Piezoelectric Actuator Moisture-Resistant Insulating Layers
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Solution Overview
Problem
Piezoelectric actuators in liquid ejection heads face deterioration in dielectric strength and failure due to moisture in high humidity environments, particularly when aluminum oxide insulating layers are exposed to moisture during etching, leading to increased leakage current and reduced displacement characteristics.
Innovation Solution
A piezoelectric actuator configuration with a silicon oxide film as the second protective layer, which provides higher moisture resistance than the aluminum oxide film, is used between the piezoelectric element and the wirings, and a silicon nitride film as the third protective layer with higher moisture resistance than the silicon oxide film, covering the wirings and the periphery of the piezoelectric layer to prevent moisture penetration and leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If an aluminum oxide film is used as the insulating layer, then dielectric strength can be obtained, but moisture resistance deteriorates when the film is exposed to moisture during etching
Solution Approach 1:
The patent uses a composite insulating layer structure combining aluminum oxide film (for dielectric strength) with silicon oxide film or silicon nitride film (for moisture resistance). This composite structure allows the aluminum oxide to provide its dielectric function while the silicon-based films protect against moisture exposure during etching and storage, resolving the contradiction between dielectric strength and moisture resistance.
Solution Approach 2:
The silicon oxide film or silicon nitride film acts as an intermediary protective layer between the aluminum oxide insulating layer and the external moisture environment. This intermediary layer prevents direct contact between moisture and the aluminum oxide surface, thereby maintaining both dielectric strength and moisture resistance simultaneously.
2Reliability
If the aluminum oxide film thickness is increased to improve dielectric strength, then reliability improves, but displacement characteristics deteriorate
Solution Approach 1:
The patent employs a composite insulating structure where thin aluminum oxide film (providing dielectric strength) is combined with silicon oxide or silicon nitride films (providing moisture protection). This allows achieving sufficient dielectric strength with thinner total film thickness compared to using only aluminum oxide, thereby preserving the displacement characteristics of the piezoelectric actuator while maintaining reliability.
Solution Approach 2:
The patent changes the material composition parameters of the insulating layer from pure aluminum oxide to a multi-layer structure including silicon oxide and silicon nitride. This parameter change allows optimizing the balance between dielectric strength and film thickness, enabling sufficient insulation with reduced total thickness that does not adversely affect piezoelectric displacement characteristics.
3Ease of manufacture
If aluminum oxide film surface is exposed to moisture during etching, then manufacturing process is simplified, but leakage current increases due to surface alteration
Solution Approach 1:
The silicon oxide film or silicon nitride film serves as an intermediary protective barrier during the etching process. This intermediary layer prevents moisture from altering the aluminum oxide surface while allowing the etching process to proceed. The protective layer can be selectively removed in contact holes to expose aluminum oxide for wiring connection, thus preventing leakage current while maintaining ease of manufacture.
Solution Approach 2:
The patent applies the silicon oxide or silicon nitride protective film before the etching process to pre-protect the aluminum oxide surface from moisture exposure. This preliminary action prevents surface alteration and subsequent leakage current generation, while the process remains manufacturable through selective removal of the protective film where needed.
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 prevents deterioration of the piezoelectric layer characteristics, suppresses dielectric breakdown, and reduces the likelihood of failure due to increased leakage current, while maintaining sufficient displacement characteristics for liquid ejection.
Implementation Method 1
a piezoelectric body whose shape is changed by application of an electric field is applied to various industrial products
Implementation Method 2
the third layer has higher moisture resistance than the second layer
Data Source
AI summary
A piezoelectric actuator includes a piezoelectric element including a first electrode, a piezoelectric layer, and a second electrode arranged in this order from a substrate, a first wiring electrically connected to the first electrode, a second wiring electrically connected to the second electrode, a first layer covering the piezoelectric element, a second layer arranged between the piezoelectric element and the first wiring and between the piezoelectric element and the second wiring, and a third layer arranged to cover the first wiring, the second wiring, and a periphery of the piezoelectric layer when viewed from a direction perpendicular to a surface of the substrate, wherein the first layer, the second layer, and the third layer are arranged in this order from the substrate, and wherein the third layer has higher moisture resistance than the second layer.