T. Lohner

Determination of complex dielectric functions of ion implanted and implanted‐annealed amorphous silicon by spectroscopic ellipsometry

Measuring with a spectroscopic ellipsometer (SE) in the 1.8–4.5 eV photon energy region we determined the complex dielectric function (ϵ = ϵ1 + iϵ2) of different kinds of amorphous silicon prepared by self‐implantation and thermal relaxation (500 °C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted a‐Si (i‐a‐Si) differs from that of relaxed (annealed) a‐Si (r‐a‐Si). Moreover, its ϵ differs from the ϵ of evaporated a‐Si (e‐a‐Si) found in the handbooks as ϵ for a‐Si. If we use this ϵ to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the Davis–Mott plot based on the relation: (ϵ2E2)1/3 ∼ (E− Eg). The results are: 0.85 eV (i‐a‐Si), 1.12 eV (e‐a‐Si), 1.30 eV (r‐a‐Si). We attribute the optical change to annihilation of point defects.

Nondestructive determination of damage depth profiles in ion‐implanted semiconductors by spectroscopic ellipsometry using different optical models

A several‐parameter fitting of spectroscopic ellipsometry data is developed to characterize near‐surface layers in semiconductors damaged by implantation. The damage depth profiles are described by either rectangular, trapezoid‐type, or coupled half‐Gaussian (realistic) optical models. The rectangular model has three parameters: the average damage level, the effective thickness of the implanted layer, and the thickness of the native oxide. The trapezoid‐type model is enhanced with a fourth parameter, the width of the amorphous/crystalline interface. The realistic optical model consists of a stack of layers with fixed and equal thicknesses. The damage levels are determined by a depth profile function (presently coupled half‐Gaussians). Five parameters are used: the position of the maximum, the height, and two standard deviations of the profile, plus the thickness of the native oxide. The complex refractive index of each layer is calculated from the actual damage level by the Bruggeman effective medium appr...

Comparative study of surface roughness measured on polysilicon using spectroscopic ellipsometry and atomic force microscopy

Abstract Polysilicon layers prepared by low-pressure chemical vapor deposition at 560°C, 620°C, 660°C, and 700°C were measured by Atomic Force Microscopy (AFM) and Spectroscopic Ellipsometry (SE). Morphology, cross-sectional profile, roughness spectral density, and roughness of the surfaces were investigated by AFM using window sizes of 1×1 μm2, 10×10 μm2, and 50×50 μm2. The layer structure and the surface roughness were determined by SE using the Bruggemann-Effective Medium Approximation (B-EMA). The Root Mean Square (RMS) and mean square (Ra) roughness values measured by AFM were compared to the thickness of the top layer of the SE model describing the surface roughness. Although AFM results depend on the used window size, good correlation was found between the roughness values determined by AFM and SE for each window sizes. The results show that SE calibrated with AFM could be used for quantitative surface roughness determination.

Ellipsometric study of polycrystalline silicon films prepared by low-pressure chemical vapor deposition

Polysilicon layers with thicknesses between 8 and 600 nm deposited by low-pressure chemical vapor deposition at temperatures ranging from 560 to 640 °C were characterized by spectroscopic ellipsometry (SE) to determine the layer thicknesses and compositions using multilayer optical models and the Bruggeman effective-medium approximation. The dependence of the structural parameters on the layer thickness and deposition temperature have been investigated. A better characterization of the polysilicon layer is achieved by using the reference data of fine-grained polysilicon in the optical model. The amount of voids in the polysilicon layer was independently measured by Rutherford backscattering spectrometry (RBS). The SE and RBS results show a good correlation. The comparison of the surface roughness measured by SE and atomic force microscopy (AFM) shows that independently of the AFM window sizes, a good correlation of the roughness determined by SE and AFM was obtained.

Characterization of ion implanted silicon by ellipsometry and channeling

Abstract A correlation between the amount of disorder measured by channeling and the trajectory of measured ellipsometric angles (ψ, Δ) is reported. The implantation was performed by 11 B + , 28 Si + , 31 P + , 40 Ar + , 72 Ge + , 75 As + , 209 Bi + ions at room temperature. For fully amorphous samples the thickness data were obtained from channeling and the complex refractive index from a 145 nm thick amorphous layer. These experimental values were used to compute a theoretical curve in the ψ-Δ plane. The good agreement between the theoretical curve and experimental data provides a fast and non-destructive and non-contact method for estimating the thickness of an amorphous layer. For buried and partially disordered layers a qualitative interpretation of different trajectories depending on the ion species and other implantation condition such as energy and dose in the ψ-Δ plane can be given, in certain cases, on the basis of channeling measurements. It is also pointed out that plasma stripping, removes the polymerized hydrocarbon film without affecting the disordered layer, plays an important role in preparing implanted samples for ellipsometry.

Improved depth resolution of channeling measurements in Rutherford backscattering by a detector tilt

Abstract An optimized geometry for Rutherford scattering and channeling is described giving an enhanced depth resolution (40–50 A). Tis resolution was achieved by a second particle detector mounted at such an angle that it would accept scattered ions emerging at glancing angles only (tilted detector). The method was shown to bring advantages of regular RBS (easy to find major channels) and glancing angle geometry (≈ tenfold enhanced depth resolution, but restricted possibility to align the crystal) to a useful compromise in investigating thin disordered structures on single crystals or, say, depth dependence of lattice location, of foreign atoms. Comparative spectra on annealing of lattice detects and location of implanted Sb in Si are presented.

Ion-implantation induced anomalous surface amorphization in silicon

Spectroscopic ellipsometry (SE), high-depth-resolution Rutherford backscattering (RBS) and channeling have been used to examine the surface damage formed by room temperature N and B implantation into silicon. For the analysis of the SE data we used the conventional method of assuming appropriate optical models and fitting the model parameters (layer thicknesses and volume fraction of the amorphous silicon component in the layers) by linear regression. The dependence of the thickness of the surface-damaged silicon layer (beneath the native oxide layer) on the implantation parameters was determined: the higher the dose, the thicker the disordered layer at the surface. The mechanism of the surface amorphization process is explained in relation to the ion beam induced layer-by-layer amorphization. The results demonstrate the applicability of Spectroscopic ellipsometry with a proper optical model. RBS, as an independent cross-checking method supported the constructed optical model.

Refractive index of sputtered silicon oxynitride layers for antireflection coating

Silicon nitride and silicon oxynitride dielectric layers were fabricated by reactive RF sputtering from an Si target in conventional equipment. Sputtering was done using a gas mixture of high-purity nitrogen and oxygen at a total pressure of 2}3 Pa. To investigate the sputtering process parameters silicon nitride was deposited on polished slices of Si. Multiple angle incidence ellipsometry was applied for the determination of the refractive index and thickness of the deposited layers. We found that the sputtered silicon oxynitride layers have excellent transparency in visible and near-infrared ranges and a wide refractive index range varying from 2.05 to 1.45. The refractive index of the layers decreases continuously with the increasing O 2 partial pressure that did not exceed the value of 0.01 Pa. Being a low-temperature process, reactive sputtering is well appropriate for the compound semiconductor devices. ( 2001 Elsevier Science Ltd. All rights reserved.

Investigation of ion-implanted semiconductors by ellipsometry and backscattering spectrometry

Abstract Ion-implanted silicon and GaP were investigated by ellipsometry and channelling effect measurements to determine the validity of the Bruggeman thoery for partially amorphous layers. It was found that the effective medium approximation (EMA) gives a satisfactory description of the disordered layer for heavy ions and low doses. The thickness of the disordered layer and the degree of amorphousness are independent parameters and can be determined from ellipsometry alone. Low dose nitrogen implantation is cited as an example of the limitations of EMA.

Ellipsometric characterization of damage profiles using an advanced optical model

Damage created by ion implantation into single crystalline silicon was characterized with an optical model based on the coupled half-Gaussian model developed by Fried et al [J. Appl. Phys. 71, 2835 (1992)]. In the improved optical model the damage profile was described by sublayers with thicknesses inversely proportional to the slope of the profile. This method allows a better resolution at the quickly changing parts of the profile, and a better approximation of the Gaussian profile with the same number of sublayers. A fitting procedure, which we call “multipoint random search,” was applied to minimize the probability of getting in a local minimum. The capabilities of our method were demonstrated for amorphizing doses using different ions and energies. The improved fit quality and the correlation with results of backscattering spectrometry basically supported the optical model.