AU647292B2

AU647292B2 - Thin strain gauge layer, method for its manufacture and its use in a pressure transducer

Info

Publication number: AU647292B2
Application number: AU20509/92A
Authority: AU
Inventors: Eric Bonnin; Luc Petitjean; Fadhel Rezgui
Original assignee: Schlumberger Technology BV
Current assignee: Schlumberger Technology BV
Priority date: 1991-07-26
Filing date: 1992-07-24
Publication date: 1994-03-17
Anticipated expiration: 2012-07-24
Also published as: ID838B; AU2050992A; DE69204936D1; NO303412B1; NO922943L; NO922943D0; EP0526290B1; DE69204936T2; EP0526290A1; FR2679651A1; JPH06212402A; FR2679651B1

Description

647292

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s) Invention Title: SCHLUMBERGER TECHNOLOGY B.V.

THIN STRAIN GAUGE LAYER, METHOD FOR ITS MANUFACTURE AND ITS USE IN A PRESSURE

TRANSDUCER.

The following statement is a full description of this invention, including the best method of performing it known to me/us: THIN STRAIN GAUGE LAYER, METHOD FOR ITS MANUFACTURE AND ITS USE IN A PRESSURE TRANSDUCER The present invention relates to a thin strain gauge layer, useful :n particular in a pressure transducer for measuring pressure in oil wells.

Pressure transducers used in oil wells have to withstand severe operating conditions, especially temperatures as high as 175 0 C and high pressures which can reach 14x10 7 Pa, all while providing reliable measurements for several weeks usage. They must in particular be able to withsta.-d cyclic deformation as well as vibration and shock, while maintaining good stability and good reliability. The reliability specifications of the transducer relate to the accuracy of its measurements, its short-term and its long-term stability and its hysteresis.

Patent 5,024,098 describes a pressure transducer usable in oil-wells, which has high accuracy, good stability with time and good dynamic characteristics. That transducer comprises a cell supporting a strain gauge device of electrical resistors formed by thin layer deposits, the cell comprising a hermetic chamber within which there prevails a predetermined low pressure, the whole of the outside surface of the cell being subjected to the pressure to be measured.

In that transducer, the cell constitutes the pressure-sensitive element and it converts pressure into deformation, and the thin layer deposits, which may be constituted by thin films of tantalum, convert the deformation into an electrical signal.

Although that transducer has given good results, research has been carried out to find other thin strain gauge layers capable of further improving the performance of pressure transducers of this type.

A specific object of the present invention is to provide a novel thin strain gauge layer useful in a pressure transducer, which allows -2the performance of the transducer to be improved.

There is provided according to the invention a thin strain gauge layer constituted by a cermet based on tantalum and tantalum nitride comprising TaN and which exhibits a resistivity from 400 microohm.cm to 1000 microohm.cm, preferably 500 microohm.cm to 600 microohm.cm, for example in the order of 550 microohm.cm.

In this cermet layer, the presence of the high resistivity tantalum nitride TaN allows the resistivity of the layer to be increased to lie in the range of 400 microohm.cm to 1000 microohm.cm, depending on its tantalum nitride content.

This is very advantageous because the three important parameters S. of a pressure transducer can be optimized in this region of resistivity, these being: 1) the gauge factor y, defined by the equation: .y (fR/R)/(AL/L) in which L is the length of the thin layer and R its resistance; this factor should be as high as possible; 2) the temperature coefficient of resistance (TCR), which is defined by the equation: TCR x (AR/AT) in which R is the resistance of the thin layer and T is the temperature; this factor should be as low as possible; and 3) the degree of internal friction or the degree of inelasticity, which should likewise be as low as possible.

Thus, according to the invention, by selecting a strain gauge layer of cermet based on tantalum and tantalum nitride, having a resistivity of 400 microohm.cm to 1000 microohm.cm, a high gauge factor can be achieved while maintaining a low degree of inelasticity and a suitable temperature coefficient.

The taftalum nitride TaN preferably has a delta structure in the thin layer of cermet.

Preferably, the tantalum nitride of the cermet is constituted almost solely of TaN.

The thickness of the layer is advantageously 0.03 microm to microm.

The thin strain gauge layer described above may be manufactured by a method comprising the following steps: a) forming a thin layer of tantalum containing a tantalum nitride phase on an insulating substrate, by reactive deposition from a tantalum target in an atmosphere formed by a mixture of an inert gas such *e as argon, with nitrogen and possibly oxygen, and b) subjecting the thin layer thus obtained to heat treatment at a eo temperature of 300°C to 600°C.

The first step of this method can be effected using conventional techniques, for example by cathode sputtering in a reactive plasma comprising for example argon, nitrogen and oxygen.

By this technique, a layer of tantalum is deposited containing a tantalum nitride phase formed in general from TaN and Ta 2 N, and the amount of the tantalum nitride phase in the deposited thin layer can be controlled by appropriately controlling the partial pressure of the nitrogen in the sputtering chamber.

The heat treatment of step b) is intended to transform the tantalum nitride Ta 2 N of the layer into TaN. This heat treatment can be effected in times running from 1 to 300 hours, preferably in two stages, namely a first stage in air and a second stage under vacuum.

The transformation of the tantalum nitride Ta 2 N into TaN, which has higher resistivity, serves to increase the resistivity of the layer. Moreover, it appears that this transformation is accompanied by the tantalum nitride structure developing towards the delta (6) structure, which enables relaxation phenomena to be limited.

-4- It is also preferred that the thin layer of cermet Is formed almost exclusively of metallic tantalum and TaN.

The invention further provides a pressure transducer comprising at least one thin strain gauge layer such as is defined above.

In the pressure transducer, the thin strain gauge layer is disposed on a cell subjected to the pressure to be measured and adapted to transform this pressure into strains.

Further characteristics and advantages of the invention will be clear from the following description, made with reference to the accompanying drawings, in which: Figure 1 is a diagram of a magnetron-type sputtering apparatus for depositing the thin layer of the invention, Figure 2 is a micrograph showing the structure of a thin cermet layer of Ta-tantalum nitride.

Figure 3 is a diagram of the active part of a pressure transducer, Figure 4 shows the complete structure of the pressure transducer in vertical section, Figure 5 is a graph showing how the gauge factor y varies as a' function of the resistivity of the strain gauge layer, Figure 6 is a graph showing how the temperature coefficient of the resistance varies as a function of the resistivity of the layer, Figure 7 is a graph showing how the drift on the first day varies as a function of the resistivity of the thin layer, and Figure 8 is a graph showing the drift of the transducer as a function of time, before heat treatment of the layer (curve I) and after heat treatment (curve II).

Figure 1 is highly diagrammatic, and shows a reactive sputtering apparatus for use in the manufacture of strain gauge layers of the invention.

In this apparatus, the tantalum target 1 raised to a suitable potential V C is atomized inside a closed chamber 2 connected to ground, in a reactive plasma of argon, nitrogen and oxygen introduced into the chamber through conduits 3, 4 and 5 respectively, in order to deposit a thin layer on insulating substrates 6 disposed on a rotating support 7.

The chamber is maintained under vacuum by a pump 8 and a pivoted mask 9 is disposed between the target 1 and the support 7.

In this apparatus, the discharge current and the partial pressures I: of the gases are controlled so as to form thin layers of tantalum containing tantalum nitride on the substrates 7, and the amount of tantalum nitride in the layers is controlled by controlling the partial pressure of nitrogen introduced through the conduit 4.

By way of example, there may Ue used a discharge current of 0.1 A to 4 A, a potential V C of 300 V to 400 V and a gas pressure of 0.135 Pa to 1.35 Pa.

Under these conditions layers of tantalum contain'og TaN and Ta 2

N

are deposited.

After this operation, the substrates coated with thin layers are subjected to heat treatment, carried out in air at 400 0 C for several tens of hours, then under vacuum at temperatures of 300°C to 600 0

C,

typically for 1 hour.

As a result of this treatment the tantalum nitride Ta 2 N initially present in the layer converts to the more resistive TaN, which increa- -6ses the resistivity of the layer.

During the heat treatment, the matrix also changes, i.e. the tantalum content in the cermet changes and the TaN structure develops towards the delta structure.

Because of the heat treatment the ratio between the two constituents TaN and Ta 2 N of the tantalum nitride phase of the cermet alters and this alteration is effected to the detriment of the metallic phase. Thus, for a thin layer comprising 70% tantalum before heat treatment, the amount of the tantalum is no more than 68% after treatment, the amounts of Ta being measured from a micrograph on the basis of the area occupied by tantalum. In the case of a thin layer containing 63% tantalum before heat treatment, the amount of tantalum o: is no more than 58% after heat treatment.

All of these changes allow the desired resistivity to be obtained, this depending not only on the amount of tantalum in the film but also on the ratio between Ta 2 N and TaN.

Thus, by choosing an appropriate nitrogen partial pressure, the amount of Ta 2 N and TaN in the film is controlled, and then the resistivity of the layer is raised to the desired value by the heat treatment.

The resistivity of the layer thus represents a parameter representative of the amounts of TaN and Ta 2 in the layer, which are difficult to determine with sufficient accuracy by current methods of analysis.

By way of example however, the content of tantalum in the cermet, measured in the manner described above, is generally in the range of to 50% for resistivities in the range from 400 microohm.cm to 1000 microohm.cm.

To obtain thin layers of Ta-TaN with the desired characteristics it is convenient firstly to prepare a series of thin layers using ni- -7trogen partial pressures between 0.135 Pa and 1.35 Pa, then to carry out the heat treatment, and then to measure the resistivities, in order to obtain a calibration curve showing how resistivity varies as a function of the nitrogen partial pressure. On the basis of this curve, the conditions for obtaining the desired resistivity can be determined.

By way of example, Figure 2 is a micrograph which shows the structure pertaining to a thin layer of 550 microohm.cm obtained with the method of the invention, under the following conditions: a) deposition discharge current: 2.8 A potential VC: 440 V argon partial pressure: 600 Pa nitrogen partial pressure: 200 Pa oxygen partial pressure: 5 Pa deposition time: 3 minutes 50 seconds b) heat treatment in air: 400 0 C, 100 hours under vacuum: 550 0 C, 1 hour.

The resulting thin layer has a thickness of 0.09 microm and exhibits a resistivity of 550 microohm.cm.

In Figure 3 shows the active part of a transducer in accordance with the invention in a very schematic manner. The transducer comprises a cell 20 made of a material sensitive to pressure, for example of sapphire, more particularly monocrystalline sapphire such as alumina. This cell has an internal cavity 22 in which the pressure has a predetermined value and it is subjected to the pressure P to be measured on its outside surface, as represented by the arrows. It is moreover provided on its outside surface with a bridge circuit of thin strain gauge layers 24, constituting the measuring element. The cell may be disposed for example in a transducer structure such as is illustrated'in Figure 4.

-8- In Figure 4, the pressure transducer mainly comprises the measuring cell 20 disposed in the interior of the body of the transducer, comprising a main body 26 and a nose body 28 which is firmly fixed thereto. The cell 20 is disposed in the interior of an internal cavity 30 provided inside the main body 26. Connecting means formed by connecting cables 32 and feed-through connectors 34 extend through the main body 26 to enter the internal cavity 30 round the cell The main function of the nose body 28 of the pressure transducer is to place the measuring cell in communication with the pressure P of the surrounding fluid. To this end the nose body 28 has an axial passage 36 opening at one end to the exterior and at the other end into the internal cavity 30 in which the measuring cell is disposed. Moreover, in order to protect the measuring cell, a membrane 38 is located between the main body 26 and the nose body 28 in their junction plane.

The membrane 38 has sufficient elasticity to transmit the pressure of the external medium to the interior of the internal cavity 30, where the measuring cell 20 is located. The volume of the cavity delimited by the membrane 38 is completely filled with oil, said oil being introduced during the assembly of the pressure transducer.

Figures 5, 6, and 7 illustrate the effect of the resistivity (in microohm.cm) of the thin layers on the gauge factor y, the temperature coefficient of resistance (TCR), and the first day drift in a pressure transducer such as that described above. The layers were obtained under conditions like those of the cell of Figure 2 while varying the partial pressure of nitrogen.

In Figure 5 it is noted that the gauge factor increases with the resistivity of the thin layer but then starts to fall when the resistivity reaches 10,000 microohm.cm.

In Figure 6 it is seen that the temperature coefficient of resistance (in ppm/°C) is very low and practically equal to zero in the range of resistivity from 5x102 microohm.cm to 103 microohm.cm. On the other hand it increases very rapidly above 103 microohm.cm.

-9- In Figure 7, which shows measurement drift during the first day (in kPa) as a function of the resistivity of the thin layer in microohm.cm, for the case of an applied pressure of 105 MPa and a temperature of 150°C, it is noted that the drift is very low from 600 microohm.cm and is acceptable from 450 microohm.cm.

The results of Figures 5, 6 and 7 show that the appropriate resistivity range is 400 microohm.cm to 1000 microohm.cm since, under these conditions, the first day drift is low (28 kPa at the most) and the temperature coefficient of resistance is in the region of zero.

Moreover the gauge factor is high in this resistivity range.

The drift of the transducer (in kPa) is shown in Figure 8 as a function of time (in hours) when it is subjected to a pressure of 7MPa (10 kpsi) at a temperature of 150°C. In this figure, curve I relates to a thin layer of Ta-TaN before heat treatment and curve II relates to the same layer after it has been subjected to heat treatment. In the light of this figure, it is concluded that drift is much lower in the case of the heat-treated layer. Thus the stability of the layer is clearly improved by the heat treatment.

*ft

Claims

1. A thin strain gauge layer, comprising a cermet based on tantalum and tantalum nitride comprising TaN and having 400 microohm.cm and 1000 microohm.cm.

2. A thin layer according to claim 1, having a microohm.cm to 600 microohm.cm.

3. A thin layer according to claim 1, having a order of 550 microohm.cm.

4. A thin layer according to claim 1, having a micron to 0.5 micron. a resistivity between resistivity of 500 resistivIty in the thickness of 0.03 *I C. S C. A thin layer according to any one of claims 1 to 4, wherein the tantalum nitride of the cermet is constituted almost solely of Tat.

6. A thin layer according to any one of claims 1 to 5, wherein the tantalum nitride TaN has a 6 structure.

7. A method of manufacturing a thin strain gauge layer according to any one of claims 1 to 6, comprising the following steps: a) forming a thin layer of tantalum containing a tantalum nitride phase on an insulating substrate by reactive deposition from a tanta- lum target in an atmosphere formed by a mixture of an inert gas with nitrogen, and possibly oxygen, and b) subjecting the thin layer thus obtained to heat treatment at a temperature in the range 300 0 C to 600 0 C.

8. A method according to claim 7, wherein the heat treatment b) is effected in two stages, of whirh the first stage is in air and the second stage is under vacuum.

9. A pressure transducer comprising at least one thin strain gauge layer disposed on a cell subjected to the pressure to be measured, 11 said layer being formed by a cermet based on tantalum and tantalum nitride comprising TaN, a-d having an electrical resistivity of 400 microohm.cm to 1000 microohm.cm. A transducer according to claim 9, wherein the tantalum nitride TaN has a 8 structure.

11. A transducer according to claim 9 or 10, wherein the tantalum nitride of the cermet is formed almost solely of TaN. S

12. A transducer according to any of claims 9 to 11, is made of sapphire.

13. A transducer according to any one of claims 9 to thin strain gauge layer is 0.03 micron to 0.5 micron wherein the cell 12, wherein the thick. 9* .5 S. 9. DATED THIS 24TH DAY OF JULY 1992 SCHLUMBERGER TECHNOLOGY B.V. By its Patent Attorneys: GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia .5 9. S. A BS T RAC T The invention relates to a thin strain gauge layer of a cermet based on tantalum and tantalum nitride, its method of manufacture and its use in a pressure transducer. This layer is formed by a cermet based on tantalum and tantalum nitride comprising T'aN, and it exhibits resistivity of 400 microohm.cm to 1000 microohm.cm.