Strippers No More: Aqueous Approaches to Residue Removal

		At a Glance
	Aqueous residue removers offer advantages of environmental friendliness, improved safety and lower cost of ownership than their solvent-based predecessors. Most importantly, residue removal efficiency is not compromised.

Organic solvents were and still are considered a 'necessary evil' of wafer cleaning. Market numbers for traditional solvent strippers are relatively flat due to replacement by dry processing and aqueous-based solutions introduced over the last several years. In aluminum interconnect structures, oxygen plasma ashing has largely replaced wet 'strippers' -- a term no longer appropriate to the specialty chemical formulations designed not so much to strip photoresist but to remove veils from via sidewalls and multiple combinations of post-etch residues in back-end wafer processing. 'Stripping is a carry-over term from years gone by when the industry used formulations that were very similar to paint strippers,' explains Gene Goebel of EKC Technology (Hayward, Calif.). When the need for residue removal first came about, people found that many strippers also could remove some residues. Today, unique formulations typically address either bulk photoresist removal or residue removal, though some do both. Current aqueous and semi-aqueous blends efficiently remove residues while improving safety and reducing disposal costs. In copper and low-k processing, engineers are deciding whether dry, wet or a combination of the two methods will be required to remove the new residues resulting from damascene process flows.

Companies such as JT Baker (Phillipsburg, N.J.) and Arch Chemicals (Norwalk, Conn.) offer mostly water-based removers, while ACSI (Milpitas, Calif.), Ashland-ACT (Easton, Pa.) and EKC offer semi-aqueous (<60% water) blends. Aqueous-based removers offer key advantages such as lower flammability, improved operator safety, reduced disposal costs and no VOC emissions that must be controlled in the exhaust plenum. NMP (n-methyl pyrollidone), a hazardous carrier solvent, is a SARA Title III chemistry, requiring the fab to account for all material brought into, disposed from or consumed in the facility. Many fabs have phased out the use of SARA Title III materials when suitable substitutes exist.

In the early 1990s, EKC created the first semi-aqueous chemistry to effectively remove post-ash, post-etch byproducts. In 1992, it introduced patented chemistry for cleaning Al, W, Ti, TiN and polysilicon surfaces after etching and ashing.

In 1997, Arch Chemicals introduced a completely water-based solution for post-etch residue removal: the Microstrip 5002 product. 'We realized that without the requirement for photoresist stripping we could develop an aqueous residue cleaning chemistry that controls particles and metal contaminants, while increasing operator safety and reducing environmental impact,' explains Vincent Leon, in applications development for Arch Chemicals' strippers and cleaners business. The 5002's mildly acidic chemistry increases thin film compatibility with Ti, TiN, Ta and TaN, and eliminates the need for an intermediate solvent rinse. This rinse step, usually in IPA (isopropyl alcohol), is needed to buffer the transition from an alkaline solvent-based chemistry to DI water, which otherwise results in 'pH shock' and metal corrosion.

Fig. 1 A semi-aqueous cleaning step removes post-etch residues. (Source: EKC)

The EKC4000 PCT solution is 'fully miscible in water, removes particles and transition metal contaminants, and eliminates 50%-60% of the water required to rinse their solvent predecessors,' according to Goebel. Current-generation residue removers (Figs. 1 and 2) must dissolve and remove residues containing a variety of species.

How residue removers work

Following aluminum etching, plasma ashing rapidly oxidizes organic resist and usually leaves an inorganic residue that may contain aluminum oxide, silicon oxide, titanium oxide and aluminum fluoride, among other compounds. The remover contains an active species, a chelating agent, which converts the residue(s) to organometallic material that will dissolve in the remover's solvent. Hydroxylamine-based removers are used widely in aluminum interconnect structures. 'Other removers are based on catechol, though newer, organic compounds with the ability to [form a] complex [with] metal-containing residues and solubilize them, are now available,' explains Steve Fine, senior scientist of ACSI. Fine adds that there are many ways of inhibiting corrosion during residue removal. 'You can add an anode inhibitor or cathode inhibitor to the stripper itself; you can bubble CO₂ through the rinse water; you can use a buffered rinse solution, which is easier than CO₂; you can design the chelating agent to inhibit corrosion; or you can buffer the stripper itself if the chemistry permits.'

Fine explains that the water in the remover can be considered an active ingredient because 'without water you completely change the performance characteristics of the residue remover. The water helps hydrate and activate the aluminum oxide-based residues -- often a major part of the residue -- while helping to solubilize the residue.' For completely aqueous cleaners, how quickly water evaporates from the chemical solutions, a function of the temperature at which the process is running, largely determines bath life. 'One of our customers qualified the bath life of our aqueous-based remover at 168 hours,' says Arch Chemicals' Leon.

Removing copper/low-k residues

Unfortunately, formulations used in aluminum interconnect processes -- based on hydroxylamines, for instance -- readily attack copper. 'Copper is being cleaned with a chemistry that is much more friendly to metals,' says Goebel.

To be compatible with low-k dielectrics, manufacturers of plasma ashing tools are developing low-energy oxygen plasmas, reducing chemistry (H₂/N₂) plasmas and fluorine-based plasmas to reduce damage to carbon-containing dielectrics. Even with these changes, however, wet residue removers may be required. 'As you move to copper and low-k dielectrics, there is at least an implication that there may be a move back to wet resist and residue removal because of the potential damage to low-k materials as a result of plasma processing,' says Brian O'Donnelly, business manager for photopolymer ancillary products at Arch Chemicals. One of the most critical cleaning steps follows dielectric etch to metal layers where sputtering of copper on sidewalls must be minimized and all residues removed before via metal deposition.

Cost of ownership

The COO of removal processes is a function of cleaning method (batch immersion processing, batch spray or single-wafer spray processing), process time, bath life (in wet benches) and post-process requirements (i.e., length of DI rinse, IPA intermediate rinse, etc.). The mechanical action induced by the high-pressure spray or by agitation in a batch process aids in removing residues. 'We can formulate our products to specific platforms, which is something customers did not require a few years ago,' explains Goebel. EKC has a joint development agreement with SEZ (Phoenix) to develop single-wafer processes on a multi-head platform for aluminum and copper-based interconnects. 300 mm processing may require such a single-wafer approach.

Fig. 2 ST-230 solution at 60°C for 30 minutes removes both inorganic and organic residues from 0.18mm TEOS-oxide vias on TiN/Al-Cu metal. (Source: ACSI)

ACSI's Fine says the strength of the remover sometimes determines the tool platform. For instance, with spray tools the process usually needs to be kept below 90° C to prevent damage to the tool's plastic seals.

Corrosion problems severely affect yield. Leon, of Arch Chemicals, explains, 'A failure in a valve can cause IPA to bleed into an alkaline chemistry or water into the alkaline chemistry, leading to corrosion.' Having a solution that can have a wide concentration range of water without affecting remover effectiveness can reduce failure modes, and increase bath life and the process window.

The number of different cleaning chemistries in a process flow also affects efficiency. Though there has been a general trend to application-specific formulations, when possible, a universal residue remover improves throughput and reduces cost. JT Baker's Dr. Wayne Cady, director of technology business development, claims his company's REZI-28 product offers low-temperature operation, short cycle time and, in some cases, an easier transition to copper and/or low-k dielectrics. Leon notes, 'Using our aqueous cleaner, we've been able to demonstrate efficient removal of residues that previously required up to six different solvents.' The company developed a joint process with Semitool (Kalispell, Mont.), MicrOzone, that uses the Microstrip 5002 chemistry and a DI water/ozone rinse to address post-etch residues while the ozone removes residual resist.

Remove solvents completely?

Though there is strong impetus to move all solvents out of wafer-processing environments, such an enormous change will not take place anytime soon. Because solvents are the basis for all positive photoresists and are used in edge bead removers, such a change would require complete replacement with alternative methods. Still, the aqueous and semi-aqueous residue removers allow significant improvements in safety and cost of ownership. Over the next year or so, engineers will decide whether wet, dry or both methods will be used with copper and low-k dielectrics.

For additional info:

Arch Chemicals	www.archchemicals.com
Ashland-ACT	www.ashland-act.com
Arch Chemicals JT Baker	www.jtbaker.com
EKC Technology	www.ekctech.com
TOK	www.tok.co.jp