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June, 2016 Demystifying the Magic of Plasma Treatment By Demetrius Chrysostomou, Ph.D., Director of Technology, PVA TePla P
lasma is a state of matter, differing from the gaseous state in that it can conduct electrici- ty. When sufficient energy is added to a gas,
some atoms and molecules eject an electron, ioniz- ing the gas into an electrically conductive state. The energizing source is normally an oscillating electric field (kHz through to MW) that accelerates the free electrons backwards and forwards. In doing so, the electrons collide with whatever gases are fed into the system, activating them chemical- ly. This action produces a highly reactive environ- ment that is used to clean and activate surfaces in preparation for downstream processing. Unlike the first three states of matter that fill
our physical world, plasma is seldom experienced. Naturally occurring plasma, such as the aurora borealis, fire and lightning were historically regarded as mysterious, dangerous or frightening events. Unfortunately, these notions have been carried forward into the industrial setting. This is not surprising in a way, since substrates to be treated are placed into a sealed vacuum chamber where their surface properties are changed “magi- cally” by the strange glow. Unfortunately, when something goes wrong
on a production line it is easy to point the finger at the plasma process, since it is the step we under- stand the least. Ironically, plasma is a very stable
The “magic” that happens on a surface exposed to plasma, is no
more mysterious than the working principle behind the internal combustion engine.
and repeatable process where reactant gases are continually pumped into the glow, discharge, and the product gases are continually pumped back out. Other parameters such as power, gas flow,
pressure and time are easily controlled and moni- tored.
Essential Pre-treatment Process optimization is necessary to minimize
the risk of a poor wire bond, or damage to the die during bonding on these increasingly fragile sub-
effectiveness of plasma on the integrity of the wire bond. The difference in bond pull/shear strength is obvious, and the more contaminated the bond pads are, the greater the observed improvement. With this information, trials are usually concluded, however further optimization can be accomplished by adjusting the wire bonder settings for the plas- ma cleaned substrates. This final optimization step is almost always overlooked. Regarding the “magic” that happens on a sur-
face exposed to plasma, it’s no more mysterious than the working principle behind the internal combustion engine. Using oxygen plasma to remove organic contamination from the surface of the material is also a combustion process. The plasma catalyzes the reaction between the surface hydrocarbon (contaminant) and the oxygen feed
Wire pull strengths plotted as a function of bonder power settings.
strates. Reducing the bonder’s power settings (force and ultrasonic energy) is critical in these sit- uations. To ensure high-quality bonding using more gentle parameters, plasma cleaning and acti- vation is an essential pre-treatment step. Gas plasma surface cleaning and activation is
a well-established technology in the electronics packaging industry. Plasma cleaning bond pads is considered a critical step prior to wire bonding, particularly for high-volume manufacturing. It not only increases ball and stitch bond strengths, but also reduces or eliminates non-stick events. The result is reduced interruption of the bonding process and increased yield and reliability of the device. It is easy to empirically demonstrate the
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gas, which produces CO2 and H2O as byproducts. This is analogous to an engine’s spark plug cat- alyzing the combustion of the air and fuel mixture in its cylinders. Although the chemical process is an exothermic one, the heat generated by the plas- ma combustion process does not have sufficient thermal load to significantly heat the device. In addition to the chemical component of
plasma, there is also a physical component. Ions can be accelerated within the primary plasma by an applied or induced bias. This results in a “mol- ecular-sandblasting” of the surface. The effect can be “dialed up” as a chemical asset, or even as a sputter agent, to remove inorganic salts and oxides. It can also be switched off to protect devices that are sensitive to electrostatic discharge.
Improving Wire Bond Performance Gas plasma technology can be used to
improve wire bond performance while simultane- ously reducing bonder power settings. A wire bond- ing trial was conducted on a standardized alu-
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