DE102013007644B4 - Arrangement for measuring an elongation, a pressure or a force with a resistance layer - Google Patents

Abstract

An arrangement for measuring strain, pressure or force with a resistive layer, said resistive layer comprising: - an amorphous insulation matrix (3) of electrically non-conductive base material; and clusters (4) of an electrically conductive cluster material which is oxidation-resistant in the insulation matrix (3) and embedded in the insulation matrix (3) and insulated from each other by the base material, the base material being SiO 2 and the cluster material Pt being wherein the resistive layer is made by simultaneous sputtering in a co-sputtering process with the base material and the cluster material, the clusters (4) being crystalline.

Description

Technical area

The present invention relates to pressure and strain sensitive resistor layers for use in pressure sensors.

State of the art

Strain gauges with sensor layers for measuring pressures are known from the prior art in many ways. The sensor layers are usually designed as resistance layers whose ohmic resistances change during a bending or pressurization.

Resistive layers for use in pressure sensors are currently almost exclusively made of CrNi-based alloys, usually CrNiSi or CrNiAl. However, such resistance layers have a very low strain sensitivity with k-factors around 2. Furthermore, increased temperatures due to post-oxidation or the addition of oxygen at the grain boundaries affect the long-term stability of the CrNi layers.

Another common disadvantage of such pressure sensitive resistive layers is the lack of temperature resistance that renders them unsuitable for use in high temperature applications. For example, a cylinder pressure measurement in a combustion chamber of a cylinder of an internal combustion engine requires a temperature resistance of the relevant sensor up to 600 ° C.

For high temperature applications is out of the document US 2009/014 5235 A1 a strain gauge with a functionality at temperatures above 1,400 ° C known. The sensor comprises a substrate having thereon a layer of an oxide semiconductor such as an indium-tin oxide alloy, aluminum-doped zinc oxide or the like, and a high-temperature melting metal of Pt, Pd, Rh, Ni, W, Ir , NiCrAlY and NiCoCrAlY are deposited on the substrate to form a pressure-sensitive element. This layer is characterized by the high metal content of z. B. 88 at% Pt, very low impedance, which is disadvantageous for sensor applications.

In the publication S.E. Dyer et al., "Improved Passivating Cr2O3 Scales for Thin Film High Temperature PdCr Strain Gages", Thin Solid Films, 312, 1998, pages 331-340, strain gages with a PdCr alloy are proposed. These are suitable for high temperature applications, but have a relatively low k-factor of 1.3. In addition, Cr must be effectively protected in this alloy against oxidation, since otherwise also a deterioration of long-term stability occurs.

As from the pamphlets WO 2009/129930 A1 and US 8,198,978 B2 known, pressure and strain-sensitive resistive layers can also by the introduction of metallic clusters, such as. As nickel, are prepared in carbonaceous layers. These resistive layers achieve high strain sensitivity with k-factors between 20 and 30. However, at temperatures above 250 ° C, oxidation occurs because the material of the metallic clusters can usually oxidize. Furthermore, the morphology of the carbonaceous layer is irreversibly modified above about 500 ° C, so that the strain sensitivity is degraded.

Furthermore, for example, the document discloses US 2007/010 7494 A1 a resistive layer in which inclusions of conductive material are introduced into a base material of TaN, the total thickness of the layer being between three and ten times the average size of the inclusions. The achievable k-factor can be up to 10.

From the publication EP 0 526 290 A1 Also, a strain-sensitive layer formed of Ta ₂ N and TaN is known.

From the publication US 4 591 417 A For example, a method and apparatus for depositing cermets are known. In this case, a metallic material and an insulating material is deposited alternately on a substrate until a desired layer thickness is reached. At least one of the two materials is deposited for each deposition step so that the formed layer is not consistently opaque. The composition of the deposit thus formed may be determined by the relative deposition rates of the insulating material and the size of the deposited metal particles by the duration of each Metal deposition can be determined. As the material for the deposition, the insulating material SiO ₂ and the metallic material Au are proposed.

From the publication US 5 510 895 A is the use of Pt and Ni known as cluster materials.

It is an object of the present invention to provide a layer structure for producing a pressure and strain-sensitive resistance sensor which has, in particular, a high temperature stability, a high k-factor and a durability at temperatures of several 100 ° C.

Disclosure of the invention

This object is achieved by the arrangement for measuring an elongation, a pressure or a force with a resistive layer according to claim 1.

Further advantageous embodiments of the present invention are specified in the dependent claims.

• – eine amorphe Isolationsmatrix aus einem elektrisch nicht leitenden Basismaterial; und
• – Cluster aus einem in der Isolationsmatrix gegenüber dem Basismaterial oxidationsfesten, elektrisch leitenden Clustermaterial, die in der Isolationsmatrix eingebettet sind und voneinander durch das Basismaterial isoliert sind,

wobei das Basismaterial SiO₂ und das Clustermaterial Pt ist, wobei die Widerstandsschicht durch gleichzeitiges Besputtern in einem Co-Sputter-Prozess mit dem Basismaterial und dem Clustermaterial hergestellt ist, wobei die Isolationsmatrix amorph und die Cluster kristallin ausgebildet werden.According to a first aspect, there is provided a resistance layer for use in an arrangement for measuring strain, pressure or force, comprising:

• - An amorphous insulation matrix of an electrically non-conductive base material; and
• Clusters of an electrically conductive cluster material which is oxidation-resistant to the base material in the insulation matrix and which are embedded in the insulation matrix and are insulated from one another by the base material,

wherein the base material is SiO ₂ and the cluster material is Pt, wherein the resistance layer is made by simultaneous sputtering in a co-sputtering process with the base material and the cluster material, the insulation matrix being amorphous and the clusters being formed crystalline.

An idea of the above arrangement with the resistive layer is to form it with a ceramic, electrically non-conductive base material. Clusters of an oxidation-resistant or inert material, such as a noble metal in a crystalline form, are embedded in the ceramic base material. In particular, the matrix of ceramic material and the clusters embedded therein form a so-called cermet material which has a surprisingly high strain sensitivity, a high temperature stability and a high operating temperature range.

The base material is provided amorphous or quasi-amorphous. As amorphous here a material is referred to, in which the atoms do not form ordered structures, but an irregular pattern and have only short-range, but not long-range order.

• – 15–55 at% Pt, und
• – die verbleibenden at% Si und O.

The base material may be SiO ₂ and the cluster material may be Pt, having the following composition:

• - 15-55 at% Pt, and
• - the remaining at% Si and O.

According to a further aspect, a method for producing a resistance layer may be provided. In this case, an electrically non-conductive substrate is sputtered simultaneously in a co-sputtering process with a base material and a cluster material, wherein the base material is selected such that it forms an electrically non-conductive, amorphous insulation matrix in the sputtering process, and wherein the cluster material with respect to the base material oxidation-resistant and electrically conductive is selected.

Furthermore, the base material can be sputtered using a target of SiO ₂ without adding reactive gases.

According to another alternative, the base material may be sputtered using a target of Si by adding O ₂ in an inert gas atmosphere so that SiO _{2 is} deposited amorphously on the surface of the substrate.

Brief description of the drawings

Preferred embodiments of the present invention will be explained in more detail with reference to the accompanying drawings. Show it:

1 a schematic representation of a cross section through a resistive layer;

2 a schematic representation of a sputtering apparatus for producing the resistance layer;

3 an XRD scan of a sensor layer of Pt / SiO ₂ ;

4 a TEM image of a top view of a Pt / SiO 2 layer with a layer thickness of about 50 nm.

5a and 5b Diagrams showing the change in resistance for several load cycles at a measurement temperature of 250 ° C in air and after a heat treatment in vacuum at 600 ° C at a measurement temperature of 30 ° C in air; and

6 a diagram showing a change in resistance of a Pt / SiO ₂ sensor layer under a hydrostatic pressure load.

Description of embodiments

In 1 schematically the basic structure of a strain-sensitive resistor layer is shown in cross section. The order 1 includes a substrate 2 , which may be formed, for example, as a sapphire substrate or as a substrate of a comparable, non-conductive material.

On a surface of the substrate 2 the resistance layer is applied. The resistance layer comprises an isolation matrix 3 as a layer of a ceramic or ceramic base material such as SiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , AlN or mixed forms thereof, such. As SiO _x N _y , or the like. The base material 3 is preferably to be selected so that it is electrically non-conductive and can be provided as an amorphous layer. The isolation matrix 3 may preferably have a thickness between 10 nm and 400 nm, preferably between 20 nm and 200 nm. Depending on the application, the layer thickness can also be greater than 400 nm or less than 10 nm.

Into the isolation matrix 3 are clusters 4 embedded in a metallic material. The cluster material of the clusters 4 is preferably a refractory metal or a refractory metal alloy, and further has above-average oxidation resistance in the material of the insulation matrix 3 on. Exemplary materials may be or include Pt, Pd, Rh, Ni, Ir and Ru. Preferably, the clusters exist 4 from a noble metal, such as Pt, or other inert, electrically conductive material. The size of the clusters 4 may for example be between 3 nm-10 nm.

Alternatively, the transition material Ni may be used, the oxidation resistance in air is not high, but in the amorphous material of the insulation matrix 3 is very high and especially at temperatures of several 100 ° C is high.

The clusters 4 are crystalline and in the amorphous isolation matrix 3 embedded in the base material, with the clusters 4 are not related, ie separated from each other by the base material or isolated. An exemplary material system for the resistive layer is SiO ₂ as a base material and Pt as a cluster material.

2 shows schematically an apparatus for producing such a resistive layer. The device corresponds to a conventional co-sputtering system 10 with a vacuum chamber 11 , In the vacuum chamber 11 there is a heatable holder 13 , The vacuum chamber 11 is evacuated by means of a pump, not shown.

On the holder 13 is a substrate 12 hung up. On the substrate 12 can use a standard magnetron from a base material target 14 by means of high frequency RF the ceramic base material on the substrate 12 be applied or sputtered. In the described embodiment, SiO _{2 is} selected as the base material.

Furthermore, from a cluster material target 15 at the same time the cluster material, in this embodiment, the noble metal Pt, are applied or sputtered by DC magnetron or RF diode method.

The base material target 14 and the cluster material target 15 are, in this example, arranged at a right angle to each other. Furthermore, the surface of the holder 13 on which the substrate 12 is hung up, at an angle of z. B. between 35 ° and 55 °, preferably of 45 °, to the base material target 14 and the cluster material target 15 inclined.

Through a supply line gases, such as the process gas argon and additional gases such as O ₂ or N ₂ , are introduced into the vacuum chamber. In particular, by adding O ₂ or N ₂ , changes in the stoichiometry of the ceramic base material can be effected.

Alternative to SiO ₂ as base material target 14 It is also possible to provide an aluminum target and together with an added process gas O _{2 it is} possible to react Al ₂ O ₃ on the substrate in a reactive manner 12 be deposited. The addition of N ₂ can sputter AlN when using an aluminum target.

Also, Si ₃ N ₄ or generally SiO _x N _y as the resulting isolation matrix 3 from a reactive sputtering process by the addition of N ₂ and use of a SiO ₂ target as base material targets 14 be deposited, so that the isolation matrix 3 amorphous forms.

In the present example, the following process parameters are used to produce a Pt / SiO ₂ resistance layer: sputtering arrangement Co-sputtering at right angles coating temperature up to 400 ° C sputtering 15 minutes Performance SiO ₂ (5 '' target) 350 watts Power Pt (2 '' - target) 60 watts Pressure in vacuum chamber 5 × 10 ^-3 mbar argon flow 20 sccm Frequency base material target 13.56 MHz

The composition of the resistance layer produced is about 40 at% Pt, 20 at% Si and 40 at% O ₂ and the measured k-factor, ie the ratio between a relative change in resistance and a relative change in length, is 16 in this composition on a silicon substrate.

A granular resistance layer of an amorphous SiO ₂ matrix with embedded crystalline Pt clusters is obtained. The crystalline Pt clusters have a size of 3 to 12 nm in the present example and are surrounded on all sides by SiO ₂ . The Pt clusters do not touch each other, which has a particular effect on the high resistivity of the layer.

A granular resistance layer of an amorphous SiO ₂ matrix with embedded crystalline Pt clusters is obtained.

3 shows an X-ray diffractogram of such a sample. Only peaks at scattering angles, ie preferential directions for the scattering of the X-radiation, which are to be assigned to the fcc phase of metallic platinum, are to be recognized. Based on the widths of the peaks, a cluster diameter of about 10 nm could be estimated.

In 4 is a transmission electron micrograph (TEM) recording of a thin layer of the resistance layer as prepared above to recognize. It can be seen that the Pt clusters (dark areas) are isolated from each other and crystalline objects in the order of about 3 to 10 nm are formed. The bright areas correspond to the SiO ₂ matrix.

The resistance layer produced has a high temperature resistance, as shown by the 5a and 5b can be seen. 5a FIG. 10 illustrates a change in resistance of a Pt / SiO ₂ resist layer made by the above sputtering process, which has been pre-aged at 300 ° C. for a total of 6 hours in air. The diagram was recorded with cyclic equal loading and unloading at a measuring temperature of 250 ° C in air. The determined k-factor is 17. It can be seen that the resistance changes repeatably in a strain-dependent manner.

In 5b a comparable diagram is shown. The graph shows the resistance changes of an Ft / SiO ₂ resistor layer prepared by the above sputtering process with cyclic equal loading and unloading at a measurement temperature of 30 ° C after the sample was vacuum annealed at 600 ° C for 1.5 hours. The k-factor is at this sample 16 and you can see here too that the resistance changes repeatably in a strain-dependent manner.

In 6 Fig. 12 is a diagram showing the dependence of a resistance layer produced by the above method on a hydrostatic pressure. For this purpose, the resistance layer is exposed to an all-round compression in an oil chamber under an oil pressure of up to 900 bar. The curve represents the course of a relative change in the ohmic resistance .DELTA.R / R over the pressure. It can be seen a linear pressure dependence of the ohmic resistance of the resistance layer.

Furthermore, SiO ₂ may be provided as the base material and Ni may be provided as the cluster material. The resistance layer is preferably formed with 50-60 at% Ni.

Claims (2)

Arrangement for measuring an elongation, a pressure or a force with a resistance layer, wherein the resistance layer comprises: an amorphous insulation matrix ( 3 ) of an electrically non-conductive base material; and - clusters ( 4 ) from one in the isolation matrix ( 3 ) with respect to the base material oxidation-resistant, electrically conductive cluster material in the isolation matrix ( 3 ) and are isolated from each other by the base material, wherein the base material is SiO ₂ and the cluster material Pt, wherein the resistance layer is produced by simultaneous sputtering in a co-sputtering process with the base material and the cluster material, the clusters ( 4 ) are crystalline. Arrangement according to claim 1, wherein the resistance layer has the following composition: - 15-45 at% Pt, and - the remaining at% O and Si.

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