XBAR Wafer Layer Transfer for Multi-Thickness Membrane Planarity
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Solution Overview
Problem
Existing techniques for fabricating transversely-excited film bulk acoustic resonators (XBARs) with different membrane thicknesses lead to undesirable resonator characteristics due to surface elevations on the wafer, degrading performance.
Innovation Solution
The use of layer transfer processes with thin etch stop layers allows for the accurate fabrication of multiple membrane thicknesses on a single die, maintaining resonator performance and planarity without significant degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If existing techniques are used to fabricate XBARs with different membrane thicknesses, then multiple thicknesses can be achieved, but surface elevations occur that degrade resonator performance
Solution Approach 1:
A planarization layer is introduced as an intermediary element between the resonators with different membrane thicknesses and the wafer surface. This layer fills in the elevation differences caused by varying membrane thicknesses, providing a flat top surface that eliminates performance degradation while preserving the different thickness characteristics needed for frequency tuning.
2Manufacturing precision
If existing techniques are used to fabricate XBARs with different membrane thicknesses, then multiple thicknesses can be achieved, but wafer surface planarity is compromised
Solution Approach 1:
The planarization layer serves as a mediator that decouples the membrane thickness variation from the wafer surface topology. It allows different membrane thicknesses to coexist on the same wafer while maintaining a uniform external surface, enabling both thickness diversity and surface planarity.
Solution Approach 2:
The solution addresses the surface planarity issue by adding a new dimensional element (the planarization layer) rather than modifying the membrane thicknesses themselves. This extra layer provides the necessary compensation in the vertical dimension to achieve overall surface flatness.
3Adaptability or versatility
If layer transfer processes with etch stop layers are used, then multiple membrane thicknesses can be fabricated on a single die, but process complexity increases
Solution Approach 1:
Etch stop layers are deposited in advance during the fabrication process, before the final membrane thicknesses are determined. These pre-positioned layers serve as reference planes that guide subsequent thinning operations, enabling precise control of different membrane thicknesses through a systematic sequence of operations rather than ad-hoc adjustments.
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 method enables the creation of XBARs with different membrane thicknesses on the same die, improving resonator performance by controlling resonant and anti-resonant frequencies, quality factor, and mechanical characteristics while maintaining wafer surface planarity.
Implementation Method 1
a first piezoelectric layer having first and second surfaces that oppose each other, with the second surface facing the substrate and coupled thereto directly or via one or more intermediate layers; a second piezoelectric layer having first and second opposing surfaces, with the first surface coupled to the first surface of the first piezoelectric layer
Implementation Method 2
first and second interdigital transducers (IDTs) on at least one of the first and second piezoelectric layers, respectively
Implementation Method 3
an important parameter that determines the resonance frequency of a transversely-excited film bulk acoustic resonator (XBAR) is the thickness of the diaphragm or piezoelectric material
Data Source
AI summary
An acoustic resonator is provided that includes a substrate; a first piezoelectric layer having first and second surfaces that oppose each other, with the second surface coupled to the substrate directly or via one or more intermediate layers; a second piezoelectric layer having first and second opposing surfaces, with the first surface coupled to the first surface of the first piezoelectric layer and opposite to the substrate; an etch stop layer disposed between the respective first surfaces of the first and second piezoelectric layers; and first and second interdigital transducers (IDTs) on at least one of the first and second piezoelectric layers, respectively. Moreover, a portion of one of the first and second piezoelectric layers is removed between the second surface of the respective piezoelectric layer and the etch stop.