Auxiliary Bumps Stabilize Ultra-Dense Interconnect Alignment
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Solution Overview
Problem
At smaller pixel sizes and pitches, the alignment and registration of interconnect bumps on micro LED displays become difficult due to limited space, leading to potential misalignment and defects such as short circuits, open circuits, and weaker mechanical bonds.
Innovation Solution
The use of auxiliary bumps, such as interdigitated or nail bumps, which are formed and bonded in the same process as interconnect bumps, stabilizes the alignment of interconnect bumps during bonding, reducing slippage and misalignment by providing lateral stability without requiring additional alignment steps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel size and pitch are reduced to increase display density, then the display resolution and density are improved, but the alignment and registration control of interconnect bumps becomes difficult
Solution Approach 1:
Alignment bumps are introduced as intermediary elements between the interconnect bumps on the LED die and the logic die. These alignment bumps serve as reference markers that facilitate precise registration and alignment during the bonding process, enabling accurate placement even at reduced pixel pitches below 5 microns.
Solution Approach 2:
The alignment bumps are formed on the LED die before the bonding process. This preliminary formation of alignment features allows the bonding system to establish precise registration and alignment prior to the actual interconnect bump bonding, ensuring accurate positioning even when working with smaller pixel dimensions.
2Productivity
If the interconnect bump size is reduced to accommodate smaller pixel pitches, then the display resolution is improved, but the mechanical stability and bonding reliability are reduced
Solution Approach 1:
Alignment bumps act as intermediary reference structures that do not carry electrical signals but provide mechanical and positional stability during bonding. This allows the actual interconnect bumps to be smaller while maintaining overall bonding reliability through the stabilizing presence of the alignment bumps.
Solution Approach 2:
The bump structure is segmented into two functional types: alignment bumps for positioning and stability, and interconnect bumps for electrical connection. This segmentation allows the interconnect bumps to be minimized for high density while the alignment bumps provide the necessary mechanical stability for reliable bonding.
3Productivity
If the pitch between bumps is decreased to increase pixel density, then the display resolution is improved, but the risk of misalignment and defects increases
Solution Approach 1:
Alignment bumps serve as intermediary reference markers that enable precise registration even when the pitch between functional interconnect bumps is reduced. This intermediary alignment system reduces the risk of misalignment defects that would otherwise increase with smaller pitches.
Solution Approach 2:
By forming alignment bumps in advance on the LED die, the system establishes a precise reference framework before bonding begins. This preliminary alignment infrastructure allows for accurate placement of interconnect bumps even at reduced pitches, thereby reducing the risk of misalignment and associated defects.
Data Source
AI summary
Additional “auxiliary” bumps are used to stabilize alignment and reduce slippage of dense arrays of interconnect bumps on opposing die during a bonding process. One example of auxiliary bumps are interdigitated bumps. Interdigitated bumps are more self-aligning and laterally stable because bumps do not meet head-to-head. Rather, the head of a bump on one die falls into the space between bumps on the other die. Another example of auxiliary bumps are nail bumps. In nail bumps, one bump is harder (the nail) and “drives” into the opposing softer bump during bonding. This constrains the lateral movement of the two bumps relative to each other and reduces lateral slippage. In some embodiments, the auxiliary bumps and interconnect bumps are formed in the same process, and also bonded in the same process.