A majority of the gas plasma equipment manufacturers for the semiconductor packaging industry rely on a primary plasma mode for generation of active plasma species. The primary plasma mode employs an energy source to ionize and dissociate a source gas creating a gas plasma consisting of physically and chemically active components. Samples to be plasma treated are placed directly in the gas discharge, on or near the electrode plates of the system with full exposure to the working species of the plasma (i.e., ions, free radicals, and byproducts). An added benefit to capacitively coupled RF based plasma systems is the generation of a negative direct-current (DC) bias on the powered electrode. The self DC-bias is useful for applications which require more aggressive and anisotropic etching.
There are two variations of primary plasma: Direct primary plasma and Reactive Ion Etch Plasma (RIE) primary plasma. When considering these two modes, an appreciation of the electrode configuration characteristics of the system is crucial. The powered electrode is inherently more aggressive and runs at a higher temperature when compared to the alternating ground electrode in the process. Both the powered and the grounded electrode offer benefits to any given application. One must consider the location of the sample within the direct plasma, the sample characteristics, such as chemical or physical characteristics, and the required throughput for process justifications. Since the powered is more aggressive than the grounded electrode, cleaning, surface activation and etching processes are accomplished at an accelerated rate when parts are placed directly on the powered electrode. RIE processing further exploits the powered electrode benefit by minimizing the space between the two electrodes to increase the high RF-electric field and DC bias levels for true etching of materials. This is evidenced by failure analysis or MEMS applications where the creation of deep features in the sample with anisotropy and aggressive ion bombardment is desired.
Typical applications for direct primary plasma include all packages that are not sensitive to direct plasma exposure due to the integrated circuit technology or material selection. Plasma is employed for substrate pre-treatment, package processing prior to die attach, wire bond, underfill, mold or encapsulation. Plasma improves wire bond pull strength and CpK through contamination removal and surface activation processes; it increases the wicking speed, the filet height, and minimizes voids in underfill via surface modification processes; and it maximizes fluid adhesion in mold/encapsulation through surface activation mechanisms. The use of plasma for these applications greatly increases the yield and reliability of the microelectronic package.