Nano-structured Materials and Thin Layers

Nano-structured Materials and Thin Layers

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Nano-structured Materials and Thin Layers are currently of considerable interest for various industrial applications, e.g. for new magnetic and or electronic devices and as coating layers to protect the surface of specific functional materials against corrosion. The macroscopic physical properties of these nano-structured materials are largely controlled by their numerous internal interfaces, which make these materials promising candidates for future applications. This change in their physical properties is often closely related to the stability of their interfaces, and hence an investigation of the chemical reactions at these interfaces is of a great importance to understand and optimize their properties. The relevant systems are currently Ni-W, Co-P, Cu-Bi and Cu-Al, Fe-Ni-Cu, Co-Al, Cu-Co-FeNi, Co-Al2O3-FeNi.
 
  1. Particularly, a number of heterogeneous metal/ceramics interfaces can be fabricated in a very easy fashion via internal oxidation. This process when applied in a homogeneous binary alloy consisting of a noble metal A (e.g. Ag, Pd, Pt, Cu) and a less noble metal B (e.g. Mg, Al, Li etc.) is a thermally activated process. Oxide precipitates of the general type BxOy are formed under certain conditions. From a thermodynamic point of view, this process can be considered as phase separation in a ternary A-B-O system. Recently, this method has been used to study the Metal/Ceramic (Me/C) interfaces. 
     
    Figure 3: The field ion micrographs illustrate the formation of small MgO- precipitates with different size in dependence of the reaction temperature and time after internal oxidization of a Ag-1 at % Mg alloy in air atmosphere. The recon-structed 3D TAP-analysis volume shows the distribution of such precipitates and enables the determination of the composition of these ceramic particles.
     


    Figure 4: 3D- volume reconstruction of a TAP analysis of nano- crystalline Co-1.2at.% P aged isothermally for 1h at 753K. In this image only the P-atoms are displayed. The segregation of P is clearly indicated in the different grain boundaries (KG) owing to the inhomogeneous distribution of P along these boundaries with different composition (left part). The second image shows with the help of composition maps the formation of P-rich clusters and also the precipitation of the Co2P phase in a triple junction (KG2).
          
    Figure 5: 3D volume reconstructtion of a TAP- analysis of an Ar- sputtered Cu-Bi multilayer on a W- substrate tip consisting of a number of parallel grain boundaries (cf. the enlargement of the yellow box on the right). The resolved atomic layers in this image correspond to the Cu-(111)-planes. The concentration map shows a significant enrichment of Bi along these grain boundaries GB.
     
     
        
    Figure 6: 3D-volume reconstruction of TAP-analysis data of thin layers containing an oxide tunnel barrier as the inter-layer (TMR-system).   (a) Ag/MgO/Ag-layer deposit, left: all atoms, right: only Mg- und O-atoms are displayed; (b) Fe/MgO/Ag-layer deposit.
     
     
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    Figure 7: A study of solid state inter-reaction in Cu-Al sandwich couple as studied via atom probe tomography. On the left a schematic arrangement of the layers and a TEM image of the tip showing these layers after Argon deposition on a pre-shaped W-tip. 3D reconstruction of the atom distribution in the as-deposited state of Al/Cu/Al tri-layer. ​ ​Figure 8: A study of solid state iter-reaction in Cu-Al sandwich couple as studied via atom probe tomography. Concentration depth profile of the Al/Cu/Al tri-layer in the as- prepared state and annealed at 110 C for 40 min. The thickness asymmetry is very clearly revealed. These investigations allow us to determine inter-diffusion coefficients of the respective species in the layers at the selected annealing temperature. The dashed lines are drawn at the composition of the intermetallic phase concentration.
     
     
    These interfaces play an important role in many technological materials, as they are considered to be the controlling parameters for the electrical and mechanical properties. Therefore, it is of great interest to study the atomic structure of these interfaces and the segregation behavior of specific gases at them. The investigation of internal interfaces in metal/ceramic compounds (Cu/MgO, Pd/MgO, Ag/MgO, Ag/MnO) is another main topic of our activities.
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  3.  A major development in sample preparation was to utilize a dual beam focused ion beam (FIB). With the help of FIB site specific preparation of specimen can be routinely done as illustrated in Fig. 7 for the investigation of segregation effects. In this case a Cu bi-crystal with a well-defined  Sigma 19a grain boundary (GB) was studied. Such kinds of analyses are still very rare in the world. In particular, the FIB allows overcoming the difficulties in forming a suitable shape required for the specimen in the APT. Gibbs excess at the GB can be exactly determined and the segregation effect can be explored. 
    The selection of various element stacking in a thin layer configuration results very often in tuning the electrical or magnetic properties of these materials as in the case of the giant magneto-resistance materials (GMR), which are nowadays considered as important parts of the reading/writing heads in a computer disk interface. On the other hand, squeezing a thin oxide tunnel barrier within a thickness of 1 to 4nm between two ferromagnetic layers results in the so called tunnel magneto resistance (TMR) junctions. These building stones are currently used in non-volatile data storage devices and in sensor applications.

    Figure 7: Single steps illustrating the lift-out method for the preparation of APT tips by means of the focused ion beam FIB technique in a Sigma 19a GB: a) protection of the Grain Boundary (GB) by means of ion assisted Pt deposition and subsequently applying stair cuts to produce a lamella b) cutting and welding of a pillar to the manipulator including the GB c)-e) transport to the W-support, welding with Pt and finally removing the manipulator f) a pillar fixed on the support and ready to produce an APT tip. ​           ​ Figure 8: a) SEM-image of the machined tip. b) In the TEM image a clear sign of the GB is resolved. c) 3D-volume reconstruction of an APT analysis of the Bi-doped Cu-Bi-crystal after the thermal treatment showing the segregation of Bi to the GB. ​
      
    In the first generations of 3D atom-probes, materials with low electrical conductivity could not be analyzed properly. High voltage pulses could not be transmitted to the tip apex. The broadening of the HV pulse transmitted to the tip apex as well as the lowering of its amplitude lead to much deteriorated mass spectra. For highly resistive or isolating materials, it was even impossible to get any data. The next major instrumental development in last decade was to impose the tip to pulsed laser beams (pico- and femtosecond laser diodes) to resemble the HV pulse and enabling the analysis of even isolating materials as discussed in the following. A TMR stack as shown in figure 10a was deposited on a Si-substrate via Argon sputtering. High resolution images of the lamella confirm that the major part of the MgO layer is also nano- crystalline. The TEM- lamellae as well as the APT-tips were prepared from the pillars again utilizing a dual-beam Focused Ion Beam (FIB) (cf. 10). Analysis of these specimens were performed at Rouen/France using a laser with a wavelength of 1030 nm, a duration of 350 fs and a repetition rate of 1 kHz. The energy of the single pulse was kept below 700 nJ.

     

     

    Figure 9: TEM-image of the multi layer stacking of the TMR- device. The sequence was: Au 10nm/ Fe 50nm/ MgO 2nm/ Fe 100nm /MgO 2nm/ Fe 50nm grown on a Si-substrate.​         ​
    Figure 10: a) Schematic drawing (side view) to illustrate the final cutting process with FIB and to compare to the b) SEM image of an APT tip after final production step. c) An enlarged view showing the different stacking of the layers resolved with SEM. d) 3D-volume reconstruction of the position of Mg atoms within the analysis volume with the LA-TAP.​
      
      
    Results of such analysis are shown in Fig. 10. The 3D reconstruction of the analyzed volume is shown for the same set of specimen as in Fig. 9. A clear identification of the layer structure and the two MgO layers can be noticed. Hence the oxide layers in the stacking were thoroughly analyzed and the complete structure as well as the correct stacking of the thin layers was measured. That is, giving the proof, with the help of the new adapted laser pulser, non- conducting materials can be very well analyzed. The reconstructions of these volumes are still under evaluation, nevertheless, the thickness of the two layers are within the estimated value of about 1 to 2 nm. These results show clearly the power of the unique LaWaTAP apparatus to acquire answers in modern scientific regimes which were not accessible by the conventional APT.

  4. Systematic studies to characterize the atomic process of mechanical alloying is another research activity of our group. With focus on Fe-Cu powders as an ideal binary model alloy and being the most attractive system for technical application, investigation were aimed to elaborate the enforced non-equilibrium enhanced solubility on the system that shows limited low miscibility at ambient temepratures and is characterized by a large positive heat of mixing.