Fabrication of 3D microwave and millimetre-wave components in LTCC technology

Rapid prototyping through infra-red laser machining is adopted. The process does not involve chemicals and a single plain screen is sufficient for printing
and via filling of any number of layers. The process does not require special masks and is performed in the ordinary laboratory environment. Optimisation
of laser power, pulse rate, and scanning speed produce high repeatability and selective thinning, cutting and drilling of any positions and layers made possible with optical camera assisted alignment. The laser ablation of low
loss LTCC 9K7 green state tape produces minimum damage with surface roughness below 1 μm which is acceptable for microwave fabrication requirements.
To fabricate hollow waveguide in LTCC, the standard LTCC process has been considered, and it was found unable to produce suitable channels/cavities. The hurdle was the lamination step. Experiments on lamination pressure, temperature, time duration and number of layers have
been conducted to find the most influential parameters. 3D structures were successfully fabricated using a novel multi-stage/progressive lamination technique. With this technique, deformation-free 3D hollow structures
laminated at pressure as low as 2 MPa have been realised. Three types of sacrificial inserts have also been prepared and examined to enhance the process variability. While organic-based and water-based sacrificial paste
are more suitable for preparing trenches and channels in the micrometre scale, wax-graphite based sacrificial insert is better for large volume of channel and cavity application.
A WR28-like Hollow SIW (HSIW) is fabricated and measured using WR28 waveguide flanges. A multimode calibration technique was used to calibrate the complex propagation constant. The HSIW has been successfully demonstrated working in the millimetre-wave region with measurements on
26.5 to 40 GHz test samples. This is believed to be a significant milestone.
A HSIW-based antenna and filter as an integrated component was fabricated. The measurement of the radiation pattern of the waveguide antenna and cavity filter is performed using a 67 GHz PNA and a standard gain horn antenna placed inside an anechoic box. The measured radiation pattern shows both E- and H-plane pattern closely fit with the modelled
performance.