Micro-Acoustic Wafer Package With Low-Temperature Polymer Bonding
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Solution Overview
Problem
Wafer-bonding of materials with different thermal expansion coefficients (CTEs) in micro-acoustic devices leads to mechanical stress due to thermal mismatch, and existing methods like using glass cap wafers with enhanced CTEs face issues with ion diffusion and packaging reliability.
Innovation Solution
A micro-acoustic wafer-level package is formed using a base wafer with piezoelectric material, a polymer frame structure, and a glass cap wafer with a UV-curable polymer coating, allowing low-temperature bonding and minimizing thermomechanical stress through a sandwich-like arrangement that encloses the device structures, preventing ion diffusion and ensuring airtight sealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high temperature wafer-bonding is used to bond glass cap wafer to base wafer, then bonding reliability is improved, but thermomechanical stress increases due to CTE mismatch
Solution Approach 1:
The patent changes the bonding temperature parameter from high temperature to low temperature process, and modifies the glass composition parameters to adjust CTE matching. This resolves the contradiction by achieving reliable bonding without the high temperature that causes thermal stress, while the modified glass composition ensures adequate CTE compatibility at the lower bonding temperature.
Solution Approach 2:
The patent uses a composite structure combining modified glass material with specific ion compositions to create a cap wafer that has both low enough bonding temperature requirements and matched CTE to the base wafer. The composite approach allows simultaneous optimization of bonding reliability and stress reduction.
2Reliability
If heavy ions (e.g., barium) are added to glass to enhance CTE matching, then CTE compatibility is improved, but ion diffusion occurs at extreme conditions compromising device reliability
Solution Approach 1:
The patent changes the processing temperature parameter to low temperature bonding, which prevents the extreme conditions that trigger ion diffusion from the glass. By lowering the bonding temperature, the patent maintains CTE compatibility through modified glass composition while avoiding the harmful ion diffusion that would occur at high temperatures.
Solution Approach 2:
The patent converts the potential harm of ion diffusion into a benefit by carefully selecting specific ion compositions and controlling the bonding temperature. The same ions that could diffuse at high temperatures are used in a controlled low temperature process where they provide CTE matching without diffusing, turning a potential defect into a useful property.
3Stress or pressure
If low temperature wafer-bonding is used to combine materials with different CTEs, then thermomechanical stress is reduced, but bonding reliability may be compromised
Solution Approach 1:
The patent uses a composite glass material with specifically tailored composition to enable low temperature bonding while maintaining bonding reliability. The modified glass composition provides both the low melting point needed for low temperature processing and the mechanical properties required for reliable bonding, resolving the contradiction between stress reduction and bonding reliability.
Solution Approach 2:
The patent changes multiple parameters simultaneously: glass composition parameters to enable low temperature processing, bonding temperature parameter to reduce thermal stress, and bonding pressure parameters to ensure reliable bonding at the lower temperature. This multi-parameter optimization resolves the contradiction between reduced stress and maintained bonding reliability.
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 solution reduces residual thermomechanical stress and ensures a reliable, air-tight package with enhanced mechanical strength and chemical resistance, maintaining device performance and reliability.
Implementation Method 1
the polymer coating is a thin homogeneous layer and consists of or comprises a UV-curable polymer. This means that no thermal step is required for curing.
Implementation Method 2
When exposing two wafers out of different materials to a high temperature and mutually fixing the wafers the both wafers usually show different thermal expansion that cause mechanical stress in the wafer-bonded arrangement after cooling to room temperature.
Data Source
AI summary
A wafer-level package for micro-acoustic devices and a method of manufacture is provided. The package comprises a base wafer with electric device structures. A frame structure is sitting on top of the base wafer enclosing particular device areas for the micro-acoustic devices. A cap wafer provided with a thin polymer coating is bonded to the frame structure to form a closed cavity over each device area and to enclose within the cavity the device structures arranged on the respective device area.