RF AlN Thin Film Bulk Acoustic Wave Resonators

Thin film bulk acoustic wave resonators (TFBAR) show considerable promise as an integrable solution for RF bandpass filters with center frequencies between 900MHz and 3GHz. TFBAR based filters are orders of magnitude smaller than current dielectric filters and can potential be manufactured on-chip, thereby allowing VLSI integration of bandpass filters into a RF portable radio circuit. TFBAR based bandpass filters can easily meet and even exceed current specifications for the front-end bandpass filters employed within portable communication systems. High quality VLSI compatible piezoelectric films are an essential component of a TFBAR. Aluminum Nitride (AlN) is one of the attractive piezoelectric materials for TFBAR application. The AlN film quality strongly affects the performance of TFBAR, and thus the bandwidth as well as the insertion loss of TFBAR-based filters. A DC magnetron reactive sputtering system was optimized to deposit high quality, (002)-oriented polycrystalline AlN films. The Rocking Curve full width at half maximum (FWHM) of 5.6° on silicon substrates was achieved. Due to relative low deposition rate of AlN, and the required low deposition temperature, the AlN film quality was found to be limited by the oxygen content and depend on the sputtering modes. For a deposition rate of 1μm/hour and a self-heating process, a low oxygen containing partial pressure and a metal mode of sputtering process are necessary for narrow Rocking Curve material. AlN etching techniques were explored to realize the TFBAR fabrication process. For the low etching speed and poor patterning result of wet etching with aluminum etchant for VLSI,RIE with HCFC-124 was introduced in device processing and can reach 700?/min. At the same time, the comparison of the qualities of AlN films on different substrates helped us to choose platinum as the electrode material for AlN TFBAR. A simple acoustic device, the composite resonator and the device model were developed to simulate the device performances. Several AlN TFBAR devices and TFBAR-based filters were fabricated and tested. A freestanding electrode-pizoelectrics-electrode sandwich structure was achieved by bulk micromachining. The best devices have resonating modes at 975MHz, 1.75GHz and 2.48GHz with the insertion loss of 5~6dB and the bandwidth of about 50MHz.