Sn Alloy Bump Plating Sequence for Uniform Ag-Cu Composition
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Solution Overview
Problem
Conventional methods for forming Sn alloy bumps, particularly in high aspect ratio patterns with fine pitching, face challenges in compositional control due to precipitation variations of Ag and Cu, leading to instability in the plating solution viscosity and reduced Ag and Cu deposition at the bottom, resulting in undesirable Sn precipitation and compositional variations in the height direction.
Innovation Solution
The method involves forming a preformed Sn layer on an electrode pad, followed by laminating alloy layers with specific compositions of Sn-Ag and Sn-Cu, which are then melted to reduce the depth of the opening and suppress compositional variations of Ag and Cu, ensuring uniform composition of the Sn alloy bump.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If electrolytic plating is performed using an Sn-Ag-Cu alloy plating solution or two layer plating is performed using an Sn-Ag plating solution and an Sn-Cu plating solution to form alloy plating in the opening of resist on high aspect ratio pattern, then the bump can be formed, but precipitation of Ag and Cu is suppressed at the bottom of the opening resulting in undesirable increase in Sn precipitation at the bottom and compositional variations in the height direction
Solution Approach 1:
The plating process is divided into multiple sequential steps with different plating solutions: first an Sn-Ag alloy plating step, then an Sn-Cu alloy plating step, and finally an Sn plating step. This segmentation allows each metal component to be deposited in a controlled manner, preventing premature precipitation and ensuring uniform composition throughout the bump height.
Solution Approach 2:
The Sn-Ag alloy plating is performed first as a preliminary action before the Sn-Cu and Sn plating steps. This preliminary deposition of Ag-containing layer ensures that Ag is properly distributed in the lower portion of the bump before subsequent plating operations, preventing Ag precipitation suppression that would occur if plating were performed in a different sequence or as a single step.
2Manufacturing precision
If electrolytic plating is repeated a number of times, then the bump can be formed with desired composition, but the acid concentration in the plating solution becomes high resulting in an increase in viscosity of the solution and making it further difficult for Ag to be precipitated at the bottom
Solution Approach 1:
The invention changes the plating parameters by using multiple different plating solutions with varying compositions and pH levels. Each plating step uses a solution optimized for depositing specific metal components, allowing repeated plating operations without the acid concentration and viscosity problems that would occur with repeated use of a single plating solution.
3Productivity
If fine pitching is implemented for microminiaturization, then the device density is increased, but the aspect ratio of the opening increases making it more difficult to control the composition uniformly in the height direction
Solution Approach 1:
The plating process is segmented into multiple steps targeting different height regions of the bump. The Sn-Ag plating step primarily deposits material in the lower portion, the Sn-Cu plating step deposits in the middle region, and the final Sn plating step completes the upper portion. This segmentation allows uniform compositional control even in high aspect ratio openings required for fine pitch applications.
Solution Approach 2:
Different metal compositions are deposited in different local regions of the opening height. The lower portion receives Ag-rich plating, the middle portion receives Cu-rich plating, and the upper portion receives Sn-rich plating. This local quality approach ensures that each region of the bump has the appropriate composition for its position, achieving overall uniformity despite the high aspect ratio.
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 allows for high compositional uniformity of the Sn alloy bump, effectively addressing the challenges of fine pitching and aspect ratio patterns by maintaining target compositions even after reflow processing, thereby enhancing the stability and consistency of the alloy bumps.
Implementation Method 1
forming a Sn layer on an electrode pad in an opening of a resist by electrolytic plating; laminating an alloy layer composed of Sn and the other metal on the Sn layer by electrolytic plating
Implementation Method 2
forming an Sn alloy bump by melting the Sn layer and the alloy layer laminated thereon by reflow processing
Data Source
Figure 1(a)~1(c)
Figure 2(a)~2(c)
Figure 3
AI summary
Provided is a method for manufacturing an Sn alloy bump, wherein composition of the Sn alloy bump can be readily controlled. The method for manufacturing an Sn alloy bump formed of an alloy composed of Sn and other one or more types of metals has a step of forming an Sn layer (4a) on an electrode pad (3) in a resist opening (2a) formed on a substrate (1) by electrolytic plating; a step of laminating Sn and an alloy layer (4b) on the Sn layer (4a) by electrolytic plating; and a step of forming an Sn alloy bump (5) by melting the Sn layer (4a) and the laminated alloy layer (4b) after removal of a resist (2).