Abstract:
The invention relates to a micro-machined fluid-flow device ( 10 ) comprising a substrate ( 12 ) possessing a flow duct ( 14 ), a deformable thin layer ( 18 ) such a pump membrane or a valve-forming membrane. According to the invention, the thin layer ( 18 ) is a rolled metal sheet, preferably made of titanium, and connected to the substrate ( 12 ) in the zone ( 20 ) overlapping the flow duct, by an anodic bonding. The invention is applicable to making a valve.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a micro-machined fluid-flow device and to a method of manufacturing it, said device comprising a substrate possessing a flow duct and a thin layer forming a deformable membrane. 
     By way of example, such a device constitutes a member for controlling liquid inlet/outlet which can be used as a check valve or in a micropump. 
     BACKGROUND OF THE INVENTION 
     Valves of this type are encountered, for example but not exclusively, in micropumps for medical use which deliver a regular and controlled quantity of medication. The manufacture of micropumps is based on the technologies of micro-machining silicon and of using a piezo-electric actuator. International patent application PCT IB 95/00028 describes a self-priming micropump. In that application, as in others, it is necessary to make an inlet valve and sometimes an outlet valve so that the leakage rate is minimized or even zero. The leakage rate from a valve corresponds to the rate at which liquid flows through the valve when the membrane is in its rest position, i.e. when the valve is closed. Furthermore, since the valve operates because of the elasticity of the membrane, with this elasticity allowing the membrane to deform when fluid is injected to the inlet of the valve at sufficient pressure, it is important not to degrade the mass and surface state of the membrane when manufacturing the valve in order to obtain a membrane that presents a minimum amount of internal stress. 
     The object of the present invention is to provide a machined liquid inlet/outlet device having a minimum leakage rate in the closed position of the valve and in which the method of manufacture leads to a membrane having good physical and mechanical properties with little internal stress. 
     When the problem arises of covering a substrate with a thin metal layer, various methods can be used. The thin metal layer can be deposited on the substrate by evaporation, or by the cathode sputtering technique. Nevertheless, those methods have certain limitations. 
     Usually, metal layers that have been deposited have physical properties that are less good than those of the same materials in solid form. Thus, the layer is usually obtained with considerable amounts of internal stress, particularly because of the crystal structure of the deposited layer which is very sensitive to deposition conditions. Furthermore, a deposited thin layer is of a thickness that is limited to about 1 micrometer, since greater thicknesses cause the method to become too expensive because the time required to make the deposit is too long. 
     Another possibility consists in depositing the metal layer electrolytically, which technique does not suffer from all of the above-mentioned drawbacks. Nevertheless, it is not possible to deposit all materials, and in particular metals, by that method, and the physical and mechanical properties of the deposited layer are often insufficient. 
     SUMMARY OF THE INVENTION 
     According to the invention, these objects are achieved by the fact that the thin layer, e.g. forming a deformable membrane, is a rolled metal sheet, preferably connected to the substrate in the overlap zone by the anodic bonding technique. According to the invention, the method of manufacturing a micro-machined fluid-flow device is characterized in that it comprises the following steps: 
     a substrate is provided that possesses a flow duct; 
     a sacrificial layer is deposited on the substrate by physico-chemical means; 
     zones of the sacrificial layer that are to give rise to a membrane which is not attached to the substrate are conserved by photolithography and chemical etching; 
     a deformable thin layer constituted by a metal sheet is made by rolling; 
     the thin layer is placed on the substrate; 
     said thin layer is connected to the zones of the substrate that are not covered by the sacrificial layer by means of a physico-chemical method; 
     said thin layer is machined by photolithography and chemical etching after it has been fixed on the substrate; and 
     the sacrificial layer is again etched, thereby releasing the membrane from the substrate. 
     Thus, according to the invention, a rolled metal sheet is used that can be connected to a substrate and can then be machined again to make microstructures. The advantages that stem from this invention are, in particular, physical and mechanical properties of the metal after rolling that are excellent and well-controlled. Thus, the stresses present in the metal are low, with the final stress state of the membrane resulting mainly from the method of bonding the membrane to the substrate. 
     Another advantage of the present invention is the possibility of fixing the sheet on a cavity of the substrate, thus making it possible to make a membrane or a bridge directly without any etching step. 
     Another important aspect of the present invention is the use of anodic bonding for fixing the sheet on the substrate. The prior art has never disclosed the use of that technique for metal sheets. 
     The anodic bonding technique is known per se and consists in raising the temperature of the parts that are to be assembled together, i.e. the substrate and the membrane, to about 300° C., and in placing the stack between two electrodes that are in contact with the substrate and with the membrane while applying a negative potential of about—1000 V to the electrode which is pressed against the substrate. A leakproof weld is thus obtained at relatively low temperature between the membrane and the substrate. 
     By using a rolled metal sheet, it is possible for the metal sheet that is subsequently to serve as a membrane in the valve or the micropump to be of a thickness that is fixed in very accurate manner and over a range of values that is quite large. 
     In the present text, the term “rolled sheet” is used to mean a sheet obtained by a metal-working method in which the sheet is obtained by successive passes between rolls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and secondary characteristics and advantages thereof will appear on reading the description of embodiments given below by way of example. 
     Naturally, the description and the drawings are given purely by way of non-limiting indication. Reference is made to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic section view of a first embodiment of a micro-machined valve of the invention; 
     FIG. 2 is a view of the FIG. 1 valve in direction II—II of FIG. 1, i.e. from above the valve; 
     FIG. 3 is a diagrammatic section view of a second embodiment of a valve of the present invention, suitable for integration in a micropump; and 
     FIG. 4 shows how a micropump of the invention can be embodied. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the first embodiment of the invention, the micro-machined valve  10  constitutes a check valve. This valve  10  comprises a substrate  12 , e.g. of “PYREX” type of glass which possesses flow duct  14 . The outlet of the flow duct  14  opens out in the top surface  16  of the substrate  12 . The membrane  18 , e.g. a sheet of rolled titanium of thickness lying in the range 2 micrometers (μm) to 10 μm, covers the outlet orifice of the duct  14  and is fixed to the top surface  16  of the substrate by a peripheral zone  20 . The membrane  18  forms a thin disk having at least one flow orifice  22  disposed in a zone of the membrane that surrounds the outlet orifice of the flow duct  14  so that when the membrane  18  is in its rest position, the duct  14  and the orifices  22  cannot communicate with one another. 
     As can be seen in FIG. 2, the flow orifices  22  through the membrane  18  can, for example, be oval in shape and can be distributed at equal distances apart from one another in the central zone  19  of the membrane and on a circle that is concentric about the outlet orifice of the duct  14 . The central zone  19  of the membrane  18  having the orifices  22  is thus not fixed to the substrate. 
     When fluid arrives at sufficient pressure via the inlet orifice of the flow duct  14 , this liquid pressure reaches the central zone  19  of the membrane  18  which then deforms elastically by curving, while the periphery  20  of the membrane  18  remains fixed to the substrate  12 . The deformation of the membrane  18  establishes a space between the membrane and the substrate  12  so that the liquid can penetrate via said space and through the flow orifices  22  of the membrane  18  (arrows in FIG.  1 ): this is the open position of the valve  10 . 
     Operation of such a valve thus requires the peripheral zone  20  of the membrane  18  to be fixed in permanent manner on the substrate  12 , it requires the membrane  18  to be capable of moving away from the substrate  12  in the central zone  19  of the membrane, and it requires the flow duct  14  through the substrate and the orifices  22  through the membrane to be in a relative position such that they do not communicate with one another when the valve is in its rest or closed position because they are relatively offset, while they are able to communicate with one another when the valve is in its open position in order to allow the liquid to flow from the duct  14  through the orifices  22 . 
     The method of manufacturing such a membrane valve is described below. The flow duct  14  is pierced through the substrate  12 , e.g. a “PYREX” wafer, e.g. by ultrasound drilling to a diameter of about 0.1 mm. A thin sacrificial layer of aluminum is deposited on the top surface  16  of the substrate  12  surrounding the outlet orifice of the duct  14 , said sacrificial layer being made by evaporation and having a thickness of about 0.1 μm. The outline of the aluminum layer is rectified by photolithography and by etching using a standard solution for attacking aluminum. The rolled titanium sheet  18  is fixed to the glass substrate by anodic bonding between the peripheral zone  20  of the titanium sheet and the top surface  16  of the glass substrate  12 , the central zone  19  of the titanium sheet being over the sacrificial layer of aluminum. The outline of the titanium sheet is rectified by photolithography and etching using a solution of diluted hydrofluoric acid. During the last step of the method of manufacturing the valve, the sacrificial layer of aluminum is completely removed or dissolved by a standard aluminum-etching solution. 
     Thus, a valve is obtained in which the active element, i.e. the membrane, is practically free from any internal stress, thereby enabling it to present better mechanical performance, such as better resistance to deformation or fatigue, and better resistance to chemical corrosion. 
     After the sacrificial layer has been dissolved, the central layer  19  of the membrane  18  is thus not fixed in any way to the substrate  12 . 
     A second embodiment is described below with reference to FIG. 3 which shows a valve  30  of the kind that can be found in a micropump, e.g. a micropump as described in the above-specified international patent application. 
     In this embodiment, the valve  30  comprises a substrate  32 , e.g. of glass, a flexible membrane  38 , and a plate  44 , e.g. of silicon. As can be seen in FIG. 3, a flow duct  34  passes through the substrate  32 , having an inlet orifice  35  placed in the top portion of the duct  34  in FIG. 3 which is obstructed by the membrane  38 . This membrane, e.g. a sheet of rolled metal and preferably of titanium, is fixed to the planar surface  36  of the glass substrate  32  adjacent to the inlet orifice  35  of the flow duct  34  of the substrate. The membrane  38  has a central zone  39  in register with the flow duct  34 , and an annular zone  40  fixed to the surface  36  of the glass substrate  32 . 
     An orifice  42  passes through the membrane  38 , preferably in the center of the central zone  39  of the membrane  38  so that the orifice  42  is in line with the flow duct  34  of the substrate  32  and preferably on the axis of the duct  34 . 
     The silicon plate  44  is also fixed to the planar surface  36  of the glass substrate  32 , beside the inlet orifice  35  of the flow duct  34 . The face of the silicon plate  44  which is in register with the membrane  38  is not entirely planar, but has a contact surface  45  connected to the surface  36  of the substrate  32  away from the membrane  38 . 
     The zone of the plate  44  which is in register with the membrane  38  is shaped so as to form a chamber  46  in which liquid can flow. Provision is made for this chamber  46  which is situated between the substrate  32  and the silicon plate  44  to extend beyond the zone of the micropump shown in FIG.  3  and constituting the valve  30  of the present invention, towards liquid inlet means. 
     The zone of the plate  44  situated in register with the orifice  42  of the membrane  38  has an annular projection  48  whose cross-section as shown in FIG. 3 appears as two tapering quadrilaterals. The inside space within the projection  48  is likewise tapering and constitutes the space  50  which is in line with and in register with the orifice  42  in the membrane  38  and with the flow duct  34  in the glass substrate  32 . 
     Provision is made for the free end of the annular projection  48  to be in contact with the central zone  39  of the membrane  38  around the orifice  42  when the valve  30  is in its rest position. Thus, when the valve  30  is in its rest position, the annular projection  48  constitutes an obstacle to the flow of liquid between the chamber  46  adjacent to the silicon plate  44  and the flow duct  34  of the glass substrate. When the valve  30  is in operation, the pressure of the liquid contained in the chamber  46  increases, thereby deforming the central zone  39  of the elastic membrane  38  downwards from the disposition shown in FIG. 3, thereby spacing the membrane  38  away from the free end of the projection  48 , and thus allowing liquid to flow from the chamber  46  towards the space  50 , and then from the chamber  50  through the orifice  42  in the deformed flexible membrane  38  into the flow duct  34 . 
     When all of the liquid coming from the liquid inlet means situated upstream from the chamber  46  has flowed via the space  50  and the orifice  42  into the flow duct  34  towards another compartment of the micropump, the pressure in the chamber  46  decreases, and because of its elasticity the membrane  38  returns to its initial position, i.e. it comes back into contact with the free end of the annular projection  48  so that the chamber  46  and the flow duct  34  are no longer in liquid communication with each other. 
     The valve  30  thus acts as a check valve since because of the above-described configuration, if liquid contained in the flow duct  34  of the substrate  32  is subjected to an increase of pressure, deformation of the membrane  38  will not allow said liquid to pass via the orifice  42  and the space  50  towards the chamber  46  because the free end of the projection  48  remains in contact with the membrane  38 , preventing it from deforming. 
     In this second embodiment, the annular projection  48  from the silicon plate serves as a valve seat for the membrane  38  which presses against the projection  48 . The periphery  40  of the membrane  38  is preferably fixed to the surface  36  of the substrate  32  by anodic bonding, and the same applies to the bond between the surface  45  of the silicon plate  44  and the surface  36  of the glass substrate  32 . 
     The valve  30  forms a check valve operating in the opposite direction to the valve  10  described above. For the valve  30 , manufacture differs from that of the valve  10  in that there is no need to use a sacrificial layer prior to fixing the membrane on the substrate. 
     In FIG. 3, it will be observed that the flow duct  34  is of transverse size that is greater than that of the orifice  42  in the membrane  38 , so the flow duct  34  constitutes a cavity. 
     FIG. 4 shows how the invention can be applied to making a micropump. The pump has a wall  60  that is made, for example, of silicon and that defines an internal cavity  62 . The bottom  64  of the micropump body is pierced by two orifices  66  and  68  respectively for fluid inlet and outlet relative to the inside of the cavity  62 . The top portion of the cavity  62  is closed by a deformable membrane  70 , preferably made of titanium using the above-described method. The periphery of this membrane is fixed as described above. The body  60  of the micropump acts as the substrate. The orifices  66  and  68  are fitted respectively internally and externally with respective deformable membranes  74  and  72  acting as check valves as defined in the description above. 
     Other, alternative embodiments can come within the ambit of the invention. Thus, the substrate can be made not only of glass, preferably a borosilicate glass of “PYREX” or other type, but also of silicon or of ceramic or indeed of other materials that match the thermal expansion coefficient of the metal used. Corning, Inc., of Corning, N.Y., is the owner of the PYREX trademark. Other fixing techniques can also be used to fix the sheet on the substrate, such as adhesive, soldering, a silicon combination (Ti Si, Pt Si, . . . ), or making a eutetic (e.g. Au Si). 
     Because the rolled metal sheet conserves physical and mechanical properties close to those of a solid material, it is possible to use the rolled sheet in an electromagnetic actuator or sensor with a membrane that then possesses magnetic properties that are much better than it would possess if it had been made by deposition; metal sheets made using shape memory alloy can also constitute another alternative embodiment. 
     Other materials can be suitable for the metal sheet: platinum, iridium, aluminum or chromium, tantalum, niobium, molybdenum, or indeed stainless steel alloys such as Fe—Ni ferro—nickels. Nevertheless, it seems that titanium is the metal that lends itself best to anodic bonding. In addition, titanium has the mechanical and chemical properties that are most suitable for the intended use. Furthermore, it withstands corrosion well and can easily be machined by chemical etching.