Abstract:
A capacitive pressure sensor of substantially ceramic material comprises a thick base plate ( 100 ), a front plate ( 140 ) having the same thickness as the base plate and a movable diaphragm ( 120 ) located between the front plate and the base plate. Capacitor electrodes ( 121   b   , 121   a ) are provided at the surface of the diaphragm facing the base plate and form a measurement capacitor. The diaphragm ( 120 ) is extremely thin and is produced by sintering a ceramic material such aluminium oxide. Owing to a pressing process used during the sintering a strong, very thin diaphragm is obtained having no mechanical stresses and fracture indications. It can be produced to have a very small thickness in order to provide pressure sensors having a high sensitivity, which can also for high-vacuum applications tolerate to be subjected to the atmospheric pressure. A shielding plate ( 110 ) can be inserted between the base plate ( 100 ) compensating the measurement capacitor. The shielding plate ( 110 ) can also be extremely thin, having a thickness down to a thickness corresponding to the thickness of a diaphragm ( 120 ).

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a ceramic, capacitive pressure transducer or sensor element for detecting small pressure differences and low pressures.  
         BACKGROUND  
         [0002]    Sensor elements to be used in pressure sensors can be designed to detect strain or capacitance, i.e. be the strain type or the capacitive type. They can be built from various ceramic materials. Ceramic materials based on aluminium oxide are often used in such sensor elements but also glass ceramic materials can be used. A ceramic capacitive sensor element for sensing pressures is usually constructed of two main parts. These parts comprise a stabile base plate and a thinner circular plate, also called a diaphragm, a part of which is movable with a pressure difference and which mounted to one of the large surface of the base plate and joined thereto by for example glass joints at the circular edge of the thin plate and the base plate. The diaphragm has the same diameter as the base plate and has a thickness which is adapted to the magnitude of the load, i.e. the pressure difference to which the diaphragm is intended to be subjected.  
           [0003]    The change of the position of the central portion of the diaphragm is detected as a change of a capacitance between two oppositely located electrodes made of e.g. gold, the electrodes being layers which are coated by means of thin film methods on facing surfaces of the base plate and the diaphragm respectively. Such a sensor element can be used for different types of pressure measurements, where the desired variable is a measurement pressure acting on the free surface of the diaphragm, i.e. on the surface which does not face the base plate. The measurement is always made in relation to some form of a reference pressure acting on the inner surface of the diaphragm which faces the base plate and is opposite the free surface. Pressure sensors can be classified based on the way in which the reference pressure is formed. Thus the pressure sensor is:  
           [0004]    a “gauge sensor” if the reference pressure=the atmospheric pressure  
           [0005]    an “absolute sensor” if the reference pressure=a technical zero pressure  
           [0006]    a “differential sensor” if the reference pressure=a second measurement pressure  
           [0007]    The diaphragm of such a sensor element is the part which mainly determines the performance of the sensor element. The diaphragm should be as thin as possible in order to provide a high sensitivity. However, a too thin diaphragm can easily break if exposed to too high pressures. Apparently a sensor element intended for measuring extremely small pressures should comprise such a very thin diaphragm but such a diaphragm cannot without taking some precautions be subjected to the atmospheric pressure. However, thin plates, which have been produced by mechanical working such as polishing, always have mechanical stresses and are not completely flat in an unloaded state, at least not for varying ambient temperatures and are thus not suitable to be used in high reliability pressure sensors which are intended for small pressures.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the invention to provide sensor elements which have an extremely high sensitivity and stability and which can stand large pressure variations.  
           [0009]    A sensor element of the capacitive type constructed of mainly ceramic materials comprises a circular ceramic plate, also called a diaphragm, which is movable with the pressure of the gas acting on it and which is extremely thin and has typically a thickness less than 0.1 mm for a diameter of 38 mm of the plate, i.e. it has a ratio of the thickness to the diameter which is less than substantially 0.26%. The ratio of the thickness to the diameter of the movable portion of the diaphragm will then be less than about 0.35%. As to its other characteristics the sensor element can be made according to the disclosure of the published International patent application WO98/37392. The extremely thin ceramic diaphragm is furthermore attached between two ceramic elements in a particular manner. Such a mounting allows that the sensor element can be made to have an insignificant temperature drift.  
           [0010]    The ceramic material used in the sensor element and in particular in the thin diaphragm is preferably aluminium oxide. Other ceramic materials such as glass ceramic materials can also be used but have not equally good properties.  
           [0011]    In a method of manufacturing such flat, extremely thin diaphragms and of mounting them in sensor elements diaphragms can be obtained which have a low helium permeability, which have no viscoelastic properties and which have no mechanic indications of fracture such as micro cracks or similar material defects which can influence the strength of such diaphragms when they are subjected to pressure variations.  
           [0012]    Between a base plate and a shielding plate preferably an electrically conductive layer of gold, applied by means of thin film methods, is located according to the disclosure of the published International patent application WO95/28624 in order to change and minimize stray capacitances around the measurement electrodes. Furthermore, on the under surface of the base plate and on the top surface of the shielding plate electrically conductive gold layers can be provided, which have a circular shape and are located opposite or facing each other and thereby form a pair in a reference capacitor.  
           [0013]    The shielding layer can be enclosed by a dotted or channelled glass pattern according to the disclosures of the published International patent application WO95/28623 and the published International patent application WO98/37393. Furthermore, the reference capacitor can be surrounded by a similar dotted or channelled glass pattern.  
           [0014]    A metallic mounting element can be attached to the sensor element according to the disclosure of the published International patent application WO95/28623 cited above.  
           [0015]    The shielding plate can be a circular ceramic plate which has the same thickness as the thin circular ceramic plate which is movable with the pressure acting on its free surface. Such a thin ceramic shielding plate can further have a small recess, which has by means of laser been cut out of the peripheral edge of the shielding plate. The recess results in that the spaces between the measurement diaphragm and the shielding plate and between the shielding plate and the base plate can be given the same reference pressure.  
           [0016]    In sensor elements intended for measurement of absolute pressures an ultra high vacuum reference pressure is provided which is integrated in the reference cavity of the sensor element according to the disclosure of the published International patent application WO98/37392 cited above. An NEG-element (Non Evaporable Getter element) which is active at ambient temperatures maintains the reference pressure at an ultra high vacuum level for long periods of time comprising several years.  
           [0017]    A sensor element according to the discussion above is built of three or four circular ceramic plates which in sequence or stacking order comprise:  
           [0018]    Case A: a base plate, a diaphragm, a front plate or front ring  
           [0019]    Case B: a base plate, a shielding plate, a diaphragm, a front plate or front ring  
           [0020]    Sensor elements according to Case A are the absolute type intended for measuring absolute pressures. Sensor elements according to Case B comprise both elements intended for measuring absolute pressure and so called gauge-elements according to the definition above, in which the pressure of the atmosphere constitutes a reference in relation to a pressure of a measurement medium which is to measured.  
           [0021]    Such capacitive sensor elements comprising extremely thin diaphragms are advantageously used when measuring small pressure differences for flow determination and controlling for example air in ventilation systems and in absolute measurements of low vacuum pressures in for example the manufacture of semiconductors.  
           [0022]    A sensor element fabricated according to the principles as indicated above will have insignificant and negligible errors as to non-linearity, repeatability and hysteresis and an insignificant temperature drift of the zero point and in the measurement range of the sensor element. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The invention will now be described as a non-limiting embodiment with reference to the accompanying drawings, in which.  
         [0024]    [0024]FIG. 1 is a sectional view of a sensor element having a NEG-element and an extremely thin diaphragm,  
         [0025]    [0025]FIG. 2 a  is a sectional view of an aluminium oxide film coated on top of a thin polymer film,  
         [0026]    [0026]FIG. 2 b  is a sectional view of a sintered circular thin aluminium oxide plate,  
         [0027]    [0027]FIG. 2 c  is a sectional view of a sintered circular thin aluminium oxide film located between two aluminium oxide blocks during a second sintering cycle,  
         [0028]    [0028]FIG. 2 d  is a sectional view of a turned-around, twice sintered thin aluminium film located between two aluminium oxide blocks during a third sintering cycle,  
         [0029]    [0029]FIG. 3 is a sectional view of jig comprising two aluminium oxide blocks, between which a green film is placed and which are coated with a suitable material in order to avoid adhesion during the sintering cycle, the jig being constructed so that the pressing force between the blocks can be varied when running a sintering process,  
         [0030]    [0030]FIG. 4 is a sectional view of a sensor element comprising an integrated reference electrode and an extremely thin circular ceramic plate which is movable with a pressure, in which sensor element the atmospheric pressure is used as a reference, and  
         [0031]    [0031]FIG. 5 is a sectional view of a sensor element comprising an integrated reference electrode and an extremely thin, circular ceramic plate which is movable with a pressure, in which sensor element an integrated technical zero pressure is used as a reference pressure. 
     
    
     DETAILED DESCRIPTION  
       [0032]    Sensor elements of the absolute type, i.e. for measuring absolute pressures, according to Case A as defined above consist of three ceramic circular plates. In FIG. 1 such a sensor element  5  is shown which is built of a base plate  10 , a front plate  12  and a circular ceramic plate  17 , also called a diaphragm, which is movable with the difference of the gas pressures acting on the its large surfaces. The diaphragm is extremely thin in relation to its diameter and is located between the base plate and the front plate. In the base plate  10  two circular through-holes  11   a  and  11   b  are provided for letting electrical conductors through. On the under side  10   a  of the base plate  10  an electrically conducting thin film  18   b  of preferably gold is provided, which is made by means of thin film methods, the free, under surface of this thin film  18   b  constituting one of two opposite electrically conductive areas which form the measurement capacitor of the sensor element. On the top surface  17   a  of the diaphragm  17  a second electrode area is constituted by a layer applied to this surface, which thus is opposite or facing the under surface of the thin film  18   b . The thin electrically conductive layer  18   a  on the top side of the diaphragm is preferably made of gold and is made by means of thin film methods. The layer  18   a , which forms the electrode area on the diaphragm  17 , has a somewhat larger diameter than the thin film layer  18   b  constituting the upper electrode area on the under side of the base plate  10 .  
         [0033]    A glass joint  19   a  between the under side  10   a  of the base plate  10  and the top side  17   a  of the diaphragm  17  and a glass joint  19   b  located between the under side of the diaphragm and the top side of the front plate  12 , which is located undermost of the plates, hold the plates to form one single unit.  
         [0034]    In the center of the front plate  12  a connection nipple  14  of metal is attached. The connection nipple  14  is attached to the front plate made of a ceramic material according to the disclosure of the cited published International patent application WO/28623. The connection nipple  14  is made of a special metal alloy. A preferred material is “Vacon  70 ”.  
         [0035]    The top side  10   b  of the base plate  10  has a recess in which an NEG-element  16  is arranged, which rests on a conical spring washer  16   a  made of an inert material. The NEG-material is enclosed in the reference cavity by a getter lid  15 . This construction is disclosed in the cited published International patent application WO98/37392. The base plate  10  and the front plate  12  has advantageously substantially the same thickness.  
         [0036]    In the sensor element  5  the ceramic plate  17  which is movable with the pressure is a unique part which has previously been beyond the technical possibility of being produced in order to be used in applications such as measuring small difference pressures in ventilation systems or small absolute pressures in vacuum systems. The ceramic plate  17  is produced of pure crystalline aluminium oxide having very small additives of materials such as e.g. MgO. It has a great importance that the plate  17  does not have viscoelastic properties caused by a possible amorphous phase in the material.  
         [0037]    The plate  17  is manufactured of an aluminium oxide powder having a selected grain size of for example an average diameter of 2 μm. It is very important in order for the plate to operate in vacuum applications that the material of the plate has a suitable grain size and thereby a low helium permeability.  
         [0038]    Furthermore, it has a great importance, that the plate is plane-parallel, i.e. has completely flat, parallel large surfaces, and has no distortions or other geometric errors. Absolutely correct geometric dimensions are further necessary if the plate is to have no temperature drift, when it in operation is located between a base plate  10  and a front plate  12 , which both have substantially the same thickness. The plate  17  has a thickness which does not allow mechanical working of type polishing in order to achieve the intended very small thickness. Furthermore, polishing causes distortions and induces stresses in the plate which when using the sensor element result in fractures.  
         [0039]    In order to produce a plane-parallel plate aluminium oxide powder having a suitable grain size is mixed with a binding agent and a dispersion agent of water soluble types and some water to form a slurry. The aluminium oxide slurry, see FIG. 2 a , is coated on top of a thin polymer film by means of a “tape-casting” method to form a film. The obtained film of aluminium oxide is dried to form a green film which can stand to be handled and which is released from the polymer film. From the green film plates  17  having a suitable diameter cut, such as by means of laser light or by shearing operations, e.g. ordinary cutting or punching. The diameter of the plates  17  is selected considering shrinking during the following sintering steps, so that for obtaining a finished diaphragm having a diameter of 38 mm the plates  17  must have a larger diameter of e.g. 41-44 mm. Thus generally, these plates have been obtained from an aluminium oxide slurry arranged on top of a thin substrate such as a polymer film.  
         [0040]    In a heating procedure the green film is sintered in an oven at 1600° C. This sintering is made applying no pressure to the green film. Then the sintered plate  20  obtained after the heating procedure will be deformed, i.e. have changes of its shape, see FIG. 2 b . To achieve a completely plane-parallel plate the plate  20  is then sintered a second time between two blocks of polished aluminium oxide having dimensions of for example 50·50·5 mm for a diameter of 38 mm of the finished plate, as exemplified above, see FIG. 2 c . The blocks press the deformed plate  20  to a flat condition owing to the weight of the upper block. The plate  20  is then turned around, so that its previously lower surface now is the top surface, and is sintered in this condition between the aluminium oxide blocks a third time, see FIG. 2 d . After the third sintering process the ceramic plate  20  is completely plane-parallel and no mechanical working of the polishing has been used in any step of the procedure. Plates having a thickness smaller than 0.1 mm can be obtained using this method.  
         [0041]    In FIG. 3 a jig is schematically illustrated, in which the pressing force F from the two blocks  21  and  22  is variable. Then a very small force is applied at the start of the heating procedure, which force is then increased to a value corresponding for example to the weight of a block  21 ,  22  as described above. The green film  23  is during the pressing and heating operation placed between bodies such as the blocks  21 ,  22 , which at their pressing surfaces have a thin film coating  24  and  25  of a special material, which is selected, so that the material of the plate, which is to be sintered, cannot adhere to the pressing surfaces during the sintering process. When using a jig as described above the sintering process can be made in one single heating step.  
         [0042]    In FIG. 4 a cross-sectional view of a sensor element according to Case B as defined above is shown, which element is intended to be used for measurements using the atmospheric pressure as a reference, in which similar or identical components have been produced substantially as described above. The sensor element is constructed of a base plate  30 , a shielding plate  40 , a thin diaphragm  50  and a front plate  60 . On the under side  30   a  of the base plate  30  an electrically conducting, thin film area  35   a  of preferably gold is disposed, which as above is produced by means of thin film methods. Opposite this area, located on the top side  40   a  of the shielding plate  40 , is an electrically conducting thin film area  35   b  of preferably gold is arranged which is also made by means of thin film methods. The electrically conducting, facing areas form a reference capacitor.  
         [0043]    The atmospheric pressure reaches the cavity around the reference capacitor through a channel  31 . The cavity is enclosed by a glass joint  32   a  and  32   b , which has been applied as a dotted or channelled pattern according to the disclosure of the cited published International patent application WO95/28623. The cavity around the reference electrode pair  35   a  and  35   b  consists of an interspace or gap  36  having a thickness of 20-50 μm. The thickness of the interspace  36  is determined by glass joints  32   a  and  32   b . A preferred gap thickness is 20 μm. If the space between the base plate  30  and the shielding plate  40  is used also for integrating thermistor elements according to what is described in the cited Swedish patent application 9700613-4, the interspace  36  can instead have a thickness of 50 μm.  
         [0044]    Further, on the under side  40   b  of the shielding plate  40  an electrically conducting thin film area  41  of preferably gold is applied. On the top side  50   a  of the thin plate  50  an electrically conducting thin film area  51  of preferably gold is also applied. These opposite or facing areas  41  and  51  form an electrode pair which constitutes the measurement capacitor of the sensor element. The atmospheric pressure reaches the cavity between the under surface  40   b  of the shielding plate  40  and the top side  50   a  of the thin plate  50  through a prolongation of the channel  31  in the base plate  30  which is formed by a channel  31   a  in the shielding plate  40 .  
         [0045]    In this preferred embodiment of a gauge-element, in which the atmospheric pressure is the reference pressure, the reference electrode pair  35   a ,  35   b  and the measurement electrode pair  41 ,  51  will be exposed to the humidity of air. Using suitably designed electronic circuits for determining suitable electric quantities of the reference electrode pair and the measurement electrode pair it can be obtained, that the influence of the air humidity on the capacitance between measurement electrode and reference electrode cancel each other. When using the element, the measured values will have no dependency on the humidity of air.  
         [0046]    In a preferred embodiment the capacitance of the reference capacitor will not change when the plate  50  which is movable with the pressure changes the electric characteristic value given by the measurement capacitor. A reference capacitor having a capacitance changing with the pressure results in linearity errors so that the measured values do not agree with the theoretical relations described in the cited published International patent application WO95/28624.  
         [0047]    A preferred distance between the measurement electrode areas  41  and  51  are as above 20 μm. The distance is defined by glass joints  57  and  58 . In the front plate  60 , which has a thickness corresponding substantially to the sum of the thicknesses of the base plate  30  and the shielding plate  40 , a connection nipple  59  of a metallic material is attached. In a preferred embodiment the nipple is made of “Vacon  70 ” as described above. The joint  59   a  between the connection nipple  59  and the front plate  60  of aluminium oxide gives a hermetic and stabile mounting of the nipple.  
         [0048]    In FIG. 5 a sensor of the same type as in FIG. 4 is illustrated in which the channel  31  is replaced by an NEG-element  70 , a spring washer  71 , a getter lid  72  and a recess  73  in the base plate  100 , compare also the sensor element of FIG. 1. The element thus belongs to Case B as defined above comprising an absolute reference pressure. The shielding plate can be designed to have a thickness of 0.5 mm. In a preferred embodiment the shielding plate is substantially thinner than in the embodiment according to FIG. 4 and can have substantially the same thickness as the plate  120  which is movable with the pressure.  
         [0049]    The glass joints  115   a  and  115   b  between the base plate  100  and the shielding plate  110  are in a preferred embodiment part of a circular ring and the channel  130  is a hole cut by means of laser light in the shielding plate  110 . When using a ceramic plate  120  which is movable with the pressure and has a thickness of the magnitude of order of 50 μm or less the influence of gravitation on such a plate will cause that the angular position of the sensor element, i.e. whether it is placed in a vertical or horizontal direction, will affect the capacitive value of the measurement capacitor formed by the electrodes  121   a ,  121   b .  
         [0050]    The own weight of the thin plate  120  results, for a rotation of the sensor element from a horizontal position to a vertical position and for an inverse movement, in a change of the capacitive value of the measurement electrodes.  
         [0051]    The reference electrode pair  122   a ,  122   b  will, in the case where the shielding plate  110  has substantially the same thickness as the thin plate  120 , cause a corresponding change of the capacitive value of the reference electrode which change has an equal magnitude. For suitably designed electronic circuits for detecting the capacitances of the measurement electrode pair and the reference electrode pair this influence of the gravitation can be compensated and thereby also the geometric orientation of the sensor element.