TaN-Al Pad Structure for Cu Low-k Wafer Stress Control
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Solution Overview
Problem
Intermetallic compound (IMC) cracking in post wire bonded dies during Cu/Low-k BEOL processing is caused by high tensile stresses in the film, leading to excessive Al diffusion into Au bonds, resulting in unstable Al-rich IMC phases and eventual cracking, which persists despite control of surface contamination and film properties.
Innovation Solution
A pad structure with a thick barrier layer, such as TaN, is introduced between the aluminum and copper layers to control macro stresses, with specific configurations like TaN (120 nm)-Al (1.2 μm) or TaN (60 nm)-Al (0.6 μm)-TaN (60 nm)-Al (0.6 μm)-TaN (60 nm), to reduce tensile stress and promote the formation of stable Au-rich IMC phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If Cu/Low-k metallization is used, then productivity and device performance are improved, but tensile stresses build up causing IMC cracking
Solution Approach 1:
The aluminum pad layer is divided into multiple thinner layers (e.g., three layers of 0.4 μm each) separated by barrier layers. This segmentation reduces the cumulative tensile stress in the aluminum structure while maintaining the required pad thickness and functionality, preventing IMC cracking during Cu/Low-k BEOL processing
Solution Approach 2:
Additional barrier layers (TaN or TiN) are introduced as intermediary structures between the aluminum pad layers and the copper interconnect layers. These barrier layers serve as stress management interfaces that control Al diffusion and modulate stress distribution, preventing direct contact between Cu and Al while managing the thermal and mechanical stress mismatch
2Reliability
If thick barrier layers are used to control stress, then IMC cracking is prevented, but device complexity increases
Solution Approach 1:
Instead of using a single thick barrier layer, the structure segments the barrier function into multiple thinner barrier layers distributed throughout the aluminum pad stack. This achieves the same stress control and diffusion barrier functionality while distributing the material volume and reducing local stress concentrations
Solution Approach 2:
The invention changes the parameters of the barrier layer configuration from a single thick layer to multiple thinner layers with specific thickness ranges (60-120 nm each). This parameter optimization maintains effective stress control and diffusion prevention while managing the overall structural complexity
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 proposed pad structures effectively reduce wafer bow and tensile stress, preventing IMC cracking by maintaining a stable Al layer for mechanical support and ensuring a uniform, gold-rich IMC formation, thus enhancing wire bonding reliability.
Implementation Method 1
Thermal stresses are generated due to a mismatch between the temperature coefficients of expansion between metal, dielectric films and substrate
Implementation Method 2
The proposed pad structures effectively reduce wafer bow and tensile stress
Implementation Method 3
the Al-diffusion rates are controlled to a low enough level that the stable Au-rich phases are formed
Implementation Method 4
The unstable Al-rich phases eventually undergo reverse phase transformations to Au-rich phases; the associated volume change (very large ~30%) in such phase transformations can result in voiding and eventual cracking of the IMC
Data Source
AI summary
A pad structure and passivation scheme which reduces or eliminates IMC cracking in post wire bonded dies during Cu/Low-k BEOL processing. A thick 120 nm barrier layer can be provided between a 1.2 μm aluminum layer and copper. Another possibility is to effectively split up the barrier layer, where the aluminum layer is disposed between the two barrier layers. The barrier layers may be 60 nm while the aluminum layer which is disposed between the barrier layers may be 0.6 μm. Another possibility is provide an extra 0.6 μm aluminum layer on the top barrier layer. Still another possibility is to provide an extra barrier layer on the top-most aluminum layer, such that a top barrier layer of 60 nm is provided on a 0.6 μm aluminum layer, followed by another harrier layer of 60 nm, another aluminum layer of 0.6 μm and another barrier layer of 60 nm.