CA2082021C

CA2082021C - Method of stabilizing the surface properties of objects to be thermally treated in a vacuum

Info

Publication number: CA2082021C
Application number: CA002082021A
Authority: CA
Inventors: Frank Hegner; Ulfert Drewes
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 1991-11-30
Filing date: 1992-11-03
Publication date: 1996-06-25
Anticipated expiration: 2012-11-03
Also published as: DK0544934T3; CA2082021A1; JPH05264384A; DE59108247D1; US5400489A; EP0544934A1; JPH0718768B2; EP0544934B1

Abstract

To avoid any deterioration of the surface properties of objects of ceramic, glass, or a single-crystal insulat-ing material which are subjected to a vacuum temperature process, a thin layer of a spin-on glass solution with a silicon-dioxide equivalent of not more than 10% is applied to the objects by spinning or spraying prior to the vacuum temperature process. This is particularly important to avoid the strong moisture dependence of capac-itive or resistive pressure sensors having a substrate and a diaphragm to be joined together, forming a chamber sealed at least at the edge. The diaphragm is covered with a layer of silicon carbide, niobium, or tantalum serving as one capacitor electrode, or the surface portion of the diaphragm which will lie within the chamber is coated with at least one strain gage; the portion of the substrate surface which will lie within the chamber is coated with at least one additional capacitor elec-trode or, in the case of the resistive pressure sensor, not coated therewith; over the entire surface portion of the substrate and diaphragm thus coated, a thin layer of the spin-on glass solution is applied and dried;

contact is made to the capacitor electrodes or strain gages through the substrate and/or diaphragm, and sub-strate and diaphragm are brazed together by means of a ring-shaped part of active brazing material, which also serves as a spacer, or by means of a sufficient amount of active brazing paste.

Description

Method of Stabilizing the Surface Properties of Objects to be Thermally Treated in a Vacuum BACKGROUND OF THE IN~ENTION

The present invention relates to a method of stabilizing the surface properties of objects which are subjected to a vacuum temperature process.

Such a process step is used, for example, in the manufac-ture ofpressure sensors. US-A-50 50 034, for example, de-scribes the manufacture of a capacitive pressure sensor having a substrate and diaphragm which are to be joined together, particularly in a defined spaced relationship and parallel to each other, forming a chamber sealed at least at the edge, the substrate and/or the diaphragm being made of ceramic, glass or a single-crystal insulat-ing material. This method comprises the following steps.
The diaphragm is covered with a layer of silicon carbide, niobium or tantalum which serves as one of the capacitor electrodes; the portion of the substrate surface which wilL lie within the chamber is covered with at least one additional layer of any of the aforementioned conductive materials which serves as the second etc. capacitor elec-trode; contact is made to the capacitor electrodes through the substrate, and substrate and diaphragm are high-vacuum-brazed together by means of a ring-shaped part of active brazing material, which also serves as a spacer, or by means of an amount of active brazing paste suf-ficient for holding the two parts at the desired distance from each other.

Pressure sensors thus manufactured are extremely moisture-sensitive, which greatly reduces the Q of the capacitor(s) in particular. Investigations have shown that this moisture sensitivity is not due to changes in the electrodes or the strain-gage material during the high-vacuum brazing, but that in this process step the uncovered surface por-tions of substrate and diaphragm change so that they, instead of retaining their very good insulating ability, become semiconducting and highly moisture-sensitive.

For example, an experimental capacitive reference pres-sure sensor with a 60-pF precision capacitor and a 60-pF
reference capacitor exhibited, at zero pressure, a capacitance difference of 1.5 pF at a tan-delta difference of 0.05 and a reference-ca.paci-tance difference of 3 pF at a tan-delta difference of 0.1 for a change in relative humidity from 30% to 85%
(at a temperature of 20C).

Furthermore, prior to the high-vacuum brazin~, an ex-perimental substrate of alumina ceramic wi:th 96% purity on which two concentric coatings.corresponding to said one electrodes of the above precision and reference capaci-tors were deposited at a distance of 1 mm from ea~ch other, with the inner electrode having a di.ameter of 16 mm, showed a resistance, measured between these two electrodes, - 2~82021 of 4X1013 ohms in a dry atmosphere (= 0% relativ;e humidity), but a resistance of 1X1011 ohms at 70% relative humidity (again at 20C).
After the high-vacuum brazjn~, the co,rresponddn~ ~re sistance values were 3X1013 ohms for a dry atmosphere and only 3X108 ohms for 70% relative humidity.

These invesigations led to the recognition that as a re-sult of the high-vacuum brazing, those surface portions of the substrate and diaphragm which will not be covered within the chamber to be formed lose oxygen or nitrogen atoms, i.e., thatthese surface portions are reduced. This re-sults in these portions becoming semiconducting, for example, which causes the above-mentioned degredation of the Q of the capacitors and the changes in capacitance.

Thus, as a rule, vacuum temperature processes do not leave the surface properties of the treated objects un-affected and mostly deteriorate them.

The invention serves to solve this problem.

SUMMARY OF THE INVENTION

Accordingly, the invention consists in a method of stabi lizing the sl~rface properties of ob-jects of ceramic, glass, or a single-crystal insulating material which are subjected to a temperature process, particularly a high-temperature process, in a vacuum, particularly a high vacuum, and to which a thin layer of a spin-on glass solution with a silicon-dioxide equiva-lent of not more than 10% is appl;ed by spinning or spraying, and dried.

2ns202l ~ he invention fur.her consists in a method of manufacturing a capacitive pressure sensor having a substrate and a diaphragm to be joined together, particularly in a defined spaced relationship and parallel to each other, forming a chamber sealed at least at the edge, with the substrate and/or the diaphragm made of ceramic, glass, or a single-crystal insulating material, comprising the following steps:
- The diaphragm is covered with a layer of silicon carb;de, niobium, or tantalum serving as one capa-citor electrode;
- the portion of the substrate surface which will lie within the chamber is covered with at least one additional layer of any of sa;d conducting materials serving as the second etc. capacitor electrode(s);
- over the entire surface portion of the substrate and diaphragm thus covered, a thin layer of a spin-on glass solution with a silicon-dioxide equivalent of not more than 10% is applied ~y spinning or spraying, and dried;
- contact is made to the capacitor electrodes through the substrate and, if necessary, through the dia-phragm,and - substrate and diaphragm are high-vacuum-brazed to-gether by means of a ring-shaped part of active brazing material, which also serves as a spacer, or by means of an amount of active brazing paste sufficient for holding the two parts a~ the de-sired distance from each other.

The invention further consists in a method of manufacturing a resistive pressure sensor having a substrate and a diaphragm to be joined together, ceramic, glass, or single-crystal insulating materials, namely in semiconductor techno;ogy in the fabrication of integrated circuits, and there for a cdifferent purpose, namely for planarizing their surfaces, i.e., for levelling surface irregularities ca~sed by the multiple process steps, cf. an article by S. K. Gupta, "Spin-On Glass for Dielectric Planarization", published in the journal "Microelectror,ic Manufacturing and Testing", April 1989.

Surprisingly, the spin-on glass solutions described in that article can be used to eliminate the above-described disadvantages associated with the materials mentioned above.

The invention will now be explained, by way of example, with reference to the accompanying drawings, in which like parts are designated by like reference characters.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a top view of a capacitive pressure sensor made in accordance with the ;nvention;

Fig. 2 is a section taken along line A-8 of Fig. 1;
.
Fig. 3 is a section, corresponding to that of Fig. Z, of a resistive pressure sensor made in accordance with the invention;
Fig. 4 shows a section of the two parts of the pres-sure sensor of Fig. 2 following the appLica-tion of the spin-on glass solution, and - 2~82021 Fig. 5 shows a secti~n of the two parts of the pres-sure sensor of Fig. 3 following the applica-tion of the spin-on glass soLution.

DETAILED DESCRIPTION OF THE INVENTION

The fundamental idea of the invention will now be described with respect to the manufacture of the pressure sensors shown in the figures, i.e., there is no separate figure f-or the subject matter of claim 1, since such a figure would only show an arbitrary object with a thin layer around it, which is obvious.

The capacitive pressure sensor 10 shown in Figs. 1 and 2 has a diaphragm 11 in the form of a circular plate with plane-parallel surfaces which is joined around the periph-ery to a circular substrate 12 at a defined distance d therefrom, thus forming a chamber 13 between the top side of the substrate 12 and the opposite surface of the dia-phragm 11.

The diaphragm 11 may be formed from ceramic, preferably alumina ceramic with a purity of 96 wt.%, glass, or a single-crystal insulating material, such as sapphire.
The materialsof substrate 12 and diaphragm 11 may differ.

The diaphragm 11 is elastic, so that it can deform when a force or pressure is applied to it. The substrate 12 may be solid or rigid, which can be achieved, for example, by a thickness greater than that of the diaphragm. It may also be designed as a flat, elastic and, hence, de-flectable plate like the diaphragm.

The surfaces of diaphragm 11-and substrate 12 which face each other are provided with circular capacitor elec-trodes 14, 15 of a suitable metal, namely niobium, tantalum or conductive silicon carbide, which lie opposite each other within the chamber 13. The electrode 14 covers the diaphragm 11 completely, but it may also be provided only in the chamber area. Each of the eLectrodes 14, 15 may be covered with a protective layer 21, 22 at its chamber-side, free surface, as is shown in Fig. 2. This protective layer is made, for example, of one of the oxides of the material from which the electrodes are formed. In the case of tantalum, this is preferably tantalum pentoxide.

Connected to the electrodes 14 and 15 are leads 16 and 17, respectively, which are brought out through the sub-strate to its rear side. The electrical connection to the diaphragm electrode 14 is made via the lead 16 and the active brazing material of the joint, but this is not mandatory.

If the diaphragm electrode does not cover the entire dia-phragm surface but is only provided in the region of the chamber 13 as mentioned above, contact must be made to it through the diaphragm 11 in the same manner as in the case of the electrode on the substrate. To this end, contactis made to the leads 16 and 17 by means of inserts 18 and 19, respectively, of active brazing material. In-stead of these inserts, leads covered with active brazing material can be used.

An active brazing solder is a hard solder which contains -_ 9 _ at least one highly reactive element, such as titanium, zirconium, beryllium, hafnium, or tantalum. During the brazing process, these reactive elements wet the sur-faces of the parts to be brazed. In the case of (aluminum-) oxide ceramic, the high affin;ty of the reactive elements for oxygen causes a reaction with the ceramic, which results in the formation of mixed oxides and free chemical valences. The reactive component of the brazing-solder is embedded in a matrix of other alloying elements, such as silver/copper, which form the active brazing material proper.

The two electrodes 14, 15 form a capacitor whose capaci-tance depends on the distance between the electrodes.
when the diaphragm 11 deforms under the action of a force or pressure, the distance between the electrodes changes, thereby changing the capacitance of the pressure sensor.
This change can be measured by means of an electronic circuit to be connected to the leads 16, 17, and can thus be a measure of the pressure or force acting on the diaphragm 11.

The resistive pressure sensor 10' shown in Fig. 3 in a sectional view has a diaphragm 11' in the form of a cir-cular plate with plane-parallel surfaces which is joined around the circumference to a circular substrate 12' in a defined spaced relationship d therefrom, so that a chamber 13' is formed between the top side of the sub-strate 12' and the opposite surface of the diaphragm 11'.
What was said above about the materials suitable for, and the elasticity of, the substrate and diaphragm ofi the capacitive pressure sensor applies equally to the resis-tive pressure sensor.

Attached to one surfaceofthe diaphragm 11' is at least one strain gage 14, e.g., a half-bridge arrangement of two strain gages or a full-bridge arrangement of four strain gages, which is connected to two leads 16', 17' brought out through the diaphragm 11', i.e., to the rear side thereof, in a gas-tight manner.

The resistance of the strain gages is dependent on the deflection of the diaphragm 11' resulting from the action of a force or pressure. This change in resistance can be measured by means of an electronic circuit to be con-nected to the leads 16', 17', and can thus be a measure of the pressure or force acting on the diaphragm 11'.

If the chamber 13, 13' is evacuated, only an external pressure is applied to the capacitive or resistive pressure sensor 10,10'. If the chamber has an external opening, e.g., a hole in the substrate 12, 12', the pressure sensor can be used as a reference-pressure sensor.

The layer of the aforementioned spin-on glass solution deposited in accordance with the invention, which is trans-formed into a cross-linked silicon-dioxide layer by the high-temperature brazing process, is not visible on the finished pressure sensor and, hence, in Figs. 1 to 3.

By contrast, in Fig. 4, the diaphragm 11 and the sub-strate 12 of the capacitive pressure sensor of Figs. 1 and 2 are shown in a condition after a spin-on glass solution with a silicon-dioxide equivalent of not more than 10% was deposited, which can be done by spinning or spraying and results in the spin-on glass layer 23. The 2082~21 latter is quite thin, namely only about 200 nm thick, so that it is not drawn to scale in Fig. 4.

Fig. 5 shows the diaphragm 11' and the substrate 12' of the resistive pressure sensor 10' of Fig. 3 in a similar view, again in the condition after a spin-on glass solu-tion with a silicon-dioxide equivalent of not more than 10% was applied by spinning or spraying, which results in the spin-on glass layer 23'. The latter, too, is only about 200 nm thick, so that it is not drawn to scale in Fig. 5,either.

The diaphragm 11, 11' and the substrate 12, 12' are brazed together in a high vacuum of at least 10 5 hPa (= mbar) better in the range - of 10 6 hPa (= mbar). A very good vacuum is necessary to avoid reactions of the respective brazing metal, particularly titanium, with the residual gas and achieve good wetting. The brazing temperature is advantageously 30C to 100C above the liquidus temperature to achieve an optimum reaction as well as high strength and gas tightness of the joint.

The spin-on glass layer 23, 23' surprisingly seals the uncovered surface portions of diaphragm 11, 11' and sub-strate 12, 12' so perfectly that the above-mentioned re-duction during the brazing process practically no longer occurs.

This is shown by the following results of measurements performed on a capacitive reference-pressure sensor fabricated according to the invention which had the same dimensions as the above-mentioned reference-pressure sensor, and which was measured under the same conditions (temperature 20C, zero pressure).

2082û21 First, as above, only à substrate 12 was examined and coated with the spin-on glass solution. After the coated substrate h~d dried and then been heated to 400 C, a resistance of 1X1013 ohms was measured at 0% relative humidity, and 1x101 ohms at 70% relative humidity. This is already a significant improvement over the above values for an uncoated substrate.

If the coated substrate was heated to approximately 900C, which temperature corresponds to the brazing-temperature range of the pressure sensor, a resistance of Sx10 ohms was measured at 70% relative humidity; the value for 0% relative humidity was unchanged.

This considerably improved resistance also has an effect on the characteristics of a capacitive pressure sensor:
Between 30% and 85% relative humidity, the capacitance difference was only 0.2 pF at a tangent-delta difference of 0.005, and the reference-capacitance difference was 0.4 pF at a tangent-delta difference of O . 0 1 .

In the case of the pressure sensors described above, care must be taken in selecting the composition of the spin-on glass soLution with the aid of manufacturer's data to ensure that after the brazing process, no hydrocarbon bonds, such as in siloxanes, will occur in the cross-linked silicon dioxide, i.e., that pure silicate, phospho-silicate, or the like is present, because otherw;se the humidity sensitivity will not be sufficiently reduced.

Claims

1. A method of stabilizing the surface properties of ob-jects of ceramic, glass, or a single-crystal insulating material which are subjected to a temperature process, particularly a high-temperature process, in a vacuum, particularly a high vacuum, and to which a thin layer of a spin-on glass solution with a silicon-dioxide equiva-lent of not more than 10% is applied by spinning or spraying, and dried.

2. A method of manufacturing a capacitive pressure sen-sor having a substrate and a diaphragm to be joined to-gether,particularly in a defined spaced relationship and parallel to each other, forming a chamber sealed at least at the edge, with the substrate and/or the diaphragm made of ceramic, glass, or a single-crystal insulating material, comprising the following steps:
- The diaphragm is covered with a layer of silicon carbide, niobium, or tantalum serving as one capa-citor electrode;
- the portion of the substrate surface which will lie within the chamber is covered with at least one additional layer of any of said conducting materials serving as the second etc. capacitor electrode(s);
- over the entire surface portion of the substrate and diaphragm thus covered, a thin layer of a spin-on glass solution with a silicon-dioxide equivalent of not more than 10% is applied by spinning or spraying, and dried;
- contact is made to the capacitor electrodes through the substrate and, if necessary, through the dia-phragm, and - substrate and diaphragm are high-vacuum-brazed to-gether by means of a ring-shaped part of active brazing material, which also serves as a spacer, or by means of an amount of active brazing paste sufficient for holding the two parts at the de-sired distance from each other.

3. A method of manufacturing a resistive pressure sensor having a substrate and a diaphragm to be joined together, particularly in a defined spaced relationship and paral-lel to each other, forming a chamber sealed at least at the edge, with the substrate and/or the diaphragm made of ceramic, glass or a single-crystal insulating material, comprising the following steps:
- The surface portion of the diaphragm which will lie within the chamber is covered with at least one strain gage;
- over the entire surface portion of the diaphragm thus covered and over the entire surface portion of the substrate which will lie opposite said sur-face portion of the diaphragm, a thin layer of a spin-on glass solution with a silicon-dioxide equiva-lent of not more than 10% is applied by spinning or spraying, and dried;
- contact is made to the at least one strain gage through the diaphragm, and _ substrate and diaphragm are high-vacuum-brazed to-gether by means of a ring-shaped part of active brazing material, which also serves as a spacer, or by means of an amount of active brazing paste sufficient for holding the two parts at the de-sired distance from each other.

4. A method as claimed in claim 2 wherein prior to the application of the spin-on glass solution, the capa-citor electrodes (14, 15) are covered with a protective layer (21).