WHY GOLD SPITS DURING EVAPORATION

Evaporation is a physical vapor deposition (PVD) process whereby material is transformed into the vapor phase using thermal energy to enable the deposition of a thin film on a substrate, often a wafer.  It is a key technology employed to manufacture devices such as power amplifiers used in wireless handsets and for high brightness light emitting diodes (LEDs).  Gold is one of the principal materials evaporated.  Unfortunately, it can be prone to a behavior known as spitting.  Spitting involves the ejection of liquid droplets from the molten pool of metal instead of the smooth transfer of material through evaporation (See figure).

A certain fraction of these liquid droplets land on the deposition wafer where they solidify into solid particles.  These particles constitute a defect since they can lead to the improper functioning of a microelectronic circuit.  Spitting therefore negatively affects yield due to the scrapping of partial or whole production lots.  There are many factors that can affect spitting and can be divided into two broad groups: 1) process conditions, and 2) material quality.  Process conditions include power input, heat transfer, crucible liner user, E-Beam sweep pattern, ramp profiles and deposition rates.  Material quality is determined by both purity and cleanliness. 

Materion excels at providing extremely high purity gold (minimum of 99.999% purity) that has been deactivated and cleaned to the highest standards and can substantially reduce spitting and significantly increase our customers’ yield.  Furthermore, Materion can assist customers in developing an understanding of the critical process parameters that can also be adjusted to minimize the likelihood of spitting.  For more information, please contact Senior Scientist - Alan.Duckham@materion.com