UBM Undercut Reduction via Rapid Etch-Rinse Switching
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Solution Overview
Problem
Conventional metal bump formation processes result in significant undercuts due to lateral etching of the titanium layer, leading to delamination of metal bumps and low yield, particularly when using ammoniacal etching for copper seed layer removal and high-concentration HF for titanium etching.
Innovation Solution
A method involving the formation of an under-bump metallurgy layer with a barrier and seed layer, followed by acidic etching of the seed layer using diluted phosphoric acid and hydrogen peroxide, and low-concentration HF etching of the barrier layer, with rapid etch-to-rinse switching to minimize undercuts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If ammoniacal etching is used to remove copper seed layer and high-concentration HF is used to etch titanium layer, then the etching effectiveness is improved, but significant undercuts are formed under metal bumps leading to delamination
Solution Approach 1:
The patent changes the concentration parameter of HF etchant from high (11%) to low (0.5-5%), and changes the etching method from ammoniacal to acidic etchants. This parameter modification reduces the lateral etching that causes undercuts while maintaining vertical etching effectiveness for removing the titanium barrier layer and copper seed layer.
Solution Approach 2:
The patent implements a multi-step periodic etching process with alternating etching and rinsing cycles. Each etching step is followed by a rinse step to remove residual etchant, preventing continuous lateral etching. The process alternates between removing copper seed layer and titanium barrier layer in controlled sequences, reducing undercut formation compared to continuous high-concentration etching.
2Speed
If high-concentration HF solution is used for titanium layer etching, then the etching speed is improved, but lateral etching increases causing undercuts to extend up to 10 μm under metal bumps
Solution Approach 1:
The patent modifies the HF concentration parameter from 11% to 0.5-5%, which reduces the etching speed but simultaneously reduces lateral etching. This parameter change trades off some etching speed for better undercut control, achieving a balance where the titanium layer is removed effectively without forming significant undercuts.
Solution Approach 2:
The patent implements continuous rinsing between etching steps to remove residual etchant immediately. This continuous action of rinsing prevents the etchant from continuing to etch laterally after the intended etching duration, maintaining control over undercut formation while preserving etching effectiveness.
3Device complexity
If conventional etching process is used, then the metal bump formation process is simple, but the yield is low due to delamination of metal bumps from metal pads
Solution Approach 1:
The patent introduces a multi-step periodic process with alternating etching and rinsing cycles. This periodic action, while increasing process steps, ensures complete removal of etchant between etching steps, preventing delamination. The systematic alternation between etching titanium and copper layers with intermediate rinses improves metal bump adhesion and overall process reliability.
Solution Approach 2:
The patent changes the etchant concentration parameters and types (from ammoniacal to acidic), which modifies the etching characteristics to reduce delamination. Although this changes the process parameters, it maintains reasonable process simplicity while significantly improving metal bump adhesion and reducing defects.
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
Significantly reduces undercuts by 30% compared to conventional methods, enhancing the reliability of metal bump formation and redistribution line processes by minimizing delamination.
Implementation Method 1
A first etch is performed to remove the seed layer portion
Implementation Method 2
A second etch is performed to remove the barrier layer portion
Data Source
AI summary
A method of forming a device includes providing a wafer including a substrate; and forming an under-bump metallurgy (UBM) layer including a barrier layer overlying the substrate and a seed layer overlying the barrier layer. A metal bump is formed directly over a first portion of the UBM layer, wherein a second portion of the UBM layer is not covered by the metal bump. The second portion of the UBM layer includes a seed layer portion and a barrier layer portion. A first etch is performed to remove the seed layer portion, followed by a first rinse step performed on the wafer. A second etch is performed to remove the barrier layer portion, followed by a second rinse step performed on the wafer. At least a first switch time from the first etch to the first rinse step and a second switch time from the second etch to the second rinse step is less than about 1 second.