Integration of Multifunctional Epitaxial (Magnetic) Shape Memory Films in Silicon Microtechnology

Magnetic shape memory alloys exhibit various multifunctional properties, which range from high stroke actuation and magnetocaloric refrigeration to thermomagnetic energy harvesting. Most of these applications benefit from miniaturization and a single crystalline state. Epitaxial film growth is so far only possible on some oxidic substrates, but they are expensive and incompatible with standard microsystem technologies. Here, epitaxial growth of Ni–Mn–based Heusler alloys with single crystal‐like properties on silicon substrates is demonstrated by using a SrTiO3 buffer. It is shown that this allows using standard microfabrication technologies to prepare partly freestanding patterns. This approach is versatile, as its applicability for the NiTi shape memory alloy is demonstrated and spintronic and thermoelectric Heusler alloys are discussed. This paves the way for integrating additional multifunctional effects into state‐of‐the‐art microelectronic and micromechanical technology, which is based on silicon.


DOI: 10.1002/adfm.202305273
due to ferromagnetism, and twin boundaries due to ferroelasticity. [3]Coupling of both ferroic properties enables various emerging applications.In particular, Ni-Mn-based Heusler alloys achieve actuation with a high stroke of up to 12%, [4] enable efficient magnetocaloric refrigeration, [5] and allow thermomagnetic harvesting of lowgrade waste heat. [6]Optimum performance is often obtained only in single crystals, for example, actuation by a magnetically induced reorientation is inhibited by large angle grain boundaries. [7]The outstanding properties in bulk single crystals have motivated intense research on epitaxial films, which represent the thin film counterpart.Compared to bulk, films exhibit a much higher surface to volume ratio, which accelerates the heat exchange and accordingly results in very high cycle frequencies and power densities for any caloric [8] or thermomagnetic [6] application.Furthermore, multifunctional Heusler alloys follow the concept "the material is the machine," [9] and thus are ideally suited for miniaturization since no additional parts like levers and valves are required, which would complicate microfabrication substantially.The required epitaxial growth has been achieved on various single crystal oxidic substrates like Al 2 O 3 , [10,11] SrTiO 3 , [12] and MgO, [13] as well as other materials like NaCl [14] and GaAs. [15]However, these substrates are both expensive and incompatible with standard silicon microtechnology, which hampered the application of multifunctional Heusler alloys within microsystems up to now.
Here, we demonstrate epitaxial growth of Ni-Mn-based Heusler alloys on Si substrates in (001)-orientation.We show that these magnetic shape memory films are compatible with standard microsystem technologies like photolithography, anisotropic etching, and silicon-on-insulator (SOI) substrates.This allows for an easy fabrication route for partly freestanding patterned structures, which we exemplify with some generic geometries.These structures are ferroelastic and ferromagneticthe two ferroic properties enabling all multifunctionalities of these Heusler alloys.][22] Many other functional materials would also benefit from such an integration and we discuss this for the rich class of Heusler alloys, which exhibit extraordinary spintronic [23] and thermoelectric [24] properties.Similar breakthroughs for system integration have been achieved recently for silicon photonics [25] and ferroelectrics. [26]Thus, our versatile approach is a decisive step for integrating a large variety of emerging functional properties in today's microelectronic and micromachining technology, which is based on silicon.

Results and Discussion
Starting point for epitaxial growth are Si (001) substrates that are covered with a 4 nm thick epitaxial SrTiO 3 (in short STO) buffer layer.This buffer was developed for optoelectronic applications of barium titanate [26] and 4″ wafers are available commercially. [27]espite the large misfit of 5% between STO and Ni-Mn-based Heusler alloys, epitaxial growth is possible. [12]This mismatch can be reduced by a Cr buffer, which also can be etched selectively. [28]or the formation of L2 1 ordered Heusler alloy, substrates are heated to 400 °C during deposition.Focusing on the ternary Ni-Mn-Ga prototype system, we will show first, that we can grow epitaxial films on STO-buffered Si substrates, second, that these films show microstructural and magnetic properties typical for Ni-Mn-Ga single crystals, and third, that we can apply standard silicon technology to produce partially freestanding microsystems.
First indications of epitaxial growth on Si substrates come from -2 XRD measurements of 500 nm thick Ni 54 Mn 19 Ga 27 films grown with a 20 nm thick Cr buffer on STO/Si and STO/SOI, which are shown in Figure 1 along with a film grown on a single crystalline STO substrate as a reference.Except for substrate reflections, we only observe a set of {00l} reflections, which indicate a preferential alignment of the unit cells.All film reflections can be indexed by non-modulated (NM) and 14M modulated martensite-two martensite structures, which are closely related by adaptivity. [29]The presence of martensitic phase at room temperature reveals that the martensitic (=ferroelastic) transition occurred at some higher temperature.In the following, we use the term martensitic instead of ferroelastic to increase the readability for the shape memory community.The martensitic transformation from the cubic austenite state results in a reduction of crystal symmetry, and accordingly, we observe a set of reflections, which represents the different lattice parameters of this reduced symmetry (tetragonal (00l) NM and (l00) NM , and orthorhombic (l00) 14M , (0l0) 14M , and (00l) 14M ).In the cubic austenitic state, this peak splitting does not exist.We demonstrate this for a film with a different composition, which remains austenitic down to room temperature (Figure S1, Supporting Information).Lattice parameters change from a A = 0.578 nm to a 14M = 0.607 nm, b 14M = 0.578 nm, c 14M = 0.554 nm, c NM = 0.658 nm, and a NM = 0.554 nm.All the austenitic and martensitic films exhibit very similar values on different substrates (The reported values are averages.Individual values are in Table S1, Supporting Information).The associated lattice distortion (1 − c/a) is often taken as the ferroelastic order parameter.
To accommodate the change in lattice parameters during a martensitic transformation, it is necessary to combine differently oriented variants of the martensite, which are connected to each other with twin boundaries.This transformation requires a slight tilt and rotation of the lattice by a few degrees, as described by the phenomenological theory of martensite, [30] and observed for this particular system in TEM. [31,32]In our measurements, we account for this by showing for each sample the sum of XRD measurements, where the substrate normal was tilted in steps of 1°between 0°and 10°away from Bragg-Brentano geometry. [33]o sum up, these diffractograms originate from a single crystalline film grown within the high temperature austenite state, which transformed to the ferroelastic martensitic state at room temperature.
To probe epitaxial growth, pole figures are depicted in Figure 2a-c for the three different substrates.All (202) 14M pole figures exhibit the four-fold symmetry induced by the cubic substrates in (001) orientation.High-intensity spots split up and cluster around a tilt angle () of 45°.As analyzed in detail before, [13,33] this splitting originates from the reduction of crystal symmetry during the martensitic transition of a single crystalline film, grown epitaxially in cubic austenite state at high deposition temperatures.The pole figure of the film grown on STO/Si is rotated by 45°around the substrate normal with respect to the films grown on STO and STO/SOI.This originates from different substrate architectures, as sketched in Figure 2d-f.To minimize the misfit between the Heusler alloy and STO, both unit cells are rotated by 45°with respect to each other. [12]The same rotation occurs when STO is grown on Si.To compensate for this, the top Si layer on SOI substrate was rotated by 45°during bonding step.The Cr buffer used for these experiments is optional, as films grown without this buffer reveal the same pole figure and microstructure (Figure S2, Supporting Information).To sum up, these measurements confirm that our approach allows growing Ni-Mn-Ga Heusler films epitaxially on Si substrates.
In the following, we will demonstrate that our films exhibit the typical microstructure and magnetic properties of singlecrystalline Ni-Mn-Ga films.Twinning is the most obvious consequence of a martensitic transformation.During the transformation, a high number of transformation twins form; in Ni-Mn-Ga films, the transformation even results in several levels of twin boundaries nested into each other. [31]The twin boundaries and their high mobility are essential for superelasticity, the shape memory effect, and magnetically induced reorientation observed in these alloys. [3]In our films, twin boundaries can be seen as characteristic parallel lines in SEM images (Figure 2g-i).These twin boundaries are at the mesoscale level of hierarchical martensite microstructure and occur during the nucleation and growth stage of the martensite.The geometrical description of these stages in epitaxial Ni-Mn-Ga films can be found in our previous works. [31,34]In Ni-Mn-Ga, twin boundaries are slightly tilted and rotated away from {110} planes (as described before) and from the six possible orientations in bulk, in these films we only observe the so-called type X subsets, where the twin boundary is inclined by 45°to the substrate normal and traces follow either [100] Ni-Mn-Ga or [010] Ni-Mn-Ga directions on the film surface. [31,35]Twin boundaries in (i) are rotated by 45°with respect to those visible in (g) and (h) due to the differences in the substrate architecture mentioned before (Figure 2d-f).Due to the four-fold symmetry of the cubic substrates, two equivalent orientations of the parallel lines on the film surface are possible, which are also clearly visible for both films grown on Si.For films grown on STO single crystals, the regions of parallel twin boundaries are much larger, and thus different orientations are not captured in the scanned region.
To probe single crystal-like behavior, we performed magnetization measurements in dependence of temperature, field, and direction (Figure 3).The properties of all three films agree within a few Kelvin; thus in the following text, we give average values (see Table S2, Supporting Information, for values of each film, and Figure S3, Supporting Information, for the measurements of the film grown on STO single crystal).A temperature sweep at a low magnetic field of 0.01 T applied in-plane reveals the following transitions during cooling: Below the Curie temperature of 328 K, magnetization increases steeply due to the magnetic transition from paramagnetic austenite to ferromagnetic austenite.A Curie transition is of second order and therefore proceeds without hysteresis.At 301 K, the magnetization drops steeply, which is commonly attributed to the martensite start temperature M S , where the low magnetic field does not allow aligning magnetization due to the high magnetocrystalline anisotropy of martensite.This  decrease of magnetization is finished at 295 K, which defines the martensite finish temperature M F .During heating, the reverse transformation to austenite occurs and accordingly, A S = 303 K and A F = 308 K are obtained.As characteristic of a first-order martensitic transformation, the reverse transformation occurs at higher temperatures than the forward transformation.The measured hysteresis of 6 K is typical for epitaxial Ni-Mn-Ga films. [12,13,33]As these measurements are performed in a field far below the anisotropy field, magnetization is not saturated and thus, slight differences in sample alignment result in differences in magnetization between the samples.To probe the field dependency, the inset in Figure 3 shows magnetization curves at 300 K within the martensitic state.Measurements along [100] Ni-Mn-Ga and [110] Ni-Mn-Ga differ due to the magnetocrystalline anisotropy of martensite, which can be probed due to the single crystalline growth of these films.Following the procedure described, [13] we obtain from the anisotropy field H A an anisotropy constant k 1 = 1.1 × 10 5 Jm −3 , which is close to the value k 1 = 1.7 × 10 5 Jm −3 of a 14M single crystal. [36]Also, the saturation magnetization of M sat = 34 Am 2 kg −1 is close to the bulk value of 35 Am 2 kg −1 . [37]o illustrate the versatility of our approach, we used STObuffered Si substrates also for the epitaxial growth of NiTi films.Up to now, either polycrystalline films [16] or epitaxial films on oxidic substrates [38,39] had been available for this system.For optimum properties, NiTi requires different deposition and heat treatment conditions compared to Ni-Mn-Ga, as described in the Experimental Section.However, we could directly use the growth parameters optimized for MgO substrates [40] for the growth on STO/SOI, which illustrates that our approach requires a minimum adaption of existing processes.The 500 nm thick film, depicted in  2e.Though polycrystalline NiTi films on Si are already used today, [16] epitaxial growth of NiTi will have the same functional advantages of bulk single crystals: a higher pseudo-elastic and -plastic strain, [41] and a higher elastocaloric effect. [19] demonstrate the feasibility of integrating epitaxial Heusler films into silicon microtechnology, we fabricated two archetypical geometries for partly freestanding microsystems.Top-down microfabrication steps are sketched in Figure 5 and are briefly described in the caption.As Si can be selectively etched, we omit the Cr buffer.Details are given in the Experimental Section.Criteria for method selection and optimization will be published elsewhere.Figure 6 shows exemplarily two microsystem geometries.The top row shows several of these microsystems to illustrate reproducibility.The bottom row is a zoom-in of one microsystem, demonstrating the presence of twin boundaries and thus proving that these microsystems are in the martensitic state at room temperature.On the left side, a bending actuator is shown, which can utilize the shape memory effect. [42]To allow bending, the cantilever must be freestanding.For Joule heating, an electric current can be passed through the double-beam cantilever.The current is provided through the contact pads, which must be connected to the substrate.This example illustrates the advantage of our approach, as it allows an easy combination of freestanding parts with parts attached to a silicon substrate.The right side image illustrates a geometry where a freestanding spring is connected with a mass-a geometry that can utilize the strong damping properties of shape memory alloys at the nanoscale. [43]ur approach can be directly adapted for other multifunctional microsystems.For example, a thermomagnetic microoscillator [6] just requires replacing the spring with a straight cantilever.To utilize the magnetocaloric properties of these Heusler alloys, one can prepare partly freestanding films, which avoids the unfavorable large heat capacity of the thick substrate compared to the thin magnetocaloric film. [44]For this application, larger freestanding areas are beneficial, which can be prepared by anisotropic etching of the silicon substrate (without SOI) from the backside.For caloric and thermomagnetic applications, the magnetic properties can be tuned by adding Cu to Ni-Mn-Ga. [45]ccordingly, we used this quaternary alloy for our microsystems.The versatility to use different compositions is decisive for Ni-Mn-based Heusler alloys since each multifunctional property is optimal at a particular alloy system and composition. [3]Last but not least, our approach is feasible for micromachining of self-folding origami, which utilizes NiTi as bending actuators and switchable Heusler alloys as fixtures. [46]These examples illustrate that monolithic micromachining of partly freestanding devices is possible, without the need of manually transferring epitaxial grown films and patterns to another substrate. [6,42,47]Thus, our approach allows for batch fabrication, which is decisive for both, further miniaturization and future mass production.

Conclusion and Outlook
To conclude, a thin epitaxial SrTiO 3 (STO) buffer enables epitaxial growth of Ni-Mn-based Heusler alloys on silicon substrates.Our approach is compatible with silicon-on-insulator substrates and thus allows using standard techniques for the monolithic fabrication of partly freestanding patterns.This enables combining multifunctional elements of these Heusler alloys with today's microelectronic and micromechanical systems.Our approach is flexible, as it allows using different film compositions and architecture to obtain optimum multifunctional properties.Moreover, it also enables epitaxial growth of NiTi, which is the most used non-magnetic shape memory alloy today.
As an outlook, we note that Heusler alloys exhibit many more outstanding functional and fundamental properties, including spintronic, [23] thermoelectric, [24] topological insulator, [48] and unconventional superconductivity properties. [49]Spintronic applications, for example, benefit from the full spin polarization of half-metallic Heusler alloys.When grown epitaxially, for example, Co 2 FeSi reaches polarized spin currents, which are one order of magnitude higher than polycrystalline films. [50]However, on silicon, epitaxial growth is limited due to interdiffusion beyond a film deposition temperature of 60 °C, which by far is not sufficient to induce the required chemical order in Heusler alloys. [51] high effort is therefore put into the epitaxial growth on GaAs and Ge substrates.[52] Our approach of using an epitaxial STO buffer promises to solve this issue and make many functional properties of Heusler alloys integrable into silicon technology.

Figure 1 .
Figure 1.Diffractograms of Ni 54 Mn 19 Ga 27 films deposited at 400 °C on different substrates.Except for substrate reflections, only {00l} reflections of the martensitic phases are visible, which reveal that the films grown on Si substrates are martensitic at room temperature and exhibit a pronounced texture.Black dots mark reflections originating from the substrate holder.

Figure 2 .
Figure 2. a-f) Epitaxial growth of Ni-Mn-Ga films is confirmed by the four-fold symmetry of (202) 14M pole figure measurements (a-c) for the three different film architectures sketched in (d-f).The orientation of Ni-Mn-Ga unit cell of the film on STO/Si is rotated by 45°with respect to the films on STO and STO/SOI, as discussed within the text.SEM micrographs in (g-i) illustrate the twinned morphology of each film.Traces of the twin boundaries are aligned in parallel to [100] Ni-Mn-Ga and [010] Ni-Mn-Ga directions on the Ni-Mn-Ga surface.For these experiments, an optional Cr buffer was used.All figures are oriented parallel to the substrate edges.

Figure 3 .
Figure 3. Single crystal-like magnetic properties of epitaxial Ni-Mn-Ga films grown on STO/Si (blue) and STO/SOI (red).Temperature dependent measurements reveal the transition temperatures, as described within the text.The inset shows field dependent measurements within the martensitic state at 300 K. Epitaxial growth allows to measure the difference between [100] Ni-Mn-Ga and [110] Ni-Mn-Ga directions and to derive the anisotropy field H A , as described within the text.

Figure 4 .
Figure 4. Epitaxially grown NiTi shape memory films on STO/SOI.a) The (110) pole figure shows that the film grows epitaxially on the silicon substrate.b) In an SEM micrograph, martensitic twins are visible as straight traces with distinct orientations to the substrate.

Figure 5 .
Figure 5. Process steps for fabricating partly freestanding patterned structures of epitaxial Heusler films in monolithic silicon microsystems.a) Starting with the epitaxial Heusler film grown on STO/SOI substrate, b) a photoresist is spin-coated, which is then c) exposed and developed by laser lithography to get the pattern design.d) The exposed regions of the film are etched by Ar ions to transfer the pattern design through the epitaxial film.e) The photoresist is then removed by acetone to obtain a patterned sample.f) The patterned sample is spin-coated again with a photoresist and undergoes laser lithography to reveal areas of the sample for under-etching the top Si layer of SOI substrate.g) The top Si layer is under-etched isotropically by XeF 2 gas.h) The photoresist is then removed by O 2 plasma to obtain partly freestanding structures.

Figure 6 .
Figure 6.Monolithic microsystems utilizing partly freestanding Ni 52 Mn 19 Ga 26 Cu 3 films epitaxially grown on STO/SOI.In the left column, we show shape memory bending microactuator with contact pads, in the right column we show micro-damper.Principle of operation for both microsystems is described within the text.The top row shows several of these microsystems, whereas in the bottom row, details of one system are depicted.At this magnification, the twinned martensitic microstructure becomes visible.