Langmuir 1990,6, 1701-1703
1701
Marangoni Drying: A New Extremely Clean Drying Process A. F. M. Leenaars, J. A. M. Huethorst,’ and J. J. van Oekel Philips Research Laboratories, P.0. Box 80,000, 5600 J A Eindhoven, The Netherlands Received June 18, 1990. In Final Form: August 27, 1990 In a new drying process (referred to as Marangoni drying), the substrate to be dried is withdrawn from a rinse bath (water) while at the same time nitrogen gas with a trace of an organic vapor is led along its surface. The organic vapor dissolves into water and introduces a surface tension gradient in the wetting film on the substrate, causing the water film to quickly drain backwards into the rinse batch (a Marangoni effect). As a result, a completely dry substrate emerges from the bath. Contrary to spin drying, almost no contamination is added to the substrate surface during drying.
Introduction In the manufacture of, for instance, integrated circuits a n d liquid crystal displays, contamination (particles, organics, and metals) of the respective substrates is t o be avoided as much as possible.’ This poses severe constraints on the applied processes. Often a large percentage of the total number of processing steps concerns wet processing. Typically, a wet-processing sequence comprises etching or cleaning, rinsing, and finally drying. Drying is normally carried out by centrifugation (spin drying). It appears that this drying technique can readily add contaminants to the substrate surface. Important sources of contamination during spin drying appear to be t h e centrifuge itself (probably particles generated as a result of friction between fast moving parts) and contaminants transferred from the liquid onto the substrate surface. T h e latter source can be explained as follows. During spin drying, most of the water on the substrate is removed by centrifugal forces. However, part of t h e liquid on the substrate surface is evaporated, and particularly in the final stages of spin drying contaminants present in this vaporized liquid film will become deposited onto the substrate. In this letter, a new drying process is presented.2 As this drying process is believed to be based on t h e Marangoni effect, we have named it “Marangoni drying”. It will be shown that this new drying process is extremely clean: during drying virtually no contamination is added to the substrate surface. The two sources of contamination presented above for spin drying are either absent or insignificant.
Experimental Section The experiments were performed with silicon wafers with a diameter of 10 cm, obtained from Wacker Chemitronic Co. (Burghausen, F.R.G.). In order to ensure that the surfaces were hydrophilic, these wafers (covered with a so-called native siliconoxyde film) were treated in an UV-ozone reactor (UVP Inc. PR-100) for 5 min to remove organic contaminants. Figure 1 gives a sketch of the experimental setup of the drying experiments. A wafer is clamped onto a lever which can be moved in a vertical direction at an adjustable speed (0.5-15 mm/s). The wafer is initially immersed in water. It is then withdrawn from the bath at a predetermined speed (normally 1 mm/s), while a t the same time a flow of nitrogen gas with a trace of an organic vapor is passed along the wafer surface. This flow is produced by leading nitrogen gas as a carrier through an aspirator bottle containing the selected organic liquid. The Nz flow rate is quantified with a flow meter. It was established that ~~
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* To whom correspondence should be addressed.
(1) Ferris-Prabhu,A. V. In Surface Contamination: Genesis,detection and control; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 2, p 925. (2) Leenaars, A. F. M.; van Oekel, J. J. Patent application.
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Figure 1. Experimentalsetup for Marangoni drying: (1)nitrogen flow; (2) bottle containing organic liquid; (3) Nz/vapor flow; (4) silicon wafer; (5) water or salt solution; (6) lever with clamp.
the organic vapor does not condense on a dry part of the wafer: the temperature of the organic liquid was always slightly lower than that of the wafer. Bubbling Nz through the aspirator bottle results in a small decrease of the temperature of the liquid compared to the temperature of the wafer. This is an important difference between Marangoni drying and another drying technique: vapor drying. During vapor drying, any adhering water film is exchanged by a condensed film of an organic liquid, which subsequently evaporate^.^ The particle addition resulting from spin drying and Marangoni drying was determined under clean-room conditions. Wafers were dried from a rinse bath containing deionized filtered water. Marangoni drying was carried out with N2/2propanol vapor at a flow rate of 12 L/min. For spin drying, a Semitool ST 260D centrifuge was used. Wafers were dried at a speed of 2500 rpm for 5 min, followed by 1 min at 500 rpm. Particles on the wafer surface were counted, both before and after drying, with a light-scattering technique4+ (using a Tencor Surfscan 4000). Particles with a scattering cross section between 0.09 and 5 pmz were measured (the excluded wafer edge was 6 mm wide). Only wafers with initially less than 10 particles were used for the drying experiments. Information on the thickness of the final evaporating liquid film (mentioned in the Introduction as a source of contamination), as a function of the drying conditions, was obtained by retracting the wafers from a 1M sodium chloride solution, instead of pure water. Marangoni drying was carried out with a N2/ vapor flow of 6 L/min for various vapors. For spin drying, a single wafer spin dryer was used at a speed of 2900 rpm. After drying, the wafers were inspected with a dark-field microscope, using 265X magnification, in order to detect salt crystals. The amount of salt deposited is a measure for the thickness of the final liquid film. (3) Ohmi, T. et al. Microcontamimtion 1989, 7 (5), 25. (4) Pecen, J.; Neukermans, A.; Kren, G.; Galbraith,L. Solid State Tech-
nol. 1987, May, 149. (5) Berger, J. In Particles on Surfaces, detection, adhesion and removal, Mittal, K. L., Ed.; Plenum Press: New York, 1988 p 247. (6)Galbraith, L.; Kren, G.; Neukermans, A.; Pecen, G. In Particles on surfaces: detection, adhesion and remoual; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 269.
0 1990 American Chemical Society
1702 Langmuir, Vol. 6, No. 11. 1990
Letters Table 1. Marangoni Drying Results Found for Various Vapors. after Drying from a I M NaCl Solution drying vapor pressure, solubility: solvent
alcohols 2-propanol 1-propanol %butanol 1-hexanol I-heptanol I-octanol
alkanes hexane
tetrachloromethane octane various diethyl ether ethyl acetate butyl acetate methyl isobutyl ketone acetone
Results I t appears that under properly chosen conditions (see below), a wafer can be withdrawn completely dry from a n aqueous bath, provided a flow of an organic vapor stream is passed along the wafer surface. Nitrogen gas, partly saturated with the organic vapor, can conveniently be applied as a carrier. When nitrogen gas is used in the absence of organic vapor, no drying occurs a t all. If a N2/ 2-propanol vapor flow of 12 L/min is applied, a dry wafer is obtained up to a speed of 2 mm/s. The polished side of the wafer can actually be withdrawn in a completely dry form up to a speed of 15 mm/s, which is the maximum speed of the present instrumentation. The number of particles introduced BS a result of drying from pure water under clean-room conditions WBS measured for 23 wafers. For Marangoni drying, an average particle addition of about 0.1 particle per wafer was found, while for drying with a conventional centrifuge the average addition was 10 particles per wafer. Figure 2 gives dark-field microscopy pictures of wafers after drying in a 1 M salt solution. I t is clear that spin drying results in a much larger salt deposition than Marangoni drying. This is evidently caused by a thicker liquid film on the wafer surface. After this film is evaporated, the dissolved salt is deposited onto the surface. The results for various organic vapors, after drying from a salt solution, are given in Table 1.
Proposed Drying Mechanism As indicated schematically in Figure 3, a hydrophilic substrate is withdrawn from a rinse bath with water, while a t the same time an organic vapor is passed along the substrate. This vapor has the capacity to reduce the
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