Low-stress MEMS packaging via asymmetric bonding
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Solution Overview
Problem
Traditional packaging of MEMS acceleration sensor chips introduces thermal stress due to mismatched thermal expansion coefficients of materials, affecting sensor performance by deforming elastic beams and proof masses.
Innovation Solution
A low-stress packaging structure featuring a MEMS acceleration sensor chip with metal layers on both sides of the bottom and a groove between them, and a chip carrier with corresponding metal layers, where only one side is bonded, allowing for stress release through a free end and providing mechanical support without bonding on the other side, along with an expansion gap for thermal expansion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional packaging with full bonding is used, then mechanical support and electrical connection are achieved, but thermal stress deforms the sensor chip affecting performance
Solution Approach 1:
The bonding interface is segmented into two distinct sides: a first side with bonding structure for mechanical support and electrical connection, and a second side with free end for stress release. This segmentation allows the package to maintain structural integrity while eliminating thermal stress transmission to the sensor chip.
Solution Approach 2:
The packaging structure employs asymmetric bonding where only one side of the sensor chip is bonded to the substrate. This asymmetric configuration creates a stress-free zone on the opposite side, preventing thermal expansion mismatch from deforming the elastic beam and proof mass while still providing adequate mechanical support through the bonded side.
2Ease of operation
If bonding material is used to connect chip and carrier, then electrical connection is established, but thermal expansion mismatch causes stress transmission to the sensor
Solution Approach 1:
The electrical connection function is localized to the bonded side only, while the free end side provides stress relief. This functional segmentation allows electrical connectivity to be maintained through the bonding structure without requiring complete bonding coverage, thereby preventing stress transmission to the sensitive sensor elements.
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
Significantly reduces thermal stress, enhances impact-resistance and reliability, and minimizes packaging stress by allowing sufficient motion space and mechanical support, as evidenced by reduced offset angle and mass center displacement of the proof mass.
Implementation Method 1
When the external temperature changes, the bonding material, the package material, and the chip material will reflect different deformations due to the disagreement over thermal expansion coefficients of three materials
Data Source
AI summary
A low-stress packaging structure for a MEMS acceleration sensor chip includes a MEMS sensor chip and a chip carrier. Two sides of the bottom of the sensor chip are provided with a first metal layer and a second metal layer respectively. Two sides of a die attach area of the chip carrier are correspondingly provided with a third metal layer and a fourth metal layer. The first metal layer of the sensor chip and the third metal layer of the chip carrier are bonded together. The second metal layer of the sensor chip and the fourth metal layer of the chip carrier are only in contact but not bonded. A groove is arranged between the first metal layer and the second metal layer at the bottom of the sensor chip. A certain gap is defined between the sensor chip and cavity walls of chip carrier.