by Michael A. Fury, Techcet Group

November 1, 2010 – Day 2 of the 7^th International Surface Cleaning and Preparation Workshop (Oct. 28-29) began a little less sharply focused than Day 1, given an excess of celebrating the previous night following the second unflattering World Series rout of the Texas Rangers by the San Francisco Giants. (The political symbolism is not lost on this blogger.)

Joel Barnett of SEMATECH talked about cleaning challenges for III-V materials, driven by the need for higher mobility and currently focused on InGaAs heterojunctions grown on Si substrates. The chemistry of Ga is particularly uncooperative with standard integration etching and cleaning processes. Passivation/depassivation strategies are proving to be useful, though many depend on the use of (NH₄)₂S, an unpleasant smelling beast. On the other hand, some ALD process paths provide for in situ surface self-cleaning. Monolayer doping from liquid solutions is proving to be a cost-effective alternative to ion implant for shallow junction doping. The outlook is still 2016-2020 for commercial devices, so GaAs remains, for now, the perpetual technology of the future.

Michael Fury of Techcet Group — yours truly — gave perspectives on cleaning-related supply chains with selected examples of recent evolutionary, revolutionary and disruptive developments. CMP pad conditioning is one arena with unexpected contributions, from new conditioning materials and substrates to a complete overhaul of the CMP consumables usage benchmark by using the conditioning arm as a waste slurry vacuum cleaner. The greatest leaps forward are being led by new metrology for non-contact charge-related and multi-spectral optical properties.

Fundamentals of nanoscale surface cleaning were reviewed by Northeastern U.’s Ahmed Busnaina, based largely on work from the NSF Center for Micro and Nanoscale Contamination Control. Megasonic cleaning is better served by increasing the liquid acoustic streaming at the wafer surface than by increasing cavitation. Fluorescence microscopy is preferred over SEM for particle inspection, as the e-beam exposure makes particles harder to remove.

Richard Reidy of the U. of North Texas looked at the effects of plasma and chemical modification on wetting of low-k films. Vapor condensation on surfaces is being investigated as metric for quantifying small differences in surface tendency to wet. Adsorption of D₂O was used to decipher surface interaction mechanisms on porous low-k surfaces following different pre-treatments, both in the liquid and vapor phases. A 20s O₂ ash pretreatment locks up the surface moieties and prevents water adsorption altogether, even on full immersion. This factoid was a hot Q&A topic.

Mark Lichty of CT Associates explained the effect of pump type on particle shedding and membrane filter particle performance. Shed particle counts did not vary with flow rate and outlet pressure as much as they varied between pump types (centrifugal, bellows, and diaphragm designs). The centrifugal pumps shed fewer particles overall than the other designs over the full range of operating conditions tested. On filter membranes, hydraulic shock is a known factor in degrading particle retention performance; some pumps generate this effect, with diaphragm having the largest pressure excursions and centrifugal pumps the smallest. In all cases, the diaphragm pumps gave the poorest filter retention results, degrading even further as flow rate increases. Over an extended filter loading period, particle retention degradation was proportional to pump pulse intensity, again with centrifugal giving the best performance, followed by bellows then diaphragm.

Dongsik Kim of Pohang U. described a novel surface cleaning process using a laser-induced micro liquid jet, also known as laser shock cleaning (LSC). The 20-30nsec laser pulse at 805nm can create a 1km/sec spray from a 1pl to 1μl water droplet, making it useful for surface particle cleaning, but also for micromachining. Power density is restricted to create a laser-induced plasma with forward momentum, rather than vaporizing the droplet in place. The liquid volume is so small that it is essentially a dry process. Extending this concept further is a startup in China, Synova, that uses a micro water jet as a total internal reflection optical guide for laser wafer dicing with less stress damage than conventional laser dicing.

Tae-Hoon Kim of Northeastern U. studied the removal mechanism of nanoscale particles in wet laser shockwave cleaning (WLSC). The liquid medium ~25μm thick is used to affect cleaning of both inorganic particles and organic thin-film contaminants. Each shock wave propagates with laminar flow from wafer center to edge, favoring particle removal from the wafer surface. WLSC particle removal efficiency was greater than LSC for both inorganic and organic particle samples.

Deog-Ju Moon of Micro Chemical Korea gave an overview of challenges in FPD glass surface preparation. The industry has evolved from knife cleaning to cleaning tapes to cleaning pads, which have surface nubs similar to PVA brush rollers to allow liquid to flow across the surface and reduce frictional heating. Abrasive rollers are used for cleaning the more sensitive ITO panels. As expected, there is zero tolerance for scratching during cleaning.

Jacob Andrews of Entegris presented some advancements in liquid purification technologies using high-purity polymer-based membrane systems. Standard filtration removes particles but not ionic contaminants, particularly metal cations. Sulfonic acid groups are grafted to a porous membrane surface to affect point-of-use ion exchange for metals removal. Metal removal efficiency to 1ppb levels in DI water at 80°C exceeds 90%. POU capture of metals has been shown to increase device minority carrier lifetime. Metal removal may be effective in organic solvents as well, but confirmation is required for each solvent choice. In some cases, the sulfonic groups can react with the solvent, degrading the purity. Likewise, acid or base ions can interfere with metal removal, requiring confirmation under use conditions.

Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; e-mail [email protected].