Capacitive pressure transducer and method of making same

• US 4,426,673 A
• Filed: 03/13/1980
• Issued: 01/17/1984
• Est. Priority Date: 03/12/1976
• Status: Expired due to Term

• Current Assignee: Kavlico Corporation (Sensata Technologies Holding Plc)
• Original Assignee: Kavlico Corporation (Sensata Technologies Holding Plc)
• Inventors: Bell, Robert L., Kavli, Fred, Willing, Robert
• Primary Examiner(s): Kellogg, Arthur D.
• Application Number:
  US06/131,417
• Time in Patent Office: 1,405 Days
• Field of Search: 65/42, 65/43, 65/59.21, 29/25-41, 73/718, 73/724, 361/283
• US Class Current: 361/283.4
• CPC Class Codes: G01L 9/0075 using a ceramic diaphragm, e.g. alumina, fused quartz, glass
• CPC Class Codes: G01L 9/0086 using variations in capacitance
• CPC Class Codes: Y10T 29/43 Electric condenser making

Abstract

A capacitive pressure transducer comprising a pair of disc shaped members having planar surfaces made from an insulator material, one of the members being several times thinner than the other plate and flexible and constituting a diaphragm and the other thicker plate constituting a stationary plate. A thin conductive film is formed on the surface of each of the members to form the plates of the capacitor. A glass frit is applied on the marginal edge of each member and when the members are held in adjacent relationship, the assembly is fired to seal the two members together while spacing them a predetermined distance apart so that the two conductive plates are opposite each other and are separated by an open gap of a predetermined distance, the two conductive plates being insulated one from the other. Leads are electrically connected to the conductive plate of each disc through the fused glass frit. When pressure is applied to the member, the diaphragm member is displaced thereby changing the capacitance of the pressure transducer.

Claims

1. A method for making a capacitive pressure transducer comprising:

preparing two members of substantially zero hysteresis nonconducting material with flat matching opposed surfaces, at least one of said members being thin enough to be flexible;

applying an electrically conductive layer onto each of two nonconductive members in a predetermined pattern, comprising a central portion defining a nonconductive cut out portion and an outer portion substantially surrounding said central portion and spaced away from said central portion;

firing the two nonconductive members;

applying glass frit about the marginal perimeter of at least one of said members;

disposing one member onto the other with the fired conductive layers facing each other, with the glass frit forming the only spacing between the flat matching surfaces of said members; and

firing the members thereby bonding the two members together such that said conductive layers are opposite each other, and separated by a gap produced only by said glass frit, insulated one from the other, and sealed around the periphery of said members by said glass frit.

2. A method as defined in claim 1 including the additional steps of mounting an integrated circuit onto one of said members and coupling said opposed conductive plates to said integrated circuit.

3. An economical, high production method for making precision capacitive pressure transducers comprising the steps of:

preparing two members of substantially zero hysteresis nonconducting material with flat matching opposed surfaces, and the opposing surfaces of each member lying substantially in a single plane, and at least one of said members being thin enough to be flexible;

screening an electrically conductive layer onto said flat surface of each of said two nonconductive members in a predetermined pattern for each member, with a main central area and outwardly extending conductive pathways for external electrical connections;

firing the two nonconductive members to bond the conductive layers to the nonconductive members;

applying a thin layer of glass frit around the main central conductive area onto said flat surface of at least one of said nonconducting members with said outwardly extending conductive pathways extending beyond said frit;

disposing one member onto the other with the fired conductive layers facing each other, and with the glass frit forming the only spacing between the planar opposing surfaces of said members;

firing the members, thereby bonding the two members together such that said conductive layers are opposite each other, and separated by a gap less than 0.010 inch in thickness produced only by said glass frit engaging both of said planar opposed surfaces over an outwardly extending distance substantially greater than the thickness of said gap, insulated one from the other, and substantially sealed around the main conductive areas on said members by said glass frit with said outwardly extending conductive pathways extending through the seal provided by the fired glass frit; and

making electrical connections to said outwardly extending pathways, outside of said glass frit to permit detection of changes in the capacitance between said conductive layers as the pressure applied to said transducer varies.

4. A method as defined in claim 3 including the step of making the outwardly extending conductive pathways thicker than the central conductive area to insure the retention of their conductivity despite contact with the glass frit during firing.

5. A method as defined in claim 3 wherein said glass frit includes a high and a lower melting point glass powder and wherein the firing of said glass frit is performed at a temperature between said two melting points, whereby the spacing of said plates is determined by thickness of the non-melting frit.

6. An economical, high production method for making precision capacitive pressure transducers comprising the steps of:

preparing two plates of substantially zero hysteresis nonconducting material with flat matching opposed surfaces, and the opposing surfaces of each member lying substantially in a single plane, at least one of said plates being thin enough to be flexible;

screening an electrically conductive layer onto each of said two plates in a predetermined pattern for each member, with a main central area and outwardly extending conductive pathways for external electrical connections;

firing the two plates to bond the conductive layers to the plates;

applying a thin layer of glass frit around the main central conductive area on at least one of said nonconducting plates with said outwardly extending conductive pathways extending beyond said frit;

disposing one member onto the other spaced apart by a distance of about one-half to several thousandths of an inch, with the fired conductive layers facing each other, and with the glass frit extending between the planar opposing surfaces of said members;

firing the members, thereby bonding the two members together such that said conductive layers are opposite each other, and separated by a gap less than 0.010 inch in thickness;

insulated one from the other, and substantially sealed around the main conductive areas on said members by said glass frit engaging both of said planar opposed surfaces over an outwardly extending distance substantially greater than the thickness of said gap, with said outwardly extending conductive pathways extending through the seal provided by the fired glass frit; and

making electrical connections to said outwardly extending pathways, outside of said glass frit, to permit detection of changes in the capacitance between said conductive layers, as the pressure applied to said transduer varies.

7. An economical, high production method for making precision capacitive pressure transducers comprising the steps of:

preparing two members of substantially zero hysteresis nonconducting material with flat matching opposed surfaces, and the opposing surfaces of each member lying substantially in a single plane, at least one of said members being thin enough to be flexible;

applying electrically conductive layer onto each of said two nonconductive members in a predetermined pattern for each member, with a main central area and outwardly extending conductive pathways for external electrical connections;

applying a thin layer of ceramic sealing material around the main central conductive area on at least one of said nonconducting members with said outwardly extending conductive pathways extending beyond said ceramic sealing material, said ceramic sealing material having a coefficient of thermal expansion substantially equal to that of said nonconductive members,

disposing one member onto the other with the conductive layers facing each other, and with the ceramic sealing material extending between the planar opposing surfaces of said members;

firing the members, thereby bonding the two members together such that said conductive layers are opposite each other, separated by a gap less than 0.010 in thickness produced only by said ceramic sealing material, insulated one from the other, and sealed around the main conductive areas on said members by said ceramic sealing material engaging both of said planar opposed surfaces over an outwardly extending distance substantially greater than the thickness of said gap, with said outwardly extending conductive pathways extending through the seal provided by the fired ceramic sealing material; and

making electrical connections to said outwardly extending pathways, outside of said ceramic sealing material to permit detection of changes in the capacitance between said conductive layers, as the pressure applied to said transducer varies.

8. An economical, high production method for making precision capacitive pressure transducers comprising the steps of:

preparing two members of substantially zero hysteresis nonconducting material with flat matching opposed surfaces, and the opposing surfaces of each member lying substantially in a single plane, and at least one of said members being thin enough to be flexible;

screening an electrically conductive layer onto said flat surface of each of said two nonconductive members in a predetermined pattern for each member, with a main central area for forming a capacitor plate;

firing the two nonconductive members to bond the conductive layers to the nonconductive members;

applying a thin layer of glass frit around the main central conductive area onto said flat surface of at least one of said nonconducting members;

disposing one member onto the other with the fired conductive layers facing each other, and with the glass frit extending between the planar opposing surfaces of said members;

firing the members, thereby bonding the two members together, such that said conductive layers are opposite each other, separated by a gap less than 0.010 inch in thickness, insulated one from the other, and sealed around the main conductive areas on said members by said glass frit engaging both of said planar opposed surfaces over an outwardly extending distance substantially greater than the thickness of said gap; and

making electrical connections to each of said conductive layers, to permit detection of changes in the capacitance between said conductive layers as the pressure applied to said transducer varies.

9. A method as set forth in claim 8 including the step of applying glass frit which has a temperature coefficient of expansion which is substantially equal to that of said nonconducting members, thereby minimizing mechanical stresses during subsequent heating and cooling steps.

10. A method as defined in claim 9 wherein said glass frit is screened onto the periphery of at least one of the opposed surfaces of one of said members.

11. A method as defined in claim 9 wherein two coats of frit are applied to surfaces of said members at the periphery thereof.

12. A method as defined in claim 8 including the step of preparing said two nonconductive members as thin flexible diaphragms with substantially parallel top and bottom surfaces.

13. A method as defined in claim 8 including the step of locating elements of material between said flat opposed surfaces of said nonconducting members, said elements of material having a melting point above that of at least a portion of said frit, to space said members apart by a predetermined distance.

14. A method as defined in claim 13 wherein said elements of material are high melting point particles of glass powder.

15. A method for making a capacitive pressure transducer as claimed in claim 8 where the step of firing the members thereby bonding the two members together is followed by an additional step of securing a laser-trimmed integrated circuit to the outer surface of one of said nonconductive members.

16. A method as defined in claim 8 including the additional steps of depositing an integrated circuit directly onto one of said members and coupling said opposed conductive plates to said circuit.

17. A method for making a capacitive pressure transducer comprising:

preparing two members of substantially zero hysteresis nonconducting material with planar matching opposed surfaces, at least one of said members being thin enough to be flexible;

applying an electrically conductive layer onto each of the two nonconductive members in a predetermined pattern to form the plates of the capacitive transducer;

applying glass frit about the marginal perimeter of at least one of said members;

disposing one member onto the other with the conductive layers facing each other with the glass frit between the flat matching surfaces of said members and the members and their respective associated conductive layers spaced a predetermined distance apart; and

firing the members thereby bonding the two members together such that said conductive layers are opposite each other, and separated by a gap having said predetermined distance less than 0.010 inch, insulated by a gap having said predetermined distance, insulated one from the other, and sealed around the periphery of said members by said glass frit engaging both of said planar opposed surfaces over an outwardly extending distance substantially greater than the thickness of said gap.

18. A method for making a capacitive pressure transducer comprising:

preparing two members of substantially zero hysteresis nonconducting material with flat matching opposed surfaces, at least one of said members being thin enough to be flexible;

screening an electrically conductive layer onto each of two nonconductive members in a predetermined pattern, comprising a central portion defining a nonconductive cut-out portion and an outer portion substantially surrounding said central portion and spaced away from said central portion;

applying glass frit having a coefficient of expansion substantially the same as the two members about the marginal perimeter of at least one of at least one of said members;

disposing one member onto the other with the conductive layers facing each other, with the glass frit forming the only spacing between the flat matching surfaces of said members to form a predetermined spacing; and

firing the members thereby bonding the two members together such that said conductive layers are opposite each other, and separated by a gap produced only by said glass frit, insulated one from the other, and sealed around the periphery of said members by said glass frit.

19. A method for fabricating a capacitive pressure sensor comprising the steps of:

providing two glass plates, at least one of said glass plates being flexible enough to be capable of responding to a change in pressure, and each having a substantially plane surface;

providing conductive electrodes on said substantially plane surfaces;

applying a sealing glass material to at least one of said two glass plates;

positioning said two glass plates to place said conductive electrodes in a substantially parallel relationship having a space therebetween;

evacuating the space between said spaced apart electrodes;

heating said glass plates to cause said sealing glass material to flow and to seal said glass plates together, said glass plates and said sealing glass material bounding an evacuated volume.

20. A capacitive sensor for measuring ambient pressure comprising:

two glass pieces, at least one of said glass pieces being capable of flexing in response to a change in pressure, each having a substantially plane surface with conductive electrodes thereon, said glass pieces being positioned to place said conductive electrodes in a spaced apart, substantially parallel relationship;

a sealing glass mixture disposed on one of said pieces to seal together said glass pieces, said mixture and said glass pieces acting together to bound a volume having a reference pressure therein whereby changes in ambient pressure cause a change in the capacitance measured between said conductive electrodes.

21. The capacitive sensor of claim 20 wherein said two glass pieces comprise a substantially rigid base piece and a thinner, flexible diaphragm piece.

22. The capacitive sensor of claim 20 wherein said reference pressure is a substantial vacuum.