Abstract:
A suspended p-GaN membrane is formed using photochemical etching which membrane can then be used in a variety of MEMS devices. In the illustrated embodiment a pump is comprised of the p-GaN membrane suspended between two opposing, parallel n-GaN support pillars, which are anchored to a rigid substrate below the pillars. The p-GaN membrane bows upward between the pillars in order to relieve stress built up during the epitaxial growth of membrane. This bowing substantially increases the volume of the enclosed micro-channel defined between membrane and substrate below. The ends of membrane are finished off by a gradual transition to the flat underlying n-GaN layer in which fluidic channels may also be defined to provide inlet and outlet channels to microchannel. A traveling wave or sequential voltage applied to the electrodes causes the membrane to deform and provide a peristaltic pumping action in the microchannel.

Description:
RELATED APPLICATIONS 
     The present application is related to U.S. Provisional patent application Ser. No. 60/224,106 filed on Aug. 9, 2000. 
    
    
     GOVERNMENT SPONSORED RESEARCH 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. N00014-99-1-0972 awarded by the Office of Naval Research. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a peristaltic micro-pump fashioned in the column III-nitride material system as well as the broad processing technology used to fabricate suspended micro-devices in this same material system. 
     2. Description of the Prior Art 
     In designing the driving system of a biochip, three approaches have been used in the prior art. They are the on-chip mechanical micropump, the on-chip electro-kinetic micropump and the external servo system. An on-chip mechanical micropump may be prepared directly by the micro-machining technology. If this approach is adopted, a moveable part is provided inside the microchannel of the chip. The “electrostatically driven diaphragm micropump” shown by Roland Zengerle et al. in their U.S. Pat. No. 5,529,465 is a typical example. In the Zengerle device, the micropump includes a pressure chamber. Reciprocal pumping power is generated by electrostatics. With the help of two passive check valve, microflows are driven with a 350 mμl/min working velocity. 
     A simplified “micromachined peristaltic pump” was disclosed by Frank T. Hartley in U.S. Pat. No. 5,705,018. In this device, a series of block flexible conductive strips are positioned in the internal wall of a microchannel. When a voltage pulse passes along the microchannel, the flexible conductive strips are uplifted in sequence by the electrostatics so generated, such that a peristaltic movement is generated. This peristaltic movement drives the microflow along the microchannel. In the Hartley device, the working velocity is about 100 mμ/min. 
     The on-chip mechanical micropump does not provide the function such that the chip may be repeatedly used for different samples. This is because a microchannel with moveable parts is difficult to clean up residual samples or biochemical reagents after the reaction. Another problem is that the on-chip mechanical micropump, especially the peristaltic pump, involves expensive material costs. These biochips are not suited for disposable applications. 
     Micro-fluidic pumps fabricated in Column III-nitride materials offer several advantages over existing implementations. For one, Column III-nitride materials offer high chemical inertness and high temperature stability, making the micropumps suitable for harsh or corrosive environments. In addition, these micropumps can be readily integrated on a single chip with the broad spectrum of opto-electronic, high speed and high power devices possible in the Column III-nitride semiconductors. As described below, these micropumps employ a comparatively simple and reliable pumping mechanism. Furthermore, they are fabricated from a versatile processing technology which enables a broad range of device layouts for superior microscopic fluid control. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a versatile processing technology for the fabrication of micro-electromechanical systems in GaN. This technology, which is an extension of conventional photo-electrochemical (PEC) etching, allows for the controlled and rapid undercutting of p-GaN epilayers. The control is achieved through the use of opaque metal masks to prevent etching in designated areas, while the high lateral etch rates are achieved by biasing the sample relative to the solution. For GaN microchannel structures processed in this way, undercutting rates in excess of 30 μm/min have been attained. 
     The invention is illustrated in the fabrication of a micropump comprising an electro-deformable membrane and a substrate disposed below the membrane and coupled thereto. A microchannel is defined between the membrane and substrate. The microchannel is formed so as to have a longitudinal axis. An electrode structure is disposed on at least one side of the membrane along side of the microchannel. 
     The electro-deformable membrane is bowed to form a curvature having a symmetrical axis in the direction of the longitudinal axis of the microchannel. 
     The micropump further comprises a drive circuit coupled to the electrode structure to apply a sequential voltage along the plurality of opposing electrodes to peristaltically deform the electro-deformable membrane in the direction of the longitudinal axis of the microchannel. 
     In the illustrated embodiment the electro-deformable membrane is composed of p-type GaN, but any material having the same or similar electro-deformable properties may be employed. 
     The micropump further comprises two opposing pillars disposed on the substrate between the substrate and the membrane generally aligned in the direction of the longitudinal axis. The two opposing pillars are composed of n-type GaN. 
     The electrode structure is comprised of two opposing electrode substructures extending parallel to the microchannel. The two opposing electrode substructures each comprise a plurality of discrete electrodes arranged and configured to provide pairs of opposing electrodes on each side of the microchannel. Many equivalent electrode structures to a series of opposing electrodes may be used, including propagation line electrodes in which a traveling wave potential may be placed. It may also be possible for a single electrode rail to be provided to provide the traveling wave potential with the opposing side of the membrane left to float or grounded by an opposing rail or any other conductive means. 
     The invention is also characterized as a method of micropumping comprising the steps of providing a bowed electro-deformable membrane disposed above a substrate and coupled thereto so that a microchannel is defined between the membrane and substrate. A traveling wave potential is propagated along the electro-deformable membrane in the direction of the longitudinal axis. As a consequence, the electro-deformable membrane is peristaltically deformed by the traveling wave potential and hence fluid is pumped in the microchannel along the longitudinal axis. 
     The step of providing a traveling wave potential comprises the step of applying a potential across the electro-deformable membrane traverse to the longitudinal axis and sequentially applied along the longitudinal axis. More specifically, in one embodiment the step of providing a traveling wave potential comprises sequentially applying a plurality of discrete potentials across the electro-deformable membrane traverse to the longitudinal axis. 
     The step of providing a bowed electro-deformable membrane comprises providing p-type GaN membrane and two opposing pillars composed of n-type GaN under the p-type GaN membrane to anchor and space the membrane apart from an underlying substrate. The illustrated method of making the bowed electro-deformable membrane comprises the step of forming the n-type GaN pillars and the p-type GaN membrane by selectively photo-electrochemical etching two adjacent n-type GaN and p-type GaN layers. 
     In general the step of providing a traveling wave potential is provided by an electrode structure of two opposing electrode substructures extending parallel to the microchannel. The electrode substructures may be continuous or discrete. In the illustrated embodiment the traveling wave potential is supplied by the two opposing electrode substructures comprises across a plurality of discrete electrodes which are arranged and configured to provide pairs of opposing electrodes on each side of the microchannel. 
     While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified diagram of the principle parts of a GaN pump devised according to the invention. 
     FIGS. 2 a - 2   d  are computer simulation perspective net drawings of the membrane in isolation of other elements of the pump shown in a time sequence to illustrate the peristaltic pumping action. 
     FIG. 3 a  is a scanning electron microscopic photograph of a side view of a GN micro-pump of the invention, wherein the shaded region corresponds to the micro-channel for fluid flow. 
     FIG. 3 b  is a scanning electron microscopic photograph which shows an enlarged view of a section of the bowed 1.2μ p-GaN membrane. 
     FIG. 3 c  is a scanning electron microscopic photograph of a top plan view along the channel of a GaN micro-pump. On the left the dark strip running down the center corresponds to the suspended p-GaN film. On the right, a voltage has been applied across the channel causing the membrane to actuate or deflect. The direction of fluid flow is vertical in the images. 
     FIGS. 4 a - 4   e  are simplified cross-sectional side view of one methodology whereby the membrane of the invention may be fabricated. 
    
    
     The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In recent years, gallium nitride has established its place in the arena of solid-state devices, with applications ranging from light-emitting diodes and visible-blind photodetectors to high power Shottky diodes and ultra-fast high electron mobility transistors (HEMTs). Several material properties of GaN also make it a promising candidate for micro-electromechanical (MEMS) applications. Among the properties which set it apart from silicon, the conventional choice for MEMS, is its large piezoelectric response. This response would provide a powerful means for the excitation and detection of acoustic waves in micro-resonators. In addition, the strong piezoresistive effect in p-GaN is ideal for electrical strain sensing in micro-positioners. Furthermore, chemical inertness and high temperature stability make GaN a suitable choice for MEMS applications in harsh environments. Transparency to visible wavelengths also allows it to feature in optical micro-switches and waveguides. The methodology of the invention allows for the fabrication of a diverse range of suspended GaN microstructures. 
     The disclosed process exploits the dopant selectivity of photo-electrochemical (PEC) etching to undercut p-GaN layers grown on sacrificial n-GaN layers. PEC etching of GaN is achieved by exposing it to above bandgap radiation while immersed in an aqueous KOH solution. It is believed that band-bending at the n-GaN/electrolyte interface causes photogenerated holes to be swept toward the surface where they participate in the chemical dissolution of the semiconductor. In p-GaN, the bands bend in the opposite sense, creating a barrier for hole migration to the surface. Undercutting of p-GaN layers has also been observed and recently studied using backside illumination through the sapphire substrate. The fabrication of complex microstructures in GaN, however, requires that the undercutting be precisely controlled and optimized. First, etching must be prevented in regions of the n-type underlayer designed to provide mechanical anchoring for the p-type membrane above. Furthermore, for structures with a large undercut span, the lateral etch rate must be high to achieve a practical total etch time. 
     Photo-electrochemical etching (PEC) of column III-nitride (GaN, AlN, InN and their ternary alloys) can be used according to the methodology of the invention to fabricate a variety of micro-electromechanical devices, including but not limited to the micropump described above. For GaN, the PEC etching process is achieved by exposing the material to above bandgap UV radiation (&lt;365 nm) in an aqueous etchant solution. Under these conditions, n-type doped GaN etches rapidly, while p-type GaN remains unaffected. This dopant selectivity of PEC etching, combined with the UV light sensitivity, allows for the fabrication of p-GaN suspended microstructures as illustrated in greater detail below in connection with FIGS. 4 a - 4   e.    
     FIG. 4 a  is a side cross-sectional view of the beginning step of the method wherein a sapphire substrate  11  is provided. A sacrificial n-GaN base layer  13  is formed on substrate  11  as illustrated in FIG. 4 b . A thin p-GaN layer or membrane  12  is grown epitaxially on sacrificial n-GaN base layer  13  as also shown in FIG. 4 b . During the PEC process illustrated in FIGS. 4 a - 4   e , a portion of n-GaN base layer  13  is selectively undercut or etched away, leaving the upper p-GaN membrane  12  freely suspended. This suspended membrane  12  is formed as follows. A patterned opaque metal mask  22  is deposited on the p-GaN over-layer  12  and is used to prevent UV exposure in certain areas of n-GaN base layer  13  during the etch step in FIG. 4 d . This allows masked regions of the n-GaN base layer  13  to be locally protected from etching in order to leave structural support or pillars  14  for the thin p-GaN film  12  above. Large p-GaN areas can be undercut in this way, with lateral etch rates approaching 100 mm/min. 
     FIG. 4 d  shows the salient features of the etch setup used for controlled undercutting. In the illustrated embodiment, p-on-n bilayer samples  12 ,  13  were immersed in 0.1 M KOH and exposed from the front side by a Xenon arc lamp (not shown) with 100 mW/cm 2  in the UV. Prior to the PEC etch, opaque metal masks  22  (Ni/Au—80 nm/20 nm) were patterned onto the samples and then annealed at 500° C. for 5 minutes in Ar to prevent peeling in the corrosive bath. As indicated in the FIG. 4 d , we observed that the n-type epilayer  13  does not etch in the areas immediately below the masks  22 . However, masked regions near the outermost periphery of overlayer  12  undercut very slowly as a result of stray UV radiation that is reflected back through the sapphire substrate  11  directly into the n-GaN layer  13 . To suppress this reflection, the samples  12 ,  13  were suspended in solution by a Ni wire epoxied near the side. This problem can be effectively eliminated by using backside polished substrates with a thin SiO 2  anti-reflection coating. 
     The Ni wire also served as an electrical contact to the p-GaN overlayer  12  during the PEC etch step. It was maintained at a positive 1.5 V bias with respect to a Pt cathode  15  in solution  17 . The application of this bias was seen to dramatically accelerate the undercutting of the unmasked p-GaN areas  13 , with lateral etch rates in excess of 30 μm/min being observed for certain geometries. The origins of this marked increase in etch rate are not well understood at this time. However, observations of the undercutting dynamics suggest that the sample bias gives rise to drift currents of the electrolyte within the narrow etched channels under the p-GaN film  12 . We suspect these currents deliver chemically active OH −  radicals to the etch front much more efficiently than diffusion alone. 
     What results is the microchannel  20  shown in FIG. 4 e  which is described in greater detail below. An example of one of the devices that are possible with this processing technology is a micro-fluidic pump  10  depicted in the perspective view of FIG.  1 . The pump  10  is comprised of a p-GaN membrane  12  suspended between two opposing, parallel n-GaN support pillars  14  which are anchored to a rigid substrate  16  below pillars  14 . As depicted in FIG. 1, the p-GaN membrane  12  bows upward between the rigid support pillars  14  to relieve compressive strain in the film resulting from the original epitaxial growth process. This bowing substantially increases the volume of the enclosed micro-channel  20  defined between membrane  12  and substrate  16  below. The amount of bowing and the strain developed in membrane  12  can be varied according to conventional means to assume a wide variety of values. The termination of the longitudinal ends of microchannel  20  may be completed in any one of a number of ways using conventional micromaching techniques, such as chemically assisted ion beam etching (CAIBE), all of which are considered equivalent for the purposes of the present invention. Opposing sets of metallic contact pads  18   a  and  18   b  can then be patterned above the support pillars on the upper surface of p-GaN membrane  12  using standard lithographic techniques. These metal pads provide electrical contact to the micro-channel for the purpose of electro-actuation of the pump. 
     The micropump having now been described in general terms, consider the fabrication of the suspended membrane  12  of FIG. 1 in greater detail. An example of the diverse microstructures which can be realized using this etch process is the GaN microchannel shown in FIG.  1 . The microchannel  20  is comprised of an 1 μm thick p-GaN membrane  12  that spans between two long anchoring strips  14  on either side. To fabricate this structure, a series of Ni/Au bars (not shown, but later divided into pads  18   a  and  18   b ) with 100 μm spacing between the bars across was to become channel  20  were patterned on a p-on-n bilayer sample  12 ,  13  using standard lithographic techniques. The sample was then exposed to the PEC etch described above, during which the unmasked regions between the bars were undercut. Etching of n-GaN underlayer  13  proceeded inward from both sides in the direction of the bars. A total undercut channel length of 5 μm etched to completion in roughly 2 hours. Afterward, the metal masks were removed in places, leaving a series of isolated contact pads  18   a  and  18   b  along the anchored sidewalls. 
     The GaN layers  13  used here were grown by molecular beam epitaxy on c-plane sapphire  11  with no buffer layer. Both the n+ (Si) and the p+ (Mg) epilayers are 1 μm thick, and the growth temperature in each case was 800° C. and 700° C. respectively. Both layers are thought to have carrier concentrations in the range of 10 18 /cm 3 . 
     The surface quality of the p-type film  12  does not appear to degrade as a result of the lengthy PEC etch. Furthermore, the underside of the suspended p-GaN film  12  is smooth and featureless. This is in marked contrast to our observations of MOCVD grown p-on-n samples, for which the undersides are rough and coated with etch-resilient whiskers. 
     As seen in FIG. 1, the p-GaN membrane  12  bows upward after release to relieve inherent stress. A maximum vertical deflection of 9.2 μm is measured at the center of the 100 μm channel width. We believe the primary origin of this stress is the thermal mismatch between the GaN epilayer  13  and the sapphire substrate  11 , integrated down from growth temperatures. Measurements of the expanded length of the bowed film correspond to a biaxial compressive strain of 1.0×10 −3  in the p-GaN layer prior to release. However, we have observed strong evidence that the stress profile in the p-layer  12  is far more complicated: p-GaN cantilever structures relax into a shape which is uniformly curved away from the substrate  11 . This bending suggests there are vertical stress gradients in the p-layer  12 , perhaps built in at the time of growth as a result of the different lattice constants for Mg and Si doped GaN. 
     Consider now the method of operating the pump of FIG.  1 . By applying a voltage across a pair of opposing metal contacts  18   a  and  18   b , membrane  12  can be made to flatten locally in the intervening region between pillars  14 . Sequential actuation of membrane  12  in this manner will induce a peristaltic wave motion along the length of device  10 , as depicted in FIGS. 2 a - 2   b - 2   c - 2   d , which are computer simulation perspective net drawings of membrane  12  in isolation of other elements of pump  10  shown in a time sequence. FIG. 2 a  shows membrane  12  in equilibrium without any applied voltage to it. The remaining images of FIGS. 2 b - 2   d  show membrane  12  actuated at successive points along its length by sequential application of a voltage to opposing pairs of contacts  18   a  and  18   b  along the edges of membrane  12 . A constriction of membrane  12  can be seen in the sequence of FIGS. 2 a - 2   d  moving from left to the right end of membrane  12  as seen in the view of the drawings. This wave motion can be used to pump fluids down the length of the micro-channel  20  with a peristaltic wave motion. 
     Several GaN micro-pumps have been successfully fabricated and tested with varying channel widths and lengths. FIG. 3 a  is a scanning electron-microscope perpendicular cross-sectional side-view image of a pump  10  with no applied bias. FIG. 3 b  is an enlarged cross-sectional side-view image of a portion of FIG. 3 a . The width of the channel measured between pillars  14  is 200 μm, and the maximum vertical deflection of the p-GaN membrane  12  above the substrate  16  is 10 μm. 
     Optical microscope images of a top plan view along the length parallel to pillars  14  of device  10  of FIG. 3 a  are displayed in FIG. 3 c  with the bowed p-GaN membrane  12  viewed from above. On the left half of FIG. 3 c , the membrane  12  is in equilibrium without external bias. On the right half of FIG. 3 c  a voltage is applied across the channel  20  has caused membrane  12  to flatten locally. With the aid of a conventional timing circuit to apply voltage sequentially along the longitudinal length of channel  20 , peristaltic motion has been successfully demonstrated in these devices. For the channel depicted in FIG. 3, the voltage required to cause full constriction of the membrane is approximately 20 V. At a fixed point along the longitudinal axis, a complete actuation cycle can be performed at a maximum rate of 100 Hz. With a spacing of 100 μm between contact pads along the longitudinal axis, the peristaltic wave velocity down the channel is roughly 1 cm/s. The corresponding pumping capacity of the channel in FIG. 3 is 0.01 μL/s. 
     Thus, it can now be readily understood that the versatile PEC processing methodology can be used to create either p or n type nitride suspended membranes of variable bowing or curvature for use in a wide variety of microdevices of which the micropump  10  is only one of a myriad of possibilities. It is to be expressly understood that the method of making the nitride suspended membrane is generally applicable as a fabrication technique for the manufacture of a membrane element in any device now known or later devised. 
     GaN micro-pump  10  provide a technologically convenient way to control fluid motion in microscopic channels  20 . These pumps  10  could find application in a large range of settings, wherever peristaltic pumping of fluid in a microfluidic device or hydraulic circuit is needed, including without limitation fuel cells, water filtration, blood regulation, and micro-chemical analysis devices. 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.