Issue



Cleaning with Aqueous TMAH: An Environmentally Friendly Alternative


04/01/2007







BY KRISHNA SACHDEV, Ph.D. IBM

Conductive paste screening on green sheet through a mask is a common technique to delineate desired circuitry for ceramic modules. Screening masks are typically made of metal such as etched molybdenum (Mo) foil, electroform mask, stainless-steel stencil/screen for solder pastes, and emulsion mask for silk screening. Common conductive pastes for ceramic substrates are dispersions of Mo, Cu, Ni, or W metal powders in cellulosic or polyacrylate binder and include wetting agent, dispersant, surfactant, plasticizers, thickening agent, and a high boiling solvent vehicle. Depending on the paste chemistry, mask surface, and feature density, varying levels of paste residue are left on the mask, which requires frequent cleaning. Organic solvents were traditionally used for cleaning in many industrial and consumer product applications, prior to environmental regulatory restrictions of certain volatile solvent classes.


Figure 1.Interaction of paste constituents with aqueous TMAH in mask cleaning.
Click here to enlarge image

Several organic solvent replacements of hazardous substances include propylene glycol alkyl ethers, dibasic esters, ethyl lactate, and oxol solvents, but these still have associated issues, such as being volatile organic compounds (VOC), combustiblity (fp.>100oF), chemical safety, cost, and waste handling, and thus are not suitable for high-volume production. Commercially available water-based cleaners, on the other hand, typically are aqueous, detergent-based compositions comprising alkali metal hydroxide, silicate, phosphate, and surfactant; and blend with organic solvents that require special waste handling and wastewater treatment. These are not as effective in removing hydrophobic organic binder residue, and due to associated foaming problem, are incompatible with high-throughput pressurized spray cleaning. Detergent-based effluent requires coagulants and flocculants to precipitate hazardous metals and environmentally undesirable chemicals. Therefore, waste disposal issues also include hazardous metal waste and landfill limitations.

Mask Cleaning with Aqueous TMAH

Aqueous TMAH [(CH3)4N+OH-] can be used as a mask cleaner to remove paste residue from screening masks in the production of ceramic chip carriers.1 The solution is non-flammable, compatible with various mask metals, and provides high-efficiency cleaning at low concentration in manual or automated mode, using high-throughput pressurized spray or ultrasonic cleaning.

Widely used in the semiconductor industry as a metal-ion-free photoresist developer in lithographic processes for IC device manufacture, aqueous TMAH is also useful for cleaning silicon wafers, eye glass lenses, and optical instruments.2 Using aqueous TMAH solution without any additive as a mask cleaner offers unique advantages, especially in high-volume production, in terms of formulation simplicity, compatibility with pressurized spray cleaning due to absence of foaming problem, and removal from effluent by the well-known ion exchange method for wastewater treatment. 3-5 Its effectiveness at low concentration, coupled with effluent re-use multiple times after filtration of paste solids, provides significant materials savings in addition to offering options for waste minimization, water and reagent recovery, and zero-waste technology. Published biological studies show TMAH to be biodegradable.6

Basically, mask cleaning involves pressurized spray wash or ultrasonic agitation to remove screening paste residue from the mask, followed by a water spray or immersion rinse to remove any adhering TMAH solution, and air or nitrogen (N2) dry. The process parameters for spray cleaning vary as spray time, volume of cleaning solution, and rinse water per-cycle/per-mask depend on the type of spray nozzles, solution concentration, temperature and pressure, amount of paste residue on the mask, and chemical characteristics of the paste. Mask drying can be accelerated by final rinse with 2-propanol (IPA). In ultrasonic cleaning, the same bath can be used multiple times before requiring replenishing. As an example of the effectiveness of aqueous TMAH in cleaning paste screening masks, it is found that - at 0.5-0.6% concentration based on (CH3)4N+OH- in de-ionized water (Di) at 60-70oC and 50-60-psi spray pressure using multi-nozzle spray head - Mo paste residue on a Mo mask is quickly washed off, after which water rinse and dry operations are completed, so the overall process cycle meets the throughput requirement for the cleaning operation. Ultrasonic cleaning using 0.5-1.0% concentration at about 60oC provides an alternate process for efficient paste removal from various metal surfaces.


Figure 2.Aqueous TMAH cleaning process flow with fresh solution every pass, and with effluent reuse.
Click here to enlarge image

A typical Mo paste constitutes 70-80% Mo powder in organic matrix comprising ethyl cellulose polymer binder; oleoyl sarcosine surfactant/dispersant and a thickening agent, such as castorwax; and 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate as the solvent vehicle. Of these, the thickening agent is hydrophobic, and ethyl cellulose binder is insoluble in water. Possible rationale for the effectiveness of aqueous TMAH in mask cleaning can be provided by the chemical-mechanical removal of paste residue from metal, where chemical interaction of some paste constituents (Figure 1) facilitates paste wetting by aqueous TMAH, causing disruption of paste integrity. This interaction at the mask surface allows mechanical removal by spray pressure or ultrasonic vibration. For example, interaction of the -COOH group of oleoyl sarcosine (I) with TMAH would form a TMA-carboxylate complex (II) that mimics the aqueous wettability and emulsifying effect of the polar quaternary ammonium surfactants. With the ester solvent vehicle, such as 2,2,4-trimethy-1,3-pentanediol monoisobutyrate (III) shown in Figure 1, TMAH promoted hydrolysis results in by-products 2,2,4-trimethyl-1,3-pentanediol (IV) and TMA-isobutyrate (V), which have enhanced solubility in aqueous alkaline medium.

Effluent Analysis and Treatment

The washed-off paste metals, inorganic solids, and insoluble organics are removed by centrifuge and filtration processes. The filtered effluent contains soluble paste organics and TMAH. In addition, there is a significant level of ionic Mo present as TMA-molybdate {[(CH3)4N+]2 MoO4=} when Mo pastes are used for screening. TMAH is removed from the filtrate by passing through a cation-exchange resin, and Mo is removed by passing through an anion exchange column.7 The remaining organics are removed, passing through activated charcoal or lime to provide contaminant-free wastewater. Quantitative analysis of the effluent and the wastewater was carried out by gel-permeation chromatography (GPC) of tetrahydrofuran extract of the salt-saturated effluent/wastewater concentrates, using tetrahydrofuran as the mobile phase. Comparison with authentic samples of the paste organics showed peaks corresponding to oleoyl sarcosine and the solvent vehicle, along with the hydrolysis products as shown in Figure 1. No significant level peaks were observed for ethyl cellulose or the thickening agent.


Figure 3. Zero-waste process flow with aqueous TMAH cleaning.
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Process sequence for mask cleaning with aqueous TMAH is outlined in Figures 2 and 3, showing various options for cost reduction and waste minimization by effluent re-use, TMAH reclaim and recycle, and process based on zero-waste technology where both TMAH and water are recovered.

Conclusion

As a single active ingredient, aqueous TMAH is effective at low concentration for cleaning screening masks in the production of ceramic chip carriers. It meets criteria for an environmentally safe alternative. Non-flammable, it cleans a variety of metal/binder composite pastes on metal masks. Additionally, aqueous TMAH can be readily removed from effluent by ion exchange, generates no hazardous waste and reclaims recovered metals, and at low concentration is biodegradable and has no ecological concern. Effluent can be filtered and re-used to reduce cost, and offers an option for water conservation and closed-loop zero-waste technology.

Acknowledgments

The author would like to thank John Knickerbocker, Renee Weisman, James Humenik and Ranee Kwong.

References

  1. U.S. Patent No. 6,280,527 (Sachdev , et al), “Aqueous quaternary ammonium hydroxide as a mask cleaner.”
  2. J. S. Jeon and S. Raghavan, Proceedings - Environmental Sciences (1993), 39th (vol. 1), 268-73, “Wettability and cleaning of silicon wafers in tetramethyl ammonium hydroxide-based solutions.”
  3. U.S. Patent No. 5,545,309 (Shimizu et al), “Method of processing organic quaternary ammonium hydroxide-containing waste liquid”
  4. Hiroshi Sugawara, Yoshinori Tajima, and Tadahiro Ohmi, Japanes Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers (2002), 41(4B), 2374-2379, “A study of reclaimed photoresist developer using an electrodialysis method”
  5. Hiroshi Sugawara and Heromi Hemmi, Semiconductor Pure Water and Chemicals Conference (1999), 18th, 295-308, CODEN: SPWCFI; ISSN: 1521-4656 “Application of TMAH reclamation in LSI/LCD” Uchitomi ; K. Tohma ; Y. Yogi ; K. Takino, Semiconductor Pure Water and Chemicals Conference (1996), 15th (UPW Reclaim proceedings, 1996, 39-51
  6. U.S. Patent No. 6, 426, 6007 (Sachdev et al) “Removal of soluble metals in wastewater from aqueous cleaning and etching processes”.


    This posthumously published feature was contributed by the author’s husband, HARBANS SACHDEV, who may be contacted at 23 Fairview Dr., Hopewell Junction, NY, 12533; 845/226-8303; E-mail: sachdevhs@aol.com.