Abstract:
A passive microvalve comprises a substrate having a first aperture. An upper plate is mounted on the substrate and has a second aperture opening into a space defined between the substrate and the upper plate. A closure member is mounted in the space and is movable between a first position in which it allows fluid flow between the first and second apertures and a second position in which it closes the second aperture.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a passive microvalve that may be used to control the flow of liquid or gas, for example, in a microfluidic system.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    In the prior art there are several types of passive microvalve that can be used for the control of flow within microfluidic systems. The fabrication of most of these valves is achieved using silicon micromachining techniques.  
           [0003]    In U.S. Pat. No. 5,375,979, microvalves were fabricated by bulk micromachining of single crystal silicon. The valves comprise a silicon cantilever beam or plate that will deflect in response to applied forward or reverse pressure to open or shut respectively. EP-A-0483469 describes the fabrication of a similar type of cantilever valve by surface micromachining techniques.  
           [0004]    U.S. Pat. No. 5,529,465 describes a micropump comprising surface micromachined valves. A cantilever bridge is hinged down on all sides and has a flap or plate in the centre supported by flexible arms. The centre plate will open and close the valve during forward and reverse pressure respectively.  
           [0005]    The above microvalves operate in the direction where fluid flows firstly through a hole in the substrate and then outwards through the valve. In applications such as micropumps this necessitates the inlet and outlet valves to be formed on opposite surfaces of the silicon wafer. The requirement for precision processing on both wafer surfaces results in complex fabrication processes and is a barrier to further miniaturization of the overall systems.  
           [0006]    U.S. Pat. Nos. 5,171,132, 5,259,737, and 5,759,015, and WO-A92/01160 describe bulk micromachined valves that allow the inlet and outlet valve to be combined for micropump applications. The valve wafer is enclosed by a cavity wafer and bonded to another wafer with inlet and outlet holes or channels.  
           [0007]    Microvalves in the prior art, which were fabricated utilising bulk or surface micromachining require processing of at least three substrates when used as stand alone devices or when integrated into other microfluidic devices, for example a micropump.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the present invention, a passive microvalve comprises a substrate having a first aperture; an upper plate mounted on the substrate and having a second aperture opening into a space defined between the substrate and the upper plate; and a closure member mounted in the space and movable between a first position in which it allows fluid flow between the first and second apertures and a second position in which it closes the second aperture. Typically, this microvalve will be fabricated as an outlet valve, the first aperture constituting an outlet aperture and the second aperture constituting an inlet aperture.  
           [0009]    The present microvalve invention minimises the number of substrates being utilised or processed. This will simplify the overall fabrication process, and further improve the miniaturisation of the overall system. The valve is preferably fabricated utilising surface micromachining techniques. The direction of flow is such that fluid flows first through the microvalve then through the substrate, eg wafer, i.e. in the opposite direction to surface valves in the prior art. A major advantage of the present invention is that in the fabrication of structures that require combinations of inlet and outlet valves, such as in micropumps, both types of valve can be fabricated on the same substrate surface. For most applications of the valve, processing is only required on two substrates.  
           [0010]    Fluid flow through the uni-directional valve (or check-valve) is in the opposite direction to that of prior art designs and is thus of considerable potential in integrated systems.  
           [0011]    The closure member can be mounted in a variety of ways on the substrate. For example, the closure member may be supported on a flexible arm in a cantilever fashion. Preferably, however, the closure member is supported by a set of flexible arms, typically four, and conveniently the closure member and arms are fabricated as a flexible diaphragm.  
           [0012]    In some cases, the closure member can be constructed so that it is physically incapable of moving so as to close the first aperture but conveniently the microvalve further comprises at least one stop to limit movement of the closure member towards the first aperture. This stop could be provided on the closure member but is preferably provided on the substrate.  
           [0013]    As mentioned above, the microvalve can be formed using conventional micromachining techniques from typical semiconductor materials such as silicon or gallium arsenide, glass, polyamide, quartz, ceramic, plastic or metal. In the preferred arrangement, the substrate is formed by a single crystal silicon wafer while one or both of the upper plate and closure member are formed by polycrystalline silicon.  
           [0014]    The microvalve according to the invention can be used in a wide variety of applications including microfluidic devices such as micropumps or multichannel analysers.  
           [0015]    Thus, a micropump may comprise a substrate and a flexible diaphragm located over the substrate to define the pump chamber; an inlet microvalve and an outlet microvalve mounted on a side of the substrate facing the flexible diaphragm and in communication with respective apertures extending through the substrate, at least one of the microvalves being constructed in accordance with the present invention; and an actuator for causing the diaphragm to flex and thereby cause fluid flow in a pumping direction through the micropump. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Some examples of passive microvalves, a micropump and methods for fabricating the microvalves will now be described with reference to the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is a schematic cross-section of an example of a microvalve;  
         [0018]    [0018]FIGS. 2 a - 2   c  are plan views of the three principle layers of the microvalve shown in FIG. 1;  
         [0019]    [0019]FIGS. 3 a  and  3   b  are cross-sections similar to FIG. 1 but showing the condition of the microvalve under different pressure distributions;  
         [0020]    [0020]FIG. 4 is a schematic cross-section of a microvalve incorporated into a microfluid device;  
         [0021]    [0021]FIG. 5 is a schematic cross-section of a microvalve incorporated into a micropump;  
         [0022]    FIGS.  6 - 13  illustrate different stages in the fabrication of a microvalve; and,  
         [0023]    [0023]FIGS. 14 and 15 illustrate graphically the results of tests carried out on a microvalve. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0024]    The passive uni-directional microvalve structure comprises a movable valve plate supported by flexible arms. The valve plate moves, according to the applied pressure, to either increase or decrease the flowrate of liquid or gas.  
         [0025]    [0025]FIG. 1 shows a schematic cross-section of the microvalve with plan views of the three principal layers shown in FIG. 2. A substrate layer  1 , FIG. 2 a , provides an outwardly tapering outlet aperture  2  for the fluid flow. A second layer  3 , FIG. 2 b , consists of a movable valve plate or closure member  4  with flexible supporting arms  5 , the outer ends of which are attached to the substrate  1 . Fluid can flow through holes  6  around these supporting arms  5  during operation of the valve. A top layer  7   a , FIG. 2 c  defines a valve closure bridge containing an inlet aperture  7   b  to enable fluid flow. The top plate  7   a  and substrate  1  define a valve chamber  8  between them.  
         [0026]    Under forward pressure the movable valve plate  4  will deflect downwards, as shown in FIG. 3 a , and allow the fluid to flow through the inlet aperture  7   b  on the bridge  7   a . Fluid will then flow through the holes  6  around the supporting arms  5  and finally through the outlet hole  2  in the substrate  1 . In order to prohibit the valve plate  4  from closing tightly onto the outlet aperture  2  at high forward pressure, hence blocking the flow, pillars  9  are fabricated in the bottom layer  1 . At reverse pressure the valve plate  4  is lifted and pushed against the valve bridge  7   a , as illustrated in FIG. 3 b , closing the gap and stopping the flow.  
         [0027]    In alternative embodiments of the device, the inlet hole  7   b , outlet hole  2  and/or the valve plate  4  may be circular. Depending on the manufacturing process it may be advantageous to incorporate holes in the flexible support arms  5  to aid removal of underlying sacrificial layers. The valve plate  4  may be fabricated in such a way that it is thicker than the flexible supporting arms  5  thus increasing its rigidity.  
         [0028]    The substrate  1  is typically a single crystal silicon wafer. A plurality of valves can be fabricated simultaneously on this wafer if desired. The technology is such that other microfluidic components can also be simultaneously fabricated to enable systems or multiple systems to be produced on each silicon wafer.  
         [0029]    [0029]FIG. 4 illustrates how the valve, indicated at  10 , may be incorporated in a flow channel as part of a microfluidic system  11 . While three substrates are employed only two require to be patterned. The upper substrate  12 , which may be glass, silicon or other suitable material, has a channel or recess  13  formed in its lower surface. The middle substrate  14 , typically a silicon wafer, contains the microvalve  10  with an additional flow channel  15  incorporated in the lower surface. The bottom substrate  16 , of silicon or glass, encloses the outlet channel  15  (The substrate  14  takes the place of substrate  1 ). During operation, fluid can flow via the inlet channel  13 , through the valve  10  and out the outlet channel  15 . If pressure is applied in the opposite direction the valve will close to reduce or prevent fluid flow.  
         [0030]    An example of how the microvalve can be integrated into a micropump is shown in FIG. 5. The micropump  20  comprises a silicon or glass substrate  21  supporting a middle substrate  22  which in turn supports an upper substrate  23 . The middle substrate  22  has been formed with inlet and outlet channels  24 , 25  which communicate with respective inlet and outlet apertures  26 , 27 . The middle substrate  22  is also formed with an inlet valve  28  which may have a similar form to the layer  3  of the microvalve shown in FIG. 1 and can be flexed so as to close the aperture  26 . In addition, the middle substrate  22  is formed with an outlet valve  29  having a construction similar to the microvalve of FIG. 1.  
         [0031]    A pump chamber  30  is formed between a diaphragm  31  of the upper substrate  23  and the middle substrate  22 . The substrate  23  is attached to the substrate  22  following formation of the diaphragm  31 . Although three substrates are required, it is only necessary to pattern two of these.  
         [0032]    To operate the pump, a means of deflecting the diaphragm  31  is required; this may be achieved through the use of a piezoelectric, thermal or other type of actuator  32 . When the diaphragm  31  is deflected downwards, the increased pressure in the pump chamber  30  will cause the outlet valve  29  to open and fluid to flow into the outlet channel  25 . As the diaphragm  31  returns to rest or is deflected upwards the reduced pressure in the chamber  30  will cause the outlet valve  29  to close and the inlet valve  28  to open allowing the pump chamber to fill with fluid from the inlet channel  24 . Continued repetition of this action will thus pump fluid from inlet to outlet.  
         [0033]    Microvalve Fabrication  
         [0034]    The microvalve structure can be fabricated using a combination of surface and bulk micromachining techniques. One method by which the microvalve can be fabricated is described here, however, those skilled in the art will recognise the possibility of alternative fabrication methods.  
         [0035]    The process described here employs Local Oxidation of Silicon (LOCOS) and Chemical Mechanical Polishing (CMP) techniques to assist in maintaining a reasonably planar surface during the surface micromachining process.  
         [0036]    The fabrication of the base substrate  1  starts by covering a silicon substrate wafer  40  with a silicon dioxide layer  41  (typically 100 nm thick) and a silicon nitride layer  42  (typically 100 nm thick) . These layers are then patterned using standard photolithography techniques and the exposed silicon is etched, typically to a depth of 1.5 μm, forming a recess  43  with four protruding pillars  9 , as shown in FIG. 6. Thermal oxidation is used to form a recessed layer of silicon dioxide  44  according to the standard LOCOS process, FIG. 7.  
         [0037]    The silicon nitride layer  42  is removed and a layer of silicon dioxide  45  (typically 1 μm thick) is deposited and etched to expose the silicon wafer surface around the perimeter of the valve, FIG. 8. Next, a thick polycrystalline silicon layer  46  (typically 3 μm) is deposited. This is polished to produce a flat surface, FIG. 9. Any polycrystalline silicon which formed on the back surface of the wafer is removed.  
         [0038]    [0038]FIG. 10 shows the formation of the valve closure plate  4  and the four supporting arms  5 . This is performed using a LOCOS technique to maintain a planar surface. The wafer from FIG. 9 is coated with silicon dioxide  47  and silicon nitride  48  layers that are then patterned. The exposed polycrystalline silicon regions are partially etched  49  to remove approximately ½-⅔ the thickness of the layer. Silicon dioxide  50  is thermally grown to consume the remaining polycrystalline silicon in the etched recesses. The oxide surface becomes approximately level with the top of the polycrystalline silicon  46 . The nitride layer  48  is subsequently removed.  
         [0039]    A silicon dioxide layer  51  (typically 1 μm) is deposited and etched to expose the first polycrystalline silicon layer  46 , FIG. 11. A second layer of polycrystalline silicon  52  (typically 2.5 μm thick) is deposited and etched to form the valve closure bridge with aperture  7   b , FIG. 12. The polycrystalline silicon at the back surface is removed by plasma etching.  
         [0040]    A layer of silicon nitride  53  (typically 200 nm) is deposited. The nitride layer at the front surface will act as protection for the polycrystalline valve structure. At the back surface, the nitride layer is patterned and the silicon, etched through in potassium hydroxide (KOH) solution FIG. 13 to form aperture  2 . Finally the oxide layers are removed using concentrated hydrofluoric acid (HF) solution to release the valve structure.  
         [0041]    Typical flow characteristics for microvalves of this design, fabricated using the above process are shown in FIG. 14 and FIG. 15. The forward and reverse flow measurements were evaluated by flowing de-ionised water through the valve. FIG. 14 represents the forward flow-pressure characteristics of a microvalve with supporting arm lengths of 400 μm and 500 μm. The reverse flow-pressure characteristics are as shown in FIG. 15. The maximum forward flow rates are approximately 10 times higher than the reverse flow for these particular devices.