Abstract:
A microneedle and a process of forming the microneedle of single-crystal silicon-based material without the need for deposited films. The microneedle comprises a piercing end, an oppositely-disposed second end, and an internal passage having an opening adjacent the piercing end. The cross-section of the microneedle, and therefore the passage within the microneedle, is defined by first and second walls formed of doped single-crystal silicon-based material and separated by the passage, and first and second sidewalls separated by the passage, sandwiched between the first and second walls, and formed of single-crystal silicon-based material that is more lightly doped than the first and second walls.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/297,775, filed Jun. 14, 2001. 

   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention generally relates to cannula and similar hollow needle-like devices. More particularly, this invention relates to a method of forming miniature needle-like devices from single-crystal silicon-based material using micromachining and wafer bonding techniques. 
   2. Description of the Related Art 
   Medical delivery of drugs has been accomplished for many years using cannula and hollow needles. Metal needles have been miniaturized to very small sizes and integrated with attachments to increase functionality. However, the extent to which metal needles can be miniaturized is limited by processing limitations and the ductility of metals, the latter of which renders metal needles with small diameters prone to bending. In contrast, cannula and needles formed of silicon and silicon-based alloys such as SiGe and SiGeB are not ductile at room temperature and can be micromachined to a much smaller size, typically less than 100 micrometers in diameter, resulting in what is termed herein a microneedle. Because of this capability for greater miniaturization, there is considerable interest in fabricating cannula and other needle-like devices from silicon-based materials. 
   Silicon microneedles have typically been formed by a combination of micromachining and deposited layers. For example, U.S. Pat. No. 5,855,801 to Lin et al. discloses a process of forming microneedle a by wet anisotropic etching single-crystal silicon and depositing silicon nitride to define a microchannel within the microneedle. U.S. Pat. No. 5,928,207 to Pisano et al. discloses a process by which a silicon microneedle is fabricated by wet isotropic etching single-crystal silicon and depositing polysilicon. Another process described in K. Papageorgiou et al., “A Shuttered Probe with In-Line Flowmeters for Chronic In-Vivo Drug Delivery” combines reactive ion etching (RIE) a pattern of diagonal openings in the surface of a silicon substrate to define a grating, undercutting the grating by anisotropic etching to define a microchannel beneath the grating, and then sealing the openings of the grating with deposited films of silicon oxide, silicon nitride or polysilicon.” 
   A drawback to the use of deposited films of silicon oxide, silicon nitride, polysilicon, etc., on a single-crystal silicon micromachined features is the stress that results from grain size variation within deposited films and differences in coefficients of thermal expansion between the deposited films an single-crystal silicon. Such stresses increase the risk of bowing, warping and cracking of the micromachined features, which can lead to mechanical problems and high scrappage rates in the case of cannula and other types of microneedles. Deposited films also limit the wall thickness and internal cross-sectional area of microneedles, thereby limiting the degree to which a microneedle can be miniaturized. 
   SUMMARY OF INVENTION 
   The present invention provides a microneedle and a process of forming the microneedle of single-crystal silicon-based material without the need for deposited films. As a result, the present invention avoids the processing and mechanical problems associated with microneedles formed of deposited films on single-crystal silicon. 
   According to a first aspect of the invention, the device of this invention includes a needle member comprising a piercing end and an oppositely-disposed second end, and an internal passage having an opening adjacent the piercing end. The cross-section of the needle member, and therefore the passage within the needle member, is defined by first and second walls formed of doped single-crystal silicon-based material and separated by the passage, and first and second sidewalls separated by the passage, sandwiched between the first and second walls, and formed of a single-crystal silicon-based material that is more lightly doped than the first and second walls. Accordingly, the structural components that define the passage within the needle member are not required to be formed of a deposited film. 
   The process of this invention generally entails providing a first wafer having a first layer of doped single-crystal silicon-based material and a top layer of doped single-crystal silicon-based material on the first layer, with the top layer being more lightly doped than the first layer. A cavity is etched in the top layer so that the top layer defines the first and second sidewalls, which this time are separated by the cavity. The first wafer is then bonded to a second wafer having a second layer of doped single-crystal silicon-based material, so that the top layer is sandwiched between the first and second layers of the first and second wafers, respectively. Similar to the first layer of the first wafer, the second layer of the second wafer is more heavily doped than the top layer of the first wafer. As a result of the bonding step, the cavity etched in the top layer of the first wafer is delimited by first and second walls defined by the first and second layers, respectively, as well as the first and second sidewalls, yielding the internal passage of the needle member. The first and second wafers are then etched to define the needle member by removing portions of the first, second and top layers to define the piercing and second ends of the needle member, and the opening to the passage adjacent the piercing end. 
   In view of the above, the present invention can be seen as forming a hollow tube using slices (wafers) of single-crystal silicon-based material and wafer bonding techniques, thereby eliminating the requirement for deposited layers and the potential for processing and mechanical problems associated with deposited films on single-crystal silicon. Plasma etching techniques are preferably used to remove portions of the first, second and top layers of the wafers to produce the desired outer perimeter shape of the needle member, including a sharp, tapered point at the piercing end of the needle member. Differences in the doping levels within the layers of the wafers enable etching techniques to be used to minimize the thicknesses of the first and second sidewalls and the first and second walls of the needle member, such that the outer and inner dimensions of the needle member can be minimized. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1 through 8  represent processing steps in the fabrication of a microneedle in accordance with a preferred embodiment of this invention. 
       FIG. 9  is a plan view of a microneedle produced by the method of FIGS.  1  through  8 . 
   

   DETAILED DESCRIPTION 
     FIG. 9  represents a microneedle, more particularly a cannula  10 , suspended within a frame  12  as a result of preferred processing steps of the present invention. While the invention will be discussed in reference to the cannula  10 , essentially any type of microneedle can be fabricated in accordance with the invention, and such microneedles can differ significantly in appearance from the cannula  10  of FIG.  9 . 
   The cannula  10  is shown as being suspended within the interior of the frame  12  by a number of tabs  14 , but otherwise separated from the frame  12  by a trench  72  that delineates the outer perimeter of the cannula  10 . In this configuration, the cannula  10  can be singulated from the frame  12  by breaking the tabs  14 . The cannula  10  can be one of any number of a cannula fabricated in a wafer, in which case the frame  12  would be one of any number of interconnected frames. The cannula  10  is depicted as having a sharp piercing end  16  and a wider second end  18  suitable for attachment to a tube or other conduit (not shown) for delivering fluid to the cannula  10 . A pair of fluid ports  20  and  22  are shown as having been formed in a wall  48  of the cannula  10 . When using the cannula  10  to deliver a fluid, the port  20  located adjacent the piercing end  16  serves as the fluid outlet, while the port  22  located adjacent the second end  18  of the cannula  10  is the fluid inlet. The cannula  10  has a shaft portion  24  between its piercing and second ends  16  and  18 , with the shaft portion  24  being narrower than the second end  18  as a result of a tapered shoulder  26  therebetween. 
   A pair of electrodes  28  are shown as having been formed on the same wall  48  as the inlet and outlet ports  20  and  22 . The electrodes  28  are optional features of the invention, and allow for biochemical monitoring, stimulation functions, etc., as the cannula  10  is used to deliver or extract a fluid. Suitable materials for the electrodes  28  include such bio-compatible metals as titanium, platinum and iridium. Electrical devices (not shown) can be fabricated in and on the surface of the wall  48  of the cannula  10  to assist in the monitoring and stimulation functions. 
   A preferred process for fabricating the cannula  10  of  FIG. 9  begins with a pair of wafers  32  and  34 , shown in FIG.  1 . The wafers  32  and  34  are represented as having lightly-doped p-type single-crystal silicon substrates  36  and  42 , respectively. Alternatively, n-type silicon substrates, silicon-on-insulator (SOI) substrates as well as other types of wafers could be used in the process of this invention. A first of the wafers  32  is represented as having two epitaxial layers  38  and  40  grown on its substrate  36 . The epitaxial layers  38  and  40  are represented as being formed of a silicon-germanium-boron (SiGeB) alloy (e.g., containing less than 30 weight percent germanium) and silicon, respectively, such that the SiGeB epitaxial layer  38  is hetero-epitaxially aligned with the single-crystal silicon substrate  36 , and the Si epitaxial layer  40  is hetero-epitaxially aligned with the SiGeB epitaxial layer  38 . Both epitaxial layers  38  and  40  and the substrate  36  are indicated as being doped p-type, i.e., with boron or another trivalent element (an “acceptor-type” impurity). According to one aspect of the invention, the epitaxial layer  38  serves as an etchstop during etching of the epitaxial layer  40 , by which sidewalls ( 50  and  52  in  FIG. 8 ) of the cannula  10  are defined as discussed below (FIG.  3 ). For this purpose, the epitaxial layer  38  is preferably heavily p-type, e.g., a boron dopant concentration of greater than 1×10 19  atoms/cc. In comparison, the substrate  36  and epitaxial layer  40  may have dopant concentrations of about 1×10 15  atoms/cc, such that the epitaxial layer  38  is more heavily doped than the substrate  36  and epitaxial layer  40 . Alternatively, the substrate  36  and epitaxial layer  40  could be doped n-type. 
   With further reference to  FIG. 1 , the second wafer  32  is represented as having an epitaxial layer  44  grown on its p-type single-crystal silicon substrate  42 . As with the wafer  32 , the epitaxial layer  44  is represented as being a SiGeB alloy, such that the epitaxial layer  44  is hetero-epitaxially aligned with the single-crystal silicon substrate  42 . Also similar to the first wafer  32 , the substrate  42  and its epitaxial layer  44  are indicated as being doped p-type, with the epitaxial layer  44  again being doped more heavily than the substrate  42 , e.g., a dopant concentration of about 1×10 19  atoms/cc for the epitaxial layer  44  and a dopant concentration of about 1×10 15  to about 1×10 17  atoms/cc for the substrate  42 . 
   The epitaxial layers  38  and  44  of the wafers  32  and  34  will define upper and lower walls ( 46  and  48  in  FIGS. 5 through 8 ) of the cannula  10 , while sidewalls ( 50  and  52  in  FIG. 8 ) of the cannula  10  will be defined by the epitaxial layer  40  of the wafer  32 . According to a preferred aspect of the invention, the thicknesses of the epitaxial layers  38  and  44  ultimately determine the thicknesses of their respective walls  46  and  48 , and the thickness of the epitaxial layer  40  ultimately determines the width of the sidewalls  50  and  52 . As a result, the outer dimensions of the cannula  10  can be controlled and minimized by selecting appropriate thicknesses for the epitaxial layers  38 ,  40  and  44 . As an example, suitable thicknesses for the epitaxial layers  38 ,  40  and  44  are in a range of about five to about twenty micrometers, such as about ten micrometers. 
     FIG. 2  represents the result of growing or depositing a pair of masking layers  54  and  56  on the epitaxial layer  40  and the backside of the substrate  36 , respectively. A suitable material for the masking layers  54  and  56  is silicon dioxide, though other materials could be used, such as silicon nitride or a photoresist material. The masking layers  54  and  56  serve to protect the wafer  32  during silicon etching, and for this purpose are grown or deposited to thicknesses of at least 0.5 micrometers. The masking layer  54  is shown in  FIG. 2  as having an opening  58  as a result of the layer  54  having been patterned and etched in any suitable manner, such as chemical etching with hydrofluoric acid (HF) if the masking layer  54  is formed of silicon dioxide. In  FIG. 3 , a cavity  60  has been formed by etching the epitaxial layer  40  through the opening  58  in the masking layer  54  (which has been stripped). The cavity  60  can be performed by plasma or wet chemical etching, or a combination of both. According to a preferred aspect of the invention, the cavity  60  is formed by a two-step etch process, a first step of which is preferably a timed plasma (anisotropic) etch, followed by a wet chemical etch that uses the heavily-doped epitaxial layer  38  as an etchstop. The plasma etch is timed to remove most but not all of the epitaxial silicon beneath the opening  58  in the masking layer  54 . The remaining epitaxial silicon is then removed by wet etching, preferably anisotropically such as with ethylenediamine pyrocatechol (EDP) or potassium hydroxide (KOH). The opposing walls  62  of the cavity  60  will subsequently define the sidewalls  50  and  52  of the cannula  10 . Using a plasma etch for the bulk of the etching process enables the sidewalls  50  and  52  of the cannula  10  to be formed substantially perpendicular to the surface of the epitaxial layer  40 . Completing the etch process with a wet chemical etching using the heavily-doped epitaxial layer  38  as an etchstop enables the thickness of the epitaxial layer  40  to determine the height of the sidewalls  50  and  52  of the cannula  10 . In combination, these etching techniques yield a two-step etching process capable of minimizing the cross-sectional dimensions of the cannula  10 . 
   In  FIG. 4 , the wafers  32  and  34  have been bonded together, with the epitaxial layer  44  of the wafer  34  being bonded to the epitaxial layer  40  of the wafer  32 , with the result that the cavity  60  in the epitaxial layer  40  is closed by the epitaxial layer  44  of the second wafer  34 , yielding a closed cavity  64  within the wafer stack. A preferred bonding technique is silicon direct bonding (SDB), such as silicon fusion bonding (SFB) to produce a hermetic, covalent bond. For this purpose, the mating surfaces of the layers epitaxial layers  40  and  44  are cleaned and then activated, such as by an HF dip. The wafers  32  and  34  are then aligned, pressed together and annealed at about 900° C. to about 1200° C. for a duration of about one to about twelve hours to permanently bond the epitaxial layers  40  and  44  together. 
   After wafer bonding, the lightly-doped substrate  42  of the second wafer  34  is removed by etching (e.g., EDP) or wafer grinding, thereby the exposing epitaxial layer  44  of the second wafer  34 . The portion of the epitaxial layer  44  over the cavity  64  defines one wall  48  of the cannula  10 , shown in plan view in FIG.  9 .  FIG. 5  represents a cross-section through a portion of the wafer stack on which the metal electrodes  28  shown in  FIG. 9  have been formed.  FIG. 6  represents a cross-section through a different portion of the wafer stack than that shown in  FIG. 5 , and shows the result of depositing and patterning an oxide mask  68  on the epitaxial layer  44 , followed by anisotropically etching the epitaxial layer  44  to form an opening  70  through the wall  48  and a trench  72  with portions to either side of the wall  48 . The opening  70  shown in  FIG. 6  is the fluid port  20  of  FIG. 9 , while the trench  72  separates the cannula  10  and the frame  12  in FIG.  9  and therefore defines the outer perimeter of the cannula  10 . As seen in  FIG. 6 , the opening  70  is completely through the epitaxial layer  44  (wall  48 ), thereby breaching the cavity  64  as required for the port  20 . The trench  72  also extends completely through the epitaxial layer  44 , but terminates within the epitaxial layer  40 . The opening  70  and trench  72  are preferably formed by a timed plasma etch that is stopped soon after the opening  70  breaches the cavity  64 . 
   In  FIG. 7 , a handle wafer  74  is shown as having been bonded to the epitaxial layer  44 . The wafer  74  serves to both mechanically support the structure formed by the epitaxial layers  38 ,  40  and  44 , and to chemically protect the etched surface of this structure. For this purpose, the wafer  74  is formed to have a recess  76  that encloses the opening  70  and trench  72 , such that the cavity  64 , opening  70  and trench  72  are protected during subsequent etching, during which the substrate  36  is removed to expose the epitaxial layer  38  (FIG.  8 ). A suitable material for the wafer  74  is glass, such as the borosilicate glass commercially available under the name PYREX. A suitable technique is anodic bonding in accordance with known practices. 
   In  FIG. 8 , the substrate  36  has been removed, and that portion the epitaxial layer  38  over the cavity  64  and exposed as a result of removing the substrate  36  is identified as defining the wall  46  of the cannula  10  opposite the ports  20  and  22  in FIG.  9 .  FIG. 8  also shows the completion of the trench  72  that defines the outer perimeter of the cannular  10 . This step entails final alignment, patterning and anisotropically etching though the surface of the epitaxial layer  38 , with the etch being aligned with the existing trench  72  so that at the completion of the etch the trench  72  extends completely through the epitaxial layer  38  (wall  46 ) and the epitaxial layer  40 . The epitaxial layer  38  is preferably masked during the etching process so that the tabs  14  remain to support the cannula  10  within the frame  12 , which is defined by the remaining portions of the epitaxial layers  38 ,  40  and  44  surrounding the trench  72 . As such, the tabs  14  are formed by the epitaxial layers  38  and  40 . The tabs  14  are preferably sufficiently narrow so that minimal effort is required to singulate the cannula  10  from the frame  12 . 
   As a result of the etch process, the wall  46  of the cannula  10  is isolated from the remainder of the epitaxial layer  38 , and the sidewalls  50  and  52  are delineated from the opposing walls  62  that were defined in the epitaxial layer  40  by the cavity  64 . The sidewalls  50  and  52  can be seen as being separated by the cavity  64  and sandwiched between the walls  46  and  48 . A suitable thickness for each of the sidewalls  50  and  52  is roughly that of the walls  46  and  48 , and therefore the epitaxial layers  38  and  44 , i.e., about five to twenty micrometers. From  FIG. 8 , the thicknesses of the sidewalls  50  and  52  can be seen as being established by the alignment, location and width of the trench  72 . For this reason, a plasma etch is again preferably used to complete the trench  72 . As a result of the walls  46 ,  48 ,  50  and  52  of the cannula  10  having substantially uniform thicknesses and the cavity  60  and the trench  72  being defined by anisotropic etching, the cavity  64  defines an internal passage within the cannula  10  having a substantially rectangular cross-section and the piercing end  16  has a tapered width in a direction parallel to the walls  46  and  48  and a substantially uniform thickness in a direction normal to the walls  46  and  48 . 
   While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in art. For example, the physical configuration of the cannula  10  could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.