MEMS Vibration Resonator Oxide Barrier for Temperature Stability
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Solution Overview
Problem
Existing vibration devices using bulk waves require thick SiO2 films for improved temperature characteristics, but thermal oxidation methods are slow, and alternative methods degrade the Q-value, while n-type Si substrates doped with phosphorus suffer from nonuniform doping due to thermal bonding, causing variations in resonant frequency with temperature changes.
Innovation Solution
A vibration device with a support member and n-type Si layer, where silicon oxide films containing impurities are in contact with both surfaces of the n-type Si layer, and a piezoelectric thin film is sandwiched between electrodes, using a thermal oxidation method to form the oxide films and maintain uniform doping, thus suppressing resonant frequency variations due to temperature changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thermal oxidation method is used to form a thick SiO2 film for improved temperature characteristics, then the temperature characteristics are improved, but the growth rate becomes significantly slow when the film thickness exceeds a constant value
Solution Approach 1:
The patent changes the formation method parameter from thermal oxidation to sputtering method, enabling rapid formation of thick SiO2 films (several micrometers) while maintaining the temperature characteristics improvement function
2Productivity
If a sputtering method or CVD method is used to easily form a thick SiO2 film, then the film formation efficiency is improved, but the film mechanical loss Qm becomes unfavorable which degrades the Q-value of the vibrator
Solution Approach 1:
The patent changes the formation method parameter to thermal oxidation method, which forms SiO2 films with favorable mechanical loss characteristics and high Q-values, while accepting the slower growth rate as a trade-off for maintaining vibration device performance
3Strength
If thermal bonding is used for processing to constitute the MEMS structure, then the bonding strength is improved, but phosphorus dopant is scattered into the air or moved to other members causing nonuniform concentration
Solution Approach 1:
The patent performs thermal oxidation to form SiO2 films on the Si semiconductor layer before the bonding process. These pre-formed films act as protective barriers that prevent phosphorus dopant scattering during subsequent thermal bonding, thereby maintaining doping uniformity while still achieving strong bonding
4Reliability
If an n-type Si substrate doped with phosphorus is used to change elastic constant and improve temperature characteristics, then the temperature characteristics are improved, but variation in resonant frequency is generated due to nonuniform phosphorus concentration
Solution Approach 1:
The patent forms SiO2 films through thermal oxidation before bonding occurs. These pre-formed films protect the phosphorus-doped regions from dopant scattering during thermal bonding, maintaining uniform phosphorus concentration and preventing resonant frequency variations while preserving temperature characteristics improvement
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 solution effectively suppresses variations in resonant frequency due to temperature changes by maintaining uniform doping and high Q-values, ensuring stable vibration performance.
Implementation Method 1
a silicon oxide film containing impurities is so provided as to be in contact with a lower surface of the n-type Si layer
Implementation Method 2
an excitation section including a piezoelectric thin film is formed on the Si semiconductor layer
Data Source
AI summary
A vibration device that includes a support member, vibration arms connected to the support member and each having an n-type Si layer which is a degenerate semiconductor, and electrodes provided so as to excite the vibration arms, and silicon oxide films containing impurities in contact with a respective lower surface of the n-type Si layers of each vibration arm.