Micralyne Newslyne - August 2008

Electroplating of AuSn eutectic solder for MEMS packaging

Micralyne Inc.
In the drive to reduce packaging cost, wafer level packaging has become an important part of MEMS manufacturing. There are different methods for performing wafer level packaging such as fusion bonding, anodic bonding and
Fig. 1: EEJA cup electroplater.
Fig. 1: EEJA cup electroplater.
eutectic bonding to be name a few. Gold-tin (AuSn) solder is routinely used for eutectic bonding. AuSn eutectic solders are unique material for wafer level packaging, mainly due to their excellent thermal properties, mechanical strength and relatively low melting temperature, which makes them an attractive choice for hermetic sealing.

Application methods for deposition of AuSn solders for MEMS wafer level packaging is
mostly concentrated on costly PVD methods such as evaporation and sputtering techniques. However, electroplating is an economic, alternate deposition option that allows for high placement and composition accuracy. Micralyne has exclusive right to a patented process for electroplating of AuSn solder alloy, at
Fig. 4: SEM image of gold-tin solder electroplated on a 6" wafer using the cup plater. A sub-micron, densely packed grain structure can be seen on a smooth plated surface
compositions from 10 to 40 Sn (wt%), including commercially important Au-20Sn eutectic composition. Visit www.goldtinsolder.com for more information on Micralyne's gold-tin solder service.

In spring 2008, Micralyne installed a production-scale cup plater, EEJA_CP-0172, to expand its electroplating capabilities, in order to meet the customer requirements for AuSn electroplating needs. The EEJA electroplater is a semi-automated tool, plating wafers using an inverted fountain system. This new system provides superior uniformity and plating rates, reduced wafer processing time and
greater process control.

Gold-tin solder plated using this electroplater
has void free dense microstructure with excellent adhesion which are important factors for hermetic seal. With the unique pre-sensitizing process in place, Micralyne has been able to plate features as small as 5 microns in
Fig. 5: SEM cross section image of the same gold tin plated sample shown in Fig. 4. Dense columnar growth can be seen extending throughout the film.

- By Siamak Akhlaghi and
Kar Mun Cheng-

email: karmun@micralyne.com
email: siamak@micralyne.com
web: www.micralyne.com

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Contact Information: Micralyne Inc.
1911-94 Street, Edmonton, Alberta, Canada, T6N 1E6
Phone: 1.780.431.4400 Fax: 1.780.431.4422
Email: info@micralyne.com