EP0466796A1

EP0466796A1 - Resonant pressure sensor

Info

Publication number: EP0466796A1
Application number: EP90906394A
Authority: EP
Inventors: Nils Göran Stemme; Nils Gustav Erik Stemme
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-04-06
Filing date: 1990-04-06
Publication date: 1992-01-22
Also published as: SE8901209A; SE8901209D0; AU5425090A; WO1990012298A1; SE462874B

Abstract

Un capteur de pression comporte une capsule plate (1) sensible à la pression, adaptée pour être actionnée par une pression devant être mesurée, et être excitée pour osciller en résonance mécanique avec une fréquence de résonance en fonction de ladite pression, ainsi que pour subir une déformation provoquée par ladite pression, afin de produire une variation de rigidité de la capsule toute entière (1) et, par conséquent, une modification de sa fréquence de résonance. Le capteur de pression est caractérisé en ce que la capsule (1) est fixe (en 3) par rapport à, et adaptée pour exécuter ladite oscillation de résonance autour de deux lignes nodales sécantes non forcées (10, 11) de ladite capsule. Lorsque la capsule exécute son oscillation de résonance, elle oscille sensiblement perpendiculairement à un plan géométrique commun aux deux lignes nodales (10, 11).A pressure sensor comprises a flat pressure-sensitive capsule (1), adapted to be actuated by a pressure to be measured, and to be excited to oscillate in mechanical resonance with a resonance frequency depending on said pressure, as well as to undergo a deformation caused by said pressure, in order to produce a variation in rigidity of the entire capsule (1) and, consequently, a modification of its resonance frequency. The pressure sensor is characterized in that the capsule (1) is fixed (at 3) relative to, and adapted to execute said resonance oscillation around two non-forced intersecting nodal lines (10, 11) of said capsule. When the capsule executes its resonance oscillation, it oscillates substantially perpendicular to a geometric plane common to the two nodal lines (10, 11).

Description

RESONANT PRESSURE SENSOR

The present invention relates to a pressure sensor, more precisely a resonant pressure sensor of the type com- prising a pressure-sensitive member adapted to be actuated by a pressure which is to be measured, and to be excited to oscillate in mechanical resonance with a resonance fre¬ quency dependent on said pressure.

Most of today's pressure sensors are based on the principle that a pressure-induced static bulge of a pres¬ sure-sensitive membrane provided over a cavity is measured in piezoresistive or capacitive manner, but as computers and other digital systems are becoming more widely used, the need for pressure sensors with digital output signal increases as well. Such sensors include resonant pressure sensors of the type stated by way of introduction, the output signal of which is a frequency signal whose fre¬ quency changes as a function of the pressure which is to be measured and which actuates the membrane. When such a sensor is used, no special analog-to-digital conversion is needed at the interface to the digital equipment.

The majority of known pressure sensors with a fre¬ quency output signal are characterised in that the change in the resonance frequency of the oscillating member is a direct consequence of pressure-induced mechanical stresses in the membrane. The situation may be compared to that of a guitar string whose resonance frequency varies if the stress in the string is altered.

In addition, the majority of previously known pres- sure sensors with a frequency output signal are charac¬ terised in that the oscillating member, i.e. the membrane, oscillates in its fundamental mode. In actual practice, this means that the membrane is fixedly stretched over the cavity along its peripheral edge, which thus forms a closed nodal line, and oscillates with maximum amplitude in its middle. These known resonant pressure sensors suffer from the following inconveniences:

1. The sensitivity, i.e. the amount by which the reso¬ nance frequency changes for a given alteration of the pressure, is not satisfactory.

2. The Q-factor, or quality factor, of the oscillating

. member, i.e. the membrane, of the sensor is compara¬ tively low. A high Q-factor, i.e. a high and narrow resonance peak, is desirable since this entails a better- resolution of the pressure sensor. A general observation is that a low Q-factor of an oscillating system is caused by losses. In the known resonant pressure sensors, there is a risk not only of mecha¬ nical losses but also of acoustic losses. The mecha- n cal losses are caused by the oscillating membrane being fastened along forced nodal points or lines and by the entire membrane being in dynamic imbalance, and this imbalance creates unwanted losses via the points or lines of attachment of the oscillating mem- ber. The acoustic losses result from the fact that the oscillating member causes acoustic radiation in the sjirrounding medium, e.g. air. In order to reduce the acoustic losses, it is known to maintain a vacuum round the oscillating member, but this also unfavour- ably restricts the field of application of the sen¬ sor. To increase the Q-factor, it is also known to make the membrane of a monocrystalline material, such as monocrystalline silicon, which has a high inherent Q-factor. 3. It is not possible, on the basis of the frequency output signal, to distinguish between a positive and a negative pressure difference across the membrane, since an increased mechanical stress in said membrane and, consequently, an alteration of the resonance frequency in one and the same direction occurs both when the membrane bulges inwardly and when it bulges outwardly. US 3,503,263 dicloses a resonant altimeter in which the resonance frequency of at least one wall of a closed capsule is measured. In one embodiment, the capsule com¬ prises two circular, undulatory membranes which are at- tached at their peripheral edges along a frame wall which has the form of a cylinder and is suspended from suspen¬ sion means along its entire circumference. Consequently, the oscillating member is not in dynamic balance since the circular nodal line is forced. Furthermore, the two membranes of the capsule oscillate in phase opposition, i.e. they both move inwardly or outwardly at the same time, which entails undesired losses in the form of acoustic radiation.

US 3,257,850 discloses a resonant pressure sensor of the type mentioned by way of introduction, whose pres¬ sure-sensitive member consists of an elongate, closed tube. The function of this pressure sensor is based on the tube undergoing a deformation caused by said pres¬ sure, for producing a change in rigidity of the entire tube and, consequently, an alteration of the resonance frequency thereof. The pressure-sensitive tube either executes a transversal oscillation (an oscillation per¬ pendicular to a plane through a main axis of the tube cross-section) or a torsional oscillation (an oscillation about the longitudinal axis of said tube). These embodi¬ ments are shown in Figs 1-8, Figs 13-22 and Figs 24-31. Furthermore, Figs 9-11 illustrate two embodiments com¬ prising a U-shaped, oscillating tube whose oscillation is neither transversal nor torsional. Compared with the pre- sent invention, the oscillating tube in US 3,257,850 lacks intersecting nodal lines, and the tube in the em¬ bodiments with torsional oscillation, which are the most relevant ones for comparison purposes, is not fixed with respect to any nodal line but only with respect to a nodal plane. The object of the present invention is therefore to provide a resonant pressure sensor of the type mentioned by way of introduction, having none of the above disadvan¬ tages of the known pressure sensors but being characteris- ed by a high sensitivity, a high Q-factor, and the possi¬ bility of distinguishing between a positive and a negative pressure difference.

The present invention also aims at providing a reso¬ nant pressure sensor of the type mentioned by way of in- troduction, which reduces the attenuating effect of the surrounding medium, i.e. which reduces acoustic radiation emanating from the sensor, said radiation causing losses and lowering the Q-factor.

Therefore, the pressure sensor according to the in- vention comprises a pressure-sensitive, flat capsule adapted to be acutated by a pressure which is to be mea¬ sured, and to be excited to oscillate in mechanical re¬ sonance with a resonance frequency dependent on said pres¬ sure, as well as to undergo a deformation caused by said pressure, for producing a change in rigidity of the entire capsule and, consequently, an alteration of the resonance frequency thereof. The novel and characteristic feature of the pressure sensor according to the invention is that the capsule is fixed with respect to, and adapted to execute said resonance oscillation about, two intersecting un¬ forced nodal lines of said capsule, the latter oscillat¬ ing, when executing said resonance oscillation, substan¬ tially perpendicular to a plane common to both nodal lines. Thus, the entire cavity-forming capsule is made to oscillate in mechanical resonance, and it is the shape of the entire capsule, and thus its rigidity, that is changed by the pressure and produces the alteration of the reso¬ nance frequency.

As mentioned above, a first object of the invention is to provide a sensor with higher sensitivity. This is achieved precisely in that the alteration of the resonance frequency of the capsule is dependent on a change of the shape thereof, instead of a change in mechanical stress.

A second object of the invention is to provide a sen¬ sor with higher Q-factor, i.e. with a higher degree of re- solution. This is achieved in that the capsule is made to oscillate in dynamic balance about one or more unforced or inherent nodal lines. By 'unforced or inherent nodal lines' are here meant such nodal lines about which the capsule, when in a free state, self-oscillates. A third object of the invention is to make it pos¬ sible to distinguish, on the basis of the frequency output signal of the sensor, between a positive and a negative pressure difference. This object is also achieved by means of the pressure sensor according to the invention since an outward bulge of the capsule induced by the pressure dif¬ ference results in an increased rigidity and, consequent¬ ly, a higher resonance frequency, while an inward bulge of the capsule also induced by the pressure difference re¬ sults in a lower rigidity and, consequently, a lower reso- nance frequency. In this context, it can be observed that the capsule may, already in its initial position before the pressure measurement, have an outward or inward bulge, but this need not prevent the distinguishing between posi¬ tive and negative pressure difference. According to the invention, the capsule oscillates about two intersecting, unforced nodal lines which, for instance, divide the capsule into four equal portions. The capsule oscillates in such manner that the capsule por¬ tions adjacent to one another along the nodal lines oscil- late in phase opposition. If, for example, the capsule is square and said nodal lines intersect the centre points of the edge sides of the square, the two quadrants along one diagonal of the square will oscillate in phase with one another and in phase opposition in relation to the two quadrants along the other diagonal of said square. This oscillation mode contributes to a higher Q-fac¬ tor not only by producing the dynamic balance but also by maximally reducing the attenuating effect of the surround¬ ing medium. If the capsule is, for instance, surrounded by air, the displacement of air caused by the resonance oscillation mainly occurs internally between the capsule portions oscillating in phase opposition. Thus, the air will chiefly move short distances along the surfaces and between the sides of the capsule, such that acoustic radiation which causes losses and lowers the Q-factor is reduced considerably.

In a preferred embodiment of the invention, the pres¬ sure-sensitive oscillating capsule is made up of two op¬ posing pressure-sensitive membranes and a non-pressure- sensitive member in the form of a frame or the like, which interconnects the peripheral edges of the membrane and from which the capsule is suspended in the above manner. To obtain maximum pressure sensitivity of the sensor, the change in rigidity caused by a given alteration of the pressure should be as large as possible. The deformation of the membranes, and consequently of the capsule, is maximised if the membranes are made as thin as possible, and if the inherent rigidity of the frame is minimised by making the frame as thin and slender as possible. A low total rigidity results in large oscillation amplitudes, which facilitates the detection of the frequency. The limit as to how thin the membranes may be is determined by the ultimate strength of the material used, e.g. mono- crystalline silicon, and by the requirement that the two membranes must not strike against another when their in¬ ward bulge is at its maximum.

Naturally, it is also important for the sensitivity and the Q-factor of the pressure sensor according to the invention that the cross-section of the suspension means of the capsule is minimised, and that these means inter¬ fere as little as possible with the mechanical resonance oscillation. It is possible to have only one pressure-sensitive membrane instead of two but, as mentioned above, the sen¬ sitivity of the sensor increases as the pressure-sensitive member of the capsule is enlarged. A possible alternative would be an embodiment with two membranes whose peripheral edges are directly attached to one another without the use of any frame.

If a vacuum is established in the inner cavity of the capsule, an absolute pressure sensor is obtained. A pos- sible negative temperature dependence of the resonance frequency of the structure can be eliminated by confining inside the capsule a controlled amount of gas contributing positively to the temperature dependence of the resonance frequency. A differential pressure gauge is obtained if a duct is arranged to communicate with the cavity of the capsule, said duct preferably extending from the capsule along one of the nodal lines so as not to interfere with the reso¬ nance oscillation. The arrangement of a duct communicating with the cavity of the capsule can also be used for pro¬ viding an absolute pressure gauge with a very high Q-fac¬ tor and, consequently, a high resolution, which is achiev¬ ed if the oscillating member, i.e. the capsule, is placed in a vacuum and the pressure which is to be measured is connected with the cavity via said duct. The Q-factor will be high because there is no air around the structure which could have an attenuating effect on the oscillation.

The excitation of the oscillating member, i.e. the manner in which the capsule is made to oscillate in mecha- nical resonance, may be brought about in various ways, for example by electrostatic excitation or thermal excitation.

Electrostatic excitation is a known technique based on the electrostatic interaction of two mutually adjacent electrically charged surfaces upon application of an elec- trie potential to these surfaces. By providing one of two such surfaces in an area of the capsule where this oscil¬ lates with a high or the highest amplitude, e.g. in a cor- ner of said square, and by arranging the other surface in the form of an outer electrode in the vicinity of and pa¬ rallel to the corner surface, a mechanical resonance os¬ cillation can be produced if an electric alternating vol- tage with a frequency identical to the mechanical reso¬ nance frequency of the capsule is applied between said surfaces. The conductive surface of said corner may con¬ sist of a thin layer of metal, or silicon may be used and made electrically conductive by doping. The mechanical resonance oscillation may also, as indicated above, be produced by means of thermal excita¬ tion. This known technique is based on the fact that a ma¬ terial expands upon heating. If specifically chosen por¬ tions of the capsule are heated pulsewise with a frequency corresponding to the resonance frequency, said capsule can be made to self-oscillate. The local heating may be car¬ ried out with the aid of electric power generation in in¬ tegrated resistors on a silicon surface, in which case the resistors can be associated with outer connections via conductive patterns extending over the capsule suspension means.

For the excitation, there should, furthermore, be some kind of feedback, such that the oscillating member is all times excited with a frequency corresponding to the resonance frequency at issue which is changed depending on the applied pressure.

There are a number of known methods for detecting the resonance frequency with which the capsule oscillates, such as capacitive sensing, piezo-resistive sensing and optical sensing. Since these methods are well-known to the expert, they need not be further explained here.

The invention will be described in more detail below with the aid of an embodiment, reference being had to the accompanying drawings illustrating different methods of making the pressure sensor, as well as modifications thereof. In the drawings,

Fig. 1 is a top view of an embodiment of a pressure sensor according to the invention, which is suspended in an outer frame, Fig. 2 is a cross-section along the line A-A of the structure in Fig. 1,

Fig. 3 is schematic perspective view illustrating how the capsule in Figs 1 and 2 oscillates about nodal lines, and showing the pressure shifts caused by said oscilla- tion,

Figs 4 and 5, which correspond to Figs 1 and 2, re¬ spectively, also show arrangements for excitation and de¬ tection,

Fig. 6 is a broken-out cross-section of the differen- tial pressure sensor according to the invention, a commu¬ nication to the inner cavity being established via a duct in a suspension means,

Fig. 7 is a cross-section along the line A-A of the suspension means in Fig. 6. Figs 8A, 8B and 8C are schematic sections illustrat¬ ing different steps in the production of a pressure sensor of silicon according to the invention,

Fig. 9 illustrates the use of a layer, e.g. of si¬ lica, which serves to define the height of the cavity and is provided between two silicon plates,

Fig. 10 is a cross-section of a suspension means mo¬ dified as shown in Fig. 9, and

Fig. 11 is a broken-out top plan view of a corner of an embodiment of the pressure sensor, an apertured and increased mass being provided in each corner of the cap¬ sule.

Fig. 1 is a schematic top view and Fig. 2 is a sche¬ matic section of a pressure sensor according to the inven¬ tion, which is made of monocrystalline silicon. An oscil- lating member, which consists of a capsule 1 with an inner cavity 2, has the form of a square and is, by means of four suspension means 3, suspended in an outer frame 4. The capsule 1 consists of two pressure-sensitive membranes 5 whose peripheral edges are held together by an inner frame 6 which forms part of said capsule 1 and to which the suspension means 3 are attached. As is apparent from Fig. 2, an upper half 8 and a lower half 9 are joined to¬ gether along a common plane 7, thus forming said struc¬ ture..

As is plain from Fig. 1, the suspension means 3 are connected with the inner frame 6 of the capsule 1 in the middle of every edge side. Thus, the devices 3 are located on two inherent, perpendicularly intersecting nodal lines 10, 11 of the capsule 1.

The nodal lines 10, 11 are drawn with broken lines in Fig_* 3 which is a schematic perspective view illustrating the mode in which the capsule is adapted to oscillate, and the way in which the surrounding medium, e.g. air, is dis¬ placed by the oscillations of said capsule.

As illustrated by means of filled arrows 12, the cap¬ sule 1 is adapted to oscillate in such manner that quad- rants adjacent one another along the nodal lines 10, 11 oscillate in phase opposition. As indicated in Fig. 3 by the arrows 12, this means that the two quadrants or corner portions of the capsule 1 along a first diagonal oscillate downwardly. Consequently, that portion of the mass of the capsule 1 which, at a certain moment, is located above the plane defined by the nodal lines 10, 11 equals that por¬ tion of the mass of the capsule 1 which, at the same mo¬ ment, is located below said plane. Thus, a dynamic balance is obtained without any mechanical stress at the suspen- sion means 3, so that energy losses that way are avoided, resulting in a high Q-factor.

This mode of oscillation also entails that the dis¬ placement of air caused by the oscillation mainly occurs in the vicinity of the capsule 1, more precisely in the manner illustrated by means of the unfilled arrows 13 and 14 in Fig. 3. The arrows 13, 14 indicate that there is both a displacement of air laterally along the two main surfaces of the capsule 1 and a displacement of air be¬ tween the main surfaces of said capsule adjacent to the corner portions. As stated in the introduction, a reduc¬ tion of acoustic radiation from the oscillating capsule 1 also results in a high Q-factor of the oscillating system. Figs 4 and 5 illustrate the structure in Fig. 1 and Fig. 2, respectively, in combination with an arrangement for excitation and detection. In the embodiment shown, excitation and/or detection resistors 15 and 16 are pro- vided on the frame 6 of the capsule 1, preferably in the form of integrated resistors which are connected, via lines 17 and 18, to outer connection surfaces 19 and 20, respectively. The resistors may, for instance, be used for producing the mechanical resonance oscillation of the cap- sule 1 in the mode illustrated in Fig. 3, in that they are heated pulsewise in accordance with the method stated in the introduction.

Another arrangement for excitation and/or detection is shown in the bottom corner to the right in Fig. 4. In this embodiment, a corner electrode 21 is provided on or integrated with the frame 6 of the capsule 1 via a line 22 associated with an outer connection surface 23. The corner electrode 21 may, in combination with a fixed outer elec¬ trode 24, be used for capacitively exciting the capsule 1 or detecting the resonance frequency of said capsule. Fig. 5 illustrates how the corner electrode is integrated with the underside of the frame 6 and cooperates with the outer fixed electrode 24.

It is preferred that the lines 17, 18 and 22 to the outer connection surfaces 19, 20 and 23, respectively, are run via the suspension means 3 so as not to disturb the resonance oscillation of the capsule 1.

Optical detection is another detection variant in which, for example, a light beam is reflected and deflect- ed by an oscillating frame corner, the deflection of the reflection being measured by a suitable light-sensitive detector.

The above detection and excitation methods makes it possible to choose an excitation and detection principle suited for the application at issue. It is, for instance, possible to have electrostatic excitation of one frame corner and capacitive sensing of the four frame corners of the other side.

Here follows a brief description of how it is pos- sible to produce, by means of conventional microelectronic production methods and by means of a few special pro¬ cesses, both an absolute pressure sensor and a differen¬ tial pressure sensor of silicon. The starting material consists of 0.38 mm thick wafers (reference numeral 30 in Figs 8A and 8B) polished on both sides and made of mono- crystalline silicon of (100)-type. A thin layer of silica (reference numeral 31 in Figs 8A and 8B) of 0.2-0.5 urn is made to grow by thermal oxidation of the wafers 30 at high temperature (1000-1100°C) in an oxygen atmosphere. By pho- tolithography and oxide etching, silica patterns are then established on both sides of the silicon wafer 30. Using these oxide patterns as a protective mask, the exposed silicon is then etched away by means of an anisotropic si¬ licon etchant, e.g. EDP which is a mixture of pyroca- techol, ethylene diamine and water. The silicon etching, which is carried out in two steps, results in the struc¬ tures shown in Figs 8A and 8B.

Alternatively, a doping-selective or electrochemical etchant can be used for producing a very exactly con- trolled membrane thickness since the silicon etching auto¬ matically stops when the etchant reaches a previously made doping change in the silicon, e.g. in the form of a heavi¬ ly boron-doped layer or a p-n-junction. After division, removal of oxide and washing, two such identical struc- tures are joined together by a special bonding process called direct silicon-to-silicon bonding, silicon fusion bonding (SFB) or atomic-bonding. Prior to bonding, the two structures are exposed to a specific surface-activating process, e.g. in hot nitric acid, whereby a joint 7 is formed when the two silicon surfaces are then compressed, as seen in Fig. 8C. The joint 7 obtains its final strength during the chemical reaction which occurs at high tempera¬ ture (1000-1150°C).

If a differential pressure sensor is to be produced, a groove 32 (see Fig. 7) is etched in the two halves of one of the suspension means 3. After the two halves have been joined, the grooves 32 form a narrow duct 33 ending in the cavity 2 and in the outer frame 4, as seen in Fig. 6. As is shown in Fig. 6, a tubular connector 34 adapted to receive an outer tube or hose connection 36 can be glued (at 35) to the outer frame 4 at the outer end of the duct 33.

Optionally, the high-temperature treatment forming part of the bonding of the differential pressure sensor can be carried out in a heavily oxidising atmosphere ("wet oxygen") in order to seal any leaks in the bond with si- lica.

Using the same bonding technique, it is possible to join silicon surfaces with a thin layer 37 of silica (Figs 9 and 10) between said surfaces. With a silica pat¬ tern on one or both of the joint surfaces, it thus becomes possible to obtain, after the joining, cavities 2 and ducts 33, as shown in Figs 9 and 10 which especially illu¬ strate how the intermediate oxide layer defines the height of the cavity 2. In this way, any uncertainty as to the thickness of the membrane 5 caused by a silicon-etched cavity 2 is avoided. By purposely providing a duct 33 in the oxide 37 between the cavity 2 (silicon-etched or oxide-height-defined) and the surroundings of the pressure sensor, the bonding may be carried out at high temperature and atmospheric pressure since, because of the duct 33, there will be no differential pressure deforming the mem¬ brane. If one wishes to manufacture an absolute pressure sensor, the duct 33 can be closed by applying a layer of a suitable material at low temperature (<600°C) and in va¬ cuum. For example, silica or polycrystalline silicon can be applied by LPCVD technique. If the bond is to be air¬ tight, said layer need to have a thickness at least equal- ling the total oxide thickness defining the height of the duct.

Finally, another embodiment of the invention is shown in Fig. 11 which is a schematic and broken-out section of a corner of a pressure sensor according to the invention. The corners of the capsule 1 are provided with increased masses, in this case enlarged corner areas. The corner surface 40 is provided with a number of through holes 41. This embodiment has the advantage that an increased mass at the end points of an oscillating member (the capsule) results in a higher Q-factor, as has been shown by experi¬ ments and described in literature. Also, the surrounding medium can flow from one side of the capsule 1 through the holes 41 to the other side of said capsule, which reduces viscous and acoustic losses and, consequently, increases the Q-factor.

In the specification, the invention has been de¬ scribed with the aid of various embodiments and manufac¬ turing methods, but it goes without saying that the in¬ vention is not restricted to the above embodiments but is defined only by the appended claims. Thus, the capsule 1 need, for example, not be square but may be circular, rec¬ tangular or star-shaped, and the nodal lines 10, 11 need not intersect at right angles to one another. Furthermore, it is also conceivable to have only one pressure-sensitive membrane 5.

The outer frame 4 (shown in Fig. 1) from which the oscillating capsule 1 is suspended by the suspension means 3, must not necessarily be a frame but may have any shape whatsoever and, optionally, form part of a larger system. Moreover, it is also conceivable to fix the suspen¬ sion means 3 on the upper side and/or the underside of the frame 6 instead of along the edge of said frame.

Claims

1. A pressure sensor comprising a pressure-sensi- tive, flat capsule (1) adapted to be actuated by a pres¬ sure which is to be measured, and to be excited to oscil¬ late in mechanical resonance with a resonance frequency dependent on said pressure, as well as to undergo a de¬ formation caused by said pressure, for producing a change in rigidity of the entire capsule (1) and, consequently, an alteration of the resonance frequency thereof, c h a r a c t e r i s e d in that the capsule (1) is fixed (at 3) with respect to, and adapted to execute said resonance oscillation about, two intersecting unforced nodal lines (10, 11) of said capsule, the latter oscil¬ lating, when executing said resonance oscillation, sub¬ stantially perpendicular to a geometrical plane common to both nodal lines (10, 11).

2. Pressure sensor as claimed in claim 1, c h a - r a c t e r i s e d in that the two intersecting nodal lines (10, 11) divide the capsule (1) into four equal portions.

3. Pressure sensor as claimed in claim 1 or 2, c h a r a c t e r i s e d in that the capsule (1) com- prises two opposing, pressure-sensitive membranes (5) whose peripheral edges are held together by a non-pres¬ sure-sensitive member (6) of said capsule (1) and which define between them a cavity (2) in said capsule (1).

4. Pressure sensor as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the capsule (1) is closed and adapted to be actuated externally by the pressure which is to be measured.

5. Pressure sensor as claimed in any one of claims 1-3, c h a r a c t e r i s e d by a duct (33) which is connected with an inner cavity (2) of the capsule (1) and extends from said capsule along one of said nodal lines (10, 11).

6. Pressure sensor as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the capsule (1) is made of a monocrystalline material, such as monocrys¬ talline silicon.

7. Pressure sensor as claimed in claim 6, c h a ¬ r a c t e r i s e d in that the capsule (1) is made of two monocrystalline silicon wafers (30) which are joined together by so-called direct silicon-to-silicon bonding.

8. Pressure sensor as claimed in claim 7, c h a ¬ r a c t e r i s e d by a layer (37) of silica which is provided between the two silicon wafers (30) and whose thickness defines the height of a cavity (2) in said capsule (1).

9. Pressure sensor as claimed in any one of claims 6-8, c h a r a c t e r i s e d in that the capsule (1) is suspended from a number of suspension means (3) which extend from the capsule (1) along said nodal lines (10, 11) and which are made of the same material as and in one piece with said capsule (1), and in that said capsule (1) supports integrated driving means (15, 16, 21) for pro¬ ducing said resonance excitation, and/or detectors (15, 16, 21) for detecting the resonance frequency at issue of the capsule (1), said driving means and/or detectors (15, 16, 21) being electrically connected to lines (17, 18, 22) extending along one or more of the suspension means (3).

10. Pressure sensor as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the capsule (1) is provided with increased masses (40) in the areas where it oscillates with a large amplitude, for producing a higher Q-factor.

11. Pressure sensor as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the capsule (1) is adapted to receive a controlled amount of gas which, in case there is a negative temperature depen- dence of the resonance frequency, eliminates this depen¬ dence by contributing positively to the temperature de¬ pendence of the resonance frequency.