Fabrication Processes for RF and Microwave Circuits
3.4 Fabrication of Planar High-Frequency Circuits
3.4.2 Thick-Film Circuits (Direct Screen Printed)
Thick-film processing of low-frequency electronic circuits has been well-established over many decades, and it has now also become one of the viable techniques for manufacturing circuits for applications well into the millimetre-wave region.
This technology has a number of advantages over simple etched circuits; it is cheap, suitable for mass production, and most notably offers the potential for producing multilayer structures. Conductors and dielectrics for use in a thick-film process are supplied in the form of thick pastes. The basic thick-film process involves these pastes through a metal screen and then firing the result in furnace. Both conductor and dielectric patterns can be processed in this way, but we will describe the basic steps in terms of a conductor pattern:
(1) The process requires the use of a screen, formed from a mesh of fine steel wires typically with 300 or 400 wires per inch (i.e.∼100 or 130 wires per cm).
(2) The mesh is covered with a photo-sensitive emulsion, and a negative image of the required conductor pattern is devel- oped in the emulsion, leaving clear areas corresponding to the conductor pattern.
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3.4 Fabrication of Planar High-Frequency Circuits 93
Figure 3.4 Magnified view of an etched 30μm gap in a 10 GHz microstrip DC break.
(a) (b)
Frame for wire mesh
Emulsion covering wire mesh
Pattern developed in emulsion
Figure 3.5 Example of a thick-film screen: (a) thick-film wire mesh screen and (b) final circuit printed on alumina substrate.
(3) The conductor, which is in the form of a paste, is then squeezed through the screen onto a ceramic substrate, using a specially designed thick-film printer, so as to form the required conductor pattern.
(4) The substrate, now coated with the wet conductor pattern, is dried at a relatively low temperature (<100∘C) to remove volatile components. It is then passed through a furnace programmed with a particular firing cycle to convert the conductor paste to solid metal. All of the common conductor metals (copper, gold, and silver) can be processed in this way, although the precious metals gold and silver are most common for high-frequency circuits.
The steps described in the previous section are very much an outline, and reference [7] is recommended for those readers requiring more detailed information on thick-film materials and the associated fabrication processes. An example of a typical thick-film screen is shown in Figure 3.5. The screen shown contains the pattern of a 10 GHz microstrip circuit.
Whilst thick-film technology provides a simple, low cost method of manufacturing high-frequency circuits, the need to print patterns though a wire mesh screen has some limitations in terms of the smallest feature size that can be achieved, and also because the edges of the conductors are serrated due to the shape of the apertures in the screen. These limitations are overcome by the thick-film developments described in Section 3.4.3.
k k circuit designer with some useful additional design techniques. Three examples where multilayer techniques can be used
to practical advantage are:
(1) Microstrip lines with a high characteristic impedance are often required in filters and matching networks. High impedance lines require narrow tracks, and producing these lines on typical substrate materials, such as alumina, is limited by practical problems involved in fabricating very narrow lines. Tian et al. [8] showed that by printing a layer of low permittivity dielectric beneath a conductor on an alumina substrate (as shown in Figure 3.6) could significantly increase the characteristic impedance of the line without decreasing its width.
Tian et al. [8] considered an alumina substrate with 𝜖r =9.9 and h=635 μm and showed that the characteristic impedance of a 50μm wide microstrip line increased from 110Ω(without underlay) to 156Ωby including a 100μm thick underlay whose relative permittivity was 3.9.
(2) Many planar components, such as filters, directional couplers, and DC breaks, rely on edge coupling between paral- lel microstrip lines to achieve a particular electrical performance. In some cases the required spacing between two microstrip lines in the same plane can be of the order of 10μm, which can be difficult to fabricate. Using multilayer techniques, edge coupling can be replaced by limited broad-face coupling by simply printing the two conductors on dif- ferent planes separated by a thin dielectric layer. Figure 3.7 shows the layout of a thick-film microstrip DC break where a multilayer approach has been used to interpose a dielectric layer between the two coupled fingers. This structure enables the two fingers to overlap, giving strong partial broad-face coupling.
Tian et al. [9] used both simulation and practical measurement to show that the configuration illustrated in Figure 3.7 was capable of giving very wide bandwidth performance. Their results showed an insertion loss<0.2 dB over a fre- quency range 2.5–10 GHz. An enlarged view of the finger coupling is given in Figure 3.8, showing details of the con- ductor overlap. It should be noted that the top conductor is made slightly wider than the embedded conductor; this is to maintain the correct impedances, since the impedance of the top conductor is affected by the presence of the printed, low impedance layer.
(3) Multilayer technology can also be usefully applied when radiating elements are included on the same package as other high-frequency circuitry, such as in the front-end of a miniature receiver. Normally a high permittivity substrate such as alumina is chosen for microwave circuitry because the substrate wavelength is small and the circuits can be made
h
h1 Dielectric
underlay, εr1
(a) (b)
εr εr
Figure 3.6 Multilayer microstrip line: (a) microstrip and (b) microstrip with underlay.
(a) (b)
s P
P1
P
P1
Additional dielectric layer
Figure 3.7 Layout of a multilayer DC break: (a) single-layer DC break and (b) tick-film multilayer DC break.
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3.4 Fabrication of Planar High-Frequency Circuits 95
εr = 9.9 εr1 = 3.9 Not to scale
380
300 180
[μm]
Figure 3.8 Details of coupling in a multilayer DC break [9].
Buried ground plane Radiating patch
VIA Low permittivity layer
High permittivity layer Planer circuitry and
surface mount components
Figure 3.9 Multilayer package.
very compact. But for radiating elements, such as patch antennas, a low permittivity substrate is desirable, since this enables energy to be radiated more efficiently (see Chapter 12). A multilayer package that satisfies the requirements for both permittivies is illustrated schematically in Figure 3.9.
An alternative to the arrangement shown in Figure 3.9 is to have the antenna also on the high permittivity substrate, and to print low permittivity material only beneath the radiating edges of the antenna. This technique is discussed in more detail in Chapter 12.
The discussion so far has focussed on printing conductors and dielectrics using thick-film. However, the range of materials that can be printed using this technology is far more extensive, and provides the RF and microwave circuit designer with useful opportunities, particularly in reducing unwanted parasitics associated with surface-mount packages. Printing of resistive thick-film pastes is a well-established technology [7] and can avoid the need to include chip resistors for biasing purposes in a high-frequency circuit. Capacitors are very easily created by printing high permittivity dielectric between two flat electrodes. Both printed resistors and capacitors can easily be buried within a multilayer structure. Another useful material for high-frequency applications is Barium Strontium Titanate (BST), which has ferroelectric properties whereby its dielectric constant can be changed by varying the electric field strength across the material. BST is available in the form of a thick-film paste, and so can easily be incorporated into multilayer thick-film circuits. The drawback to BST material is that it exhibits a very high-loss tangent (∼0.01), but this is not a significant problem if only a small amount of the material is used, for example in a thin layer within a multilayer structure. Osman and Free [10] showed that the measured dielectric constant of a thick-film BST sample could be changed by around 15% over the frequency range 1–8 GHz, through the application of an electric field of strength 2.5 V/μm. They subsequently used BST to form a capacitor within a miniature ring-resonator filter, to provide tuning of the filter’s centre frequency.