Abstract:
A pump having: a cavity formed inside an insulating substrate, the upper part of the substrate being situated near the cavity having an edge; a conductive layer covering the inside of the cavity up to the edge and optionally covering the edge itself; a flexible membrane made of a conductive material placed above the cavity and resting against the edge; a dielectric layer covering the conductive layer or the membrane whereby insulating the portions of the conductive layer and of the membrane that are near one another; at least one aeration line formed in the insulating substrate that opens into the cavity via an opening in the conductive layer, and; terminals for applying a voltage between the conductive layer and the membrane.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to micropumps and in particular to their use in an integrated circuit cooling device.  
         [0003]     2. Discussion of the Related Art  
         [0004]     A known cooling device is a metal heat sink placed against a surface of an integrated circuit chip. The carrying off of the heat from a “hot” area of the circuit to the heat sink is performed through a portion of the circuit generally exhibiting poor thermal conductivity.  
         [0005]     As a complement to such a heat sink, or if no other cooling device is present, the integrated circuit is placed in an enclosure comprising a blower generating a permanent air current around the circuit.  
         [0006]     These two cooling devices, associated or not, may not provide sufficient cooling down of a circuit having a high active density of components.  
         [0007]     An object of the present invention is to provide a cooling device capable of maintaining at an acceptable level the temperature of an integrated circuit comprising a large number of active components.  
         [0008]     A more general object of the present invention is to provide a micropump.  
         [0009]     To achieve this, the present invention provides a pump comprising: a cavity formed in an insulating substrate, the upper portion of the substrate located in the vicinity of the cavity forming a border, a conductive layer covering the inside of the cavity all the way to the border and possibly covering the border, a flexible membrane, formed of a conductive material, placed above the cavity and bearing against the border, a dielectric layer covering the conductive layer or the membrane to insulate the portions of the conductive layer and of the membrane which are close to each other, at least one ventilating duct formed in the insulating substrate which emerges into the cavity through an opening of the conductive layer, and terminals of application of a voltage between the conductive layer and the membrane.  
         [0010]     According to an embodiment of the above-mentioned pump, said cavity has substantially the shape of a cup such that the interval between the conductive layer and the membrane progressively increases from the border to the bottom of the cavity.  
         [0011]     According to an embodiment of the above-mentioned pump, the membrane is in an idle state when no voltage is applied between said terminals, the application of a voltage deforming the membrane by drawing it closer to the conductive layer, the removal of the voltage bringing the membrane back to its idle state.  
         [0012]     According to an embodiment of the above-mentioned pump, the pump comprises a single ventilating duct emerging substantially at the bottom of the cavity.  
         [0013]     According to an embodiment of the above-mentioned pump, the pump comprises two ventilating ducts, one emerging substantially at the bottom of the cavity, the other one emerging close to the border.  
         [0014]     According to an embodiment of the above-mentioned pump, the pump is connected to an assembly of ventilating ducts formed in the semiconductor substrate of the integrated circuit.  
         [0015]     The present invention also provides a method for forming a pump in an integrated circuit, comprising the steps of: forming a cavity in an insulating substrate, the upper portion of the substrate located in the vicinity of the cavity forming a border; covering the inside of the cavity all the way to the border and possibly the border with a first conductive layer; forming an opening of the conductive layer emerging into a ventilating duct previously formed in the insulating substrate; filling the cavity with a sacrificial portion; covering the sacrificial portion and the portion of the first conductive layer placed above the border with a first insulating layer and with a second insulating layer; forming a small opening in the second conductive layer and in the first insulating layer; removing the sacrificial portion; and covering the second conductive layer with a second insulating layer to close back the opening.  
         [0016]     According to an embodiment of the present invention, the step of forming a cavity in an insulating substrate comprises the steps of: forming insulating pads on a first insulating layer; covering the first insulating layer and the insulating pads with a second insulating layer; and performing a chem.-mech. polishing of the second insulating layer to expose the insulating pads, the etch method of the polishing being such that it etches the second insulating layer more than the insulating pads, the insulating pads forming said border.  
         [0017]     The present invention also provides a method for actuating a pump such as described hereabove, in which a voltage is applied at regular or irregular intervals between said terminals.  
         [0018]     The foregoing object, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0019]      FIGS. 1 and 2  are cross-section views of a pump according to the present invention in two operating states;  
         [0020]      FIG. 3  is a top view of the pump shown in  FIGS. 1 and 2 ;  
         [0021]      FIGS. 4 and 5  are cross-section views of another example of a pump according to the present invention in two operating states;  
         [0022]      FIGS. 6A  to  6 I are cross-section views of structures obtained in successive steps of a method for forming a pump according to the present invention; and  
         [0023]      FIG. 7  is a cross-section view of an example of an integrated circuit comprising a pump according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0024]     As usual in the representation of integrated circuits, the various drawings are not to scale.  
         [0025]     1. Pump  
         [0026]      FIGS. 1 and 2  are cross-section views of a pump according to the present invention respectively in an idle state and in an activation state.  FIG. 3  is a top view of the pump shown in  FIGS. 1 and 2 . The pump is formed above an insulating substrate  1  and more specifically in an upper cavity  2  of substrate  1 . Cavity  2  has in this example the shape of a cup. The upper portion of substrate  1  located in the vicinity of the cavity forms a border, having in this example a circular shape such as visible in  FIG. 3 . The inside and the border of cavity  2  are covered with a conductive layer  3 , for example, made of copper or of aluminum. An opening O 1  of conductive layer  3  is formed substantially at the bottom of cavity  2  above a ventilating duct  4  formed in substrate  1 . Ventilating duct  4  emerges outside of the substrate. A flexible membrane  6 , formed of a conductive material, is placed above cavity  2  and bears against the border of cavity  2  above conductive layer  3 . Membrane  6  and conductive layer  3  are insulated from each other by an insulating layer  7  covering in this example the lower surface of flexible membrane  6 . Conductive layer  3  and flexible membrane  6  are connected to two terminals between which a control circuit V applies a voltage when ordered to do so.  
         [0027]     In the idle state, when control circuit V applies no voltage, membrane  6  and insulating layer  7  are substantially horizontal, as shown in  FIG. 1 . In the activation state, when control circuit V applies a voltage, membrane  6  deforms by coming closer to conductive layer  3 , as shown in  FIG. 2 . When membrane  6  deforms, the volume of the air pocket located between membrane  6  and conductive layer  3  decreases, which results in chasing the air towards ventilating duct  4 . When control circuit V stops applying a voltage, membrane  6  separates from conductive layer  3  to return to its idle state horizontal position. The volume of the air pocket then progressively increases, which results in letting air into ventilating duct  4 . By successively repeating the operations of deformation and release of membrane  6 , it is thus possible to alternately let in “fresh” air and let out “hot” air.  
         [0028]     According to an alternative embodiment of the above-described pump, insulating layer  7  covers conductive layer  3 . Opening O 1  is then formed through insulating layer  7  and conductive layer  3 .  
         [0029]      FIGS. 4 and 5  are cross-section views of another example of a pump according to the present invention respectively in an idle state and in an activation state. The pump has a structure substantially identical to that of the pump shown in FIGS.  1  to  3 . The pump further comprises a second ventilating duct  10  connected to a second opening of the substrate and emerging into a second opening O 2  of conductive layer  3  formed close to the border of cup-shaped cavity  2 .  
         [0030]     When control circuit V applies a voltage, membrane  6  progressively deforms and by coming closer to conductive layer  3 , it covers opening O 2 . Then, the increasing deformation of the membrane reduces the air pocket volume and chases hot air out of ventilating duct  4 . When control circuit V stops applying a voltage, membrane  6  progressively relaxes to return to its idle state. As long as membrane  6  covers opening O 2 , air enters the cavity through ventilating duct  4 . As soon as opening O 2  is uncovered, air enters cavity  2  through the two ventilating ducts  4  and  10 .  
         [0031]     In the case where the size of opening O 2  is much larger than that of opening O 1 , the air volume entering through opening O 2  is much larger than that entering through opening O 1 . Thus, upon relaxation of membrane  6 , it is possible to fill cavity  2  with air principally coming from ventilating duct  10 . Accordingly, the coming in of “fresh” air into cavity  2  mainly occurs through ventilating duct  10  and the coming out of “hot” air mainly occurs through ventilating duct  4 .  
         [0032]     As an example, the sizes of the various pump elements are the following: 
        diameter of the cup-shaped cavity: from 100 to 1000 μm     maximum depth of the cavity (at the center): 15 μm     diameter of ventilating duct  4  (opening O 1 ): from 1 to 10 μm     diameter of ventilating duct  10 : from 1 to 10 μm     thickness of conductive layer  3 : from 100 nm to 2 μm     thickness of membrane  6 : 2 μm        
 
         [0039]     When the control circuit applies a voltage between layer  3  and membrane  6 , the deformation of membrane  6  is progressive. The portions of layer  3  and of membrane  6  located in the vicinity of the border of cavity  2  are close to each other and a small voltage enables drawing them closer. Once these first portions have been drawn closer to each other, the portions of layer  3  and of membrane  6  located right next to them are then close to each other and a small voltage enables drawing them closer, and so on. The maximum deformation is that for which the “mechanical” membrane restoring force becomes equal to the electrostatic force created between layer  3  and membrane  6  by application of a voltage by control circuit V.  
         [0040]     An advantage of the above-described pumps is that they can be activated with a small voltage.  
         [0041]     The above-described pumps have a cup shape, which has the above-mentioned advantage. However, other cavity shapes may be imagined in which the conductive layer placed inside of the cavity and the flexible membrane placed above the cavity are not necessarily in contact on the cavity border.  
         [0042]     2. Pump Manufacturing Method  
         [0043]     A pump according to the present invention can be formed according to the method described hereafter.  
         [0044]     In an initial step, illustrated in  FIG. 6A , a cup-shaped cavity  20  is formed in an insulating substrate  21 . The upper part of the substrate located close to the cavity forms a border. The cavity will preferably be “cup”-shaped so that the cavity depth progressively increases from the border to the bottom of the cavity.  
         [0045]     The cup shape can be obtained according to the following method. Insulating pads  23  and  24  are formed on an insulating layer  22 . Insulating layer  22  and possibly pads  23  and  24  are then covered with a second insulating layer  25 . A chem.-mech. polishing of second insulating layer  25  is then performed to expose insulating pads  23  and  24 . The etch method implemented in the polishing is selected to that it “etches” insulating layer  25  more than pads  23  and  24 . When pads  23  and  24  are relatively spaced apart, a recess forms in insulating layer  25  between pads  23  and  24 . This phenomenon, known as “dishing”, is generally not desirable since it results in the forming of non-planar surfaces. However, advantage is taken of this phenomenon in the method of the present invention to form a cup-shaped cavity.  
         [0046]     At the next step, illustrated in  FIG. 6B , the inside and the border of cavity  20  are covered with a conductive layer  30 , for example, made of aluminum.  
         [0047]     At the next step, illustrated in  FIG. 6C , conductive layer  30  is etched to form an opening O 3  at the bottom of cavity  20  above a ventilating duct  31  previously formed in substrate  21 .  
         [0048]     At the next step, illustrated in  FIG. 6D , cavity  20  is filled with a sacrificial portion  32 . Sacrificial portion  32  does not cover the border of cavity  20 . A method of sacrificial layer deposition which conforms as little as possible may be used to avoid filling ventilating duct  31 . An etch or a chem.-mech. polishing of the sacrificial layer is then performed to remove the portions covering the border of cavity  20 .  
         [0049]     At the next step, illustrated in  FIG. 6E , an insulating layer  33  is formed above sacrificial portion  32  and above the portions of conductive layer  30  located on the border of cavity  20 .  
         [0050]     At the next step, illustrated in  FIG. 6F , insulating layer  33  is covered with a conductive layer  34 .  
         [0051]     At the next step, illustrated in  FIG. 6G , a small opening O 4  is formed in conductive layer  34  and in insulating layer  33  to reach underlying sacrificial portion  32 .  
         [0052]     At the next step, illustrated in  FIG. 6H , sacrificial portion  32  is removed through opening O 4 , for example, by etching.  
         [0053]     At the next step, illustrated in  FIG. 61 , conductive layer  34  is covered with a thin insulating layer  35  according to a method which is as little conformal as possible so that the deposited insulating layer penetrates as little as possible through opening O 4 .  
         [0054]     3. Pump Placed in an Integrated Circuit  
         [0055]     A pump according to the present invention may be used to have air or another fluid flow through an assembly of ventilating ducts formed in an integrated circuit to cool it down. An example of ventilating ducts and a method for forming such ventilating ducts are described in “Micromachining of Buried Micro Channels in Silicon”, JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, Vol. 9, N°1, March 2000, which is incorporated herein by reference.  
         [0056]      FIG. 7  is a cross-section view of an example of an integrated circuit comprising a pump according to the present invention. Components  40 , such as MOS transistors, are formed at the surface of a semiconductor substrate  41 . A network of ventilating ducts  42  is provided in semiconductor substrate  41 . A network of metal interconnects  43  is placed above components  40  and substrate  41 . Interconnect network  43  comprises in this example five metallization levels on which are formed various conductive lines. Conductive vias enable connecting conductive lines placed on two adjacent levels. A micropump according to the present invention is placed in this example above interconnect network  43  and more specifically in a cup-shaped cavity  45  formed in the upper insulating layer of the last metallization level. A conductive layer  46  covers the inside and the border of cavity  45 . A conductive layer  47 , covered at its lower surface with an insulating layer  48 , is placed above cavity  45  by bearing against the border. A vertical opening, corresponding to a duct  49 , is formed through interconnect network  43 . Duct  49  emerges on the one hand into cavity  45  of the pump through an opening of conductive layer  46  and on the other hand into ventilating duct  42  provided in semiconductor substrate  41 . The pump is placed under a protection “bell” formed of an insulating portion  54  substantially having the shape of a hemisphere laid on interconnect network  43 .  
         [0057]     On one of the sides of cavity  45 , insulating layer  48  extends to partially cover the upper insulating layer of interconnect network  43 . Conductive layer  47  continues above the extension of insulating layer  48  to cover a portion of the upper insulating layer in which is placed a conductive via  50  connected to a conductive line  51  of interconnect network  43 . Conductive layer  46  is connected to a conductive line  52  of the interconnect network via a conductive via  53  placed under conductive layer  46 . Conductive lines  51  and  52  enable connecting conductive layers  46  and  47  to a control circuit V formed in the integrated circuit substrate.  
         [0058]     Such an integrated circuit may comprise a temperature sensor. The control circuit may activate more or less rapidly the pump according to the measured temperature.  
         [0059]     Other embodiments of an integrated circuit comprising a pump according to the present invention may be imagined. The pump may for example be placed right above semiconductor substrate  41  under interconnect network  43 .  
         [0060]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, those skilled in the art may imagine other methods for manufacturing a pump according to the present invention. Further, the number and the location of the openings formed in the lower conductive layer of the pump will be determined according to the ventilating ducts provided in the integrated circuit.  
         [0061]     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.