Removal of Nanoparticles from Plain and Patterned Surfaces Using

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Removal of Nanoparticles from Plain and Patterned Surfaces Using Nanobubbles Shangjiong Yang and Anton Duisterwinkel* Netherlands Organization for Applied Scientific Research (T.N.O.), Postbus 155, 2600AD Delft, The Netherlands

bS Supporting Information ABSTRACT: It is the aim of this paper to quantitatively characterize the capability of surface nanobubbles for surface cleaning, i.e., removal of nanodimensioned polystyrene particles from the surface. We adopt two types of substrates: plain and nanopatterned (trench/ridge) silicon wafer. The method used to generate nanobubbles on the surfaces is the so-called alcohol water exchange process (use water to flush a surface that is already covered by alcohol). It is revealed that nanobubbles are generated on both surfaces, and have a remarkably high coverage on the nanopatterns. In particular, we show that nanoparticles are—in the event of nanobubble occurrence— removed efficiently from both surfaces. The result is compared with other bubble-free wet cleaning techniques, i.e., water rinsing, alcohol rinsing, and water alcohol exchange process (use alcohol to flush a water-covered surface, generating no nanobubbles) which all cause no or very limited removal of nanoparticles. Scanning electron microscopy (SEM) and helium ion microscopy (HIM) are employed for surface inspection. Nanobubble formation and the following nanoparticle removal are monitored with atomic force microscopy (AFM) operated in liquid, allowing for visualization of the two events.

’ INTRODUCTION It is very difficult to remove particles from nanostructured surfaces such as extreme ultraviolet lithography (EUV) reticles, micro-electro-mechanical systems (MEMS), and hard disk drives.1 Current cleaning techniques involve use of hazardous chemicals and consume a lot of energy. Also, collapse of the structures during the drying process is a serious concern. We propose here a new technique that involves little chemicals or energy, and reduces the risk of collapse. This technique engages the deliberate use of nanobubbles, i.e., nanoscopic gas bubbles located at the liquid solid interface.2 21 Nanobubbles have been intensively studied over recent years. Most studies employ atomic force microscopy (AFM),3 7,12 17,19 while other methods such as rapid cryofixation freeze fracture22 and neutron reflectometry23 have been used as well. The popular substrates adopted are atomically flat, including gold,4 silicon surfaces hydrophobized by silanation,13,22 polystyrene,6,12 highly oriented pyrolytic graphite (HOPG),5,7,14 and bare silicon (with a native oxide layer).13 In addition, nanopatterned surfaces are employed for location control and spatial extent of nanobubbles.12 A number of methods including solvent exchange, liquid temperature change, heating substrate, and pressurizing liquid are used to generate nanobubbles.3 For the advantage of a greater control over nanobubble production and a higher tolerance in substrate selection, electrolysis is preferred in a number of experiments.5,15 Regarding liquid, highly purified water (Milli-Q) is mainly used, though some experiments are done with alcohols6 or NaCl solutions.3,5 We must highlight the so-called alcohol water exchange process, i.e., the surface is first covered by alcohol (ethanol or propanol) r 2011 American Chemical Society

which is then flushed away by water.3,7,16,17 It is revealed that the process enhances nanobubble density dramatically. The finding has been confirmed for hydrophilic bare silicon surface13 as well, although a hydrophobic substrate (contact angle θ > 90°) is more favored in the formation of nanobubbles. Nanobubbles are in many ways fascinating objects in surface science and nanofluidics. There has been much effort in understanding their properties. Fundamentally, nanobubbles should not exist: according to the experimental data, these bubbles have a radius of curvature R smaller than 1 μm, and therefore they should dissolve on time scales far below a second,24 due to the large Laplace pressure inside the bubbles, which in a bubble with radius of, e.g., 200 nm amounts to approximately 5 atm. In a significant contrast, the experiments show that nanobubbles are stable for periods as long as hours, up to even days.3,14,25 On the basis of the convincing experimental evidence for the existence and stability of nanobubbles, there are a few mechanisms proposed, e.g., the dynamic equilibrium theory.26 On the application side, especially in the field of micro- and nanofluidics, nanobubbles are a potential candidate to explain various phenomena associated with the liquid solid interface, such as the stability of colloidal systems, the anomalous attraction of hydrophobic surfaces,8,27,28 and liquid slippage at walls.29 32 Received: March 24, 2011 Revised: June 14, 2011 Published: August 01, 2011 11430

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Figure 1. (a) 3D AFM (tapping mode, dry surface) image of the 100 nm polystyrene particles deposited on a plain wafer. Scan size 3 μm  3 μm, height range 200 nm. Nanoparticles appear to be randomly distributed on the surface. (b) 3D tapping mode AFM image (in liquid) taken at the same location as (a), immediately after ethanol water exchange process has been performed. Nanobubbles (the bright dots) are generated on the surface, while nanoparticles have been removed. Note that liquid rinsing or gas blowing cannot remove these particles. Shown in (b), two objects (dash-arrow pointed) larger than the nanoparticles are added onto the surface.

compared with conventional methods. In practice, cleaning efficiency is highly influenced by the different contamination surface interaction, e.g., the amount of proteins adsorbed and the strength of the attachment. Treatment of nanobubbles in combination with the common methods is suggested to be able to achieve a higher cleaning result, in the case of removing proteins from a hydrophilic surface.34 However, the findings are still limited, and more studies are necessary. Industrial applications often involve particles of a submicrometer size, which are challenging to remove with conventional wet methods, because these methods rely on the interaction of a liquid flow over the surface. Since the liquid typically does not move at the surface, it is almost impossible to remove those small particles physically. In this paper, we elucidate these cleaning issues by performing AFM measurements (in liquids) of nanobubbles on a plain silicon surface as well as on a nanopatterned surface which have been deposited with nanoparticles prior to the nanobubble generation. To this end, we demonstrate the improvement of the removal efficiency of nanoparticles by the occurrence of nanobubbles, in comparison with other flushing methods. Surface inspections by scanning electron microscopy (SEM) and helium ion microscopy (HIM) are used to confirm the results.

’ EXPERIMENTAL SECTION

Figure 2. (a) SEM image of the plain wafer surface deposited with the 100 nm polystyrene particles, the same surface as shown in Figure 1. There are 65 nanoparticles presented in the image where 12 of them are paired. (b) SEM image of the same surface after it has been treated with the ethanol water exchange process and dried again. Clearly, the nanobubble process completely removed (100% removal rate) the nanoparticles with no extra damage to the surface. The two triangleshaped objects are thought to be added contaminations, which presumably are introduced by the flushing process. These objects differ from the deposited nanoparticles in shape and size. The result is consistent with the previous AFM observation of the surface.

As recently suggested, it is also possible to use nanobubbles in the processing industry for cleaning a surface.33 36 It has been demonstrated that electrolytically produced nanobubbles facilitate defouling and antifouling of bovine serum albumin (BSA) proteins on a hydrophilic surface.33,35 Similar results have also been shown on both hydrophobic and hydrophilic surfaces, by the quartz crystal microbalance technique.34,36 With a few cycles of treatment, the protein can be completely removed and cleaning efficiency of a nanobubble treatment is relatively higher,

Surface Preparation. In our experiments, we employ two types of substrates. Substrate 1 is a plain silicon wafer (single polished 4 in. process wafer, rms value 0.15 nm) deposited with spherical polystyrene particles of diameter 100 nm (see Figures 1 and 2). The surface is prepared as follows. The bare silicon wafer (with a native dioxide layer on top) is rinsed with acetone (>99.9%, Sigma Aldrich, Germany), and then rinsed with ethanol (>99.8%, Sigma Aldrich, Germany). Both treatments are respectively followed by blowing dry with nitrogen gas. Thereafter, the substrate is placed into a spin-coating machine (BHG Hermle Z320, Germany) in which a drop (0.5 mL) of nanoparticle solvent (polystyrene latex spheres, diameter 100 nm, NIST Standard, Duke Scientific, USA) is injected. The particles are then spin-distributed on the surface (speed 6100 rpm, duration 10 s). The sample is dried naturally during the spinning at room temperature. Substrate 2 is a nanopatterned silicon wafer covered with spherical polystyrene particles of diameter 70 nm (see Figures 4 and 5). It is prepared as follows. The same plain wafer as Substrate 1 is developed with a silicon dioxide layer of 2 μm thickness, which is then litho-etched to achieve periodic nanotrenches (the depth 150 nm, the trench width 200 nm, the ridge width 200 nm). The surface is then acetone- and ethanol-rinsed, respectively. Thereafter, the spherical nanoparticles (diameter 70 nm) are distributed onto the surface in the same fashion as Substrate 1. The surface preparations are performed under a clean-room condition (ISO Class 5, National Van Leeuwenhoek NanoLab, Delft, The 11431

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in this paper. All experiments are carried out in a general lab environment with a temperature between 20 °C and 23 °C.

’ RESULTS AND DISCUSSION

Figure 3. Removal rate of nanoparticles as a function of treatment cycles, for ethanol water exchange, ethanol rinsing, and water rinsing. The ethanol water exchange process has removed up to nearly 90% of the nanoparticles from the plain surface after one cycle of treatment. The cleaning efficiency is further enhanced after more cycles. In marked contrast, washing only with ethanol or water results in a removal of nanoparticles less than 5% after the first cycle and very little enhancement after more treatments with a total removal rate less than 10%. Netherlands). The samples are stored in a clean-room sample cell thereafter until use. Fluidic Process. In our experiments, pure water is prepared by a Milli-Q Synthesis A10 system (Millipore SAS, France). Alcohols, i.e., ethanol and isopropanol, are of HPLC grade (>99.8% for ethanol, >99.9% for isopropanol, Sigma Aldrich, Germany). The experimental setup is established in compliance with fluidic AFM measurements for the alcohol water exchange process.3,7 A fluidic cell is connected with a syringe-pumped inlet and outlet, which creates flows simultaneously during AFM scanning. The sample is placed underneath the fluidic cell; the alcohol water exchange process is then performed on the AFM stage. Thus, AFM measurement can take place in liquid immediately after the fluidic process where the nanobubbles are produced. To compare with the results of nanobubble cleaning, other non-nanobubble methods such as water washing, water alcohol exchange, and alcohol washing are performed as well. AFM, SEM, HIM Imaging. Excitation of the tip vibration is done acoustically, using AFM measurements with the NanoWizard II (JPK Instrument AG, Germany) in tapping mode with a small piezo-element in the tip-holder. A hydrophilic Si3N4 ultrasharp AFM tip is used, with radius of curvature less than 10 nm, height about 22 μm, and full tip cone angle of 30°. For scanning in both dry and wet conditions, the scanning speed is 4 μm/s, the tapping mode free amplitude as applied to the cantilever is 400 mV, the set-point amplitude is 200 mV, and the frequency of the cantilever and the spring constant are approximately 20kHz and 0.9 N/m, respectively. An integrated live CCD camera gives good control of the scanning position on the sample. The cantilever is cleaned by immersion in ethanol and pure water before use. Besides AFM, surface inspections are done with SEM and HIM, before and after the treatments, respectively. SEM imaging is performed with the SEM XL 50 (Philips, Netherlands), at acceleration voltage 5 kV and magnification 25 000. HIM surface inspection is performed with the Orion Plus (Carl Zeiss SMT, Germany), with an acceleration voltage 25 kV, magnification 11 430, blanker current 0.5 pA, dwell time 100 μs, tilt angle 0°, and working distance 10.7 mm. HIM is capable of imaging and fabrication of nanostructures thanks to its subnanometer-sized ion probe.37 The HIM probe is scanned over the sample surface, and the secondary electron signal is recorded to create an image. The unique interaction of the helium ions with the sample material provides very localized secondary electron emission, thus providing a valuable signal for high-resolution imaging as well as a mechanism for very precise nanofabrication.38 Statistics of nanoparticle coverage is based on a number of SEM, AFM, and HIM images. Typical images are presented

Removal of Nanoparticles on a Plain Wafer. In the study of nanoparticle removal on the plain wafer, the particle-covered dry surface is first imaged by AFM. Subsequently, the ethanol water exchange process is performed. At exactly the same surface location as in the dry measurement, AFM imaging (in liquid) takes place immediately after the process. We see that nanoparticles are randomly distributed on the surface, as shown in Figure 1a. At the same surface area, nanobubbles are observed after the fluidic process (Figure 1b). The size and density of nanobubbles is similar to the previous findings reported in refs 3,14. At the same time, all of the nanoparticles are removed from the surface. Two objects as indicated by dashed arrows in Figure 1b are added on the surface: these are larger (one has height 105 nm and width 240 nm, the other has height 90 nm and width 150 nm) than the originally distributed nanoparticles. They could be clustered nanoparticles, large nanobubbles, or contaminants from the experimental setup. The ethanol water exchange process leads to the removal of nanoparticles with no visible damage to the surface. SEM characterization provides a larger field of view. A typical image of the plain wafer surface is shown in Figure 2. Image (a) presents the dry surface that is covered with nanoparticles, while (b) shows the same surface after one cycle of nanobubble cleaning. We see that the nanoparticles are completely removed by the treatment. The two triangular contaminations left on the surfacare are presumably introduced by the liquid flushing. These two pieces are clearly not the deposited nanoparticles, with respect to shape and size. SEM and AFM consistently show that the ethanol water exchange process removes the nanoparticles. A single treatment removes on average 90% of the particles, see statistics in Figure 3—the removal fraction of nanoparticles (namely, cleaning efficiency) is plotted as a function of treatment cycles, with respect to different methods. By repeated treatment, cleaning efficiency of nanobubbles is even improved to almost 100%. In marked contrast, flushing with either ethanol or water removes less than 10% of the particles. Removal of Nanoparticles on a Nanopatterned Surface. Next, experiments have been performed on the samples with parallel nanotrenches; see Figure 4. The nanoparticles predominantly are deposited inside the trenches. After the dry imaging, the isopropanol water exchange process is used to remove the nanoparticles. AFM (tapping mode, in liquid) measurement starts immediately when the fluidic process has been performed, at exactly the same surface area as in Figure 4a. We see that nanobubbles are produced with a large density, covering nearly the entire surface (Figure 4b). In comparison to Figure 1b, nanobubble density is much higher. This may be due to either the nanostructure or the fact that isopropanol is used rather than ethanol. Subsequently, the sample is dried by nitrogen gas blow, and then inspected with SEM and AFM. Most of the contaminations—both on the nanoridges and inside the nanotrenches— have disappeared (Figure 4c,d). This is remarkable because it is very difficult to remove the particles inside nanotrenches, since the particles can attach to several surfaces and the impact of (local) flow within the trenches is heavily reduced. Several portions of the surface are imaged with HIM, showing comparable images to those from AFM. The typical images are 11432

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Figure 4. (a) AFM tapping mode in air, 3D image of the nanopatterned surface, height range 224 nm. Nanoparticles (examples pointed by arrows) are shown on the ridges, even more are closely packed inside the trenches. Almost 50% of the trench area is filled with nanoparticles. Isopropanol water exchange process is then performed on the surface, and AFM imaging (in liquid) takes place immediately. (b) 3D AFM image of the wetted surface (tapping mode in liquid), with a height range 69 nm. Surface nanobubbles are observed, and the nanostructure is still visible. Nanobubbles are formed on ridges as well as in trenches, with an extremely large coverage. (c,d) AFM images of the same surface that is dried again, after it has been treated with nanobubbles (tapping mode in air, height range 66 nm). In comparison with (a), most of the particle clusters inside the trenches are removed. However, a number of single nanoparticles are still observed. Possibly, these remaining particles are due to the resettlement of the nanoparticles. The cleaning efficiency is enhanced by greater processing times, as image (c) presents the surface after one treatment while image (d) presents it after two treatments, which indicates a slightly better cleaning result. Note that the other nanobubble-free flushing methods do not or hardly remove the nanoparticles. Scan size is 10 μm  10 μm in all images.

shown in Figure 5: (a) presents an initial dry nanopatterned surface before the isopropanol water exchange process, which is covered with nanoparticles. Image (b) shows the same sample after three cycles of the nanobubble cleaning. We find that the removal of nanoparticles takes place for the entire area, i.e., 10 μm  10 μm. The enlarged view allows good observation inside the trenches, as shown in image (c). No nanoparticles are observed on the surface. The HIM and AFM measurements show good agreement. To compare the nanobubble cleaning results, rinsing with water or isopropanol was performed. Both washings do not remove a significant number of nanoparticles. Analogous to Figure 3, cleaning efficiencies are presented in Figure 6. Nearly 80% of nanoparticles have been removed from the patterned surface after the first treatment with the isopropanol water exchange process. The cleaning efficiency is further enhanced by more cycles of nanobubble treatment. In marked contrast, washing with either isopropanol or water only, and a simple water ethanol mixing (this process produces no nanobubbles) leads to very limited nanoparticle removal, i.e., less than 5%. The result of the non-nanobubble method is not improved by more cycles. As reported in previous studies,3,14,16 the larger gas solubility in alcohol compared to that in water is responsible for the sufficient nanobubble density, in an alcohol water exchange process. This is why we must use water to replace alcohol, in order to extract gas out of the liquid (in other words, to create an oversaturation in liquid) to form nanobubbles. Also, the process is an exothermic reaction releasing heat which enhances the formation of nanobubbles. Structures on the surface appear to

promote an even higher nanobubble density as well. Observed in our experiments, the exchange process leads indeed to a large amount of nanobubble formation which is more pronounced on the nanotrench patterns. Therewith, we see the dramatic enhancement of nanoparticle removal on the surfaces, which cannot be achieved by other nonbubble wet cleaning methods. There are seemingly no other possible effects in the process for cleaning to this extent, apart from nanobubbles. The flow itself is far too weak (especially inside the trenches), dissolution does not occur, and the effect of the difference in surface tensions of alcohol and water is not expected due to rapid intermixing. How do nanobubbles clean then? Previous work has shown that an emerging nanobubble rapidly expands horizontally on the surface (in other words, a nanoscopic gas layer accumulates on the surface), increasing its surface coverage on a time-scale of seconds, and then it develops in height.5 This behavior is thought to detach the nanoparticle. We estimated the adhesion force (mostly van der Waals forces) between the polystyrene nanoparticle and the surface, which is in the range of a few nanonewtons, whereas the capillary force of the expanding nanobubble is one or two orders of magnitude stronger, given the same surface contact area of the nanobubble and the nanoparticle. The exact principle of nanobubble formation is still under debate; however, the mechanism of dynamic equilibrium has increasingly been recognized recently.26 The gas accumulation and outflux over a nanobubble described in this mechanism supports the cleaning effect of nanobubbles. Moreover, nanobubbles are stable, which helps to prevent the resettling of contaminants. However, we see that a number of nanoparticles remain on the 11433

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cleaning efficiency. Furthermore, note that in our experiments polystyrene particles are used, which are fairly easy to remove. In real industrial applications, more types of particles, such as aluminum, aluminum oxide, gold, or copper, are more often presented. Real structures of EUV reticles, MEMS, and other functional nanostructures are more complex and contain more types of all different materials. More research is necessary. Clearly, nanobubble cleaning holds a number of advantages over conventional methods, which are highly favored in applications. For example, it contains less chemicals or waste, consumes less energy, and causes less damage.

Figure 5. (a) HIM image of the initial dry surface prior to an isopropanol water exchange process, deliberately contaminated with nanoparticles (examples indicated by arrows). (b) HIM image of the same sample after three cycles of nanobubble process. Clearly, we see that removal of the nanoparticles has been achieved, with a good cleaning efficiency. Note that HIM characterizations provide views on a relatively large surface area with a high resolution. (c) Enlarged view gives an observation inside the nanotrenches where most nanoparticles were deposited, as shown in Figure 4a. Nanoparticles are not observed inside the trenches after the nanobubble treatment.

’ CONCLUSION We have presented the experimental studies of surface cleaning by using nanobubbles on both plain wafer and nanopatterned (periodic ridges and trenches) substrates. Our results have shown that the spherical polystyrene nanoparticles deposited on the surfaces are efficiently removed with the process of alcohol water exchange, which leads to massive nanobubble production. The same cleaning result cannot be achieved with other fluidic processes that generate no nanobubbles. SEM, HIM, and AFM measurements have been performed for surface inspection. It is highlighted that tapping-mode AFM operating in liquid can simultaneously demonstrate the generation of nanobubbles and the nanoparticle removal caused by these nanobubbles. We must stress the cleaning result in terms of the nanotrenches. These trenches contain a large contamination density, yet have been efficiently cleaned with nanobubbles, which commonly is very difficult for other cleaning techniques. In addition, our findings have shown that increasing treatment cycles allows an enhancement of cleaning efficiency. Finally, we have not observed any damage to the nanosized surface structures in a nanobubble cleaning process. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM, AFM, HIM images of the surfaces before and after the nanobubble treatments, AFM images of nanobubbles. Data of nanoparticle coverage, description of nanobubble treatment for surface cleaning. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Figure 6. Removal percentage of nanoparticles on the nanostructured surface by different cleaning techniques is plotted as a function of treatment cycle. Nearly 80% of the nanoparticles have been removed from the surface after the first treatment with the nanobubble process. The cleaning efficiency is further enhanced up to a removal rate of 90% after 3 cycles of treatment. As a comparison, washing with isopropanol or water leads to a very limited removal of the particles, i.e., less than 5%. This poor cleaning efficiency is not further improved after a few more cycles of treatment.

patterned surface after nanobubble treatments. Presumably, this is because nanobubbles do not have a 100% surface coverage in our experiments. In addition, the direction of injecting flow is random. Therefore, control of flow direction (e.g., parallel to the trenches), other nanobubble formation processes rather than the solvent exchange (e.g., electrolysis, temperature increase, gas supersaturation), or a combination with the conventional cleaning methods can be the following steps to further improve the

Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank Detlef Lohse, James Seddon, Jacques van der Donck, and Harold Zandvliet for stimulating discussions. S. Y. acknowledges Jetske Stortelder, Anne Heyer, and Emile van Veldhoven for their experimental support. ’ REFERENCES (1) Lithography, In The International Technology Roadmap For Semiconductors; 2009 (2) Ball, P. Nature 2003, 423, 25. (3) Yang, S.; Dammer, S. M.; Bremond, N.; Zandvliet, H. J. W.; Kooij, E. S.; Lohse, D. Langmuir 2007, 23, 7072. (4) Holmberg, M.; K€uhle, A.; Garnæs, J.; Mørch, K. A.; Boisen, A. Langmuir 2003, 19, 10510. 11434

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