Quantitative analysis of backscattered electron (BSE) contrast using low voltage scanning electron microscopy (LVSEM) and its application to Al0.22Ga0.78N/GaN layers.

A novel method to quantify and predict the material contrast using Backscattered Electron (BSE) imaging in Scanning Electron Microscopy (SEM) is presented while using low primary electron beam energies (Ep). In this study, the parameters for BSE imaging in Low Voltage Scanning Electron Microscopy (LVSEM) are optimized for the layer system Al0.22Ga0.78N/GaN, which is typically used in High Electron Mobility Transistors (HEMTs). The layers are imaged at high resolution and the compounds are identified based on the quantitative BSE material contrast between Al0.22Ga0.78N and GaN. The quantification process described in this study is based on an analytical description that predicts the material contrast using a function that correlates the effective backscattering coefficient (η) with the atomic number Z.

Si amorphization by focused ion beam milling: Point defect model with dynamic BCA simulation and experimental validation.

A Ga focused ion beam (FIB) is often used in transmission electron microscopy (TEM) analysis sample preparation. In case of a crystalline Si sample, an amorphous near-surface layer is formed by the FIB process. In order to optimize the FIB recipe by minimizing the amorphization, it is important to predict the amorphous layer thickness from simulation. Molecular Dynamics (MD) simulation has been used to describe the amorphization, however, it is limited by computational power for a realistic FIB process simulation. On the other hand, Binary Collision Approximation (BCA) simulation is able and has been used to simulate ion-solid interaction process at a realistic scale. In this study, a Point Defect Density approach is introduced to a dynamic BCA simulation, considering dynamic ion-solid interactions. We used this method to predict the c-Si amorphization caused by FIB milling on Si. To validate the method, dedicated TEM studies are performed. It shows that the amorphous layer thickness predicted by the numerical simulation is consistent with the experimental data. In summary, the thickness of the near-surface Si amorphization layer caused by FIB milling can be well predicted using the Point Defect Density approach within the dynamic BCA model.

Mechanical characterization of porous nano-thin films by use of atomic force acoustic microscopy.

The indentation modulus of thin films of porous organosilicate glass with a nominal porosity content of 30% and thicknesses of 350nm, 200nm, and 46nm is determined with help of atomic force acoustic microscopy (AFAM). This scanning probe microscopy based technique provides the highest possible depth resolution. The values of the indentation modulus obtained for the 350nm and 200nm thin films were respectively 6.3GPa±0.2GPa and 7.2GPa±0.2GPa and free of the substrate influence. The sample with the thickness of 46nm was tested in four independent measurement sets. Cantilevers with two different tip radii of about 150nm and less than 50nm were applied in different force ranges to obtain a result for the indentation modulus that was free of the substrate influence. A detailed data analysis yielded value of 8.3GPa±0.4GPa for the thinnest film. The values of the indentation modulus obtained for the thin films of porous organosilicate glasses increased with the decreasing film thickness. The stiffening observed for the porous films could be explained by evolution of the pore topology as a function of the film thickness. To ensure that our results were free of the substrate influence, we analyzed the ratio of the sample deformation as well as the tip radius to the film thickness. The results obtained for the substrate parameter were compared for all the measurement series and showed, which ones could be declared as free of the substrate influence.

Mechanical characterization of nanoporous materials by use of atomic force acoustic microscopy methods.

We have used the atomic force acoustic microscopy (AFAM) method to determine the indentation modulus of nanoporous thin-film materials with ultralow values of dielectric permittivity (dielectric constant k < 2.4). The AFAM method is based on the contact mode of atomic force microscopy (AFM) and as such is able to characterize materials with high spatial resolution. The tested material was porous organosilicate glass with nominal porosity ranging from 27% to 40%. The values obtained for the indentation modulus varied from 4 to 7 GPa depending on the pore concentration. The values obtained for the indentation modulus by use of the AFAM method were in very good agreement with those determined by nanoindentation. In addition, a part of the AFAM results obtained for the sample with the highest porosity content showed dependence of the effective indentation modulus on the applied load. Preliminary data analysis suggests that the stress rate is the critical factor in triggering this particular mechanical response of the porous material.

Measuring the dielectric constant of materials from valence EELS.

Valence EELS combined with STEM provides an approach to determine the dielectric constant of materials in the optical range of frequencies. The paper describes the experimental procedure and discusses the critical aspects of valence electron energy-loss spectroscopy (VEELS) treatment. In particular, the relativistic losses might affect strongly the results, and therefore they have to be subtracted from the spectra prior the analysis. The normalization of the energy-loss function is performed assuming an uniform thickness of the investigated area, which is reasonably fulfilled for carefully prepared FIB samples. This procedure requires the presence of at least one reference material with known dielectric properties to determine the absolute thickness. Examples of measuring the dielectric constant for several materials and structures are presented.

Comparison of the annealing behavior of thin Ta films deposited onto Si and SiO2 substrates.

Structural changes at annealing temperatures (T(an)) of 500-1,100 degrees C were investigated for thin Ta films which were sputter-deposited onto pure Si substrates and onto thermally oxidized Si. In the as-deposited state, the Ta layers predominantly consist of metastable tetragonal beta-Ta, whereby the [001] texture is independent of the substrate material. At lower annealing temperatures, the microstructural evolution is essentially the same for both Ta films. Incorporation of O atoms causes an increase of the intrinsic compressive stress, and diffusion of C atoms into the Ta layer leads to the formation of Ta(2)C. Additionally, a partial transformation of the original beta-Ta phase into a second phase with tetragonal unit cell (denoted as beta'-Ta) occurs. For the Ta/Si system, the formation of a Ta-Si intermixing layer is initiated at T(an)=550 degrees C, and nucleation of crystalline TaSi(2) occurs at T(an)=620 degrees C. The formation of a second Ta silicide was not detected up to T(an)=900 degrees C. In the case of the Ta film deposited onto the SiO(2) substrate, the metastable beta-Ta and the beta'-Ta transform completely into the thermodynamically stable cubic alpha-Ta at T(an)=750 degrees C. A marked reaction with the substrate indicated by the formation of Ta(2)O(5) and Ta(5)Si(3) occurs at T(an)=1,000 degrees C.