Acoustic Wave Finger Dielectric Layout for Spurious Mode Control
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Solution Overview
Problem
Conventional acoustic wave devices utilizing the piston mode face challenges in suppressing spurious modes while minimizing polarization reversal in the piezoelectric substrate, as either vertical or tapered dielectric shapes compromise either the acoustic velocity difference or stress reduction.
Innovation Solution
The acoustic wave device employs a piezoelectric substrate with interdigitated electrodes and dielectrics having specific acoustic velocity distributions and tilted dielectric surfaces to balance stress reduction and sharp velocity differences, utilizing a layered structure with piezoelectric and high-acoustic velocity supporting substrates to enhance Q value and reduce spurious modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the side surface of the dielectric has the vertical shape, then a sharp difference in the acoustic velocity can easily be formed between the middle portion and the first and second edge portions, but stress easily acts on the piezoelectric substrate around the dielectric and polarization reversal in piezoelectric material included in the piezoelectric substrate easily occurs
Solution Approach 1:
The dielectric's side surface is designed with different tilt angles in different regions: a first tilt angle in the first direction and a second tilt angle in the second direction. This local differentiation allows the first edge portion to have a sharper acoustic velocity transition (smaller tilt angle) while the second edge portion has a gentler transition (larger tilt angle), thereby suppressing spurious modes in one direction while reducing stress and polarization reversal in the other direction.
2Reliability
If the side surface of the dielectric has the tapered shape, then the stress which acts on the piezoelectric substrate around the dielectric can be reduced, thus an occurrence of the polarization reversal is difficult, but since formation of the sharp acoustic velocity difference between the middle portion and the first and second edge portions becomes difficult, there is a concern that suppressing the occurrence of the spurious mode becomes difficult
Solution Approach 1:
Rather than uniformly tapering the entire side surface, the invention applies different tilt angles to different regions: the first tilt angle (smaller) in the first direction maintains sharp acoustic velocity difference for spurious mode suppression, while the second tilt angle (larger) in the second direction reduces stress on the piezoelectric substrate. This localized quality differentiation resolves the contradiction between the two opposing requirements.
3Manufacturing precision
If the acoustic wave velocity in edge portions is made lower than in the middle portion to suppress spurious modes, then spurious modes are suppressed, but stress concentrates on the piezoelectric substrate around the dielectric causing polarization reversal
Solution Approach 1:
The dielectric structure creates asymmetric acoustic velocity distribution with different tilt angles in different directions. The smaller first tilt angle creates a sharper velocity gradient in the first direction for spurious mode suppression, while the larger second tilt angle in the second direction creates a gentler gradient that reduces stress concentration on the piezoelectric substrate, preventing polarization reversal.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces spurious modes and prevents polarization reversal by optimizing acoustic wave velocity distribution and dielectric surface angles, enhancing the acoustic wave device's performance and reliability.
Implementation Method 1
an acoustic wave device which utilizes a piston mode... provided with a piezoelectric substrate and an interdigital transducer (IDT) electrode provided on the piezoelectric substrate
Implementation Method 2
an acoustic wave velocity in a first edge portion including tip-end portions of the plurality of first electrode fingers and an acoustic wave velocity in a second edge portion including tip-end portions of the plurality of second electrode fingers are configured to be lower than an acoustic wave velocity in a middle portion between the first edge portion and the second edge portion
Data Source
AI summary
An acoustic wave device includes an electrode finger on a principal surface of a piezoelectric substrate and extending in a Y-axis direction. In the acoustic wave device, an acoustic wave velocity is distributed in an order of an intermediate velocity, a low velocity, and a high velocity from a center of the electrode finger toward outer side portions in the Y-axis direction. The acoustic wave device further includes a dielectric between the piezoelectric substrate and a tip-end portion of the electrode finger. An end surface of the dielectric in the Y-axis direction includes first and second side surfaces. A tilt angle of the first side surface is smaller than a tilt angle of the second side surface.