Pure Tin Micropad Formation via Immersion Plating
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Solution Overview
Problem
The existing methods for chip bonding in stacked integrated circuits face reliability issues due to the formation of brittle intermetallic compounds like Cu3Sn and Cu6Sn5, which are prone to shear and stress fractures, and become non-bondable at high temperatures, requiring additional tin that increases thickness and is undesirable.
Innovation Solution
A method involving the formation of a pure tin micropad by replacing copper with tin in a semiconductor wafer using a tin immersion plating process, followed by thermo-compression bonding with a copper micropad at a temperature above the melting point of tin, preventing intermetallic compound formation and maintaining a lower, more flexible micropad height.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If copper and tin bond pads are used for thermocompression bonding, then mechanical and electrical inter-strata connections are provided, but brittle intermetallic compounds (Cu3Sn and Cu6Sn5) form at low temperatures causing reliability issues and shear fractures
Solution Approach 1:
The patent extracts the problematic copper element from the bond pad composition, using only pure tin for the bond pad. This eliminates the source of intermetallic compound formation while maintaining the bonding function through tin's inherent properties.
Solution Approach 2:
The patent changes the material composition parameter from copper-tin alloy to pure tin, fundamentally altering the chemical properties to prevent intermetallic formation. This parameter change resolves the reliability issue while maintaining bonding capability.
2Reliability
If additional tin is added to maintain bondability when intermetallic compounds form, then bonding capability is preserved, but micropad thickness increases which is undesired
Solution Approach 1:
The patent removes copper from the bond pad composition entirely, eliminating the need for additional tin to compensate for intermetallic formation. This maintains bondability while preventing thickness increase.
Solution Approach 2:
The patent applies a tin immersion plating process that pre-coats the copper interconnect with a controlled layer of tin, preventing excessive tin consumption during bonding and maintaining precise thickness control.
3Reliability
If high bonding temperatures are used to ensure bondability, then bonding capability is maintained, but other portions of the integrated circuit are degraded or fail
Solution Approach 1:
The patent changes the bonding temperature parameter to a lower range that is compatible with circuit survival, made possible by using pure tin which remains bondable at these lower temperatures without forming brittle intermetallics.
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 approach enhances the shelf-life and reliability of the semiconductor bond by avoiding intermetallic compound formation, allowing for a thinner, more flexible micropad that remains bondable even at high temperatures, thus improving the stacking and packaging of integrated circuits.
Implementation Method 1
The copper stud is immersed in a solution of tin in which the tin replaces at least 95 percent of the copper of the stud
Implementation Method 2
A second semiconductor device having a copper micropad is bonded to the semiconductor device by pressing the copper micropad against the tin micropad in an ambient temperature in excess of the melting point of tin
Data Source
AI summary
A method forms a micropad to an external contact of a first semiconductor device. A stud of copper is formed over the external contact. The stud extends above a surface of the first semiconductor device. The stud of copper is immersed in a solution of tin. The tin replaces at least 95 percent of the copper of the stud and preferably more than 99 percent. The result is a tin micropad that has less than 5 percent copper by weight. Since the micropad is substantially pure tin, intermetallic bonds will not form during the time while the micropads of the first semiconductor device are not bonded. Smaller micropad dimensions result since intermetallic bonds do not form. When the first semiconductor device is bonded to an overlying second semiconductor device, the bond dimensions do not significantly increase the height of stacked chips.