Composite Substrate Mirror Polishing for Acoustic Wave Devices
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Solution Overview
Problem
Existing methods for forming ultrathin piezoelectric layers, such as CVD and Smart Cut technology, face issues with poor crystallinity, limited crystal axis directions, and crystal defects, as well as difficulties in achieving uniform thickness without breaking the films during polishing.
Innovation Solution
A composite substrate production method involving mirror polishing of a substrate stack, creating thickness distribution data, and using an ion beam machine to achieve a piezoelectric substrate with a thickness of 3 μm or less and a crystallinity that ensures a full width at half maximum of an X-ray diffraction rocking curve of 100 arcsec or less, with a difference in thickness of 60 nm or less across the surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If CVD method is used to form thin piezoelectric layers, then uniform thickness can be achieved, but crystallinity becomes very poor and crystal axis directions are limited
Solution Approach 1:
A support substrate is introduced as an intermediary carrier to enable mechanical polishing of ultrathin piezoelectric layers. The support substrate provides mechanical strength during polishing, allowing achievement of uniform thickness without direct handling of the fragile thin layer, thereby resolving the contradiction between thickness uniformity and crystallinity preservation
Solution Approach 2:
The invention replaces chemical deposition (CVD) with mechanical polishing to achieve thickness control. By using mechanical polishing on a supported substrate, uniform thickness can be achieved while preserving the crystalline structure, avoiding the poor crystallinity inherent in CVD methods
2Manufacturing precision
If Smart Cut technology is used to form thin piezoelectric layers, then thin layers with uniform thickness can be formed, but crystal defects remain due to insufficient recovery of damages by ion implantation
Solution Approach 1:
The invention replaces ion implantation-based thickness control with mechanical polishing. This substitution eliminates the crystal damage inherent in ion implantation while achieving the same thickness control objective, thereby resolving the contradiction between thickness uniformity and crystal defect reduction
Solution Approach 2:
The invention changes the fundamental parameter of thickness control from chemical/ion-based (Smart Cut) to mechanical removal. This parameter change allows achievement of uniform thickness through controlled mechanical polishing without introducing crystal defects through ion implantation damage
3Length of moving object
If polishing is performed on piezoelectric films to reduce thickness, then thin layers can be obtained, but breakage occurs during polishing or uniform thickness cannot be achieved
Solution Approach 1:
A support substrate serves as a mediator that provides mechanical strength during the polishing process. This allows ultrathin piezoelectric layers to be polished without breakage, as the support substrate prevents mechanical failure while enabling precise thickness control through polishing
Solution Approach 2:
The piezoelectric layer is bonded to a support substrate before polishing begins. This preliminary action of bonding provides the mechanical strength needed to withstand the polishing process, preventing breakage and enabling achievement of uniform thickness that would be impossible with standalone thin films
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 produces piezoelectric single-crystal thin layers with high crystallinity and uniform thickness, addressing the limitations of CVD and Smart Cut technology, and enabling their use in acoustic wave devices with improved frequency stability and filter characteristics.
Implementation Method 1
a step of performing machining with an ion beam machine based on the data of the thickness distribution
Implementation Method 2
the piezoelectric substrate has such crystallinity that the full width at half maximum of an X-ray diffraction rocking curve is 100 arcsec or less
Data Source
AI summary
A composite substrate production method of the invention includes (a) a step of mirror polishing a substrate stack having a diameter of 4 inch or more, the substrate stack including a piezoelectric substrate and a support substrate bonded to each other, the mirror polishing being performed on the piezoelectric substrate side until the thickness of the piezoelectric substrate reaches 3 μm or less; (b) a step of creating data of the distribution of the thickness of the mirror-polished piezoelectric substrate; and (c) a step of performing machining with an ion beam machine based on the data of the thickness distribution so as to produce a composite substrate have some special technical features.