A comparative analysis on physical and chemical plasma cleaning effects on surfaces

Plasma cleaning has been widely used to clean and improve adhesion of a given surface. In semiconductor industry, plasma cleaning is being applied before capacitor bonding, die bonding, wire bonding and molding process. The most common types of plasma cleaning being used are physical cleaning and chemical cleaning. In physical cleaning process, the surface of the substrate is being bombarded by ions with high velocity and energy. During this process, molecules of the contaminant from the substrate have been broken free through ion bombardment and can be discharged by the vacuum pump. Most physical cleaning processes require high power supply and low chamber pressure in order to achieve maximum acceleration or energy level of the ions before it hits the substrate surface. In chemical cleaning process, the free radicals or byproducts chemically react with compounds on the substrates; which can lead to the formation of small volatile molecules that are pumped out by the vacuum pump [1]. In this study, physical plasma cleaning and chemical plasma cleaning are being compared. The outcome of study is plasma cleaning effectiveness on copper (Cu) layer of lead frame and glue bonding adhesiveness on Silver (Ag) plated layer of lead frame. The effectiveness of plasma cleaning can be correlated to copper oxide thickness change on Cu layer. The thickness of oxide layer being removed shows how each of the plasma cleaning technology helps to clean Cu layer. In production, lead frames are usually exposed to heat source in atmospheric environment, hence promoting oxide layer on the surface. Examples of heat source are curing oven and pre heat plate in molding machine. Measurement of copper oxide layer will be done using EDX machine where the atomic percentage of oxygen will be calculated and be fit to a model of metal oxide to estimate the thickness of copper oxide layer. On the other hand, adhesiveness of epoxy glue is a function of surface energy change and surface roughness cha- ge of the Ag layer. Plasma cleaning helps to increase surface energy of Ag layer to overcome surface tension of epoxy glue; hence improving wet ability of epoxy glue on Ag layer. Apart from increasing surface energy of the Ag layer, Ag layer is being roughened through ion bombardments, to increase effective area for epoxy glue to adhere. In this study, Dataphysics OCA20 contact angle measurement device is being used to calculate surface energy of Ag layer, using two different liquid solutions in validating surface energy on Ag layer. Meanwhile, Sensofar PLu is being used to calculate surface roughness of Ag layer. Overall, these two types of plasma cleaning can be compared by distinguishing effectiveness of plasma cleaning through removal of oxide layer and better adhesion of epoxy glue through surface energy and surface roughness change.