Charge build-up and its reduction in plasma cleaning process

Charge build-up in the plasma cleaning process has been investigated from the viewpoints of 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 MNOS capacitors and 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 plasma cleaning is negligibly small with regard to test chips placed without substrates. Use of substrates was found to increase the amount of charge build-up. Charge build-up was found to depend on the size, material and structure of the substrate. It was found that plasma cleaning with substrates with a conductive surface film such as a plated gold film resulted in considerable charge build-up. We found that use of an insulator mask or insulating die-bonding paste was very effective for charge build-up minimization