Abstract:
A process for producing a micromachined tube (microtube) suitable for microfluidic devices. The process entails isotropically etching a surface of a first substrate to define therein a channel having an arcuate cross-sectional profile, and forming a substrate structure by bonding the first substrate to a second substrate so that the second substrate overlies and encloses the channel to define a passage having a cross-sectional profile of which at least half is arcuate. The substrate structure can optionally then be thinned to define a microtube and walls thereof that surround the passage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/067,882, filed Mar. 3, 2008, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to micromachining processes and devices formed thereby. More particularly, this invention relates to a process of forming a micromachined tube (microtube) suitable for a microfluidic device, including but not limited to Coriolis mass flow sensors, density sensors, specific gravity sensors, fuel cell concentration meters, chemical concentration sensors, temperature sensors, drug infusion devices, fluid delivery devices, gas delivery devices, gas sensors, bio sensors, medical sensors, and other devices capable of making use of a stationary or resonating microtube. 
     Processes for fabricating resonant mass flow and density sensors using silicon micromachining techniques are disclosed in commonly-assigned U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628. 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. In the processes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628, wafer bonding and etching techniques are used to produce a micromachined tube supported above a surface of a substrate. The tube can be vibrated at resonance, by which the flow rate, density, and/or other properties or parameters of a fluid flowing through the tube can be measured. 
     According to one embodiment of U.S. Pat. No. 6,477,901, a tube is formed using p-type doped layers and selective etching techniques. The doped layers form the walls of the tube, and therefore determine and limit the size of the tube walls as well as the cross-sectional dimensions of the tube. According to another embodiment of U.S. Pat. No. 6,477,901, a tube is formed with the use of a silicon-on-insulator (SOI) wafer. The buried oxide layer of the SOI wafer is used as an etch stop in a manner that determines and can limit the thickness of the tube. In U.S. Pat. No. 7,351,603, an epitaxial wafer is employed to avoid the higher cost of SOI wafers. 
     The micromachined tubes produced by the processes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628 have roughly rectilinear cross-section passages as a result of using an anisotropic dry etching technique, such as reactive ion etching (RIE), dry etching, or deep reactive ion etching (DRIE), or a wet etching technique if the wafer is formed of a (110) oriented silicon. As known in the art, anisotropic etching processes produce a substantially one-directional etch, yielding the vertical walls of the passages shown within the microtubes of U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a process for producing a micromachined tube suitable for microfluidic devices, nonlimiting examples of which include resonating microtubes for mass flow and density sensors, stationary microtubes, diaphragms, and passages for such microfluidic devices as needles, cannula, pressure sensors, temperature sensors, motion sensors, drug infusion devices, fluid delivery devices, gas delivery devices, gas sensors, bio sensors, medical sensors, and other devices that can employ microtubes. 
     According to a first aspect of the invention, the process entails isotropically etching a surface of a first substrate to define therein a channel having an arcuate cross-sectional profile, and forming a substrate structure by bonding the first substrate to a second substrate so that the second substrate overlies and encloses the channel to define a passage having a cross-sectional profile of which at least half is arcuate. The substrate structure can optionally then be thinned to define a microtube and walls thereof that surround the passage. 
     A second aspect of the invention is the various types of microtubes produced by the process described above. 
     In view of the above, the present invention provides a process by which microtubes with at least partially arcuate passages can be micromachined. According to preferred aspects of the invention, a channel can be formed in the second substrate to have an arcuate cross-sectional profile, with the result that the passage has an entirely arcuate cross-sectional profile, nonlimiting examples of which include circular and elliptical cross-sectional shapes. Arcuate passages produced by this invention are capable of exhibiting improved dynamic fluid flow as a result of reduced turbulence and stagnant regions within these passages, resulting in lower pressure drops and higher flow rates through their microtubes without necessitating an increase in the in-plane (width) and out-of-plane (height) dimensions of the microtubes. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows cross-sectional views of two wafers suitable as starting material for producing a micromachined tube in accordance with an embodiment of this invention. 
         FIG. 2  shows the wafers of  FIG. 1  bonded together to form a wafer stack, and 
         FIG. 3  depicts the wafer stack following etching to form an arcuate channel in a surface of the wafer stack. 
         FIG. 4  depicts the result of bonding the wafer stack of  FIG. 3  to a third wafer having an arcuate channel formed in its surface, with the result that an enclosed round passage is defined within the resulting wafer structure by the two arcuate channels. 
         FIG. 5  depicts the result of thinning the wafer structure of  FIG. 4 . 
         FIG. 6  depicts the result of selectively thinning portions of the wafer structure to either side of the round passage. 
         FIGS. 7 and 8  depict the wafer structure of  FIG. 6  bonded to a substrate. 
         FIG. 9  depicts the result of removing remaining portions of the wafer structure of  FIGS. 7 and 8  to define external walls of a microtube having a freestanding portion suspended over the substrate. 
         FIG. 10  depicts the result of bonding a capping wafer to the substrate to enclose the microtube. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 10  represent steps in a process carried out to produce a micromachined tube (microtube)  50  ( FIGS. 9 and 10 ) suitable for a variety of microfluidic devices. The drawings are drawn for purposes of clarity when viewed in combination with the following description, and therefore are not necessarily to scale. 
       FIG. 1  depicts limited portions of a pair of wafers  10  and  12  selected for processing in accordance with the invention. The wafers  10  and  12  are both preferably silicon, though other materials can be used including but not limited to Ge, SiC, GaAs, Si/Ge, diamond, sapphire, glass, ceramic materials, plastic materials, and titanium or other metallic materials. In addition, the wafers  10  and  12  can be single crystal or polycrystalline. According to one embodiment of the invention, the wafers  10  and  12  may be undoped, though in a preferred embodiment both wafers  10  and  12  are doped similar in type and level. The type (n or p-type) and doping level can be tailored as may be required or desired by one skilled in the art. Suitable doping levels for the wafers  10  and  12  generally achieve resistivities of about 1 to about 0.01 ohm-cm, with the exception of an etchstop region  14  overlying a substrate region  16  of the wafer  12 . The etchstop region  14  can be an oxide layer or a heavily doped epitaxial or diffused layer using known dopants and doping techniques, such as p-type doping with boron, boron-germanium, etc. While shown as a surface region, the etchstop region  14  could instead be formed as a buried layer of the wafer  12 . The role of the region  14  as an etchstop will become apparent from the following discussion, though it will also become apparent that the region  14  could be replaced by various other materials capable of serving as an etchstop. To obtain a desired configuration and thickness, the wafers  10  and  12  can undergo various processes, including wet chemical etching (selective, timed, etc.), dry etching (e.g., ion milling, plasma enhanced etching, reactive ion etching (RIE), deep reactive ion etching (DRIE), mechanical removal (grinding, polishing, etc.), chemical-mechanical polishing (CMP), etc. The thickness of the wafer  10  will subsequently limit the maximum height dimension of the microtube  50  (as measured in a direction normal to the wafer surface) of  FIGS. 9 and 10 . For microtubes of particular interest to the invention, the thickness of the wafer  10  is preferably in a range of about 100 to about 1500 micrometers, though lesser and greater thicknesses are also within the scope of this invention. 
       FIG. 2  shows the result of cleaning and then bonding the wafer  10  to the etchstop region  14  of the wafer  12  to form a wafer stack  18 , resulting in the etchstop region  14  effectively becoming a buried layer within the wafer stack  18 . Bonding can be accomplished by a variety of techniques, such as fusion, direct, anodic, solder, eutectic, and adhesive bonding. Silicon fusion bonding is the preferred method if the wafers  10  and  12  are formed of silicon, as this technique can be performed at room temperature under vacuum or at ambient pressures with a plasma-assist bonding mechanism. As the intent of the bonding step is in part to bury the etchstop region  14 , it is foreseeable that a wafer with a buried etchstop region could be formed by various other processes that are also within the scope of the invention. Furthermore, it will become apparent from the following discussion that the etchstop region  14  could be omitted. 
     Following bonding, a high temperature anneal/oxidation can be employed to strengthen the silicon fusion bond.  FIG. 2  shows the wafer stack  18  provided with masks  19  to protect its surfaces from attack during a subsequent etching step, the result of which is shown in  FIG. 3 . The masks  19  can be oxide layers formed during the high temperature anneal/oxidation step, though other processes known in the art can be used to mask the wafer stack  18  with a variety of masking materials, including but not limited to silicon nitride, combined silicon oxide and silicon nitride, photoresists, polymers, metals, dielectrics, etc.  FIG. 3  shows the result of etching a channel  20  in a surface of the wafer stack  18  formed by the wafer  10 , for example, after removing part or all of the mask  19  overlying this surface. The channel  20  is represented as having an arcuate or curvilinear profile in cross-section. According to a preferred aspect of the invention, the etching technique used to define the channel  20  is an isotropic process, preferably a dry etching technique and more preferably a plasma etching technique using SF 6 , CF 4 , Cl 2 , XeF 2 , etc., though an isotropic wet etching technique could also be used. In each case, the isotropic etching process proceeds into the wafer  10  in all directions from the point at which etching is initiated to achieve an arcuate or curvilinear profile shape, including but not limited to the semicircular shape of the channel  20  shown in  FIG. 3 . As shown in  FIG. 3 , the etching process is preferably terminated prior to encountering the etchstop region  14 . 
     In  FIG. 4 , the etched wafer stack  18  of  FIG. 3  is shown bonded to a wafer  22  having a semicircular channel  28  defined in a substrate region  24 , and a mask  26  (for example, an oxide layer) on the surface of the substrate region  24  opposite the channel  28 . The wafer  22  may be a standard silicon wafer, an epitaxial wafer, or processed as a wafer stack similar to the wafer stack  18  of  FIG. 3 . Bonding of the wafer stack  18  and wafer  22  can be accomplished by a variety of techniques, such as fusion, direct, anodic, solder, eutectic, and adhesive bonding. Silicon fusion bonding is again a preferred method if the wafer stack  18  and wafer  22  are formed of silicon, and a high temperature anneal/oxidation can be employed to strengthen the silicon fusion bond. The wafer  22  is preferably selected on the basis of having a channel  28  of substantially equal width to the channel  20  of the wafer stack  18 . The channels  20  and  28  can be matched via an alignment technique, and the wafer stack  18  and wafer  22  bonded together to produce a wafer structure  30  within which the semicircular channels  20  and  28  define a circular passage  32  within the structure  30 . It should be noted that channels  20  and  28  having cross-sectional profiles that deviate from a semicircular shape will yield passages  32  that deviate from a circular shape, for example, elliptical shapes. Furthermore, it is foreseeable that the wafer stack  18  could be bonded to a flat surface of another wafer, yielding a semicircular passage. These and other cross-sectional shapes incorporating a round profile are also within the scope of this invention. 
       FIG. 5  shows the result of thinning the wafer structure  30  by removing material at both surfaces of the structure  30 . The removal process at the lower surface of the structure  30  (as viewed in  FIG. 5 ) is represented as having been terminated at the etchstop region  14 . Suitable etchants for this process will depend on the type of material used to form the etchstop region  14 , for example, an oxide layer or a heavily doped p-type silicon. Those skilled in the art will appreciate that lapping, polishing, grinding, wet or dry etching, or a combination of these techniques could be used to thin the wafer structure  30 , with or without the presence of the etchstop  14 . In the embodiment shown in  FIG. 5 , the thickness of the etchstop region  14  affects the minimum lower wall thickness of the tube  50  below the passage  32 . Removal of the substrate region  24  opposite the etchstop region  14  can be by lapping, polishing, grinding, wet or dry etching, or a combination of these techniques, or through the presence of a buried etchstop (not shown) originally present in the wafer  22  similar to the wafer stack  18 . A timed etch or timed mechanical removal process can also be used to ensure the remaining surface region  24  defines a suitably thick wall above the passage  32 . Suitable thicknesses for the tube wall will depend on the particular application for the microtube  50 , with particularly suitable thicknesses believed to be about ten to a few hundred micrometers. 
     Following bonding of the wafer stack  18  and wafer  22  ( FIG. 4 ) and optionally thinning the resulting wafer structure  30 , the passage  32  and the surrounding structure can conceivably have a form suitable for use in a variety of microfluidic devices. According to a preferred aspect of the invention,  FIGS. 6 through 10  depict further processing steps suitable for further defining a microtube  50  and a microfluidic device that utilizes the microtube  50 .  FIG. 6  shows the result of masking and etching the surface of the wafer structure  30  opposite the etchstop region  14  to establish what will become the lateral walls of the microtube  50 . For this step, a plasma etch process and a resist mask (not shown) may be employed, though other masking materials and techniques could foreseeably be used, such as an oxide layer, combination of resist and oxide layer, etc. As evident from  FIG. 6 , the etching step is used to only partially etch through the thickness of the wafer structure  30 . The depth of this etch is dependent on the thickness and strength of the wafer structure  10  desired for subsequent fabrication and handling. In  FIG. 6 , less than half the thickness of the wafer structure  30  on either side of the passage  32  has been etched, leaving side portions  34  that interconnect the passage  32  to the remainder of the wafer structure  30 . 
       FIGS. 7 and 8  represent a particular example in which the passage  32  has been defined within the wafer structure  30  to have a U-shaped configuration (when viewed from above) comprising a pair of leg portions  32 A and an interconnecting distal portion  32 B. As will be discussed below, other configurations are possible and within the scope of the invention.  FIGS. 7 and 8  represent cross-sections of the entire wafer structure  30  (instead of the partial sections represented in  FIGS. 1 through 6 ) that are taken transverse to each other, with  FIG. 7  being a cross-sectional view transverse to and through the leg portions  32 A of the U-shaped passage  32  and  FIG. 8  being a cross-sectional view parallel to the leg portions  32 A and transverse to and through the distal portion  32 B. 
       FIGS. 7 and 8  represent the wafer structure  30  after being flipped and bonded to a micromachined and metallized substrate  36 , with the result that the portion of the wafer structure  30  containing the tube passage  32  is cantilevered over a recess  44  in the substrate  36 . The tube wall of the wafer structure  30  that faces the substrate  36  is represented as having an electrically conductive layer  40 . As evident from  FIG. 8 , the conductive layer  40  provides an electrical path that connects the microtube  50  and a metal contact  42  on the substrate  36 , enabling electrical grounding or biasing of the microtube  50 . Various conductive materials can be used as the conductive layer  40 , which may or may not be electrically insulated from the remainder of the wafer structure  30 . Furthermore, the wafer structure  30  may be sufficiently doped (if formed of a semiconductor material) or otherwise formed of an electrically conductive material to render the layer  40  unnecessary. The substrate  36  may be formed of a variety of materials, including Pyrex, borofloat, quartz, or other glass-type wafer, silicon, SOI, plastic, ceramic, or another material. According to a preferred aspect of the invention, the substrate  36  is a glass wafer. A variety of bonding techniques can be employed for this purpose, with anodic bonding being preferred. According to an alternative aspect of the invention, the substrate  36  is a silicon wafer on which a dielectric coating or oxide layer has been formed to provide an electrical insulating layer  38  for the metal contact  42  as well as other metallization on the substrate  36  forming electrical runners, bond pads, etc., for the microfluidic device. A variety of bonding techniques can be employed for this purpose, with fusion bonding being preferred. 
     In the U-shaped configuration of the passage  32  represented in  FIGS. 7 and 8 , a side portion  34 A of the wafer structure  30  is surrounded on three sides (in the plane of the wafer structure  30 ) by the leg and distal portions  32 A and  32 B of the tube passage  32 , and another side portion  34 B surrounds the tube passage  32  along its outer perimeter (again, in the plane of the wafer structure  30 ). By removing the side portions  34 A and B, the external shape of the microtube  50  will also approximate a U-shape similar to that of the tube passage  32 . Inlet and outlet holes  46  (one of which is shown in  FIG. 8 ) can be formed at this time by etching, preferably DRIE. 
       FIG. 9  shows the result of masking and etching the remainder of the wafer structure  30  to remove the side portions  34  and complete the microtube  50  and its outer periphery, including the external surfaces of the walls that surround the tube passage  32 . For this process, a mask (not shown) can be aligned through the substrate  36  to the edges of the side portions  34  or to metallization or the recess  44  on the surface of the substrate  36  using double-side alignment tools or another similar technique known in the art. Alternatively, IR alignment can be employed. After alignment and development, the side portions  34  of the wafer structure  30  can be removed, preferably by DRIE plasma etching. As an alternative method, a single plasma etch could be employed before or after bonding of the wafer structure  30  to the substrate  36 , and tabs or thick scribe street rims could be employed to mechanically reinforce the wafer structure  30  after etching prior to bonding. 
     With the microtube  50  cantilevered over the recess  44  in the substrate  36  as represented in  FIG. 9 , the microtube  50  can be vibrated and its movement induced relative to the substrate  36  in a direction perpendicular to the plane of the microtube  50 . For this purpose,  FIGS. 7 ,  8  and  9  show drive and sensor electrodes  52  and  54  formed within the recess  44  for electrostatic coupling with the microtube  50 . The conductive layer  40  on the lower surface of the microtube  50  facing the electrodes  52  and  54  can serve as an electrode on the microtube  50  for electrostatically driving the freestanding portion of the microtube  50  with the drive electrode  52 . Alternatively, the microtube  50  could be formed to be electrically conductive, such as doped silicon, to enable electrostatic driving of the microtube  50  without a separate electrode. It should be noted that vibration or other desired movement of the microtube  50  relative to the substrate  36  can be induced in the tube  50  by means other than electrostatically, including but not limited to piezoelectrically, piezoresistively, acoustically, ultrasonically, magnetically, optically, or another actuation technique. Movement of the tube  50  can be sensed capacitively, piezoelectrically, piezoresistively, acoustically, ultrasonically, magnetically, optically, or another sensing technique. These actuation and sensing techniques can be applied to the microtube  50  in combination with the substrate  36 , or without the presence of the substrate  36 , or in combination with another substrate such as a product package. 
     In addition to the U-shape represented in the Figures, the microtube  50  and its passage  32  can have a variety of other shapes (in plan view), including but not limited to the C-shaped tubes of U.S. patent application Ser. Nos. 11/620,908, 12/267,263 and 12/369,118, double tubes of U.S. patent application Ser. Nos. 12/143,942 and 12/267,263, S-shaped tubes of U.S. patent application Ser. Nos. 11/620,411 and 12/267,263, and straight tubes of U.S. patent application Ser. No. 12/369,510. The contents of these applications relating to the configurations and use of their microtubes are incorporated herein by reference. Notably, such configurations for the microtube  50  do not necessarily require the presence of the substrate  36 . For example, using straight tubes of the type disclosed in U.S. patent application Ser. No. 12/369,510, the microtube  50  can be vibrated and its vibration sensed within the plane containing the microtube  50 . 
     Finally,  FIG. 10  shows the result of bonding a capping wafer  56  to the substrate  36  to enclose the microtube  50 , preferably vacuum sealing the microtube  50  between the substrate  36  and capping wafer  56  in order to enhance the dynamic performance of the microtube  50  if the microtube  50  is desired to vibrate, for example, in accordance with U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628. A variety of materials can be considered for the capping wafer  56 , including but not limited to silicon, glass, ceramic, and plastic wafers that can be processed to have a sufficiently deep cavity sufficient to accommodate the microtube  50 . The capping wafer  56  is shown as having an integrated getter  58  to improve vacuum quality in accordance with known practices. Depending on the materials of the substrate  36  and capping wafer  56 , sealing of the capping wafer  56  to the substrate  36  can be by glass frit sealing, eutectic bonding, solder bonding, anodic bonding, or other bonding technique known in the art. Alternatively, this step can be omitted if an acceptable vacuum can be formed without wafer-to-wafer bonding. In addition, the capping wafer  56  can be omitted and enclosure of the microtube  50  can be performed in a subsequent packaging step, such as but not limited to IC packaging (e.g., an IC package with a Kovar lid) or product packaging. 
     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.