Ceramic carrier and sensor element, heating element and sensor module, each with a ceramic carrier and method for manufacturing a ceramic carrier

Abstract

An Al.sub.2O.sub.3 carrier has a thin-film structure of platinum or a platinum alloy arranged thereon. The carrier and/or the thin-film structure are adapted to reduce mechanical stresses owing to different thermal expansion coefficients. The carrier and/or the thin-film structure include a surface of the carrier in the region of the thin-film structure is smoothed at least in sections to reduce the adhesion and/or a surface of the carrier has an intermediate layer on which the thin-film structure is arranged. The thermal expansion coefficient of the intermediate layer is from 8*10.sup.6/K to 16*10.sup.6/K, in particular from 8.5*10.sup.6/K to 14*10.sup.6/K, and/or the thin-film structure has at least one conductor path that is undular at least in sections, said conductor path extends laterally along the surface of the carrier.

Claims

1. An Al.sub.2O.sub.3 carrier for reducing mechanical stresses, the Al.sub.2O.sub.3 carrier comprising: an Al.sub.2O.sub.3 body; and a thin-film structure comprising platinum or comprising a platinum alloy, the thin-film structure comprising a first coefficient of thermal expansion; and a surface of the Al.sub.2O.sub.3 carrier having an intermediate layer on which the thin-film structure is arranged, the intermediate layer comprising a second coefficient of thermal expansion from 8*10.sup.6/K to 16*10.sup.6/K; wherein Al.sub.2O.sub.3 is at least 99% by weight of the Al.sub.2O.sub.2 carrier; wherein the second coefficient of thermal expansion is greater at most by a factor of 1.5 than the first coefficient of thermal expansion; and wherein the intermediate layer consists of an electrically insulating metal oxide.

2. The Al.sub.2O.sub.3 carrier as claimed in claim 1, wherein the surface in a region of the thin-film structure forms at least one sliding portion and at least one adhesive portion.

3. The Al.sub.2O.sub.3 carrier as claimed in claim 1, wherein the surface in the region of the thin-film structure has a strip-shaped depth profile which forms at least one recess.

4. The Al.sub.2O.sub.3 carrier as claimed in claim 3, wherein the thin-film structure has at least one conductor track, the at least one conductor track being undular at least in portions and extending laterally along the surface of the Al.sub.2O.sub.3 carrier, wherein the amplitude of the undular conductor track is from 0.2*B to 2*B, and a wavelength of the undular conductor track is from 3*B to 10*B, where B is the width of the conductor track, wherein at least one conductor track of the thin-film structure is arranged at an angle relative to the strip-shaped depth profile.

5. The Al.sub.2O.sub.3 carrier as claimed in claim 3, wherein the at least one recess has a trapezoidal cross section with two inclined flanks and a base between the flanks, wherein at least one flank rises at an angle of 10 to 80 relative to the base.

6. The Al.sub.2O.sub.3 carrier as claimed in claim 3, wherein the at least one recess has a depth of 0.4 m to 1.2 m or a width of 5 m to 20 m.

7. The Al.sub.2O.sub.3 carrier as claimed in claim 3, wherein the strip-shaped depth profile has a plurality of parallel recesses, and wherein the spacing between the recesses is in each case from 5 m to 20 m.

8. The Al.sub.2O.sub.3 carrier as claimed in claim 3, wherein at least one conductor track of the thin-film structure is arranged at an angle of 30 to 90 relative to the strip-shaped depth profile.

9. The Al.sub.2O.sub.3 carrier as claimed in claim 1, wherein a thickness of the intermediate layer is from 0.2 m to 3 m.

10. The Al.sub.2O.sub.3 carrier as claimed in claim 1, wherein the intermediate layer comprises MgO or BaO.

11. The Al.sub.2O.sub.3 carrier as claimed in claim 1; wherein the thin-film structure has at least one conductor track, the at least one conductor track being undular at least in portions and extending laterally along the surface of the Al.sub.2O.sub.3 carrier, wherein the amplitude of the undular conductor track is from 0.2*B to 2*B, and a wavelength of the undular conductor track is from 3*B to 10*B, where B is the width of the conductor track, wherein the undular conductor track comprises a plurality of arcs extending laterally along the surface, wherein an undular substructure is formed at least in the conductor track portions between the arcs, or the undular conductor track forms a plurality of fingers of an electrode which are arranged in a comb-like manner.

12. The Al.sub.2O.sub.3 carrier as claimed of claim 1, wherein the thin-film structure has at least one conductor track, the at least one conductor track being undular at least in portions and extending laterally along the surface of the Al.sub.2O.sub.3 carrier, wherein the amplitude of the undular conductor track is from 0.2*B to 2*B, and a wavelength of the undular conductor track is from 3*B to 10*B, where B is the width of the conductor track, wherein the undular conductor track is embodied in the form of a sine wave or a sawtooth-shaped wave or a trapezoidal wave.

13. The Al.sub.2O.sub.3 carrier as claimed in claim 1, further comprising a first cover layer comprising oxidic nanoparticles of Al.sub.2O.sub.3 or MgO and which is applied directly to the thin-film structure.

14. The Al.sub.2O.sub.3 carrier as claimed in claim 13, wherein the first cover layer is sealed hermetically by a second cover layer comprising glass.

15. A sensor module, the sensor module comprising: an Al.sub.2O.sub.3 carrier for reducing mechanical stresses, the Al.sub.2O.sub.3 carrier comprising an Al.sub.2O.sub.3 body; and a thin-film structure comprising platinum or comprising a platinum alloy the thin-film structure comprising a first coefficient of thermal expansion; and a surface of the Al.sub.2O.sub.3 carrier having an intermediate layer on which the thin-film structure is arranged, the intermediate layer comprising a second coefficient of thermal expansion from 8*10.sup.6/K to 16*10.sup.6/K; wherein the first cover layer is sealed hermetically by a second cover layer comprising glass; wherein Al.sub.2O.sub.3 is at least 99% by weight of the Al.sub.2O.sub.3 carrier; wherein the second coefficient of thermal expansion is greater at most by a factor of 1.5 than the first coefficient of thermal expansion, and wherein the intermediate layer consists of an electrically insulating metal oxide.

16. The sensor module as claimed in claim 15, wherein a plurality of sensor structures are arranged on the Al.sub.2O.sub.3 carrier, wherein the thin-film structure comprises platinum or a platinum alloy and forms at least one sensor structure and an electrode structure forms at least one further sensor structure.

17. The Al.sub.2O.sub.3 carrier as claimed in claim 15, further comprising a first cover layer comprises oxidic nanoparticles of Al.sub.2O.sub.3 or MgO and which is applied directly to the thin-film structure.

18. A method of making an Al.sub.2O.sub.3 carrier, the Al.sub.2O.sub.3 carrier for reducing mechanical stresses, the Al.sub.2O.sub.3 carrier comprising an Al.sub.2O.sub.3 body; and a thin-film structure comprising platinum or comprising a platinum alloy the thin-film structure comprising a first coefficient of thermal expansion; and a surface of the Al.sub.2O.sub.3 carrier having an intermediate layer on which the thin-film structure is arranged, the intermediate layer comprising a second coefficient of thermal expansion from 8*10.sup.6/K to 16*10.sup.6/K; wherein Al.sub.2O.sub.3 is at least 99% by weight of the Al.sub.2O.sub.3 carrier; wherein the second coefficient of thermal expansion is greater at most by a factor of 1.5 than the first coefficient of thermal expansion, and wherein the intermediate layer consists of an electrically insulating metal oxide; the method comprising the steps of: applying the intermediate layer to the surface of the Al.sub.2O.sub.3 carrier by a thin-film method, or applying the undular conductor track to the surface of the Al.sub.2O.sub.3 carrier by the thin-film method.

19. The Al.sub.2O.sub.3 carrier as claimed in claim 18, further comprising a first cover layer comprises oxidic nanoparticles of Al.sub.2O.sub.3 or MgO and which is applied directly to the thin-film structure.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will be explained with further details hereinbelow on the basis of exemplary embodiments and with reference to the appended drawings.

(2) In said drawings, schematically,

(3) FIG. 1 shows a section through a carrier according to one exemplary embodiment according to the invention, in which the surface is structured by a depth profile;

(4) FIG. 2 shows a section through a carrier according to a further exemplary embodiment according to the invention, the surface of which is structured in the same way as in FIG. 1 and is additionally coated with the platinum thin-film structure;

(5) FIG. 3 shows a plan view onto the carrier shown in FIG. 1;

(6) FIG. 4 shows a section through a carrier according to a further exemplary embodiment according to the invention, in which an intermediate layer is arranged between the platinum thin-film structure and the carrier;

(7) FIG. 5 shows a plan view onto an undular conductor track compared with a rectilinear conductor track;

(8) FIGS. 6a-6d show plan views onto undular conductor tracks having different geometries;

(9) FIG. 7 shows a section through a carrier according to a further exemplary embodiment according to the invention, in which the platinum thin-film structure is protected with a cover layer, and

(10) FIG. 8 shows an exploded illustration of a sensor module comprising various thin-film structures which are arranged on a carrier according to one exemplary embodiment according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(11) FIG. 1 shows a section through a ceramic carrier according to one exemplary embodiment according to the invention. Specifically, the ceramic carrier is an Al.sub.2O.sub.3 carrier (aluminum oxide carrier). The carrier serves as a substrate, or as a ceramic support, for a thin-film structure (not shown in FIG. 1). Al.sub.2O.sub.3 has proved to be expedient as the material for the ceramic carrier, in particular with at least 96% by weight and preferably more than 99% by weight Al.sub.2O.sub.3. The carrier can be in the form of a plate with a thickness in the range of 100 m to 1000 m, in particular 150 m to 650 m. Other plate thicknesses are possible. With a view to the thermal response behavior, the thickness of the carrier should be chosen to be as thin as possible. Particularly in applications in the automotive sector, in which severe vibrational loading often occurs, the mechanical stability of the carrier determines the lower limit of the plate thickness. The ceramic carrier can be in the form of a rectangular plate. Other shapes of the carrier are possible.

(12) The above statements in relation to the general shape of the carrier and in relation to the material composition apply in general terms to the invention and are disclosed in conjunction with all exemplary embodiments.

(13) The carrier as shown in FIG. 1 is adapted for reducing the mechanical stresses owing to different coefficients of thermal expansion of the materials used. To this end, the surface of the carrier in the region of the thin-film structure is smoothed. There are two possibilities for this purpose. Either the carrier is smoothed in the entire region of the thin-film structure, which is easy to realize in terms of production, or the carrier is smoothed partially in the region of the thin-film structure.

(14) The smoothed surface 11 has the effect that the adhesion of the thin-film structure is reduced, and therefore the latter can slide on the surface 11 of the carrier in order to compensate for differences in linear expansion. If the surface 11 is smoothed only partially in the critical regions, the untreated surface regions ensure the adhesion for the thin-film structure. One example of this is shown in FIG. 1, in which the surface 11 has a strip-shaped depth profile which forms at least one recess 17, wherein the surface of the recess 17 is smoothed. The regions of the surface 11 which directly adjoin the recess 17 on both sides are untreated. As a result, the surface 11 of the carrier forms a sliding portion 15 in the region of the recess 17, said sliding portion being laterally delimited in each case by an adhesive portion 16. The adhesive portion 16 is formed by the surface regions which adjoin the recess 17.

(15) In the region of the sliding portion 15, or of the recess 17, the adhesion between the platinum thin-film structure shown in FIG. 2 and the carrier is reduced. In the event of linear expansion of the platinum thin-film structure 10, this can lead to detachment of the latter in the region of the recess 17. Those surface regions of the carrier which adjoin the recess 17 are untreated, and therefore the roughness is higher in these regions than in the region of the recess 17. The adhesive portions 16 thus formed fix the platinum thin-film structure 10 between the sliding portions 15, or the recesses 17. In the event of linear expansion of the platinum thin-film structure, the latter breaks away in the region of the recess 17 and can be stretched. This change in the geometry of the platinum thin-film structure 10, together with the detachment from the carrier, has the effect that deformation of the platinum thin-film structure 10 owing to the different coefficients of thermal expansion of the carrier and of the structure 10 is largely avoided.

(16) The detachment of the platinum thin-film structure 10 from the surface 11 is facilitated by the fact that the recess 17 has a trapezoidal cross section. The cross section is determined by two flanks 18, which are arranged in an inclined manner and laterally delimit a base 19 of the recess 17. The flanks 18 rise at an angle of 10 to 80, in particular of 45 to 60. The angle is determined by a first imaginary plane running through the base 19 and a second imaginary plane running through the flank in question. The depth of the recess 17 can lie in the range of 0.4 m to 1.2 m, in particular in the range of 0.6 m to 1.0 m. The width can be 5 m to 20 m, in particular 10 m to 15 m.

(17) As can furthermore be seen in FIG. 1, the strip-shaped depth profile has a plurality of parallel recesses 17 extending along the surface 11 of the carrier. The spacing between the recesses 17 can be from 5 m to 20 m, in particular from 10 m to 15 m. The recesses 17 are spaced apart equidistantly and in a parallel manner.

(18) Instead of the partially smoothed surface resulting from the formation of the depth profile, the surface can be smoothed without a profile. This means that the surface is smoothed uniformly, without the formation of a depth profile.

(19) The smoothing can be effected by removal of the surface. The removal can be effected by ion etching, in particular plasma ion etching, with a removal depth of 0.2 m to 2 m. The partial removal, or the partial smoothing, can be achieved by a resist mask, which is applied prior to the ion etching and protects the covered regions during the etching operation.

(20) FIGS. 2 and 3 show how the platinum thin-film structure 10 adapts to the depth profile of the carrier. In this respect, FIG. 2 specifically shows that the shape of the depth profile is reflected in the shape of the platinum thin-film structure 10. The method gives rise to a trapezoidal formation of the depth profiles, and this avoids steps (continuous profiling). The platinum thin-film structures 10 follow the depth profile over the entire substrate with an approximately constant layer thickness.

(21) It can be seen in the plan view as shown in FIG. 3 that the conductor track 13 intersects the recess 17 in a transverse manner, i.e. at an angle of 90. Other angles of intersection are possible, for example depending on the meandering shape of the conductor track 13. The conductor track 13 can intersect the recess 17 at an angle in the range of 30 to 90.

(22) FIG. 2 moreover illustrates the layer structure above the platinum thin-film structure. A first cover layer 14a is applied directly to the platinum thin-film structure and serves for the passivation of the platinum thin-film structure 13. A second cover layer 14b is applied to the first cover layer 14a and seals the first cover layer 14a hermetically.

(23) FIG. 4 shows a further exemplary embodiment of the invention, in which an intermediate layer 12 is arranged between the carrier and the platinum thin-film structure 10. The intermediate layer 12, which is also referred to as the interface layer, is formed from an electrically insulating metal oxide. This has the function of a buffer, which absorbs the stresses caused by the mismatch and conducts them at least partially into the carrier. The intermediate layer 12 has a greater coefficient of thermal expansion than the ceramic carrier, in particular than Al.sub.2O.sub.3, and this may be up to 50% greater than the coefficient of thermal expansion of platinum. A magnesium oxide layer (MgO) applied by thin-film technology and having a layer thickness in the range of 0.2 m to 3 m has proved to be expedient in practice. The coefficient of thermal expansion of magnesium oxide is 13*10.sup.6/K. This coefficient of expansion is greater than the coefficient of expansion of Al.sub.2O.sub.3, with 6.5*10.sup.6/K, and of platinum, with 9.1*10.sup.6/K. Instead of magnesium oxide (MgO), barium oxide (BaO) can be used for the intermediate layer 12. For setting the coefficient of thermal expansion of the intermediate layer 12, use can be made of a mixture of an electrically insulating metal oxide, for example magnesium oxide, and Al.sub.2O.sub.3. The coefficient of thermal expansion of the intermediate layer 12 changes depending on the Al.sub.2O.sub.3 content.

(24) A further exemplary embodiment, in which the shape of the conductor track or of the conductor tracks is modified, is shown in FIGS. 5 and 6a-6d. The concept on which this exemplary embodiment is based involves forming the conductor track 13 not in a linear manner, as shown at the top in FIG. 5, but rather in a non-linear manner, in particular in an undular form, as shown at the bottom in FIG. 5. The undulation extending in the lateral direction, i.e. along the surface 11 of the carrier, has the effect that the differences in linear expansion of the platinum thin-film structures are separated into an X and Y component. It has been found that this separation has a positive effect on the stability of the platinum thin-film structures in the event of severe fluctuating thermal loads.

(25) The amplitude of the undular conductor track 13 is from 0.2*B to 2*B, in particular from 0.4*B to 1*B. The wavelength is from 3*B to 10*B, in particular from 4*B to 7*B. Here, B denotes the width of the conductor track 13. The terms amplitude and wavelength are to be understood as meaning the variables which are customary in conjunction with the description of oscillations. The amplitude corresponds to the peak value with respect to the zero line of the undulation. The wavelength corresponds to an oscillation period likewise with respect to the zero line of the wave. The zero line is the axis of symmetry in the longitudinal direction of the wave.

(26) As is shown in FIG. 8, the conductor track 13 can have a superordinate meandering shape, which is to be distinguished from the undulation of the conductor track 13. The meandering shape of the conductor track forms a superstructure, which is superposed by the undulation of the conductor track 13. In this respect, the undulation of the conductor track 13 forms a substructure, which is provided at least in the conductor track portions between the arcs of the meandering shape (superstructure). It is also possible for the arcs of the meandering shape to be provided themselves in the substructure. The term arc is also to be understood as meaning a rectangular change in direction in the conductor track, as shown in FIG. 8. The undular conductor track 13 can also form the fingers of the electrode, which are arranged in a comb-like manner (shown in FIG. 8). In this case, the rectilinear finger shape forms the superstructure, which is superposed by the undulation as the substructure.

(27) FIGS. 6a to 6d show various geometries of the undulation, these each being contrasted with a rectilinear, wave-free conductor track, similar to in FIG. 5. Thus, FIG. 6a shows that the conductor track 13 can have the shape of a sine wave. A plurality of conductor tracks 13 are arranged alongside one another in phase. FIG. 6b shows an undular conductor track 13, in the case of which the wave has a trapezoidal form. A further example for the wave is shown in FIG. 6c. In this example, the wave has a sawtooth-shaped form. It is also possible to refer to a rectangular meandering shape as the substructure here. The change in direction of the conductor track 13 is effected at an angle of 90. A mixture of the sine wave shown in FIG. 6a and the sawtooth-shaped wave shown in FIG. 6c is shown in FIG. 6d. In this case, the flanks of the sawtooth-shaped wave are approximated to the sine shape and rounded.

(28) FIG. 7 shows a further exemplary embodiment, in which the first cover layer is modified by the addition of oxidic nanoparticles. This has the effect that the volume of the first cover layer 14a changes upon a change in temperature, this reducing the resistance drift. The first cover layer 14a is sealed hermetically by a second cover layer made of glass.

(29) The exemplary embodiments described above each seen individually improve the dimensional stability of the platinum thin-film structure 10 and thus counteract the resistance drift. The exemplary embodiments are therefore each disclosed independently of one another. In addition, the exemplary embodiments can also be combined with one another, as is shown by way of example with reference to the exemplary embodiment as shown in FIG. 2. The combination of the various exemplary embodiments leads to a synergy effect, which is manifested in an increased reduction in the resistance drift.

(30) Specifically, the first cover layer 14a comprising the oxidic nanoparticles can be combined with all exemplary embodiments, because the generally required passivation of the platinum thin-film structure 10 can thereby be effected in such a way that, in addition to the passivation, the resistance drift is improved. As shown in FIG. 2, the first cover layer 14a is combined with the depth profile and the partially smoothed surface 11. It is also possible, as shown in FIG. 4, to combine the first cover layer 14a with the intermediate layer 12. In addition, the intermediate layer 12 can be used together with the undular conductor track 13. Moreover, it is possible to combine the depth profile as shown in FIG. 2 with the intermediate layer as shown in FIG. 4 and also with the undular conductor track 13 as shown in FIG. 5 or one of the undulations as shown in FIGS. 6a to 6d. The combination of all exemplary embodiments is possible.

(31) The carrier can be used for building up various sensors. By way of example, it is expedient to use the carrier for a temperature sensor having a platinum thin-film structure. The use of a flow measurement sensor is similarly possible, in the case of which a heating element and a temperature measurement element are combined in accordance with the anemometric principle. A further example for the use of the invention is shown in FIG. 8 in conjunction with a sensor module. The sensor module forms a multi-sensor platform and has a carrier substrate 23. The carrier substrate 23 can be modified according to one of the exemplary embodiments explained above. By way of example, a strip-shaped depth profile (not shown) can be formed in the carrier substrate. The intermediate layer 12 is arranged on the carrier substrate 23 and serves as a buffer for the platinum thin-film structure 10 arranged on the intermediate layer 12. The platinum thin-film structure 10 can be a heater and/or sensor in each case with contact connections. An insulation layer 20 is applied to the platinum structure 10, with an interdigital electrode structure 21 for conductivity measurement (IDE) being arranged on said insulation layer. The interdigital electrode structure 21 is provided with an active functional layer 22, which can be applied for example by the customer.

LIST OF REFERENCE SIGNS

(32) 10 Thin-film structure 11 Surface 12 Intermediate layer 13 Conductor track 14a First cover layer 14b Second cover layer 15 Sliding portion 16 Adhesive portion 17 Recess 18 Flanks 19 Base 20 Insulation layer 21 Electrode structure 22 Functional layer 23 Carrier substrate