Elastic Wave Filter Stack Using SiNx to Suppress High-Order Modes
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Solution Overview
Problem
Elastic wave devices with stacked supporting substrates, high-acoustic-velocity films, and piezoelectric films generate high-order-mode unwanted waves and frequency changes due to variations in the physical properties of the high-acoustic-velocity film materials, particularly silicon nitride, which can interfere with other filter devices connected to an antenna.
Innovation Solution
The elastic wave device is designed with a high-acoustic-velocity film made of SiNx where x<0.67, a low-acoustic-velocity film of silicon oxide, and a piezoelectric layer, where the acoustic velocities are optimized to confine energy within the piezoelectric layer, reducing high-order-mode unwanted waves and frequency changes, and optionally includes a second low-acoustic-velocity film and an insulating film for improved temperature characteristics and protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a high-acoustic-velocity film made of silicon nitride is used in the stacked structure, then the acoustic velocity of bulk waves is increased, but high-order-mode unwanted waves are generated and frequency changes occur due to variations in physical properties
Solution Approach 1:
The patent changes the compositional parameter x in SiNx from the conventional range (x≥0.67) to a specific range (0.1≤x<0.67). This parameter change reduces the acoustic velocity of bulk waves in the high-acoustic-velocity film, preventing high-order-mode unwanted waves while maintaining the desired acoustic wave propagation characteristics in the piezoelectric layer.
Solution Approach 2:
The patent applies different materials with specific acoustic velocity characteristics to different layers: SiNx with 0.1≤x<0.67 for the high-acoustic-velocity film, silicon oxide for the low-acoustic-velocity film, and aluminum nitride for the piezoelectric layer. This local differentiation of material properties confines acoustic wave energy to the piezoelectric layer and eliminates high-order-mode interference.
2Speed
If the physical properties of silicon nitride in the high-acoustic-velocity film are changed, then the acoustic velocity is adjusted, but a change in high-order-mode frequency occurs
Solution Approach 1:
The patent establishes a specific compositional parameter range (0.1≤x<0.67) for SiNx that simultaneously achieves the desired acoustic velocity and prevents high-order-mode frequency changes. By controlling the nitrogen content within this range, both acoustic velocity and frequency stability are optimized.
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 high-order-mode unwanted waves and frequency changes, minimizing interference with other filter devices and enhancing temperature stability, as demonstrated by reduced frequency shifts and wave levels in high-humidity environments.
Implementation Method 1
a piezoelectric layer disposed on the low-acoustic-velocity film
Implementation Method 2
the acoustic velocity of a bulk wave that propagates through the high-acoustic-velocity film is higher than the acoustic velocity of an elastic wave that propagates through the piezoelectric layer, the acoustic velocity of a bulk wave that propagates through the low-acoustic-velocity film is lower than the acoustic velocity of an elastic wave that propagates through the piezoelectric layer
Data Source
AI summary
An elastic wave device includes a supporting substrate, a high-acoustic-velocity film disposed on the supporting substrate, a low-acoustic-velocity film disposed on the high-acoustic-velocity film, a piezoelectric layer disposed on the low-acoustic-velocity film, and an interdigital transducer electrode disposed on the piezoelectric layer. The acoustic velocity of a bulk wave that propagates through the high-acoustic-velocity film is higher than the acoustic velocity of an elastic wave that propagates through the piezoelectric layer. The acoustic velocity of a bulk wave that propagates through the low-acoustic-velocity film is lower than the acoustic velocity of an elastic wave that propagates through the piezoelectric layer. The high-acoustic-velocity film is composed of SiNx, where x<about 0.67.