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State-of-the-art extreme ultraviolet lithography requires the use of ultrathin photoresists (or resists) due to pattern stability concerns and reduced depth of focus of the extreme ultraviolet lithography scanners. Current resists for extreme ultraviolet lithography are less than 50 nm thick. These ultrathin resists further complicate pattern transfer as unintended plasma-induced damage during dry etching is more pronounced. A better understanding of the interaction of plasma species with ultrathin resists is critical for enabling pattern transfer of sub-10 nm features. Here, we study the impact of vacuum ultraviolet photons, argon ions, and argon plasma on a 40 nm thick polymethylmethacrylate film. Using a deuterium lamp, an industrial ion beam etch tool, and an industrial inductively coupled plasma etch tool, we exposed the polymer to photons, ions, and plasma, respectively. The exposed samples were then analyzed for chemical and physical changes using different characterization techniques. It was observed that the vacuum ultraviolet photons interact with the entire bulk of polymer film, while the ions only affect the surface and subsurface region. The photon exposed samples formed smaller polymer fragments at low exposure doses and further started to cross-link at high doses. In contrast, the ion modification leads to carbonization of only the top few nanometers of the polymer film, leaving the bottom bulk intact. The plasma exposed sample showed changes characteristic to both vacuum ultraviolet photons and ions and their synergism. It was stratified with a 1.34 ± 0.03 nm thick ion-caused carbonized layer on top of a 13.25 ± 0.12 nm photon-induced cross-linked layer. By studying the impact of plasma photons on ultrathin polymethylmethacrylate, we were able to establish a baseline for a testing methodology that can be extended to novel ultrathin resist platforms. [ABSTRACT FROM AUTHOR]

Atomic layer etching (ALE) schemes are often deemed economically unviable due to their slow pace and are not suited for every material/hard-mask combination. Conversely, plasma etching presents pattern profile challenges because of its inability to independently control ion and neutral flux. In this work, we introduce a new cyclic transient-based process, called transient-assisted plasma etching (TAPE). A cycle of TAPE is a short exposure step to a sustained flow of reactant before the reactant gas injection is stopped in the second step, resulting in a plasma transient. As the plasma ignites and a substantial amount of etchant remains, a chemically driven etching process occurs, akin to conventional etching. Later in the transient, the modified surface is exposed to a reduced etchant quantity and a sustained ion bombardment, in a similar way to ALE. The cointegration of conventional etching and atomic layer etching allows interesting compromises between etch control and processing time. Going for a transient plasma allows to provide the time and conditions needed for the necessary plasma-surface interactions to occur in one step. In this perspective, the mechanisms behind etch rate, profile correction, and conservation of surface composition using amorphous carbon, as a benchmark, are discussed. [ABSTRACT FROM AUTHOR]

We report the synthesis and characterization of high-quality thin films of the topological semimetal (Bi2)5(Bi2Se3)7. Cryogenic magnetotransport experiments reveal strong metallic character and spin--orbit coupling in the films. By studying the temperature dependence of the electrical resistance of the topological semimetal, we observe a pronounced Kondo effect, which points toward the presence of magnetic scatterers. With the aid of density functional theory calculations, we identify Bi vacancies as intrinsic magnetic scatterers in this topological semimetal. [ABSTRACT FROM AUTHOR]

The growth of interfacial carbon (IFC) layers at the epitaxial sapphire/metal interface could unlock the transfer process for large-scale epitaxial thin films, such as graphene. To assess the feasibility of the application of IFC, understanding of the growth mechanisms of IFC layers at the epitaxial metal/sapphire interface is required. In this study, atmospheric pressure chemical vapor deposition is used to directly synthesize wafer-scale multilayer carbon at the interface between the Al2O3(0001) substrate and epitaxial Cu(111) layer. Characterization by Raman spectroscopy, AFM, and XRD, indicates the IFC layers exhibit low defect density and sp2 bonding character. Furthermore, IFC as thick as 100 nm could be synthesized. The effect of the partial pressure of reaction gases is systematically investigated, and a growth mechanism is proposed based supported by experimental observations and simulation results. Finally, a mechanical lift-off of a Cu(111) film with the aid of the IFC layer is demonstrated as a proof-of-concept application. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenide (TMD) semiconductors have risen as an important material class for novel nanoelectronic applications. Molybdenum disulfide (MoS2) is the most representative TMD compound due to its superior stability and attractive properties for (opto-) electronic devices. However, the synthesis of single-crystalline and functional MoS2 across large-area substrates remains crucial for its successful integration in semiconductor industry platforms. Therefore, this work focuses on the study of MoS2 epitaxy via two well-established industry-compatible synthesis methods, promising for the large-area and single-crystalline integration of van der Waals (vdW) materials. These methods are molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) and have studied MoS2 quasi-vdW heteroepitaxy on reconstructed sapphire substrates and MoS2 vdW homoepitaxy on exfoliated MoS2 flakes. By examining the MoS2 structural properties using diffraction and spectroscopy techniques, the epitaxial relation and crystal quality are assessed, which reveals insights into the prevalence of inter- and intragrain defects such as grain boundaries and sulfur vacancies. The MBE method yields superior epitaxial MoS2 registry on both sapphire and MoS2 surfaces as compared to MOVPE, although inferior defectivity arises from the typical lower MBE growth temperature and chalcogen partial pressure. Moreover, both synthesis methods generate high densities of twinned MoS2 grain boundaries, which hamper defect-free integration. As a result, this challenging integration might become an important bottleneck for industrial TMD-based applications with a low tolerance for material defects. [ABSTRACT FROM AUTHOR]

The spectra of optical constants, index of refraction (n), and extinction coefficient (k) of graphene and graphite are obtained in the wavelength range of 190–1000 nm (6.53–1.24 eV) using broadband optical spectrophotometry in conjunction with the Forouhi–Bloomer dispersion relations for n and k. Measurement is made possible by the use of a multilayer substrate consisting of bulk Si and a 3000 Å SiO2 film. The effect of multiple internal reflections between the Si/SiO2 and SiO2/graphene interfaces amplifies the attenuating effect of the graphene layer, thereby improving the sensitivity of the reflectance measurement by a factor of 27 in the deep ultraviolet region of the spectrum. Maximum sensitivity is observed in the deep ultraviolet region of the spectrum, where a strong peak in the spectrum of the extinction coefficient of graphene is identified. The proposed method enables fast nondestructive angstrom-level thickness measurements of graphene and graphite. In this work, layers ranging in thickness between 3.8 Å (graphene) and 792.8 Å are detected, measured, and characterized. Reflectance spectra of graphene and graphite on Ni, Co, and Fe substrates are calculated. Differences of 1.1%–2.0% between the bare substrate and the graphene on the substrate are predicted in the deep ultraviolet region of the spectrum, which makes graphene detectable and measurable on these substrates. [ABSTRACT FROM AUTHOR]

Atomic layer deposition (ALD) is used in applications where inorganic material layers with uniform thickness down to the nanometer range are required. For such thicknesses, the growth mode, defining how the material is arranged on the surface during the growth, is of critical importance. In this work, the growth mode of the zirconium tetrachloride/water and the trimethyl aluminum/water ALD process on hydrogen-terminated silicon was investigated by combining information on the total amount of material deposited with information on the surface fraction of the material. The total amount of material deposited was measured by Rutherford backscattering, x-ray fluorescence, and inductively coupled plasma–optical emission spectroscopy, and the surface fractions by low-energy ion scattering. Growth mode modeling was made assuming two-dimensional growth or random deposition (RD), with a “shower model” of RD recently developed for ALD. Experimental surface fractions of the ALD-grown zirconium oxide and aluminum oxide films were lower than the surface fractions calculated assuming RD, suggesting the occurrence of island growth. Island growth was confirmed with transmission electron microscopy (TEM) measurements, from which the island size and number of islands per unit surface area could also be estimated. The conclusion of island growth for the aluminum oxide deposition on hydrogen-terminated silicon contradicts earlier observations. In this work, physical aluminum oxide islands were observed in TEM after 15 ALD reaction cycles. Earlier, thicker aluminum oxide layers have been analyzed, where islands have not been observed because they have already coalesced to form a continuous film. The unreactivity of hydrogen-terminated silicon surface towards the ALD reactants, except for reactive defect areas, is proposed as the origin of island growth. Consequently, island growth can be regarded as “undesired surface-selective ALD.” [ABSTRACT FROM AUTHOR]

In this work, the epitaxy of SrTiO3 and BaTiO3 perovskites on the (001)-oriented silicon substrate by molecular beam epitaxy is investigated. The heterostructures are studied by means of various structural and electrical characterization techniques. In this study especially, the authors reveal experimentally by nanobeam electron diffraction analysis the critical thickness prior relaxation of BaTiO3 grown on an SrTiO3/Si pseudosubstrate. They also propose to use a strain mediated superlattice composed of stacked [BaTiO3/SrTiO3] bilayers to prevent misfit dislocation formation. Using this approach, they could demonstrate high quality and dislocation free BaTiO3 ferroelectric layers integrated on silicon as confirmed by piezo-force microscopy techniques. [ABSTRACT FROM AUTHOR]

The integration of van der Waals materials in nanoelectronic devices requires the deposition of few-layered MX2 films with excellent quality crystals covering a large area. In recent years, astonishing progress in the monolayer growth of WS2 and MoS2 was demonstrated, but multilayer growth resulted often in separated triangular or hexagonal islands. These polycrystalline films cannot fully employ the specific MX2 properties since they are not connected in-plane to the other domains. To coalesce separated islands, ultrahigh-temperature postdeposition anneals in H2S are applied, which are not compatible with bare silicon substrates. Starting from the deposition of stoichiometric short-ordered films, the present work studies different options for subsequent high-temperature annealing in an inert atmosphere to form crystalline films with large grains from stoichiometric films with small grains. The rapid thermal annealing, performed over a few seconds, is compared to excimer laser annealing in the nanosecond range, which are both able to crystallize the thin WS2. The WS2 recrystallization temperature can be lowered using metallic crystallization promoters (Co and Ni). The best result is obtained using a Co cap, due to the circumvention of Co and S binary phase formation below the eutectic temperature. The recrystallization above a critical temperature is accompanied by sulfur loss and 3D regrowth. These undesired effects can be suppressed by the application of a dielectric capping layer prior to annealing. A SiO2 cap can suppress the sulfur loss successfully during annealing and reveals improved material quality in comparison to noncapped films. [ABSTRACT FROM AUTHOR]

Ultra-thin MoS2 film doping through surface functionalization with physically adsorbed species is of great interest due to its ability to dope the film without reduction in the carrier mobility. However, there is a need for understanding how the thickness of the MoS2 film is related to the induced surface doping for improved electrical performance. In this work, we report on the relation of MoS2 film thickness with the doping effect induced by the n-dopant adsorbate poly(vinyl-alcohol). Field effect transistors built using MoS2 films of different thicknesses were electrically characterized, and it was observed that the ION/OFF ratio after doping in thin films is more than four orders of magnitudes greater when compared with thick films. Additionally, a semi-classical model tuned with the experimental devices was used to understand the spatial distribution of charge in the channel and explain the observed behavior. From the simulation results, it was revealed that the two-dimensional carrier density induced by the adsorbate is distributed rather uniformly along the complete channel for thin films (<5.2 nm) contrary to what happens for thicker films. [ABSTRACT FROM AUTHOR]

Defect generation in HfO[sub 2]/SiO[sub 2] gate dielectric stacks under constant voltage stress is investigated. It is found that the stress induced electrical degradation in HfO[sub 2]/SiO[sub 2] stacks is different than in the SiO[sub 2] layer. The variation of the gate leakage current with different polarities shows different degradation characteristics after stress. Positive charge generation is also observed under both negative and positive gate voltage polarities. These degradation phenomena are explained by the composite effect of three components: neutral trap generation, electron trapping, and positive charge generation in the gate stacks. [ABSTRACT FROM AUTHOR]

For Cu/low-k interconnects, the reversed damascene is an alternative integration approach where the metal wires are patterned first and then the spacing filled with a flowable dielectric. In this paper, the replacement of a sacrificial template by gap-filling ultralow-k dielectrics is studied, focusing on yield and transport performance ("replacement dielectric" scheme). On non-passivated copper, the low-k curing processes induce severe damage to the metal lines, leading to the degraded electrical properties. This is confirmed by chemical inspection on the blanket Cu films and morphological inspection on patterned structures. In order to avoid Cu oxidation and out-diffusion at elevated temperature, Cu passivation by plasma-enhanced chemical vapor deposition SiCN is proposed and studied in detail. The inter-metal dielectric properties of replacement low-k are evaluated by resistance-capacitance and IV measurements using a Meander-Fork structure. By tuning the passivation layer thickness and ultraviolet curing time, high electrical yield is obtained with integrated porous low-k showing promising effective k-values (keff) and breakdown voltages (Ebd), confirming the interest of this specific integration scheme. [ABSTRACT FROM AUTHOR]

Achieving low resistance contacts to graphene is a common concern for graphene device performance and hybrid graphene/metal interconnects. In this work, we have used the circular Transfer Length Method (cTLM) to electrically characterize Ag, Au, Ni, Ti, and Pd as contact metals to graphene. The consistency of the obtained results was verified with the characterization of up to 72 cTLM structures per metal. Within our study, the noble metals Au, Ag and Pd, which form a weaker bond with graphene, are shown to result in lower contact resistance (Rc) values compared to the more reactive Ni and Ti. X-ray Photo Electron Spectroscopy and Transmission Electron Microscopy characterization for the latter have shown the formation of Ti and Ni carbides. Graphene/Pd contacts show a distinct intermediate behavior. The weak carbide formation signature and the low Rc values measured agree with theoretical predictions of an intermediate state of weak chemisorption of Pd on graphene. [ABSTRACT FROM AUTHOR]

The problem of k-value degradation (plasma damage) is a key issue for the integration, and it is becoming more challenging as the dielectric constant of low-k materials scales down. One way to circumvent this issue is temporarily conversion of low-k material from a porous to a dense state by filling the pores with a sacrificial polymer after the deposition and curing of the low-k material. A detailed process scheme for the pore stuffing and postetch polymer removal of PMMA is described in this work. The filling temperature was optimized according to the molecular weight of the PMMA. To remove the polymer after plasma-etching in a purely thermal environment, a temperature of at least 430 °C had to be applied. Annealing assisted by variable frequency microwaves could remove the polymer already at 380 °C and with a He-H2 afterglow plasma the polymer could be removed at 280 °C. Laser annealing allowed the removal at a stage temperature of 200 °C with an only surface-limited heating of about 500 °C and higher to prevent the FEOL structures from damage. This work presents the results of the detailed study of stuffing and unstuffing processes, discusses mechanisms, and provides background for a robust stuffing and polymer removal process for the plasma damage reduction in porous low-k dielectrics. [ABSTRACT FROM AUTHOR]

The effect of narrow-band 172 nm and broad-band >200 nm UV sources in the new curing scheme of the plasma-enhanced chemical vapor deposition (PECVD) dielectrics is studied. The new curing scheme is based on porogen removal (organic sacrificial phase introduced to generate open porosity) from PECVD dielectric before its final UV curing. The results are compared with the PECVD films fabricated in the conventional scheme in which porogen is still present during the UV curing. The same curing time of porogen-containing conventional PECVD films with 172 nm and >200 nm UV sources results in only 10% difference in their Young's modulus (YM): 5.84 and 5.32 GPa, respectively. However, the porogen-free films cured with 172 nm UV source show a YM of 6.64 GPa (k100 kHz∼2.2, 44% open porosity), approximately twice as large as those cured with >200 nm UV having a YM of 3.38 GPa (k100 kHz∼2.0, 48% open porosity). The mechanical properties, optical properties in the range of 150--800 nm, dielectric constants at 100 kHz and 4 GHz, porosities, pore size distributions, and bonding structure are evaluated. The impact of porogen on optical characteristics and, therefore, on the photochemical UV-hardening mechanism is discussed. The achieved mechanical properties are explained on a basis of the percolation of rigidity theory and random network concepts [ABSTRACT FROM AUTHOR]

Uniformity and wafer-to-wafer reproducibility of plasma etch processes are often related to the conditioning of the plasma etch chamber walls. For advanced complementary metal-oxide semiconductor fabrication, numerous metals are used which might deposit on the chamber walls during etch processes and as these metals are not always straightforward to remove, process instabilities can occur. This happens because recombination of atomic species on the chamber walls determines to a certain degree the plasma composition. Therefore, in this article, the impact of metal etch residues, especially titanium and tantalum residues, on plasma composition and uniformity is studied. The chamber walls are analyzed by x-ray photoelectron spectroscopy analysis of so-called floating samples and the densities of Cl, Br, O and F in Cl2, HBr, O2, and SF6 plasmas are monitored by optical emission spectroscopy. Plasma uniformity is checked by measuring etch rates across 300 mm silicon wafers. It is found that chlorine and bromine have similar recombination probabilities on the metals than on anodized aluminum. Fluorine and oxygen recombination, however, is strongly influenced by the presence of metal residues. Accordingly, for fluorine and oxygen based plasmas, metal residues showed to have an impact on the plasma uniformity. [ABSTRACT FROM AUTHOR]

Downscaling equivalent oxide thickness (EOT) by decreasing the physical thickness or increasing the permittivity of the gate dielectric is required to reach the complementary metal-oxide semiconductor (CMOS) (sub) 45 nm node performance specifications. In this work, we study the atomic layer deposition (ALD) of hafnium silicate gate dielectrics from HfCl4, SiCl4, and H2O both experimentally and theoretically. Hafnium silicate is characterized by Rutherford backscattering, time-of-flight secondary ion mass spectroscopy, and x-ray photoelectron spectroscopy. The interaction of the precursors with the surface sites is investigated by first-principles calculations. The electrical properties are evaluated on TaN gated capacitors. The HfCl4/H2O reactions create surface sites that enable the chemisorption of SiCl4 in doses much smaller than the doses required for SiO2 ALD from SiCl4/H2O. The hydrolysis of Si–Cl is slower than the hydrolysis of Hf–Cl. Optimization of the hafnium silicate deposition results in a leakage current reduction of one order of magnitude for ultrascaled hafnium silicate gate stacks. At 0.9 nm EOT, a leakage current of <0.1 A/cm2 is obtained for hafnium silicates with Hf content higher than 90%. [ABSTRACT FROM AUTHOR]

Physical cleaning methods are applied in the semiconductor industry and have become increasingly challenging due to the continued scaling of semiconductors device elements. Cavitation and acoustic phenomena are known to play a fundamental role in megasonic cleaning. Hence, a better understanding of cavitation phenomena in multi-bubble systems is crucial. Here, a study on the effects of lower bulk surface tension and different O2 concentrations on the bubble activity in the megahertz range is presented. A lower bulk surface tension (45 mN/m) with respect to ultra pure water (72 mN/m) is obtained by adding a non-ionic surface-active agent (Triton X-100). After a thorough surfactant characterization, a Triton X-100-containing cleaning solution is investigated under pulsed and continuous acoustic fields, for different acoustic amplitudes and gas concentrations. It is demonstrated that cavitation activity, measured by means of ultraharmonic cavitation noise, is enhanced in presence of a lower surface tension, under continuous acoustic fields. In addition, cavitation measurements performed under pulsed fields reveal the existence of optimal pulse-off times, for which a maximum of activity is observed. These optimal pulse-off time values are linked to the bubble dissolution theoretical times and experimentally verified. To end, cavitation noise measurements are correlated to cleaning performance in megasonic fields by means of particle removal and damage tests on patterned wafers. A clear increase in particle removal efficiency of 78 nm SiO2 particles is obtained when Triton X-100 is employed, at the optimized process conditions. In addition, the number of defects due to cavitation bubbles is significantly reduced for lower surface tension, at particle removal efficiencies <60%. The results here reported constitute a different approach towards more efficient megasonic cleaning processes. [ABSTRACT FROM AUTHOR]

In this paper, we report the fabrication and characterization of Schottky rectifying junctions between semiconducting, modified single-layer graphene and a metal. The pristine, semimetallic behavior of graphene is altered by controlled exposure to an oxygen plasma, resulting in the opening of an optical band gap as shown by photoluminescence spectroscopy. The occurrence of a Schottky barrier between semiconducting graphene and metals with different work functions (Al, Cr, Pd, and Yb) is investigated by electrically characterizing the as-fabricated junctions. The rectifying properties of our Schottky diodes show the potential of semiconducting, modified graphene as building block of elementary logic circuits. [ABSTRACT FROM AUTHOR]

The energy band-alignment of a multimetal-layer gated metal-oxide-semiconductor structure is investigated. An analytical expression for the Fermi level of the metal stack is derived. It is shown that the thin metal layer contacting the gate dielectric plays a critical role in determining the work function of the whole metal stack, in agreement with experimental results. [ABSTRACT FROM AUTHOR]

The density of hafnium oxide films grown by atomic layer deposition for high-κ gate dielectric applications was investigated for films with thickness in the nanometer range. The density, measured by combining the film thickness from transmission electron microscopy with the amount of hafnium deposited from Rutherford backscattering, decreased with decreasing film thickness. The dielectric constant of hafnium oxide remained constant with decreasing film thickness, however. The main reason for the decrease in the measured density seemed not to be a decrease in the inherent material density. Instead, the relative importance of interface roughness in the density measurement increased with decreasing film thickness. [ABSTRACT FROM AUTHOR]

A strong polarity effect on the temperature dependence of the leakage current in TiN/HfO[sub 2]/SiO[sub 2]/Si capacitors is reported. A model is proposed to explain these experimental results that combines tunneling through the stack and Frenkel-Poole hopping in the HfO[sub 2] layer, depending on the value of the gate voltage. It is shown that the polarity effect most probably results from the anisotropy of the band diagram of the HfO[sub 2]/SiO[sub 2] stack, as well as from the location of the shallow traps with respect to the conduction band of the HfO[sub 2] layer. Comparison of the model with the experimental results allows an estimate of the trap depth to be between 0.5 and 0.8 eV. [ABSTRACT FROM AUTHOR]

Graphene films demonstrating low sheet resistance and high transparency in the visible light range are promising to be used as electrodes for light-emitting applications. In this work, we report the implementation of single layer graphene as hole injecting electrode for CdSe/ZnS quantum dot-light emitting diodes (QD-LED). We compare graphene vs. indium-tin-oxide (ITO)-based anode junctions by electroluminescence intensity performance of QD-LEDs. Our results demonstrate better hole injection efficiency for the graphene-based electrode at technologically relevant current densities J < 0.4 A/cm2, therefore, recommending single layer graphene as a valuable alternative to replace ITO in QD-LED technology. [ABSTRACT FROM AUTHOR]