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PVD Magnetron Sputtering Parameters and their Effect on the Composition of AuSn Solder

80Au20Sn wt% eutectic solder is frequently used in the semiconductor packaging process due to its high strength, creep resistance, and electrical and thermal conductivities.  With new applications in back-side metallization, and particularly products such as LEDs, there is an increasing demand for thin (several microns) physical vapor deposited (PVD) solder layers.  For these applications, the composition of the solder is critical for proper flow and strength.  Materion, and one of their customers, NEXX Systems, partnered to optimize the manufacturing methods for producing AuSn sputtering targets and PVD magnetron sputtering parameters, to control the composition of the sputtered solder. 

During the development trials, many aspects of the solder material and manufacturing processes were studied.  Included were: the functioning of the eutectic solder in compound semiconductor applications, consideration of the entire soldering system and its effect on the chemistry, and wetting and strength of the solder. 

There are several sputtering parameters that affect the composition of the deposited layer including deposition power and chamber pressure (Argon flow).  There is a significant composition shift between the chemistry of the AuSn sputtering target and the deposited AuSn film, due primarily to the loss of Sn during sputtering.  The magnitude of this composition shift varies quite considerably for different sputtering systems. 

Studies conducted by Materion and NEXX found that some of the shift (1-2%) can be controlled by the process parameters of deposition power and chamber pressure (Argon flow).  Higher deposition power and lower chamber pressure result in greater Sn loss during the PVD sputtering process.  However, the majority of the shift should be compensated for by adjusting the composition of the sputtering target.  Typically, Sn-rich sputtering targets are used due to the anticipated loss of tin.  The aim is to deposit a film that is slightly more Sn rich (79.5Au 20.5Sn wt%) compared to the eutectic composition (80.25Au 19.75Sn wt%) due to the existence of a gold layer on either substrate or the soldered assembly which will interact with the deposited AuSn layer during soldering.  Thus during soldering the AuSn alloy composition will shift through the eutectic composition towards the Au-rich side of the eutectic. 

A complete paper with all the findings from this study will be presented at CS Mantech 2012 in Boston, MA, April 23-26.