GB2025692A - Method of producing a piezo- resistance pressure sensor - Google Patents

Abstract

A piezo-resistance pressure sensor, comprising a body part 1 and a diaphragm part 11, both of silicon (part 1 being formed with a cavity and an aperture for the admission of a medium the pressure of which is to be measured and the diaphragm being formed with different conductivity-type diffusion regions), is produced by applying glass powder 10 to the lapped faces of part 1 and connecting the diaphragm thereto by melting the glass powder at a temperature below the diffusion temperature of the silicon. The diaphragm is reduced to the required thickness mechanically or chemically from its surface opposite that in which the diffused regions are formed. <IMAGE>

Description

SPECIFICATION Method of producing a piezo-resistance pressure sensor This invention relates to a method of producing a pressuresensorworking with a piezoelectric or piezo-resistance effect, each sensor comprising at least two silicon parts connected to one another, one part being a pressure-sensitive, piezoelectric diaphragm of monocrystalline silicon with diffusion regions of different types of conductivity and with metal contacts, and a part to which it is connected being a semiconductor body provided with a cavity, preferably of polycrystalline silicon.

Such pressure sensors are used as pressure transducers in the measurement and regulating arts.

Methods of producing semiconductor pressure sensors have already been proposed, wherein these are composed of a plurality of silicon parts - see German Patent applications P 27 53378.4 and P 27 53 273.6. In the first of these earlier proposals the parts with an electronic function consist of monocrystalline silicon and the other parts of polycrystalline silicon. In the second of these earlier proposals, all parts consist of monocrystalline silicone. In both cases, the joining together of the parts is effected by thermomigration. Aluminium, which migrates into silicon, is used for the connection.

These metallic connections are more stable than the previously known connections by means of metal alloys - see German Journal "Regelungstechnische Praxis" 1977, No. 6. pages 162-165. Reference should also be made to this literary source with regard to the basic construction and mode of operation of the pressure sensors. It is possible, however, that a metallic connection will not withstand alternating temperature stress over long periods of time. Since it is necessary to go down to diaphragm thicknesses of a few micro-meters, according to the pressure range for which the sensors are intended, the requirements regarding the accuracy of the thickness, the reducibility and the planeness ofthe diaphragm are high in the production of the diaphragm. With very small thicknesses, the brittel silicon easily breaks.

In accordance with this invention there is provided a method of producing a pressure sensor working with a piezo-electric or piezo-resistance effect, the sensor comprising at least two silicon parts connected to one another, one part comprising a pressure-sensitive, piezo-electric or piezo-resistance diaphragm of monocrystalline silicon with diffusion regions of different types of conductivity and with metal contacts and another part comprising a semiconductor body provided with a cavity, said method comprising the step of applying glass powder to the lapped end faces of the semiconductor body intended to be connected to the diaphragm and then connecting the semiconductor body to the diaphragm, which is reduced to the required diaphragm thickness mechanically or chemically from its back surface after diffusion at its front, by melting the glass at a temperature below the diffusion temperature of the silicon.

By an embodiment of this method to be described herein, pressure sensors resultwhich are reproducible in their technical values and stable for a long time.

In orderto remove bubbles from the glass, the glass powder is preferably melted onto the end faces during a first melting process before the application of the diaphragm. Then the connection of the diaphragm to the semiconductor body is effected in a second melting process.

Furthermore, a glass may advantageously be used with an annealing point between 570"K and 680"K, a melting point below 11 000K and with a coefficient of heat expansion substantially corresponding to that of the silicon.

The use of a zinc-boron-silicate composite glass, the coefficient of heat expansion of which comes particularly close to that of silicon, is particularly advantageous.

The use of glass to connect the semiconductor body to the diaphragm and particularly the use of the last-mentioned glass, leads to a component wherein the loading of the connection point by mechanical stresses, particularly caused by alternating temperature stressing, is largely avoided.

The starting point is preferably a silicon disc which is suitable for the simultaneous diffusion of a plurality of diaphragms, which is covered with lacquer at its front after the diffusion, then reduced to the required diaphragm thickness by etching and or lapping from its back, and is finally divided up.

Using this method, it is an advantage to start with a silicon disc having an initial thickness of 150 to 200 lim, that is to say a usual working thickness in the semiconductor art, without any particular risk of breakage. Nevertheless, it is possible to achieve diaphragms with thicknesses of a few micrometers and to adjustthe thickness precisely. Production starting form a large silicon disc, a so-called wafer, is known from the above-mentioned journal.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 and 2 are respectively diagrammatic section and end views of a sensor made by a prior art method; Figure 3 is a diagrammatic section through a semiconductor body, which is used as the starting point in a method in accordance with this invention; Figure 4 is a similar section through a finished pressure sensor having the semiconductor body of Figure 3; Figure 5 is an enlarged edge view showing a detail of production of a diaphragm in accordance with this invention; and Figure 6 shows a large silicon disc from which a plurality of diaphragms is produced.

Figures 1 and 2 show diagrammatically the former production of a pressure sensor. Such a pressure sensor consists of a silicon body which is composed of a semiconductor body 1 and a part 2. An opening 3 is provided in the semiconductor body 1 for connection to a medium of which the pressure is to be measured, for example a liquid. The liquid pressure arches a diaphragm section 4 of the part 2, and a piezo-electric or piezo-resistance effect results from this deformation. The thickness of the diaphragm may amount to 10 cm to 1 mm. The connection of the semiconductor body 1 to the part 2 is effected, according to the earlier proposals already described, bythermomigration of aluminium. It is further known to solder the semiconductor body 1 to the part 2 by means of a gold alloy.In all cases, there is a metallic connection which is designated by the reference numeral 5. The silicon is of n-typè conductivity and contains thin meandering doping sections which are highly doped top-type conductivity.

Furthermore, corresponding metal contacts or takeoff electrodes 6 are provided on this diaphragm. The doping is applied in the lateral direction and in the vertical direction. The doping 7, as illustrated in Figure 2, is carried out by the usual diffusion technology with oxide masks. The cavities 8a and 8b in the parts 1 and 2 (Figure 1) are produced by a mechanical method or by an erosion process. At the end, an etching process may be effected to improve the surface quality of the inner surface of the two parts. The adjustment of the diaphragm thickness is effected in the above-mentioned known case by a micro-spark-erosion.

With the method according to the invention, the starting point is likewise the semiconductor body 1 with the opening 3, but the end faces 9 of the semiconductor body 1 are lapped (polished). A glass powder 10 is applied to these end faces 9 and melted on. The glass powder 10 should have a melting point which is, if possible, below 1100 K, preferably below 1073 K, so that diffusion is not involved as well. The glass powder 10 is applied to a thickness of about 10 lim and bubbles escape from this layer of glass 10 during the melting-on process. Following this, the fully diffused semiconductor tablet provided with metal contacts (not illustrated) or diaphragm 11, which is already in the required thickness of, for example a few Fa to 1 mm depending on the pressure range, is melted on.

After the melting on the diaphragm 11, the holding point ofthe glass 10 is ulitized in particular. Glasses which have an annealing point between 570"K and 680"K are favourable. In the selection of the glass, particular care should be taken to ensure that the coefficient of heat expansion corresponds substantially to that of the silicon. A zinc-boron-silicate composite glass of Messrs. Schott und Gen., Mainz, is suitable for example. This glass has a component which compensates the glass in the coefficient of expansion so that it corresponds to that of silicon.

Finally, Figure 5 indicates how a diaphragm 11 can be produced. Its doping lines are indicated at 12.

Experience has shown that such diffusions can be carried out on plane-lapped and strongly etched silicon in a thickness of about 150 to 200 cm satisfactorily. With less thicknesses, breakages occur very easily in production. For the method in accordance with the invention, therefore, the starting point comprises silicon discs 13 of the usual thickness of 150 to 200 ism. These discs 13 are then covered with a lacquer 15 at the side 14 of the metallizing or diffusion (here simply called the front although the orientation may vary). The "back" 16 of the silicon disc 13 is removed to the required diaphragm thickness by means of an etching process or a lapping process coupied with subsequent etching.

For the homogeneous removal by etching an etching treatment is recommended during which the disc 13 is moved.

Finally, Figure 6 shows the silicon disc 13 from which a plurality of active parts or diaphragms 141 for the pressure sensors can be produced by the diffusion and etching or lapping process. The diameter of such a silicon disc 13 may be between s to 10 mm for example.

Claims (8)

1. A method for producing a pressure sensor working with a piezo-electric or piezo-resistance effect, the sensor comprising at least two silicon parts connected to one another, one part comprising a pressure-sensitive, piezo-electric or piezoresistance diaphragm of monocrystalline silicon with diffusion regions of different types of conductivity and with metal contacts and another part comprising a semiconductor body provided with a cavity, said method comprising the step of applying glass powder to the lapped end faces of the semiconductor body intended to be connected to the diaphragm and then connected the semiconductor body to the diaphragm, which is reduced to the required diaphragm thickness mechanically or chemically from its back surface after diffusion at its front, by melting the glass at a temperature below the diffusion temperature of the silicon.

2. A method as claimed in claim 1, in which, before the application of the diaphragm to said end faces of the semiconductor body, the glass powder is melted onto said end faces and, during a second melting process, the semiconductor body is connected to the diaphragm.

3. A method as claimed in claim 1 or 2, in which a glass is used with an annealing point between 570"K and 680"K, a melting point below 11 000K and with a coefficient of heat expansion substantially corresponding to that of the silicon.

4. A method as claimed in claim 3, in which the glass used is a zinc-boron-silicate composite glass.

5. A method as claimed in any preceding claim, in which a silicon disc is formed by simultaneous diffusion into a plurality of diaphragms, is then covered with lacquer after the diffusion at its front, then reduced to the required diaphragm thickness by etching and/or lapping from its back and finally is divided up into the individual diaphragms.

6. A method as claimed in claim 5, in which said silicon disc has an initial thickness of 150 to 200 clam.

7. A method of producing a pressure sensor, said method being as claimed in claim 1 and substantially as herein described.

8. A pressure sensor produced by a method as claimed in any preceding claim.