Ursula Meyer and James D Getty look at the use of gas plasma in semiconductor packaging and assembly.

The use of gas plasma in semiconductor packaging and assembly is often perceived to be a ablack box' cure-all without consideration for the chemistry, configuration, or capability.
The application of gas plasma requires knowledge about the substrate materials and the process required, as well as an understanding of the influence of key plasma parameters. Without proper plasma process optimisation, the full potential of plasma treatment will not be realised. Therefore, only if a step away from the ablack box' mentality concerning plasma technology is taken, can plasma be applied to increase product quality, reliability and yield.
There is a wide variety of possible applications of plasma technology in advanced packaging. For example, rendering the surface properties can increase the product quality and reliability by decreasing delamination, increasing wire bonding strengths and CpK.
Ttypical applications of plasma treatment include cleaning and activation before die attach and wire bonding. Prior to molding, plasma can be used to remove contaminations and to activate the surfaces to improve the product quality and reliability. In addition to these well-known applications, plasma can furthermore be used to improve marking, clean optical surfaces, facilitate fluxless soldering and improve flow for underfill applications. Plasma can also be applied for etching of glass, silicon wafers or photo resists, and cross-linking f polymers.
Special challenges in microelectronics are decreasing feature sizes and the usage of new advanced materials. In these cases especially, plasma treatment is not only a product reliability enhancing technology, but can be essential for successful production.

Plasma technology

Plasma is a partially ionised gas that is electrically conductive and can be controlled magnetically. It is often described as the fourth state of matter, next to solid, fluid and gaseous. Plasma consists of electrons, ions, free radicals, photons (UV and visible light) and neutral gas molecules. An example for oxygen plasma is given in Fig.1.
Plasma discharge is initiated by injecting a process gas into a vacuum chamber and applying energy to it. Energy can be applied in the form of temperature, radiation, or an electrical field. For effective plasma processing of advanced technology products, a low temperature plasma ignited by an electric field offers optimum conditions. In this case, the plasma has a temperature of about 30--50°C and a pressure between 100--1000mTorr.
The reactions between the active species of the plasma and the substrate are not only influenced by the choice of the process gas, but also by the key parameters of the plasma process, such as applied power, chamber pressure and process time. The occurring reactions can be divided into physical and chemical processes. In chemical plasma processes, the free radicals chemically react with compounds on the samples surface. This process can lead to the formation of small volatile molecules that are pumped out by the vacuum pump, it is known as chemical cleaning. Due to the chemical reactions, new functional groups can be introduced on the surface of the samples (eg OH, COOH, NH2 and F), known as surface activation. The addition of new functional groups can be controlled by the selection of the process gas.
In physical processes, ions with high velocity and energy bombard the surface of the sample. During this process, molecules are broken free and can be removed by the vacuum pump. Most physical cleaning processes require high power and low chamber pressure. The high power allows ions with high velocity and energy, and the low pressure maximises the average distance that each ion can travel before colliding with another plasma species.
Each mechanism, chemical and physical, has distinct advantages and disadvantages with regard to cleaning and activation.
The key process parameters, applied power, chamber pressure and process time, dictate whether the process is physical, chemical or a combination of these mechanisms. Variation of the applied power and the chamber pressure in a plasma treatment recipe can strongly influence the process outcome. To achieve optimal results, each process has to be tailored exactly for the specific product and the requirements of the packaging process.

Plasma treatment in advanced packaging

New materials, packaging technologies, device technologies, and geometries are challenges for the packaging industry, and incorporation of plasma cleaning and activation in the process can help to overcome the challenges and increase product quality. Certain packaging techniques will only give satisfactory results when plasma treatment is used, making plasma an enabling technology.
Plasma prior to die attach improves both adhesion of the die attach epoxy and the bond between the die and substrate. Better bonds improve heat dissipation. In addition, plasma processing efficiently removes oxidation from the metal surface to ensure void-free die attachment. Oxidation can adversely affect die attachment when eutectic solder is used as an adhesive for die bonding.
Plasma processing prior to wire bonding maximises wire-pull strength and uniformity (CpK), and increases product reliability and production yield. Manufacturing challenges in the wire bond process, where plasma treatment can positively influence the results, include fine pitch wire bonding, decreasing dimensions, the application of new advanced materials, and flash gold bond pads.
Plasma processing prior to molding will improve the adhesion of the mold compound to the many package interfaces. Organic-based substrates, like those used in BGA packages, are susceptible to delamination. Delamination is caused by poor adhesion of the encapsulant compound to the multiple interfaces on the BGA package. Peel tests and C-Mode scanning acoustic microscopy (C-SAM) have proven that delamination issues are significantly reduced when BGA packages are plasma treated prior to molding.
Plasma processing prior to underfill will increase the wicking speed and result in a more uniform fillet height, which increases product stability. Furthermore, a significant decrease of voiding can be observed. See Figs.2 and 3 for plasma equipment examples.

Conclusion

Plasma pre-treatment proves more and more to be an enabling technology in back-end packaging. By applying a tailored plasma process, product reliability, quality and yield can be significantly improved. A thorough understanding of the application and the plasma process is crucial to successful implementation of plasma treatment for advanced packaging.

Ursula Meyer, PH.D. is with March Plasma Systems, Maastricht; The Netherlands, and James D Getty, PH.D. is with March Plasma Systems, Concord, CA, USA. www.marchplasma.com