Published: August 01, 2011

r 2011 American Chemical Society 11430 dx.doi.org/10.1021/la2010776 | Langmuir 2011, 27, 11430–11435

ARTICLE

pubs.acs.org/Langmuir

Removal of Nanoparticles from Plain and Patterned SurfacesUsing NanobubblesShangjiong Yang and Anton Duisterwinkel*

Netherlands Organization for Applied Scientific Research (T.N.O.), Postbus 155, 2600AD Delft, The Netherlands

bS Supporting Information

’ INTRODUCTION

It is very difficult to remove particles from nanostructuredsurfaces such as extreme ultraviolet lithography (EUV) reticles,micro-electro-mechanical systems (MEMS), and hard diskdrives.1 Current cleaning techniques involve use of hazardouschemicals and consume a lot of energy. Also, collapse of thestructures during the drying process is a serious concern. Wepropose here a new technique that involves little chemicals orenergy, and reduces the risk of collapse. This technique engagesthe deliberate use of nanobubbles, i.e., nanoscopic gas bubbleslocated at the liquid�solid interface.2�21 Nanobubbles havebeen intensively studied over recent years. Most studies employatomic force microscopy (AFM),3�7,12�17,19 while other methodssuch as rapid cryofixation�freeze fracture22 and neutron reflec-tometry23 have been used as well. The popular substrates adoptedare atomically flat, including gold,4 silicon surfaces hydrophobizedby silanation,13,22 polystyrene,6,12 highly oriented pyrolytic gra-phite (HOPG),5,7,14 and bare silicon (with a native oxide layer).13

In addition, nanopatterned surfaces are employed for locationcontrol and spatial extent of nanobubbles.12 A number of methodsincluding solvent exchange, liquid temperature change, heatingsubstrate, and pressurizing liquid are used to generate nano-bubbles.3 For the advantage of a greater control over nano-bubble production and a higher tolerance in substrate selection,electrolysis is preferred in a number of experiments.5,15 Regard-ing liquid, highly purified water (Milli-Q) is mainly used, thoughsome 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)

which is then flushed away by water.3,7,16,17 It is revealed thatthe process enhances nanobubble density dramatically. The find-ing has been confirmed for hydrophilic bare silicon surface13 aswell, although a hydrophobic substrate (contact angle θ > 90�) ismore favored in the formation of nanobubbles.

Nanobubbles are in many ways fascinating objects in surfacescience and nanofluidics. There has been much effort in under-standing their properties. Fundamentally, nanobubbles shouldnot exist: according to the experimental data, these bubbleshave a radius of curvature R smaller than 1 μm, and thereforethey should dissolve on time scales far below a second,24 due tothe large Laplace pressure inside the bubbles, which in a bubblewith radius of, e.g., 200 nm amounts to approximately 5 atm. Ina significant contrast, the experiments show that nanobubblesare stable for periods as long as hours, up to even days.3,14,25 Onthe basis of the convincing experimental evidence for theexistence and stability of nanobubbles, there are a few mechan-isms proposed, e.g., the dynamic equilibrium theory.26 On theapplication side, especially in the field of micro- and nano-fluidics, nanobubbles are a potential candidate to explainvarious phenomena associated with the liquid�solid interface,such as the stability of colloidal systems, the anomalous attrac-tion of hydrophobic surfaces,8,27,28 and liquid slippage atwalls.29�32

Received: March 24, 2011Revised: June 14, 2011

ABSTRACT: It is the aim of this paper to quantitativelycharacterize the capability of surface nanobubbles for surfacecleaning, i.e., removal of nanodimensioned polystyrene particlesfrom the surface. We adopt two types of substrates: plain andnanopatterned (trench/ridge) silicon wafer. The method usedto generate nanobubbles on the surfaces is the so-calledalcohol�water exchange process (use water to flush a surfacethat is already covered by alcohol). It is revealed that nano-bubbles are generated on both surfaces, and have a remarkablyhigh 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 withatomic force microscopy (AFM) operated in liquid, allowing for visualization of the two events.

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

compared with conventional methods. In practice, cleaningefficiency is highly influenced by the different contamina-tion�surface interaction, e.g., the amount of proteins adsorbedand the strength of the attachment. Treatment of nanobubbles incombination with the common methods is suggested to be ableto achieve a higher cleaning result, in the case of removingproteins from a hydrophilic surface.34 However, the findings arestill limited, and more studies are necessary. Industrial applica-tions often involve particles of a submicrometer size, which arechallenging to remove with conventional wet methods, becausethese methods rely on the interaction of a liquid flow over thesurface. Since the liquid typically does not move at the surface, itis almost impossible to remove those small particles physically. Inthis paper, we elucidate these cleaning issues by performing AFMmeasurements (in liquids) of nanobubbles on a plain siliconsurface as well as on a nanopatterned surface which have beendeposited with nanoparticles prior to the nanobubble generation.To this end, we demonstrate the improvement of the removalefficiency of nanoparticles by the occurrence of nanobubbles, incomparison with other flushing methods. Surface inspections byscanning electron microscopy (SEM) and helium ion micro-scopy (HIM) are used to confirm the results.

’EXPERIMENTAL SECTION

Surface Preparation. In our experiments, we employ two types ofsubstrates. Substrate 1 is a plain silicon wafer (single polished 4 in.process wafer, rms value 0.15 nm) deposited with spherical polystyreneparticles of diameter 100 nm (see Figures 1 and 2). The surface isprepared as follows. The bare silicon wafer (with a native dioxide layeron top) is rinsed with acetone (>99.9%, Sigma Aldrich, Germany), andthen rinsed with ethanol (>99.8%, Sigma Aldrich, Germany). Bothtreatments are respectively followed by blowing dry with nitrogen gas.Thereafter, the substrate is placed into a spin-coating machine (BHGHermle Z320, Germany) in which a drop (0.5 mL) of nanoparticlesolvent (polystyrene latex spheres, diameter 100 nm, NIST Standard,Duke Scientific, USA) is injected. The particles are then spin-distributedon the surface (speed 6100 rpm, duration 10 s). The sample is driednaturally during the spinning at room temperature. Substrate 2 is ananopatterned silicon wafer covered with spherical polystyrene particlesof diameter 70 nm (see Figures 4 and 5). It is prepared as follows. Thesame plain wafer as Substrate 1 is developed with a silicon dioxide layerof 2 μm thickness, which is then litho-etched to achieve periodicnanotrenches (the depth 150 nm, the trench width 200 nm, the ridgewidth 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

Figure 1. (a) 3DAFM (tappingmode, dry surface) image of the 100 nm polystyrene particles deposited on a plain wafer. Scan size 3 μm� 3 μm, heightrange 200 nm.Nanoparticles appear to be randomly distributed on the surface. (b) 3D tappingmode 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, whilenanoparticles 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.

Figure 2. (a) SEM image of the plain wafer surface deposited with the100 nm polystyrene particles, the same surface as shown in Figure 1.There are 65 nanoparticles presented in the image where 12 of them arepaired. (b) SEM image of the same surface after it has been treated withthe ethanol�water exchange process and dried again. Clearly, thenanobubble process completely removed (100% removal rate) thenanoparticles with no extra damage to the surface. The two triangle-shaped objects are thought to be added contaminations, which pre-sumably are introduced by the flushing process. These objects differfrom the deposited nanoparticles in shape and size. The result isconsistent with the previous AFM observation of the surface.

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Netherlands). The samples are stored in a clean-room sample cellthereafter 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 experimentalsetup is established in compliance with fluidic AFM measurements forthe alcohol�water exchange process.3,7 A fluidic cell is connected with asyringe-pumped inlet and outlet, which creates flows simultaneouslyduring AFM scanning. The sample is placed underneath the fluidic cell;the alcohol�water exchange process is then performed on the AFMstage. Thus, AFM measurement can take place in liquid immediatelyafter the fluidic process where the nanobubbles are produced. Tocompare with the results of nanobubble cleaning, other non-nanobubblemethods such as water washing, water�alcohol exchange, and alcoholwashing are performed as well.AFM, SEM, HIM Imaging. Excitation of the tip vibration is done

acoustically, using AFM measurements with the NanoWizard II (JPKInstrument AG, Germany) in tapping mode with a small piezo-elementin the tip-holder. A hydrophilic Si3N4 ultrasharp AFM tip is used, withradius of curvature less than 10 nm, height about 22 μm, and full tip coneangle of 30�. For scanning in both dry and wet conditions, the scanningspeed is 4 μm/s, the tapping mode free amplitude as applied to thecantilever is 400 mV, the set-point amplitude is 200 mV, and thefrequency of the cantilever and the spring constant are approximately20kHz and 0.9 N/m, respectively. An integrated live CCD camera givesgood control of the scanning position on the sample. The cantilever iscleaned by immersion in ethanol and pure water before use. BesidesAFM, surface inspections are done with SEM and HIM, before and afterthe treatments, respectively. SEM imaging is performed with the SEMXL 50 (Philips, Netherlands), at acceleration voltage 5 kV and magni-fication 25 000�. HIM surface inspection is performed with the OrionPlus (Carl Zeiss SMT, Germany), with an acceleration voltage 25 kV,magnification 11 430�, blanker current 0.5 pA, dwell time 100 μs, tiltangle 0�, and working distance 10.7 mm. HIM is capable of imaging andfabrication of nanostructures thanks to its subnanometer-sized ionprobe.37 The HIM probe is scanned over the sample surface, and thesecondary electron signal is recorded to create an image. The uniqueinteraction of the helium ions with the sample material provides verylocalized secondary electron emission, thus providing a valuable signalfor high-resolution imaging as well as a mechanism for very precisenanofabrication.38 Statistics of nanoparticle coverage is based on anumber of SEM, AFM, and HIM images. Typical images are presented

in this paper. All experiments are carried out in a general lab environ-ment with a temperature between 20 �C and 23 �C.

’RESULTS AND DISCUSSION

Removal of Nanoparticles on a Plain Wafer. In the study ofnanoparticle removal on the plain wafer, the particle-covered drysurface is first imaged by AFM. Subsequently, the ethanol�waterexchange process is performed. At exactly the same surfacelocation as in the dry measurement, AFM imaging (in liquid)takes place immediately after the process. We see that nanopar-ticles are randomly distributed on the surface, as shown inFigure 1a. At the same surface area, nanobubbles are observedafter the fluidic process (Figure 1b). The size and density ofnanobubbles is similar to the previous findings reported in refs3,14. At the same time, all of the nanoparticles are removed fromthe surface. Two objects as indicated by dashed arrows inFigure 1b are added on the surface: these are larger (one hasheight 105 nm and width 240 nm, the other has height 90 nm andwidth 150 nm) than the originally distributed nanoparticles.They could be clustered nanoparticles, large nanobubbles, orcontaminants from the experimental setup. The ethanol�waterexchange process leads to the removal of nanoparticles with novisible 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 nanobubblecleaning. We see that the nanoparticles are completely removedby the treatment. The two triangular contaminations left on thesurfacare are presumably introduced by the liquid flushing. Thesetwo pieces are clearly not the deposited nanoparticles, withrespect to shape and size. SEM and AFM consistently show thatthe 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 treatmentcycles, with respect to different methods. By repeated treatment,cleaning efficiency of nanobubbles is even improved to almost100%. In marked contrast, flushing with either ethanol or waterremoves less than 10% of the particles.Removal of Nanoparticles on a Nanopatterned Surface.

Next, experiments have been performed on the samples withparallel nanotrenches; see Figure 4. The nanoparticles predomi-nantly are deposited inside the trenches. After the dry imaging,the isopropanol�water exchange process is used to remove thenanoparticles. AFM (tapping mode, in liquid) measurementstarts immediately when the fluidic process has been performed,at exactly the same surface area as in Figure 4a. We see thatnanobubbles are produced with a large density, covering nearlythe entire surface (Figure 4b). In comparison to Figure 1b,nanobubble density is much higher. This may be due to either thenanostructure or the fact that isopropanol is used rather thanethanol. Subsequently, the sample is dried by nitrogen gas blow,and then inspected with SEM and AFM. Most of the contamina-tions—both on the nanoridges and inside the nanotrenches—have disappeared (Figure 4c,d). This is remarkable because it isvery difficult to remove the particles inside nanotrenches, sincethe 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

Figure 3. Removal rate of nanoparticles as a function of treatmentcycles, 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 markedcontrast, washing only with ethanol or water results in a removalof nanoparticles less than 5% after the first cycle and very littleenhancement after more treatments with a total removal rate lessthan 10%.

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shown in Figure 5: (a) presents an initial dry nanopatternedsurface before the isopropanol�water exchange process, which iscovered with nanoparticles. Image (b) shows the same sampleafter three cycles of the nanobubble cleaning. We find that theremoval of nanoparticles takes place for the entire area, i.e.,10 μm � 10 μm. The enlarged view allows good observationinside the trenches, as shown in image (c). No nanoparticles areobserved on the surface. The HIM and AFM measurementsshow good agreement. To compare the nanobubble cleaningresults, rinsing with water or isopropanol was performed. Bothwashings do not remove a significant number of nanoparticles.Analogous to Figure 3, cleaning efficiencies are presented inFigure 6. Nearly 80% of nanoparticles have been removed fromthe patterned surface after the first treatment with the isopropa-nol�water exchange process. The cleaning efficiency is furtherenhanced by more cycles of nanobubble treatment. In markedcontrast, washing with either isopropanol or water only, and asimple water�ethanol mixing (this process produces nonanobubbles) leads to very limited nanoparticle removal, i.e.,less than 5%. The result of the non-nanobubble method is notimproved 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 thesufficient nanobubble density, in an alcohol�water exchangeprocess. This is why we must use water to replace alcohol, inorder to extract gas out of the liquid (in other words, to create anoversaturation in liquid) to form nanobubbles. Also, the processis an exothermic reaction releasing heat which enhances theformation of nanobubbles. Structures on the surface appear to

promote an even higher nanobubble density as well. Observed inour experiments, the exchange process leads indeed to a largeamount of nanobubble formation which is more pronounced onthe nanotrench patterns. Therewith, we see the dramatic en-hancement of nanoparticle removal on the surfaces, whichcannot be achieved by other nonbubble wet cleaning methods.There are seemingly no other possible effects in the process forcleaning to this extent, apart from nanobubbles. The flow itself isfar too weak (especially inside the trenches), dissolution does notoccur, and the effect of the difference in surface tensions ofalcohol and water is not expected due to rapid intermixing. Howdo nanobubbles clean then? Previous work has shown that anemerging nanobubble rapidly expands horizontally on the sur-face (in other words, a nanoscopic gas layer accumulates on thesurface), increasing its surface coverage on a time-scale ofseconds, and then it develops in height.5 This behavior is thoughtto detach the nanoparticle. We estimated the adhesion force(mostly van der Waals forces) between the polystyrene nano-particle and the surface, which is in the range of a few nano-newtons, whereas the capillary force of the expanding nanobub-ble is one or two orders of magnitude stronger, given the samesurface contact area of the nanobubble and the nanoparticle. Theexact principle of nanobubble formation is still under debate;however, themechanism of dynamic equilibrium has increasinglybeen recognized recently.26 The gas accumulation and outfluxover a nanobubble described in this mechanism supports thecleaning effect of nanobubbles. Moreover, nanobubbles arestable, which helps to prevent the resettling of contaminants.However, we see that a number of nanoparticles remain on the

Figure 4. (a) AFM tapping mode in air, 3D image of the nanopatterned surface, height range 224 nm. Nanoparticles (examples pointed by arrows) areshown on the ridges, even more are closely packed inside the trenches. Almost 50% of the trench area is filled with nanoparticles. Isopropanol�waterexchange process is then performed on the surface, and AFM imaging (in liquid) takes place immediately. (b) 3D AFM image of the wetted surface(tappingmode in liquid), with a height range 69 nm. Surface nanobubbles are observed, and the nanostructure is still visible. Nanobubbles are formed onridges 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 withnanobubbles (tappingmode in air, height range 66 nm). In comparison with (a), most of the particle clusters inside the trenches are removed.However, anumber of single nanoparticles are still observed. Possibly, these remaining particles are due to the resettlement of the nanoparticles. The cleaningefficiency 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. Scansize is 10 μm � 10 μm in all images.

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patterned surface after nanobubble treatments. Presumably, thisis because nanobubbles do not have a 100% surface coverage inour experiments. In addition, the direction of injecting flow israndom. Therefore, control of flow direction (e.g., parallel to thetrenches), other nanobubble formation processes rather than thesolvent exchange (e.g., electrolysis, temperature increase, gassupersaturation), or a combination with the conventional clean-ing methods can be the following steps to further improve the

cleaning efficiency. Furthermore, note that in our experimentspolystyrene particles are used, which are fairly easy to remove. Inreal industrial applications, more types of particles, such asaluminum, aluminum oxide, gold, or copper, are more oftenpresented. Real structures of EUV reticles, MEMS, and otherfunctional nanostructures are more complex and contain moretypes of all different materials. More research is necessary.Clearly, nanobubble cleaning holds a number of advantages overconventional methods, which are highly favored in applications.For example, it contains less chemicals or waste, consumes lessenergy, and causes less damage.

’CONCLUSION

We have presented the experimental studies of surface clean-ing by using nanobubbles on both plain wafer and nanopatterned(periodic ridges and trenches) substrates. Our results haveshown that the spherical polystyrene nanoparticles depositedon the surfaces are efficiently removed with the process ofalcohol�water exchange, which leads to massive nanobubbleproduction. The same cleaning result cannot be achieved withother fluidic processes that generate no nanobubbles. SEM,HIM, and AFM measurements have been performed for surfaceinspection. It is highlighted that tapping-mode AFM operating inliquid can simultaneously demonstrate the generation of nano-bubbles and the nanoparticle removal caused by these nanobub-bles. We must stress the cleaning result in terms of thenanotrenches. These trenches contain a large contaminationdensity, yet have been efficiently cleaned with nanobubbles,which commonly is very difficult for other cleaning techniques.In addition, our findings have shown that increasing treatmentcycles allows an enhancement of cleaning efficiency. Finally, wehave not observed any damage to the nanosized surface struc-tures in a nanobubble cleaning process.

’ASSOCIATED CONTENT

bS Supporting Information. SEM, AFM, HIM images ofthe surfaces before and after the nanobubble treatments, AFMimages of nanobubbles. Data of nanoparticle coverage, descrip-tion of nanobubble treatment for surface cleaning. This materialis available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: anton.duisterwinkel@tno.nl.

’ACKNOWLEDGMENT

The authors thank Detlef Lohse, James Seddon, Jacquesvan der Donck, and Harold Zandvliet for stimulating discussions.S. Y. acknowledges Jetske Stortelder, Anne Heyer, and Emile vanVeldhoven for their experimental support.

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Figure 5. (a) HIM image of the initial dry surface prior to anisopropanol�water exchange process, deliberately contaminated withnanoparticles (examples indicated by arrows). (b) HIM image of thesame sample after three cycles of nanobubble process. Clearly, we seethat removal of the nanoparticles has been achieved, with a goodcleaning efficiency. Note that HIM characterizations provide views ona relatively large surface area with a high resolution. (c) Enlarged viewgives an observation inside the nanotrenches where most nanoparticleswere deposited, as shown in Figure 4a. Nanoparticles are not observedinside the trenches after the nanobubble treatment.

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