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

Abstract

Measuring chip for determining the thermal conductivity of a thin layer, comprising a substrate (14) with a recess (15), a membrane (16) made of electrically insulating material, which is applied to the substrate (14) so ​​as to cover the recess (15), and a heating wire applied to the membrane (16) which overlies the recess (15), characterized in that a measuring field boundary (18, 19) which partially covers the recess (15) is arranged on the membrane (16).

Description

[0001] The invention relates to a measuring chip suitable for determining the thermal conductivity of a thin layer. The measuring chip comprises a substrate with a recess and a membrane that is applied to the substrate, covering the recess. A heating wire is applied to the membrane. The invention also relates to a method for determining the thermal conductivity of a thin layer. DE 44 10 315 A1 discloses a microsensor for measuring the specific thermal conductivity of thin layers, which is intended for in-situ measurement during the layer deposition of said layers.

[0002] The thermal conductivity of a thin layer (sample) can be determined by measuring the amount of heat dissipated through the layer by a heating wire arranged adjacent to the thin layer. By taking the measurement in the area of ​​a recess in the measuring chip, heat dissipation occurs largely through the sample, while heat dissipation through other components of the measuring chip is kept as low as possible. By subtracting the other components of the heat dissipation, the proportion of heat dissipated through the sample can be determined.

[0003] The measuring chip is equipped with a heating structure that, on the one hand, generates a defined heat flow in the material and, on the other hand, serves as a sensor to detect a temperature increase of the heating structure. The calculation of the sample's thermal conductivity takes into account the area over which the sample dissipates heat. Determining this area size is not entirely straightforward. In particular, it requires considerable effort to determine the size of this contact area experimentally through additional measurements.

[0004] The invention is based on the object of presenting a measuring chip and a method with which the thermal conductivity of a thin layer can be determined reproducibly and without further measurements. Based on the cited prior art, this object is achieved by the features of the independent claims. Advantageous further developments are the subject of the dependent claims.

[0005] According to the invention, a measuring field limitation is provided on the membrane, which is arranged partially overlapping the recess in the substrate.

[0006] First, some terms will be explained in more detail: A thin layer is defined as a continuous layer made of one or more solid materials. The thickness of such a layer typically ranges from a few micrometers to a few nanometers. A thin layer can be produced, for example, by physical processes (e.g., sputtering or thermal vapor deposition), chemical processes (e.g., atomic layer deposition or molecular beam epitaxy), or other coating methods (e.g., drop casting or spin coating) and deposited directly onto a measurement chip. In this sense, the membrane, in particular, can also represent a thin layer.

[0007] A measurement field refers to a section of the measurement chip where the thermal conductivity of the sample is measured. The measurement field preferably comprises areas in which the membrane or the combination of membrane and sample is not covered by any other components except the heating wire. In particular, in the measurement field area, the membrane is not covered by the substrate or the measurement field boundary. Starting from the heating wire, a defined heat flow is generated through the membrane in this area, which can be used to draw conclusions about the thermal conductivity of the membrane or an applied sample.

[0008] The invention recognizes that the measurement field limitation according to the invention makes it possible to create a defined boundary for the measurement field in specific regions of the measurement field. This facilitates the determination of the relevant area of ​​the sample. In particular, it becomes possible to measure the thermal conductivity without additional measurement of the recess geometry. Such recesses are typically produced using etching processes, which are difficult to control because they depend on a multitude of process parameters (time, substrate, temperature). The use of a measurement field limitation allows for a known and reproducible size of a measurement field despite deviations in the recess geometry.

[0009] The measuring chip according to the invention comprises a substrate with a recess. The substrate is preferably a silicon wafer. The recess is preferably a rectangular trench with edge lengths in the range of 0.1 to 10 millimeters. A membrane made of electrically insulating material is applied to the substrate, covering the recess. The membrane is preferably silicon nitride with a layer thickness of at most 200 nanometers, preferably at most 100 nanometers, more preferably at most 50 nanometers. A small layer thickness of the membrane is advantageous for determining thermal conductivity by keeping parasitic heat flows outside the sample as low as possible.

[0010] The heating wire is preferably applied to the side of the membrane facing away from the substrate. The heating wire preferably extends over a length in the range of several millimeters and has a width of less than 5 micrometers. The heating wire is preferably made of an electrically conductive material. The heating wire should be designed so that at least one of its ends, preferably both ends, lie in the region of the recess. If an overlap between the heating wire and the substrate is avoided, the undesirable heat dissipation that could otherwise occur between the heating wire and the substrate is reduced. Terms such as cover and overlay refer, within the scope of the invention, to a projection in a direction perpendicular to the membrane.

[0011] The measurement field boundary is preferably made of a material with high thermal conductivity. When applying the heating wire and the measurement field boundary, additional adhesion promoters such as titanium or chromium can preferably be used between the membrane and the structures. Conventional lithographic processes from microstructure technology are preferably used for structuring the measurement chips.

[0012] In a preferred embodiment, the measuring field boundary comprises at least one, preferably two, sections that are electrically insulated from the heating wire. The design of the measuring field boundary is thus spatially independent of the heating wire and, due to the poor thermal conductivity of the membrane, is also thermally insulated from the heating wire. A temperature increase of the heating wire is thus decoupled from a temperature increase of the measuring field boundary, and vice versa.

[0013] The heating wire can extend between the electrically insulated sections. By extending the heating wire within two sections of the measuring field, it is possible to construct a symmetrical measuring field and thus create a defined area for heat flow from the heating wire to two edges of the measuring field boundary.

[0014] In one embodiment of the invention, the measuring field boundary comprises at least one, preferably two contact areas electrically connected to the heating wire. The contact areas are preferably significantly wider than the heating wire, so that each contact area simultaneously defines one end of the heating wire. The electrically connected contact areas allow a measuring field boundary directly at the ends of the heating wire. The contact areas are usually significantly wider than the heating wire and can absorb heat well. A further advantage of the contact areas is that they can be used to electrically contact the chip and thus the heating wire. This contact can be achieved, for example, using bonding wires or spring contact pins. The large contact areas are significantly less sensitive to the pressure exerted by the bonding needle or spring contact pin than the narrow heating wire.In addition, the contact areas are easier to see under a light microscope.

[0015] Preferably, the heating wire and the measurement field boundary are arranged on the same side of the membrane. In this case, the two structures can be deposited in a single processing step, which facilitates the production of the measurement chips. Furthermore, it is possible to align the measurement field boundary directly with the heating wire, which facilitates the determination of the measurement field and increases measurement accuracy. A further advantage is that the back of the membrane remains free, preventing a sample applied to the measurement chip from coming into contact with these structures.

[0016] The measuring field boundary can generally be designed in one or more parts. If the measuring field boundary is made of an electrically conductive material, it will usually be designed in several parts, as otherwise an electrical connection could occur that bypasses the heating wire. The interruptions between the individual parts of the measuring field boundary should be as small as possible so that the measuring field is enclosed as far as possible by the measuring field boundary.

[0017] In a preferred embodiment, a circumferential line of the measuring field lying within the recess is formed over at least 70%, preferably at least 80%, more preferably at least 90% by an edge of the measuring field boundary. The larger the area in which the edge of the measuring field is formed by the measuring field boundary, the more precisely the size of the measuring field can be controlled and determined. The geometry of the measuring field is largely independent of the geometry of the recess on the underside of the membrane. Particular care should be taken to ensure that the heating wire is not short-circuited by the measuring field boundary. To this end, a distance of 10 to 20 micrometers is preferably maintained between electrically non-insulated parts of the measuring field boundary.The measuring field boundary can be composed of sections that are electrically connected to the heating wire (in particular the contact areas) and sections that are electrically insulated from the heating wire.

[0018] It can be advantageous to manufacture the measurement field boundary from gold or platinum. Both materials have high thermal conductivity compared to the poorly thermally conductive membrane and can be applied to the membrane at the same time as the heating wire. Gold and platinum are also extremely stable over wide temperature ranges. Furthermore, both materials have the necessary hardness to be connected with bonding wires or spring contact pins, even at thin layers.

[0019] In an advantageous embodiment, an insulating layer is applied covering the heating wire and the measurement field boundary. The insulating layer preferably consists of an electrically insulating photoresist. The insulating layer makes it possible to determine the thermal conductivity of a sample layer applied in the region of the recess on the side of the membrane facing away from the substrate. A further advantage is that a sample layer can be applied to the measurement chip, for example, by spin coating, since there is no recess on the side of the membrane facing away from the substrate.

[0020] In a preferred embodiment of the measuring chip according to the invention, a first measuring field with a first measuring field boundary and a second measuring field with a second measuring field boundary are provided, wherein the area of ​​the first measuring field is smaller than the area of ​​the second measuring field. Such a configuration of the measuring chip offers the possibility of measuring simultaneously in two differently sized measuring fields when determining the thermal conductivity of a sample. This allows a heat flow based on thermal conductivity and thermal radiation (thermal conductivity and emissivity) to be taken into account, thus allowing the thermal conductivity to be determined with greater accuracy.

[0021] Each of the measuring fields is equipped with a heating wire. Preferably, one heating wire overlying the first measuring field and one overlying the second measuring field are of equal length. The heating wires therefore have virtually the same electrical resistance and exhibit virtually identical heating behavior. Furthermore, the geometry of the measuring fields depends only on their width, i.e., their extension in a direction perpendicular to the direction of the heating wire.

[0022] In thermoelectric generators, heat can be converted directly into electrical energy using the Seebeck effect. The Seebeck effect occurs when a temperature gradient is applied across a thermoelectric layer, i.e., the layer has cold and warm areas. The efficiency with which a material can convert heat into electrical energy is generally determined by a dimensionless figure of merit ZT, which is proportional to the square of the Seebeck coefficient S, the electrical conductivity σ, and the temperature T, and inversely proportional to the thermal conductivity κ of the material. To determine the figure of merit, it is desirable to measure all quantities of a thin layer under fixed ambient conditions.

[0023] It is an object of the present invention to provide a measuring chip that allows all of the above-mentioned parameters of a thin layer to be determined essentially simultaneously over a wide temperature range. The figure of merit (ZT) can then be determined directly with the measuring chip, without the need for further measurements, such as determining the sample geometry. The measuring chip is pre-structured in such a way that only a single sample deposition process is necessary. During this deposition process, all measurement fields of the measuring chip are covered with a thin layer.

[0024] For this purpose, the measuring chip according to the invention preferably comprises a measuring device designed to determine the thermal conductivity of a sample layer applied to the membrane in the region of the recess. A measuring device on the measuring chip can ensure, for example, that the heating wire is correctly contacted during a measurement. Furthermore, the measuring device is capable of applying and measuring currents and voltages.

[0025] The measurement chip can further comprise a measuring device designed to determine the electrical conductivity and / or 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 determined on the chip under the same given conditions. Because all measuring devices are present on the chip, the sample can be fully characterized during a first temperature or magnetic field run. The measuring devices comprise means for applying and / or tapping voltages as well as means for generating a temperature gradient across a wider area of ​​the sample.

[0026] The invention also relates to a method for determining the thermal conductivity of a thin layer. In the method according to the invention, heating power is applied via a heating wire applied to a membrane, and an average temperature rise of the heating wire is measured. The heating wire is arranged within a measurement field boundary that partially covers a recess in the substrate on the back of the membrane.

[0027] When a current is passed through the heating wire in the method according to the invention, part of the electrical energy is converted into Joule heat, which leads to an increase in the temperature of the heating wire. The temperature increase depends on how much of the heat is dissipated (through convection, conduction, and radiation). Under given boundary conditions, the in-plane thermal conductivity of the membrane can be determined from this.

[0028] In a preferred embodiment of the method according to the invention, a sample layer can be applied to the membrane. The sample layer is preferably arranged on a different side of the membrane than the heating wire and the measurement field boundary. Conventional methods for depositing thin layers can be used to apply the sample layer. The product of thermal conductivity and thickness of the applied layer is preferably less than 2 10 -7 Watts per Kelvin are not. If a sample layer is additionally applied to the membrane, the method is sensitive to the component of thermal conductivity that lies in the common plane of the membrane and the sample (in-plane).

[0029] Preferably, a cover layer can be applied to the sample layer, wherein the thickness of the cover layer is preferably at least 10 times greater, more preferably at least 20 times greater, and more preferably at least 100 times greater than the thickness of the membrane. The cover layer can also be applied using conventional methods for depositing thin layers. However, it is also possible to spray the cover layer on. The cover layer serves as a heat sink and causes a change in the direction of the heat flow in the membrane and / or sample. The heat flow is then deflected towards the cover layer, and the measurement of the thermal conductivity is sensitive to the component of the thermal conductivity that is oriented perpendicular to the common plane of the membrane and sample (out-of-plane).

[0030] For ease of handling, it may be advantageous for the cover layer to be made of a material with high thermal conductivity, particularly graphite. The graphite can preferably be sprayed directly onto the measuring chip from a spray can.

[0031] The method can be further developed with additional features that are described in connection with the measuring chip according to the invention. The measuring chip can be further developed with additional features that are described in connection with the method according to the invention.

[0032] The invention is described below by way of example with reference to advantageous embodiments in the accompanying drawings. They show: Fig. 1: a schematic representation of a front side of a part of a measuring chip according to the invention; Fig. 2: a schematic representation of a back side of the measuring chip from Fig. 1 without samples; Fig. 3: an enlarged sectional view of parts of the measuring chip according to the invention from Fig. 1 with a sample; Fig. 4: an enlarged sectional view of a second embodiment of a measuring chip according to the invention with a second measuring field; Fig. 5: an enlarged sectional view of a third embodiment of a measuring chip according to the invention with two measuring fields and a cover layer; Fig. 6: a schematic view of a method according to the invention for determining a thermal conductivity of a thin layer; Fig. 7: a schematic representation of the front side of a measuring chip according to the invention for determining the thermal conductivity, electrical conductivity and Seebeck coefficient of a sample; Fig. 8: a schematic representation of the back of the measuring chip from Fig. 8; Fig. 9: an enlarged sectional view of a measuring chip according to the invention in an alternative embodiment.

[0033] A measuring chip according to the invention is Fig. 1 from above and in Fig. 2 shown from below. The measuring chip comprises a substrate 14 with a recess 15. A membrane 16 made of electrically insulating material is applied to an upper side of the substrate 14 such that the membrane 16 completely covers the recess 15. A heating wire 17 is applied to the membrane 16 such that it overlies the recess 15. Furthermore, a measuring field boundary 18, 19 is provided, which consists of a material with high thermal conductivity compared to the thermal conductivity of the membrane 16, for example gold or platinum, and which is applied to the membrane 16. A part 18 of the measuring field boundary extends electrically insulated along both sides of the heating wire 17, while another part of the measuring field boundary 19 extends electrically connected to the heating wire along the end faces of the heating wire 17.

[0034] Fig. 2 shows the substrate with the self-supporting membrane 16, which completely covers the recess 15.

[0035] From the sectional view in Fig. 3 of the measuring chip according to the invention Fig. 1 shows that, in this embodiment, the measuring field boundary 18 partially covers the recess 15 on both sides of the heating wire 17. A circumferential line of the measuring field can be constructed along the edges of the measuring field boundary 18 located within the recess 15. The measuring field defined in this way is therefore always smaller than the area defined by the recess on the underside of the membrane 16. Fig. 3 also shows a sample 20 in the form of a thin layer applied to an underside of the membrane 16 in the region of the recess 15.

[0036] In Fig. Figure 4 shows a second embodiment of the measuring chip according to the invention 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 recesses 15, 25 have different widths. Within each recess 15, 25, a sample layer 20 is applied to the self-supporting membrane 16. On the opposite side of the membrane 16, a heating wire 17, 27 is applied overlying the recess 15, 25. In this embodiment, the heating wires 17, 27 have an identical cross-section and an identical length, as in Fig. 8. The measuring field limitation 18 shown is arranged on both sides of the heating wire 17 at the same distance from the heating wire 17 and partially overlapping the recess 15 and extends almost over the entire length of the heating wire 17, as shown in Fig. 1. In this embodiment, the circumferential line of the measuring field lying within the recess 15 extends over more than 90% of an edge of the measuring field boundary 18, 19, since the two contact regions 19 are also arranged so as to partially overlap the recess 15. The same applies to the measuring field boundary 28 with regard to the heating wire 27; however, the distance between the heating wire 27 and the measuring field boundary 28 is not identical. The measuring fields in this embodiment have different sizes. This makes it possible to carry out the same measurement in differently sized measuring fields in both measuring fields with a sample layer 20 applied once to the measuring chip. This makes it possible to experimentally determine not only the thermal conductivity of the sample layer 20 but also the emissivity of the heating wire 17, 27.

[0037] Fig. Figure 5 shows another embodiment of the measuring chip, in which a cover layer 30 was applied to the sample layer 20. Like all other illustrations, this illustration is not to scale and is intended merely to indicate that the cover layer 30 is at least a factor of 10 larger than the thickness of the membrane 16. The cover layer 30 was applied using a graphite spray. The measuring chip with the cover layer 30 enables the determination of the out-of-plane component of the thermal conductivity of a thin layer, as explained in the context of the method according to the invention.

[0038] In addition, Fig. 5 an alternative embodiment of the measuring field limitation 18, 19. A part of the measuring field limitation 18 overlaps with the measuring field limitation 28 and is arranged such that this part of the measuring field limitation 18 partially covers both recesses 15, 25.

[0039] Fig. Figure 6 schematically shows the method according to the invention for determining the thermal conductivity of a thin layer. A heating power 35, for example, via a constant current, is applied to the heating wire 17 via the two contact areas 19, so that the heating wire 17 heats up. At the same time, an average temperature increase 36 is measured across the heating wire 17 between the two contact areas 19, for example, via a resistance measurement of the heating wire 17. The thermal conductivity of a sample layer 20 can be determined from the temperature change 36, knowing the applied power 35 and the size of the measuring field. The recess 15 is not visible in this view. The arrangement of the measuring field boundary 18 according to the invention is identical to that shown in Fig. 1 and is arranged partially overlapping the recess 15.

[0040] Fig. 7 and Fig. 8 show a further embodiment of the measuring chip according to the invention, wherein, in addition to the two measuring fields for thermal conductivity measurement, a third measuring field for a sample layer 20 is provided. In this embodiment, a sample layer 20 has already been applied to all measuring fields simultaneously during a single deposition process. On the third measuring field, the sample layer 20 is applied directly to the membrane 16 without a recess 15 in the substrate 14 being provided. A measuring device is provided that is designed to determine an electrical conductivity 40 and / or a Seebeck coefficient 41 of the sample layer 20.

[0041] In Fig. Figure 8 also shows that, for a parallel thermal conductivity measurement in two measurement fields, the heating wires 17 and 27 are connected in series to obtain an identically supplied heating power 35 with identical dimensions of the heating wires 17, 27. With identical sample layers 20, an average temperature increase 36 of the heating wire 17 is then different from an average temperature increase of the heating wire 27 due to the different size of the measurement fields.

[0042] An alternative embodiment of the measuring chip according to the invention is shown in Fig. 9. An additional insulating layer 22 is applied to the heating wire 17 and the measuring field boundary 18, so that the insulating layer 22 covers the heating wire 17 and the measuring field boundary 18, 19. As shown in Fig.As shown in Figure 9, the insulating layer 22 also extends laterally from the heating wire 17 and the measuring field boundary 18 to ensure electrical insulation from a sample layer 22, which is applied in the region of the recess 15 on the side of the membrane 16 facing away from the substrate 14. The insulating layer 22 consists of an electrically insulating material, for example a photoresist. A sample layer 20, which is applied over a large area on the measuring chip, is then not in direct contact with the heating wire 17 or the measuring field boundary 18, 19. In this embodiment, too, the measuring field is at least partially defined by a measuring field boundary 18, 19 covering the recess 15.

[0043] It is intended to arrange the measuring chip according to the invention in a machine (not shown) such that the contact surfaces 19 of the measuring chip and the machine are electrically connected to one another. The connection can be established, for example, via spring contact pins or bonding wires. A measurement of the thermal conductivity can take place within the machine in a vacuum to exclude convection as a heat transfer. Furthermore, the chip is thermally coupled to the machine, for example, to generate a temperature gradient across the sample. Furthermore, the temperature and the magnetic field can be adjusted by the machine as parameters.

Claims

[1] Measuring chip for determining the thermal conductivity of a thin film, comprising a substrate (14) with a recess (15), a membrane (16) made of electrically insulating material which is applied to the substrate (14) covering the recess (15), and a heating wire applied to the membrane (16) which overlaps the recess (15), characterized by , that a measuring field boundary (18, 19) partially covering the recess (15) is arranged on the membrane (16). [2] Measuring chip according to claim 1, characterized by , that the measuring field boundary (18, 19) comprises at least one, preferably two sections (18) electrically insulated from the heating wire (17). [3] Measuring chip according to claim 2, characterized by , that the heating wire (17) extends between the electrically insulated sections (18). [4] Measuring chip according to claim 1 or 2, characterized by, that the measuring field boundary (18, 19) comprises at least one, preferably two contact areas (19) electrically connected to the heating wire (17). [5] Measuring chip according to any one of claims 1 to 4, characterized by , that the heating wire (17) and the measuring field limit (18, 19) are arranged on one side of the membrane (16). [6] Measuring chip according to any one of claims 1 to 5, characterized by , that a circumferential line of the measuring field lying within the recess (15) is formed over at least 70%, preferably at least 80%, further preferably at least 90% of an edge of the measuring field boundary (18, 19). [7] Measuring chip according to any one of claims 1 to 6, characterized by , that the measuring field boundary (18, 19) consists of gold and / or platinum. [8] Measuring chip according to any one of claims 1 to 7, characterized by , that an insulating layer (22) is applied covering the heating wire (17) and the measuring field boundary (18, 19). [9] Measuring chip according to any one of claims 1 to 8, characterized by a first measurement field with a first measurement field boundary (18, 19) and a second measurement field with a second measurement field boundary (28), wherein the area of ​​the first measurement field is smaller than the area of ​​the second measurement field. [10] Measuring chip according to claim 9, characterized by , that a heating wire (17) superimposed on the first measuring field and a heating wire (27) superimposed on the second measuring field have the same length. [11] Measuring chip according to any one of claims 1 to 10, characterized by a measuring device designed to determine the thermal conductivity of a sample layer (20) applied to the membrane (16) in the area of ​​the recess (15). [12] Measuring chip according to any one of claims 1 to 11 characterized bya measuring device designed to determine an electrical conductivity (40) and / or a Seebeck coefficient (41) of a sample layer (20) applied in a third measuring field. [13] Method for determining the thermal conductivity of a thin layer, wherein a heating power (35) is supplied via a heating wire (17) applied to a membrane (16) and a temperature increase (36) of the heating wire (17) is measured, wherein the heating wire (17) is arranged on the membrane (16) within a measuring field boundary (18, 19) which is arranged partially overlapping a recess (15) of a substrate supporting the membrane (16). [14] Method according to claim 13, characterized by , that a sample layer (20) is applied to the membrane (16). [15] Method according to one of claims 13 or 14, characterized by, that a cover layer (30) is applied to the sample layer (20), wherein the thickness of the cover layer (30) is at least by a factor of 10, preferably by a factor of 20, further preferably by a factor of 100 greater than the thickness of the membrane (16). [16] Method according to claim 15, characterized by , that the cover layer (30) consists of a material with high thermal conductivity, in particular graphite.

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