively, of a microfluidic device of the prior art.  
         [0010]      FIG. 3  is a plan view of a microfluidic device in accordance with a first embodiment of this invention.  
         [0011]      FIGS. 4 and 5  are cross-sectional views showing in greater detail a freestanding member of the microfluidic device of  FIG. 3 .  
         [0012]      FIG. 6  is a cross-sectional view analogous to  FIG. 5 , but showing a freestanding member configured in accordance with a second embodiment of this invention.  
         [0013]      FIG. 7  is a plan view of the freestanding member of  FIG. 6 .  
         [0014]      FIGS. 8 and 9  are cross-sectional and plan views, respectively, of an interface between a base member from which the freestanding member of  FIG. 3  is cantilevered and an inlet port within the substrate to which the base member is bonded.  
         [0015]      FIGS. 10 and 11  are alternative cross-sectional and plan views, respectively, of the interface between the base member and the inlet port of  FIGS. 8 and 9 .  
         [0016]      FIGS. 12 and 13  depict two packaging options for the microfluidic device of  FIG. 3 .  
         [0017]      FIG. 14  depicts an alternative configuration for the microfluidic device of  FIG. 3 .  
         [0018]      FIG. 15  depicts an alternative configurations for the freestanding member of  FIGS. 3 and 14 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 3  represents a microfluidic device  40  whose fabrication, construction, and operating principles can be similar to microfluidic devices disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., whose contents relating to micromachining techniques and microfluidic device operation are incorporated herein by reference. As such, the device  40  can be fabricated using wafer bonding and silicon etching techniques to produce a microelectromechanical system (MEMS) comprising a suspended micromachined freestanding structure  44  through which fluid flows. However, in contrast to the tube of Tadigadapa et al., the freestanding structure  44  of the device  40  of this invention has an internal passage  48  made up of multiple channels  50  and  52  through which a fluid under evaluation flows. In preferred embodiments, the freestanding structure  44  has an entirely closed configuration such that openings and voids are not present in its exterior, in contrast to the devices taught by Tadigadapa et al. whose U or D-shaped tubes define a large central opening. An advantage of the closed configuration of the freestanding structure  44  of this invention is the improved miniaturization to which it can be fabricated while maintaining the ability to precisely analyze very small quantities of fluids. The compact configuration of the freestanding structure  44  also reduces the amount of structural material required in its construction, thereby increasing the density sensitivity of the device  40 .  
         [0020]     In  FIG. 3 , the microfluidic device  40  is represented as including a substrate  42  that may be formed of silicon or another semiconductor material, quartz, glass, ceramic, metal, a polymeric material, a composite material, etc. The freestanding structure  44  extends from a base  46  bonded to the substrate  42  so that the structure  44  is suspended above a surface  78  of the substrate  42 , defining a gap between the structure  44  and substrate  42  that permits the structure  44  to deflect in a plane normal to the surface of the substrate  42 , as evident from  FIGS. 4 and 5 . The surface  78  can be the result of etching an opening in the base  46  to expose the substrate  42  beneath, or can be further defined by a recess etched into the surface of the substrate  42 . Electrodes  64  and  66  are shown as being located on the surface  78  of the substrate  42  directly beneath the freestanding structure  44 , and electrically interconnected with bond pads  68  for transmitting input and output signals to and from the device  40 . In  FIG. 1 , the electrode  64  is a drive electrode for inducing vibration in the freestanding structure  44 , and the electrode  66  is a sensing electrode for sensing the position (deflection) of the freestanding structure  44  relative to the substrate  12 , as discussed in more detail below. Bond pads  70  are also provided for ground contacts  72  connected to the base  46 .  
         [0021]     According to a preferred aspect of the invention, the freestanding structure  44  and base  46  are micromachined from silicon, doped silicon or another semiconductor material, though other materials can be used including but not limited to sapphire, quartz, or another glass material, ceramic materials, plastic, metallic materials, and composite materials. The freestanding structure  44  and base  46  can be micromachined together or individually and then bonded (for example, by fusion, direct, anodic, solder, eutectic, or adhesive bonding) as a unitary structure to the substrate  42 .  FIG. 3  shows the top of the freestanding structure  44  removed to expose its interior construction, which includes the continuous internal passage  48  defined by two straight and parallel channels  50  and  52  interconnected with a curved channel  51 , such that the channels  50 ,  51 , and  52  are in fluidic series. Though not required, the smooth and rounded shape of the curved channel  51  is preferred to reduce the trapping and nucleation of bubbles within the fluid present in the freestanding structure  44 , the presence of which would degrade the performance of the device  40 . The channels  50  and  52  are separated within the freestanding structure  44  by a single wall  54 , whose opposite surfaces contact the fluid within the channels  50  and  52 . As represented in  FIGS. 3 and 5 , the wall  54  is the only structure that separates the channels  50  and  52  within the freestanding structure  44 , including the inlet and outlet of the freestanding member  44  coupled to inlet and outlet passages  56  and  58  micromachined in the base  46 .  
         [0022]     From  FIGS. 3 through 5 , it can be seen that fluid enters and leaves the freestanding structure  44  through the fluid inlet and outlet passages  56  and  58  within the base  46 , and exit the device  40  through inlet and outlet ports  60  and  62  located in the substrate  42 , for example, at the bottom surface of the substrate  42 . As a result of this configuration, fluid enters the device  40  through the inlet port  60 , flows through the inlet passage  56  to the freestanding structure  44 , where the fluid enters the channel  50  and flows in a first direction toward the curved channel  51 . The curved channel  51  reverses the flow direction of the fluid, such that fluid flow through the second channel  52  is opposite that of the first channel  50 . From the channel  52 , the fluid exits the freestanding structure  44  and enters the outlet passage  58  within the base  46 , and exits the device  40  through the outlet port  62 .  
         [0023]     From the above, it should be understood that the internal passage  48  of the freestanding structure  44  can serve as a conduit through which a fluid flows while the cantilevered freestanding structure  44  is vibrated for the purpose of ascertaining certain properties of the fluid using Coriolis force principles, as explained in Tadigadapa et al. As indicated in  FIGS. 4 and 5 , the freestanding structure  44  is vibrated in a direction perpendicular to the surface  78  of the substrate  42 , preferably at or near its resonant frequency. During half of the vibration cycle in which the freestanding structure  44  moves upward, the freestanding structure  44  has upward momentum as the fluid travels around the tube bends, and the fluid flowing out of the freestanding structure  44  resists having its vertical motion decreased by pushing up on that part of the freestanding structure  44  nearest the outlet passage  58 . The resulting force causes the freestanding structure  44  to twist. As the freestanding structure  44  moves downward during the second half of its vibration cycle, the freestanding structure  44  twists in the opposite direction. This twisting characteristic is referred to as the Coriolis effect, and the degree to which the freestanding structure  44  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 freestanding structure  44 , while the density of the fluid is proportional to the frequency of vibration and the damping and amplitude of the peak is proportional to the viscosity of the fluid.  
         [0024]     The resonant frequency of the freestanding structure  44  is controlled by its mechanical design (shape, size, construction and materials). Typical resonant frequencies for the micromachined freestanding structure  44  represented in  FIG. 3  will generally be in the range of about 1 kHz to about 100 kHz. The amplitude of vibration is adjusted through the drive electrode  64 . In a preferred embodiment, the freestanding structure  44  is formed of doped silicon and can therefore serve as an electrode that can be capacitively coupled to the drive electrode  64 , enabling the electrode  64  to capacitively (electrostatically) drive the freestanding structure  44 . However, it is foreseeable that the freestanding structure  44  could be formed of a nonconductive material, and a separate electrode formed on the freestanding structure  44  opposite the drive electrode  64  for vibrating the freestanding structure  44  electrostatically. An alternative driving technique is to provide a piezoelectric element on an upper surface of the freestanding structure  44  to generate alternating forces in the plane of the freestanding structure  44  that flex the structure  44  in directions normal to the plane of the structure  44 . Other alternatives are to drive the freestanding structure  44  magnetically, thermally, or by another actuation technique. In addition to sensing the deflection of the freestanding structure  44  relative to the substrate  42 , the sensing electrode  66  provides feedback to the drive electrode  64  to enable the vibration frequency to be controlled with appropriate circuitry (e.g.,  100  in  FIGS. 12 and 13 ). The sensing electrodes  66  can sense the freestanding structure  44  capacitively or in any other suitable manner capable of sensing the proximity or motion of the structure  44 .  
         [0025]     A sealing ring  88  is represented in  FIG. 3  as surrounding the freestanding structure  44  and base  46  to permit bonding of a capping wafer (not shown) to the substrate  42  to protect the freestanding structure  44 . In the preferred embodiment of this invention, the bond between the cap and the substrate  42  is hermetic, and the enclosure formed by the cap is evacuated to enable the freestanding structure  44  to be driven efficiently at high quality (Q) values without damping. In such an embodiment, a getter material (not shown) is preferably placed in the enclosure to assist in reducing and maintaining a low cavity pressure.  
         [0026]     The device  40  is also shown in  FIG. 3  as including bond pads  74  to a temperature sensing element  76  for measuring the temperature of the fluid flowing through the freestanding structure  44 . Properties such as densities of materials change with temperature, as do the Young&#39;s and shear moduli of materials. Placement of the temperature sensing element  76  on the substrate  42  enables the temperature of the freestanding structure  44  and its fluid contents to be monitored with suitable accuracy under many operating conditions. A suitable construction for the sensing element  76  can make use of one or more metal layers of the type employed to form the electrodes  68 ,  70 , and  74 , and their associated conductive runners. For example, a resistive-based temperature sensing element  86  can be formed by a thin-film metal layer of platinum, palladium or nickel, in accordance with known practices. With the temperature sensing element  76 , changes in mechanical properties of the freestanding structure  44  and properties of the fluid therein attributable to temperature changes can be compensated for with appropriate circuitry (e.g., the circuitry  100  in  FIGS. 12 and 13 ). Alternatively or in addition, an electrical potential could be applied to pass a current through the freestanding structure  44  to raise and maintain the temperature of the freestanding structure  44  and the fluid flowing therethrough by Joule heating, with the sensing element  76  used as feedback for appropriate control circuitry (not shown).  
         [0027]     While the freestanding structure  44  is represented in  FIGS. 3 through 5  as containing a single pair of straight and parallel channels  50  and  52 , the structure  44  can be fabricated to contain any number channels. As an example,  FIGS. 6 and 7  show the freestanding structure  44  modified to contain two additional straight channels  80  and  82  fluidically coupled to the channels  50  and  52  via a curved channel  81 , and separated from the channels  50  and  52  by a wall  84 . The channels  80  and  82  are fluidically coupled to each other via a curved channel  83 , and separated from each other by an additional wall  86 .  
         [0028]      FIGS. 8 through 11  represent additional techniques for reducing the likelihood of bubbles being trapped, nucleated, or injected into the internal passage  48  of the freestanding structure  44 . In  FIGS. 8 and 9 , the connection between the inlet passage  56  (within the base  46 ) and inlet port  60  (within the substrate  42 ) is shown. The inlet passage  56  is configured to be narrower in width than the inlet port  60 , and to have a tubular extension  90  that projects transversely into the inlet port  60 . As a result, bubbles entrained in the fluid entering the device  40  through the inlet port  60  tend to be trapped within the inlet port  60  and thereby prevented from entering the freestanding structure  44 . Flow turbulence within the inlet port  60  tends to break up bubbles into finer ones that would have a much reduced negative effect on the performance of the freestanding structure  44 . The embodiment of  FIGS. 10 and 11  achieves a similar effect with the inlet passage  56  having roughly the same width as the inlet port  60  by forming slots  92  in the extension  90 , effectively creating a sieve that can filter bubbles, trapping them in the inlet port  60  and/or breaking up larger bubbles.  
         [0029]      FIGS. 12 and 13  represent packaging techniques that capitalize on the miniaturization achieved with the freestanding structure  44 . In  FIGS. 12 and 13 , the device  40  is shown mounted to a package header  94  and the freestanding structure  44  enclosed by a capping wafer  96  to form a MEMS package. In  FIG. 12 , a chip  98  carrying an application specific integrated circuit (ASIC)  100  for the device  40  is bonded to the top of the capping wafer  96 , with wire bonds connecting the ASIC  100  to the bond pads  68 ,  70 , and  74  on the device  40 . In  FIG. 13 , a separate ASIC chip is omitted, and the ASIC  100  is formed directly in the surface of the capping wafer  96 .  
         [0030]     While the compact configurations shown in  FIGS. 3 through 7  are preferred for the freestanding portion  44 , other compact configurations are possible. As examples,  FIG. 14  shows a microfluidic device  102  essentially similar to that of  FIG. 3 , but modified to have a freestanding structure  104  that is not cantilevered, and instead configured as an S-shaped tube. Drive electrodes  110  are placed under curved segments  106  of the freestanding structure  104 , and a sensing electrode  112  is located beneath the generally straight intermediate segment  108  between the curved segments  106 . The drive electrodes  110  are operated to cause the freestanding structure  104  to twist, causing the intermediate segment  108  to periodically deflect toward and away from the substrate surface  114  beneath the freestanding structure  104 .  FIG. 15  depicts another non-cantilevered freestanding structure  116  in the form of a straight tube with drive electrodes  118  beneath the ends of the tube and a sensing electrode  120  beneath the tube between the drive electrodes  118 . Though achieving the smallest size for a freestanding structure, a straight tube is stiffer than the configurations shown in  FIGS. 1 through 14  and therefore requires more amplification of the signal from the sensing electrode  120 . Because higher sensitivity to twisting is key for Coriolis mass flow sensors, the higher sensitivity capabilities of the embodiments in  FIGS. 3-14  are preferred.  
         [0031]     The small chip size and low cost capability of the devices of this invention are extremely valuable for consumer and high-volume applications. For example, the devices represented in  FIGS. 3 through 15  can be used in a wide variety of applications, including chemical concentration meters, drug concentration and type identification, sensing air bubble in drug delivery equipment, and other applications. Consistent with the teachings of commonly-assigned U.S. Patent Application Publication No. 2006/0175303, the devices of this invention can be used in fuel cell systems to sense the concentration of fuels in a fuel cell solution, such as a mixture of water and methanol, ethanol, ethylene glycol, isopropyl alcohol (IPA), formic acid, sulfuric acid, gasoline, or other organic liquid, and in combustion fuel systems to sense the concentrations in the fuel mixture, such as a mixture of gasoline and an alternative fuel such as ethanol (e.g., E85) or methanol (e.g., M85).  
         [0032]     The devices can be modified in accordance with commonly-assigned U.S. Patent Application Publication No. 2006/0169038 and 2006/0213552 to be capable of operating in a bypass mode for use in relatively large flow rate systems, such as to monitor the concentration of chemicals in a small sample of a fluid. In this manner, the devices can be used to evaluate a variety of fluids used in vehicle fluid systems, such as fuels, intake air, lubricating oils, transmission, hydraulic and brake fluids, coolants, exhaust gases, window washing fluids, etc., for land-based, aquatic-based, and aerospace vehicles. Furthermore, a variety of fluid properties can be measured with the devices, including but not limited to flow rate (including mass and volumetric flow rates), density and properties that can be correlated to density, such as specific gravity, relative chemical concentrations of intended fluid constituents, and the presence of undesirable contaminants such as liquids (e.g., fuel or water in engine oil), gas or air bubbles (e.g., in a fuel or brake fluid), solid particles (e.g., in engine oil), etc.  
         [0033]     While the invention has been described in terms of a particular embodiment, 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.