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Aluminum Pedestal With Copper Channels for PbFree Solder Bump

IP.com Disclosure Number: IPCOM000181691D
Original Publication Date: 2009-Apr-09
Included in the Prior Art Database: 2009-Apr-09
Document File: 4 page(s) / 249K

Publishing Venue

IBM

Abstract

The core idea of the invention comprises an electromigration current-spreading pedestal structure that consists of an aluminum plate with copper-channels which are strategically placed so as to effect optimal current spreading through the PbFree solder bump structure. They are carefully designed and matched with the copper film thickness in the BLM (TiW-Cu-Ni) so that the channel thickness is less than 2x the copper thickness, such that the channel is completely filled with copper.

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Aluminum Pedestal With Copper Channels for PbFree Solder Bump

Pb Free solder chip:package connections with standard final via / BLM designs are sensitive to C4 electromigration problems. These problems are generally due to limited current spreading (edge effects, voiding, etc.) to solder Sn reaction with BLM Cu to form high resistance intermetallics and due to the risk of Sn penetration into BEOL copper. So design and structure modifications that enhance current spreading and limit BLM inter-reaction (to form high resistance IMCs) are needed in order to enable PbFree solder technology. The core idea of the invention comprises an electromigration current-spreading pedestal structure that consists of an aluminum plate with copper-channels which are strategically placed so as to effect optimal current spreading through the PbFree solder bump structure. They are carefully designed and matched with the copper film thickness in the BLM (TiW-Cu-Ni) so that the channel thickness is less than 2 tiems the copper thickness. Thus the the channel is completely filled with copper. This results in a near-planar surface for top surface Ni coverage, which preserves Ni barrier integrity. (Ni barrier integrity degrades with normal C4/Via topography). The final pedestal structure combines the integration advantages of aluminum and copper into a single novel structure, which is better for "white bumps" and long-term electromigration reliability than a standard copper pedestal. This makes it better for current capacity and spreading than an aluminum-only pedestal.

Figure 1. Basic Aluminum Pedestal w/Cu channels for Preferred (for Em) current feed to solder bump.

PbFree Solder Bump

~3um wide Cu channels in BLM

This Al-Cu Pedestal combines the feature of an Al nonreactive current spreader with Cu channels for centralized enhanced signal conduction within the solder/BLM, such that the Ni BLM top-layer has near planarity - which enables full barrier integrity at a minimal Ni thickness.

TiW-Cu-Ni BLM

Al / Cu Pedestal

Final Passivation

10

um

Dielectric

1

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PSPI

Top-down view of

Aluminum pedestal design with copper channels

Al / Cu Pedestal 10

Final Passivation

Al Pedestal w.

Cu trenches

TiW-Cu-Ni BLM

Cross-section of Aluminum pedestal design with copper channels

Advantages:

- Combines benefits of both aluminum and copper into pedestal structure

Al pedestal benefit
1. nonreactive structure, no IMC formation, reduces thickness requirement for Ni barrier in BLM
2. combines TD reliability benefit and pedestal into one stru...