RF and Microwave Component Development in LTCC

Low Temperature Co-fired Ceramic (LTCC) technology is a multi-layer ceramic process that can be used to fabricate low cost, high performance RF and microwave components. It is an extremely versatile technology that can be used to realise a wide range of components from simple passive filter structures and packages to complex sub-system assemblies containing discrete SMT (Surface Mount Technology) components, bare die and printed passives. This paper presents an overview of the LTCC fabrication process and details an on-going programme of work aimed at providing the detailed RF characterisation data that designers require in order to fully exploit the benefits of the technology. Measured material characterisation data is presented for a number of different LTCC substrate materials and metallisations. Two different band-pass filter structures, one at 2.4GHz the other at 28GHz, have been developed to assess the accuracy of the characterisation data. Details of the design, layout and measured performance of these filters is also presented. Further development work, underway at the time of writing, is also described and results from this will be presented at the conference. Overview of LTCC Technology LTCC technology is a multi-layer ceramic process. The ceramic layers are tape-cast in their pre-fired “green-state” and the tape is cut to the required size. Registration holes, via holes and cavities are then punched or drilled into the different tape layers. The via holes are normally filled, often with silver, and then thick film processing is used to print metallisation patterns on each, or selected tapes. When thick film processing is used, the minimum line width/gap is around 100μm. If finer line geometries are required a photo-imagable process can be used [1]. The different layers are then inspected, registered and laminated and then co-fired at around 850°C. Post fired processing of the top layer is also an option that is sometimes used. This description of the LTCC process makes it sound similar to conventional multi-layer circuit boards fabricated using laminate materials such as FR4. However, LTCC has a number of advantages: • Lower loss dielectric (lower tanδ) • Better controlled dielectric properties (εr, tanδ and thickness) • It is well suited to producing modules in low-cost SMT packages, including BGA topologies • LTCC processes can produce modules, which are well suited to incorporating bare die. Cavities and integral heat-sinks can be easily realised • Many processes allow the integration of printed passive components (resistors, capacitors and inductors) A 3-dimensional image of an LTCC assembly incorporating a bare die, SMT components and integrated passives is shown in Figure 1.