Photoimageable thick-film technology
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Photoimageable thick-film technology is a combination of conventional thick film technology with elements of thin film technology, and it provides a low cost solution to producing high quality microwave circuits. The ability to directly photoimage the printed layers means that the technology can provide the high line and gap resolution required by high frequency planar components.[1][2] It provides a feasible fabrication process to produce circuits operating at microwave and millimetre-wave frequencies. Circuits made using this technology meet the modern requirements for high density packaging, whilst yielding the high quality components required for very high frequency applications, including wireless communication, radar, and measurement systems.
This technology also enables both single-layer and multi-layer filters to be produced conveniently. Research work has investigated the combination of conventional thick film and fine line photoimageable technologies in order to accommodate fine pitch and high density applications on the ceramic substrates.[3] Furthermore, previous work has shown that this technology is capable of realizing the circuit quality necessary for high performance microwave components.[4]
Edge coupled band-pass filters were chosen for this study as they are one of the most common and useful microwave and millimetre-wave planar components. The filter performance is based on the coupling between the resonant sections and controlled by the size of the gap.[5] This characteristic makes edge coupled band-pass filters very sensitive to fabrication errors.
Another reason of choosing this structure in a multi-layer form is due to limitation on the structure when it has been fabricated on a single layer. The gap between the two resonant structures becomes very small and cannot easily be fabricated due to the limitations of low-cost fabrication technologies. In multi-layer circuits, the coupling between resonant sections is achieved by overlapping conductors which are separated by a thin dielectric layer. However, to some extent the problem of fabricating small gaps has been exchanged for that of achieving high alignment between the conductor layers. Normally a modern mask aligner will be needed to achieve the required degree of resolution.