MEMS Device Packaging with Cantilevered Substrate and Cap Layer

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Solution Overview

Problem

Microelectromechanical systems (MEMS) devices face challenges in packaging due to thermal stress, fragility, and cost constraints, requiring smaller size and environmental protection without affecting mechanical motion, and existing solutions like silicon gel coating add complexity and cost.

Innovation Solution

The method involves creating MEMS device assemblies with cantilevered substrate platforms and a cap layer to isolate stress, using low-cost overmolded packaging that prevents the molding compound from contacting the MEMS device, eliminating the need for silicon gel coating and reducing material costs.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If traditional packaging materials with dissimilar coefficients of thermal expansion are used, then environmental protection is provided, but high thermally induced stress develops during manufacture or operation

Engineering Contradiction:

Improveenvironmental protectionVSAvoidthermally induced stress

Core Design Contradiction:

Object-affected harmful factorsVSStress or pressure

Solution Approach 1:

A stress isolation layer is introduced between the MEMS device and the packaging structure as an intermediary element. This layer has a coefficient of thermal expansion matched to the MEMS device, serving as a mediator that prevents stress transmission from the dissimilar packaging materials while maintaining environmental protection.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The coefficient of thermal expansion parameter is strategically matched between the stress isolation layer and the MEMS device. By changing the material parameter of the isolation layer to match the device, thermal stress is eliminated during temperature variations while the outer packaging still provides environmental protection.

Inventive Principle:

Principle #35Parameter changes

2Volume of stationary object

If smaller size packaging is used to meet cost targets, then packaging size and cost are reduced, but stress isolation and device stability become more challenging

Engineering Contradiction:

Improvepackaging sizeVSAvoiddevice stability

Core Design Contradiction:

Volume of stationary objectVSReliability

Solution Approach 1:

The packaging structure is segmented into distinct functional layers: an outer packaging layer for environmental protection and a separate stress isolation layer for mechanical decoupling. This segmentation allows the compact outer packaging to provide protection while the specialized isolation layer maintains device stability regardless of overall package size.

Inventive Principle:

Principle #1Segmentation

3Stress or pressure

If silicon gel coating is used for stress isolation, then stress transmission is reduced, but manufacturing complexity and cost increase

Engineering Contradiction:

Improvestress transmissionVSAvoidmanufacturing complexity

Core Design Contradiction:

Stress or pressureVSDevice complexity

Solution Approach 1:

The stress isolation layer is implemented as a simple, inexpensive structural layer integrated into the packaging process, replacing the complex and costly silicon gel coating process. This disposable-like layer provides sufficient stress isolation functionality without requiring sophisticated manufacturing steps.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

4Ease of manufacture

If packaging materials with dissimilar coefficients of thermal expansion are used, then cost constraints are met, but package stress variations cause instability and output shifts

Engineering Contradiction:

Improvecost constraintsVSAvoidoutput stability

Core Design Contradiction:

Ease of manufactureVSManufacturing precision

Solution Approach 1:

The stress isolation layer acts as a mediator between the low-cost dissimilar packaging materials and the MEMS device. It allows the use of cost-effective packaging materials with different thermal expansion coefficients while preventing their stress variations from affecting device output stability.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS8304275B2MEMS device assembly and method of packaging same

Publication Date: 2012.11.06 NORTH STAR INNOVATIONS

AI summary

A MEMS device assembly (20) includes a MEMS die (22) and an integrated circuit (IC) die (24). The MEMS die (22) includes a MEMS device (36) formed on a substrate (38) and a cap layer (34). A packaging process (72) entails forming the MEMS device (36) on the substrate (38) and removing a material portion of the substrate (38) surrounding the device (36) to form a cantilevered substrate platform (46) at which the MEMS device (36) resides. The cap layer (34) is coupled to the substrate (38) overlying the MEMS device (36). The MEMS die (22) is electrically interconnected with the IC die (24). Molding compound (32) is applied to substantially encapsulate the MEMS die (22), the IC die (24), and interconnects (30) that electrically interconnect the MEMS device (22) with the IC die (24). The cap layer (34) prevents the molding compound (32) from contacting the MEMS device (36).