Real-time Ellipsometry Research Articles

Real-time artificial intelligence enhanced defect engineering in CeO2 nanostructures

CeO2 nanostructures have been utilized for various biomedical, sensor, and catalysis applications owing to their unique defect structure, enabling them to have regenerative oxidative properties. Defect engineering in CeO2 nanostructures has major importance, enabling them to be utilized for specific applications. Despite various synthesis methods, it is challenging to have precise and reversible control over defect structures. Against this backdrop, in the current work, we have explored machine learning (ML) enhanced defect engineering of CeO2 nanofilms. In our earlier work [J. Vac. Sci. Technol. A 39, 060405 (2021)], we have developed an atomic layer deposition process for CeO2 using in situ ellipsometry measurements. In the current work, data collected through in situ spectroscopic ellipsometry and ex situ XPS have been correlated using two ML algorithms (gradient boost and random forest regressor) to exert better control over the chemical properties. Defect structures are one of the desired properties in CeO2 nanomaterials, characterized by the Ce3+/Ce4+ oxidation state ratio leading to its regenerative properties. We have shown that the defect structure of the CeO2 nanofilms can be predicted using in situ ellipsometry data in real time using a trained ML algorithm using two different methods. The first method involves an indirect approach of thickness prediction using an ML algorithm (k-nearest neighbors) followed by Ce3+/Ce4+ estimation using an experimental calibration curve. The second method with a more direct approach involves Ce3+/Ce4+ prediction using real-time ellipsometry data (amplitude ratio ψ and phase difference Δ) using gradient boost and random forest regressor.

Real-Time Ellipsometry at High and Low Temperatures.

Among the many available real-time characterization methods, ellipsometry stands out with the combination of high sensitivity and high speed as well as nondestructive, spectroscopic, and complex modeling capabilities. The thicknesses of thin films such as the complex dielectric function can be determined simultaneously with precisions down to sub-nanometer and 10-4, respectively. Consequently, the first applications of high- and low-temperature real-time ellipsometry have been related to the monitoring of layer growth and the determination of optical properties of metals, semiconductors, and superconductors, dating back to the late 1960s. Ellipsometry has been ever since a steady alternative of nonpolarimetric spectroscopies in applications where quantitative information (e.g., thickness, crystallinity, porosity, band gap, absorption) is to be determined in complex layered structures. In this article the main applications and fields of research are reviewed.

Simplified CVD route to near-zero thickness silicon nitride films

Silicon nitride (SiNx, x ∼ 1) thin films were deposited by chemical vapor deposition on silicon oxide (SiO2) substrates by combining controlled pulses of the precursor 1,3,5-tri(isopropyl)cyclotrisilazane (TICZ, C9H27N3Si3) with a continuous ammonia (NH3) plasma. This plasma-assisted pulsed CVD (PPCVD) process enables the integration of the nanoscale thickness and uniformity control achieved in atomic layer deposition with the efficiency of plasma-enhanced CVD (PE-CVD). TICZ was selected because it is a nonpyrophoric stable liquid with a high vapor pressure (∼133 Pa at 70 °C) and could act as a single source for SiNx with both high Si and N contents. An optimized PPCVD process window was identified consisting of a substrate temperature of 350 °C, a TICZ pulse of ≤0.2 s, and a TICZ purge pulse ≥10 s in a continuous direct NH3 plasma at a NH3 flow rate and a power of 40 SCCM and 3000 W, respectively. The as-deposited films were analyzed by x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. XPS analysis confirmed the absence of any C inclusion and demonstrated the existence of the 1:1 Si:N ratio. In situ, real-time ellipsometry measurements indicated that SiNx growth occurred in a typical PE-CVD regime. They also yielded an as-grown SiNx average refractive index of ∼1.75.

The low-temperature remote-plasma-activated pulsed chemical vapor deposition route to SiNx from 1,3,5-tri(isopropyl)cyclotrisilazane

High-quality silicon nitride (SiNx) thin films were grown by remote-plasma-activated pulsed chemical vapor deposition (P-CVD) from the source precursor 1,3,5-tri(isopropyl)cyclotrisilazane (TICZ, C9H27N3Si3) and remote ammonia (NH3) plasma on silicon oxide (SiO2) substrates within an optimized substrate temperature window ranging from 200 to 350 °C. TICZ was selected because of its chemical stability, non-pyrophoric nature, good vapor pressure (~127 Pa at 70 °C), and its chemical structure that incorporates alkyl groups with three C atoms on each N atom, which provides a clean elimination mechanism for low temperature SiNx deposition. P-CVD consisted of a four-step process: TICZ pulse with no plasma, N2 purge, NH3 plasma pulse, and N2 purge. The as-deposited films were analyzed using spectroscopic ellipsometry and x-ray photoelectron spectroscopy (XPS). Wet etch rates were determined using a standard solution consisting of 0.5% hydrofluoric acid in deionized water. XPS analysis revealed a Si:N ratio of ~1:1 within the entire substrate temperature range and validated the formation of the SiNx phase. In situ, real-time ellipsometry measurements confirmed that SiNx growth exhibited a non-self-limiting P-CVD behavior. They also yielded an as-grown SiNx average refractive index of ~1.8 for the films grown at substrate temperatures above 200 °C.

In situ real-time and ex situ spectroscopic analysis of Al2O3 films prepared by plasma enhanced atomic layer deposition

In situ real-time ellipsometry (irtE) with a very high time resolution of 24 ms was applied to monitor the inductively coupled plasma enhanced atomic layer deposition (ALD) process of Al2O3 thin films to precisely resolve each step of the ALD process and its complete cycle. The influence of plasma power, plasma pulse duration, and deposition temperature on the film growth characteristics was investigated. Ex situ ellipsometry [UV-VIS-NIR-SE (ultraviolet-visible-nearinfrared-spectroscopic ellipsometry) and IR-SE (infrared spectroscopic ellipsometry)] and x-ray photoelectron spectroscopy revealed the bulk properties (thickness, refractive index, chemical composition, and carbon incorporation) of the films, which together with the in situ results are compared to those of the films prepared by thermal ALD (T-ALD). The ICPEALD (inductively coupled plasma enhanced ALD) films were deposited at substrate temperatures between 80 and 250 °C and the role of plasma power (50–300 W) and its pulse duration (1–20 s) was investigated at 250 °C. The reference T-ALD layers were prepared at 200 °C. The ICPEALD process of Al2O3 shows an increased growth rate, and the produced films exhibit higher carbon contaminations than the T-ALD Al2O3 films. Plasma pulse times of up to 15 s further increase the content of carbon and CH species; at the same time, the refractive index decreases. The optical properties of ICPEALD deposited Al2O3 films are comparable with those of the T-ALD films for low plasma power and short plasma pulse durations. For the ICPEALD films, UV absorption is found and it is dependent on the deposition parameters. irtE resolves process effects that correlate with the bulk properties of Al2O3, such as impurities and oxygen deficiencies.

New route for selective etching in remote plasma source: Application to the fabrication of horizontal stacked Si nanowires for gate all around devices

This letter proposes a new route to achieve selective etching using remote plasma (RP) source processes that offer purely chemical etching by the action of plasma neutral species only. The proposed approach relies on cycling a two-step process comprising a surface treatment step and an etching step. The concept is based on the strong sensitivity of plasma neutrals' absorption to the chemistry of the substrate. By modifying the material surface state with a treatment, the etching can be delayed or even suppressed, converting a nonselective surface into one for which the etching process is area selective. This approach has been applied to the isotropic and selective removal of SiGe relative to Si in order to fabricate horizontal stacked-Si nanowires for gate all around devices. The RP process alternates an oxidation step using remote He/O2 plasma and a short etching step using remote NH3/NF3/O2 plasma. Real-time ellipsometry measurements indicate that in remote NH3/NF3/O2 plasma, there exists an incubation time before the etching starts, which is greater on Si than on SiGe films, allowing highly selective SiGe etching in the transient regime. Moreover, the Si surface modification through oxidation can inhibit reactive sites, and thus delay or even suppress the Si etching, allowing infinite SiGe over Si selectivity. By using this approach, 10 nm-thick horizontal stacked-Si nanowires could be obtained.

Stages of polymer transformation during remote plasma oxidation (RPO) at atmospheric pressure

The interaction of cold temperature plasma sources with materials can be separated into two types: ‘direct’ and ‘remote’ treatments. Compared to the ‘direct’ treatment which involves energetic charged species along with short-lived, strongly oxidative neutral species, ‘remote’ treatment by the long-lived weakly oxidative species is less invasive and better for producing uniformly treated surfaces. In this paper, we examine the prototypical case of remote plasma oxidation (RPO) of polymer materials by employing a surface micro-discharge (in a N2/O2 mixture environment) treatment on polystyrene. Using material characterization techniques including real-time ellipsometry, x-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, the time evolution of polymer film thickness, refractive index, surface, and bulk chemical composition were evaluated. These measurements revealed three consecutive stages of polymer transformation, i.e. surface adsorption and oxidation, bulk film permeation and thickness expansion followed by the material removal as a result of RPO. By correlating the observed film thickness changes with simultaneously obtained chemical information, we found that the three stages were due to the three effects of weakly oxidative species on polymers: (1) surface oxidation and nitrate (R-ONO2) chemisorption, (2) bulk oxidation, and (3) etching. Our results demonstrate that surface adsorption and oxidation, bulk oxidation, and etching can all happen during one continuous plasma treatment. We show that surface nitrate is only adsorbed on the top few nanometers of the polymer surface. The polymer film expansion also provided evidence for the diffusion and reaction of long-lived plasma species in the polymer bulk. Besides, we found that the remote plasma etched surface was relatively rich in O–C=O (ester or carboxylic acid). These findings clarify the roles of long-lived weakly oxidative plasma species on polymers and advance the understanding of plasma–polymer interactions on a molecular scale.

Electrical and optical degradation study of methylammonium-based perovskite materials under ambient conditions

Hybrid perovskites have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells which combine the advantages of organic and inorganic semiconductors. One of the main obstacles to their commercialization is their poor stability under light, humidity, oxygen, and high temperatures. In this work, we compare the optical and the electrical light-induced degradation of CH3NH3PbI3 (“MAPI”)-based solar cells using real-time ellipsometry measurements, electrical measurements and X-Ray Diffraction (XRD) techniques. We evidence that while the electrical degradation takes place on a short time scale (2–3 days of exposure to ambient light conditions in a nitrogen atmosphere), no optical degradation is observed before 10 days when the dissociation reaction of methylammonium lead iodide starts acting. We find a very good agreement between XRD and ellipsometry measurements; both show the appearance of PbI2 after 1 week of exposure. We also confirm that the main mechanism at play is a light-induced degradation affecting the edges of the stack and the interfaces between the perovskite and the neighbouring layers. Last, a very good match is obtained on the optical constants of MAPI between our ellipsometry measurements and density functional theory calculations we performed, and we confirm the behavior of MAPI as an inorganic semiconductor.

Characterizing fluorocarbon assisted atomic layer etching of Si using cyclic Ar/C4F8 and Ar/CHF3 plasma.

With the increasing interest in establishing directional etching methods capable of atomic scale resolution for fabricating highly scaled electronic devices, the need for development and characterization of atomic layer etching processes, or generally etch processes with atomic layer precision, is growing. In this work, a flux-controlled cyclic plasma process is used for etching of SiO2 and Si at the Angstrom-level. This is based on steady-state Ar plasma, with periodic, precise injection of a fluorocarbon (FC) precursor (C4F8 and CHF3) and synchronized, plasma-based Ar+ ion bombardment [D. Metzler et al., J. Vac. Sci. Technol., A 32, 020603 (2014) and D. Metzler et al., J. Vac. Sci. Technol., A 34, 01B101 (2016)]. For low energy Ar+ ion bombardment conditions, physical sputter rates are minimized, whereas material can be etched when FC reactants are present at the surface. This cyclic approach offers a large parameter space for process optimization. Etch depth per cycle, removal rates, and self-limitation of removal, along with material dependence of these aspects, were examined as a function of FC surface coverage, ion energy, and etch step length using in situ real time ellipsometry. The deposited FC thickness per cycle is found to have a strong impact on etch depth per cycle of SiO2 and Si but is limited with regard to control over material etching selectivity. Ion energy over the 20-30 eV range strongly impacts material selectivity. The choice of precursor can have a significant impact on the surface chemistry and chemically enhanced etching. CHF3 has a lower FC deposition yield for both SiO2 and Si and also exhibits a strong substrate dependence of FC deposition yield, in contrast to C4F8. The thickness of deposited FC layers using CHF3 is found to be greater for Si than for SiO2. X-ray photoelectron spectroscopy was used to study surface chemistry. When thicker FC films of 11 Å are employed, strong changes of FC film chemistry during a cycle are seen whereas the chemical state of the substrate varies much less. On the other hand, for FC film deposition of 5 Å for each cycle, strong substrate surface chemical changes are seen during an etching cycle. The nature of this cyclic etching with periodic deposition of thin FC films differs significantly from conventional etching with steady-state FC layers since surface conditions change strongly throughout each cycle.

Fluorocarbon assisted atomic layer etching of SiO2 and Si using cyclic Ar/C4F8 and Ar/CHF3 plasma

The need for atomic layer etching (ALE) is steadily increasing as smaller critical dimensions and pitches are required in device patterning. A flux-control based cyclic Ar/C4F8 ALE based on steady-state Ar plasma in conjunction with periodic, precise C4F8 injection and synchronized plasma-based low energy Ar+ ion bombardment has been established for SiO2 [Metzler et al., J. Vac. Sci. Technol. A 32, 020603 (2014)]. In this work, the cyclic process is further characterized and extended to ALE of silicon under similar process conditions. The use of CHF3 as a precursor is examined and compared to C4F8. CHF3 is shown to enable selective SiO2/Si etching using a fluorocarbon (FC) film build up. Other critical process parameters investigated are the FC film thickness deposited per cycle, the ion energy, and the etch step length. Etching behavior and mechanisms are studied using in situ real time ellipsometry and x-ray photoelectron spectroscopy. Silicon ALE shows less self-limitation than silicon oxide due to higher physical sputtering rates for the maximum ion energies used in this work, ranged from 20 to 30 eV. The surface chemistry is found to contain fluorinated silicon oxide during the etching of silicon. Plasma parameters during ALE are studied using a Langmuir probe and establish the impact of precursor addition on plasma properties.

HgCdTe structures for dual-band photodetectors operating in the 3–5 and 8–12 µm spectral ranges

A bilayer cadmium-mercury-tellurium (CMT) heterostructure was designed consisting of photosensitive layers of compositions xCdTe = 0.29–0.32 and xCdTe = 0.220–0.230, sensitive in the spectral ranges of 3–5 and 8–12 µm, a barrier layer between them, and wide-band variable-gap layers on the heterojunction and the surface grown on a GaAs substrate with ZnTe and CdTe buffer layers. The molecular beam epitaxial (MBE) growth of the heteroepitaxial structure (HES) was controlled by real time ellipsometry. After the growth, the composition distribution throughout the thickness was measured by reflection spectra with layer-by-layer chemical etching. There is good agreement between the results of composition measurements using ellipsometry and reflection spectra. P-type conductivity of bilayer MBE CMT HESs was obtained after thermal annealing at 220–240 °C in an inert gas (helium) for 24 h. The concentration of holes in the photosensitive layers is (4–10) ·1015 cm−3 and (8–20) · 1015 cm−3 at 78 K.

Interaction of sphingomyelinase with sphingomyelin-containing supported membranes

We have studied the interaction of the enzyme sphingomyelinase with sphingomyelin-containing supported membranes using quantitative applications of real-time epifluorescence microscopy and imaging optical ellipsometry. The enzymatic action converts sphingomyelin into ceramides by cleaving the phosphodiester bond. Our results confirm previous studies establishing a gross morphological transformation of lipid bilayers involving a multi-step process consisting of lag-burst type of enzyme activation and in-plane reorganization of membrane components attributed to the formation of ceramide-enriched domains. A unique finding of our study is the evidence for the existence of an additional out-of-plane deformation following lateral reorganization resulting in membrane voids disrupting the laterally contiguous bilayer. Taken together, the in-plane and out-of-plane deformations suggest a mechanistic picture in which lateral diffusional processes of translational mobility and phase separation couple with out-of-plane interactions across the membrane leaflet to induce irreversible membrane disruption in response to SMase action. Remarkably, lipid monolayers supported on hydrophobic substrates exhibit no such large-scale deformation despite ceramide generation by enzymatic activity of sphingomyelinase, possibly suggesting the importance of coupling across membrane leaflets in inducing out-of-plane deformations.

Correlation between Tribological Properties, sp²/sp³-Ratio and H-Content of Low-Wear Diamond-Like Carbon (DLC) Layers

It has been established that hardness and density of diamond-like carbon (DLC) layers can be raised by increasing ion energy during deposition, decreasing H-content and by increasing sp3-fraction. To confirm differences in hydrogen content of hydrogen containing and hydrogen free DLC films deposited at different bias voltages, layers were etched in oxygen atmosphere in a capacitively coupled plasma device. By employing real-time ellipsometry measurements, the H-content of the hydrogen containing a-C:H layers were estimated by determining the optical constants n and k (n-real part and k-imaginary part of the refractive index). In addition, DLC layers were analyzed by X-ray photoelectron spectroscopy to estimate the ratio of sp²- and sp³-hybridization. The mechanical and tribological properties of the coatings were evaluated by means of nanoindentation and ball-on-disc-tests. Finally correlations between these properties, H-content and sp3/sp2-ratio were obtained in an effort to explain different tribological behaviors of DLC-layers.

Assessment of interface roughness during plasma etching through the use of real-time ellipsometry

Real-time in situ ellipsometry was used to investigate the etching of SiO2/silicon wafers with a high concentration of Cl2. We monitored the temporal trajectory of the ellipsometric parameter Δ and then selected several points for ex situ study using atomic force microscopy (AFM). There was a clearly observable transition period in the trajectory near the endpoint of the SiO2/Si interface. We studied the relationship between the ellipsometric parameter Δ and the same point in the AFM ex situ measurements. Three stages, thin film, the interface layer, and the substrate, were analyzed in this work.

In Situ Analysis of Pb Electrodeposition on Polycrystalline Cu Using Ellipsometry

In this study, electrodeposition of Pb on polycrystalline Cu surface has been investigated by cyclic voltammetry (CV). Simultaneous in real time ellipsometry (SE) have been performed to study the characterization and morphological evolution of the Pb layer. The results demonstrated that the rate at which deposition thickness is increasing is going up and revealed that thin Pb deposition layer has already acquired at all the polarization process. So the Cu substrate is not optically ignored during the total polarization process.

Stages in the interaction of deuterium atoms with amorphous hydrogenated carbon films: Isotope exchange, soft-layer formation, and steady-state erosion

We report studies of the interactions of quantified deuterium (hydrogen) atom beams with hard amorphous hydrogenated carbon films at a substrate temperature of ∼330 K in an ultrahigh-vacuum chamber. The modification/erosion of a-C:H (a-C:D) films was monitored in situ by ellipsometry in real time. By interpreting the ellipsometric information and combining it with measurements of the absolute D areal density changes in the a-C:H (a-C:D) films by ion beam analysis as a function of D (H) atom fluence, we are able to distinguish three sequential stages of D interaction with hard a-C:H films. The first stage is replacement of bonded hydrogen by deuterium up to an areal density of ∼5×1015 D cm−2 to a depth of ∼1.4 nm from the surface. This phase is complete after a deuterium fluence of ≈2×1018 cm−2. The effective cross section for isotopic exchange of H with D atoms for the a-C:H layer is found to be σ=2.0×10−18 cm2, and is close to the cross section for H abstraction from a carbon surface. This may indicate that H abstraction by D from the a-C:H surface is the rate limiting step for isotope exchange in this situation. Hydrogen replacement is followed by creation of additional C–D bonds in the near-surface region and increases the D areal density by about 2.5×1015 D cm−2. By ellipsometry this process can be observed as the formation of a soft a-C:D layer on top of the hard a-C:H bulk film, with the soft layer extending about 1.4 nm from the surface. This stage is complete after a deuterium fluence of about 2×1019 cm−2. Subsequently, steady-state erosion of the a-C:H film takes place. Here, a soft a-C:D layer with roughly constant thickness (∼1.4 nm) remains on the hard a-C:H substrate and is dynamically reformed as the underlying hard a-C:H film becomes thinner. A similar sequence of processes takes place at a substrate temperature of 650 K, albeit at a much faster rate.

Real-time measurement of photo-induced effects in 9,10-phenanthrenequinone-doped poly(methyl methacrylate) photopolymer by phase-modulated ellipsometry

The photo-induced effects in 9,10-phenanthrenequinone-doped (PQ-doped) poly(methyl methacrylate) (PMMA) photopolymer were studied by phase-modulated ellipsometry and polarimetry in real time. The PQ-doped PMMA was exposed to an Ar/Kr tunable laser at the wavelength of 488 nm and measured by a HeNe laser (632.8 nm). We measured the induced birefringence and the variation of the refractive index separately during exposure; there was a difference of three orders of magnitude between these two effects. This suggested that the physical mechanism of holographic recording in PQ-doped PMMA is mainly due to the photo-induced variation in the refractive index.

Ellipsometry investigation of the amorphous-to-microcrystalline transition in a-Si:H under hydrogen-plasma treatment

We have investigated by ellipsometry the structural evolution of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) thin films during their exposure to a hydrogen plasma in conditions of chemical transport. The real-time kinetic ellipsometry trajectory at 4.2 eV allowed a precise determination of the amorphous-to-microcrystalline (a→μc) phase transition time. In situ spectroscopic ellipsometry spectra were collected at both sides of the transition to identify the hydrogen induced changes in the film structure and its time evolution under hydrogen-plasma exposure. The whole ellipsometry diagnostics reveal that, while intrinsic and phosphorus-doped a-Si:H present a similar trend during the plasma treatment, boron-doped a-Si:H differs by the following special features: (i) a rapid formation of the hydrogen-rich subsurface layer with a high hydrogen excess. (ii) An early amorphous-to-microcrystalline phase transition. (iii) A less porous and small-grains formed transition layer. Such a particular behavior of the p-type material is attributed to the effects of boron which, on the one hand, reduces the etching rate by hindering the formation of volatile species such as SiH4 and, on the other hand, promotes quick and repeated nucleation of nanoparticles and/or crystallites which contribute to the growth film.

Real-time and post-plasma studies of influence of low levels of tungsten on carbon erosion and surface evolution behaviour in D2 plasma

A profound influence of monolayer tungsten coverage of hard carbon films on the evolution of carbon surface erosion behaviour, surface chemistry and morphology in D2 plasma has been established by real-time ellipsometry, x-ray photoelectron spectroscopy and atomic force microscopy measurements. The erosion of tungsten-covered carbon showed two distinct stages of plasma material interactions: rapid tungsten removal during the initial erosion period and steady-state amorphous carbon removal accompanied by large-scale surface roughness development. The initial removal of tungsten takes place at a rate that significantly exceeds typical sputter yields at the ion energies used here and is attributed to elimination of weakly bonded tungsten from the surface. The tungsten remaining on the a-C : H film surface causes surface roughness development of the eroding carbon surface by a masking effect, and simultaneously leads to a seven fold reduction of the steady-state carbon erosion rate for long plasma surface interaction times (∼100 s). Results presented are of direct relevance for material transport and re-deposition, and the interaction of those films with plasma in the divertor region and on mirror surfaces of fusion devices.

Real-time evaluation of thickness, optical properties and stoichiometry of SiO x gas barrier coatings on polymers

Silicon oxide (SiO x ) coatings have been exploited for packaging applications due to their exceptional properties. They can be fabricated by a number of techniques, including Electron Beam Evaporation (EBE); an easily applicable method in industrial scale. The SiO x films' thickness and stoichiometry are the key parameters for the achievement of the final functional properties of the systems. The need to reduce material waste as well as the time required for the evaluation of the systems, make real-time monitoring and control requisite. This work highlights a methodology based on real-time Multi-Wavelength Ellipsometry for the monitoring of the EBE processes of SiO x coatings on poly(ethylene terephthalate) membranes. It is presented a thorough correlation between the basic optical parameters, such as refractive index and Penn gap, and the SiO x films' thickness, stoichiometry, composition and functional properties, such as oxygen permeation.