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New ferrite technology improves integrated circuits for microwaves
Microelectronics and Reliability
Pergamon Press 1970. Vol. 9, p. 434.
Printed in Great Britain
New Ferrite Technology Improves Integrated Circuits for Microwaves U SING ferrite substrates which are not magnetic but containmagnetically active domains, Philips Zentrallaboratorium in Hamburg can now incorporate magnetic components such as circulators and phase-shifters in planar integrated microwave circuits. The microwave properties of this new substrate material are comparable with those of the hitherto widely used aluminium oxide (alumina). The improvement in microwave applications of semiconductor devices has in recent years led to a fast development in miniaturized microwave systems. The passive components required for this are, in a planar construction, made in the form of microstrip lines by means of photo-etching. Many microwave circuits as used for radar and for microwave transmission links in telecommunications, depend for their operation on certain magnetic components, which are usually based on the magnetic oxide material ferrite. Up to now the integration of such ferrite components has come up against a variety of problems. Attempts have been made, for example, to apply a microwave circuit of this type in integrated form on a substrate consisting entirely of magnetic ferrite, so that the magnetic material is available where it is locally needed. At the places where no magnetic function is needed, however, the operation of such a circuit is unnecessarily poor owing to the magnetic high-frequency losses in the substrate. If, in order to reduce these losses, a substrate is used consisting entirely of alumina, it becomes difficult to apply magnetically active ferrite components on or inside the substrate at the required places. Both materials are not easily workable, and their different coefficients of expansion make them difficult to join. Dr. Holst and Dr. W. Tolksdorf of this laboratory have found a solution to these problems by using a substrate consisting of non-magnetic ferrite material. Every ferromagnetic material has a characteristic temperature, called the Curie temperature, above which the ferromagnetic properties are absent. Using a material of
Relative dielectric constant Total loss factor Quality factor of a resonator Attenuation over one wavelength
appropriate composition the investigators mentioned were able to produce a ferrite substrate with a Curie temperature far below room temperature, so that it behaves like a conventional electrical insulator when used as the substrate of an integrated circuit. Before the substrate is sintered, holes can easily be punched into it and these are filled, where necessary, with islands of magnetically active ferrite. After sintering, a flat substrate is obtained upon which the rest of the circuit can be deposited by vacuum evaporation, etching, etc., by the normal techniques of integrated circuitry. Some important properties of this new non-magnetic substrate, consisting of a ferrite in spine1 form, are given in the table below and compared with those of a magnetic ferrite (in garnet form) and those of the conventional aluminium oxide. The figure for the attenuation over the distance of one wavelength relates to a microwave transmission line, a socalled microstrip line, applied to the relevant substrate. Compared with the aluminium oxide the ferrite substrates have only one drawback-their thermal conductivity is lower. This is not, however, of much importance in practice. The fact that this does not present problems even in circuits with significant dissipation appears from the following experiment. A microstrip circulator, based entirely on a garnet-type ferrite, was operated at 10 GHz (wavelength 3 cm) with a dissipation of 10 W. The temperature rise was no more than 30°C which is a permissible value for practically all applications. In the same laboratory the new technique has been tried out with success by M. Lemke in widely different experimental microwave circuits, as, for example, in a Doppler radar and in X-band phase shifters (wavelength approx. 3 cm). Large-scale production of the new substrates is not yet envisaged. H. C. M. E D E L M A N Philips Research Laboratories Eindhoven, Netherlands
Nonmagnetic ferrite
Magnetic ferrite
11.2 3.5 x10-4 255 0.10 dB
15.8 1 x 10-s 266 0.11 dB
434
Aluminium oxide 9.7 4 x 10-d 258 0.11 dB
FIG. 1. Photomicrograph of the boundary between a magnetic island and the nonmagnetic ferrite material of the substrate, after sintering.
FIG. 2. A 4-bit version of an X-band phase shifter, seen from above. Wires can be led through the four holes to exert a local influence on the magnetic ferrite material.
Pergamon Press 1970. Vol. 9, p. 434.
Printed in Great Britain
New Ferrite Technology Improves Integrated Circuits for Microwaves U SING ferrite substrates which are not magnetic but containmagnetically active domains, Philips Zentrallaboratorium in Hamburg can now incorporate magnetic components such as circulators and phase-shifters in planar integrated microwave circuits. The microwave properties of this new substrate material are comparable with those of the hitherto widely used aluminium oxide (alumina). The improvement in microwave applications of semiconductor devices has in recent years led to a fast development in miniaturized microwave systems. The passive components required for this are, in a planar construction, made in the form of microstrip lines by means of photo-etching. Many microwave circuits as used for radar and for microwave transmission links in telecommunications, depend for their operation on certain magnetic components, which are usually based on the magnetic oxide material ferrite. Up to now the integration of such ferrite components has come up against a variety of problems. Attempts have been made, for example, to apply a microwave circuit of this type in integrated form on a substrate consisting entirely of magnetic ferrite, so that the magnetic material is available where it is locally needed. At the places where no magnetic function is needed, however, the operation of such a circuit is unnecessarily poor owing to the magnetic high-frequency losses in the substrate. If, in order to reduce these losses, a substrate is used consisting entirely of alumina, it becomes difficult to apply magnetically active ferrite components on or inside the substrate at the required places. Both materials are not easily workable, and their different coefficients of expansion make them difficult to join. Dr. Holst and Dr. W. Tolksdorf of this laboratory have found a solution to these problems by using a substrate consisting of non-magnetic ferrite material. Every ferromagnetic material has a characteristic temperature, called the Curie temperature, above which the ferromagnetic properties are absent. Using a material of
Relative dielectric constant Total loss factor Quality factor of a resonator Attenuation over one wavelength
appropriate composition the investigators mentioned were able to produce a ferrite substrate with a Curie temperature far below room temperature, so that it behaves like a conventional electrical insulator when used as the substrate of an integrated circuit. Before the substrate is sintered, holes can easily be punched into it and these are filled, where necessary, with islands of magnetically active ferrite. After sintering, a flat substrate is obtained upon which the rest of the circuit can be deposited by vacuum evaporation, etching, etc., by the normal techniques of integrated circuitry. Some important properties of this new non-magnetic substrate, consisting of a ferrite in spine1 form, are given in the table below and compared with those of a magnetic ferrite (in garnet form) and those of the conventional aluminium oxide. The figure for the attenuation over the distance of one wavelength relates to a microwave transmission line, a socalled microstrip line, applied to the relevant substrate. Compared with the aluminium oxide the ferrite substrates have only one drawback-their thermal conductivity is lower. This is not, however, of much importance in practice. The fact that this does not present problems even in circuits with significant dissipation appears from the following experiment. A microstrip circulator, based entirely on a garnet-type ferrite, was operated at 10 GHz (wavelength 3 cm) with a dissipation of 10 W. The temperature rise was no more than 30°C which is a permissible value for practically all applications. In the same laboratory the new technique has been tried out with success by M. Lemke in widely different experimental microwave circuits, as, for example, in a Doppler radar and in X-band phase shifters (wavelength approx. 3 cm). Large-scale production of the new substrates is not yet envisaged. H. C. M. E D E L M A N Philips Research Laboratories Eindhoven, Netherlands
Nonmagnetic ferrite
Magnetic ferrite
11.2 3.5 x10-4 255 0.10 dB
15.8 1 x 10-s 266 0.11 dB
434
Aluminium oxide 9.7 4 x 10-d 258 0.11 dB
FIG. 1. Photomicrograph of the boundary between a magnetic island and the nonmagnetic ferrite material of the substrate, after sintering.
FIG. 2. A 4-bit version of an X-band phase shifter, seen from above. Wires can be led through the four holes to exert a local influence on the magnetic ferrite material.