Patent Description:
Electronic devices such as optoelectronic devices are frequently formed in assemblies which contain components which are arranged in multilayer structures. There is therefore a need for structures comprising angled transitions to connect electronic components located in layers of differing heights. For example, surface mounted devices are typically powered via electrical connections which pass through a package body to connect radiofrequency transmission layers disposed on opposing faces of the package body. Commonly, the electrical connection may be orthogonal to the radiofrequency transmission layers, resulting in a pair of angled transitions of <NUM>°.

Such angled transitions are known to be problematic for high performance radiofrequency (RF) devices. In particular, angled transitions comprising interconnections to a circuit or radiofrequency transmission layer housed within a package may require careful impedance matching to ensure high RF performance.

Several techniques and materials are known in the art for forming structures with angled transitions. Ceramic materials may be used as a supporting structure for the radiofrequency transmission layers, in which case electrical connections can be located in holes through the ceramic material. Alternatively, <CIT> describes conductors bent within a plastic material to realise a transition between internal and external conducting layers in an optoelectronic device. The bending of the conductors avoids inductance parasitic effects which may degrade the radiofrequency performance.

A further approach for forming angled transitions uses a coaxial glass bead mounted within the package body. The coaxial glass bead comprises an insulating glass cylinder with a central conducting pin which may be connected to a radiofrequency transmission layer by soldering. This approach has the advantage that a metal package body can be used and frequently results in good radiofrequency performance. Unfortunately, the approach can suffer from problems related to tolerances and process control, particularly when multiple circuits must be interconnected simultaneously.

<CIT> describes a coupling device for electrically connecting a strip transmission line to another transmission line. <CIT> describes a solderless right-angle RF interconnect. <CIT> describes a coaxial connector that interfaces a launch end of the connector with a planar circuit mounted in a microcircuit package. <CIT> discloses a plug in package for microwave integrated circuits.

The skilled person would therefore understand the desirability of an improved angled transition that is suitable for use in high performance RF devices.

It is an object of the present invention to provide an angled radiofrequency transition which addresses, or at least alleviates, the problems described above.

Aspects of the present disclosure are defined by the appended claims. Also provided herein is an RF transition assembly for enabling a radiofrequency transition between an RF transmission layer of an electronic device and a conductor which is electrically connected to the RF transmission layer. The conductor extends generally orthogonal to the RF transmission layer. The assembly comprises an open coaxial structure located adjacent to an edge of the RF transmission layer. The open coaxial structure comprises a cavity extending therethrough for receiving the conductor. The cavity comprises an opening facing the edge of the RF transmission layer so as to direct electromagnetic radiation towards the RF transmission layer. Optionally, at least a portion of the open coaxial structure extends in the plane of the RF transmission layer.

The cavity of the open coaxial structure guides the electromagnetic field generated by radiofrequency structure across the angled transition such that losses through microwave emission can be reduced. Electrical losses at the interfaces of the conducting elements are also reduced by the guidance effect, thereby improving RF transmission through the angled transition.

The guidance of the electromagnetic field provided by the cavity of the open coaxial structure can be varied by altering the size and shape of the cavity. For example, the cavity may have a cross section corresponding to a circular segment and the angle enclosed by the cavity may be greater than <NUM>°. As another example, an angle of between about <NUM>°and <NUM>°, optionally about <NUM>°, may be used.

The open coaxial structure may further comprise one or more ground interconnections between the open coaxial structure and one or more grounding regions associated with the RF transmission layer.

The cavity of the open coaxial structure may extend beyond an end of the conductor adjacent to the RF transmission layer to improve the guidance of the electromagnetic field. For example, the cavity of the open coaxial structure may extend beyond the end of the conductor by a length of at least the radius of the cavity.

The open coaxial structure may be provided with one or more stepped grounding regions adjacent to the conducting layer which are approximately coplanar with one or more grounding regions of the conducting layer. Grounded interconnections may then be made between the one or more stepped grounding regions and one or more grounding regions of the conducting layer.

The RF transition may be used in a package for an electronic device, including for example, an electro-optical modulator.

Also provided herein is an electronic device comprising an RF substrate mounted on a face of a package body. An RF transmission layer is mounted on the RF substrate such that the RF substrate forms a layer between the RF transmission layer and the package body. A conductor is electrically connected to the radiofrequency transmission layer and extends through the package body in a direction generally orthogonal to the RF transmission layer. An open coaxial structure is mounted on the face of the package body adjacent to an edge of the RF transmission layer. The open coaxial structure comprises a cavity extending therethrough for receiving the conductor. The cavity comprises an opening facing the edge of the radiofrequency transmission layer so as to direct electromagnetic radiation towards the radiofrequency transmission layer.

Further provided herein is an RF transition assembly for enabling radiofrequency transitions between a stacked arrangement of RF transmission layers and a conductor. The conductor is electrically connected to each RF transmission layer and extends orthogonally to the stacked arrangement of RF transmission layers. An open coaxial structure is located adjacent to the stacked arrangement of RF transmission layers and comprises a cavity extending therethrough for receiving the conductor. The cavity comprises one or more openings facing the edges of the RF transmission layers so as to direct electromagnetic radiation towards each of the RF transmission layers.

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:.

<FIG> is a perspective view of a <NUM>° radiofrequency transition <NUM> known in the art in which an upper RF transmission layer <NUM>, such as a transmission line or RF circuit, mounted in or on an RF substrate <NUM> is interconnected to a coaxial glass bead (not shown in <FIG>). The RF substrate <NUM> is disposed on the upper surface of a package body <NUM>, whilst an external RF transmission layer <NUM>, such as a flexible circuit, is located on the lower surface of the package body <NUM>. In order to allow transmission of an RF signal from the external RF transmission layer <NUM> to the upper RF transmission layer <NUM>, the external RF transmission layer <NUM> and the RF circuit <NUM> are electrically connected by a pin or central conductor <NUM> of the coaxial glass bead which passes vertically through the RF substrate <NUM> and the package body <NUM>, and into the external RF transmission layer <NUM>. The upper RF transmission layer101 is connected to the central conductor <NUM> by a solder interconnection <NUM> to form the RF transition with an angle of <NUM>°.

It will be appreciated that "upper" and "lower" are relative terms, used with respect to the package body <NUM> as illustrated in the drawings. In use, the device or apparatus may be oriented in any direction.

<FIG> is a section view of the <NUM>° RF transition <NUM> shown in <FIG>. The coaxial glass bead <NUM> has a cylindrical body formed of an insulating material and provided with a central bore through which the central conductor <NUM> is securely mounted. The coaxial glass bead <NUM> is itself fixed within a void of the package body <NUM> located between the RF substrate <NUM> and the external RF transmission layer <NUM>. There may be additional voids <NUM>, <NUM>, in the package body <NUM> located above and/or below the body of the coaxial glass bead <NUM> which also serve to insulate the central conductor <NUM> from the package body <NUM>. The central conductor <NUM> is electrically connected to the RF circuit by a solder interconnection <NUM>.

<FIG> shows a perspective view of an exemplary RF transition <NUM>. In this arrangement, an open coaxial structure <NUM> is mounted on an upper face of a package body <NUM> adjacent to an RF transmission layer <NUM> mounted on an RF substrate <NUM>, which is itself mounted on the upper face of the package body <NUM>. In this example the open coaxial structure <NUM> is shown as a layer formed on the upper face of the package body <NUM>. The upper end of a central conductor <NUM> of a coaxial glass bead (not shown in <FIG>) is also located adjacent to the RF transmission layer <NUM> and accommodated within a cavity <NUM> formed in the open coaxial structure <NUM>. The cavity <NUM> of the open coaxial structure <NUM> comprises an opening facing the RF transmission layer <NUM> and may have a cross section corresponding to a circular segment. For example, the cavity <NUM> may be a cylindrical segment formed by truncating a cylinder in a plane parallel to the axis of the cylinder. The central conductor <NUM> of the coaxial glass bead and the RF transmission layer <NUM> may be interconnected by a bonding interconnection <NUM>, such as a ribbon or wire bonding interconnection, or an alternative interconnection technique may be used. In the example shown here, the central conductor <NUM>, the RF transmission layer <NUM> and the open coaxial structure <NUM> extend to a similar height above the upper surface of the package body <NUM>.

The cavity <NUM> of the open coaxial structure <NUM> guides the electromagnetic field across the <NUM>° angle from the central conductor <NUM> of the coaxial glass bead towards the RF transmission layer <NUM>, thereby improving the transition feature by reducing RF electrical losses, and leading to improvements in circuit (and device) electrical performance. The angled transition is not limited to a <NUM>° transition and the present invention may also be used for angled transitions of greater than or less than <NUM>°.

The open coaxial structure may be formed from a conductive material or from a dielectric or ceramic which is plated with a conductive material. The open coaxial structure may also be integrated with a conducting package body.

The cavity <NUM> of the open coaxial structure <NUM> may enclose an angle of <NUM>°, as would be the case for a semi-cylindrical cavity, or the cavity <NUM> may enclose an angle that is greater than <NUM>° and less than <NUM>°.

The cavity <NUM> may enclose an angle greater than <NUM>° as it has been found that this is particularly beneficial in guiding the electromagnetic field. The optimal size or shape of the cavity <NUM> or the optimal angle enclosed by the cavity <NUM> may depend on the diameter of the central conductor <NUM>, the material from which the open coaxial structure <NUM> is formed and any RF performance requirement, such a particular impedance or required transmission frequency. An angle of about <NUM>° has been found to be particularly suitable in forming a <NUM> Ohm RF transition, for example.

<FIG> illustrates a modification to the arrangement of <FIG> in which the open coaxial structure <NUM> is electrically grounded to one or more grounding regions <NUM> located adjacent to the RF transmission layer <NUM> on the upper surface of the substrate <NUM>. The electrical grounding may be provided by one or more ground interconnections <NUM> extending between a grounding region <NUM> of the RF substrate <NUM> and the open coaxial structure <NUM>. In the arrangement shown here, as in <FIG>, the cavity encloses more than <NUM>°, and this has the further beneficial effect of improving the ground reporting between the open coaxial structure <NUM> and the grounding regions <NUM> by minimising the length of the ground interconnections <NUM>.

<FIG> illustrates a modification to the arrangement of <FIG> in which the open coaxial structure <NUM> is separated from the RF substrate <NUM> by a gap <NUM>. In this example, the cavity <NUM> of the open coaxial structure <NUM> encloses an angle of <NUM>° and the grounded interconnections <NUM> are wire interconnections.

<FIG> shows an alternative arrangement for an RF transition <NUM>, in which the open coaxial structure <NUM> extends in a direction away from the upper surface of the package body <NUM> such that the open coaxial structure <NUM> extends above the upper end of the central conductor <NUM>. In this arrangement, the end of the central conductor <NUM> and the RF transmission layer <NUM> are a similar height above the upper surface of the package body <NUM>. The extended height of the open coaxial structure <NUM> with respect to the central conductor <NUM> produces an additional guidance effect for the electromagnetic field associated with the RF transition. The height h of the open coaxial structure <NUM> with respect to the central conductor may be optimised to improve the performance of the RF transition such that the height h of the open coaxial structure <NUM> is approximately equal to or greater than the radius r of the cavity <NUM> formed in the open coaxial structure <NUM>. Additional height beyond about h=<NUM>. 1r has relatively small additional benefit.

<FIG> illustrates a further modification of the arrangement of <FIG>, in which the upper surface of the open coaxial structure <NUM> comprises one or more stepped grounding regions <NUM> adjacent to the substrate <NUM> which are lower in height than the remainder of the open coaxial structure <NUM>. Preferably, the one or more stepped grounding regions <NUM> of the open coaxial structure <NUM> should be located adjacent to a grounding region <NUM> of the RF substrate <NUM> to minimise the length of the grounding interconnections <NUM> and facilitate ground reporting.

Although the various RF transition assemblies have been exemplified using a coaxial glass bead, alternative conducting structures known in the art can also be used.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It would be apparent to one skilled in the relevant art(s) that various changes in form and detail could be made.

Claim 1:
An RF transition assembly (<NUM>, <NUM>) for enabling a radiofrequency transition between an RF transmission layer (<NUM>) of an electronic device and a conductor (<NUM>) extending orthogonally to the RF transmission layer (<NUM>) and being electrically connected to the RF transmission layer (<NUM>), the assembly comprising:
an open coaxial structure (<NUM>, <NUM>, <NUM>) located adjacent to an edge of the RF transmission layer (<NUM>) and comprising a cavity (<NUM>, <NUM>, <NUM>) extending therethrough for receiving the conductor (<NUM>),
the cavity (<NUM>, <NUM>, <NUM>) comprising an opening facing the edge of the RF transmission layer (<NUM>) so as to direct electromagnetic radiation towards the RF transmission layer (<NUM>), and characterised by
a height of the open coaxial structure (<NUM>, <NUM>, <NUM>), with respect to the conductor (<NUM>), being:
equal to or greater than a radius of the cavity (<NUM>, <NUM>, <NUM>), and
less than or equal to <NUM> times the radius of the cavity (<NUM>, <NUM>, <NUM>).