Freestanding Ni-Mn-Ga Microstructures
5.2. Preparation by Electron Beam Lithography
Figure 5.4.: Schematics of the process flow of EBL based preparation of free-standing microstructures. Details are given in the text.
In order to be able to study the properties of freestanding Ni-Mn-Ga films
5.2. Preparation by Electron Beam Lithography using integral methods (e.g. SQUID, XRD) a microscopic amount of freestand-ing material is needed. For this purpose the previously described process flow is changed in a way, that the Ni-Mn-Ga layer is patterned using electron beam lithography (EBL) and wet-chemical etching. Wet-chemical etching is used since dry etching of Ni-Mn-Ga requires physical etching, which can possibly cause some undesired effects such as sidewall deposition [149] and a possible damaging of the martensitic microstructure by bombarding ions.
Process Flow Fig. 5.4 shows schematically the process flow. The main process steps1are as it follows:
a) DC magnetron sputtering
b) Spincoating of EBL resist: First, the samples are cleaned with acetone to re-move organic contaminants. The surface is then immediately rinsed with isopropanol to remove acetone. Afterward the sample is rinsed with deion-ized water, dried with compressed nitrogen and the whole procedure is repeated again. In case of intense contamination of the surfaceN -Methyl-2-pyrrolidone (NMP) is used instead of acetone. The sample is dried on a hotplate at120◦C for 5 minutes to improve the resist adhesion. A positive tone PMMA resist is spin-coated on the Ni-Mn-Ga layer at 5000 rpm. The PMMA resist thickness is approximately150nm.
c) Electron beam lithography: The PMMA resist layer is patterned by EBL with an electron energy of10kV and an electron dose of250µC/cm2. The ex-posed resist layer is developed in a solution of IPA and methyl isobutyl ketone (MIBK) at a ratio of 3:1 for30s.
d) Wet-etching of Ni-Mn-Ga: The most critical step is the transfer of the pat-terned structures from the EBL resist to the Ni-Mn-Ga layer. A solution of 1.25% HCl,37% HNO3 and H2O, mixed at a ratio of1.5 : 0.3 : 10, is used for wet chemical etching of Ni-Mn-Ga. This solution has an etch rate of
≈ 90nm/min. The etch time depends on the thickness of the Ni-Mn-Ga layer. After the etching procedure the sample is rinsed with Di water.
1 The first and the last steps are identically to the ones described for the FIB-based process flow and are not nearly specified here
e) EBL resist stripping: Prior to wet-etching of the sacrificial Cr layer the PMMA is stripped with acetone and the sample is rinsed with deionized water.
f) Wet-etching of sacrificial layer
In the following paragraphs the layout of the Ni-Mn-Ga test structures is in-troduced and details of the fabrication are explained.
Figure 5.5.: (a)Optical micrograph of a developed PMMA mask showing an array of bridges.(b)SEM image showing an array of freestanding Ni-Mn-Ga bridges on sample EF8 after wet-etching of the sacrificial Cr layer. Attached parts of the film appear brighter.
Layout To study the effect of MFIS Ni-Mn-Ga bridges are designed (see Fig.
5.5a). The width of the bridges is varied between 5 and 10 µm and the length is varied between 25 and 55 µm. The bridges are arranged as arrays with a distance of 15 µm. This Ni-Mn-Ga stripe between the bridge arrays serves as an anchor structure and has to be designed larger than the largest bridges width to prevent total release of all bridges during underetching of the sacrificial layer (see Fig. 5.5b2). With this layout a maximum proportion of freestanding Ni-Mn-Ga film of85% can be achieved assuming an isotropic etching behavior of the Ni-Mn-Ga etchant. However due to errors in the process flow somewhat lower proportions were achieved.
2 Parts of the film which are still attached to the underlying Cr layer appear brighter in SEM images due to higher backscattering intensity of this areas. This contrast formation was also confirmed by EDX measurements.
5.3. Freestanding Ni-Mn-Ga Microstructures
Anisotropic Etching Wet etching experiments on Ni-Mn-Ga demonstrated that the used etchant results in an isotropic etching behavior. It was found out that the etching rate of the Ni-Mn-Ga[110] direction is 1.17 higher than that of the Ni-Mn-Ga[100] direction (see Fig. 5.6a). This fact should be considered for the layout of the freestanding test structures. In order to fabricate smooth edges they should be aligned along the Ni-Mn-Ga[100] direction (see Fig. 5.6b).
Figure 5.6.: (a)SEM images of a wet-etched Ni-Mn-Ga layer demonstrating an anisotropic etching. Ni-Mn-Ga[110] is preferred over Ni-Mn-Ga[100] as the etch-ing direction. Red areas mark the pattern of the used EBL mask.(b)SEM image of an array of freestanding Ni-Mn-Ga bridges on sample EF8 with showing the influence of the anisotropic etching.
5.3. Freestanding Ni-Mn-Ga Microstructures
After the preparation of freestanding Ni-Mn-Ga microstructures their proper-ties were studied. It was observed that the release of the Ni-Mn-Ga film from the substrate leads to a change ofTM. This effect can be observed in Figure 5.7b.
The surface morphology of the film, which defines the contrast of SEM mea-surements, is determined by the existence of twin planes and their orientation.
In Fig. 5.7b the freestanding part of the film exhibits a characteristic martensitic microstructural pattern, while the attached film shows a flat sample surface originating from the austenitic cubic phase. The coexistence of the martensi-tic and the austenimartensi-tic phase is possible due to the fact thatTMof the attached
Figure 5.7.: (a)Temperature dependance of the magnetization for the epitaxial film sample EF6 indicating that the phase transition occurs close to room temper-ature (TM= 281K,TA= 282K).(b)SEM image obtained at room temperature showing freestanding and attached parts of the sample EF6 after partial removal of the Cr layer. The freestanding part of the film transformed to the martensitic phase.
sample is slightly below room temperature (see Fig. 5.7a). The release of the Ni-Mn-Ga film shiftsTMabove room temperature, resulting in the existence of the martensitic state at room temperature for the freestanding parts of sample EF6. This shift of the transition temperature can be explained by the release of compressive stress present in the attached epitaxial films. The stress release manifests itself also in the bending of the freestanding microbridges, as can be observed e.g. in Figs. 5.3 and 5.6.
In further experiments the configuration of the martensitic twin variants of freestanding Ni-Mn-Ga films was studied. Differently oriented twin variants are connected by twin boundaries (TB), which are well defined crystallographic planes. When TB cross the film surface, they leave linear traces and one can observe a characteristic microstructural pattern at the film surface.[151] De-pending on the orientation of the{101}TB with respect to the film surface two different types of surface pattern arise. The typeXpattern shows linear traces that run at an angle of45◦towards MgO[100] or parallel to Ni-Mn-Ga[100] (see Figs. 5.8b and c). This pattern type is caused by{101}TB which are inclined by45◦ with respect to the substrate normal. The typeYcan be identified by linear traces which run parallel to the MgO[100] or Ni-Mn-Ga[110] direction
5.3. Freestanding Ni-Mn-Ga Microstructures
Figure 5.8.: SEM images of a freestanding microbridge prepared using sample EF7. The images show different configurations of twin boundaries (schemes are shown on the right hand side) obtained after following sample treatment pro-cedures:(a)Removal of the Cr buffer layer.(b)Annealing at573K.(c)Heating above the phase transition temperature for several times.(d)Attached part of the film exhibiting the typeYtwin variant arrangement.
(see Fig. 5.8d). This pattern type is a consequence of{101}TB oriented parallel to the substrate normal. This pattern type is characterized by weak contrast in SEM images due to very low surface tilt for different twin variants. It was observed that the variant distribution is not affected by the release process.
However, thermal treatment had an impact on the twin variant arrangement.
Annealing of sample EF7 at573K changed the twin variant arrangement from typeYto typeX, as can be observed in Fig. 5.8a→b. This effect is also visible for sample EF8 in Figure 5.9. It should be also noted that temperature treatment affected only the TB configuration on the freestanding parts of the sample. The change of the TB type arrangement is also accompanied by the change of the twin variant size. The mean twin variant size3accounted to≈90nm for sam-ple EF7 for the TB arrangement of typeY. After the change of TB arrangement
3 Twin variant size denotes here the mean distance between two TB measured perpendicular to the TB direction on film surface.
to typeXthe mean twin variant size increased to≈420nm. Considering the film thickness of500nm this large value of the twin variant size suggests that the twin variant formation is not governed by the substrate-film interface.[152]
However, the resulting twin variant pattern of the freestanding film is very reg-ular. Also there seems to be a thickness dependency of the twinning periodicity of freestanding films: for sample EF8 with a film thickness of250nm a twin variant size of≈210nm was measured (see Fig. 5.9b). An austenite-martensite
Figure 5.9.: SEM images of freestanding microstructures on sample EF8 ex-hibiting different types of twin variants arrangement.(a): TypeYtwin variant arrangement after the removal of the Cr buffer layer.(b): TypeXtwin variant arrangement after an annealing of the sample in UHV conditions at673K for 12h.
transformation of the film can result in a rotation of the TB by90◦, as can be seen in Fig.5.8b→c. However, the type of the twin variant arrangement does not change after the phase transformation.
Experiments were performed in order to address the influence of an exter-nal magnetic field on the shape and the twin variant arrangement of the free-standing Ni-Mn-Ga structures. However, no reliable evidence of magnetically induced reorientation of variants could be demonstrated. Following possible reasons can explain this:
I Used sample compositions do not allow TB movement due to high twin-ning stress.
II TB movement is hindered by already existing (from growth) or induced (by preparation of freestanding structures) defects.
III The magnetically induced reorientation is blocked by the twinning mi-crostructure and/or the design of freestanding mimi-crostructures.