Dynamic Chemically Driven Dewetting, Spreading, and Self-Running

Article pubs.acs.org/Langmuir

Dynamic Chemically Driven Dewetting, Spreading, and Self-Running of Sessile Droplets on Crystalline Silicon Steve Arscott* Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), CNRS UMR8520, The University of Lille, Cité Scientifique, Avenue Poincaré, 59652 Villeneuve d’Ascq, France ABSTRACT: A chemically driven dewetting effect is demonstrated using sessile droplets of dilute hydrofluoric acid on chemically oxidized silicon wafers. The dewetting occurs as the thin oxide is slowly etched by the droplet and replaced by a hydrogen-terminated surface; the result of this is a gradual increase in the contact angle of the droplet with time. The time-varying work of adhesion is calculated from the timevarying contact angle; this corresponds to the changing chemical nature of the surface during dewetting and can be modeled by the well-known logistic (sigmoid) function often used for the modeling of restricted growth, in this case, the transition from an oxidized surface to a hydrogen-terminated silicon surface. The observation of the time-varying contact angle allows one to both measure the etch rate of the silicon oxide and estimate the hydrogenation rate as a function of HF concentration and wafer type. In addition to this, at a certain HF concentration, a self-running droplet effect is observed. In contrast, on hydrogen-terminated silicon wafers, a chemically induced spreading effect is observed using sessile droplets of nitric acid. The droplet spreading can also be modeled using a logistical function, where the restricted growth is the transition from hydrogen-terminated to a chemically induced oxidized silicon surface. The chemically driven dewetting and spreading observed here add to the methods available to study dynamic wetting (e.g., the moving three-phase contact line) of sessile droplets on surfaces. By slowing down chemical kinetics of the wetting, one is able to record the changing profile of the sessile droplet with time and gather information concerning the time-varying surface chemistry. The data also indicates a chemical interface hysteresis (CIH) that is compared to contact angle hysteresis (CAH). The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena. (e.g., patterned and structured) silicon surfaces;52−59 in contrast, the literature is somewhat sparse in terms of dynamic wetting on silicon. A very thin silicon oxide layer is known to play a key role in wetting,31 as is a hydrogen-terminated silicon surface resulting from the exposure of crystalline silicon to hydrofluoric acid (HF) solutions.39 Williams and Goodman31 showed that the contact angle of water is dependent on the silicon oxide thickness. Stoneham and Tasker32 and Philipossian36 demonstrated that the wetting of oxidized silicon surfaces can give information about the fixed charge near the silicon/ oxide interface. Gould and Irene33 used static contact angle measurements to study the etching properties of silicon oxide by HF solutions. Morita et al.34 showed that wetting could be used to identify termination species during native oxidation34,60 of the specific crystalline silicon surface. Hermansson et al.35 and Backlund et al.37 showed that the static wetting contact angle of a droplet depends on the specific chemical treatment of the silicon. Vera-Graziano et al.38 demonstrated that the

1. INTRODUCTION Crystalline silicon is the key element in the ongoing microelectronics1 and microsystems2 revolutions. The material is also emerging as a major player in future technologies based on nanoscience and nanotechnologies. 3 Wet processes involving silicon are very important in all of these areas; they include inter alia surface treatments,4 self-assembled monolayers,5 surface cleaning and preparation,6 surface roughening7 and smoothing,8 chemical etching,9,10 chemical oxidation,11 high-temperature oxide growth,12 crystal growth,13 soft lithography,14 bottom-up processes,15 nanowire growth,16 selfassembly,17 material characterization,18 printing,19 plating,20 anodization,21 wafer bonding,22 silicon on an insulator,23 sacrificial etching,24 evaporation,25 and drying.26 Such wet processes often involve surfaces with a changing chemical and morphological nature; in as much, a knowledge of static as well as dynamic wetting27−29 on silicon surfaces is very important, as this can reveal novel underlying physical and chemical processes and also give useful technological information for engineers.30 The literature contains many studies concerning the static wetting of smooth silicon surfaces31−51 and physically modified © 2016 American Chemical Society

Received: September 6, 2016 Revised: November 11, 2016 Published: November 14, 2016 12611

DOI: 10.1021/acs.langmuir.6b03287 Langmuir 2016, 32, 12611−12622

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Figure 1. Examples of chemically driven dewetting of dilute hydrofluoric acid droplets on chemically oxidized, crystalline silicon. The HF concentrations are (a−c) 0.1% (v/v) and (d−f) 0.3% (v/v) with DI water diluted from a 10% (w/w) HF/DI water stock solution. The silicon wafer [(100)-oriented and resistivity >10 000 Ω cm] surfaces were chemically oxidized using 65% HNO3 (10 min at 20 °C). solutions were used for the experiments: aqueous 10% (w/w) 5.747 mol L−1 hydrofluoric acid (HF) solution (VLSI grade, Technic), aqueous 65% (w/w) 14.576 mol L−1 nitric acid (HNO3) solution (electronic grade, Carlo-Erba), and deionized (DI) water (resistivity 18 MΩ cm). The pH of the 10% HF solution is 1.21, and the pH of the 65% HNO3 solution is near zero. The following test liquids were prepared for the wetting experiments: (i) a 0.1% HF (v/v) or 0.0574 mol L−1 solution, a 0.2% HF (v/v) or 0.1148 mol L−1 solution, a 0.3% HF (v/v) or 0.1722 mol L−1 solution, and a 0.4% HF (v/v) or 0.2296 mol L−1 solution. All solutions were prepared using the 10% HF stock solution and DI water. The pH values of the HF solutions are calculated to be 2.23 (0.1% solution), 2.07 (0.2% solution), 1.98 (0.3% solution), and 1.92 (0.4% solution). Three dilute nitric acid solutions were prepared for the wetting tests: 4:1, 3:1, and 2:1 (v/v) 65% HNO3/DI water. These correspond to concentrations of 12.099, 11.273, and 9.621 mol L−1 and have pH values near zero. The silicon wafers were used directly from the commercial supplier’s box because they are pretreated by the manufacturers (Siltronix, France) with stringent RCA cleaning6 (SC-1, NH4OH/ H2O2/H2O and SC-2, HCl/H2O2/H2O), resulting in an ∼7-nm-thick layer of protection oxide on the silicon wafer surfaces prior to processing (information kindly supplied by Siltronix, see Acknowledgments). All beakers used for the experiments (surface treatment and solution preparation) were precleaned using a piranha treatment [3/1 (v/v) 97%H2SO4/30%H2O2] and were rinsed thoroughly with DI water. Two types of chemically modified silicon surfaces were formed for the experiments. First, a hydrogen-terminated silicon surface67,68 (denoted here as H−Si) was obtained by exposing the silicon wafers to the 10% HF stock solution for 2 min. The wafers were then rinsed in a flow of DI water for 1 min and dried in pure nitrogen for 1 min. Note that no high-temperature annealing was performed following this. Second, a chemically oxidized silicon surface (denoted here as O−Si) was obtained by exposing the silicon wafers (pretreated to be H−Si as described above) to the 65% HNO3 solution for 10 min,69 followed by a 1 min rinse in a flow of DI water and a 1 min dry using pure nitrogen. Again, no high-temperature annealing was performed following this. It is well documented that the exposure of a polished, crystalline silicon surface to nitric acid11 forms a thin, amorphous chemically induced oxide that renders the silicon surface hydrophilic.70 Note that only one chemical oxidation processes was used in the study. Asuha et al.69 showed that a nonannealed chemical oxide has higher interface states than does a thermal oxide. Because Stoneham and Tasker32 and Philipossian36 have clearly demonstrated that the wetting of oxidized silicon surfaces can give information about the fixed charge near the silicon/oxide interface, it seems likely that the wetting will be affected by the specific chemical oxidation process, so this needs to be

wetting of bare silicon surfaces is dependent on illumination. Sata and Maeda39 showed that the wetting of pure water on crystalline silicon is dependent on the impurity doping density and orientation. They measured a static contact angle of 60° for heavily doped p-type (100) using 10% HF and a contact angle equal to 80° for heavily doped n-type (100) and (111). Adachi et al.,41 Kobyashi et al.,42 Tomita and Adachi,45 and Noguchi and Adachi47 plotted the static wetting contact of water on Si(111) as a function of oxide removal using timed immersion in dilute HF, in good agreement with Williams and Goodman.31 Extrand and Kumagai43 studied the contact angle hysteresis of liquids on silicon, measuring Δθ ≈ 14° for water. Miller et al.49 studied the effect of wetting and silicon micromachining. Ramos-Alvarado et al.51 modeled wetting on silicon and graphene-covered silicon surfaces. Here, in an effort to add to the literature concerning the wetting of silicon surfaces, the dynamic chemically driven dewetting and chemically driven spreading of sessile droplets on hydrogen-terminated and chemically oxidized crystalline silicon are reported and studied. The observations are recorded using the common chemicals often used to create these surfaces (hydrofluoric acid and nitric acid) to produce useful data and to enable others to easily repeat the measurements. This dynamic chemical wetting/dewetting method adds to the established methods of studying dynamic contact angles, which include hydrophobicity,61 evaporation,25,62 impact,63 effects of gravity,64 and surface heterogeneity.65,66

2. EXPERIMENTAL SECTION 2.1. Silicon Wafers Used for the Study. Commercial 3-in.diameter/380 ± 25-μm-thick, polished Czochralski (CZ) grown and float-zone (FZ) silicon wafers were used for the experiments (Siltronix, France). Six types of wafers were used for the study: (i) intrinsic FZ (100)-oriented silicon (ρ > 10 000 Ω cm), (ii) boron-doped CZ (100)-oriented p-type (5−10 Ω cm), (iii) phosphorus-doped CZ (100)-oriented n-type (5−10 Ω cm), (iv) boron-doped CZ (100)oriented p-type (0.009−0.01 Ω cm), (v) antimony-doped CZ (100)oriented n-type (0.01−0.02 Ω cm), and (vi) phosphorus-doped CZ (111)-oriented n-type (0.01 Ω cm). The surface roughness of the polished wafers is 10 000 Ω cm), and the chemical oxidation used a 65% (w/ w) HNO3 solution for 10 min. The solid red squares were obtained using a 3 min chemical oxidation with 65% HNO3. The dashed lines indicate a fit using a logistic (sigmoid) function; see the text. 2.3. Wetting Experiments. The silicon wafers were mounted into the contact angle meter (Digidrop, GBX) sample holder immediately following the silicon surface treatments described above, and droplets of the preprepared test liquid were rapidly (10 000 Ω cm). (a) Initial droplet wetting and spreading, (b, c) rapid self-running droplet motion in the plane of the wafer. The silicon surfaces were chemically oxidized using 65% HNO3 (10 min). The red arrow indicates the droplet motion. The droplet speed is ∼1.4 mm s−1.

environment101 during the transition from an O−Si to an H−Si surface (open purple diamonds in Figure 2a). The droplet initially wets the surface as with lower concentrations (Figure 4a) and begins to dewet as with lower concentrations but then begins to move rapidly in the plane of the wafer surface. (The red arrows in Figure 4b,c indicate the direction of movement of the droplet across the wafer surface). The initial droplet speed is measured to be 1.4 mm s−1, although this process results in the droplet moving in a haphazard way across the whole wafer surface. It is possible that faster dewetting on one part of the contact line leads to the opposite side of the contact line moving onto a part of the oxide that has not been exposed to HF. This will cause the droplet to wet the new oxide surface with a low contact angle; the process could rapidly repeat, leading to self-running. More work is needed to study this effect 12616


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Figure 5. Examples of chemically driven spreading of nitric acid droplets on hydrogen-terminated crystalline silicon. The HNO3 solution is (a−c) 65% (w/w) stock solution and (d−f) 3:1 (v/v) HNO3 (65%)/DI water. The hydrogen-terminated silicon wafer surfaces were prepared using a 10% (w/w) HF solution (2 min). The red arrows indicate a precursor film. The silicon wafer is (100)-oriented, and its resistivity is >10 000 Ω cm.

lower than the hydrogen-termination rates estimated for the HF solutions on O−Si. Figure 7 shows the results of the experiments for droplets of nitric acid (65% w/w) on different silicon wafer types that have been treated with HF (10% w/w) to achieve a hydrogenterminated surface. First, a higher doping density (p- or n-type) results in a slightly lower initial contact angle; remember that these results are for 65% HNO3, implying a constant surface tension. Indeed, it has been observed that the doping type and density play a role in the contact angle of water on crystalline silicon.39 In addition, the results indicate that the oxidation of the H−Si surface occurs more rapidly for a highly doped p-type wafer. Indeed, highly doped p-type (100) material was found to produce the thickest chemical oxide by Asuha et al.,73 although they concluded that the influence of the dopant atoms on the oxidation reaction was not catalytic. Note that analysis is more difficult when considering spreading droplets of nitric acid on H−Si surfaces as the wetting phases are not as well-defined as with dilute HF droplets on the chemically derived O−Si surfaces. The physical properties of silicon oxide are known to be complicated and vary with thickness below 4 nm.110 Therefore, one cannot make simple assumptions and approximate the oxidation site rate or oxidation rate of a uniform film as one can above. To conclude this section, again in terms of the effect of evaporation a 65% w/w nitric acid solution is a quasi-azeotrope, meaning that we can consider the droplet concentration to be unchanged over the time of the experiment and the changes in the contact angle to be predominantly due to chemically driven spreading; the evaporation rate of 65% HNO3 solution is determined to be 112.9 ± 36 nL min−1 from the data. This is lower than that measured for the HF solutions; 65% HNO3 has a higher boiling point than water (∼121 °C). The level of the evaporation rate implies that the droplet volumes are little changed during the experiments. 3.3. Contact Angle Hysteresis and Chemically Driven Dewetting/Spreading. The results can be discussed in the context of contact angle hysteresis (CAH).48,111 CAH can be understood by considering a droplet wetting a surface with a contact angle θ and an initially static three-phase contact line. If this contact angle changes, meaning that it decreases because of

contact angle is higher as the HNO3 concentration decreases because of an increase in the surface tension with changing concentration.108 The surface tensions of the nitric acid solutions are calculated, using γc = −0.8c + 72.8 mJ m−2, to be 61.1, 63.1, 63.8, and 65.1 mJ m−2 for increasing dilution. Using these values of surface tension, Figure 6c plots WA versus time for the chemically driven spreading of nitic acid solutions on an H−Si surface. As with the chemically driven dewetting effect above, the spreading here again suggests the observation of a transition from a hydrogen-terminated silicon surface to a uniform oxidized surface passing by an intermediate surface, which is well documented for very thin oxide films.109 In the case of 65% HNO3, the experimental data indicates that the complete oxidation of the surface occurs in ∼200 s. Using the same reasoning as above for the surface coverage, the value of WA should also follow a sigmoid function, indicating the changing chemical nature of the surface during the chemically induced droplet spreading due to an H−Si to O−Si transition followed by progressive chemical oxidation of a uniform film, increasing in thickness;69 this is seen in the dashed lines in Figure 6c. The results here can be compared to those of Morita et al.,34 who showed that static wetting could be used to identify termination species during native oxidation34,60 of the silicon surface, and Hermansson et al.35 and Backlund et al.,37 who showed that the static wetting contact angle of a droplet depends on the specific chemical treatment of the silicon. It is also interesting to compare the findings here to superspreading,106 where a physiochemical modification of a surface by the presence of surfactants in a droplet modifies the wetting. The dynamic chemical dewetting (using dilute HF on O−Si) observed here can be considered to be the opposite to superspreading; the presence of HF in the droplet chemically modifies the oxidized silicon surface, resulting in dynamic dewetting of the droplet. In contrast, the presence of the HNO3 in the droplet chemically modifies the H−Si surface toward O− Si, resulting in enhanced wetting and spreading, as with superspreading. Again, in a first approximation, if we consider the case of the uniform oxidation of all surface sites before island growth, the oxidation rate of the silicon surface can also be estimated from the data. For 65% HNO3 on H−Si, the oxidation rate corresponds to 7.4 × 1011 sites s−1, which is 12617


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Figure 7. Results for chemically driven spreading of nitric acid droplets on hydrogen-terminated crystalline silicon as a function of the silicon wafer type. Plots of (a) the droplet contact angle (CA), (b) the droplet base radii (rb), and (c) the work of adhesion (WA). Hydrogen termination was achieved on all wafers using a 10% (w/w) HF solution for 2 min. The dashed lines indicate a fit using a logistic (sigmoid) function (see the text).

Figure 6. Results for chemically driven spreading of nitric acid droplets on hydrogen-terminated crystalline silicon as a function of HNO3 concentration. Plots of (a) the droplet contact angle (CA), (b) the droplet base radii (rb), and (c) the work of adhesion (WA). The wafers are have intrinsic (100) orientation (resistivity >10 000 Ω cm), and the hydrogen termination was obtained by immersing the wafers in a 10% (w/w) HF solution for 2 min. The dashed lines indicate a fit using a logistic (sigmoid) functions (see the text).

droplet’s surface) leads to a reduced contact angle whereas the liquid−solid−vapor line is pinned (i → ii). When the contact angle falls to be equal to the receding contact angle, the contact line begins to move inward (ii → iii). The dynamics of the CAH depend on the evaporation rate and the value of the receding contact angle, which depends on the chemical and topographical nature of the surface, which are fixed in this case. In the case of the hysteresis of a chemically driven dewetting droplet, the situation is different. With reference to Figure 8b, the initial contact angle θ1 remains constant (i → ii) until the contact line begins to move as a result of a change in surface chemistry (ii → iii), finally arriving at θ2 (iv) as the chemical nature of the surface no longer evolves with time. The colored surfaces in (ii → iv) illustrate the changing chemical nature of

evaporation or increases because of condensation, then for some given contact angle the contact line begins to move.28 These two angles are termed the receding contact angle θR and the advancing contact angle θA for the specific liquid−solid− vapor system. CAH is observed in sessile droplet−solid surface systems where the underlying surface is not varying (chemically or topographically) during the experiment.48 The situation here is different: the chemical reaction between the liquid droplet and the solid surface changes the underlying surface locally. Figure 8 shows schematic diagrams comparing common CAH of an evaporating droplet (Figure 8a) with the hysteresis of a droplet that is chemically dewetting (Figure 8b). In the case of evaporation, the loss of volume (due to evaporation from the 12618


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contact angle equals θR; in this case, θR is an observable parameter. In contrast, in the case of the moving contact line in chemically driven dewetting, the contact line recedes when the liquid−solid surface energy reaches γRsl, which is not directly observable. In this way, we have chemical interface hysteresis (CIH) rather than contact angle hysteresis. For the CIH, the observable parameter is the time to begin the dewetting (i.e., the time when the contact line begins to move). Once γsl > γRsl, we can look at the system from the perspective of the moving three-phase contact line, as illustrated in Figure 8d, and one can write down the dynamic version of Young’s equation as follows: γsv(t ) = γ cos θ(t ) + γsl(t )

(4)

Using the same reasoning, it is possible to write down an expression for the net force (per meter) that causes the droplet position to be unstable and result in a self-running droplet F(t ) = γsvl(t ) − γsvr(t ) + [γslr(t ) − γsll(t )] + γ[cos θ r(t ) − cos θ l(t )]

(5)

where superscripts l and r refer to the left- and right-hand sides of the droplet. The instability could arise from wetting nonsymmetry (e.g., a defect).113 3.4. Comparison with Previous Work. The observed variation of wetting from a hydrogen-terminated to a partially oxidized and fully oxidized silicon surface and vice versa, which are observed in the experiments, is explained by the change in surface energy from H−Si44 to oxidized silicon114,115 and back again; the ratio of surface energies is between 5 and 10. It is known that a very thin silicon oxide layer plays a key role in wetting,31 as does a hydrogen-terminated silicon surface resulting from the exposure of the silicon to HF.39 Williams and Goodman31 showed that the static contact angle of water is dependent on the oxide thickness. For a room-temperature native oxide (thickness between ∼0.6 and 1.8 nm), they measured a smooth contact angle variation from ∼70 to ∼30°, a range similar to that observed here. The results here can also be compared to those of Adachi et al.,41 Kobyashi et al.,42 Tomita and Adachi,45 and Noguchi and Adachi,47 who plotted the static wetting contact of water on silicon as a function of oxide removal using immersion in HF and the measurement of a static contact angle evolution of 20−70° over a progressive 100 s dip in 1.5% HF. In the current experiment, the use of ultradilute HF solutions and HNO3 solutions enables this to be observed in a single dynamic experiment as the droplet gradually dewets (HF on O−Si) or wets (HNO3 on H−Si) the surface, revealing the changing chemical nature of the underlying surface.

Figure 8. Contact angle hysteresis for (a) an evaporating droplet and (b) a chemically driven dewetting droplet. (c) Contact line surface energies and (d) illustration of the dynamic Young’s equation that describes the system. In panel a, θR is the receding contact angle and the black horizontal line indicates that the chemical nature of the surface is not changing. In panel b, the colored lines (green, yellow, and blue) indicate the changing chemical nature of the surface due to the chemical reaction between the liquid and the surface. θ1 and θ2 correspond to the initial and final contact angles, and θi is an intermediate contact angle. The red arrows correspond to the moving contact line.

4. SUMMARY AND CONCLUSIONS This study has revealed dynamic chemically driven dewetting, spreading, and self-running of hydrofluoric acid and nitric acid sessile droplets on hydrogen-terminated and oxidized crystalline silicon wafers. The chemically driven dewetting effect is observed when dilute sessile droplets of an aqueous HF solution are deposited onto a chemically oxidized silicon wafer. The dilute HF solution slowly etches away the thin chemically derived oxide layer, which leads to a gradual change in the surface energy. This is manifested as an increase in the macroscopic contact angle of the sessile droplet with time; the higher the HF concentration, the smaller the dewetting transition time. Above a certain HF concentration, a selfrunning droplet effect is observed where the droplet’s center of mass moves horizontally on the wafer surface.

the surface. There are two major differences compared to CAH: first, it is the changing surface that governs the hysteresis, and second, the contact line recedes while the contact angle increases, although this can occur for micropatterned surfaces.112 This difference is further illustrated in Figure 8c, which shows the forces due to the three surface energies acting at the contact line. When the contact angle is changed by some external influence (e.g., evaporation or gravity), then the balance of forces means that the contact line recedes when the 12619


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(7) Biswas, K.; Kal, S. Etch Characteristics of KOH, TMAH and Dual Doped TMAH for Bulk Micromachining of Silicon. Microelectron. J. 2006, 37 (6), 519−525. (8) Sparacin, D. K.; Spector, S. J.; Kimerling, L. C. Silicon Waveguide Sidewall Smoothing by Wet Chemical Oxidation. J. Lightwave Technol. 2005, 23 (8), 2455−2461. (9) Williams, K. R.; Muller, R. S. Etch Rates for Micromachining Processing. J. Microelectromech. Syst. 1996, 5 (4), 256−269. (10) Williams, K. R.; Gupta, K.; Wasilik, M. Etch Rates for Micromachining Processing-Part II. J. Microelectromech. Syst. 2003, 12 (6), 761−778. (11) Asuha; Kobayashi, T.; Maida, O.; Inoue, M.; Takahashi, M.; Todokoro, Y.; Kobayashi, H. Ultrathin Silicon Dioxide Layers with a Low Leakage Current Density Formed by Chemical Oxidation of Si. Appl. Phys. Lett. 2002, 81 (18), 3410−3412. (12) Tang, M.; Ramos, A. V.; Jud, E.; Chung, S.-Y.; Gautier-Soyer, M.; Cannon, R. M.; Carter, W. C.; Chiang, Y.-M. Nanometer-Scale Wetting of the Silicon Surface by Its Equilibrium Oxide. Langmuir 2008, 24 (5), 1891−1896. (13) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4 (5), 89−90. (14) Xia, Y.; Whitesides, G. M. Soft Lithography. Annu. Rev. Mater. Sci. 1998, 28 (1), 153−184. (15) Cui, Y.; Lieber, C. M. Functional Nanoscale Electronic Devices Assembled Using Silicon Nanowire Building Blocks. Science 2001, 291 (5505), 851−853. (16) Misra, S.; Yu, L.; Chen, W.; Roca i Cabarrocas, P. Wetting Layer: The Key Player in Plasma-Assisted Silicon Nanowire Growth Mediated by Tin. J. Phys. Chem. C 2013, 117 (34), 17786−17790. (17) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295 (5564), 2418−2421. (18) Blood, P. Capacitance-Voltage Profiling and the Characterisation of III-V Semiconductors Using Electrolyte Barriers. Semicond. Sci. Technol. 1986, 1 (1), 7−27. (19) Lim, T.; Han, S.; Chung, J.; Chung, J. T.; Ko, S.; Grigoropoulos, C. P. Experimental Study on Spreading and Evaporation of Inkjet Printed Pico-Liter Droplet on a Heated Substrate. Int. J. Heat Mass Transfer 2009, 52 (1−2), 431−441. (20) Lennon, A.; Yao, Y.; Wenham, S. Evolution of Metal Plating for Silicon Solar Cell Metallisation. Prog. Photovoltaics 2013, 21 (7), 1454−1468. (21) Yamani, Z.; Thompson, W. H.; AbuHassan, L.; Nayfeh, M. H. Ideal Anodization of Silicon. Appl. Phys. Lett. 1997, 70 (25), 3404− 3406. (22) Niklaus, F.; Stemme, G.; Lu, J.-Q.; Gutmann, R. J. Adhesive Wafer Bonding. J. Appl. Phys. 2006, 99 (3), 031101−031128. (23) Danielson, D. T.; Sparacin, D. K.; Michel, J.; Kimerling, L. C. Surface-Energy-Driven Dewetting Theory of Silicon-on-Insulator Agglomeration. J. Appl. Phys. 2006, 100 (8), 083507−083510. (24) Tas, N.; Sonnenberg, T.; Jansen, H.; Legtenberg, R.; Elwenspoek, M. Stiction in Surface Micromachining. J. Micromech. Microeng. 1996, 6 (4), 385−397. (25) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Contact Line Deposits in an Evaporating Drop. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 62 (1), 756−765. (26) Park, J.-G.; Pas, M. F. Effects of Drying Methods and Wettability of Silicon on the Formation of Water Marks in Semiconductor Processing. J. Electrochem. Soc. 1995, 142 (6), 2028−2031. (27) De Gennes, P.-G. Wetting: Statics and Dynamics. Rev. Mod. Phys. 1985, 57 (3), 827−863. (28) Blake, T. D. The Physics of Moving Wetting Lines. J. Colloid Interface Sci. 2006, 299 (1), 1−13. (29) Quéré, D. Wetting and Roughness. Annu. Rev. Mater. Res. 2008, 38 (1), 71−99. (30) Extrand, C. W. Origins of Wetting. Langmuir 2016, 32, 7697. (31) Williams, R.; Goodman, A. M. Wetting of Thin Layers of SiO2 by Water. Appl. Phys. Lett. 1974, 25 (10), 531−532.

In contrast, a chemically driven spreading effect is observed when a droplet of nitric acid is placed on a hydrogenterminated silicon surface. In this case, the droplet contact angle is seen to slowly decrease with time, indicating that the surface energy is gradually changing as the surface transforms from a hydrogenated surface to a partially oxidized surface and finally a fully oxidized surface. The experimental data allows the work of adhesion to be plotted as a function of time for the two systems, i.e., dilute HF droplets dewetting on O−Si and HNO3 droplets spreading on H−Si. The time-varying work of adhesion can be interpreted as the changing chemical nature of the silicon surface as the dewetting or spreading proceeds with time; the data plots can be fitted accurately using a logistic (sigmoid) function. One can assume here that the restricted growth is the gradual transition from fully oxidized silicon to a hydrogenated silicon surface, presumably passing through a transition surface. The hydrogenation and oxidation rates of the silicon surface using dilute HF and HNO3 solutions can be calculated from the experimental data. The chemically driven dewetting and spreading observed here is another way to study the dynamic wetting (e.g., the moving contact line) of droplets on surfaces by slowing down the chemical kinetics enough to be able to record the changing profile of the sessile droplet with time. The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena, something that is potentially technologically useful.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Steve Arscott: 0000-0001-9938-2683 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author thanks Elisabeth Gallopin (IEMN) for her help with the ellipsometry measurements to measure the oxide thicknesses. The author also thanks Nathalie Thomas and JeanCharles Mermoud (both from Siltronix, France) for their kindness in offering information concerning the commercial RCA cleaning performed on the wafers and the resulting surface of the commercially supplied wafers. This work was partially supported by the French RENATECH network.

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REFERENCES

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Dynamic Chemically Driven Dewetting, Spreading, and Self-Running

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