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 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.