Seed layer deposition in microscale features

Abstract

A method of forming a metal feature on a workpiece with deposition is provided. The method includes providing an under bump metal layer for solder of an electronic device on the workpiece, depositing a substantially pure tin layer directly to the under bump metal layer, and depositing a tin silver alloy layer onto the substantially pure tin layer.

Description

BACKGROUND[0001]1. Field[0002]The disclosed embodiments relate generally to a method and apparatus for applying metal structures to a workpiece, and more particularly to a method and apparatus for depositing a lead-free solder into micro-scale patterns in the surface of a workpiece coated with a photo-resist patterning film, and more particularly to a method and apparatus for electroplating tin-silver alloy solder bumps.[0003]2. Brief Description of Related Developments[0004]The semiconductor industry has been working towards eliminating lead in electronics, as required under the European Union's Restriction of Hazardous Substances (RoHS) Directive. The industry is moving faster than the regulation to offer “green” consumer's electronics with lead-free packaging. Electrodepositon of lead-free solder such as using through mask patterned deposition, is a technology capable of providing tight pitch bumping (connection pitch less than approximately 300 microns) or microbumping for advan...

AI Technical Summary

Problems solved by technology

Substantially pure tin has many desirable properties of a solder metal, for example fatigue, resistance, thermal cycling and ductile mechanical properties, however the industry has found that tin whisker growth in substantially pure tin solder makes it an unreliable joining solder for advanced packaging applications.

Tin-silver alloy (SnAg) solder plating in a conventional manner is more difficult than substantially pure tin electroplating or lead-tin (PbSn) electroplating because of the large difference in electrochemical reduction potential between tin (−0.13 volts SHE) and silver (+0.799 volts SHE).

The organic Ag+ ion complex in the plating solution does not eliminate the likelihood of unwanted Ag immersion deposition on the Under Bump Metal (UBM), which is typically Nickel or Copper, when electroplating SnAg lead free solder on such UBM structures.

This unwanted immersion deposition may cause void defects at the UBM / SnAg interface, said voids are observable after reflowing the solder, and such voids can cause mechanical and electrical failures of the chip to package joint.

Due to the high cost of the Ag-complexor and other components in commercial SnAg plating chemistries, the typical cost of SnAg plated bumps is several multiples of the PbSn bumps.

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