Study of ferroelectric based varactors for RF applications

This internship was carried out at Thales Research & Technology and focused on the fabrication and electrical characterization of ferroelectric varactors for radio-frequency (RF) applications. The varactors were based on a metal–ferroelectric–metal (MFeM) structure using Al-doped HfO₂ as the ferroelectric layer, deposited by Atomic Layer Deposition (ALD). Two different thicknesses were studied to com-pare their dielectric and tunable properties. The first part of the internship took place in the cleanroom, where I participated in the fabrication of the devices. I learned to perform photolithography, and carry out metal deposition and etching steps. After fabrication, I used different electrical setups to characterize the varactors. Low-frequency measurements confirmed the ferroelectric behavior of the capacitors and showed how electrical cycling can improve their performance. High-frequency (RF) measurements using a Vector Network Analyzer (VNA) allowed us to extract key parameters such as relative permittivity, loss tan-gent, and tunability. The influence of annealing time and electrode materials on these parameters was also analyzed. This work showed that Al:HfO₂ is a promising material for tunable RF components, with some sam-ples achieving performance comparable to or better than current state-of-the-art materials.