Enhancing film bulk acoustic resonators performance by optimizing AlN seed layer crystallinity and polarity alignment

Area of Science:

• Materials Science
• Electrical Engineering
• Acoustic Devices

Background:

• Aluminum nitride (AlN)-based film bulk acoustic resonators (FBARs) are crucial for wireless communication but suffer from limited electromechanical coupling.
• Scandium (Sc) incorporation in AlN enhances piezoelectric properties, yet often leads to degraded crystallinity and polarity inversion, hindering device performance.
• Theoretical analysis identified the AlN/ScAlN interface as a critical factor in piezoelectric degradation due to polarity mismatch.

Purpose of the Study:

• To address the limitations of Sc-doped AlN resonators by proposing a novel dual-optimization strategy.
• To improve crystalline alignment and eliminate polarity inversion at the AlN/ScAlN interface.
• To enhance the effective electromechanical coupling coefficient and overall performance of FBARs for advanced wireless communication.

Main Methods:

• Employed a single-crystalline AlN seed layer to promote highly c-axis oriented ScAlN films.
• Utilized a subsequent seed layer removal step to eliminate the polarity inversion interface.
• Fabricated and characterized FBARs and filters using the developed dual-optimization strategy.

Main Results:

• Resonators fabricated with the seed layer achieved a maximum quality factor of 736.
• Removal of the seed layer eliminated the polarity inversion interface, increasing the effective electromechanical coupling coefficient from 6.0% to 13.2%.
• Filters demonstrated a center frequency of 6.4 GHz, a 740 MHz bandwidth, and >40 dB out-of-band rejection.

Conclusions:

• The proposed dual-optimization strategy effectively improves crystalline quality and eliminates polarity mismatch in ScAlN films.
• This approach significantly enhances the electromechanical coupling and performance of FBARs.
• The developed filters show great potential for next-generation Sub-7 GHz wireless communication systems.