Tapered UBM Electrical Connectors via Selective Light Absorption

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Solution Overview

Problem

The existing metal bump formation process in semiconductor wafer manufacturing faces challenges in achieving a tapered profile for Under-Bump Metallurgy (UBM) openings, leading to uneven stress distribution on low-k dielectric layers and increased solder bridging due to inconsistent light exposure during photoresist processing.

Innovation Solution

A method involving a photoresist with a photoactive compound (PAC) that absorbs light selectively, allowing for controlled light exposure to create openings with a bottom lateral dimension greater than the top, enabling the formation of non-reflowable metal bumps with a tapered profile, reducing stress on low-k dielectric layers and minimizing solder bridging.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If conventional photoresist processing is used with uniform light exposure, then the photoresist can be easily processed, but the UBM openings cannot achieve a tapered profile leading to uneven stress distribution

Engineering Contradiction:

Improvetapered profile of UBM openingsVSAvoidphotoresist processing complexity

Core Design Contradiction:

Manufacturing precisionVSEase of manufacture

Solution Approach 1:

The photoactive compound (PAC) concentration is varied through the thickness of the photoresist layer, creating local quality differences. The PAC concentration is highest at the bottom interface with the UBM layer and decreases toward the top surface, enabling selective light absorption and tapered opening formation in specific regions of the photoresist layer.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent changes the physical parameter of PAC concentration distribution within the photoresist layer. By controlling the gradient of PAC concentration from bottom to top, the light absorption characteristics are modified, allowing the formation of tapered UBM openings with bottom lateral dimension greater than top lateral dimension during photoresist processing.

Inventive Principle:

Principle #35Parameter changes

2Stress or pressure

If uniform metal bumps are formed, then the plating process is simpler, but stress distribution on low-k dielectric layers becomes uneven

Engineering Contradiction:

Improvestress distribution on dielectric layersVSAvoidmetal bump dimensional consistency

Core Design Contradiction:

Stress or pressureVSManufacturing precision

Solution Approach 1:

The metal bumps are formed with an asymmetric tapered profile where the bottom lateral dimension is greater than the top lateral dimension. This asymmetric geometry is achieved through the tapered UBM openings created by selective light exposure, and it provides more uniform stress distribution on the underlying low-k dielectric layers compared to uniform cylindrical bumps.

Inventive Principle:

Principle #4Asymmetry

3Reliability

If consistent light exposure is applied to photoresist, then the exposure process is uniform, but solder bridging increases due to inadequate tapered profile formation

Engineering Contradiction:

Improvesolder bridging preventionVSAvoidlight exposure distribution

Core Design Contradiction:

ReliabilityVSIllumination intensity

Solution Approach 1:

The photoactive compound concentration is made non-uniform through the photoresist thickness, with higher concentration at the bottom and lower concentration at the top. This local quality variation causes differential light absorption, enabling the formation of tapered openings that prevent solder bridging while maintaining a simplified uniform light exposure process.

Inventive Principle:

Principle #3Local quality

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 method effectively reduces stress on low-k dielectric layers and minimizes solder bridging by creating metal bumps with a greater bottom dimension and smaller top dimension, enhancing the bonding process and reliability of electrical connectors.

Implementation Method 1

a photoresist with a photoactive compound (PAC) that absorbs light selectively, allowing for controlled light exposure

Methodology Applied

Scientific EffectLight absorption: Absorption (EM radiation)

Data Source

PatentUS8853071B2Electrical connectors and methods for forming the same

Publication Date: 2014.10.07 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A method includes coating a photo resist over an Under-Bump Metallurgy (UBM) layer and exposing the photo resist. In the step of exposing, a light amount reaching a bottom of the photo resist is less than about 5 percent of a light amount reaching a top surface of the photo resist. The method further includes developing the photo resist to form an opening in the photo resist. A portion of the UBM layer is exposed through the opening. The opening has a bottom lateral dimension greater than a top lateral dimension. An electrical connector is formed in the opening, wherein the electrical connector is non-reflowable.