United States Patent 19
`Renaud
`
`54 CAPACITIVE ABSOLUTE PRESSURE
`MEASUREMENT SENSOR AND METHOD OF
`MANUFACTURING A PLURALITY OF SUCH
`SENSORS
`
`75) Inventor: Philippe Renaud, Ochettaz,
`Switzerland
`73) Assignee: CSEM Centre Suisse d’Electronique
`et de Microtechnique SA, Neuchatel,
`Switzerland
`
`21 Appl. No.: 187,677
`(22
`Filed:
`Jan. 28, 1994
`30
`Foreign Application Priority Data
`Feb. 12, 1993 (FR)
`France ................................... 93 O1700
`(51) int. Cl. ........................................... GOL 7700
`(52) U.S. C. .................................. 73/724; 73/718; 73/754
`58) Field of Search .............................. 73/715, 718, 724,
`73/754
`
`(56
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`... T3/718
`4,815,472 3/1989 Wise et al. ...
`... 73/724
`4,838,088 6/1989 Murakami .....
`... 73/724
`5,041,900 8/1991 Chen et al. ...
`... 73/78
`5,113,868 5/1992 Wise et al. ....
`5,186,054 2/1993 Sekimura .................................. 73/718
`
`
`
`||||III IIII
`US005488869A
`11
`Patent Number:
`5,488,869
`(45) Date of Patent:
`Feb. 6, 1996
`
`9102748 7/1991
`
`5/1993 Wise et al. ................................ 73/78
`
`5,207,103
`FOREIGN PATENT DOCUMENTS
`Germany.
`OTHER PUBLICATIONS
`Elektronik, E. Habokotte et al., Integrierbare Funktions
`blocke und Systeme, vol. 39, 5 Jan. 1990, pp. 80-87
`XP86381. No translation.
`Primary Examiner-Richard Chilcot
`Assistant Examiner-Ronald Biegel
`Attorney, Agent, or Firm-Pollock, Vande Sande & Priddy
`57
`ABSTRACT
`The invention relates to a capacitive absolute pressure
`measurement sensor having a first element (2) in which is
`arranged a mobile electrode (4), a second element (6) in
`which is arranged a fixed electrode (8) situated facing and
`insulated from said mobile electrode (4), a connecting frame
`(10) interposed between the first element and the second
`element to define a chamber (12) insulated from the external
`environment and inside of which there is substantially zero
`pressure, and a reference volume (14) connected to the
`chamber. The first element (2) has the configuration of a
`membrane of substantially constant thickness and the refer
`ence volume (14) is arranged at least partially in the second
`element (6).
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`Feb. 6, 1996
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`1.
`CAPACTIVE ABSOLUTE PRESSURE
`MEASUREMENT SENSOR AND METHOD OF
`MANUFACTURING A PLURALITY OF SUCH
`SENSORS
`
`TECHNICAL FIELD
`The invention relates to a capacitive absolute pressure
`measurement sensor and in particular to Such a sensor
`intended to measure absolute pressure and which provides a
`large reference volume minimizing the residual internal
`pressure.
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`O
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`BACKGROUND OF THE INVENTION
`The invention also relates to a method of manufacturing
`a plurality of such sensors using semiconductor material
`micro-machining technologies.
`The pressure measurement sensors, made of semiconduc
`tor based materials, most commonly used to measure abso
`lute pressure are silicon sensors of the piezo-resistive type.
`These piezo-resistive sensors generally comprise a silicon
`membrane in which are incorporated piezo-resistive ele
`ments, that is to say elements whose resistivity varies
`according to the pressure to which they are subjected.
`These sensors, although having a very simple structure
`and small dimensions, present a number of disadvantages.
`Their lack of temperature stability necessitates the use of
`a temperature compensating circuit in order to obtain an
`accurate measurement. Added to this are reactions such as
`interdiffusions between the silicon of the membrane and the
`piezo-resistive elements, which accelerate the aging process
`of the sensors.
`This is why, when the use of semiconductor sensors
`requires durability, temperature stability, very high sensitiv
`ity and low consumption, capacitive sensors are generally
`used to measure absolute pressure.
`Capacitive sensors generally comprise a first element in
`which is made a membrane forming a mobile electrode and
`a second elementin which is arranged a Zone forming a fixed
`electrode, known as a counter-electrode. An insulating frame
`is interposed between the first and the second elements to
`create a chamber, insulated from the external environment,
`inside which there is zero pressure. A capacitive absolute
`pressure sensor is thus produced as the external pressure is
`measured in relation to the zero pressure, or almost zero
`pressure, prevailing inside the chamber.
`One disadvantage of these sensors arises from the tech
`nology for the manufacture of the latter, in this case the
`semiconductor material micro-machining technology.
`In the course of manufacture it is noted that degassing can
`occur within the structure of the sensor leading to the
`creation of residual pressure inside the chamber. The sensor
`does not, thus, under these conditions give an exact reading
`of the absolute pressure. Furthermore, the volume of gas
`contained in the chamber varies according to the tempera
`ture, so that the stability and/or the reproductibility of the
`readings are affected.
`A known solution to eliminate this problem consists of
`placing a cavity in the first element in order to create a
`so-called reference volume. The object of this reference
`volume in connection with the chamber volume is to reduce
`the residual pressure caused by the degassing.
`However, this solution in itself presents disadvantages.
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`When the reference volume is located outside the active
`surface of the membrane, that is to say outside the surface
`of the sensor which is subjected to pressure, the total surface
`of the sensor is increased by an amount corresponding to the
`surface occupied by the reference volume so that, for sensors
`providing active surface data, the number of realizable
`sensors on the same wafer of silicon is reduced, which in
`itself increases the cost of each sensor.
`Further, the inclusion of the reference volume in the first
`element requires machining the latter on two faces which
`complicates the process considerably.
`As these sensors are manufactured in batches from silicon
`substrate, at the end of the process they must be separated
`from each other in the course of a cutting operation across
`the first and second substrates. To this end, it is necessary to
`provide a supplementary surface at the periphery of the
`membrane which also reduces the number of sensors which
`can be produced on the surface of a wafer. Further, this
`cutting is generally by mechanical sawing, which causes
`stresses, or even cracks, within the membrane of the Sensors,
`which generally lead to irreparable damage to the sensors.
`
`SUMMARY OF THE INVENTION
`It is thus the main object of the invention to overcome the
`above mentioned disadvantages of the prior art by provid
`ing, on the one hand, a capacitive absolute pressure mea
`suring sensor which provides an enlarged reference volume
`in its active zone and which provides great sensitivity at the
`same time creating a sensor with a very simple structure, and
`on the other hand, a simplified and economical method of
`manufacturing a plurality of these sensors, which eliminates
`a large number of the risks of damage to the active parts of
`the sensors, notably at the time of cutting.
`It is thus an object of the invention to provide a capacitive
`absolute pressure measuring sensor comprising a first ele
`ment in which is arranged a mobile electrode, a second
`element in which is arranged a fixed electrode situated
`facing and insulated from said mobile electrode, a connect
`ing frame interposed between the first and the second
`element to define a chamber, insulated from the external
`environment and inside which there is substantially zero
`pressure, and a reference volume connected to said chamber,
`said sensor being characterised in that the first element has
`the configuration of a membrane of substantially constant
`thickness and in that said reference volume is arranged at
`least partially in the second element.
`Thus the ratio of the reference volume to its surface is
`increased and the sensor presents a temperature stability and
`a high sensitivity for a total sensor surface substantially
`equal to that of its active part.
`According to a feature of the invention, the reference
`volume is formed by a groove running around the fixed
`electrode.
`This feature provides the advantage of reducing the size
`of the active part of the measuring capacitator without
`reducing the sensitivity of the sensor so that the relative
`sensitivity of the sensor according to the invention is
`increased.
`The object of the invention is also to provide a method of
`manufacturing a plurality of integrated capacitive absolute
`pressure measuring sensors each comprising a mobile elec
`trode formed by a membrane and a fixed electrode, charac
`terised in that it comprises the steps of:
`-supplying a first substrate of a semiconductor material,
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`-supplying a second substrate of a semiconductor mate
`rial,
`-forming a connecting insulating layer on at least one
`first face of one of said first and second substrates,
`-structuring the connecting layer to expose a part of said
`first face to form at least one connecting frame,
`-structuring the exposed part of said first face to define
`the fixed electrode and a reference volume,
`-assembling by soldering under vacuum said first and
`second substrates using the connecting frame in such a way
`that that a first face of said second substrate is situated facing
`said first substrate and so as to define a chamber, insulated
`from the external environment, inside which the pressure is
`substantially zero, said chamber being in contact with said
`reference volume,
`-thinning the second substrate so as to produce a mem
`brane forming the mobile electrode,
`-structuring the second substrate to define the outline of
`said membrane, and for each of the sensors,
`-forming an electrical contact for each of the fixed
`electrode and the mobile electrode.
`The use of conventional semiconductor material micro
`machining techniques allows, in addition to maximum min
`iaturization of the sensors, minimisation of the internal
`stresses and the drift in temperature of these sensors by the
`implementation of a series of very simple steps.
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`The connecting frame 10 also forms a spacing frame
`which creates a dielectric space between the mobile elec
`trode 4 and the fixed electrode 8 and thus forms a conven
`tional measuring capacitor. The connecting frame 10 is
`closed and defines with the membrane 2 and the substrate 6
`a chamber 12 which is insulated from the external environ
`ment and inside which the pressure is zero or almost Zero so
`that any movement of the mobile electrode 4 is representa
`tive of the external pressure, or, in other words, of the
`pressure to which the active surface of the sensor is subject,
`and so that the measurement indicated is representative of
`the absolute pressure.
`The sensor also comprises a reference volume 14 in
`contact with the chamber 12 to reduce the pressure of the gas
`contained in the chamber 12 which result from the degassing
`which occurs during the manufacturing of the sensor 1.
`Furthermore, the sensor 1 comprises contact means 16 to
`establish contact with an external measuring circuit (not
`represented) which, conventionally, interprets the variations
`in capacity of the capacitor formed by the mobile electrode
`4 and the fixed electrode 8 when the external pressure
`displaces the mobile electrode 4.
`The contact means 16 are formed, in the example illus
`trated, by metallizations 16a, 16b placed respectively on a
`contact pin 18 linked to the mobile electrode 4 and on a
`exposed part 20 of the second element 6. In this regard, it
`will be noted that the pin 18 is supported by a contact stud
`22 of identical thickness to the connecting frame 10.
`In the example illustrated, the first and second elements
`are made of monocrystalline silicon and the connecting
`frame is made of silicon oxide.
`Moreover, the first element 2 comprising the mobile
`electrode 4 has the configuration of a membrane which
`presenting substantially constant thickness over its entire
`surface.
`This membrane has a general square shape and remains
`linked in one of its corners to the contact pin 18. Further
`more, the membrane extends substantially within the edge of
`the second element 6 for reasons which will become clear in
`the course of this description.
`It is thus easily understood that, for an identical total
`surface of the sensor, the active surface of the sensor is
`increased in relation to that of the sensors of the prior art in
`which the membrane is produced by a chemical etching of
`a part of the surface of the first silicon element 2 which is
`achieved according to the crystallographic planes inclined
`approximately 57% in relation to this surface and which
`consequently leads to a reduction in the active surface
`vis-a-vis the total surface of the sensor.
`The thickness of this membrane may vary and can be
`adjusted according to the sensitivity and/or the range of
`pressures that is to be measured.
`Furthermore, the sensor according to the invention has a
`cavity forming the reference volume 14 which is placed in
`the second element 6. More particularly, the volume 14 is
`composed of a groove which extends around the fixed
`electrode 8 and whose walls 24 are bell-mouthed.
`The method of manufacturing of a plurality of sensors 1
`according to the invention will now be described in con
`nection with FIGS. 4 to 10.
`It will be noted that, taking account of the small dimen
`sions of the sensors and for practical reasons which will be
`easily understood, the method of manufacturing according
`to the invention applies to the simultaneous manufacture of
`a large number of sensors each comprising a fixed electrode
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`Other features and advantages of the invention will appear
`more clearly from study of the following description of an
`embodiment of the invention, given purely by way of
`illustrative and non-limiting example, this description being
`made in connection with the drawings in which:
`FIG. 1 is a partially broken plan view of an absolute
`pressure measuring sensor according to the invention;
`FIGS. 2 and 3 are sectional views taken along line II-II
`and line III-III respectively of the sensor of FIG. 1; and
`FIGS. 4 to 10 are sectional views taken along line II-II
`of FIG. 1 of a capacitive absolute pressure measuring sensor
`according to the invention represented at different stages of
`the method of manufacturing according to the invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`Referring first to FIGS. 1 to 3, a capacitive absolute
`pressure measuring sensor according to the invention can be
`seen designed by the general numeral reference 1. In the
`following description, in the interests of simplification, the
`sensor defined above will be referred to as a 'sensor'.
`The sensor 1 which is generally of rectangular shape,
`comprises a first semiconductor element 2 comprising a
`mobile electrode 4, a second semiconductor element 6
`comprising a fixed electrode 8, known as a counter-elec
`trode, which is located more or less facing the mobile
`electrode 4. The surface of the electrode 4 defines the
`so-called active surface of the sensor 1, that is to say, the
`surface sensitive to the pressure which is required to be
`measured.
`The electrode 4 is insulated from the counter-electrode 8
`and, to this end, the first element 2 is separated from the
`second element 6 by an insulating connecting frame 10.
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`8 and a mobile electrode 4 separated by an open space 12 of
`slight thickness, from two complementary wafers defining a
`first substrate 30 and a second substrate 32 as shown in FIG.
`4. However, the description and the drawing will, for sim
`plification, refer only to the manufacture of a single sensor
`1.
`It is, moreover, important to note that the values of the
`various parameters, such as the temperatures, the times, the
`etchant used, etc., which will be mentioned below, are in no
`way limiting and mainly depend on the materials and
`apparatus used. These values can easily be determined by a
`person skilled in the art.
`The wafers (not shown) defining the first and second
`substrates 30, 32 from which the sensor 1 is manufactured
`are made of a semiconductor material such as monocrystal
`line silicon and, preferably, having the orientation <100>.
`FIG. 4 shows the first substrate 30 and the second
`substrate 32 after their preparation and the forming of a
`connecting layer 34 in an insulating material on a first face
`36 of the second substrate 32.
`The preparation of the substrates consists for example of
`cleaning their surfaces such as in the way described in the
`publication entitled "RCA Review” No 31 page 187, 1970.
`The connecting layer 34 comprises a lower layer 38 in a
`material which reacts to a first etchant and an upper layer 40
`in a material which reacts to a second etchant but which
`hardly reacts to the first.
`In this case, the lower layer 38 is made of silicon oxide
`(SiO2) and the upper layer 40 is made of silicon nitride
`(SiNx).
`In the example disclosed, the lower layer 38 has been
`formed by thermal oxidation of the second substrate 32 in an
`oven at around 1100° C. and under an oxidising atmosphere
`for several hours. By way of example, to obtain a layer 38
`having a thickness of approximately 2000 A, this second
`substrate 32 needs to be left in an oven for about 10 hours.
`It will be noted that the second face 42 of the second
`substrate 32 was protected during this step of formation of
`the layer 38. The presence or absence of this protection has
`no effect, however, on the development of the process of the
`invention.
`It goes without saying that, in one embodiment, the
`formation of the layer 38 can be achieved by chemical or
`physical vapour phase deposition (CVD or PVD).
`The upper layer 40 is then deposited on the surface of the
`lower layer 38, for example by low pressure chemical
`vapour phase deposition, to a thickness of approximately
`0.15 um.
`FIGS. 5 to 7 show the structuring step of the connecting
`layer 34 and of a part of the surface 36 of the second
`substrate 32 to define respectively the connecting frame 10
`and the groove 14 which forms the reference volume and
`which delimits the outline of the fixed electrode 8 of the
`sensor 1. As is shown in FIG. 1, the connecting frame is of
`a general square shape and the fixed electrode 8 has the
`shape of a square surface which extends concentrically to
`the connecting frame 10.
`To do this, a first layer of photoresist (not shown) is
`deposited on the entire surface of the upper layer 40, this first
`layer of photoresist is insolated through a mask (also not
`shown), the insolated parts of the first photosensitive layer
`are conventionally eliminated, for example by using a humid
`etchant, and the exposed parts of the upper layer 40 are
`etched using the first etchant to the surface of the lower layer
`38, so that the remaining parts 44 of the upper layer 40 in the
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`form of a frame make up a mask for the formation of the
`connecting frame 10 at the next step as is shown in FIG. 5.
`A second layer of photoresist (not shown) is then depos
`ited on the remaining parts 4 and on the parts 46 of the lower
`layer 38 exposed at the preceding step. This second layer of
`photoresist is insolated through a second mask (also not
`shown), the insolated parts of the second layer of photoresist
`are eliminated by conventional means, for example by using
`a humid etchant, so that the only photoresist left is disposed
`more or less concentrically to the frame 10 and covers a
`square 48 of the lower layer 38 whose surface corresponds
`to Chat of the fixed electrode 8. Using the second etchant the
`exposed parts of the lower layer 38 are etched until the face
`36 of the second substrate is exposed. The remaining parts
`of the photosensitive layer are then eliminated.
`The upper layer 40 is etched, for example, by means of a
`chlorine plasma, whilst the etching operation of the lower
`layer 38 is achieved, for example, by using a solution of
`hydrofluoric acid (HF). The elimination of the remaining
`parts of the first and second layers of photoresist is achieved
`by conventional means, for example using a humid etchant
`in a suitable solvent or by means of a plasma under oxygen
`atmosphere.
`This leads to the structure illustrated in FIG. 6 in which
`the surface 36 of the second substrate comprises the con
`necting frame 10 and the square 48 of the lower layer
`protected by the photoresist in the preceding step.
`The parts of the face 36 not protected by the connecting
`frame and the square 48 will be etched in a later step to
`define the groove 14 constituting the reference volume of the
`sensor according to the invention.
`By using the connecting frame 10 and the square 48
`formed by the lower layer as etching masks, the step
`illustrated in FIG. 7 consists of an anisotropic etch of the
`exposed parts of the face 36 using a third etchant which
`reacts mainly with the material of the second substrate but
`does not react with the materials of the lower and upper
`layers. The square 48 is then eliminated by etching using the
`second etchant.
`By way of example, the etching of the groove 14 is
`achieved using a humid etchant in a solution of KOH to a
`depth of the order of 100 um and the elimination of the
`square 48 is achieved by an etch using a humid etchant in a
`solution of HF.
`As shown in FIG. 7, the lateral walls 24 of the groove 14
`are bell-mouthed outwards because of the difference in
`speed of etching of the different crystalline planes of the
`substrate by the etchant,
`The second substrate 32 being structured, the next step
`consists of eliminating the remaining parts 44 of the upper
`layer 40 (SiN) with a view to the last step of assembly with
`the first substrate 30 and to form the second element 6.
`In this case, the elimination of the parts 44 of the upper
`layer 40 is achieved, for example, by means of a plasma
`etch.
`The next step consists of putting in place the first substrate
`30 so that a first 50 of its faces is situated facing the
`structured face 36 of the second substrate 32 or second
`element 6.
`The two substrates 30, 32 being prepared, the latter is then
`assembled by autogeneous soldering under a vacuum. To do
`this, the two substrates 30, 32 are placed in an oven
`preheated to a temperature of about 1100° C. in which there
`is substantially zero pressure.
`At the following step illustrated in FIG. 8, the first
`substrate 30 is thinned, that is to say that the second face 52
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`of this second substrate 32 which is exposed, is etched until
`the first substrate 30 reaches a predetermined thickness.
`To do this, the two substrates 30 and 32, assembled by the
`connecting frame 10 are dipped in a solution of etchant at a
`specific concentration, at a specific temperature and for a
`specific period of time to obtain the desired thickness,
`approximately 80 um in the example described.
`In order to avoid etching the face 42 of the second
`substrate 32 during this thinning step of the first substrate 30,
`the face 42 is mechanically protected from any contact with
`the etchant. According to another embodiment, a protective
`layer of a material which does not react to an etch by the
`third etchant can also be deposited.
`FIG. 9 illustrates the etching step of the first substrate 30
`with a view to defining the outline of the membrane con
`stituting the electrode 4. This stage consists of depositing a
`protective layer 54 which will be configured by conventional
`photolithographic techniques to etch the exposed parts of the
`first substrate and to eliminate the protective layer 54.
`In the example disclosed, silicon nitride (SiNx) is depos
`ited on the entire surface of the face 52, for example by low
`pressure chemical vapor deposition.
`Next, a third layer of photoresist (not shown) is deposited
`on the protective layer 54. This third layer of photoresist is
`insolated through a third mask (also not shown), the inso
`lated parts of the third layer are eliminated conventionally,
`for example using a humid etchant, in this case one part of
`a strip extending along one side of the face 52 as well as a
`narrow border of said face 52 to configure the electrode as
`it is shown in outline on FIG. 1.
`Using an etchant, the exposed parts of the protective layer
`are then etched, then the remaining parts of the photoresist
`are eliminated. This leads to the structure illustrated at FIG.
`9 in which the face 52 is completely covered by the
`protective layer 54 with the exception of one strip extending
`along the edge of the face 52 of the first substrate and the
`narrow border of the face 52 which were covered by the
`photoresist.
`The unprotected parts by the silicon nitride, of the face 52
`of the first substrate 30 are then etched on their entire
`thickness, for example, using the same etchant used during
`the thinning step of the first substrate 30.
`According to one feature of the manufacturing process of
`the invention, and as emerges from FIG. 10 in which an
`adjacent sensor is shown in dotted line, it will be noted that
`during the etching which has just been effected, the periph
`eral parts of the first substrate 30 formed by the first element
`2 and which itself forms the mobile electrode 4 of each of
`the sensors 1, separate from each other, in such a way that
`the individual sensors can be separated from each other by
`a simple sawing step of the sole second substrate 32 or
`element 6.
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`The next step consists of forming the contact means
`respectively for the first 2 and the second 6 elements.
`As emerges from FIG. 1, the contact 16a is formed on the
`lug 18 linked to the mobile electrode 4 and placed along the
`edge of the latter and the contact 16b is formed on the
`exposed edge 20 of the second element 6. The contact pads
`16a, 16b are achieved by vacuum evaporation of a metal, for
`example aluminium, through a mask (not shown). Of course,
`these contact pads 16a, 16b can also be produced by cathode
`sputtering.
`The sensors thus formed, each having a mobile electrode
`4 and a fixed electrode 6 composed respectively of the parts
`2 and 6 of the first substrate 30 and of the second substrate
`32, are then separated from each other at the time of the
`conventional sawing step and are each encapsulated in a
`housing or package, not shown, provided for this purpose. It
`should be noted that the figures do not represent the exact
`relative dimensions of the various elements in proportion to
`one another, these dimensions being sometimes highly exag
`gerated for greater clarity. To give an idea of the dimensions,
`a sensor produced according to the process of the invention
`has the general dimensions 1.9x2.2x0.5 mm, the surface of
`the membrane is approximately 1.7x1.7 mm', the thickness
`of the membrane is approximately 60x10 m and the
`thickness of the connecting frame is approximately 2x10
`m. With such dimensions, the measuring capacity and the
`parasitic capacity are respectively in the order of 6pF and
`5pF.
`What is claimed is:
`1. A capacitive absolute pressure measuring sensor com
`prising a first element forming a mobile electrode, a second
`element forming a fixed electrode positioned facing and
`separated from said mobile electrode, and a connecting
`frame interposed between said first and second elements,
`said first and second elements being electrically conductive,
`said connecting frame constituting a part distinct from said
`first and second elements and being made of an insulating
`material such that said first and second elements are elec
`trically insulated from each other, said connecting frame
`together with said first and second elements defining a
`chamber isolated from the external environment and inside
`of which there is substantially zero pressure, said second
`element having a surface facing said first element, said
`surface comprising an active part and a cavity disposed
`outside of said active part, and said cavity being formed by
`a groove extending around said active part.
`2. A measuring sensor according to claim 1, wherein the
`first and second elements are made of at least one semicon
`ductor material and said connecting frame is made of an
`oxide of said material.
`
`Abbott
`Exhibit 1011
`Page 008
`
`