Abstract:
A pressure sensor includes a pressure sensor house assembly which contains a reference cavity, in which a vacuum exists, and a getter capable of being thermally activated. The getter is activated by directly contacting the getter with an exterior heated body, conducting heat from the exterior heated body, maintaining the exterior heated body in direct contact with the getter for a predetermined period of time, and removing the exterior heated body.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to sensor elements having integrated reference pressures for measuring pressures such as for absolutely measuring pressures and in particular low or small pressures and methods for producing such sensor elements.  
           [0002]    BACKGROUND  
           [0003]    A ceramic capacity sensor element  1  for pressure sensing is usually constructed of two parts, see FIG. 1. These parts are a stable circular base plate  3  having typically a diameter of 20-30 mm and a thickness of 4-5 mm and a thinner circular plate  5 , also called diaphragm, having a movable central portion and mounted at one of the large surfaces of the base plate  3  and joined thereto by means of for example glass joints  6  at the circular edges of the diaphragm and base plate. The diaphragm  5  has the same diameter as the base plate and has a thickness which is adapted to the magnitude of the load to which the diaphragm is to be subjected. The change of the position of the central portion of the diaphragm  5  is detected as a change of the electrical capacitance between two electrodes  7 ,  9  of i.e. gold which are located in parallel with and opposite each other and which are coated using thin film methods on the facing, inner surfaces of the base plate  3  and the diaphragm  5  respectively. The sensor element  1  can be used for different types of measuring pressures, in which the variable searched for is the pressure P meas , which acts on the free, outer surface of the diaphragm  5  facing away from the base plate  3 . The measurement is always made in relation to a reference pressure P ref  in some form. Pressure sensors can be classified according to the way in which the reference pressure is formed. Thus the following types are obtained:  
                                       “a gauge-sensor”   if P ref  = the atmospheric pressure       “an absolute sensor”   if P ref  = technical zero pressure       “a differential sensor”   if P ref  = a second pressure to be measured                  
 
           [0004]    Of course, in a more strict sense all these types use a differential or relative measurement.  
           [0005]    A gauge-sensor uses a circular hole  11  through the base plate  3  from the free surface thereof into the cavity  13  formed between the inner surfaces of the base plate  3  and the diaphragm  5  as a channel for conducting the ambient atmospheric pressure to the inner side of the diaphragm  5 . If the pressures on the two sides of the sensor element of a gauge-sensor are equal and particularly the pressures on the two sides of the diaphragm  5  are equal, i.e. if P meas =P ref =the atmospheric pressure, the diaphragm  5  will rest in a flat position. If the pressure to be measured is larger than the reference pressure, i.e. if P meas &gt;P ref , where P ref =the atmospheric pressure, the diaphragm  5  will bend in towards the base plate  3  and the capacitance between the electrode surfaces  7 ,  9  will be changed, what is electrically detected. Is If the inner cavity  13  between the base plate  3  and the diaphragm  5  and the channel  11  are evacuated from air and other foreign gases and is closed by e.g. a tin plug, a situation is obtained in which the pressure to be measured is for example of the magnitude of order of an ambient pressure, i.e. P meas =the atmospheric pressure, and the reference pressure is substantially equal to zero (vacuum or technical zero pressure), i.e. P ref =0. Here an exact zero pressure is not considered but a zero pressure which can be produced technically, i.e. of the magnitude of order of 10 −6  torr.  
           [0006]    The diaphragm  5  then bends in towards the base plate  1 , since the pressure P meas  to be measured, which then e.g. is approximately equal to the atmospheric pressure, is larger than the reference pressure P ref . If the pressure P meas  to be measured is increased to become larger then the atmospheric pressure, the diaphragm  5  bends even more in towards the base plate  3 . If the pressure P meas  to be measured instead is reduced from the atmospheric pressure in order to approach the vacuum range, the diaphragm bends less and less in towards the base plate. When the pressure to be measured reaches a technical zero pressure, i.e.P means =P ref =technical zero pressure, the diaphragm  5  will rest in a flat position. This type of absolute sensors is apparently based on the fact that the inner reference pressure P ref  is maintained intact and is maintained at a substantially constant, very low pressure during a long time period.  
           [0007]    If e.g. air and/or other gases slowly leak into the sensor element  1  into the reference cavity  13  of the sensor element, the sensor element will gradually loose the possibility to operate as an absolute sensor. Leakage can take place by for example permeation of gas molecules through the ceramic material in the base plate and diaphragm, through the glass joint or through the tin plug which closes the channel  11 . If the reference cavity  11  has a small volume, the increase of pressure therein can occur rapidly, what can be counteracted by increasing the volume of the cavity in order to for example also comprise a room on the top side of the base plate, what results in a more complex structure. The cavity can also be provided by some form of device, which maintains a correct level of the reference pressure P ref  during a longer time. Such a device can e.g. be a “non-evaporable getter” (NEG), i.e. in principle a body of a gas absorbing material. A suitable choice can be a porous sintered material such as e.g. Zr and/or an alloy of Zr, V and Fe. The material can then act as a small in-situ vacuum pump, which absorbs foreign gases in the reference cavity. For an NEG integrated in the reference cavity a high qualitative reference pressure P ref  of the magnitude of order of 10 −8 -10 −10  torr or lower is obtained.  
           [0008]    An NEG is activated by specific high vacuum/temperature processes. If the activating process is executed for a sensor element, which mainly is under atmospheric pressure, it can be executed e.g. in the following way, see FIGS. 2 and 3. The sensor element  1 , which is here provided with a stabilizing ring  15  mounted at the margin region of the diaphragm  5 , see the published International patent application WO 95/28624, is mounted in a recess  17  in a jig  19 , which is mounted on top of a heating plate  21 . A channel  22  connects the free surface of the diaphragm  5  to a high vacuum pump, not shown. A tip-off tube  23  is attached in a recess in the free surface of the base plate  3  having a connection to the channel  11 . The tip-off tube  23  is connected to the high vacuum pump so that the inner surface of the diaphragm will also be subjected to the vacuum. Thereby the diaphragm will be located in a flat position all the time during the activation and closing and will be exposed to minimum mechanical stresses. The sensor element  1  is slowly heated to temperatures about 200-300 E C, by energizing the heating plate  21 . Gas molecules inside the reference cavity  13  and at the surfaces of the cavity are “shaken” and thus detached from the surfaces and are then pumped out by the high vacuum pump. The tip-off tube  23 , which can be made of e.g. copper or glass, is closed by heating it locally to a very high temperature and then pinching it off, when this so called bake-out procedure is finished.  
           [0009]    A closed getter tube  25  is mounted in another low recess on the free surface of the base plate  3 , the recess being connected to a second channel  27  extending in to the reference cavity  13 . The getter tube can made be of e.g. copper or preferably of glass and contains an NEG  28  having the shape of a rod, which is located in a transverse position inside the getter tube  23  and has a resistive inert wire  26  of e.g. platinum wired around it. The platinum wire  26  is introduced in an electrically isolated way through the getter tube, for example molten into glass, in the case the tube  25  is made thereof, so that an electric current can be conducted through the wire  26 . The resistive wire  26  can also be integrated in the NEG-element  28  and then be located inside it. The capability of the NEG-element  28  of in-situ pumping (strictly absorbing) gas molecules is activated by the gradual heating. When the bake-out approaches its end, first the tip-off tube  23  is closed, see  29  in FIG. 3, and then a short, intense final activation of the NEG-element  28  is executed. A current is now conducted through the platinum wire  26 , which then starts to glow and intensely increases the temperature of the NEG-element  28 . This temperature increase of the NEG-element  28  results in a final activation of the NEG-element, which thereby increases its capability of absorbing foreign undesired gases in the reference cavity.  
           [0010]    The obtained reference pressure is of the magnitude of order of 10 −8 -10 −10  torr. The NEG-element  28  will maintain its function also when the entire sensor element  1  and then also the NEG-element included therein has cooled to ambient temperature. The function is maintained until the NEG-element  28  has been saturated with foreign gases originating from e.g. leakage into the cavity. If impermeable ceramic materials are used, the saturation time of the getter will clearly exceed the commercial technical lifetime of the element. A reference cavity in which e.g. an NEG-element according to the discussion is used provides a high qualitative pressure having a long lifetime. However the process, which has been described above, has a number of complications and disadvantages as to the production method, the design and the method of operation.  
         SUMMARY  
         [0011]    It is an object of the invention to provide a sensor element design having a low, accurately defined reference pressure and to provide a method of producing it and in particular to obtain a high qualitative reference pressure to be used also in a minimum reference cavity in the sensor element.  
           [0012]    It is another object of the invention to provide a sensor element, which has a simple, vacuumtight closing of a reference cavity and which thereby obtains a configuration which can be easily handled and has substantially no projecting parts.  
           [0013]    The problems associated with gases leaking into the reference cavity thereby shortening the lifetime of the sensor element can be overcome by an enlargement of the reference cavity as described above. This can result in the draw-back mentioned above comprising that a more complex design is obtained. Therefore it is preferred that an in-situ operating unit of the type an NEG-element is used for capturing foreign gases in the minimum reference cavity, as will be described hereinafter. Also the choice of materials in the different components of the sensor elements has importance for maintaining the reference pressure and this is in particular true in regard of leakage of inert gases such as helium into the cavity.  
           [0014]    The sensor element is preferably built of a ceramic material, which mainly consists of an amorphous glass phase and a crystalline phase, i.e. the material is of type glass ceramic material. Glass ceramic materials have the advantage of being very impermeable to gases and in particular also to helium. However, glass ceramic materials have a disadvantage, in the respect of some types of long time mechanical behaviour and this is in particular true for very thin diaphragms. As very thin diaphragms here diaphragm having thicknesses of less than 0.1 mm (&lt;100 μm) are considered. This type of thin diaphragms is of interest in e.g. measurement of very small pressure differences or pressures in e.g. ventilation applications or gas measurements in a vacuum. One can then advantageously use diaphragms of aluminium oxide manufactured in some suitable way. Also other plates included in the sensor element can then be manufactured of aluminium oxide and in particular this is true for the shielding plate.  
           [0015]    A compact pressure sensor of the capacitive and absolute type includes a pressure sensor house assembly. The assembly contains a reference cavity in which a high vacuum exists and is thus an absolute pressure sensor according to the discussion above. The sensor house assembly has in principle the same small outer dimensions as a sensor house assembly used in gauge-type and differential pressure sensors, for which no closed reference cavities are required. This means that for plate-shaped sensors the reference cavity has a small volume. The pressure sensor house comprises an upper thick base plate, a thinner shielding plate and a movable thin diaphragm. A low, small cavity is formed between the shielding plate and the diaphragm. A narrow channel extends from the cavity to the outside of the assembly and forms together with the cavity the reference cavity. At the mouth of the channel a recess is provided, in which an elastically arranged getter body which is capable of being thermally activated is placed. A portion of the surface of the getter body is a wall surface in the reference cavity. A closing lid having a low projecting profile is arranged in a gastight way in the recess and is engaged with the getter body. When simultaneously closing the reference cavity and activating the getter body the lid is attached to a heating probe at some distance from the sensor house and the getter body is located in the recess. The whole assembly is placed in a vacuum chamber ( 57  and the temperature is then increased to the temperature, at which the getter starts to be activated, and thereupon the probe is moved to a position in which the closing lid is in contact with the getter body but not in tight contact with the sensor house. The temperature of the probe is increased further for a final activation and in order to melt the joint material on the lid. Heat is then conducted through the lid to the getter body. Thereupon the lid is moved to be attached to the house in a tight and sealed way and is fixed in this position.  
           [0016]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:  
         [0018]    [0018]FIG. 1 is a sectional view of a previously known sensor element,  
         [0019]    [0019]FIG. 2 is a sectional view of a previously known device having a sensor element mounted therein in order to execute the final steps when manufacturing a pressure sensor of absolute type having a separate getter and vacuum closing device,  
         [0020]    [0020]FIG. 3 is a sectional view similar to that of FIG. 2 but in which the cavity of the sensor element has been closed,  
         [0021]    [0021]FIG. 4 a  is a sectional view of a pressure sensor of absolute type having a combined mounting of getter and vacuum closing device,  
         [0022]    [0022]FIG. 4 b  is a partial sectional view in a larger scale of the pressure sensor in FIG. 4 a,  which shows the vacuum closing device including a getter before closing,  
         [0023]    [0023]FIG. 5 is a view from below of a closing lid having joint material arranged on contact surfaces,  
         [0024]    [0024]FIG. 6 is a side view of the closing lid in FIG. 5,  
         [0025]    [0025]FIG. 7 a  is a sectional view of a spring washer,  
         [0026]    [0026]FIG. 7 b  is a plan view of the spring washer in FIG. 7 a,  and  
         [0027]    [0027]FIG. 8 is a sectional view of a device for producing a pressure sensor of absolute type having a sensor element mounted therein. 
     
    
     DETAILED DESCRIPTION  
       [0028]    In FIG. 4 a  a sensor element  1  is shown constructed of three circular flat plates, a base plate  3 , a diaphragm  5  and a shielding plate  3  arranged between the base plate and the diaphragm and a front ring or front plate  15  associated with the sensor element and arranged at a marginal region of the free surface of a diaphragm  5  for reinforcing mechanically the diaphragm, see the above-cited International patent application WO 95/28624 and also the published International patent application WO 95/28623. All parts are concentrically arranged and have the same exterior diameter. In the base plate  3  and the shielding plate  5  a number of circular holes  3  are arranged for letting electrical conductors through, which holes are arranged on top of each other for forming through-channels into the joint region  6 , and holes  35  also arranged straight on top of each other, so that a channel into the reference cavity  13  is obtained. At the free surface of the base plate a recess  37  is provided, which is concentric with the reference cavity holes  35  and has a larger diameter than that of these holes.  
         [0029]    The recess  37  can have a suitable depth and for example extend in to approximatively half the thickness of the base plate  3 . Furthermore, the recess  37  can have some suitable shape. For example, it can have a top or edge region  39  bevelled to some suitable angle, for example an angle of about 45 E C as illustrated in FIG. 4 b,  and an axial length corresponding to for example about half the depth of the recess  37 . The edge region  39  can also have other angles and it can often be advantageous that it forms an angle rather close to 90 E  to the surface of the base plate  3 , compare also the configuration of the corresponding lid in FIG. 6, so that the angle of the walls of the strictly speaking frusto-conical top region can be within the range of 45-85 E  in relation to the surface of the base plate  3 . The lower portion  40  of the recess  37  can have circular-cylindrical shape. In the recess  37  an NEG-element  41  is mounted and a closing device comprising a getter lid  43 , see in particular FIG. 4 b  and also FIG. 6.  
         [0030]    In the cylindric portion  40  of the recess  37  in the sensor element thus the NEG-element  41  is located and has or example the shape of a cylindric washer. To the circular frusto-conical, bevelled region  39  and a circular annular region of the exterior surface of the base plate  3  surrounding opening of the recess  37  is the getter lid  43  attached. The getter lid rests with its bottom surface against the top side of the NEG-element  41 . The getter lid  43  has a shape corresponding to the shape of the regions to which it is attached. It thus has a lower frusto-conical portion  42  and an upper cylindric, plate-shaped portion  44 , which laterally extends over the conical portion and thereby at its bottom forms a circular annular surface  46  engaged with the free surface of the base plate  3  at the region thereof at the recess  37 . The cylindrical portion  44  of the getter lid thus forms a low projection on the top side of the base plate. The height of this projection can be very small and is in most cases smaller than the thickness of any plate  3 ,  31 ,  5  forming the main building blocks of the sensor element  1 . It is also possible to make the getter lid as a purely frusto-conical body omitting the top cylindrical portion  44  of the lid. Then the sensor element  1  will have a completely flat surface without any projecting or protruding parts. The getter lid  43  can be made of a ceramic material or of special metal alloys.  
         [0031]    To the circular annular bottom surface  46  of the cylindrical portion  44  of the getter lid  43  and to the circular frusto-conical portion of the getter lid glass material  45  is applied by means of screen printing or by means of pad/brush printing for forming a glass joint. The glass joint material  45  is applied to the surfaces of the getter lid in the shape of a dotted pattern. The glass joint material is thus applied by means of printing, is then dried and is finally sintered before the final heating in order to “burn” the components to each other, when a glass joint is formed and the getter lid  43  is glassed to the corresponding surfaces of the base plate  3 . Bake-out and activation of the NEG-element and finally the closing of the reference cavity is made in a chamber, in which a vacuum pressure of the magnitude of order of 10 −6 -10 −8  torr exists, as will be described in detail hereinafter.  
         [0032]    The glass joint material  45  sintered on the getter lid  43  contains substances and occlusions of some gas or gases, which are evaporated or released respectively during a heating procedure called bake-out executed directly before the final activation of the NEG-element and the closing (“tip-off”) of the reference cavity. When the sensor element  1  and the getter lid  43  comprising glass joint material each but simultaneously are located in a high vacuum chamber, this degassing and release of non-desired gaseous substances can be very sudden and abrupt. Sudden gas explosions of gas enclosed in the glass joint material  45  can destroy the finished glass joint. In the purpose of counteracting this effect the glass joint material  45  is applied as a dotted pattern. The dotted pattern facilitates the transport of gas molecules out of the glass joint material to the high vacuum chamber in order to then be removed by a high vacuum pump. The dotted pattern further results in that the glass joint material  45  at the peak temperature of the process flows out and/or forms a thinner joint than what can be obtained from a uniformly thick layer of joint material. The obtained thin joint reduces temperature induced mechanical stresses in the finished sensor element.  
         [0033]    The glass joint material  45  can be applied in different kinds of dotted patterns. The glass joint material, which is located on the circular annular bottom region  46  of the getter lid, has a preferred pattern of small regular hexagons, see FIG. 5. The glass joint material  45  on the circular frusto-conical portion  42  of the getter lid  43  has as a preferred pattern circular narrow rings, which are concentric with each other and with the getter lid, see FIG. 6.  
         [0034]    The NEG-element  45  rests against a spring washer  47  made of an inert material, which is placed on the bottom of the recess  37 . The spring washer  47  lifts the NEG-element  41  somewhat, so that a slot is formed between the getter element and the bottom surface of the recess  37  in the base plate  3 . Gas molecules, which are “shaken” and thus detached during the final activation of the NEG-element  41 , can thereby leave the reference cavity and be pumped away by the high-vacuum pump. The spring washer  47  can be etched or laser cut to the correct configuration, see FIG. 7. The portion  49  of the bottom side of the spring washer  47 , which rests against the bottom of the recess  37 , consists of a flat circular ring at the edge of the washer  47 . This portion  49  continues at its inner edge into a frusto-conical ring  51 , which extends upwards from the exterior circular ring  49  and at its upper edge, having a smaller diameter, is terminated in a flat, inner circular whole portion  53 , which rests against the bottom side of the NEG-element  41 . The portion  51  having the shape of a frusto-conical ring contains a number of recesses and slots  55  etched in uniformly spaced, radial directions.  
         [0035]    The slots  55  make a transport of molecules from the reference cavity possible and they in addition reduce the conduction of heat between the getter material  41  and the base plate  3  when during bake-out and final activation pumping foreign gases away from the inner of the reference cavity. The reference cavity generally comprises the space  13  between the diaphragm and the shielding plate and the circular channel formed by the holes  35  up to the bottom of the recess  37 .  
         [0036]    The above-mentioned bake-out of the reference cavity and of the glass joint material, activation of the NEG-element and closing the reference cavity are executed in a high-vacuum chamber  57 , see FIG. 8, and will now be described in detail. The sensor element  1  is placed resting with its support ring  15  against a heating plate  59 , which rests on support blocks  61  on the bottom of the high-vacuum chamber  57 , which is connected to a high-vacuum pump, not shown, by an outlet  63  centrally at its bottom. The getter element  41  rests on the spring washer  47  in the recess  37 . The getter lid  43  is at the start located at a distance from the sensor element  1 . It is attached to the bottom surface of a heating probe  65 , which is also located in the high-vacuum chamber and can be moved and operated therein. The heating probe  65  is provided with its own separate integral heating means, not shown, which are activated later during the procedure. The sensor element  1  is slowly heated to 200-300 E C during about 1-2 hours by supplying energy to the heating plate  59 . The temperature is then maintained constant for  1 - 2  hours, the high vacuum pump then working for pumping away the released gases.  
         [0037]    The heating probe  65  having the getter lid  43  retained thereby also follows the temperature of the heating plate  59  during the heating and the bake-out. The NEG-element  41  is gradually activated during the slow temperature increase. At the end of the bake-out period the heating probe  65  together with the getter lid  43  and the glass joint material  45  applied to the lid is lowered towards the opening of the recess  37  and towards the upper surface of the NEG-element  41 . During the final phase, when the heating probe  65  is lowered to the NEG-element  41 , the temperature of the heating probe is increased rapidly by activating its own heating source up to the temperature at which the glass joint material  45  on the getter lid  43  flows. At the same time the bottom surface of the central, projecting portion  42  of the getter lid  43  is made to come in contact with the top side of the NEG-element  41 , whereby the temperature of the NEG-element rapidly increases. The peak temperature of the heating probe  65  is then allowed to finally activate the NEG-element  41  during about six minutes.  
         [0038]    The heating probe  65  is during this period pressed down to the NEG-element  41 . During the first two minutes of the final activation of the NEG-element  41  the slot formed between the getter lid  43  and the walls of the recess  37  must be somewhat open. Thereupon the getter lid  43  including the now flowing glass joint material  45  is pressed down further into the recess  37 , so that the glass joint material  45  on the circular ring  46  under the top cylindrical portion of the lid and on the frusto-conical surface  42  comes in contact with corresponding surfaces on the top side of the base plate  3  and in the recess  37 , whereby the reference cavity is closed. The NEG-element  41  has such a thickness that then its bottom surface presses the projecting portions of the spring washer  47  down nearly to the bottom of the recess  37  in the base plate  3 . The recess  37  and the spring washer  47  are designed so that however a small slot remains between the central portion of the bottom surface of the spring washer  47  and the bottom surface of the recess  37 , when the getter lid  43  is pressed down to be engaged with corresponding portions of the base plate  3  and in particular the circular frusto-conical portion  39  of the recess  37 .  
         [0039]    The heating probe  65  is maintained at the peak temperature for four minutes more for final activation of the NEG-element  41  and closing the reference cavity. After executing heating, bake-out, final activation and closing the reference cavity, the temperature of the now gettered sensor element  1  is slowly lowered, the temperature decrease taking place during a number of hours.  
         [0040]    The procedure thus has the following main steps:  
         [0041]    1. The sensor element  1  is placed in the high vacuum chamber  57  on the heating plate  57 . The getter lid  43  including glass joint material  45  is attached to the heating probe  65 . The high vacuum chamber  57  is evacuated by the high vacuum pump.  
         [0042]    2. The temperature of the heating plate  57  and the heating probe  65  is increased slowly and is maintained constant during a long period.  
         [0043]    3. The temperature of the heating probe  65  is increased at the same time as the heating probe is moved down to be engaged with the top surface of the NEG-element  41 . After a short period of time the getter lid  43  is pressed down into the recess  37  in the base plate  3 .  
         [0044]    4. The temperature of the heating probe  65  is lowered to the temperature of the heating plate  59  and thereupon the temperature is lowered to the ambient temperature.  
         [0045]    In the procedure as described above the final gettering of the NEG-element is made during six minutes. However, in practical embodiments of the procedure the specific circumstances during the heating process must be considered. A delicate thermal balance exists between the heating probe and the heating plate in the high vacuum chamber and the control of the heating must be very accurate. A correct control can shorten the time but in many cases the period of time required for the final gettering is longer than that mentioned above and can in practical cases amount to 30-40 minutes. In principle there are no effects adverse to the sensor element associated with using long activation periods. However, for production reasons all processes involving heating should of course be as short as possible.  
         [0046]    Instead of using a heating probe as described above, the closing lid can be attached to some other operating means which can move the lid down to the recess. The intense heating required in the final stage of activating the getter can be provided to the lid in some other way, such as by means of laser radiation or IR-radiation.  
         [0047]    In the heating process for making the bake-out and the final activation of the getter material also various or even all components of the pressure sensor element can be finally attached to each other. However, many cases a divided process is to be preferred. Then the components of the sensor element  1  are already attached to other by a previous heating process. A condition thereof is that the already formed joints in the sensor element will not be affected by the later heating treatment. Practically then, e.g. the glass joint  6  between the diaphragm  5  and the shielding plate  7  must have melting characteristics different from those of the material of the joint  45  between the lid  43  and the base plate  3 . Thus, the joint  6  can comprise a high temperature glass and the material of the joint  45  a low temperature glass.  
         [0048]    In a practical example of producing a sensor element a getter material was used which is satisfactorily activated at 400 E C. A high temperature glass was used in the joint  6 . It had a melt temperature of 630 E C and started to soften at 500 E C. A low temperature glass having a melt temperature of 470 E C was used in the joint  45 . However, for making the low temperature glass material to appropriately flow in the joint for sealing and attaching the lid, a temperature of 550 E C is required. The following heating procedure was used:  
         [0049]    The temperature of the heating plate  59  and the probe  65  was slowly raised to about 350 E C, the system then being baked-out and the joints being degassed. The heating plate  59  was then maintained at 350 E C. The temperature of the heating probe  65  was increased to 550 E C and the lid attached thereto is then moved to contact the getter body  41  and the bas plate  1  in order to be attached to the latter one. Then a temperature gradient exists over the sensor element from 550 E C down to 350 E C. The glass joint  6  can cope with this when carefully controlling the system. The reason thereof is the positive thermal characteristics of the ceramic materials, in particular a combination of heat capacity, density and thermal conductivity of the materials.  
         [0050]    According to the description above, an absolute pressure sensor element has been presented which has a compact shape and a low, accurately defined reference pressure in a minimum reference cavity. The sensor element has a simple, vacuumtight closing of a reference cavity.  
         [0051]    While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.