Acoustic Wave Package Structure for Higher-Mode Suppression
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Solution Overview
Problem
Acoustic wave devices with silicon support substrates face issues where higher modes leak to the support substrate, affecting filter characteristics due to higher acoustic velocities, leading to increased response and interference with main mode frequencies.
Innovation Solution
The acoustic wave device incorporates a silicon oxide film and a piezoelectric body on a silicon support substrate with a thickness greater than or equal to 3λ, and an interdigital transducer electrode, where the acoustic velocity of the support substrate is controlled to match or exceed the higher mode velocity, reducing or preventing the response of higher modes by optimizing the film thickness and crystal orientation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the support substrate thickness is increased to reduce higher mode leakage, then filter characteristics are improved, but device size and integration density decrease
Solution Approach 1:
The patent embeds multiple functional layers within a compact structure: a first piezoelectric layer is formed on the support substrate, and a second piezoelectric layer is formed on the first piezoelectric layer. This nested configuration allows the device to achieve higher mode suppression equivalent to thicker substrates while maintaining a compact overall form factor, effectively nesting functionality within limited space.
Solution Approach 2:
Instead of solving the higher mode leakage problem by increasing substrate thickness in the vertical dimension, the patent introduces a lateral dimension solution by forming a reflective electrode pattern on the support substrate. This reflective electrode creates acoustic reflection in the lateral direction, suppressing higher mode propagation without requiring increased substrate thickness, thus resolving the contradiction through dimensional transformation.
2Reliability
If the acoustic velocity of bulk waves in the support substrate is increased to match or exceed higher mode velocity, then higher mode response is reduced, but manufacturing precision requirements increase
Solution Approach 1:
The patent changes the acoustic velocity parameter of the support substrate by selecting specific crystal orientations (such as <100>, <110>, or <111> directions) and materials (silicon, sapphire, or GaAs). By adjusting these fundamental material parameters, the bulk wave acoustic velocity is optimized to match or exceed the higher mode velocity, thereby suppressing higher mode response while using commercially available materials with well-defined properties.
Solution Approach 2:
The patent employs composite material structures combining the support substrate with multiple piezoelectric layers having different acoustic velocities. The support substrate is designed with specific acoustic velocity characteristics, while the piezoelectric layers are engineered with complementary properties. This composite approach allows the overall device to achieve higher mode suppression through velocity matching without requiring extreme precision in any single material layer.
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
This configuration effectively reduces or prevents the response of higher modes, improving filter characteristics by ensuring the higher modes are enclosed within the silicon oxide film and piezoelectric body, enhancing energy concentration and reducing losses.
Implementation Method 1
a piezoelectric body provided on or above the silicon oxide film
Implementation Method 2
an interdigital transducer electrode provided on or above one of main surfaces of the piezoelectric body
Implementation Method 3
the acoustic velocity of a first higher mode that propagates through the piezoelectric body is equal to an acoustic velocity VSi of bulk waves that propagate in the support substrate
Implementation Method 4
an acoustic velocity VSi of the following mathematical expression (1), which is an acoustic velocity of bulk waves that propagate in the support substrate
Data Source
AI summary
An acoustic wave device includes a silicon oxide film, a piezoelectric body, and an interdigital transducer electrode laminated on a support substrate made of silicon. Where a wave length that is determined by an electrode finger pitch of the interdigital transducer electrode is λ, a thickness of the support substrate is greater than or equal to about 3λ. An acoustic velocity of the first higher mode that propagates through the piezoelectric body is an acoustic velocity VSi=(V1)1/2 of bulk waves that propagate in the support substrate, which is determined by V1 out of solutions V1, V2, and V3 of x derived from the mathematical expression Ax3+Bx2+Cx+D=0, or higher than VSi.