Taming the Distribution of Light in Gate-All-Around Semiconductor Devices.

Optical metrology is ubiquitous, but image-based methods cannot resolve features of dimensions much smaller than the wavelength. However, it has recently been demonstrated that light can be nanofocused into subwavelength semiconducting lines by setting the incident polarization along the direction of these lines. This Letter extends the previous studies to systems with two perpendicular gratings, as found e.g. after replacement gate processing of gate-all-around (GAA) field-effect transistors (FETs). We show that besides the nanofocusing effect, the incident polarization also offers control over which array of lines the light couples into. The interaction of the incident light occurs with the semiconducting lines to which the polarization is parallel with remarkably low interference from the existence of another perpendicular grating. We demonstrate the use of this effect with Raman spectroscopy to simultaneously extract the SiGe volume and the strain in the Si forksheet channels and in the SiGe layers of GAA FETs.

Reference-free x-ray fluorescence analysis with a micrometer-sized incident beam.

Spatially resolved x-ray fluorescence (XRF) based analysis employing incident beam sizes in the low micrometer range (µXRF) is widely used to study lateral composition changes of various types of microstructured samples. However, up to now the quantitative analysis of such experimental datasets could only be realized employing adequate calibration or reference specimen. In this work, we extent the applicability of the so-called reference-free XRF approach to enable reference-freeµXRF analysis. Here, no calibration specimen are needed in order to derive a quantitative and position sensitive composition of the sample of interest. The necessary instrumental steps to realize reference-freeµXRF are explained and a validation of ref.-freeµXRF against ref.-free standard XRF is performed employing laterally homogeneous samples. Finally, an application example from semiconductor research is shown, where the lateral sample features require the usage of ref.-freeµXRF for quantitative analysis.

Influence of the Emitter Shape on the Field-of-View in Atom Probe Tomography.

Atom probe tomography (APT) is a unique analytical technique that offers three-dimensional elemental mapping with a spatial resolution down to the sub-nanometer. When APT is applied on complex heterogenous systems and/or under certain experimental conditions, that is, laser illumination, the specimen shape can deviate from an ideal hemisphere. Insufficient consideration of this aspect can introduce artifacts in the reconstructed dataset, ultimately degrading its spatial accuracy. So far, there has been limited investigation into the detailed evolution of emitter shape and its impact on the field-of-view (FOV). In this study, we numerically and experimentally investigated the FOV for asymmetric emitters and its evolution throughout the analysis depth. Our analysis revealed that, for asymmetric emitters, the ions evaporated from the topmost region of the specimen (summit) project approximately to the detector center. Furthermore, we demonstrated the implications of this finding on the FOV location for asymmetric emitters. Based on our findings, the location of the center of the FOV can deviate from the specimen central axis with an evolution depending on the evolution of the emitter shape. This study highlights the importance of accounting for the specimen shape when developing advanced data reconstruction schemes to enhance spatial resolution and accuracy.

Photonic metamaterial with a subwavelength electrode pattern.

The next generation of tunable photonics requires highly conductive and light inert interconnects that enable fast switching of phase, amplitude, and polarization modulators without reducing their efficiency. As such, metallic electrodes should be avoided, as they introduce significant parasitic losses. Transparent conductive oxides, on the other hand, offer reduced absorption due to their high bandgap and good conductivity due to their relatively high carrier concentration. Here, we present a metamaterial that enables electrodes to be in contact with the light active part of optoelectronic devices without the accompanying metallic losses and scattering. To this end, we use transparent conductive oxides and refractive index matched dielectrics as the metamaterial constituents. We present the metamaterial construction together with various characterization techniques that confirm the desired optical and electrical properties.

A Bottom-Up Volume Reconstruction Method for Atom Probe Tomography.

This paper describes a reconstruction method for atom probe tomography based on a bottom-up approach accounting for (i) the final tip morphology (which is frequently induced by inhomogeneous evaporation probabilities across the tip surface due to laser absorption, heat diffusion effects, and inhomogeneous material properties), (ii) the limited (and changing) field of view, and (iii) the detector efficiency. The reconstruction starts from the final tip morphology and reverses the evaporation sequence through the pseudo-deposition of defined small reconstruction volumes, which are then stacked together to create the full three-dimensional (3D) tip. The subdivision in small reconstruction volumes allows the scheme to account for the changing tip shape and field of view as evaporation proceeds. Atoms within the same small reconstruction volume are reconstructed at once by placing atoms back onto their possible lattice sites through a trajectory-matching process involving simulated and experimental hit maps. As the ejected ion trajectories are simulated using detailed electrostatic modeling inside the chamber, no simplifications have been imposed on the shape of the trajectories, projection laws, or tip surface. We demonstrate the superior performance of our approach over the conventional reconstruction method (Bas) for an asymmetrical tip shape.

Size-dependent optical properties of periodic arrays of semiconducting nanolines.

We study the size-dependent optical properties of periodic arrays of semiconducting nanolines in the near-infrared to near-ultraviolet spectral range, where the absorption of the semiconductor increases. Using band structure calculations, we demonstrate that specific dimensions allow the slow down of the light, resulting in an enhanced absorption as compared to bulk material once the extinction coefficient of the semiconductor becomes comparable to its refractive index. Further, the refractive properties of the arrays can be tailored beyond the values of the constituting materials when the extinction coefficient of the semiconductor exceeds its refractive index. To confirm our theoretical findings, we propose a simple semi-analytical model for the light interactions with such structures and validate it with experimental reflectance spectra collected on arrays for the next-generation transistors.

Potential sources of compositional inaccuracy in the atom probe tomography of InxGa1-xAs.

With the objective of applying laser-assisted atom probe tomography to compositional analysis within nanoscale InGaAs devices, experimental conditions that may provide an accurate composition estimate were sought by extensively studying an InGaAs blanket film. Overall, the determined arsenic atomic fraction was found to exhibit an electric field dependent deficiency, which was more pronounced at low field conditions. Although the determined group III site-fraction also showed a (weak) field-dependent deficiency at low field conditions, it remained invariant with analysis conditions and in close agreement with the nominal value at higher field. In this study, we investigate and discuss the mechanisms that could potentially contribute to As underestimation. Given the field dependence observed, the phenomena occurring between low and high field conditions are compared. At low field, the tendency of As to field evaporate in significant amounts as multiply charged cluster ions (Asni+ with n as large as 9 and i = 1,2,3) is shown to be a significant source of compositional inaccuracy. These clusters may lead to peak overlap in the mass spectrum (e.g. the peak at 150 Da may represent As42+ or As2+ or both), thereby creating an uncertainty in the quantification. Emitted clusters may also dissociate with the likelihood of neutral generation and multi-hit losses being non-negligible. Experimental studies and density functional theory calculations are presented to characterize cluster stability and its contribution to measurement uncertainty. Under high field conditions, although fewer clusters are detected and the composition appears more accurate, the emergence of two additional mechanisms, i.e., multi-hits and DC evaporation, may degrade the data quality. The challenges in evaluating the impact of all these loss mechanisms are examined in detail. Finally, we show that for InGaAs under UV illumination, due to the laser-tip interaction, the resulting asymmetric electric field distribution across the apex introduces local atomic fraction variations.

A variable probe pitch micro-Hall effect method.

Hall effect metrology is important for a detailed characterization of the electronic properties of new materials for nanoscale electronics. The micro-Hall effect (MHE) method, based on micro four-point probes, enables a fast characterization of ultrathin films with minimal sample preparation. Here, we study in detail how the analysis of raw measurement data affects the accuracy of extracted key sample parameters, i.e., how the standard deviation on sheet resistance, carrier mobility and Hall sheet carrier density is affected by the data analysis used. We compare two methods, based primarily on either the sheet resistance signals or the Hall resistance signals, by theoretically analysing the effects of electrode position errors and electrical noise on the standard deviations. We verify the findings with a set of experimental data measured on an ultrashallow junction silicon sample. We find that in presence of significant electrical noise, lower standard deviation is always obtained when the geometrical analysis is based on the sheet resistance signals. The situation is more complicated when electrode position errors are dominant; in that case, the better method depends on the experimental conditions, i.e., the distance between the insulating boundary and the electrodes. Improvement to the accuracy of Hall Effect measurement results is crucial for nanoscale metrology, since surface scattering often leads to low carrier mobility.

Electrical characterization of single nanometer-wide Si fins in dense arrays.

This paper demonstrates the development of a methodology using the micro four-point probe (µ4PP) technique to electrically characterize single nanometer-wide fins arranged in dense arrays. We show that through the concept of carefully controlling the electrical contact formation process, the electrical measurement can be confined to one individual fin although the used measurement electrodes physically contact more than one fin. We demonstrate that we can precisely measure the resistance of individual ca. 20 nm wide fins and that we can correlate the measured variations in fin resistance with variations in their nanometric width. Due to the demonstrated high precision of the technique, this opens the prospect for the use of µ4PP in electrical critical dimension metrology.