Fabrication of a GHz band surface acoustic wave filter by UV-nanoimprint with an HSQ stamp

Abstract

A GHz band surface acoustic wave (SAW) filter was fabricated by UV-nanoimprint lithography (UV-NIL). The key techniques to produce a SAW filter include stamp and interdigital transducer (IDT) fabrication. For the stamp, high-aspect-ratio (AR) hydrogen silsesquioxane (HSQ)/ITO/glass stamps were first exposed by low e-beam dose. Adequate post-exposure bake, tetramethylammoniumhydroxide concentration, and etch time were utilized to pattern the HSQ stamps with a perfect vertical sidewall. HSQ/ITO/glass IDT stamps with widths in the range 46–168 nm were fabricated. The stamps were then cured with a novel step-like heating cycle as hard-bake. The modified HSQ stamps have a high hardness of 19 GPa. The HSQ/ITO/glass layered structure shows 80% transmittance at 365 nm UV light. Afterward, the IDT patterns were transferred on UV-curable resist/lithium niobate (LiNbO3) by UV-NIL at room temperature and low pressure. After removing the residual layer, Al/Ti films were deposited on UV-curable resist/LiNbO3. IDTs with widths in the range 62–219 nm and thickness of 20 nm could be obtained by following a lift-off process. The center frequency of 4.3–8.6 GHz and insertion loss of −14.1 to −61.7 dB of the SAW filters were measured by a network analyzer HP 8510C. The reactive ion etching step for residual layer elimination has significant implication for the electrical characterization of a SAW filter. The thorough removal of the residual layer not only ensures a clean interface between IDTs and LiNbO3 for SAW filters with minimum insertion loss, but also keeps intact IDT patterns for small center frequency shift.

Damage imaging method based on circular DE-IDT array for phased array data fusion

“…The NCD film deposited on the Si substrate (NCD/Si) not only had a smooth surface but also exhibited good compatibility with the ZnO material. In 2011, Chen et al 8 used ultraviolet-nanoimprint lithography to fabricate hydrogen silsesquioxane HSQ/ITO/glass IDT molds with widths ranging from 46 to 168 nm by imprinting high aspect ratio HSQ/ITO/glass. This enabled the fabrication of GHz band surface acoustic wave (SAW) filter IDTs.…”

Ultrahigh-frequency surface acoustic wave transducers on ZnO/SiO2/Si using nanoimprint lithography

“…A multilayer lift-off process using UV-NIL was also developed to fabricate 140 nm IDT fingers on LiNbO 3 [18]. Recently a UV-NIL process with a high-aspect-ratio stamp has been reported to fabricate 100 nm IDT fingers on LiNbO 3 with 8.6 GHz center frequency [19]. However, further improvement of the finger resolution of IDTs and reproducibility of the process is limited by the ability to get an appropriate undercut profile, which is essential for metal lift-off.…”