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Giant Blistering of Nanometer-Thick Al2O3/ZnO Films Grown by Atomic Layer Deposition: Mechanism and Potential Applications Hongfei Liu, Shifeng Guo, Ren Bin Yang, Coryl Jing Jun Lee, and Lei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08260 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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ACS Applied Materials & Interfaces

Giant Blistering of Nanometer-Thick Al2O3/ZnO Films Grown by Atomic Layer Deposition: Mechanism and Potential Applications Hongfei Liu,* Shifeng Guo, Ren Bin Yang, Coryl J. J. Lee, and Lei Zhang Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Singapore 138634, Singapore ABSTRACT: Giant circular blisters of up to 300 µm in diameter and 10 µm in deflection have been produced on nanometer-thick Al2O3-on-ZnO stacks grown by atomic layer deposition at 150 °C followed by annealing at elevated temperatures. Their shape changes upon varied ambient pressures provide evidence that their formation is related to an anneal-induced outgassing combined with their impermeability. The former mainly occurs in the bottom ZnO layer that recrystallizes and releases residual hydroxide ions at elevated temperatures while the latter is dominantly contributed by the pinholefree Al2O3 layer on top. Vibrations at a resonant frequency of ~740 kHz are mechanically actuated and optically probed from an individual blister. By modulating the thickness and stacking sequence of Al2O3 and ZnO, we further demonstrate a localized circular film swelling upon electron-beam irradiation and its recovery after reducing the irradiation flux. The elastic blistering and the recoverable swelling of the nanometer-thick films represent a miniaturized event-driven mechanical system for potential functioning applications. Keywords: Al2O3/ZnO, atomic layer deposition, post-growth annealing, thin film blistering, mechanical responses

*Author to whom correspondence should be addressed; electronic mail: [email protected]

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1. INTRODUCTION Surface blistering, i.e., localized delamination and bulging, has long been observed along with material degradations of coated or native oxide thin films on metals and alloys at elevated temperatures and/or upon high-energy irradiations.1,2 The thermal- and/or irradiation-induced surface blistering of thin films on rigid substrate is usually related to relaxations of biaxial compressive stress and/or gases desorption at the film/substrate interface.3-4 However, the dominant blistering mechanism is complex and case dependent. For example, Malerba et al. recently reported that the stress-driven effect is dominant over the gas-driven effect on the film blistering of sputter-grown Cu2ZnSnS4 upon postgrowth annealing.5 The same phenomenon has been observed in blistering of sputtergrown gold films upon low-flux hydrogen plasma treatment at low temperatures.6 In contrast, the blistering of Al2O3 thin films grown by atomic layer deposition (ALD), which have been extensively investigated as high-k dielectrics for electronic devices and effective surface passivation agents for silicon-based photovoltaic devices, is generally attributed to the gas-driven mechanism due to their low gas permeablility.7-10 Although the post-growth annealing induced surface blistering of Al2O3 thin films is undesired in high-k dielectric and surface passivation applications, its impermeable character might be employed, in combination with graphene and/or other optoelectronic thin films, e.g., α-Si/SiO2,11 parylene-C,12 etc., for producing novel miniaturized electromechanical and/or optomechanical systems.11-15 A typical microscale bulging test for ALD-grown Al2O3 with the thickness down to ~1 nm using a graphene template has been recently demonstrated by Wang et al.13 From the pressurized blister test, they obtained a Young’s modulus of 154 ± 13 GPa for the ultrathin Al2O3 films, which is

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comparable to those of 168-220 GPa reported in the literature for ALD-grown Al2O3 films of much larger thicknesses, i.e., tens to hundreds of nanometers.16-18 Dedicated processes, including fabricating through holes in rigid wafers and transfer of the templating graphene, are basically necessary for creating such blisters for the bulging test.12-13,

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These processes may hinder the practical applications of the blisters,

especially when integrating them on chips with conventional devices and/or modules. However, it has also been reported that localized blisters can be produced by introducing weakly chemisorbed molecules, e.g., ClF3, underneath gas impermeable films or localized low-adhesion layers, e.g., CxFy, in between delicately designed multilayers;11, 14,21

upon heat treatment or irradiation (e.g., laser illuminations), release of the

chemisorbed molecules or localized buckling/delamination (defined by the low-adhesion layers) give rise to the blisters. Based on these studies, we were interested in determining whether robust blisters could be produced on ALD-grown Al2O3 thin films by monolithically introducing an underlying ZnO layer since the recrystallization and gas desorption temperature of ZnO is lower than that of Al2O3. We then attempted to explore whether these blisters could be applied for electromechanical and/or optomechanical systems. The former can have wide applications in acoustic modules such as sensors and transducers while the later, making use of the half symmetric cavity/lens of the sphere cap-shaped blisters, can be integrated with active semiconductor devices, e.g., laser diodes, to create novel light modulation modules. In this light, we have grown ZnO, Al2O3, and their stacking structures with varied thickness and stacking sequence by ALD at 150 °C. They were then annealed at elevated temperatures under various atmospheres including oxygen, sulfur, nitrogen,

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argon, and a mixture of argon and hydrogen. Giant blisters of up to 300 µm in diameters and 10 µm in deflections have been observed on a 120-cycle Al2O3/120-cycle ZnO structure in spite of the different annealing environments. A resonant vibration frequency of ~740 kHz has been obtained for such individual blisters. Electron-beam igniting and recovery of localized circular film swelling have also been realized by changing the layer thickness and stacking periods. The elastic blistering and the recoverable irradiationinduced swelling of the nanometer-thick films represent a miniaturized event-driven mechanical system that can be compatibly integrated with conventional electrical and optoelectronic devices and/or modules on chips for potential functioning applications.

2. EXPERIMENTAL PROCEDURE 2.1. ALD Growth of Oxide Thin Films and Bilayer Stacks. The ZnO, Al2O3, and their bilayer stacking structures were grown in an Azimuth Flex ALD chamber using diethylzinc (DEZ), trimethylaluminum (TMA), and H2O as the source precursors for zinc, aluminum, and oxygen, respectively. Pure nitrogen (99.999%) was used as the precursor carrier as well as the purging gas. The growth was performed at 150 °C on sapphire substrates. The detailed reactor setup and the oxide growth procedures can be found in our earlier publication;22 a slight difference in this study is that the purge durations after the half cycles of H2O are decreased from 40 to 30 s to increase the residual hydroxide ions while keeping the gas impermeability intact. Single layer ZnO and Al2O3 thin films were first grown for 240 cycles separately and were used as the control samples. Two bilayer stacks Al2O3-on-ZnO (Stack A) and ZnO-on-Al2O3 (Stack B) with individual sublayers of 120 cycles each were then grown for producing film blisters. In another two

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stacks, the thickness of sub-layers was further reduced to 40 cycles while the stacking was repeated two more times, i.e., to form six-layer stacks starting with Al2O3 (Stack C) and ZnO (Stack D), respectively. The layer thicknesses of the 240-cycle Al2O3 and ZnO thin films measured by X-ray reflection (see Supporting Information, Figure S1) are 31.9 and 36.0 nm, respectively. Likewise, the nominal growth rates of Al2O3 and ZnO in this work are ~0.13 and ~0.15 nm/cycle, respectively. 2.2. Post-Growth Heat Treatment. Post-growth annealing was carried out in atmospheric pressures under various environments including oxygen, sulfur, nitrogen, argon, and a mixture of argon and hydrogen. However, we found that changes in the gas environments do not have any significant influence on the blister formations. In this regard, for the sake of brevity, we will only focus on thermal vapor sulfurization (TVS), i.e., annealing under the sulfur vapor environment.23-25 The main reason for choosing TVS is to make use of the fact that if the underlying ZnO layer turns into ZnS, it can be a direct evidence of the permeability of the Al2O3 layer on top, and vice versa. The TVS under atmospheric pressure was performed, for 60 min, at 550 and 750 °C, respectively. 2.3. Characterizations. Optical microscopy, scanning-electron microscopy (SEM, JEOL JSM-7600F, working at 9.6 × 10-5 Pa), and atomic-force microscopy (AFM, tapping mode) have been employed to compare the morphological and potential structural changes before and after the post-growth heat treatments, i.e., TVS. X-ray diffraction (Bruker-D8, general-area-detector diffraction system, and Cu-Kα1), together with X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF, Bruker M4 TORNADO), was used to evaluate the anneal-induced recrystallizations of the oxide

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layers. Fourier-transform infrared (FTIR) micro-spectrometer (Bruker VERTEX 80v & HYPERION) was used to identify possible gases in the blisters. The blister deflection profile is measured using an Alpha-Step D-600 Stylus Profiler at a force of 0.05 mg.

3. RESULTS AND DISCUSSION 3.1. Morphological and Structural Properties. Figures 1a-1l present the SEM images recorded from the oxide thin films and the bilayer stacks before and after TVS; the scale bars are of 200 nm. Comparisons of the SEM images revealed that (i) the asgrown 240-cycle Al2O3 film does not exhibit any surficial features (Figure 1a) while fine particle-like structures emerged when incorporating of ZnO (Figures 1d, 1g, and 1j); (ii) the particle-like structures of stack B (ZnO-on-Al2O3, Figure 1g) are a bit coarser than those of stack A (Al2O3-on-ZnO, Figure 1d) but similar to those of the 240-cycle ZnO film (Figure 1j); (iii) neither the Al2O3 thin film (Figures 1a-1c) nor stack A (Figures 1d1f) exhibits any significant morphology changes after TVS; and (iv) the fine particle-like structures of stack B and the ZnO thin film underwent an apparent coarsening upon TVS and resulted in grain structures with clear facets and sharp edges after the TVS at 750 °C (Figures 1g-1l). Figures 1m-1p present the XRD intensity distributions in the 2θ-ϕ frames collected from the respective samples after TVS at 750 °C. The same 2θ-ϕ distributions in Figures 1o and 1p, together with the increased XRD intensity (represented by the strengthened brightness at ϕ = 0) in Figure 1p than that in Figure 1o, indicate that the same crystalline structures as that created in stack B (Figure 1o) have been crystalized with larger grain sizes in the ZnO layer (Figure 1p). However, the XRD intensity

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distributions in Figure 1n are apparently different from those in Figures 1o-1p and no XRD intensity can be detected at all in Figure 1m. The former indicates crystals different from those created in stack B and the ZnO layer have been crystalized in stack A while the Al2O3 film is still in its amorphous state after the TVS at 750 °C for 60 min. A combination of the SEM and the XRD comparisons in Figure 1, as well as the XPS spectra collected from stacks A and B after TVS at 550 °C (see Supporting Information, Figure S2, which revealed Al2O3 and ZnS on the surface of stack A and stack B, respectively), leads to the conclusion that the ALD-grown Al2O3 is much more stable than ZnO; the Al2O3 layer is amorphous and does not experience any crystallizations under the post-growth TVS up to 750 °C; the TVS-induced crystallizations mainly occurred in the ZnO layer; and an Al2O3 cover on the ZnO layer apparently modified the crystallizing reactions that, in turn, resulted in different crystals. To verify these conclusions, we have further compared the XRD patterns for the oxide thin films and the bilayer stacks A and B before and after TVS. The results are presented in Figure 2. The absence of any XRD features in Figure 2a confirms the amorphous state of the 240-cycle Al2O3 before and after the TVS. Figure 2b shows an onset of crystallization reactions occurred in stack A upon TVS at 550 °C; an increase in the TVS temperature to 750 °C resulted in well distinguished hexagonal wurtzite ZnO (JCPDS 036-1451) crystals. In comparison, Figures 2c and 2d show that the ZnO layers in stack B and the 240-cycle ZnO film somewhat crystalized during the ALD growth, which is consistent with their more coarsened particle-like structures in the SEM images (Figures 1g and 1j). The crystallization of ZnO in the as-grown samples is attributable to the Al2O3 templating

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effect and/or the increased cycles as those in typical ALD process of oxides.26-29 More OH terminals provided by the ALD Al2O3 and the earlier cycles of ALD ZnO than those by the original substrate surface tend to promote the nucleation and crystal growth of ZnO. However, the post-growth TVS resulted in well distinguished cubic ZnS (JCPDS 080-0020) crystals at 550 °C; the crystal sizes of the recrystallized ZnS are further increased, manifested by the increased peak intensity accompanied with the narrowed linewidth (see Figures 2c-2d), by increasing the TVS temperature to 750 °C. These results provide evidence that the Al2O3 top layer is not permeable to gas, which can effectively prevent the ZnO in stack A from being sulfurized upon TVS. The TVSinduced crystallization of ZnO, in the absence of sulfur, is dynamically controlled by the annealing temperatures. 3.2. Giant Blisters and Their Response to Pressure Variations. When reducing the SEM magnifications, we observed circular blisters of up to 300 µm in diameter on the surface of stack A after TVS at 550 °C (Figure 3a). These diameters are much larger than those in the range of a few to tens of micrometers reported for dielectric and/or semiconductor thin film blisters in the literature.7-9,

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However, such giant

blisters are absent from the surface of stack B (Figure 3b) and they are also absent from the surface of the 240-cycle ZnO and Al2O3 films after TVS. Instead, surface blisters with diameters smaller than 25 µm emerged on the surface of stack B after TVS at 550 °C (see Figure 3b). As mentioned above, these blister sizes are consistent with those reported in the literature for ALD-grown Al2O3 thin films thicker than ~10 nm.7, 9-10, 33-34

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Most of the giant blisters, exhibiting spherical cap shapes in the SEM chamber at a pressure of ~9.6 × 10-5 Pa, partially collapsed at atmospheric pressure (i.e., 1.0 × 105 Pa) in the air environment. A dedicated comparison of the same blisters upon varied pressures is presented in Figures 3c and 3d (see Supporting Information, Figure S3, for more detailed comparisons). Figure 3c is an SEM image recorded at lower pressure while Figure 3d is a microphotograph recorded under atmospheric pressure. When the partially collapsed blisters were reloaded into the SEM chamber, they either bulge out again to the spherical cap shapes or break (see Supporting Information, Figure S4). The pressureinduced shape changes and the wrinkles induced by the blister collapsing (see Figure 3d) provide evidence that gases accumulated in the giant blisters during their formation upon TVS at elevated temperatures. From the gas-driven ‘blister growth’ point of view, its final shape at the end of TVS should be a spherical cap to minimize its surface tension. However, in terms of pV = nRT and assuming the volume V and the gas moles n of the blisters do not change upon cooling the samples, the inner pressure p thus decreases with the temperature T, leading to an increase in the outer-to-inner pressure difference, ∆pout −in = pout − pin , of the blisters. When the pressure difference increases to a critical point, the blister starts to collapse, leading to various blister shapes (see Supporting Information, Figure S3). Next, when loading the samples into the SEM chamber, the pressure outside the blisters decreases as the chamber’s pumping down, leading to an increase in the inner-to-outer pressure difference, ∆pin −out = pin − pout . As a result, the blisters bulged out to the spherical cap shapes (Figures 3a and 3c).

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A small number of blisters are robust; they kept their spherical cap shapes unchanged after unloading from the SEM chamber out to the atmospheric environment. Figures 3e and 3f present the tilt- and cross-section-view SEM images recorded from such a blister. A careful SEM measurement revealed that this blister has a diameter of d ≈ 297.5 µm and a maximum deflection of h ≈ 10.1 µm in the SEM chamber. A microphotograph taken from this blister under atmospheric pressure is shown in Figure 3g, where, one also sees a concentric circle (indicated by the arrow) with a larger diameter surrounding the blister. This concentric circle is most likely a trace of the original size of the blister just after the TVS at 550 °C. A step-profile has further been measured across the maximum deflection of this blister (see Figure 3h), which reveals a diameter of d ≈ 288.5 µm and a maximum deflection of h ≈ 8.4 µm, both are smaller than those measured in the SEM chamber. This comparison indicates that such robust blisters shrink their sizes, instead of partially collapse, in response to the increased outer-to-inner pressure difference. The larger concentric circle surrounding the blister also gives rise to a small hump, indicated by the arrow, in the step-profile, from which the diameter of the original blister can be estimated to be d ≈ 406.5 µm. Assuming the h/d ratio of the spherical cap-shaped blister does not change upon cooling and in terms of

 d 2 h2  V = πh  −  , the ratio of the blister volume at 550 °C (i.e., 823 K) over that at room 8 6   temperature (i.e., 300 K) is then V823K V300 K = 2.8 . This ratio is quite close to the absolute temperature ratio between 550 °C and room temperature, i.e., T823K T300 K = 2.74 . In such a particular case, according to pV = nRT , the inner pressure of the blister would not apparently change upon sample cooling. This estimation implies that desorption and

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accumulation of gases stopped at the beginning of sample cooling, which in turn indicates the gas impermeability of the Al2O3-on-ZnO bilayer stack upon TVS at 550 °C. We also found that some of the blisters were broken (see the SEM image in Figure 4a), which is helpful for in-depth studies of the surface localized at the area under the original blister, the intact conformal film, and the flipped blister wall (i.e., the inner surface of the original blister); they are labeled i, ii, and iii, in Figure 4a, respectively. A sharp film edge caused by the blister breakage is enlarged in Figure 4b and an AFM image recorded around this edge is presented in Figure 4c. From the AFM image, we derived a height profile (Figure 4d), which revealed a thickness of ~29.2 nm for the film, corresponding to the wall thickness of the original blister. This thickness is about the same as the nominal thickness of the bilayer stack A (i.e., 120-cycle Al2O3/120-cycle ZnO). We have also recorded AFM images, at large magnifications, from both sides of the film edge, i.e., on the surface of the intact conformal film and on the surface that was under the original blister (Supporting Information, Figure S5). Root-mean-square analysis revealed roughnesses of 0.48 and 0.12 nm for the intact conformal film and the surface of the delaminated area. The latter is quite close to that of the bare sapphire substrate after annealing at 1050 °C for 60 min in air, i.e., 1.0 nm.35 Both the surface roughnesses and the layer thickness in Figure 4d confirm that the blistering of the bilayer stack occurred at the interface in between the bottom ZnO layer and the sapphire substrate. Figures 4e-4g present the elemental distribution mappings collected by XRF across the broken blisters in Figure 4a for aluminum, zinc, and sulfur, respectively.

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Likewise, Figure 4h presents the XRF spectra collected in point-focus, which has higher elemental resolutions, from the locations i-iii in Figure 4a. Enlarged XRF spectra, addressing the concentration of sulfur, are presented in Figure 4i. Both the XRF mappings and the XRF spectra revealed an absence of zinc at the delaminated areas (i.e., the areas under the original blisters) while sulfur only presents at the surface of the flipped pieces. These results further confirm that the film blistering occurred at the interface in between the bottom ZnO layer and the sapphire substrate of the bilayer stack. They also suggest that the blisters were broken, most likely, during the TVS treatment since sulfur-related reactions only occurred at the flipped pieces. 3.3. Gas Desorption, Gas Accumulation, and Blister Formation. Based on the results discussed above, it is quite clear that localized gas accumulations occurred at the interface between the bottom ZnO and the sapphire substrate of the bilayer stack upon TVS at elevated temperatures. When the gases accumulate to a critical pressure, delamination and bulging out of the bilayer stack give rise to the blisters. Since the TVSinduced giant blisters are absent not only in the Al2O3 and ZnO thin films but also in stack B (i.e., the ZnO-on-Al2O3 bilayer stack), which, together with the TVS-induced crystallizations in the ZnO layer rather than in the Al2O3 layer (see Section 3.1), indicates that the gas desorption mainly occurred in the ZnO layer. The gas desorption, gas accumulation, and blister formation mechanism is schematically shown in Figure 5. The much smaller bonding energy of Zn-O (66 kcal/mol) than that of Al-O (116 kcal/mol) leads to a larger amount of hydroxide complexes, substituting oxygen, in ZnO than that in Al2O3 grown by ALD and, as a result, a larger amount of zinc vacancies are formed in ZnO than that in Al2O3 to compensate the substitutional hydroxide complexes (Figure

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5a).22,

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During heat treatment, i.e., TVS at elevated temperature in this work, the

hydroxide complexes may either have broken down individually to release an interstitial hydrogen or react with one other in pairs to generate one oxygen ion (bonded to zinc), one oxygen vacancy, and one H2O molecule (Figure 5b). As the annealing continues, the TVS-induced oxygen vacancy and the intrinsic zinc vacancies diffuse and cluster to form pores, where the interstitial hydrogen and the H2O molecule are then trapped and accumulated (Figure 5c). The interstitial hydrogen, when trapped in the pores, subsequently form hydrogen platelets via bonding to the dangling bonds on the inner wall of the pores, which eventually form H2.6, 31 The coexistence of H2 and H2O in the giant blisters has been confirmed by FTIR with the H2- and OH-related stretching vibration bands at 3750-3800 cm-1 and 3650-3700 cm-1, respectively (see Supporting Information, Figure S6). They are closely matching those of H2 in silicon voids and localized vibrational mode of atmospheric H2O vapor measured by Raman scattering.37 Structural characterizations of the broken blisters (Section 3.2) revealed that the blisters were delaminated at the interface between the bottom ZnO layer and the sapphire substrate, which indicates that the situation in Figure 5c is only a transition state that finally develops into the situation in Figure 5d. In general, the diffusion and clustering of the vacancies, the effusion of the gas molecules, and the diffusion of the hydrogen interstitials are dynamically random in directions until they reach the interfaces.1-2, 31, 38-40 This is also why reducing the layer thickness of Al2O3 can suppress its blistering upon annealing.7-10, 33 In the Al2O3/ZnO bilayer stack A, we believe that the adhesion between the top Al2O3 layer and the bottom ZnO layer is greater than that between the bottom ZnO and the sapphire substrate due to the monolithic ALD growth as well as the anneal-

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induced Al/Zn atomic interdiffusion.22 On the other hand, the crystallization and degassing in the bottom ZnO layer cause a strain change in the Al2O3/ZnO bilayer stack, leading to tensile strains built in the top Al2O3 layer that can be derived from the upwardrolling of the broken edges of the blisters (see Figure 4b and Supporting Information, Figure S7, for an enlarged SEM image). Relatively, the tensile strained Al2O3 on top would generate compressive stress to the underlying ZnO layer and, as a result, the moving vacancies, gas molecules, and hydrogen interstitials were squeezed and pushed towards the interface in between the bottom ZnO layer and the rigid sapphire substrate (Figure 5d).38-40 3.4. Mechanical Response and Potential Application. To explore the potential application of such film blisters, we have further studied the fundamental resonant frequency of an individual blister. The experimental setup is schematically shown in Figure 6a, where a sweeping voltage signal with frequency components from 0-1.0 MHz was applied to the piezoceramic plate through the top and bottom electrodes. The surface displacement of the blister, which is glued on the top electrode using adhesive epoxy, is measured by an ultrahigh frequency laser-scanning vibrometer (Polytec, UHF-120). The frequency response obtained from an individual blister is shown in Figure 6b. After scanning the surface of the blister for the resonant frequencies shown in Figure 6b, we found that the vibrations of the blister distinguished itself from its surroundings only at a sweeping frequency of 740.3 kHz (Figure 6c). This indicates that the mode at ~740.3 kHz in Figure 6b is a resonant mode of the blister while the others are the resonances of the combined system (i.e., the combination of the film sample, the epoxy fixture, and the piezoceramic). Movie comparisons of the blister vibrations at an intrinsic mode of ~224.7

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kHz and the resonant mode of 740.3 kHz are provide in Supporting Information (Movies S1-S2). Plotted in Figure 6d are vibration mode of the blister at the resonant frequency of 740.3 kHz. It is seen that the maximum magnitude, at the center of the blister, is about 65 pm larger than those of its surrounding areas. Such an electromechanical blister-on-chip system might have important applications, e.g., in selective cell culturing and/or lysis.41-42 Via structural engineering of the gas impermeable layer on top of the gas desorption ZnO layer, electromechanically controlled micromirrors and/or half symmetric microcavities might be fabricated by the anneal-induced blistering of the engineered layer structures.11, 43-45

We also attempted engineering the ZnO/Al2O3 and Al2O3/ZnO stacks by reducing the individual layer thicknesses from 120 to 40 cycles while repeating the stacking by two more times, i.e., stacks C and D, respectively. TVS processes were then carried out for stacks C and D at 550 and 750 °C for 60 min (see Supporting Information, Figure S8, for the detailed morphological and structural changes). Recoverable circular film swelling induced by electron-beam irradiation has been observed on stack C after TVS at 750 °C. For this observation, the surface of the stack was irradiated by an electron-beam (5 kV and 10 µA) focused at a working distance of 7.5 mm and a magnification of 100 k times for 5 s and then observed at a low-magnification mode, i.e., 25 times, keeping the other setting intact. The electron-beam flux is thus significantly reduced due to the reduced magnification during the film observation. Presented in Figures 7a-7f are the low-magnification SEM images that show the surface evolution after the electron-beam irradiations. A movie recorded just after lowering the SEM magnification is supplied in Supporting Information (Movie S3). The scale bars in Figure 7 are of 1 mm. One sees

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that the circular film swelling with a diameter of ~1-mm has been induced by the electron-beam irradiation and after irradiation the film swelling gradually shrank back and completely recovered within 9 s. Although the detailed film swelling and recovery mechanism is unclear at this stage, they are most likely related to the electron-beam irradiation-induced gas desorption and trapping in the multi-interface stacks. In comparison, the electron-beam irradiation induced film swelling of stack D is unrecoverable. A typical SEM image of the unrecoverable film swelling is shown in the Supporting Information (Figure S9), which provides evidence that the electron-beam irradiation induced circular structure is indeed film swelling rather than electrical charging. The swelling and shrinking film can be thought of as a kind of piston with the irradiation acting as the ignition plug, similar to the membrane-piston made of graphene/ClF3 upon laser irradiation.14 Both the recoverable irradiation-induced film swelling and the elastic film blistering could be useful in miniaturized event-driven mechanical system. They can also be compatibly and monolithically integrated with conventional electrical and/or optoelectronic devices on chips for potential functioning applications. 4. CONCLUSION In conclusion, giant blisters have been created on ALD-grown nanometer-thick Al2O3/ZnO stacks by post-growth thermal annealing at elevated temperatures. The blisters are localized formed with the bottom ZnO layer as the gases deposition source while the top Al2O3 layer as the gas impermeable cap. During the ALD growth, more hydroxide ions were incorporated, in the form of substitutional complexes and/or hydrogen interstitials, into the ZnO layer than those into the Al2O3 layer. To

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compensation, more zinc vacancies were incorporated in ZnO than those of aluminum vacancies in Al2O3 during ALD growth. Upon post-growth heat treatments, crystallizations occurred in the ZnO layer, leading to release of hydroxide ions and formation of pores. The release of hydroxide ions generates H2O molecules via reactions between adjacent hydroxide ions; it also generates H2 by driving the hydrogen interstitials into the pores while the formation of pores is dynamically controlled by diffusion, coalescence, and clustering of vacancies. In comparison, the Al2O3 top layer does not experience any crystallization at all and keeps its amorphous state throughout the postgrowth annealing process. The impermeability of the Al2O3 layer to gas and its in-plane compression onto the underlying ZnO drive the migrations of H2O, H2, hydrogen interstitials, and vacancies towards the interface in between the ZnO layer and the sapphire substrate, where the interlayer adhesion is weaker than that in between the ZnO and the Al2O3 on top due to the monolithic ALD growth and the anneal-induced atomic interdiffusions of Zn/Al across the Al2O3/ZnO interface.22 When the accumulated gas researches a critical pressure, localized delamination and film blistering will occur. Electromechanically actuating, via a piezoceramic plate, and optically probing the blister vibrations revealed a resonant vibration frequency of ~740 kHz for an individual blister. Such blister-on-chip structures and the anneal-induced film blistering can find potential applications, e.g., in selective cell culturing and/or lysing, fabricating micromirror and/or half symmetric microcavities. By reducing the individual layer thickness and increasing the stacking period, we also demonstrated a recoverable electron-beam irradiationinduced circular film swelling, i.e., a kind of film-piston with the irradiation acting as the ignition plug.

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ASSOCIATED CONTENT Supporting Information XRR curves of 240-cycle Al2O3 and ZnO (Figure S1), XPS spectra bilayer stacks A and B after TVS at 550 °C (Figure S2), SEM image and Microphotograph comparing the shape changes of blisters upon pressure changes (Figure S3), SEM image of the bulge out of the partially collapsed blisters (Figure S4), AFM images from the conformal surface and a blister broken area (Figure S5), FTIR-reflectance of an individual blister (Figure S6), SEM image of upward-rolling layers of a broken blister (Figure S7), SEM images and XRD 2θ-ϕ mappings of stacks C and D after TVS at 550 and 750 °C for 60 min (Figure S8), SEM image of TVS treated (TVS 750 °C for 60 min) stack D upon electronbeam irradiation (Figure S9), Movies of surface vibrations of an individual blister and its near surroundings at non-resonant and resonant frequencies (Movies S1 and S2), and Movie of electron-been induced film swelling and its recovery after reducing the irradiation flux. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Hongfei Liu. Email: [email protected]. Phone: (+65)-68720785. Fax: (+65)63194885. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors wish to thank P. C. Lim for his help in collecting the XRD mappings and curves as well as the XRF mappings and spectra.

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28. Liu, H. F.; Ansah-Antwi, K. K.; Wang, Y. D.; Ong, L. T.; Chua, S. J.; Chi, D. Z., Atomic Layer Deposition of Crystalline Bi2O3 Thin Films and Their Conversion into Bi2S3 by Thermal Vapor Sulfurization. RSC Advances 2014, 4 (102), 58724-58731. 29. Liu, H., Recent Progress in Atomic Layer Deposition of Multifunctional Oxides and Two-Dimensional Transition Metal Dichalcogenides. Journal of Molecular and Engineering Materials 2016, 04 (04), 1640010. 30. Dingemans, G.; Kessels, W. M. M., Status and Prospects of Al2O3-Based Surface Passivation Schemes for Silicon Solar Cells. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2012, 30 (4), 040802. 31. Kuznetsov, A. S.; Gleeson, M. A.; Bijkerk, F., Temperature Dependencies of Hydrogen-Induced Blistering of Thin Film Multilayers. Journal of Applied Physics 2014, 115 (17), 173510. 32. Kuznetsov, A. S.; Gleeson, M. A.; Bijkerk, F., Hydrogen-Induced Blistering Mechanisms in Thin Film Coatings. Journal of Physics: Condensed Matter 2012, 24 (5), 052203. 33. Vermang, B.; Goverde, H.; Uruena, A.; Lorenz, A.; Cornagliotti, E.; Rothschild, A.; John, J.; Poortmans, J.; Mertens, R., Blistering in ALD Al2O3 Passivation Layers as Rear Contacting for Local Al BSF Si Solar Cells. Solar Energy Materials and Solar Cells 2012, 101, 204-209. 34. Acero, M. C.; Beldarrain, O.; Duch, M.; Zabala, M.; González, M. B.; Campabadal, F. In Effect of the Blistering of ALD Al2O3 Films on the Silicon Surface in Al-Al2O3Si Structures, 2015 10th Spanish Conference on Electron Devices (CDE), 11-13 Feb. 2015; 2015; pp 1-4. 35. Liu, H. F.; Wong, S. L.; Chi, D. Z., CVD Growth of MoS2-Based Two-Dimensional Materials. Chemical Vapor Deposition 2015, 21 (10-11-12), 241-259. 36. Richardson, J. J.; Goh, G. K. L.; Le, H. Q.; Liew, L.-L.; Lange, F. F.; DenBaars, S. P., Thermally Induced Pore Formation in Epitaxial ZnO Films Grown from Low Temperature Aqueous Solution. Crystal Growth & Design 2011, 11 (8), 3558-3563.

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Figure captions: Figure 1: SEM images (a-l) and XRD 2θ-ϕ mappings (m-p) in the frame of 2θ = 15°~60° and ϕ = -90°~90° of nanometer-thick films and bilayer stacks grown by atomic layer deposition (ALD) before and after thermal vapor sulfurization (TVS) at 550 and 750 °C for 60 min. (a-c, m) Al2O3 thin film of 240-cycle; (d-f, n) Stack A: 120-cycle Al2O3/120-cycle ZnO; (j-i, o) Stack B: 120-cycle ZnO/120-cycle Al2O3; and (j-l, p) ZnO thin film of 240-cycle. The scale bars are of 200 nm. Figure 2: XRD patterns of the ALD-grown Al2O3 and ZnO nanometer-thick films as well as their stacks before and after TVS at 550 and 750 °C for 60 min. (a) Thin film: 240-cycle Al2O3, showing absent of any features; (b) Stack A: 120-cycle Al2O3/120-cycle ZnO, exhibiting crystallizations in the bottom ZnO layer; (c) Stack B: 120-cycle ZnO/120-cycle Al2O3, exhibiting recrystallization of ZnO to ZnS; and (d) Thin film: 240-cycle ZnO thin film, exhibiting ZnO-to-ZnS recrystallizations. Figure 3: Blisters and their shape and size changes upon pressure variations of stacks A (120-cycle Al2O3/120-cycle ZnO) and B (120-cycle ZnO/120-cycle Al2O3) after TVS at 550 °C for 60 min. (a) SEM image showing giant spherical cap shaped blisters on stack A; (b) Blisters of tens of microns on stack B; (c, d) Dedicated comparisons of the same giant blisters in the SEM chamber at low pressures (9.6 × 10-5 Pa) and at atmospheric pressures (1.0 × 105 Pa), see Supporting Information, Figure S3, for more detailed comparisons ; (e, f) Tile- and crosssection-view SEM images of a robust giant blister that keeps its spherical cap

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shape in air; (g) Microphotograph of the robust giant blister under atmospheric pressure; and (h) step profile collected from the robust giant blister. The scale bars are of 100 µm. The arrows indicate a concentric cycle surrounding the robust blister, which is most likely a trace of its original size at 550 °C just after TVS. Figure 4: Broken blisters and the localized delamination interface of the 120-cycle Al2O3/120-cycle ZnO bilayer stack (Stack A) after TVS at 550 °C for 60 min. (a) SEM image showing the broken blisters and the flipped pieces; (b) Enlarged SEM image of the broken blister, showing the upward rolling of the blister pieces after broken; (c) AFM image recorded at the film edge of the broken blister; (d) Height profile extracted from the AFM image at locations across the broken edge; (e-g) XRF mappings showing the elemental distributions of Al, Zn, and S around the broken blister; (h) XRF spectra recorded from the locations labeled in (a); and (i) enlarged XRF spectra, addressing the presence/absence of sulfur. Figure 5: Schematic illustrations of the formation of surface blisters in the ALD-grown the 120-cycle Al2O3/120-cycle ZnO bilayer stack (Stack A) upon thermal vapor sulfurization. (a) Residual hydroxide ions in ZnO with hydrogen forming the substitutional O-H complexes and/or hydrogen interstitials; to compensate the hydroxide ions, zinc vacancies, VZn, are also incorporated; (b) Thermal-induced release of hydrogen via reactions with lattice oxygen adjacent to VZn, leading to H2O molecules and onset of cavities via vacancies coalescence; (c) Transition

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state of interstitial hydrogen, H2O molecules, and vacancies (or small cavities), they are migrating in the ZnO layer during its crystallization upon thermal annealing; and (d) Segregation and diffusion of vacancies, hydrogen interstitials, and H2O molecules into the interface in between the bottom ZnO layer and the rigid sapphire substrate, giving rise to the localized gas accumulations and film blistering. Figure 6: Electromechanically actuating and optically probing the resonant frequency of an individual blister on the bilayer stack A. (a) Schematic illustration of the blister resonance measurement, where the piezoceramic plate , driven by a sweeping voltage to the top/bottom electrodes, transfers its vertical expansionshrinkage vibrations to the thin film sample glued on the top surface; the vibration of the film/blister is probed by a laser scanning vibrometer; (b) Frequency response of the blister; they, except for the mode at ~740.3 kHz, exhibit the resonances of the Sample-Epoxy fixture-Piezoceramic plate system; (c) Typical vibration magnitudes of the mode at ~740.3 kHz, exhibiting the characteristic resonance of the blister at its surface center; and (d) The magnitude profile across the blister at its vibrational peak (~740.3 kHz). Movies comparing the non-resonant and resonant blister vibrations driven at the frequencies of 224.7 kHz and 740.3 kHz, respectively, are supplied in the Supporting Information (Movies S1 and S2). Figure 7: Low-magnetization SEM images showing the electron-beam irradiationinduced elastic blistering of the ALD grown stack C (i.e., 3-periof 40-cycle ZnO/40-cycle Al2O3) after thermal vapor sulfurization at 750 °C for 60 min. 26


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The irradiation was carried out in the SEM chamber with the electron-beam (5 kV and 10 µA) focused at a working distance of 7.5 mm and a magnification of 100 k times for 5 s. (a) The initial film surface before electron-beam irradiation and (b-f) Evolutions of the irritation-induced film swelling immediately after stopping the irradiations and switching to the low-magnification mode. The scale bars are of 1.0-mm. A movie recording the recovery of the electron-beam irradiation-induced film swelling is supplied in the Supporting Information (Movie S3).

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TOC

Giant circular blisters of up to 300 µm in diameter and 10 µm in deflection have been produced on nanometer-thick Al2O3-on-ZnO stacks. Modulated stacks exhibit electronbeam induced circular film swelling and recovery after cessation of the irradiations. Both mechanical systems can be integrated with conventional devices on chip for functioning applications.

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Figure 1: SEM images (a-l) and XRD 2Θ-φ mappings (m-p) in the frame of 2Θ = 15~60o and φ = -90°~90o of nanometer-thick films and bilayer stacks grown by atomic layer deposition (ALD) before and after thermal vapor sulfurization (TVS) at 550 and 750 oµC for 60 min. (a-c, m) Al2O3 thin film of 240-cycle; (d-f, n) Stack A: 120-cycle Al2O3/120-cycle ZnO; (j-i, o) Stack B: 120-cycle ZnO/120-cycle Al2O3; and (j-l, p) ZnO thin film of 240-cycle. The scale bars are of 200 nm. 115x162mm (300 x 300 DPI)



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Figure 2: XRD patterns of the ALD-grown Al2O3 and ZnO nanometer-thick films as well as their stacks before and after TVS at 550 and 750 oC for 60 min. (a) Thin film: 240-cycle Al2O3, showing absent of any features; (b) Stack A: 120-cycle Al2O3/120-cycle ZnO, exhibiting crystallizations in the bottom ZnO layer; (c) Stack B: 120-cycle ZnO/120-cycle Al2O3, exhibiting recrystallization of ZnO to ZnS; and (d) Thin film: 240-cycle ZnO thin film, exhibiting ZnO-to-ZnS recrystallizations. 71x62mm (300 x 300 DPI)


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Figure 3: Blisters and their shape and size changes upon pressure variations of stacks A (120-cycle Al2O3/120-cycle ZnO) and B (120-cycle ZnO/120-cycle Al2O3) after TVS at 550 oC for 60 min. (a) SEM image showing giant spherical cap shaped blisters on stack A; (b) Blisters of tens of microns on stack B; (c, d) Dedicated comparisons of the same giant blisters in the SEM chamber at low pressures (9.6 x 10-5 Pa) and at atmospheric pressures (1.0 x 105 Pa), see Supporting Information, Figure S3, for more detailed comparisons ; (e, f) Tile- and cross-section-view SEM images of a robust giant blister that keeps its spherical cap shape in air; (g) Microphotograph of the robust giant blister under atmospheric pressure; and (h) step profile collected from the robust giant blister. The scale bars are of 100 µm. The arrows indicate a concentric cycle surrounding the robust blister, which is most likely a trace of its original size at 550 oC just after TVS. 57x39mm (300 x 300 DPI)



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Figure 4: Broken blisters and the localized delamination interface of the 120-cycle Al2O3/120-cycle ZnO bilayer stack (Stack A) after TVS at 550 oC for 60 min. (a) SEM image showing the broken blisters and the flipped pieces; (b) Enlarged SEM image of the broken blister, showing the upward rolling of the blister pieces after broken; (c) AFM image recorded at the film edge of the broken blister; (d) Height profile extracted from the AFM image at locations across the broken edge; (e-g) XRF mappings showing the elemental distributions of Al, Zn, and S around the broken blister; (h) XRF spectra recorded from the locations labeled in (a); and (i) enlarged XRF spectra, addressing the presence/absence of sulfur. 81x47mm (300 x 300 DPI)


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Figure 5: Schematic illustrations of the formation of surface blisters in the ALD-grown the 120-cycle Al2O3/120-cycle ZnO bilayer stack (Stack A) upon thermal vapor sulfurization. (a) Residual hydroxide ions in ZnO with hydrogen forming the substitutional O-H complexes and/or hydrogen interstitials; to compensate the hydroxide ions, zinc vacancies, VZn, are also incorporated; (b) Thermal-induced release of hydrogen via reactions with lattice oxygen adjacent to VZn, leading to H2O molecules and onset of cavities via vacancies coalescence; (c) Transition state of interstitial hydrogen, H2O molecules, and vacancies (or small cavities), they are migrating in the ZnO layer during its crystallization upon thermal annealing; and (d) Segregation and diffusion of vacancies, hydrogen interstitials, and H2O molecules into the interface in between the bottom ZnO layer and the rigid sapphire substrate, giving rise to the localized gas accumulations and film blistering. 65x51mm (300 x 300 DPI)



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Figure 6: Electromechanically actuating and optically probing the resonant frequency of an individual blister on the bilayer stack A. (a) Schematic illustration of the blister resonance measurement, where the piezoceramic plate , driven by a sweeping voltage to the top/bottom electrodes, transfers its vertical expansion-shrinkage vibrations to the thin film sample glued on the top surface; the vibration of the film/blister is probed by a laser scanning vibrometer; (b) Frequency response of the blister; they, except for the mode at ~740.3 kHz, exhibit the resonances of the Sample-Epoxy fixture-Piezoceramic plate system; (c) Typical vibration magnitudes of the mode at ~740.3 kHz, exhibiting the characteristic resonance of the blister at its surface center; and (d) The magnitude profile across the blister at its vibrational peak (~740.3 kHz). Movies comparing the non-resonant and resonant blister vibrations driven at the frequencies of 224.7 kHz and 740.3 kHz, respectively, are supplied in the Supporting Information (Movies S1 and S2).µ 66x54mm (300 x 300 DPI)


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Figure 7: Low-magnetization SEM images showing the electron-beam irradiation-induced elastic blistering of the ALD grown stack C (i.e., 3-periof 40-cycle ZnO/40-cycle Al2O3) after thermal vapor sulfurization at 750 oC for 60 min. The irradiation was carried out in the SEM chamber with the electron-beam (5 kV and 10 µA) focused at a working distance of 7.5 mm and a magnification of 100 k times for 5 s. (a) The initial film surface before electron-beam irradiation and (b-f) Evolutions of the irritation-induced film swelling immediately after stopping the irradiations and switching to the low-magnification mode. The scale bars are of 1.0-mm. A movie recording the recovery of the electron-beam irradiation-induced film swelling is supplied in the Supporting Information (Movie S3). 82x82mm (300 x 300 DPI)


ZnO Films Grown by

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