MEMS Sensor Stress Isolation via Suspended Spring Frame

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Solution Overview

Problem

Microelectromechanical Systems (MEMS) sensors face challenges with high thermal stress due to dissimilar materials used in fabrication and packaging, leading to thermal coefficient of offset (TCO) issues and substrate deformation, which affect the accuracy and reliability of the sensors.

Innovation Solution

The MEMS sensor design incorporates a stress isolating frame mounted to substrate anchors via spring elements, suspending movable and fixed elements above the substrate, allowing the frame to move as a rigid body and reducing thermal stress impacts, while using two structural layers to maintain sense capability without direct contact with the substrate.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If dissimilar materials are used in fabrication and packaging, then manufacturing flexibility and cost are improved, but thermal stress and substrate deformation increase

Engineering Contradiction:

Improvemanufacturing flexibilityVSAvoidthermal stress

Core Design Contradiction:

Ease of manufactureVSObject-affected harmful factors

Solution Approach 1:

The sensor structure is divided into separate functional layers (sensing layer, structural layer, sacrificial layer) that can be independently fabricated and released. This segmentation allows each layer to be optimized for its specific function while reducing thermal stress coupling between dissimilar materials.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A sacrificial layer is introduced as an intermediary during fabrication that is selectively removed to create voids and release the sensing structure from the substrate. This intermediary enables the use of dissimilar materials while preventing thermal stress transmission from the substrate to the sensitive sensing elements.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of manufacture

If dissimilar materials are used in fabrication and packaging, then manufacturing flexibility and cost are improved, but thermal coefficient of offset increases

Engineering Contradiction:

Improvemanufacturing flexibilityVSAvoidthermal coefficient of offset

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The sensing structure is extracted and released from the substrate by removing the sacrificial layer, creating a suspended configuration. This extraction eliminates the direct thermal coupling between the substrate and sensing elements, thereby reducing thermal coefficient of offset while maintaining manufacturing flexibility with dissimilar materials.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The sensing structure transitions from a planar configuration to a three-dimensional suspended structure with voids beneath the sensing elements. This dimensional change allows thermal isolation while maintaining electrical connectivity and sensing functionality, reducing TCO effects despite using dissimilar materials.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Object-affected harmful factors

If the sensing structure is released from the substrate, then thermal stress isolation is improved, but device complexity increases

Engineering Contradiction:

Improvethermal stress isolationVSAvoidfabrication process complexity

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The sacrificial layer is deposited and patterned during the initial fabrication stages, preparing the structure for subsequent release. This preliminary action integrates the release mechanism into the standard fabrication process flow, achieving thermal stress isolation without requiring complex post-fabrication processing steps.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The sacrificial layer is temporarily incorporated during fabrication and then selectively removed to achieve the desired suspended structure. This temporary incorporation followed by selective removal simplifies the overall process compared to attempting to create the suspended structure directly, as it uses standard deposition and etching techniques.

Inventive Principle:

Principle #34Discarding and recovering

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 design effectively isolates the MEMS sensor from thermal stresses, reducing thermal coefficient of offset and improving accuracy and reliability by minimizing substrate-induced deformations and stress effects on the sensor's output.

Implementation Method 1

spring elements connecting the support structure to substrate anchors

Methodology Applied

Scientific EffectElasticity: Elasticity

Data Source

PatentUS8925384B2MEMS sensor with stress isolation and method of fabrication

Publication Date: 2015.01.06 STMICROELECTRONICS INT NV

AI summary

A MEMS sensor (20, 86) includes a support structure (26) suspended above a surface (28) of a substrate (24) and connected to the substrate (24) via spring elements (30, 32, 34). A proof mass (36) is suspended above the substrate (24) and is connected to the support structure (26) via torsional elements (38). Electrodes (42, 44), spaced apart from the proof mass (36), are connected to the support structure (26) and are suspended above the substrate (24). Suspension of the electrodes (42, 44) and proof mass (36) above the surface (28) of the substrate (24) via the support structure (26) substantially physically isolates the elements from deformation of the underlying substrate (24). Additionally, connection via the spring elements (30, 32, 34) result in the MEMS sensor (22, 86) being less susceptible to movement of the support structure (26) due to this deformation.