Abstract:
A process for producing a tube suitable for microfluidic devices. The process uses a uniformly-doped first material having a first portion into which a channel is etched partially through the first material between second and third portions of the first material. The first material is then bonded to a second material so that a first portion of the second material overlies the first portion of the first material and encloses the channel to define a passage. The second and third portions of the second material and part of the second and third portions of the first material are then removed, and the first portion of the second material is bonded to a substrate such that the passage projects over a recess in the substrate surface. The second and third portions of the first material are then removed to define a tube with a freestanding portion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/603,156, filed Aug. 20, 2004. 
    
    
     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 suitable for a microfluidic device. 
     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 and 6,647,778. 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 and 6,647,778, 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. Another consideration of this embodiment is the higher cost of SOI silicon wafers as compared to standard silicon wafers. 
     In view of the above, while well suited for producing micromachined tubes for microfluidic devices, it would be advantageous if other micromachining processes were available that avoid the size restraints of previous processes, as well as potentially simplify processing and reduce cost and processing time. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a process for producing a micromachined tube suitable for microfluidic devices. While resonating tubes for mass flow and density sensors of the types disclosed in U.S. Pat. Nos. 6,477,901 and 6,647,778 are notable examples, other tubular structures within the scope of this invention include stationary tubes, diaphragms, and channels for such microfluidic devices as needles, cannula, pressure sensors, temperature sensors, motion sensors, drug infusion devices, and other devices that can employ microtubes. 
     The process of this invention entails the use of a first material having a thickness throughout which the first material is substantially uniformly doped. A channel is etched in a first portion of the first material in a direction of the thickness thereof. The channel is etched to extend not entirely through the thickness of the first material, through preferably through more than half the thickness, and is between second and third portions of the first material. The first material is then bonded to a second material so that a first portion of the second material overlies the first portion of the first material and encloses the channel therein to define a passage, and so that the second and third portions of the second material overlie the second and third portions of the first material. The second and third portions of the second material and some but not all of the second and third portions of the first material underlying the second and third portions of the second material are then removed, such that the first portions of the first and second materials define a protrusion. The first portion of the second material is then bonded to a substrate adjacent a recess in a surface of the substrate such that a portion of the passage projects over the recess. Finally, the second and third portions of the first material are removed such that the first portion of the first material and the passage therein define a tube, and a freestanding portion of the tube projects over the recess in the substrate so as to be capable of movement relative thereto. 
     In view of the above, it can be seen that the depth and width of the channel in the first material determine the height and width, respectively, of the passage within the tube, and the remaining thickness of the first material following etching of the channel determines the thickness of one of the walls of the tube. Therefore, the height of the tube passage and the thickness of the tube walls are not limited by doped layers or buried oxide layers on or within the first material, permitting the micromachining of larger tubes. Larger tubes produced by this process achieve lower pressure drops and permit higher flow rates within microfluidic systems containing the tubes, without necessitating an increase in the in-plane (width) dimensions of the tube or the substrate carrying the tube. As such, the invention enables tube passage dimensions to be increased vertically (with or without an increase in in-plane dimensions) to achieve higher flow rates, and if horizontal dimensions are held constant can achieve higher flow rates without an increase in chip size, and in some cases while even permitting smaller chip sizes. The elimination of SOI wafers in the manufacturing process is another advantage of the present invention. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  show cross-sectional views of a uniformly-doped first wafer and a second wafer with an epitaxial layer used to produce a micromachined tube in accordance with an embodiment of this invention. 
         FIGS. 2 and 3  depict masking and etching steps performed on the first wafer of  FIG. 1  to form a channel in the first wafer. 
         FIG. 4  depicts the result of bonding the first wafer of  FIG. 3  to the second wafer of  FIG. 1  to enclose the channel and form a passage within the first wafer. 
         FIG. 5  depicts the result of removing all but the epitaxial layer of the second wafer. 
         FIGS. 6 and 7  depict the results of removing the epitaxial layer and underlying portions of the first wafer to either side of the passage, and then bonding the remaining portion of the epitaxial layer to a substrate. 
         FIG. 8  depicts the result of removing the remaining underlying portions of the first wafer to yield a tube with a freestanding portion suspended over the substrate. 
         FIG. 9  depicts the result of bonding a capping wafer to the substrate to enclose the tube. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 9  represent steps in a process carried out to produce a micromachined tube ( 40  in  FIGS. 8 and 9 ) suitable for a variety of microfluidic devices. It should be noted that 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 a pair of wafers  10  and  12  (only a single chip region of which are shown for convenience) selected for processing in accordance with the invention. The wafers  10  and  12  are both preferably silicon, though other materials can be used. The wafer  10  is preferably of constant doping throughout its thickness to permit a uniform rate of etching of the wafer  10 , as discussed below. The type (n or p-type) and doping level can be tailored as may be required or desired by one skilled in the art. Heavy p-type doping (e.g., with boron, aluminum, or gallium) is preferred for improving etching and corrosion resistance, though lighter doping can be used to enable the wafer  10  to be more readily inspected for defects by infrared (IR) radiation. While silicon is preferred, the wafer  10  can be formed from materials that include but are not limited to Ge, SiC, GaAs, Si/Ge, sapphire, glass, ceramic materials, and metallic materials, and can be single crystal or polycrystalline. The thickness of the wafer  10  will typically vary from about 100 to about 1500 micrometers. According to a preferred aspect of the invention, the thickness of the wafer  10  determines the height dimensions of the tube  40  (dimensions measured in a direction normal to the wafer surface). As such, a particularly suitable thickness is about 500 micrometers. 
     The second wafer  12  is represented as an epitaxial wafer, in which an epitaxial layer  14  is supported on a substrate  16 , though it is foreseeable that a SOI wafer or doped single-crystal wafer could be used instead. The epitaxial layer  14  is can be doped similar in type and level to the wafer  10 . For example, in  FIG. 1  the wafer  10  and the epitaxial layer  14  are represented as being doped P+. While an epitaxial layer  14  is shown and preferred, it could be replaced by a diffused layer, a boron or B/Ge doped layer, or a buried doped layer, as will be appreciated by those skilled in the art. From the following discussion it will become evident that the epitaxial layer  14  establishes the thickness of one wall of the tube  40 . Therefore, suitable thicknesses for the epitaxial layer  14  will depend on the desired thickness of the tube walls, which in turn will depend on the particular application for the tube  40 . The substrate  16  supports the epitaxial layer  14  during initial process, and as such its thickness and doping level are not critical to the invention. 
       FIG. 2  shows the wafer  10  as being provided with masks  18  to protect its surfaces from attack during an etching step, the result of which is shown in  FIG. 3 . The masks  18  can be formed by depositing or growing an oxide on the surfaces of the wafer  10 , though other materials known in the art can be used to mask the wafer  10 .  FIG. 3  shows the result of removing a portion of one of the masks  18  and then etching a channel  20  into the wafer  10 . To obtain the roughly rectilinear cross-section shown for the channel  20 , a preferred etching technique is to use a plasma that is anisotropic, in some cases via sidewall deposition. This type of vertical etching is known as reactive ion etching (RIE), dry etching, or deep reactive ion etching (DRIE), as well known in the art. An alternative method of forming the desired vertical/straight sidewalls of the channel  20  is to form the wafer  10  of a ( 110 ) oriented silicon and use a wet etch technique to form the channel  20 . Throughout this description, wet etching is considered an alternative method to the etching steps that have been or will be noted. 
     As previously noted, the entire thickness of the wafer  10  is used to form the tube  40  and determines its height dimensions. Furthermore, the depth of the channel  20  determines the inner height dimension of the passage  36  ( FIGS. 7 and 8 ) within the tube  40 . As such, etching of the channel  20  is a generally long process, and preferably extends through about 75% to about 90% of the thickness of the wafer  10 . As such, a tube passage  36  with a height of up to about 1350 micrometers is possible with this invention if a conventional silicon wafer (thickness of up to 1500 micrometers) is used. 
       FIG. 4  represents the result of cleaning and then bonding the wafer  10  to the epitaxial layer  14  of the wafer  12 . Bonding can be accomplished with 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 mechanism. A high temperature anneal/oxidation can be employed to strengthen the silicon fusion bond. 
       FIG. 5  shows the lighter-doped substrate  16  of the epitaxial wafer  12  as having been removed, such as by lapping, polishing, grinding, wet or dry etching with an etchant selective to lightly-doped substrate  16 , or a combination of these techniques. Selective etching that stops at the heavily-doped epitaxial layer  14  is preferred. In an alternative embodiment in which a SOI wafer is used as the material for the wafer  12 , selective etching that stops at the buried oxide layer of the SOI wafer can be used. 
       FIG. 6  shows the result of masking and etching the epitaxial layer  14  to define an outer wall  24  of the tube  40  and form portions of the sidewalls  26  of the tube  40 . For this step, a resist mask  22  is represented as being 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 , this etching step is used to etch entirely through the epitaxial layer  14  but only partially etch through the thickness of the wafer  10 . As a result, the final depth of this etch is dependent on the total thickness of the epitaxial layer  14  and the wafer  10 , and the strength desired for the remaining portion of the wafer  10 . Leaving a significant amount of the thickness of the wafer  10  will enable the wafer stack to more readily survive manufacturing handling. For this reason, less than half the thickness of the side portions  28  and  30  of the wafer  10  are shown in  FIG. 6  as having been etched away. For example, if the wafer  10  is about 500 micrometers in thickness, this etch might remove up to about 150 micrometers of the wafer  10 . 
     In  FIG. 7 , the wafer stack is shown as having been flipped and bonded to a micromachined and metallized substrate  32 , such as Pyrex, borofloat, quartz, silicon, or other glass-type wafer. A variety of bonding techniques can be employed for this purpose, with anodic bonding being preferred.  FIG. 7  shows the heavily-doped outer wall  24  (formed by the remnant of the epitaxial layer  14 ) as making electrical contact with a metal pattern  34  on the substrate  32  to enable electrical grounding or biasing of the tube  40 . As a result of this step, a portion of the passage  36  formed when the channel  20  was closed by the epitaxial layer  14  is partially suspended above a recess  38  in the surface of the substrate  32 . While inlet and outlet holes  44  (one of which is shown in  FIG. 7 ) can be formed at this time by etching, such holes  44  can be formed during or after any of the following steps. 
       FIG. 8  shows the result of masking and etching the remainder of the wafer  10  to finish defining the tube  40  and its outer periphery, including its sidewalls  26  and its outer wall  42 . For this process, a mask (not shown) can be aligned to the sidewalls  26  of the tube  40  (or to metallization or the recess  38  on the surface of the substrate  32 ) through the substrate  32  using double-side alignment tools known in the art. Alternatively, IR alignment can be employed. After alignment and development, the remaining outer portions  28  and  30  of the wafer  10  are removed, preferably by DRIE plasma etching. As an alternative method, a single plasma etch could be employed before or after bonding of the wafer stack to the substrate  32 , and tabs or thick scribe street rims could be employed to mechanically reinforce the wafer  10  after etching prior to bonding. 
     The tube  40  can have a variety of shapes, including the U-shape (in plan view) of the resonating tubes of U.S. Pat. Nos. 6,477,901 and 6,647,778. If the tube  40  is intended to be vibrated, as is the case for the resonating tubes of U.S. Pat. Nos. 6,477,901 and 6,647,778, the portion of the tube  40  suspended above the recess  38  is a freestanding portion in which movement can be induced relative to the substrate  32 . For this purpose,  FIG. 8  shows a drive electrode  50  formed within the recess  38  for electrostatic coupling with the tube  40 . Because the lower wall  24  of the tube  40  facing the electrode  50  is conductive as a result of being formed by the doped epitaxial layer  14 , a separate electrode is not required on the tube  40  for electrostatically driving the freestanding portion of the tube  40  with the electrode  50 . It should be noted that vibration or other desired movement of the tube  40  relative to the substrate  32  can be induced in the tube  40  by means other than electrostatically, such as piezoelectrically, piezoresistively, acoustically, ultrasonically, magnetically, optically, or another actuation technique. 
     Finally,  FIG. 9  shows the result of bonding a capping wafer  46  to the substrate  32  to enclose the tube  40 , preferably vacuum sealing the tube  40  between the substrate  32  and capping wafer  46  in order to enhance the dynamic performance of the tube  40  if the tube  40  is desired to vibrate in accordance with U.S. Pat. Nos. 6,477,901 and 6,647,778. As evident from  FIG. 9 , the capping wafer  46  must be thicker than the tube  40 , so for a full wafer-thickness tube  40 , a special thick wafer must be employed for the capping wafer  46 . The capping wafer  46  is shown as having an integrated getter  48  to improve vacuum quality. Sealing of the capping wafer  46  to the substrate  32  can be by glass frit sealing, eutectic bonding, solder bonding, anodic bonding, or other bonding technique known in the art. The capping wafer  46  can be formed of a silicon, glass, or other material known and used for this purpose. Alternatively, this step can be omitted if an acceptable vacuum can be formed without wafer-to-wafer bonding. 
     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.