Flexible Polyimide Dielectric for Interconnect Stress Management
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Solution Overview
Problem
Extreme low-k (ELK) materials used in semiconductor interconnect structures are brittle and have thermal expansion coefficients mismatched with the IC substrate, leading to delamination and cracking issues during testing.
Innovation Solution
Incorporating a flexible film, such as polyimide, as a high-level intermetal dielectric material in one or more layers to act as a stress buffer, reducing mechanical stress and thermal expansion mismatch between ELK materials and the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If ELK materials are used to reduce dielectric constant, then signal propagation delay is reduced, but mechanical reliability deteriorates due to brittleness and thermal expansion mismatch
Solution Approach 1:
The patent uses a composite structure combining ELK dielectric material with a flexible stress buffer layer. The ELK material (e.g., carbon-doped silicon dioxide with k<3.0) provides low dielectric constant for fast signal propagation, while the flexible layer (e.g., polyimide or silsesquioxane) provides mechanical compliance and thermal expansion matching, creating a composite interconnect structure that achieves both electrical performance and mechanical reliability
Solution Approach 2:
The patent changes the mechanical and thermal parameters of the interconnect structure by introducing a flexible layer with specific properties: elastic modulus of 1-10 GPa (much lower than ELK), and coefficient of thermal expansion matched to the substrate. This parameter modification allows the structure to accommodate thermal cycling without cracking while maintaining the low-k electrical performance
2Strength
If SiO2 is used as dielectric material, then mechanical strength is high, but dielectric constant is too high causing increased propagation delay
Solution Approach 1:
The patent applies different materials with different properties to different locations/levels of the interconnect structure. The ELK material is used in specific IMD layers where low dielectric constant is critical for signal speed, while the flexible stress buffer layer is placed in locations where mechanical compliance is needed. This local differentiation allows optimization of both speed and strength in different parts of the structure
3Object-generated harmful factors
If low-k materials are used to reduce dielectric constant, then crosstalk is reduced, but thermal expansion mismatch with substrate increases causing delamination
Solution Approach 1:
The flexible stress buffer layer acts as an intermediary between the low-k/ELK dielectric layers and the substrate. This intermediate layer has a coefficient of thermal expansion that is substantially matched to the substrate, serving as a thermal expansion buffer that prevents delamination of the low-k layers during thermal cycling, while allowing the low-k materials to maintain their low crosstalk performance
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 flexible film layer effectively reduces cracking and improves yield by providing stress release and matching thermal expansion coefficients, enhancing the mechanical stability and reliability of the interconnect structure.
Implementation Method 1
a flexible film, such as polyimide, as a high-level intermetal dielectric material in one or more layers to act as a stress buffer, reducing mechanical stress
Implementation Method 2
many low-k and ELK materials have CTE values that differ substantially from that of the package substrate. As a result, during tests, de-lamination and cracking of the top IMD layers have been observed
Data Source
AI summary
An integrated circuit device has a dual damascene structure including a lower via portion and an upper line portion. The lower via portion is formed in a polyimide layer, and the upper line portion is formed in an inter-metal dielectric (IMD) layer formed of USG or polyimide. A passivation layer is formed on the IMD layer, and a bond pad is formed overlying the passivation layer to electrically connect the upper line portion.