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Plasma Cleaning

Plasma cleaning is techniques involving the use of gas plasma to remove organic contaminants from wire bonding or die attach surfaces. By removing surface contamination, plasma cleaning increases the bonding or adhesive properties of the bond site surface.

WHAT'S A PLASMA?

Plasma is one of the four fundamental states of matter (the others being solid, liquid, and gas). When air or gas is ionized a plasma forms with similar conductive properties to that of metals. Plasma is the most abundant form of matter in the Universe, because most stars are in a plasma state. (See Figure 1)

Plasma Cleaning
Figure 1 Gas Plasma - the Fourth State of Matter (Henniker Plasma)

A plasma is a collection of positive, negative, and neutral particles in which the density of the negatively charged particles is equal to the density of the positively charged particles. When an energetic electron strikes a neutral gas molecule, it can cause dissociation and form free radicals and ions. The free radicals cause chemical reactions for destroying contaminants. For example, with oxygen, the dissociation process produces the free radical atomic oxygen (O). This reactive species has enough energy to break a carbon-carbon bond. (See Figure 2)

Most hybrids contain IC chips that are epoxy bonded using silver filled epoxy. Oxygen plasma will turn silver opsxy black because silver oxide is black in color.  Besides being unsightly this also changes the surface resistivity of the epoxy. 

Argon gas which is inert is often used or Argon with just a small percentage of oxygen gas to clean hybrids prior to die attach and wirebond.
Plasma Cleaning
Figure 2 Plasma Cleaning Process

PLASMA CLEANING EQUIPMENT


Plasma cleaning involves placing the pieces to be bonded into a plasma chamber, removing air from the chamber to create a vacuum, introducing a gas or gaseous mixture into the chamber, and applying energy to the chamber to produce the plasma. (See Figure 3) 

Commonly used gases include O2, N2, Ar, H2, and CF4. These five gasses used singularly or in combination are used in the majority of applications for activation, cleaning, etching and modification of surface chemistry.
Ionization of the gas (to create a plasma is generated by applying an energy field using one of three government regulated source frequencies:
  1. Low frequency or < 100 KHZ.
  2. RF frequency or 13.56 MHZ.
  3. Microwave frequency or 2.45 GHZ.
In the presence of the plasma, organic contaminants on the bond site surfaces are converted to carbon monoxide, carbon dioxide, water vapor, and/or other gasses, which are pulled out of the plasma chamber by a vacuum pump. After a suitable amount of time, the gas flow and energy are shut off, and the chamber is then purged with a nonreactive gas, such as nitrogen, to remove all traces of volatile compounds. Finally, the chamber is returned to atmospheric pressure. A cleaning cycle usually lasts from between 30 seconds to 15 minutes and is largely a function of the work piece loading in the plasma chamber.

Only a minute amount of contamination is cleaned up in a plasma cleaner, on the order of one  to maybe a few hundred angstroms of  organic contamination.  Research and experience has shown plasma cleaning  be effective,  especially for cleaning up surfaces prior to fine wire gold bonding processes

Plasma Cleaning
Figure 3:  Bench Top  Plasma Cleaner (AP-300 MARCH-NORDSON)

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