GB2109640A - Waveguide construction - Google Patents

Abstract

One method currently being used to construct millimetric waveguides involves the screen printing onto the appropriate surface of a thick film of dielectric in the required pattern. However, certain difficulties have arisen; the special pasty film-forming compositions that have had to be developed to allow guides to be formed by a screen printing process make it almost impossible to form a printed image of the required high resolution and fine detail, primarily because as the screen is removed so the pastiness of the composition causes it to be dragged off the dielectric surface, so destroying the fine detail. The present invention provides an alternative mode of construction of dielectric waveguides, wherein a correspondingly-shaped and dimensioned channel (36) is formed in a substrate (34, 35) which either is or is then rendered conductive, a suitable dielectric paste or frit (37) is placed into the channel so as to fill it, and "fixed" into position, and the "open" surface of the fixed dielectric (37) is then smoothed off, and optionally closed with a conductive cover (38), to form the desired waveguide. <IMAGE>

Description

SPECIFICATION Waveguide construction This invention concernes waveguide construction, and relates in particular to methods for the construction of integrated circuit components in the form of millimetric waveguides.

While DC and low, medium and high and even ultra high frequency AC electrical signals may be transmitted from point to point around a circuit along conductive wires, those high frequencies resulting in "microwave" signals, specifically those having frequencies measured in Gigahertz (thousands of millions of cycles per second), can most conveniently be sent around a circuit using waveguides. The nature of a waveguide depends somewhat upon the frequency of the signal; waveguides suitable for the transmission of the lower frequency microwave signals without substantial loss tend to be elongate hollow metal tubes, often of rectangular cross-section, while waveguides more suitable for the higher frequency (millimetric wave) signals, albeit with some loss, tend to be elongate dielectric rods or slabs, again often of rectangular cross-section.

The present invention is concerned with a form of waveguide that is in a sense a combination of the two, and is especially suited for the combination of the two, and is especially suited for the transmission of millimetric wave signals (having frequencies of the order of 60 GHz and more, and wavelengths of the order of five millimeters and less)-and relates in particular to a method for the construction of such waveguides as parts of integrated circuits.

Over the last few years there has been increasing research into, and increasing development and production of, millimetric wave components and systems, and substantial efforts have been directed towards producing relatively low cost integrated circuit for operation at Gigahertz frequencies. These seek to provide compact rugged and reasonably low cost components and subsystems particularly for applications where relatively large numbers of small units are required, and it is generally considered that the use of dielectric image guides (where the dielectric strip is mounted directly on a metal layer ground plane carried by a suitable circuit board/substrate) and insular guides (where the dieletric strip is mounted on a dielectric layer which is itself mounted directly upon the metal layer ground plane carried by the circuit board/substrate) are the best option for use in integrated circuits at frequencies where conventional microstrip losses are unacceptably high. However both image guides and insular guides suffer considerable signal loss at corners and junctions (that is, where the guide turns a corner, or where it joins another guide), and it would be of great benefit if there could be used a waveguide construction which did not result in such losses.

There are also other problems. One method currently being used to construct millimetric waveguides involves the screen printing onto the appropriate surface of a thick film of dielectric in the required pattern. However, certain difficulties have arisen. The printed guides must be relatively thick (typically about 0.5mm, though the exact value depends upon the dielectric constant and the intended signal frequency) and have an aspect ratio (height: width) approaching 1; special pasty film-forming compositions have had to be developed to allow them to be formed by a screen printing process.Unfortunately, the use of such compositions in a screen printing process makes it almost impossible to form a printed image of the required high resolution and fine detail, primarily because as the screen is removed so the pastiness of the composition causes it to be dragged off the dielectric surface, so destroying the fine detaii.Indeed, the best results obtainable are waveguides with about +5% tolerance on width and 12% tolerance on thickness, and with "acceptable" guide cross-section, but when couplers (comprising close, parallel, waveguide sections) are required, laser or other machining is necessary to produce the gap between guides, since the closest printable guide separation using the available printing methods is of the order of the waveguide width (thus, a millimeter or so, the actual value again being dependent upon the dielectric constant and the intended signal frequency). Holes through the waveguide to the ground plane (to provide through-connections for semiconductor devices) must also be provided by laser or mechanical machining.Some problems with voids in the dielectric, also probably resulting from the screen printing technique and the thick patterns required, still occur.

It will be seen that the present waveguide construction is not entirely satisfactory, and it would be useful to have an alternative construction that deals with the problems of signal loss, narrow gaps, through-holes, voids, dimensional accuracy and fidelity of crosssection. It is an object of the present invention to provide such an alternative construction of dielectric waveguide, wherein a correspondinglyshaped and dimensioned channel is formed in a substrate which either is or is then rendered conductive, a suitable dielectric paste or frit is placed into the channel so as to fill it, and "fixed" into position, and the "open" surface of the fixed dielectric is then smoothed off, and optionally closed with a conductive cover, to form the desired waveguide.

In one aspect, therefore, this invention provides a a method of constructing a waveguide of the type constituted by a dielectric-filled channel or tube of electrically-conductive material, in which a) an open channel having a shape and size corresponding to the desired waveguide is formed out of a suitable substrate material, and if necessary the inner wails of the channel are provided with an electrically-conductive linging; b) the thus-formed channel is then filled with a suitable dielectric composition; and c) the open surface of the dielectric filling is where necessary appropriately shaped and smoothed off, and optionally provided with an electrically-conductive layer cover, so as to form the desired waveguide.

In the first stage of the method according to the invention there is formed from a suitable substrate material an open channel of a shape and size corresponding to that of the desired waveguide. The channel is open in the way that the letter U is open, and in that the shape and size of the waveguide may be any of those used, or suggested for use, for microwave guides, so the channel shape and size may be any appropriate.

Generally, however, the channel is such that it is of uniform, rectangular cross-section, with an aspect ratio approaching unity and an absolute width of about 1 mm.

The substrate material employed to form the channel may be any conductive or nonconductive material which is suitable for use in the chosen forming process (about which more is said hereinafter). Where the material is conductive then the channel walls can themselves act as the waveguide reflector boundary, while where the material is non-conductive it is necessary, after forming the channel, to provide its walls with a layer of conductive material which can act as the waveguide reflector boundary. In fact, even when the substrate material is conductive it may often be desirable to provide the channel walls with a layer of another conductive material which is more suitable either as a reflector or from the point of view of its mechanical/thermal properties.

The material used for the substrate depends primarily on the process chosen to form the channel. Several processes are useful in the method of the invention, including scribing (cutting and scraping the substrate surface into the required shape), spark machining (in which sparks between a suitably shaped tool are used to varpourise, and erode away, the substrate) and anodic dissolution (preferential electrolytic dissolution of a metal surface using an appropriately shaped cathode in immediate proximity thereto), amongst which electroforming, pressing, moulding/casting and etching deserve especial mention.

Electroforming This process starts with a precision machined and polished tool in the form of a negative of the desired waveguide (i.e., the eventual channel is a ridge on the tool) made of a conductive material (or of an insulating material with a conductive surface layer).

The tool is made the cathode in an electroplating bath, and a thick layer of metal is plated onto the tool. The tool material is chosen to have poor adhesion properties to the plating (stainless steel is such a material), or alternatively is precoated with a release agent, so that on completion of the plating stage the formed layer can be removed physically intact. The removed layer is the inverse replica of the tool (i.e.. a channelled ground plane) with the same finish and precision as the tool.

A typical conductive material that can be satisfactorily electroformed in this way is copper.

If the resultant electroformed layer is thin and flimsy then it is usually desirable to provide a suitable rear support in order to give it robustness and dimensional stability in subsequent processing. This can conveniently be done by moulding or casting a plastics substance-an epoxy resin, for example-onto the rear surface.

Pressing This process also requires a precision machined and polished negative tool but uses it as a die, pressing it into contact with a plate, sheet or foil of a ductile or otherwise flowable material. For example, it may be pressed into a ductile metal foil sheet, or it may have a thermoplastic plastics sheet vacuum-formed onto it (and thereafter coated "interiorly" with a conductor to provide the required conductive walls).

Suitable metals for use in plate, sheet or foil form in this process are copper, aluminium, and their alloys; suitable thermoplastics for vacuum forming by this process are polyethylene and related polyalkylenes.

Casting Again employing a similar negative tool, but this time as a mould, the substrate material can be cast thereon. Though notionally useable with low melting point metals, this proces is most suited to the cold casting of plastics such as epoxy resins.

The formed channelled substrate is then interiorly provided with a thin film/plated coating of a good conductor.

Etching It is known that relatively deep (i.e., with a substantial depth-to-width ratio) channels can be accurately chemically etched in silicon by the use of so-called anisotropic etches or by ion beam etching. In this fabrication process, the waveguide 'tool' would be a photolithography mask which would in turn define the required etching mask in a photo-resist layer on the surface of the silicon.

The nature of the antisotropic etches will only allow 'bends' made up of discrete facets associated with certain crystallographic directions, but by suitable circuit design this need not be a major problem. On the other hand, the use of silicon as the substrate material allows scope to directly integrate active electronic devices within the waveguide circuit-this could prove to be an extremely important factor.

The details of these two etching techniques are well known per se, and need no comment here.

After etching the channels, the required areas of silicon would, of course, be coated with a conducting film. A suitable material for this film is gold.

In accordance with the inventive method, the thus-formed channel in the substrate (after any necessary subsequent coating with a conductive material) is filled with a suitable dielectric.

Preferably the filling is dimensionally uniform in the propagation direction (except where circuit design requires otherwise) and most conveniently the channel is simply filled completely, so that no part of the depth/width of the channel is left unfilled-there are no voids, and the surface of the dielectric is at least up to the top of the channel-and may even be over-filled (so that the dielectric stands proud therefrom).

The actual physical emplacement of the dielectric substance into the channel may be done in any convenient manner compatible with the materials used both for the dielectric and the channel substrate (and any lining it may have); though this may involve injecting the substance in paste or gel form into the channel, or even depositing it from a solution or suspension, an advantageous way is simply to squee-gee a paste of the substance across the channel-bearing surface with a roller or a scraper blade in a conventional manner, and no more need be said about that here.

The dielectric composition employed may be any suitable such substance-the major proviso being that its use must not result in damage to the channel substrate (or its lining). By and large the dielectric composition can be any of those used, or suggested for use, in millimetric waveguide construction (especially in image guide or insular guide construction). Preferably the composition is such as to "dry", "cure" or otherwise "set" into a firm state, simply in air at ambient temperature, or after low temperature warming in an oven. The composition will generally be a paste comprising a dielectric material-for example, alumina, silica ortitania, or barium titanate--suspended in a thick binder.

Suitable binders of the required general nature are well known by virtue of their use in other applications. Suitable organic binders are plastics such as polyethylene (in melt form), polystyrene and polymethylmethacrylate (both in a solvent such as chloroform) or their monomers, while inorganic binders are certain glasses such as EMCA Overglass Paste 2274 and EMCA Glass 101 (both vitreous binders available from Electro Materials Corp., of America); these binders can be mixed with an amount of an acceptable dielectric, such as 1 micron alumina, sufficient to give the resulting composition the desired dielectric properties (which might require, say, 90% by weight or more of dielectric), together perhaps with a small amount of an appropriate thinner solvent.

The composition may, of course, be such that after the initial "drying" stage it is both sufficiently stable, dimensionally and chemically, and electrically suitable to be used as a waveguide dielectric in that form, but most likely it will not be so, and will need a further "curing" stage. The paste made using EMCA Glass 101, for example, needs to be fired into place, this being done at about 3700C.

Once the channel has been filled with the chosen dielectric composition, and if necessary after the latter has been "fixed" into place by whatever process is appropriate, the open surface of the dielectric filling is as necessary suitably shaped and smoothed off.

It will generally be required that a rectangular cross-section guide be filled with dielectric flush to the top, and in such a case the open surface can simply be lapped down (or otherwise smoothed) until it is level with the main substrate surface. Sometimes, however, it may be required that the dielectric either stand proud of or be recessed below the substrate surface, and that the open face be shaped (profiled) to support particular modes. In each case standard shaping and smoothing techniques are available.

After having its open dielectric surface shaped and smoothed, the waveguide-the channelled substrate (with conductive lining if required) filled with dielectric composition-is ready for use. In order to reduce losses still further, however, it may well be desirable to cap the dielectric with a conductive cover conductively sealed to the channel side walls (or their lining), so as to form a complete tube of conductive material. Though the cover may be applied employing any suitable technique, nevertheless, in order to ensure that throughout its length it is conductively joined to the upper edge of the channel side walls (or their lining) it is preferred to deposit the cover in place using processes such as vacuum evaporation, sputtering, or electroless plating.

The cover material may be any suitable conductive material (provided that it is compatible with the substrate, wall layer and/or dielectric composition, and that it can indeed be provided by the chosen technique). Particularly suitable cover materials are, however, gold, aluminium and copper.

A waveguide prepared according to the invention is a tube or channel of conductive material filled with a dielectric, and as so far described the dielectric is a solid dielectric composition that has been suitably fixed in position. It is possible, however, to employ as waveguides tubes filled not with solid dielectric but with fluid dielectric-specifically, with a gaseous dielectric such as air-resulting in even lower losses. Such a waveguide can be prepared using the method of the invention if, after first filling the channel with an appropraite material, and providing the conductive cover layer, the internal material is removed and replaced by the chosen fluid.Thus, for example, the channel filling could be a plastics substance soluble in an appropriate solvent, and could be simply dissolved away (by immersing the waveguide/substrate combination in a bath of that solvent), air at NTP then being allowed to take its place after the solvent has been emptied out. Such a soluble plastics substance is methyl methacrylate, the appropriate solvent being acetone.

The invention extends, of course, to a waveguide whenever made by a method as described and claimed herein.

The invention is now described, though only by way of illustration, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic part see-through, part sectional representation of an inventive substrate carrying a millimetric waveguide segment thereon; Figure 2 is a diagrammatic sectional view of another inventive substrate; and Figures 3A to F form a sequence of diagrammatic sectional views showing the steps in the production of a substrate carrying millimetric waveguide segments thereon such as is shown in Figure 1 A.

The waveguide illustrated in Figure 1 comprises a ground plane, formed of a thick, solid metal substrate portion (11), bearing on its upper (as viewed) surface (1 2) a rectangular channel (13), the aspect ratio of which is about 0.6, which is filled with a dielectric composition (14) that has been smoothed off flush with the substrate surface 12, then capped with a thin metal layer (15) that makes a conductive join with the surface 1 2 along the entire length of either side of the channel 13.

The column defined by the channel 13 walls and base and by the metal cap 1 5 lower (as viewed) surface is filled with the dielectric composition 14; the whole is a dielectric-filled, rectangular section, conductive tube, and constitutes the waveguide.

Figure 2 shows a different sort of waveguide.

Here there is a thin copper substrate (21) formed so as to have a rectangular-section channel (22) therein, this thin copper substrate being given both a solid backing (23) of plastics materials and (within and extending either side of the channel 22) a gold lining (24), the gold-lined channel 22 then being filled with a dielectric composition (25) the upper (as viewed) surface (26) having an arcuate shape because of surface tension effects during curing (three other possible surface shapes are shown by the dotted lines).

The sequence of Figures 3A and 3F shows the theoretical steps employed in one particular method of making out of copper a waveguide like that of Figure 1. The steps may be described and explained as follows:- Step A There is first manufactured out of a hard metal (such as stainless steel) a die-a tool (31) in the shape of a negative version of the desired waveguide, and thus having a ridge (32) projecting out of a planar substrate (33).

Steps B/C This tool 31 is stamped with considerable force into a relatively soft copper sheet (34); the copper, being soft and malleable, is forced into a negative version of the tool 31 (a "positive" version of the desired waveguide), and thus there is formed a planar substrate (35) having a channel (36) pressed into its surface.

Steps D/E The channel 36 is filled with a dielectric composition (37), and-after any necessary "fixing" procedure-the surface of the filling is smoothed off flush with the surface of the substrate 35.

Step F Finally, a thin metal layer (38) is placed over the filled channel 36/37 with a small overlap (as 39) on each side, so that it makes conductive contact along, and seals, the entire length of each side of the channel.

Claims (14)

Claims

1. A method of constructing a waveguide of the type constituted by a dielectric-filled channel or tube of electrically-conductive material, in which a) an open channel having a shape and size corresponding to the desired waveguide is formed out of a suitable substrate material, and if necessary the inner walls of the channel are provided with an electrically-conductive lining; b) the thus-formed channel is then filled with a suitable dielectric composition; and c) the open surface of the dielectric filling is where necessary appropriately shaped and smoothed off, and optionally provided with an electrically-conductive layer cover, so as to form the desired waveguide.

2. A method as claimed in claim 1, in which the channel is such that it is of uniform, rectangular cross-section, with an aspect ratio approaching unit and an absolute width of about 1 mm.

3. A method as claimed in either of the preceding claims, in which the substrate material employed to form the channel is a non-conductive material, and after forming the channel its walls are provided with a layer of conductive material which acts as the waveguide reflector boundary.

4. A method as claimed in claim 3, in which the nonconductive material is silicon or a plastics material, and the conductive material coating the formed channel walls is gold or copper.

5. A method as claimed in any of the preceding claims, in which the formed channel in the substrate is thereafter filled with the dielectric composition so that the filling is dimensionally uniform in the propagation direction (except where circuit design requires otherwise).

6. A method as claimed in claim 5, in which the channel is filled completely.

7. A method as claimed in any of the preceding claims, in which the actual physical emplacement of the dielectric composition into the formed channel is effected by squee-geeing a paste of the composition across the channel-bearing surface.

8. A method as claimed in any of the preceding claims, in which the dielectric composition employed is such as to "dry", "cure" or otherwise "set" into a firm state, in air at ambient temperature, or after low temperature warming in an oven.

9. A method as claimed in any of the preceding claims, in which the dielectric composition is a paste comprising, as the dielectric material, alumina, silica or titania, or barium titanate, suspended in a thick binder.

10. A method as claimed in claim 9, in which the binder is a plastics material or a glass frit,

11. A method as claimed in claim 10, in which the binder is polyethylene (in melt form), or polystyrene or polymethylmethacrylate (both in a solvent) or their monomers.

12. A method as claimed in any of the preceding claims, in which, once the channel has been filled with the chosen dielectric composition, and if necessary after the latter has been "fixed" into place by whatever process is appropraite, the open surface of the dielectric filling is smoothed off.

13. A method as claimed in any of the preceding claims, in which, after any required smoothing and shaping of the dielectric surface, the channelled substrate filled with dielectric composition is capped with a conductive cover conductively sealed to the channel side walls (or their lining), and in order to ensure that throughout its length it is conductively joined to the upper edge of the channel side walls (or their lining) the cover is deposited in place by a vacuum evaporation, sputtering or electroless plating process.

14. A method as claimed in claim 13, in which the cover material is gold, aluminium or copper.

1 5. A method as claimed in either of claims 13 and 14, in which, after providing the conductive cover layer, the internal material is removed and replaced by air.

1 6. A method as claimed in any of the preceding claims and substantially as described hereinbefore.

1 7. A waveguide whenever made by a method claimed in any of the preceding claims.