Rolled-up tubes and cantilevers by releasing SrRuO3-Pr0.7Ca0.3MnO3 nanomembranes
© Deneke et al; licensee Springer. 2011
Received: 25 August 2011
Accepted: 7 December 2011
Published: 7 December 2011
Three-dimensional micro-objects are fabricated by the controlled release of inherently strained SrRuO3/Pr0.7Ca0.3MnO3/SrRuO3 nanometer-sized trilayers from SrTiO3(001) substrates. Freestanding cantilevers and rolled-up microtubes with a diameter of 6 to 8 μm are demonstrated. The etching behavior of the SrRuO3 film is investigated, and a selectivity of 1:9,100 with respect to the SrTiO3 substrate is found. The initial and final strain states of the rolled-up oxide layers are studied by X-ray diffraction on an ensemble of tubes. Relaxation of the sandwiched Pr0.7Ca0.3MnO3 layer towards its bulk lattice parameter is observed as the major driving force for the roll-up of the trilayers. Finally, μ-diffraction experiments reveal that a single object can represent the ensemble proving a good homogeneity of the rolled-up tubes.
PACS: 81.07.-b; 68.60.-p; 68.37.Lp; 81.16.Dn.
Perovskite oxides have become a fascinating class of materials because of the wide variety of electronic properties including an intriguing ferroic (magnetic or ferroelectric) response for potential use in memory or sensor applications. At the same time, an epitaxial strain has been demonstrated to massively change the fundamental properties of such oxides, in particular, affecting their electronic behavior [1–4]. A recent sensor design includes freestanding cantilevers for electromechanical devices . An elegant way to form three-dimensional structures based on the release and deterministic rearrangement of two-dimensional films has been established over the last years [5–7]. An inherently strained layer stack is deposited on top of a sacrificial layer (or substrate) and is released by selective removal of this sacrificial layer. Due to cunning strain design and patterning, the layer stack bends up forming cantilevers or rolls up into nano- and microtubes. The technique has been employed to form fluidic systems , optical resonators [9–11], microtube lasers , metamaterial waveguides , and even microrobots [14, 15] from various material systems [16, 17]. Due to the strain relaxation driving the bending and roll-up processes, the three-dimensional micro-objects exhibit a unique strain state , influencing the properties of the microtubes .
In this work, an approach for the fabrication of three-dimensional micro-objects (freestanding cantilevers, rolled-up microtubes) from perovskite oxides, i.e., ferromagnetic SrRuO3 [SRO] known for its chemical stability  and antiferromagnetic Pr0.7Ca0.3MnO3 [PCMO], is reported. The diameter of the obtained tubes varies between 6 and 8 μm, and a preferred <100> rolling direction is observed. The etching selectivity between the SRO film and the SrTiO3 [STO] substrate is estimated as 1:9,100. X-ray diffraction [XRD] is carried out to evaluate the original and final strain states. Unlike our previous studies using μ-focus XRD , diffraction is carried out for an ensemble of microtubes using a conventional single crystal diffraction beamline setup. Results clearly reveal the change in the strain state after roll-up, with the PCMO layer relaxing towards its bulk lattice parameter, whereas the upper SRO layer is compressed. Finally, μ-XRD is carried out on the same beamline, allowing for comparison of the ensemble properties with a single object. We find that a single tube can represent the ensemble indicating a good overall homogeneity of the roll-up process.
Results and discussion
Figure 1a shows an SEM image after underetching a single SRO layer. The central part of the pattern is a circle with fingers in different crystallographic directions. The emerging etching pattern (the initial pattern is round; see Figure 1b) reveals that the solution etches anisotropically. Clear etching facets in the <110> crystal direction of the STO substrate are observed, indicating the slowest etching direction. The <100> direction is the fastest etching direction as seen in the underetched fingers (Figure 1a, b). From the etching time (2 min) and the mean underetching distance in <110> directions (1.1 μm, marked for two facets in Figure 1a), an average etching velocity of 0.55 μm/min for the <110> directions is calculated. Using the height difference between the bottom and the top of the mesa, we determine a nearly three times higher velocity of 1.45 μm/min along <001>. Since no bending or curling of the single SRO layer is observed, the strain gradient in the film is low as expected for the good lattice match between cubic lattice parameters of a STO = 3.905 Å and the pseudocubic lattice parameter a SRO = 3.928 Å [22, 23].
To obtain rolled-up structures, the chemically inert SRO layer was combined with another oxide, creating a layer stack with pronounced built-in differential stress. For this purpose, trilayers with a functional oxide layer sandwiched between a bottom and a top SRO layer for protection against the acid have been grown. For the middle sandwiched layer, PCMO with a pseudocubic bulk lattice parameter of a PCMO = 3.85 Å  has been found to work well. Freestanding SRO/PCMO/SRO trilayer cantilever structures (with a total thickness of 120 nm) are shown in Figure 1b. The underetching was deliberately stopped after only fingers are detached in the fast etching <100> direction. The curvature of the cantilevers in Figure 1b is around 0.0625 μm-1. This value indicates the relatively large stiffness of the oxides.
In summary, the approach of fabricating three-dimensional micro-architectures by deterministic release and rearrangement of strained films has been extended to ferroic oxides. Careful investigation of the etching behavior shows a high selectivity of 1:9,100 for an SRO film against the STO substrate. Bent-up cantilevers have been prepared by releasing pseudomorphic SRO/PCMO/SRO trilayers from an STO substrate. Patterning straight long trenches into such SRO/PCMO/SRO trilayers allows one to fabricate well-positioned rolled-up microtubes with large aspect ratios. The strain states of the oxide layers before and after roll-up have been analyzed by XRD, and the ensemble homogeneity has been checked by comparing the microdiffraction pattern of a single tube to the pattern obtained from the ensemble. This approach enables strain tailoring of three-dimensional oxide heterostructures in order to tune the magnetic, electrical, or optical properties. The layers in a microtube experience a strong linear radial strain gradient (Figure 4b) which can be tuned continuously by varying the layer thicknesses, whereas the longitudinal lattice parameter is roughly fixed to that of the substrate. The effect of such kind of strain gradient in complex ferroic oxides is rather unknown and may lead to a new behavior such as a flexoelectric effect . Furthermore, cantilevers and microtubes are less clamped by the substrate. Their thus expected larger strain responses towards electric or magnetic fields may enable an improved function for strain-coupled systems such as two-phase magnetoelectric heterostructures.
J. Fontcuberta is acknowledged for pointing out the potential of SRO for this kind of experiment for its chemical inertness. We thank for the experimental help and fruitful discussions with D. J. Thurmer, Ch. Mickel, X. Kong, T. Dienel, and K. Nenkov. M. D. Biegalski and B. Rellinghaus are acknowledged for providing some SRO samples and access to Tecnai T20, respectively. Beamtime was granted by the LNLS under proposal number D10A - XRD2 - 9948.
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