Transducer element, method for its manufacture and its use in a pressure pick-up device

p0001The invention relates to a transducer element with at least one membrane region comprising diaphragm plate of semiconductor material with at least one piezoresistor and an insulating plate on which the diaphragm plate is attached. The invention further relates to methods for the production of the transducer element as well as a pressure transducer using the transducer element.

p0002The development and application of piezoresistive pressure sensors based on a semiconductor material such as diffused resistors have acquired in recent years comprehensive significance in the case of the automation. The sensing element of such a pressure sensor is on an insulating substrate such as glass arranged plate, preferably of silicon (Si) single crystal, in which an elastic membrane portion is formed, which undergoes a deflection under the acting pressure measurement. In the places with the highest radial and tangential resistors on the diaphragm portion attached or diffused, which are connected in a known manner to the hour or half Messbrükken.

p0003Single crystal silicon as has excellent mechanical properties by it practically does not show any Kreich- and Alterungserschneinungen. Silicon is therefore an ideal semiconductor material for forming the membrane plates, especially since it can be machined with high precision mechanical or chemical, and has particularly good electrical properties. The essential disadvantage of the silicon is by the factor 4 to 6 lower thermal expansion coefficient compared to the steels commonly used for the pressure transducer housing, in particular the austenitic corrosion resistant steels. Therefore, temperature changes can easily lead to tensions and distortions of attached to such steels Si-membrane plates, which may have as zero deviations.

p0004The development work in recent years have therefore to deal with these highly undesirable bracing difficulties and various measures have been proposed to overcome this drawback of piezoresistive pressure sensors based on Si-membrane plates at least partially. So it is suggested in EP-AI-0033749, the stretchable Si membrane plate by a plastic or silicone rubber and shear elastic to attach to the metal mounting plate of the transducer so that the expansion between silicon and metal are intercepted elastic. However, this measure has still reaction forces are placed on the Si membrane plate result, so that it is suitable only for pressure, the relative stability and low zero stability requirement. In addition, plastic and in particular silicone rubber is characterized by a very high thermal expansion coefficient and a pronounced hysteresis with respect to the stress / strain behavior. Both properties can cause additional errors in pressure measurement.

p0005From DE-AI-29 38 240 is a piezoresistive pressure transducer on Bais a transducer element is known in which the Si-membrane plate by anodic bonding, which include a heating in the temperature range of 360 ° C-500 ° C is necessary, on an insulating glass plate is attached. Since glass has a higher coefficient of expansion than silicon, tilt the laminate of the two panels to become tense during the cooling by the Si-membrane plate is laterally displaced under compressive stress. This can in extreme cases lead to the membrane of a stable position to another "flips" (tin-lid effect). Such instability, even if the extreme is not reached changed in highly undesirably the steepness or Kati Brier curve and is the cause of various deviations from the Linierität. In addition, the zero point of the temperature is greatly pending.

p0006From DE-AI-30 09 163 is a piezoresistive pressure transducer further based on a Si-membrane plate is known which is mounted on a thick soft solder layer on a mounting plate of an Fe-Ni alloy. Solders have substantially higher expansion coefficient than silicon and therefore also cause a change in temperature stressing of the diaphragm plate. Such pressure transducers daber not suitable for precision measurements.

p0007In GB-A-20 80 541 a receiver is described with a Si membrane plate fixed on one side of a glass plate. On the opposite side of the glass plate, a tubular holder is attached from an Fe-Ni alloy. In order to minimize internal stresses in the anodic connection of the parts, the Wäremeausdehnungskoeffizienten adjacent components are matched. Although at a specific composition of the Fe-Ni alloy can be achieved such that their coefficient of thermal expansion substantially corresponds to that of Pyrex glass, but is not a perfect match can be achieved. This has the result that although pressure sensor can be produced in which no cracking is to befürehten during the anodic connection, subject to certain upper temperature limits are not exceeded, but have the remaining differences in thermal expansion coefficients with the result that no regard to the zero stability and measuring accuracy substantial improvement over other known arrangements can be obtained.

p0008The invention is based on the object to provide a transducer element of the aforementioned type, which can be preisgünstigt and manufactured with high precision and at the temperature changes in a wide temperature range only geginge or no distortion of the measurement membrane entail.

p0009This object is characterized in that on the membrane plates facing away from the surface of the insulating plate, a base plate is begestigt which has a substantially equal coefficient of thermal expansion as the diaphragm plate and the dimensions of which are matched to the dimensions of the diaphragm plate so that when the temperature change, the exerted by the diaphragm and the base plate on the insulating plate deformation forces for achieving a voltage symmetrical state are substantially equal.

p0010Further developments of the invention are set forth in the dependent claims 2-9.

p0011With the inventive measures is achieved that when the temperature changes, the force exerted by the two plates on the insulating plate deformation forces are substantially the same sense, for example, when the thermal expansion coefficient of the semiconductor material is smaller than that of the insulation material, the thermal expansion coefficient of the material of the base plate is also to be smaller than that the lsolationsmaterials. The base plate can also be anodically bonded to the insulator or used to bond glass frits. Both methods require heating to 400 ° C to 500 ° C. At a temperature change, the dynamic effects of both bonded to the insulator plates in the same direction and are mutually supportive. A glass plate, which is also attached on one side to a silicon diaphragm plate and on the other side of a base plate made of silicon is prevented at a cooling of both sides at a thermal contraction and the force of the glass plate on the diaphragm plate is smaller than because no base plate would be present. The thermally-induced force of the base plate on the glass plate causes the cooling process in the same tensions that continue through under attenuation through the glass plate and retained. By choosing a relatively thick base plate of material of low expansion coefficient, a compensation of the stress of the silicon diaphragm can be achieved. In practice, it is usually not necessary to go through to complete compensation. It is sufficient to reduce the voltage spikes, thus the elongation of the material in the elastic range Hooke's remains and no permanent deformations occur.

p0012The transducer element according to the invention is thus distinguished by the fact that temperature changes in a wide temperature range of at least 100 ° C little or no change in bias of the diaphragm games entail. Fitted with the transducer element according to the invention, pressure sensors are therefore characterized by high accuracy, temperature independence of the sensitivity and zero stability.

p0013The base plate and the diaphragm plate can be made of same or different materials, as long as their coefficients of thermal expansion are similar to that of the insulator boards or of the membrane. The preferred material for the base and diaphragm plate is silicon, while the insulating plate is preferably made of a suitable glass material. This has inter alia the advantage that the plates can be fixed by rigid anodic compounds with one another.

p0014According to another embodiment of the invention may be attached to an intermediate carrier of a metallic material such as an iron-nickel alloy with a similar coefficient of expansion the base plate. The base plate causes an expansion compensation between the insulating plate and the intermediate carrier, even if that of a material with a higher coefficient of expansion than the material of the insulating plate should be. The connection of the base plate with the mushroom-shaped intermediate carrier may be a bond, a glass frit or solder. At the intermediate support has a solderable surface coating may be provided. Transducer elements with such intermediate carriers can therefore be soldered particularly economical in so-called. Batch process to the mounting plates or brackets of the pressure sensor soft.

p0015The method of manufacturing a transducer element of the kind mentioned is characterized in that one anodically connecting a a plurality of membrane regions having Water from the semiconductor material, and a plate substrate made of the insulating material with each other, cutting the laminate formed into units each having at least one diaphragm portion, base plates made of a material having substantially the same coefficient of thermal expansion as the diaphragm plate and adapted to the diaphragm plate of each unit dimensions are prefabricated and mounted on the insulating plate of each unit a prefabricated baseplate, which when the temperature changes, the pressure exerted by the base plate and diaphragm plate on the insulating board deforming forces to achieve a voltage ymmetrischen state are substantially equal.

p0016This approach has the advantage that the formation of the anodic bonding between the membrane wafer and the insulating generally transparent substrate can be easily monitored visually. The attachment of the base plate to the insulating plate is preferably carried out simultaneously in one and the same heat treatment process together with the fastening of the base plate to the support of the transducer or on a mushroom-shaped intermediate carrier. Preferably, the base plate is bonded to the insulating plate or the holder with glass frits, but it would also be an adhesive bond possible.

p0017An alternative to the aforementioned method is characterized, that one arranges a plurality of membrane regions having wafers of the semiconductor material and a base wafer, a disk substrate made of the insulating material between and connects anodically with the wafers, said base wafer of a material having substantially the same coefficient of thermal expansion as the diaphragm wafer and has a coordinated on the membrane wafer dimension has, and the laminate thus formed is cut into units with at least one diaphragm portion, whereby when the temperature changes, the force exerted by the diaphragm and the base plate of each unit on the insulating plate deformation forces to achieve a voltage symmetrical state are substantially equal.

p0018In this way, can be produced economically at the same time a plurality of transducer elements with sophisticated principle to the expert known methods of semiconductor technology. The transducer element can then be fixed in a further step, for example by bonding or by glass frit, to a mounting plate or mounting of the pressure sensor. Under glass frits refers to the abandonment of a glass powder onto the surfaces to be joined and the subsequent melting of the glass.

p0019A pressure sensor using a transducer element of the aforementioned type is characterized in that the transducer element is attached directly or via a to the base plate-fixed intermediate carrier at a arranged in the transducer housing Haiterung. The attachment can be by gluing, soldering or glass frits successes. With respect to other developments, reference is made to the dependent claims 14-18.

p0020To keep the effects of thermal stresses between the mounting of the transducer element and the transducer housing as low as possible, the support should preferably be connected elastically extensible to the transducer housing. therefore a particularly advantageous development of the invention is characterized in that the holder is connected via a stretch-elastic tubular member to the transducer housing. The holder is replaced by a substantially U-shaped cross section. Not only does the thermal expansions of the transducer are intercepted elastic, but also the additional advantage is achieved that the sensitive sealing by, which are mounted in the holder, the electrical conductors, which dissipate the measuring signals to the outside, placed sufficiently far from the location away could be where the tubular member is welded to the transducer housing. This avoids damaging the sealing by local overheating.

p0021The tubular elastic member may be disposed on the same side of the support as the transducer element. In this case the installation of the attitude with the thereon transducer element from below into the transducer housing is done. Therefore, can be attached one to be provided on the housing waterproofing membrane for a pressure-transmitting medium before installing the bracket and checked for proper operation.

p0022The resilient tubular member may also be arranged on the side facing away from the transducer element of the posture. This opens the possibility to let the pressure transmission medium and laterally acting on the holding device or the tubular member, so that the electrical, glazed-in of the holder conductors are subjected to a lateral pressure. This avoids that, especially at high measurement pressing the electrical conductors are pressed out of the sealing by.

p0023The invention will now be described by way of embodiments with reference to the drawing measure. Show it:<ul><li>FIG. 1 in a sectional view of an inventively constructed transducer element showing the force exerted by measuring compressive forces.</li><li>FIG. 2 fragmentary partial views of transducer elements ählich Fig. 1 for absolute pressure measurements (left half of the drawing) and relative pressure measurements (right half of the drawing) and their mounting on appropriately designed mounting plates or brackets and with marked deformation forces.</li><li>FIG. 3 shows fragmentary sectional views of transducer elements similar to FIG. 2, for absolute or relative pressure readings in connection that with accordant formed intermediate carriers, wherein the base plate is formed substantially stronger than the membrane plate</li><li>Fig. 4 is a perspective partial sectional view of a laminate of a membrane wafer and an insulating substrate,</li><li>Fig. 5 is a perspective sectional view of a cut from the laminate according to Fig. 4 transducer element with subsequently fixed base plate,</li><li>Fig. 6 is a perspective partial sectional view of a laminate consisting of a membrane wafer, a base wafer and a substrate disposed therebetween,</li><li>Fig. 7 is a perspective sectional view of a cut from the laminate according to Fig. 6 transducer element for pressure measurements,</li><li>Fig. 8 is a sectional view of a pressure transducer according to the invention according to a first embodiment of a transducer element according to the invention, and</li><li>FIGS. 9 to 11 are sectional views of preferred embodiments of the inventive pressure transducers of transducer elements according to the invention.</li></ul>

p0024In the drawings bear the same or similar parts the same reference numerals. The terms "top" and "bottom" refer to the reproduced in the drawing position of the parts.

p0025In Fig. 1 is provided with the general reference numeral 8 is a symmetrical forces or dehungskompensiertes transducer element shown according to the invention, comprising an upper membrane plate 1, an interlayer insulating plate 2 and a lower base plate 3. The membrane plate 1 consists of a semiconductor material, preferably silicon, in the central region of the bottom a recess 4 is introduced, which diluted the wall thickness of the plate 1, diaphragm-like. The membrane plate 1 has piezoresistors can be diffused by known methods of semiconductor technology in the plate or attached to it.

p0026With its thick edge portion the membrane plate 1 is mounted on the insulating plate. 2 The insulating plate 2 should consist of a material with suitable thermal expansion coefficient with respect to the membrane plate 1, which can be also combined well with the membrane plate. 1 Suitable for the insulating plate 2 are particularly glass materials, preferably borosilicate glass, whose coefficient of thermal expansion, although lower than that of other glasses, but is still higher than that of silicon.

p0027The base plate 3 can in principle be made of any suitable material which has an equally low or lower coefficient of thermal expansion as the material of the diaphragm plate. 1 Silicon has been found to be suitable for the base plate 3rd

p0028The connection of the panels 1 and 3 can be carried out by known methods of semiconductor technology, wherein in particular the anodic compound is preferred because of their high strength and rigidity. The anodic bonding technique is known in the art and therefore need not be explained in more detail.

p0029In a preferred embodiment of the invention, the base plate 3 is so tuned to the membrane disc 1, that in case of temperature changes, the forces occurring in the transducer element acting approximately symmetrical, as indicated for example in FIG. 2. This means that in particular the surface area of ​​the forces acting between the membrane plate 1 and the insulating plate 2 on the one hand, and between the base plate 3 and the insulating plate 2 on the other hand, are essentially the same, and because of uniform shear stress distribution on both sides of the glass plate, the low most spikes occur. Also contact any distortions of the transducer element, in particular its diaphragm plate on. This is achieved by the above-mentioned selection of the materials used for the diaphragm plate 1 and the base plate 3 having substantially the same coefficient of thermal expansion, and further characterized in that the configuration and dimension, ie in particular the thickness, the base plate 3 is adapted to the diaphragm plate 1, so that the surface area of ​​forces that occur in the two plates 1 and 3 to compensate each other substantially. As a result, a conversion element is obtained, which does not or only slightly deformed when the temperature changes, that is, retains its original shape.<ul><li>Fig. 2 shows on the left cartoon wind_Hvoltage half a transducer element 8 according to Fig. 1 for absolute pressure measurements, while the transducer element shown on the right half of the drawing of Fig. 2 is formed for pressure measurements. For this purpose, the base plate 3 and the insulating plate 2 penetrated by a through bore 6 which opens into the recess 4 of the membrane plate first The recess 4, and thus the underside of the membrane area of ​​the membrane plate 1 can thus be put under pressure. The bore may be incorporated into the transducer element in any suitable manner, for example by ultrasound.</li><li>FIG. 2 shows further how the transducer element 8 is fixed to a base or support 7 by an intermediate layer 5. A preferred method for attaching the transducer element 8 on the attitude 7 is the so-called. Glass frits, wherein the surfaces to be joined is sprinkled and melted glass powder. This technique is known in the art and need not be explained in more detail. A simpler method is gluing.</li><li>Fig. 3 shows a transducer element for absolute pressure measurements or relative pressure measurements (left and right Zeichungshälfte), wherein the base plate 3 thickened and on a cross-sectionally T-shaped or mushroom-shaped metallic intermediate support 9, 9 'in a suitable manner, for example by means of an epoxy adhesive, by glass frit or solder attached. The intermediate support 9, 9 'is preferably made of a material that behaves under temperature change similar to the material of the insulating plate. 2 But even with significantly different thermal expansion coefficients of the materials for the intermediate support 9 and the insulating plate 2, the base plate 3 of the transducer element has a balancing such that cracking of the insulating plate 2 is avoided. This risk exists if the insulating plate 2 directly. would be attached to the metallic intermediate support 9, 9 ', since all types of glass allow only very low tensile stresses.</li></ul>

p0030As shown, it can be advantageous to the base plate 3 is thicker than the membrane plate 1 run. For certain temperature ranges by asymmetrical distribution of forces lower stress peaks in the membrane plate 1 can be achieved.

p0031The intermediate support 9.9 ', which is also penetrated in connection with a transducer element for pressure measurements from an aligned to the bore 6 through bore may have at its shaft portion 10 a solderable surface layer eleventh This opens the possibility, for example, in the so-called. Batch process at the same time a plurality of intermediate supports 9, 9 'to be soldered with attached transducer elements 8 at a corresponding plurality of supports 7.

p0032Referring to FIG. 4, 5, 6 and 7, the following preferred method for manufacturing the transducer elements are described by the invention. It is basically made use of methods of semiconductor technology, such as are described in US-PS-37 64 950th

p0033According to FIG. 4, on an insulating plate-shaped substrate 17, for example from a suitable glass material, a so-called. Wafer 18 from a material suitable for the formation of the membrane plates 1 of semiconductor material, such as silicon, applied by means of an anodic compound 16. The wafer is, as shown, is divided into a plurality of units, which have a dimension and configuration corresponding to the diaphragm plate 1 of a transducer element to be produced.

p0034For the creation of transducer elements for gauge pressure measurements, the disc-shaped substrate 17, as in 15 can hinted be pierced.

p0035The two-layer laminate according to Fig. 4 has the advantage that the Verbundungszone 16 between the wafer 18 and the insulating substrate 17 which is preferably a glass plate can be easily visually monitored. One recognizes immediately the zones that have formed a perfect combination of the two plates. Units 5 as are corresponding respectively to a transducer element with a membrane plate 1 and an insulating plate 2 according to FIG from the two-layer laminate according to Fig. 4. Cut with a diamond saw. is applied to each unit so formed then a prefabricated base plate 3, as described in connection with Fig. 1, for example, fixed by a glass frit compound 14.

p0036There is also the possibility of the base plate 3 in one and to attach the same heat treatment process at the same time to the insulating plate 2 and on the bracket 7 of the transducer, as is indicated in Fig. 5 by the intermediate layer 5, which is also a can act glass frit compound. By the reference numeral 19 connection points are indicated on the diaphragm plate 1, via which the electric signal of the resistors can be removed.

p0037The method shown in Fig. 6 differs from the method of FIG. 5 in that on the opposite side of the wafer 18 of the insulating substrate 17, another wafer 20 is applied, from which the base plates 3 are formed. The wafers 18 and 20 are connected to the insulating substrate 17 preferably by anodic compounds sixteenth A three-layer laminate according to from the Fig. 6 excised transducer element is shown in Fig. 7 and corresponds to the element that has been explained in connection with FIGS. 1 and 2 in more detail. The transducer element may be attached via an intermediate layer 5, for example a glass frit compound on the holder 7 of the transducer, or it can also be an adhesive bond can be provided.

p0038Fig. 8 shows a pressure sensor having a symmetrical transducer element 8 according to FIG. 1. The transducer element 8 is attached via an intermediate layer 5 directly on the plate-shaped bracket of the transducer or glued. The bracket 7 has holes in which electrical conductors 22 are molded into glass eyes 26th The electrical conductors 22 are connected to the connection points 19 of the resistors on the diaphragm plate 1 by wires 27 made of gold or aluminum.

p0039The holder 7 is, as illustrated, on an annular shoulder surface at an intermediate point of the transducer 32 and is connected by a circumferential welding seam 24 hermetically to the transducer housing. The pressurization of the transducer element 8 takes place via a pressure-transmitting medium 25, which is preferably is silicone oil. The pressure transfer medium substantially fills the housing 32 from above the holder 7 of the enclosed space, as shown, is sealed at the top by a housing 32 fixed to the elastic metal membrane 28th To keep the volume of space or the amount of filled pressure transmission medium 25 small, a ring 21 is inserted from a ceramic material in the housing. The holder 7 is made because of the better processing ability and the necessary implementing holes for the conductors 22 from a non-corrosion-resistant steel having a temperature expansion coefficient which is smaller than the expansion coefficient of austenitic steels are preferably used for the housing of the pressure transducer. With temperature change can thus distortions occur due to different coefficients of expansion between the holder and the housing of the transducer, which also adversely affect the measurement accuracy and repeatability. To prevent this, in the illustrated in FIGS. 9-11 the preferred pressure transducers according to the invention, the supporting bracket, the transducer element via a tubular elastic member 30 is connected to the transducer housing 33rd

p0040This stretching elastic connection causes thermal stresses much lower impact on the intermediate layer between support and transducer element than in the embodiment of Fig. 8 is the case with the simple plate-shaped bracket. Therefore, the strain elastic connection provides another important contribution with regard to the creation of piezoresistive pressure sensors for precision measurements in which the measurement accuracy is not affected by temperature changes substantially.

p0041In the pressure sensor of Fig. 9, the tubular elastically extensible portion 30 is integrally formed on the same side as the transducer element 8 to the support 7 and indicated on its free upper end, as at 31, is welded via a butt welding with a horizontal step surface on the transducer housing 33rd Otherwise, the pickup corresponds to Fig. 9 to that of FIG. 8, so that in this respect can be made thereto. The transducer of FIG. 9 has the advantage that the assembly of the holder 7 and transducer element 8 can be inserted from below into the housing 33 after the membrane 28 has been attached to the housing. The proper mounting and configuration of the membrane 28 can therefore be checked before the holder 7 is attached to the transducer element 8 via the tubular part 30 on the housing 33rd

p0042Fig. Figure 10 shows a pressure transducer with a holder 7, which has at its upper side a recess in which the transducer element is fixed, for example, 8 by a layer 40 on the basis of an epoxy resin. Of the element facing away from the transducer 8 lower side of the holder 7 the tubular portion 30 is elastically extensible from which is stumpfverschweisst at its free end face with a shoulder surface on the transducer housing 39th Since the tubular member 30 weger infected by the membrane 28, the assembly of the holder 7 with the attached transducer element 8 must first be attached to the housing 39, before the membrane 28 can be mounted. As a result of the mounting of the diaphragm side, opens up the possibility, the pressure transmission medium 25 is also in an annular space 38 between the holder and the inner peripheral surface of the housing 39 and thus to blank radially act on the bracket 7th The electrical conductors 22 are therefore not leak even under high medium pressures or pressed out of the glass eyes 26th The pressure sensor shown in FIG. 10 is therefore particularly suitable for high pressure measurements.

p0043As shown, may be further secured with 33 compensation resistors 34 at a lower portion of the transducer 39 a plate. The electrical conductors 22 are guided on slide bushings 37 through the plate 33rd

p0044Wires 36 of the electrical conductors 22 are used for signal transfer. The plate 33, the resistors 34 and electrical conductors 22 can be embedded in a plastic material 35, which as shown, is introduced in the lower region of the housing. 39

p0045Fig. Figure 11 shows a pressure sensor for pressure measurements using an array of transducer element and the intermediate carrier, as shown in FIG. 3 on the right half of the drawing. The symmetrical transducer element 8, as described in connection with FIG. 3, fixed to the T-shaped intermediate support which in turn is soldered with its shaft portion 10 in a hole in the holding device 7. Instead of solder also Kittverbindung can be provided.

p0046As with the embodiment of FIG. 9, the bracket 7 is inserted with the elastic tubular member 30 and the soldered assembly of the intermediate support and transducer element from below into the transducer housing 53, according to which the free upper end face of the tubular part 30 with an annular shoulder surface on the transducer housing 53 , as indicated at 31, stumpfverschweisst. The time required for the relative pressure measurement pressure supply to the underside of the diaphragm plate of the transducer element 8 takes place via a fixed to the shaft portion 10 of the intermediate support tube 50 extends out downward from the pickup. The length L of the tubular resilient member 30 can be selected according to the specific circumstances, so that always ensured that the electrical conductors 22 holding glass eye 26 from overheating by the welding at the point 31 while the tubular elastic member 30 the desired can exert strain compensating effect. Otherwise, the receiver shown in FIG. 11 essentially the embodiment of FIG. 9 and with respect to the foreseen in the lower part of the transducer parts corresponding to the parts 33 to 37 can be so made thereto in the embodiment of FIG. 10.

p0047A common feature of the pressure sensors shown in FIG. 9 to 11, that it due to the combination of a symmetrical and thus warp-free transducer element with a stress-free connection of the brackets with the Aufnehmergehäusen via a tubular elastically extensible portion having an exceptional stability and zero stability, thus representing precision pressure. This pressure sensor can be manufactured inexpensively, thanks to the advanced semiconductor technology in conjunction with the known batch process yet.