By Gene Dunn, Panasonic Factory Solutions of America
Plasma technology is often characterized as a “dry” cleaning process, using ionized gases in vacuum chambers. In contrast to chemically-based wet technologies, which have their role removing thicker contaminants in the micron range such as flux residues, plasma deals with contamination in the nanometer range on substrate and wafer surfaces. While these oxides are invisible to the naked eye and quite thin, if left on bond pads, they can contribute to bond strength degradation in both wire bonding and gold bump flip chip applications.

This article describes the basics of vacuum-based plasma cleaning and surface modification technology, and demonstrates how plasma etching can removes contaminants to improve yields in gold bonding processes. In addition, auger electron spectroscopy (AES) is explained as a useful analytical technique for determining the elemental surface characteristics and the effectiveness of plasma treatment to remove contaminants. The surface modification aspect of plasma deals with altering the molecular makeup of the substrate and die surfaces of interest. Plasma treatment here will increase the number of C-O-H molecules resulting in a “rougher” surface that becomes more hydrophilic, thereby improving underfill and encapsulant wetting and adhesion.

Vacuum Based Plasma Technology
A plasma etching system consists of an RF generator and a vacuum chamber with a parallel plate arrangement that keeps the lower electrode at a negative voltage bias with respect to the upper electrode (Figure 1). This design is most effective for directional ion flow from the grounded upper electrode to the negatively biased lower electrode where the object to be treated is located. Typical vacuum pressures are set at 8-12Pa (60-90mtorr). The electric field helps accelerate the free electrons to collide with the argon atoms. Ionization occurs as a free electron dislodges an electron from the Ar atom, resulting in a positively charged Ar+ ion plasma. These highly directional ions move in parallel fashion toward the negatively biased chamber electrode and thus provide a physical etching effect to the Au pads on the substrate. Argon is particularly effective since it is relatively heavy and has a high sputter yield* with respect to gold and nickel.

Contamination Analysis of Gold Bond Pads
Die attach and subsequent adhesive cure steps can cause contamination to form on substrate or package gold bond pads. Untreated, this can lead to no-sticks or low ball shear at the wire bonding operation. Oftentimes the problem is mistakenly assigned to the wire bonder, resulting in attempts to alter the recipe to improve bondability. However, before assuming the wire bonder is at fault, it is important to analyze the condition of the gold bond pads by performing a ball shear test and if possible, an elemental analysis of the bond pad surface. Named after Pierre Auger, AES is an analytical technique that offers reliable surface analysis by using an electron beam and an electron spectrometer to measure energy levels of reflected Auger electrons. These particular electrons are of interest because their origin is from the first 5-50