Lithium Niobate SAW Filter Structure for Temperature-Stable Reflection
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Solution Overview
Problem
Conventional surface acoustic wave devices using lithium tantalate substrates face significant temperature-related frequency shifts due to large thermal expansion coefficients and varying elastic constants, affecting their temperature characteristics and electrical performance.
Innovation Solution
A surface acoustic wave device is designed with a lithium niobate substrate, interdigital transducers (IDTs), and a protective silicon oxide film with specific uneven shapes and thicknesses, ensuring that the pitch width and electrode finger widths satisfy certain ratios to achieve optimal reflection characteristics and improved temperature and electrical performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a lithium tantalate substrate is used in a surface acoustic wave device, then the device can be manufactured with standard processes, but the frequency characteristic shifts greatly with temperature changes due to large thermal expansion coefficient and varying elastic constant
Solution Approach 1:
The patent changes the substrate material parameter from lithium tantalate to lithium niobate, which has different thermal and elastic properties. This material substitution fundamentally alters the temperature-stability parameter while maintaining manufacturability through established SAW device fabrication processes
Solution Approach 2:
The patent employs a composite structure combining lithium niobate substrate with silicon oxide protective film. This composite material system leverages the low thermal expansion coefficient of lithium niobate while the silicon oxide layer provides additional thermal stability and protection, collectively improving temperature characteristics
2Ease of manufacture
If a flat protective film is used over the IDT, then the manufacturing process is simple, but the reflection characteristic is insufficient
Solution Approach 1:
The patent introduces surface unevenness (curvature variation) in the protective film rather than using a completely flat surface. The film thickness varies across the surface with specific proportions, creating a curved profile that enhances acoustic wave reflection characteristics while still using standard deposition processes
Solution Approach 2:
The protective film has different thicknesses in different locations: thicker at the IDT region and thinner at the reflector electrode region. This local variation in film quality optimizes the reflection characteristic by providing appropriate acoustic impedance matching in different areas of the device
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 configuration results in a surface acoustic wave device with enhanced temperature stability and electrical characteristics, allowing for stable operation across a wider temperature range with improved frequency accuracy and reduced spurious signals.
Implementation Method 1
a surface acoustic wave device used as a resonator or a band-pass filter
Implementation Method 2
an LT substrate has a large thermal expansion coefficient of a substrate in the direction in which a surface acoustic wave propagates
Implementation Method 3
a piezoelectric substrate, an electrode film and an insulating film
Data Source
AI summary
A surface acoustic wave device includes a substrate including lithium niobate; a IDT being provided on an upper surface of the substrate and including a plurality of electrode fingers; and a protective film covering the IDT and having an uneven shape on an upper surface thereof. When a pitch width of one pitch of the IDT is p, a width of one of the electrode fingers is p1, a width between the electrode fingers is p2, and a thickness of the IDT is h, following relations are satisfied, p1+p2=p, and h/(2×p)≧4.5%. With this configuration, an appropriate reflection characteristic is realized, and the surface acoustic wave device having excellent temperature coefficient of frequency and electrical characteristic can be obtained.