Fabrication Processes for RF and Microwave Circuits
3.4 Fabrication of Planar High-Frequency Circuits
3.4.3 Thick Film Circuits (Using Photoimageable Materials)
k k
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.
k k Spray
developer (e.g. Na2Ca3)
Ceramic substrate
layer
UV Mask
Furnace
A ceramic substrate is coated with the photoimageable material, which is then levelled and dried.
The dried layer of photoimageable material is exposed to UV through a photographic glass mask.
The circuit is sprayed with developer to remove unwanted material.
The circuit is fired in a furnace for a pre-determined time.
Figure 3.10 Basic photoimageable thick-film fabrication process.
The process of making thick-film circuits using photoimageable materials is outlined in Figure 3.10, and involves four basic steps:
(i) A layer of photoimageable material (conductor or dielectric) is printed so as to cover the surface of a supporting sub- strate. The printed layer is left a short while to level, and is then dried.
(ii) The photoimageable layer is then exposed to UV through a suitable (negative) mask.
(iii) The areas of the layer that have been developed through exposure to UV are washed away, normally using a special unit that rotates the substrate while applying the developer through a fine spray.
(iv) The circuit with the desired pattern of conductor or dielectric is fired in a furnace to achieve the final thick-film pattern.
Steps 1–4 can be successively repeated to build up a multilayer structure of conductor and dielectric material. If desired, dielectrics having different permittivities can be included. Also, materials forming passive components can be used, as previously described.
One particularly useful feature of the photoimageable process is the ability to easily create vertical interconnections (VIAs), by simply imaging holes in the dielectric layers and subsequently filling them with metal through a suitable mask.
The VIAs can be used as simple electrical connections between conductors in different levels, or as thermal VIAs to conduct excess heat from within the multilayer package. Thermal VIAs are particularly useful for removing heat from high-power components such as power amplifiers, which tend to be rather inefficient at high frequencies.
Whilst photoimageable thick-film technology can be used to fabricate a wide range of variety of electronic circuits, one particular feature that has attracted a lot of attention is the ability to fabricate surface integrated waveguides (SIWs). This is a waveguide that is integrated within the printed dielectric layers, with the top and bottom broad faces of the waveguide formed from printed layers of conductor, and the vertical side walls formed from VIA fences. A VIA fence is simply a closely spaced linear array of conducting VIAs. If the spacing between adjacent VIAs is small compared to the wavelength in the dielectric, the VIA fence will act like a solid conductor. Figure 3.11 illustrates the basic SIW structure.
Figure 3.11a shows the integrated rectangular waveguide with the dimensions identified using the conventional nota- tions,aandb. The required waveguide height is achieved through the use of a number of printed dielectric layers. One such layer is shown in Figure 3.11b. The method of fabricating the SIW is to first print and fire a conducting layer on the supporting substrate to form the bottom broad wall of the waveguide. A layer of photoimageable dielectric is then printed on the ground plane, and photo-imaged with the array of holes forming the two VIA fences. The circuit is then developed and fired, so as to leave a layer of dielectric with an array of holes. The holes are then filled with metal, through a suitable
k k
3.4 Fabrication of Planar High-Frequency Circuits 97
Supporting substrate, e.g.
alumina
a
a
b VIA fence
Multiple layers of printed
dielectric s
(a) (b)
Figure 3.11 Structure of surface integrated waveguide (SIW): (a) cross-section of SIW and (b) one layer of printed dielectric showing filled VIAs.
Coplanar–SIW coupling
Inter-cavity coupling irises
a
Sidewall
VIA fence One of the
printed dielectric
layers Figure 3.12 Schematic view of VIA arrays in a four-cavity SIW filter.
mask, and the circuit fired again. This process of printing and firing the dielectric layers is repeated until the desired waveg- uide height,b, is achieved. Typically, each layer of dielectric is of the order of 10μm thick, and between 10 and 20 firings are required to achieve a suitable waveguide height.
Two particular uses of SIW have attracted a lot of attention in the literature, namely for filters and antennas. Figure 3.12 illustrates one of the printed dielectric layers in a four-cavity filter, and shows how the VIA arrays are arranged. The input and output connections to the SIW filter are normally provided through coplanar coupling probes on the top surface of the structure.
A particularly good introduction to SIW filters is provided through a series of articles in IEEE Microwave Magazine [11–13], which provide a wealth of theoretical and practical data, together with an extensive list of references.
Whilst SIW cavity filters have the advantage of being fully integrated within the ceramic package, they suffer from the disadvantage that the cavities are necessarily filled with dielectric, and the losses associated with the dielectric have detri- mental effects on the insertion loss and roll-off of the filter. One technique that overcomes these losses is to couple the integrated SIW line to a surface mount waveguide (SMW) which contains air-filled metallic cavities that form the filter, as shown in Figure 3.13.
The combination of SIW and SMW is particularly useful at millimetre-wave frequencies where the dielectric losses become very significant and where the wavelength is small enough to make the size of the SMW cavities a realistic proposi- tion. Consequently, at these frequencies the absence of dielectric filling the filter cavities gives them a much higherQ-value, and at the same time the short wavelength makes the cavities relatively small. Schorer et al. [14] compared the performance of SIW filters with those using SMW cavities at K-band. They found significant benefits accrued from using SMW filters, and showed that the insertion loss and roll-off improved by a factor of around 3.7 using SMW cavities that were only a few millimetres high.
The other area in which SIW technology has received significant attention is that of slot antennas. A very compact antenna can be made by cutting slots in the upper surface of an SIW; this type of antenna is discussed in more detail in Section 12.15 of Chapter 12.
k k
SIW SIW
SMW
Supporting substrate Printed dielectric
Figure 3.13 Four-cavity filter using a surface mount waveguide (SMW)