By Terrence E. Thompson, Senior Editor
Until a few years ago, cleaning for IC assembly and packaging was not a priority. The reasons were simple enough: there were not many fine-pitch chips, interposers or microvia-like substrates that needed to be super clean. It's Not Just ICs Anymore Today, cleanliness is a priority. I/O pitches are steadily decreasing. The use of multiple devices in one package is gaining momentum, such as in SiP variants, where cleanliness is more stringent. The fact is the level of cleanliness needed for placing semiconductors or photonic parts in the same package is very challenging. Packages for both the pre- and post-assembly/packaging steps commonly need wet or plasma cleaning systems, and it is not unusual to use both techniques.
'Cleaner' Is Not an Option Speakers at a recent chip developers' forum noted that there is little point in pursuing ever-smaller IC geometries if the packaging infrastructure is not in place to move that performance increase through the package levels into the system. Consider that most cleaning techniques came from two ends of the electronics spectrum: PWBs or wafer cleaning. As these ends converge, they bring a formidable usage history to the cleaning party, and any one process will not do it all. If you lean toward wet cleaning, a solid base of chemical suppliers will be delighted to discuss your cleaning challenges. With lead-free solders now common, all the materials from wafer I/O pads and under-bump metallization to the final PWB traces need re-evaluation from soldering, fluxing and cleaning perspectives. 'Benign' Residue For instance, some of those "no-clean" fluxes leave a seemingly benign residue. While this might not be a problem with wide pitches and low I/O counts, it can be problematic with tighter pitches. Wetting is necessary, but it has physical limits with the chemistries used, as well as with device and substrate geometries. Wetting also presents challenges for cleaning solder bumps under an attached CSP. The reason? There is no easy way for fluids to flow into every crevice that needs cleaning. What seems obvious when digging into cleaning alternatives is that there are an incredible number of possibilities offered by wet chemistries.
Some plasma processes are better than others for certain applications. In other uses, most approaches will work. Some companies use both plasma and wet cleaning, as required, on the same wafer or device at appropriate stages. Ironically, wet or plasma may be used as a follow-up cleaner for the other process! There is no shortage of cleaning tools available, as the photos-representing only a few of the better-known models-indicate.
Plasma: Not New for Bumps For plasma cleaning, Tom Bergman of Matrix Integrated Systems says, "Over the years, we've seen an almost universal adoption of single-wafer plasma processing in WLP. For adhesion promotion, organic contamination removal and significant yield improvements, plasma processing is near ubiquitous in wafer and substrate cleaning. "Most WLP fabs have acquired single-wafer systems that are more capable than batch systems in delivering optimized processing solutions at high throughputs." WLP requires precise process control to successfully integrate multiple steps, Bergman observes. "Plasmas and plasma chemistries are enabling, but outside the acceptable process window they can be detrimental." Help in understanding the complex interactions between plasma processing and substrate materials is what equipment suppliers bring to the customer. Jim Marshall, Matrix's president, notes, "WLP and bumping technology have been available for quite some time. I bumped wafers at HP in 1977." Is the Future Wet? "What is the future of cleaning?" ponders Jim Mello of SEZ America. "Treating each wafer individually and being able to manage the same conditions repeatedly is why single-wafer processing is the future. "Yet, compared to batch processing, throughput and cost for comparable applications are drawbacks for single-wafer processing." Some single-wafer tools can clean both wafer sides at once. This is uncommon, he notes, because it is more difficult to build double-sided tools inexpensively. In those cases, batch cleaning may be the best choice. Why look at front-end-of-line cleaning tools for back-end-of-line answers? "It's a matter of defect density-the need to reduce particles and surface roughness," says Mello. Applications requiring surface conditioning for bonding are the drivers. They require tools that keep up with other equipment.
"I do not foresee more integration that would eliminate cleaning steps, but the technology roadmap does change faster than the solutions." Nor does he foresee a slowdown in technology innovation, despite the current production slowdown. Where batch excels is in its ability to run more wafers through simpler applications at a reduced price because of volume. However, batch will have to reduce its overall cost and footprint to remain competitive. "The decision to run batch processes is made because single wafer processes are still too expensive for certain technology nodes, and because most users are familiar with batch processing. In time, single-wafer cleaning will be in greater demand," Mello adds. Many Options When looking at the possible number of cleaning processes, it is clear that wet systems have very customizable and versatile platforms. Wet systems can immerse, spray, agitate or scrub. They provide a range of chemistries including aqueous, semi-aqueous and solvents. Who Will Win? The winners are those who package devices. The equipment tables following this article provide insights regarding the many options and divergent choices available. The time when you will require better cleaning is immediate for many and right around the corner for the rest of you. Narrow the options to fit your applications and budgets. As equipment vendors will show you, there are many solutions for common cleaning challenges.
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