GB2054958A

GB2054958A - Substrate for a microwave integrated circuit

Info

Publication number: GB2054958A
Application number: GB8022414A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1979-07-23
Filing date: 1980-07-09
Publication date: 1981-02-18

Abstract

A substrate for a microwave integrated circuit comprising a self- supporting electrically conductive plate with a relatively thin layer of dielectric material adhering to a main face thereof. Typically the plate consists of molybdenum and the dielectric material is borosilicate glass. In use, the plate serves as circuit earth plane conductor.

Description

SPECIFICATION Microwave integrated circuits This invention relates to microwave integrated circuits.

The invention relates particularly to substrates for use in such circuits.

For circuits operating at lower frequencies, e.g.

below 20 GHz, such substrates conventionally comprise a wafer of dielectric material, such as alumina ceramic, having an earth plane conductor deposited on one main face, the microwave circuit component and interconnections being carried on the other main face. For higher frequencies, e.g. in the range 20 GHz to 100 GHz, such a substrate is still satisfactory although dielectric materials of low permittivity must be used for the wafer in order that the wafer may be sufficiently thick and therefore strong to be handled without breakage and support the earth plane conductor, components and interconnections. However, at a frequency of 100 GHz, even when using a dielectric material of low permittivity the required wafer thickness is typically only 0.1 millimetre, and at higher frequencies progressively thinner wafers must be used which are very difficult to handle without breakage.

It is an object of the present invention to provide a substrate for a microwave integrated circuit whereby this difficulty is overcome.

According to the present invention a substrate for a microwave integrated circuit comprises a self-supporting electrically conductive plate with a relatively thin layer of dielectric material adhering to a main face thereof.

It will be appreciated that in use the plate serves as the circuit earth plane conductor.

In a preferred arrangement in accordance with the invention said dielectric layer comprises a fused glass layer.

The dielectric layer is suitably formed by covering said face of the plate with a layer of the dielectric material in particulate form and heating.

The plate and layer preferably consist of materials having closely matched thermal expansion characteristics, One substrate in accordance with the invention and its manufacture will now be described by way of example.

The substrate comprises a molybdenum plate on one main face of which there is an adherent relatively thin borosilicate glass layer.

The substrate is formed by covering the relevant face of the plate with a borosilicate glass frit and heating. The fused glass layer so formed is then ground and polished to the required thickness.

The required microwave circuit can be fabricated on the substrate in conventional manner.

In one particular substrate for use in fabricating an integrated circuit for operation at 100 GHz the plate has a thickness of 1 millimetre and the glass layer a thickness of 0.1 millimetre.

It might be thought that since the dielectric loss of suitable glasses such as borosilicate glass is higher than that of alumina the dielectric loss in the substrate might be excessive. However, as operating frequency is increased, other factors being constant, the fraction of the total loss attributable to dielectric loss falls. Thus, for a circuit element which is scaled according to the operating wavelength, and assuming constant effective conductivity, permittivity and loss tangent, the conductor loss increases as the square root of frequency, whereas the dielectric loss contribution is constant. Thus the dielectric loss obtained with glass at high frequencies e.g.

above 100 GHz is in fact quite tolerable.

Claims

1. A substrate for a microwave integrated circuit comprising a self-supporting electrically conductive plate with a relatively thin layer of dielectric material adhering to a main face thereof.

2. A substrate according to Claim 1 wherein said dielectric layer comprises a fused glass layer.

3. A substrate according to Claim 2 wherein said plate consists of molybdenum and said dielectric material is borosilicate glass.

4. A substrate according to Claim 2 or Claim 3 wherein said plate has a thickness of 1 millimetre and said glass layer has a thickness of 0.1 millimetre.

5. A substrate for a microwave integrated circuit substantially as hereinbefore described by way of example.

6. A microwave integrated circuit incorporating a substrate according to any one of the preceding claims, the plate serving as the circuit earth plane conductor and the microwave circuit components and interconnections being carried on the face of said thin layer remote from the plate.

7. A method of making a substrate according to any one of Claims 1 to 5 comprising covering a main face of a plate of electrically conductive material with a layer of dielectric material in particulate form and heating so as to cause the particles of dielectric material to fuse.

8. A method according to Claim 7 wherein the fused layer is ground and polished to the required thickness.

9. A method of making a substrate for a microwave integrated circuit substantially as hereinbefore described by way of example.