Charge build-up and its reduction in plasma cleaning process

Charge build-up in plasma cleaning process has been investigated from the view points of the plasma uniformity, gas species and pressure, plasma exposure time, anode-cathode distance and substrate (circuit board) configuration. The spatial distribution of plasma parameters in plasma cleaning was diagnosed using a Langmuir probe. The charge build-up was evaluated using metal/nitride/oxide silicon (MNOS) capacitors and metal/oxide/silicon (MOS) capacitors. Gases such as Ar, Ar+H₂, Xe and O₂ have been investigated. It has been found that the spatial distribution of plasma parameters is uniform, and that the charge build-up in the plasma cleaning is negligibly small as far as test chips are placed without substrates. Use of substrates was found to increase the amount of the charge build-up. The charge build-up was found to depend on size, material and structure of the substrate. It was found that the plasma cleaning with substrates having a conductive surface film such as plated gold film resulted in considerable charge build-up. We found that use of insulator mask or insulating die-bonding paste was very effective to minimize the charge build-up.