POI Substrate With Graded Acoustic Impedance for Parasitic Mode Reduction
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Solution Overview
Problem
The significant difference in acoustic impedance between silicon-based substrate materials and silicon oxide dielectric layers in piezoelectric substrates leads to performance losses and parasitic modes in acoustic wave devices, such as sensors and filters.
Innovation Solution
A piezoelectric substrate on insulator (POI) design featuring a silicon oxynitride intermediate layer with a variable composition along its thickness, gradually matching the acoustic impedance between the dielectric layer and the support substrate, thereby reducing the impedance difference and minimizing parasitic effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a silicon-based substrate with silicon oxide dielectric layer is used in POI substrates, then good adhesion and structural stability are achieved, but significant acoustic impedance difference creates parasitic modes and performance loss
Solution Approach 1:
A nitrogen-containing intermediate layer is introduced between the silicon oxide dielectric layer and the silicon-based support substrate. This intermediate layer serves as an acoustic impedance mediator, having lower acoustic impedance than silicon oxide but higher than silicon, thereby gradually transitioning the acoustic impedance and reducing the sharp impedance mismatch that causes parasitic modes
Solution Approach 2:
The acoustic impedance parameter of the intermediate layer is specifically optimized to be lower than that of silicon oxide dielectric layer. By controlling the nitrogen content and thickness of the intermediate layer, the acoustic impedance is tuned to create a gradual transition, reducing parasitic mode generation while maintaining structural integrity
2Reliability
If silicon oxide dielectric layer is used, then good adhesion is achieved, but acoustic wave device performance is degraded due to impedance mismatch
Solution Approach 1:
The nitrogen-containing intermediate layer acts as an acoustic mediator that reduces energy loss from parasitic modes. It maintains the adhesion benefits of silicon oxide while compensating for its high acoustic impedance through the impedance-matching function of the intermediate layer
Solution Approach 2:
The intermediate layer is strategically positioned only at the critical interface where acoustic impedance transition is needed. This localized modification preserves the bulk properties of silicon oxide dielectric layer while addressing the specific impedance mismatch problem at the substrate interface
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 design enhances the stability and performance of acoustic wave devices by reducing acoustic impedance differences, leading to improved characteristics and reduced parasitic modes, thus enhancing the functionality of sensors and filters.
Implementation Method 1
the acoustic impedance of the intermediate layer varies, in particular gradually, between the values of the first and second acoustic impedances. Thus, varying the acoustic impedance of the intermediate layer allows the acoustic impedance difference between the dielectric layer and the support substrate to be gradually reduced
Implementation Method 2
a piezoelectric layer, in particular a layer of Lithium Tantalate (LTO), Lithium Niobate (LNO), Aluminium Nitride (AIN), Lead Titano-Circonate (PZT), Langasite or Langatate
Data Source
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Figure 5a
AI summary
The invention relates to a piezoelectric-on-insulator (POI) substrate (100) comprising a supporting substrate (102) having a first acoustic impedance; a piezoelectric layer (104), in particular made of lithium tantalate, lithium niobate, aluminium nitride, lead zirconate titanate, langasite or langatate; a dielectric layer (110) having a second acoustic impedance and sandwiched between the piezoelectric layer (104) and the supporting substrate (102); and an intermediate layer (106) positioned between the supporting substrate (102) and the dielectric layer (110), characterised in that the intermediate layer (106) is a layer with a variable composition, in particular along its thickness, such that the acoustic impedance of the intermediate layer varies, in particular gradually, between the values of the first and second acoustic impedances. The invention also relates to a method for producing such a POI substrate and to a surface acoustic wave device comprising such a POI substrate.