Secondary plasma provides two additional plasma modes: Downstream secondary plasma and Ion-Free Plasma (IFP) secondary plasma. Secondary plasma is typically used when a less intense plasma exposure is desired or the sample is sensitive to specific components of a primary plasma. Downstream secondary plasma is a more benign, but similar alternative to primary plasma.
Downstream secondary plasma (below) contains the same types of active species as a primary discharge, but with lower kinetic energy. The configuration relies on the transfer of the active species (ions, electrons, radicals, and byproducts) from an upstream primary discharge to a secondary process chamber or sample placement area. The secondary downstream plasma is initiated by the diffused ions and electrons and sustained with additional process gas in the secondary chamber. This configuration relies on both the physical and chemical plasma mechanisms. It has the advantage of relatively benign plasma processing however the success applications often suffer from treatment uniformities.
Ion-Free Plasma (IFP) (below) is a purely chemical plasma and is free of both ions responsible for the physical component and photons. The IFP process consists of the generation of active species upstream of the sample processing area followed by diffusion of active species through a gas baffle assembly. The gas baffle removes the ions, electrons, and photons allowing only the plasma generated free radicals and byproducts through to the sample area for chemically reactive plasma processing. The removal of ions and light eliminates the concerns of processing extremely sensitive devices, as there is no ion bombardment or UV light exposure at the sample.
Specific applications of the IFP technology include those where the exposure to active species of a traditional primary plasma may induce damage due to ion bombardment sensitivity or UV light exposure susceptibility. Specifically devices such as pre-programmed ASIC or memory devices, and CMOS image detectors may require an unfamiliar strategy for plasma processing – the isolation and use of specific components within the plasma. The newly defined secondary mode of IFP plasma has enabled applications success where more traditional plasma configurations have failed. The IFP plasma approach while it is not necessary for most traditional advanced packaging applications, it as proven to be an enabling technology for advanced memory components and hybrid packages. Additional plasma applications with similar issues of damage concern that benefit from an IFP plasma include: preprogrammed ASIC’s susceptible to erasure, CMOS image detectors, thin film substrates with bond pad thicknesses that are sensitive to sputtering, flip chip and wafer level devices.
Qualification and implementation of a plasma process in most cases will lend on the configurations familiar to most of us, utilizing a directional direct plasma mode that relies on ions, electrons, and free radicals of a specific gas chemistry for process enhancement. There is a growing realization that traditional methods and configurations that do not work for new advanced memory components can now benefit from a newly defined plasma mode. This plasma mode that is absent of electrons, ions and light, but full of chemically reactive species and capabilities beyond which were previously available, may be the answer to the challenges we face now and in years to come.