CN107037076B

CN107037076B - Measuring chip and method for determining the thermal conductivity of a thin layer

Info

Publication number: CN107037076B
Application number: CN201611151226.7A
Authority: CN
Inventors: 林塞斯·文森特; 弗克雷·弗里德曼
Original assignee: Linseis Mebgerate GmbH
Current assignee: Linseis Mebgerate GmbH
Priority date: 2015-12-15
Filing date: 2016-12-14
Publication date: 2022-02-01
Anticipated expiration: 2036-12-14
Also published as: CN107037076A; DE102015225284A1

Abstract

The invention relates to a measuring chip suitable for determining the thermal conductivity of thin layers. The measurement chip comprises a substrate (14) with a recess (15) and a membrane (16) applied to the substrate (14) in a manner covering the recess (15). A heating wire (17) is applied on the membrane (16). According to the invention, a measuring field limiting element (18, 19) is arranged on the membrane (16) and is arranged in the substrate (14) in such a way that it partially covers the recess (15). The invention also relates to a method for determining the thermal conductivity of a thin layer.

Description

Measuring chip and method for determining the thermal conductivity of a thin layer

Technical Field

The invention relates to a measuring chip suitable for determining the thermal conductivity of thin layers. The measurement chip includes a substrate having a groove and a diaphragm applied to the substrate in a manner to cover the groove. Heating wires are applied on the membrane. The invention also relates to a method for determining the thermal conductivity of a thin layer.

Background

The thermal conductivity of the thin layer (sample) can be determined by: it is measured how much heat is released by the heating wire arranged adjacent to the thin layer through this layer. By carrying out the measurement in the region of the recess of the measurement chip, heat dissipation takes place substantially by this sample while heat dissipation by the other components of the measurement chip is kept as small as possible. The component of heat dissipation by this sample can be inferred by calculating the other components of heat dissipation.

The measuring chip is equipped with a heating structure which both generates a defined heat flow in the material and serves as a sensor in order to determine the temperature rise of the heating structure. The thermal conductivity of the sample is calculated taking into account the information on the size of the face of the sample that is used to dissipate heat. It is not very simple to determine the size of this area. In particular, the complexity is high when the size of this contact surface is determined experimentally by additional measurements.

Disclosure of Invention

The object of the invention is to provide a measurement chip and a method for determining the thermal conductivity of a thin layer that can be repeated and without further measurements. Starting from the prior art mentioned above, the solution of the invention for achieving the object described above is characterized by the features of the independent claims. Further developments are found in the dependent claims.

According to the invention, a measuring field limiting element is arranged on the membrane, which is arranged in the substrate in such a way that it partially covers the recess.

Some concepts are first explained in detail:

a thin layer refers to a closed layer composed of one or more solid materials. The thickness of such a layer is typically in the range of a few micrometers to a few nanometers. The thin layer can be produced, for example, by physical methods (such as sputtering or thermal evaporation), chemical methods (such as atomic layer deposition or molecular beam epitaxy) or other coating methods (such as drop plating or spin coating), and the thin layer can be deposited directly on the measurement chip. In this sense, the membrane may in particular also be a thin layer.

The measurement field represents the section of the measurement chip used to perform the thermal conductivity measurement of the sample. The measuring field preferably comprises regions in which the membrane or the combination of membrane and sample is not provided with further components, apart from the heating wire. In particular, in the region of the measurement field, the membrane is not covered by the substrate or the measurement field limiting member. Starting from the heating wire, a defined heat flow is generated in this region through the membrane, from which the thermal conductivity of the membrane or of the applied sample can be inferred.

The invention recognizes that the measuring field limiting element according to the invention can delimit the measuring field in a defined manner in a specific region of the measuring field. This simplifies the determination of the relevant side of the sample. In particular, the thermal conductivity can be measured without additional measurements of the groove geometry. Such recesses are usually produced by etching methods, which are difficult to control because they are dependent on a number of process parameters (time, substrate, temperature). Despite deviations in the geometry of the grooves, known and repeatable magnitudes of the measuring field can be achieved using measuring field limiters.

The measurement chip of the present invention includes a substrate having a groove. The substrate is preferably a silicon wafer. The recess preferably refers to a rectangular groove having a side length in the range of 0.1 to 1 mm. A diaphragm made of an electrically insulating material is applied on the substrate so as to cover the recess. The membrane preferably refers to silicon nitride, the layer thickness of which does not exceed 200 nm, preferably does not exceed 100 nm, further preferably does not exceed 50 nm. The small layer thickness of the membrane is advantageous for determining the thermal conductivity in that the parasitic heat flow outside the sample is kept as small as possible.

The heating wire is preferably applied to the side of the membrane remote from the substrate. The heating wire preferably extends over a length in the range of a few millimeters and has a width of less than 5 micrometers. The heating wire is preferably made of an electrically conductive material. The heating wire is designed in such a way that at least one, preferably both, of its ends are in the region of the groove. In case of avoiding an overlap between the heating filament and the substrate, an undesired heat dissipation is reduced, which otherwise may occur between the heating filament and the substrate. The information such as overlay and overlay in the present invention relates to the projection in the direction perpendicular to the membrane.

The measuring field limiting element is preferably made of a material with a high thermal conductivity. In applying the heating wire and the measuring field limiter, it is preferably possible to use an additional adhesion promoter, such as titanium or chromium, between the membrane and the structure. Conventional lithography methods in microstructure technology are preferably used for structuring the measurement chip.

In a preferred embodiment, the measuring field limiting element comprises at least one, preferably two, sections which are electrically insulated from the heating wire. This embodiment of the measuring field limiting element is spatially independent of the heating wire and is also thermally insulated from the heating wire on the basis of the less thermally conductive membrane. The heating of the heating wire is thus decoupled from the heating of the measuring field restriction and vice versa.

The heating wire may extend between the electrically insulated sections. The heating wire extends in two sections of the measuring field, which makes it possible to establish a symmetry of the measuring field and to generate defined regions for the heat flow from the heating wire to the two edges of the measuring field limitation.

In one embodiment of the invention, the measuring field limiting element comprises at least one, preferably two, contact areas which are electrically connected to the heating filament. The contact areas are preferably substantially wider than the heating filament, so that one end of the heating filament is simultaneously defined by each contact area. These electrically connected contact areas bring the measuring field limiter in close proximity to the ends of the heating wire. The contact area is usually much wider than the heating wire and can absorb heat well. Another advantage of the contact areas is that these contact areas can be used to electrically contact the chip and the heating wire. The contacting may be achieved, for example, by a bond wire or a spring contact pin. In this case, the sensitivity of the larger contact area is much lower than the narrower heating wire for the pressure exerted by the engaging needle or the spring contact pin. Furthermore, under an optical microscope, the contact area is more easily recognized.

Preferably, the heating wire and the measuring field limiter are arranged on the same face of the membrane. In this case, the two structures can be deposited in one processing step, thereby simplifying the manufacture of the measurement chip. Furthermore, the measurement field limiting element can be aligned directly with the heating wire, thereby simplifying the determination of the measurement field and increasing the measurement accuracy. Another advantage is that the back side of the membrane remains exposed and the sample applied on the measurement chip does not come into contact with these structures.

The measuring field limiting element can in principle be of one-piece or composite design. In the case of measuring field limiters made of electrically conductive material, these are generally of a composite type, which otherwise would result in an electrical connection around the heating wire. The interruptions between the components of the measuring field limiting member should be as small as possible, so that the measuring field is delimited as widely as possible by the measuring field limiting member.

In a preferred embodiment, a circumferential boundary of the measuring field within the recess is formed by at least 70%, preferably at least 80%, further preferably at least 90%, of the edge of the measuring field limiting element. The larger the area for the measuring field limiting member forming the edge of the measuring field, the more closely the size of the measuring field can be controlled and determined. In this case, the geometry of the measuring field is largely independent of the geometry of the recess on the bottom side of the diaphragm. It is particularly noted that the heating wire is not short-circuited by the measuring field restriction. For this purpose, a distance of 10 to 20 micrometers is preferably followed between the electrically non-isolated parts of the measurement field limiter. The measuring field limiting element can be composed of a section which is electrically conductively connected to the heating wire (in particular the contact region) and a section which is electrically insulated from the heating wire.

According to an advantageous embodiment, the measuring field limiting element is made of gold or platinum. Both materials have a higher thermal conductivity than a less thermally conductive membrane, and can be applied to the membrane simultaneously with the heating wire. In addition, gold and platinum are also very stable over a wide temperature range. In addition, the two materials also have the necessary stiffness for connecting together the bond wires or spring contact pins in the case of small layer thicknesses.

In an advantageous embodiment, an insulating layer is applied to the heating wire and the measuring field limiting element in a covering manner. The insulating layer is preferably composed of an electrically insulating photoresist. The thermal conductivity of the sample layer applied to the side of the membrane facing away from the substrate in the region of the recess can be determined by means of the insulating layer. A further advantage is that the sample layer can be applied to the measurement chip, for example by means of Spin Coating (Spin Coating), since no recess is provided on the side of the membrane remote from the substrate.

In a preferred embodiment of the measurement chip according to the invention, a first measurement field having a first measurement field limiting element and a second measurement field having a second measurement field limiting element are provided, wherein the area of the first measurement field is smaller than the area of the second measurement field. This embodiment of the measuring chip enables simultaneous measurement in two measuring fields of different sizes when determining the thermal conductivity of a sample. This allows the thermal conductivity to be measured with greater accuracy taking into account the heat flow based on thermal conduction and thermal radiation (thermal conductivity and emissivity).

Each of the measuring fields is provided with a heating wire. The heating wires superposed with the first measuring field and the heating wires superposed with the second measuring field are preferably the same in length. Therefore, the heating wires have almost the same resistance and exhibit almost the same heating characteristics. Furthermore, the geometry of the measuring field is also related to its width, i.e. the expansion in the direction perpendicular to the extension direction of the heating wire.

In thermoelectric generators, heat can be converted directly into electrical energy using the seebeck effect. The seebeck effect occurs when a temperature gradient is created across a thermoelectric layer, i.e. a layer having cooler and hotter regions. The efficiency of a material for converting heat into electrical energy is generally determined by a dimensionless figure of merit ZT, which is proportional to the quadratic of the seebeck coefficient S, the electrical conductivity σ and the temperature T, and inversely proportional to the thermal conductivity k of the material. In terms of determining the quality factor, it is desirable to measure all parameters of the thin layer under fixed environmental conditions.

The object of the invention is to provide a measuring chip for substantially simultaneously determining all the above-mentioned parameters of a thin layer over a large temperature range. This allows the figure of merit ZT to be determined directly with the measurement chip without the need to perform other measurements, such as determining the geometry of the sample. The measurement chip is pre-structured in such a way that only one single deposition process of the sample is required. All measurement fields of the measurement chip are covered by a thin layer by this deposition process.

Furthermore, the measuring chip of the invention preferably comprises first of all a measuring device which is suitable for determining the thermal conductivity of the sample layer applied to the membrane in the region of the recess. By means of the measuring device on the measuring chip, it can be ensured that the heating wire is actually contacted, for example, during the measurement. Furthermore, the measuring device is capable of applying and measuring currents and voltages.

Furthermore, the measurement chip can have a measurement device which is suitable for determining the conductivity and/or the seebeck coefficient of a sample layer applied in a separate measurement field. This ensures that all three parameters (thermal conductivity, electrical conductivity and seebeck coefficient) can be measured on-chip under the same given conditions. All measuring devices are arranged on the chip, so that the sample can be fully characterized in the first temperature rise or magnetic field test. Wherein the measuring device comprises means for applying and/or taking a voltage and means for generating a temperature gradient in other regions of the sample.

The invention also relates to a method for determining the thermal conductivity of a thin layer. In the method according to the invention, a heating power is supplied by a heating wire applied to a membrane and the average temperature rise of the heating wire is measured. The heating wire is arranged inside a measurement field limiting piece, and the measurement field limiting piece partially covers the groove of the substrate on the back surface of the membrane.

When using the method of the invention, if an electric current is directed through the heating wire, a part of the electric energy is converted into joule heat, causing the heating wire to heat up. The temperature rise is related to how much heat is dissipated (by convection, heat conduction and heat radiation). This allows the in-plane (in-plane) measurement of the thermal conductivity of the membrane under given boundary conditions.

In a preferred embodiment of the method of the invention, a sample layer may be applied to the membrane. The sample layer is preferably arranged on the side of the membrane which is different from the heating filament and the measuring field limiter. For applying the sample layer, conventional methods of depositing thin layers can be used. The product of the thermal conductivity and the thickness of the applied layer is preferably not less than 2 · 10^-7Value of watts per kelvin. If a sample layer is additionally applied to the membrane, this method is sensitive to thermally conductive components that are in-plane with the membrane and the sample.

Preferably, a cover layer may be applied to the sample layer, wherein the thickness of the cover layer is preferably at least 10 times, further preferably at least 20 times, further preferably at least 100 times greater than the thickness of the membrane sheet. The masking layer may also be applied by conventional methods of depositing a thin layer. But the masking layer may also be sputtered. The cover layer acts as a heat sink and causes a change in the heat flow in the membrane and/or the sample. The heat flow is deflected in the direction of the cover layer and the measurement of the thermal conductivity is sensitive to components with thermal conductivity oriented (out-of-plane) perpendicular to the same plane of the membrane and the sample.

For reasons of simplicity of handling, the cover layer is advantageously made of a material with a high thermal conductivity, in particular graphite. The graphite can preferably be sprayed directly from a spray can onto the measurement chip.

The method can be improved by other features described in connection with the measurement chip of the invention. The measurement chip can be improved by other features described in connection with the method of the invention.

Drawings

The invention is now described, by way of example, with reference to the accompanying drawings, in connection with advantageous embodiments. Wherein:

FIG. 1 is a schematic front view of a component of a measurement chip of the present invention;

FIG. 2 is a schematic view of the back side of the measurement chip of FIG. 1 without a sample;

FIG. 3 is an enlarged cross-sectional view of the components of the measurement chip of the present invention shown in FIG. 1 with a sample;

FIG. 4 is an enlarged cross-sectional view of a second embodiment of the measurement chip of the invention with a second measurement field;

FIG. 5 is an enlarged cross-sectional view of a third embodiment of a measurement chip according to the invention with two measurement fields and a cover layer;

FIG. 6 is a schematic diagram of a method of determining thermal conductivity of a thin layer according to the present invention;

FIG. 7 is a schematic front view of a measuring chip for measuring thermal conductivity, electrical conductivity and Seebeck coefficient of a sample according to the present invention;

FIG. 8 is a schematic diagram of the backside of the measurement chip shown in FIG. 7;

FIG. 9 is an enlarged cross-sectional view of an alternative embodiment of a measurement chip of the present invention.

Detailed Description

Fig. 1 is a top view and fig. 2 is a bottom view of a measurement chip according to the present invention. The measurement chip comprises a substrate 14 with a recess 15. A membrane 16 of an electrically insulating material is applied to the top surface of the substrate 14 in such a way that the membrane 16 completely covers the recess 15. A heating wire 17 is applied to the membrane 16 in such a way that it overlaps the groove 15. Furthermore,

measurement field limiters

18, 19 are provided, which are made of a material with a higher thermal conductivity than the membrane 16, such as gold or platinum, and which are applied to the membrane 16. One part 18 of the measuring field limiting element extends electrically insulated along both sides of the heating wire 17, while the other part 19 of the measuring field limiting element extends along the end side of the heating wire 17 in an electrically connected manner with the latter.

Fig. 2 shows a substrate with a self-supporting membrane 16 which completely covers the recess 15.

As shown in fig. 3, which is a cross-sectional view of the measuring chip of the present invention shown in fig. 1, the measuring field limiter 18 covers the groove 15 in this embodiment with respect to both side portions of the heating wire 17. The circumferential boundary of the measuring field in the recess 15 can be formed along the edge of the measuring field limiting element 18 in the recess 15. The measuring field thus defined is therefore always smaller than the surface defined by the recess on the bottom surface of the diaphragm 16. Fig. 3 furthermore shows a sample 20 in the form of a thin layer, which is applied in the region of the recess 15 on the bottom side of the membrane 16.

Fig. 4 shows a second embodiment of the inventive measuring chip with a second measuring field. In addition to the first recess 15 in the substrate 14, a second recess 25 is provided in the substrate 14, wherein the

grooves

15, 25 differ in width. Within each

recess

15, 25, a sample layer 20 is applied to the self-supporting membrane 16. On the opposite sides of the membrane 16,

heating wires

17, 27 are applied, respectively, overlapping the

grooves

15, 25. As shown in fig. 8, the

heating wires

17, 27 have the same cross section and the same length in the embodiment described. As shown in fig. 1, the measuring field limiting member 18 is shown to be arranged on both sides of the heating wire 17 at the same distance from the heating wire 17 and partly covering the recess 15 and to extend over almost the entire length of the heating wire 17. In this embodiment, the circumferential line of the measuring field within the recess 15 extends over 90% of the edge of the measuring

field limiting element

18, 19, since the two contact regions 19 are likewise arranged so as to partially cover the recess 15. As far as the heating wire 27 is concerned, the above-described solution is equally applicable to the measuring field confining element 28, but the distance between the heating wire 27 and the measuring field confining element 28 is different. In the present embodiment, these measuring fields are of different sizes. This makes it possible to carry out the same measurement in measurement fields of different sizes with the sample layer 20 applied to the measurement chip in two measurement fields at a time. Thus, in addition to the thermal conductivity of the sample layer 20, the emissivity of the

heating wires

17, 27 can be experimentally determined.

Fig. 5 shows a further embodiment of a measurement chip, in which a cover layer 30 is applied to the sample layer 20. This figure (as with all other figures) is not to scale and represents only the following: the masking layer 30 is at least 10 times greater than the thickness of the membrane 16. Masking layer 30 is spray coated with graphite. The measurement chip with the cover layer 30 is able to determine the out-of-plane component of the thermal conductivity of the thin layer, as can be explained in connection with the method of the invention.

Fig. 5 furthermore shows an alternative embodiment of the measuring

field limiting members

18, 19. The part 18 of the measurement field limiter differs from the measurement field limiter 28 and is arranged in such a way that this part 18 of the measurement field limiter partially covers the two

recesses

15, 25.

Fig. 6 is a schematic diagram of the method of determining thermal conductivity of a thin layer of the present invention. Wherein a heating power 35 is supplied above the heating wire 17 via the two contact regions 19, for example by means of a constant current, so that the heating wire 17 heats up. At the same time, also between the two contact areas 19, an average temperature change 36 is measured over the heating wire 17, for example by resistance measurement of the heating wire 17. Knowing the input power 35 and the magnitude of the measuring field, the thermal conductivity of the sample layer 20 can be determined from the temperature change 36. The recess 15 is not recognizable in this figure. The arrangement of the measuring field confining element 18 according to the invention is the same as the measuring field confining element 18 described in fig. 1 and the measuring field confining element according to the invention is arranged in such a way that it partly covers the recess 15.

Fig. 7 and 8 show a further embodiment of a measurement chip according to the invention, in which, in addition to the two measurement fields for the thermal conductivity measurement, a third measurement field for the sample layer 20 is provided. In the described embodiment, the sample layer 20 has been applied simultaneously on all measurement fields in a single deposition process. In this third measurement field, the sample layer 20 is applied directly on the membrane 16 without providing the recess 15 of the substrate 14. A measuring device is provided which is adapted to determine the electrical conductivity 40 and/or the seebeck coefficient 41 of the sample layer 20.

Furthermore, as shown in fig. 8, when the thermal conductivity measurement is carried out simultaneously in two measurement fields, the

heating wires

17 and 27 are connected in series in order to obtain a heating power 35 which is input in the same manner when the

heating wires

17, 27 are of the same size. In this case, the average temperature rise 36 of the heating wire 17 differs from the average temperature rise of the heating wire 27 on the basis of the different magnitudes of these measuring fields when the same sample layer 20 is used.

Fig. 9 shows an alternative embodiment of the measurement chip according to the invention. An additional insulating layer 22 is applied to the heating wire 17 and the measuring field limiting member 18, so that the insulating layer 22 covers the heating wire 17 and the measuring

field limiting members

18, 19. As shown in fig. 9, the insulating layer 22 also extends laterally of the heating wire 17 and the measuring field limiting member 18 in order to ensure electrical insulation from the sample layer 22 applied on the side of the membrane 16 remote from the substrate 14 in the region of the recess 15. The insulating layer 22 is made of an electrically insulating material, for example, photoresist. In this case, the sample layer 20 applied to the measuring chip over a large area is not in direct contact with the heating filament 17 or the measuring

field limiting members

18, 19. In the embodiment, the measurement field is at least partially defined by

measurement field restraints

18, 19 covering the recess 15.

The measuring chip of the invention is arranged in a machine (not shown) in such a way that it is electrically connected to the contact surface 19 of the machine. This connection can be established, for example, by spring contact pins or bond wires. Thermal conductivity measurements can be made in this machine in vacuum to exclude convection in heat transfer. Furthermore, the chip is thermally coupled to this machine in order to generate a temperature gradient, for example, through the sample. Furthermore, the temperature and the magnetic field of this machine can be set as parameters.

Claims

1. A measurement chip for determining the thermal conductivity of a thin layer, comprising a substrate (14) with a recess (15), a membrane (16) of an electrically insulating material applied to the substrate (14) in such a way as to cover the recess (15), and a heating wire applied to the membrane (16) which is superimposed on the recess (15), characterized in that at least one end of the heating wire is located in the region of the recess, a first measurement field limiter being arranged on the membrane (16) which partially covers the recess (15), the first measurement field limiter comprising two sections (18) which are electrically insulated from the heating wire (17), the first measurement field limiter comprising two contact regions (19) which are electrically connected to the heating wire (17).

2. Measuring chip according to claim 1, characterized in that the heating wire (17) extends between the sections (18) which are electrically insulated from the heating wire (17).

3. Measuring chip according to claim 1, characterized in that the heating wire (17) and the first measuring field restriction are arranged on one face of the membrane (16).

4. Measuring chip according to claim 1, characterized in that a circumferential line of the measuring field within the recess (15) is formed by at least 70% of the edge of the first measuring field limiting member.

5. The measurement chip according to claim 1, wherein the first measurement field limiter is composed of gold and/or platinum.

6. Measuring chip according to claim 1, characterized in that an insulating layer (22) is applied in a covering manner to the heating wire (17) and the first measuring field restriction.

7. Measurement chip according to claim 1, characterized by a first measurement field with a first measurement field restriction and a second measurement field with a second measurement field restriction (28), wherein the area of the first measurement field is smaller than the area of the second measurement field.

8. Measuring chip according to claim 7, characterized in that the heating wire (17) superimposed with the first measuring field and the heating wire (27) superimposed with the second measuring field have the same length.

9. Measuring chip according to claim 1, characterized by a measuring device adapted to determine the thermal conductivity of a sample layer (20) applied to the membrane (16) in the region of the recess (15).

10. The measurement chip according to any of claims 1 to 9, characterized by a measurement device adapted to determine the electrical conductivity (40) and/or the seebeck coefficient (41) of the sample layer (20) applied in the third measurement field.

11. Method for determining the thermal conductivity of a thin layer, wherein a heating power (35) is supplied by means of a heating wire (17) applied to a membrane (16) and the temperature rise (36) of the heating wire (17) is measured, wherein the heating wire (17) is arranged on the membrane (16) within a measuring field limitation, at least one end of which is located in the region of a recess, which measuring field limitation is arranged in such a way that it partially covers the recess (15) of a substrate carrying the membrane (16), wherein the measuring field limitation comprises two sections (18) which are electrically insulated from the heating wire (17), and wherein the measuring field limitation comprises two contact regions (19) which are electrically connected to the heating wire (17).

12. The method according to claim 11, characterized in that a sample layer (20) is applied to the membrane (16).

13. The method according to any one of claims 11 or 12, characterized in that a cover layer (30) is applied to the sample layer (20), wherein the thickness of the cover layer (30) is at least 10 times greater than the thickness of the membrane sheet (16).

14. The method according to claim 13, wherein the cover layer (30) is composed of a material having a relatively high thermal conductivity.