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Protection of copper from intermetallic copper-tin formation for flipchip bumping applications

IP.com Disclosure Number: IPCOM000007639D
Original Publication Date: 2002-Apr-10
Included in the Prior Art Database: 2002-Apr-10
Document File: 5 page(s) / 38K

Publishing Venue

Motorola

Related People

Varughese Mathew: AUTHOR [+5]

Related Documents

US5937320 A1: PATENT [+4]

Abstract

A process to minimize/eliminate copper-tin intermetallic formation, during electroplated flip chip bump processing, is described. An area is defined for the electrolytic or electroless deposition of nickel by photolithography method. Nickel is deposited in such a way that it wraps around the copper underbump metallurgy. It eliminates the possibility of direct contact of tin with copper during the high temperature reflow process and hence reduces the copper-tin intermetallic formation which is considered as a major cause of bump failure. Since no electroplated copper is exposed , this process also enhances the selectivity of the etching of the sputtered bus layer vs.plated copper, as the etchant exposed to copper and nickel.

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Protection of copper from intermetallic copper-tin formation for

flipchip bumping applications

Varughese Mathew, Douglas G Mitchell, John G Franka,

Rebecca G Cole and Ken G Sklorenko

Abstract:        

A process to minimize/eliminate copper-tin intermetallic formation, during electroplated flip chip bump processing, is described. An area is defined for the electrolytic or electroless deposition of nickel by photolithography method. Nickel is deposited in such a way that it wraps around the copper underbump metallurgy. It eliminates the possibility of direct contact of tin with copper during the high temperature reflow process and hence reduces the copper-tin intermetallic formation which is considered as a major cause of bump failure. Since no electroplated copper is exposed , this process also enhances the selectivity of the etching of the sputtered bus layer vs.plated copper, as the etchant exposed to copper and nickel.

Introduction:

Flip chip bump formation by an electroplating process is emerging as a leading technology for high-density electronic packaging 1,2. Electroplate bump formation 3 involves the following steps,

(1) Formation of under bump metallurgy:

UBM formation typically includes three steps, (a) Deposition of adhesion/barrier and seed layer (b) Definition of plating area and  (c) Plating of copper stud.

Deposition of adhesion/barrier and seed layer:  A continuous layer is deposited on to a cleaned incoming wafer to form the adhesion and barrier layer. Typically a Ti based film is deposited as the adhesion / barrier layer e.g. TiWN. Since copper and solder deposition in the following process steps are by electrolytic plating, appropriate path should be provided for the electric current. For this purpose copper is deposited by sputter method on the top of the adhesion layer as a seed layer.

Area definition for plating: Once the adhesion /seed layer is deposited the next step is the definition of the area where the plating has to be occurred. Areas not to be plated have to be covered so that no electrolyte contact will occur. A photolithography process is used to define the area where material needs to be plated and to mask the unwanted areas.

Electroplating of copper:  Next step in the UBM formation process is plating of a copper stud. This stud has several useful and critical functions. If sufficient copper is not available all copper will be consumed by forming imtermetallic compound like Cu3Sn or Cu6Sn5 during reflow process. Since the intermetallic compounds have no or very little adhesion to the barrier layer, dewetting will occur and the bump could fall of at the interface of TiWN and Cu.

 (2) Bump Formation:

Electroplating of Solder: Electroplating of the solder follows the copper plating. Target solder composition depends on the customer and the product. When the plating occurs outside photoresisrt well the area changes and growth occurs isotropically to assumes a mushroom shape.

Post – Plating Process:

Photoresist Strip and Sputte...