Situ Spectroscopic Ellipsometry Research Articles

Monitoring the evolution of optical coatings during thermal annealing with real-time, in situ spectroscopic ellipsometry

Abstract Thermal annealing plays a key role in optimizing the properties of amorphous optical coatings. In the field of gravitational wave detection (GWD), however, the effects of annealing protocols on the interferometry mirror coatings have been explored primarily by ex post analysis. As a result, the dynamics of the coatings properties during annealing is still poorly known, potentially leading to suboptimal performance. Here, using real-time, in situ spectroscopic ellipsometry (SE) we have tracked the refractive index and thickness of a titania-tantala coating during controlled annealing. We have tested the material and the annealing protocol used in
current GWD mirrors. The annealing cycle consisted of a heating ramp from room temperature to 500 °C, followed by a 10-hour plateau at the same temperature and the final cooling ramp. SE measurements have been run continuously during the entire cycle. Significant variations in the thickness and refractive index, which accompany the coating structural relaxation, have been recorded during the heating ramp. These variations start around 200 °C, slightly above the deposition temperature, and show an increased rate in the range 250-350 °C. A smaller, continuous evolution has been observed during the 10-hour high-temperature plateau. The results offer
suggestions to modify the current annealing protocol for titania-tantala coatings, for example by increasing the time duration of the high-temperature plateau. They also suggest an increase in the substrate temperature at deposition. The approach presented here paves the way for systematic, real-time investigations to clarify how the annealing parameters shape the properties of optical coatings, and can be leveraged to define and optimize the annealing protocol of new candidate materials for GWD mirrors.

Electron-enhanced atomic layer deposition of Ru thin films using Ru(DMBD)(CO)3 and effect of forming gas anneal

Ruthenium (Ru) thin films were deposited utilizing electron-enhanced atomic layer deposition (EE-ALD). Sequential exposures of Ru(DMBD)(CO)3 (DMBD = 2,3-dimethylbutadiene) and low-energy electrons at ∼125 eV were used to grow the Ru films at temperatures ≤160 °C. The electrons were obtained from a hollow cathode plasma electron source that provided an electron current of ∼200 mA over a surface area of ∼4 cm2. Low-energy electrons can desorb surface ligands derived from Ru(DMBD)(CO)3, such as CO, through electron-stimulated desorption. The desorbed surface ligands leave chemically reactive sites for subsequent Ru(DMBD)(CO)3 precursor absorption. Ru EE-ALD film growth was monitored utilizing in situ spectroscopic ellipsometry (SE). The electron exposures resulted in rapid Ru film nucleation and growth. Under saturation conditions at 160 °C, the growth rate for Ru EE-ALD was 0.2 Å/cycle. The electron efficiency factor for Ru EE-ALD was ∼21 500 electrons/deposited Ru atom. There was no film growth without electron exposures. Ru growth was observed on various substrates including silicon with native oxide and titanium. Ru growth was also obtained on insulating substrates such as 400 nm thick thermal SiO2 substrates. XPS analysis measured <1 at. % oxygen in the deposited Ru films. XRD, x-ray reflectivity, and SE were used to characterize the Ru films before and after forming gas anneal (FGA). FGA successfully removed carbon impurities from the as-deposited Ru films. The resistivity of the Ru EE-ALD films after FGA was determined to be as low as 17 μΩ cm for a film thickness of 6.7 nm. SE measurements of the imaginary part of the pseudodielectric function, 〈ɛ2〉, were utilized to characterize the as-deposited Ru films and the high purity Ru films after FGA. The low resistivity of the Ru films after FGA was consistent with a prominent Drude absorption in the ⟨ε2⟩ spectrum at ≤1 eV. Various reactive background gases such as H2, NH3, and H2O were utilized during EE-ALD to attempt to remove the carbon from the as-deposited Ru EE-ALD films.

Monitoring of the Initial Stages of Diamond Growth on Aluminum Nitride Using In Situ Spectroscopic Ellipsometry.

The high thermal conductivity of polycrystalline diamond makes it ideally suited for thermal management solutions for gallium nitride (GaN) devices, with a diamond layer grown on an aluminum nitride (AlN) interlayer atop the GaN stack. However, this application is limited by the thermal barrier at the interface between diamond and substrate, which has been associated with the transition region formed in the initial phases of growth. In this work, in situ spectroscopic ellipsometry (SE) is employed to monitor early-stage microwave plasma-enhanced chemical vapor deposition diamond growth on AlN. An optical model was developed from ex situ spectra and applied to spectra taken in situ during growth. Coalescence of separate islands into a single film was marked by a reduction in bulk void fraction prior to a spike in sp2 fraction due to grain boundary formation. Parameters determined by the SE model were corroborated using Raman spectroscopy and atomic force microscopy.

Controlling the surface silanol density in capillary columns and planar silicon via the self-limiting, gas-phase deposition of tris(dimethylamino)methylsilane, and quantification of surface silanols after silanization by low energy ion scattering

Surface silanols (Si-OH) play a vital role on fused silica surfaces in chromatography. Here, we used an atmospheric-pressure, gas-phase reactor to modify the inner surface of a gas chromatography, fused silica capillary column (0.53 mm ID) with a small, reactive silane (tris(dimethylamino)methylsilane, TDMAMS). The deposition of TDMAMS on planar witness samples around the capillary was confirmed with X-ray photoelectron spectroscopy (XPS), ex situ spectroscopic ellipsometry (SE), and wetting. The number of surface silanols on unmodified and TDMAMS-modified native oxide-terminated silicon were quantified by tagging with dimethylzinc (DMZ) via atomic layer deposition (ALD) and counting the resulting zinc atoms with high sensitivity-low energy ion scattering (HS-LEIS). A bare, clean native oxide – terminated silicon wafer has 3.66 OH/nm2, which agrees with density functional theory (DFT) calculations from the literature. After TDMAMS modification of native oxide-terminated silicon, the number of surface silanols decreases by a factor of ca. 10 (to 0.31 OH/nm2). Intermediate surface testing (IST) was used to characterize the surface activities of functionalized capillaries. It suggested a significant deactivation/passivation of the capillary with some surface silanols remaining; the modified capillary shows significant deactivation compared to the native/unmodified fused silica tubing. We believe that this methodology for determining the number of residual silanols on silanized fused silica will be enabling for chromatography.

The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air.

MXenes, a family of 2D transition-metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes' thermal stability can enable deeper insights into their structure-property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti3 C2 Tx , Mo2 TiC2 Tx , and Ti2 CTx ) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near-IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti3 C2 Tx and Ti2 CTx , respectively, have the highest and lowest thermal stability, indicating the role of transition-metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure-property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials.

Tunable Encapsulation and Doping of Monolayer MoS2 by In Situ Probing of Excitonic Properties During Atomic Layer Deposition

AbstractHere, it is shown that in situ spectroscopic ellipsometry (SE) is a powerful method for probing the effects of reactant adsorption and film formation on the excitonic properties of 2D materials during atomic layer deposition (ALD), thus allowing optimization of both film growth and opto(electronic) characteristics in real time. Facilitated by in situ SE during ALD on monolayer MoS2, a low temperature (40 °C) process for encapsulation of the 2D material with a nanometer‐thin alumina (AlOx) layer is investigated, which results in a 2D/3D interface governed by van der Waals interactions rather than chemical bonding. Charge transfer doping of MoS2 by AlOx is found to be an interfacial phenomenon that initiates from the earliest stages of film formation, but saturates upon deposition of a closed layer. However, the lack of chemical binding interactions at the 2D/3D interface enables physical removal of the AlOx that results in a reversal of the charge transfer doping effect. Overall, it is demonstrated that in situ SE of 2D materials during ALD can precisely probe the impact of film formation on sensitive optoelectronic characteristics of 2D materials, which is of key importance in the development of integrated 2D/3D systems.

In Situ and Ex Situ diagnostics of ALD-Ultrathin Lithium-Ion Films

All solid-state ion layers has an ever-growing interest in order to accomplish on-chip energy storage devices. As such, lithium-ion synaptic transistor to build artificial neural networks, integrated area-enhanced capacitors and lithium-ion microbatteries for energy supply are suitable candidates fulfilling energy-efficient devices. Atomic layer deposition (ALD) is one of the most promising techniques for manufacturing ultrathin layers for lithium-ion devices with exceptional capabilities to enable conformal growth at atomic scale. In this area, in situ diagnostics has become of vital importance for the growth process understanding especially on view of the air-sensitivity character of lithium ultra-thin layers.The purpose of this work is to present an in situ spectroscopic ellipsometry (SE) study of ALD thin films used on the fabrication of lithium-ion devices. We will consider several aspects of ALD ultrathin films growth, such as film thickness, saturation curves and initial films nucleation. We shall compare in situ SE results with those of ex situ diagnostics such as XRR (x-ray reflectometry) and XPS (x-ray photoelectron spectroscopy).We focus on the characterization of 10-100 nm thin lithium phosphate electrolyte with and without nitrogen doping deposited at ALD temperature range 300-350°C using LiHMDS-TMPO and LiHMDS-DEPA precursors, respectively. On the other hand, the realization of silicon integrated lithium ionic devices requires the implementation of essential barrier films to prevent Li-ion into the Si substrate. We will explore here the ability of titanium nitride depositions to act as lithium-ion diffusion barriers. TiN deposition (10-20 nm thin) by ALD process (350°C-400°C) uses TiCl4 and NH3 precursors.First, we investigate the growth process of LiPO, LiPON and TiN intrinsic layers deposited using thermal ALD. Secondly, we apply in situ and ex situ diagnostics on exemplary LiPO/LiPON deposited on top of the 10 nm ALD TiN film (Figure 1: Monitoring of LiPON thickness vs deposition time using in-situ Ellipsometry). The film thickness deduced from in situ SE measurements was corroborated by thickness measurements with XRR technique (Figure 2: Evolution of LiPON thickness vs ALD cycles using Ellipsometry and SE characterization techniques). Ex situ SE was used most often for WiW uniformity evaluating (Figure 3: 8” Wafer thickness mapping of ALD LiPON layers (a: 10nm TiN film; b: 17nm LiPON film). The preliminary results underline the fact that ALD growth rate and nucleation of lithium electrolyte and titanium nitride obviously depends on the ALD process parameters (temperature, pulse and purge times, number of cycles) but on the substrate nature (silicon, thermal oxide, and metallic). In situ and ex situ diagnostics highlight that initial film growth of lithium electrolytes and TiN layers strongly depends on the precursors kind used in the ALD process and the substrate material employed. Figure 1

Thermal atomic layer etching of silicon nitride using an oxidation and “conversion etch” mechanism

Thermal atomic layer etching (ALE) of silicon nitride was achieved using sequential exposures of oxygen (O2) or ozone (O3), hydrofluoric acid (HF), and trimethylaluminum [TMA, Al(CH3)3]. Thermal Si3N4 ALE will be useful to etch Si3N4 in semiconductor, optoelectronic, and MEMS devices. Thermal Si3N4 ALE was performed with Si3N4 thin films deposited on silicon wafers using low pressure chemical vapor deposition. In situ spectroscopic ellipsometry (SE) was employed to monitor the changes in the Si3N4 film thickness as well as the SiO2 layer thickness. The SE results at 290 °C yielded an Si3N4 etch rate of 0.25 Å/cycle with an O2-HF-TMA reactant sequence using partial pressures of 250, 0.65, and 1.2 Torr for O2, HF, and TMA, respectively. The O2, HF, and TMA reactants were held statically at the indicated partial pressures for 10, 5, and 5 s, respectively. Larger etch rates were observed using O3 instead of O2 as the oxidant. A higher Si3N4 etch rate of 0.47 Å/cycle was measured at 290 °C using an O3-HF-TMA reactant sequence at the same partial pressures and static exposure times as the O2-HF-TMA sequence. The Si3N4 etch rate was observed to decrease at lower temperatures. An Si3N4 etch rate of 0.07 Å/cycle was measured at the lowest temperature of 210 °C using an O3-HF-TMA reactant sequence. The Si3N4 surface roughness was reduced after Si3N4 ALE. The SiO2 layer on Si3N4 could be removed using sequential HF and TMA exposures. These sequential HF and TMA exposures could also very slowly etch the Si3N4 substrate. The Si3N4 etch rate was dependent on the reaction sequence. When an O3-TMA-HF sequence was employed with reactant partial pressures of 250, 0.65, and 1.2 Torr for O3, HF, and TMA, respectively, the Si3N4 etch rate was 0.20 Å/cycle at 290 °C. Thermal Si3N4 ALE adds to the growing list of materials that can be etched with atomic layer control using thermal chemistry.

Effect of HF Pressure on Thermal Al2O3Atomic Layer Etch Rates and Al2O3Fluorination

Thermal Al2O3 atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF can be employed as the fluorination reactant, and Al(CH3)3 can be utilized as the metal precursor for ligand exchange. This study explored the effect of HF pressure on the Al2O3 etch rates and Al2O3 fluorination. Different HF pressures ranging from 0.07 to 9.0 Torr were employed for Al2O3 fluorination. Using ex situ spectroscopic ellipsometry (SE) measurements, the Al2O3 etch rates increased with HF pressures and then leveled out at the highest HF pressures. Al2O3 etch rates of 0.6, 1.6, 2.0, 2.4, and 2.5 A/cycle were obtained at 300 °C for HF pressures of 0.17, 0.5, 1.0, 5.0, and 8.0 Torr, respectively. The thicknesses of the corresponding fluoride layers were also measured using X-ray photoelectron spectroscopy (XPS). Assuming an Al2OF4 layer on the Al2O3 surface, the fluoride thicknesses increased with HF pressures and reached saturation values at the highest HF pressures. Fluori...

Opto-chemical control through thermal treatment of plasma enhanced atomic layer deposited ZnO: An in situ study

Properties and performance of materials are closely connected. In order to obtain piezoelectric and lasing optical quality, ZnO has to be free of defects and highly crystalline. Instead, conductivity depends upon such defects, making it not trivial to aim at a specific set of properties in a single step. In this regard, we studied in situ the effect of temperature as an additional knob to finely control such properties. In this contribution, plasma enhanced atomic layer deposited (PE-ALD) zinc oxide (ZnO) layers, deposited between 25 °C and 250 °C, were studied in situ during annealing in air, and the opto-chemical and structural characteristics of the oxides were followed as a function of temperature. In situ spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted to identify temperature windows where major structural and optical changes in the material occurred. Two temperature regions were identified for the effusion of adsorbed gases and minor structural rearrangements (180–280 °C) and for the growth/coalescence of ZnO crystals and its densification (360–500 °C). The results were corroborated by ex situ SE, XRD, UV–Vis and X-ray photoelectron spectroscopy. The in situ study revealed differences among the ZnO layers deposited at different temperatures, giving additional insights on the material properties deposited by PE-ALD.

Nucleation Enhancement and Area-Selective Atomic Layer Deposition of Ruthenium Using RuO4 and H2 Gas

Inherent substrate selectivity is reported for the thermal RuO4 (ToRuS)/H2 gas atomic layer deposition (ALD) process on H-terminated Si (Si–H) versus SiO2. In situ spectroscopic ellipsometry (SE) detected Ru growth from the first cycle on blanket Si–H, whereas on blanket SiO2, 60 cycles were needed to detect growth. Area-selective growth was evaluated on a patterned substrate with 1–10 μm wide Si–H lines separated by 10 μm wide SiO2 regions. Ex situ planar scanning electron microscopy and cross-sectional high-resolution transmission electron microscopy measurements showed that a smooth, continuous Ru film of 4.5 nm could be deposited on Si–H, with no Ru detected on SiO2. The proposed mechanism behind the inherent substrate selectivity is the oxidation of the Si–H surface by RuO4, which was confirmed by in vacuo X-ray photoelectron spectroscopy (XPS) experiments. A methodology to enhance the nucleation of the RuO4/H2 gas process on oxide substrates is also reported. In situ SE and in vacuo XPS experiments ...

Spectroscopic ellipsometry characterization of multilayer optical coatings

The performance of an optical coating stack depends on thickness and refractive index of each layer. In situ spectroscopic ellipsometry (SE) can track coating properties in real-time, but ex situ SE characterization is limited by the large number of unknown sample properties. We review various SE characterization strategies for multilayer structures using 37-layer alternating high-low index stacks of Ta2O5 and SiO2. A “tooling factor” for each coating material is developed to inform how the actual layer thicknesses compare to nominal specifications.We also introduce tests for sources of error that can occur during multilayer processing. Process drift may produce a gradual change in film properties, such as a slight increase or decrease in coating thickness or optical constants. This is modeled by adding a gradient to each tooling factor. User-error is also considered, where incorrect parameter entry produces an unintentional stack design. An erroneous layer is identified by systematically testing each layer. A “blind” test was performed to determine SE sensitivity to such errors, where a single layer was intentionally altered within a 37-layer stack, and systematic testing was able to resolve the altered layer thickness and position in the stack.

Comparision of in situ spectroscopic ellipsometer and ex situ x-ray photoelectron spectroscopy depth profiling analysis of HfO2/Hf/Si multilayer structure

A HfO2 film was grown by RF magnetron sputtering technique on a Si substrate Using in situ Spectroscopic Ellipsometry (SE), the film thickness and refractive index were examined as a function of deposition time. Ex situ x-ray Photoelectron Spectroscopy (XPS) was used in depth profile mode to determine the phase evolution of HfO2/Hf/Si multilayer structure after the growth process. The chemical composition and the crystal structure of the film were investigated by Fourier Transform Infrared (FTIR) spectroscopic measurements and x-ray Diffraction in Grazing Incidence (GI-XRD) mode, respectively. The results showed that the film was grown in the form of HfO2 film. According to SE analysis, reactive deposition of HfO2 directly on Hf/Si results to SiO2 interface of about 2 nm. The final HfO2 films thickness is 5.4 nm. After a certain period of time, the XPS depth profile revealed that the film was in the form of Hf-rich Hf silicate with SiO2 interfacial layer. In reference to XPS quantification analysis from top to bottom of film, the atomic concentration of Hf element reduces from 19.35% to 7.13%, whereas Si concentration increases from 22.99% to 74.89%. The phase change of HfO2 film with time is discussed in details.

Inherent Area-Selective Growth and Nucleation Enhancement during Ru ALD Using the RuO4-Precursor and H2-Gas

Ruthenium is a candidate to replace copper in future sub-10 nm interconnects. At these dimensions the resistivity of Ru lines is expected to be lower compared to Cu due to the lower sensitivity to size effects.1 In addition, it is likely that Ru interconnects won’t require a diffusion barrier, and will show a better electromigration performance.2 At feature sizes below 10 nm it will be difficult to align subsequent lithography steps, and the conformality of the deposition method is increasingly important, such that area selective atomic layer deposition (ALD) of ruthenium is of high interest.3 We first report inherent area selective ALD of Ru on H-terminated Si (Si-H) versus SiO2, using the thermal RuO4 (ToRuSTM)/ H2-gas ALD process.4 In situ spectroscopic ellipsometry (SE) on blanket substrates shows that Ru growth initiation occurs from the first cycle on Si-H, while on SiO2 the growth is delayed, resulting in a substrate selectivity window of ~ 70 cycles. Area selective Ru ALD was evaluated using a patterned substrate of 1-10 µm wide Si-H lines separated by 10 µm wide SiO2 regions, and exposing it to 20 cycles of the RuO4 / H2-gas ALD process. Ex situ scanning electron microscopy (SEM) and cross section high resolution transmission electron microscopy (HRTEM) measurements show that a 4.5 nm Ru film could be deposited on the Si-H, with no Ru detected on the SiO2. In vacuo X-ray photoelectron spectroscopy (XPS) experiments showed that exposure of Si-H to a single RuO4 pulse leads to the oxidation of the Si surface, together with the deposition of RuO2. On SiO2 however, the surface is already oxidized, and in vacuo XPS shows that for the same exposure to RuO4 no Ru is deposited on the surface. Therefore, we propose that the mechanism behind the inherent substrate selectivity is the oxidation of the Si-H surface by RuO4. Secondly, we report for a methodology to enhance the nucleation of the RuO4 / H2-gas process on oxide substrates. In vacuo XPS and in situ SE experiments show that a single exposure of SiO2 to trimethylaluminum (TMA) makes the surface reactive towards RuO4, which allows for Ru growth initiation from the first cycle. We propose that this is due to the combustion of surface CH3-groups by RuO4. As TMA is known to be reactive towards many oxide substrates, this methodology presents a way to achieve Ru metallization of virtually any surface. [1] S. Dutta et al. IEEE Elec. Dev. Lett. 2017, 38, 949. [2] O. V. Pedreira et al . 2017 IEEE IRPS , Monterey, CA, 6B-2.1. [3] P. C. Lemaire et al. J. Chem. Phys. 2017, 146, 052811. [4] M. M. Minjauw et al. J. Mater. Chem. C. 2015, 3, 132.

WO3 and W Thermal Atomic Layer Etching Using "Conversion-Fluorination" and "Oxidation-Conversion-Fluorination" Mechanisms.

The thermal atomic layer etching (ALE) of WO3 and W was demonstrated with new "conversion-fluorination" and "oxidation-conversion-fluorination" etching mechanisms. Both of these mechanisms are based on sequential, self-limiting reactions. WO3 ALE was achieved by a "conversion-fluorination" mechanism using an AB exposure sequence with boron trichloride (BCl3) and hydrogen fluoride (HF). BCl3 converts the WO3 surface to a B2O3 layer while forming volatile WOxCly products. Subsequently, HF spontaneously etches the B2O3 layer producing volatile BF3 and H2O products. In situ spectroscopic ellipsometry (SE) studies determined that the BCl3 and HF reactions were self-limiting versus exposure. The WO3 ALE etch rates increased with temperature from 0.55 Å/cycle at 128 °C to 4.19 Å/cycle at 207 °C. W served as an etch stop because BCl3 and HF could not etch the underlying W film. W ALE was performed using a three-step "oxidation-conversion-fluorination" mechanism. In this ABC exposure sequence, the W surface is first oxidized to a WO3 layer using O2/O3. Subsequently, the WO3 layer is etched with BCl3 and HF. SE could simultaneously monitor the W and WO3 thicknesses and conversion of W to WO3. SE measurements showed that the W film thickness decreased linearly with number of ABC reaction cycles. W ALE was shown to be self-limiting with respect to each reaction in the ABC process. The etch rate for W ALE was ∼2.5 Å/cycle at 207 °C. An oxide thickness of ∼20 Å remained after W ALE, but could be removed by sequential BCl3 and HF exposures without affecting the W layer. These new etching mechanisms will enable the thermal ALE of a variety of additional metal materials including those that have volatile metal fluorides.

Investigation of polymer–solvent interactions in poly(styrene sulfonate) thin films

ABSTRACTThe swelling behavior of acid form poly(styrene sulfonate) (PSS‐H) thin films were investigated using in situ spectroscopic ellipsometry (SE) to probe the polymer–solvent interactions of ion‐containing polymers under interfacial confinement. The interaction parameter (χ), related to the polymer and solvent solubility parameters in the Flory–Huggins theory, describes the polymer‐solvent compatibility. In situ SE was used to measure the degree of polymer swelling in various solvent vapor environments, to determine χ for the solvent‐PSS‐H system. The calculated solubility parameter of 40–44 MPa1/2 for PSS‐H was determined through measured χ values in water, methanol, and formamide environments at a solvent vapor activity of 0.95. Flory–Huggins theory was applied to describe the thickness‐dependent swelling of PSS‐H and to quantify the water‐PSS‐H interactions. Confinement had a significant influence on polymer swelling at low water vapor activities expressed as an increased χ between the water and polymer with decreasing film thickness. As the volume fraction of water approached ∼0.3, the measured χ value was ∼0.65, indicating the water interacted with the polymer in a similar manner, regardless of thicknesses. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1365–1372

Thermal Atomic Layer Etching of SiO2 by a "Conversion-Etch" Mechanism Using Sequential Reactions of Trimethylaluminum and Hydrogen Fluoride.

The thermal atomic layer etching (ALE) of SiO2 was performed using sequential reactions of trimethylaluminum (TMA) and hydrogen fluoride (HF) at 300 °C. Ex situ X-ray reflectivity (XRR) measurements revealed that the etch rate during SiO2 ALE was dependent on reactant pressure. SiO2 etch rates of 0.027, 0.15, 0.20, and 0.31 Å/cycle were observed at static reactant pressures of 0.1, 0.5, 1.0, and 4.0 Torr, respectively. Ex situ spectroscopic ellipsometry (SE) measurements were in agreement with these etch rates versus reactant pressure. In situ Fourier transform infrared (FTIR) spectroscopy investigations also observed SiO2 etching that was dependent on the static reactant pressures. The FTIR studies showed that the TMA and HF reactions displayed self-limiting behavior at the various reactant pressures. In addition, the FTIR spectra revealed that an Al2O3/aluminosilicate intermediate was present after the TMA exposures. The Al2O3/aluminosilicate intermediate is consistent with a "conversion-etch" mechanism where SiO2 is converted by TMA to Al2O3, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO2 + 4Al(CH3)3 → 2Al2O3 + 3Si(CH3)4. Ex situ X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction of silicon species after TMA exposures. Following the conversion reactions, HF can fluorinate the Al2O3 and aluminosilicates to species such as AlF3 and SiOxFy. Subsequently, TMA can remove the AlF3 and SiOxFy species by ligand-exchange transmetalation reactions and then convert additional SiO2 to Al2O3. The pressure-dependent conversion reaction of SiO2 to Al2O3 and aluminosilicates by TMA is critical for thermal SiO2 ALE. The "conversion-etch" mechanism may also provide pathways for additional materials to be etched using thermal ALE.

Spectroscopic ellipsometry for analysis of polycrystalline thin-film photovoltaic devices and prediction of external quantum efficiency

Complete polycrystalline thin-film photovoltaic (PV) devices employing CuIn1−xGaxSe2/CdS and CdS/CdTe heterojunctions have been studied by ex situ spectroscopic ellipsometry (SE). In this study, layer thicknesses have been extracted along with photon energy independent parameters such as compositions that describe the dielectric function spectra ε(E) of the individual layers. For accurate ex situ SE analysis of these PV devices, a database of ε(E) spectra is required for all thin film component materials used in each of the two absorber technologies. When possible, database measurements are performed by applying SE in situ immediately after deposition of the thin film materials and after cooling to room temperature in order to avoid oxidation and surface contamination. Determination of ε(E) from the resulting in situ SE data requires structural information that can be obtained from analysis of SE data acquired in real time during the deposition process. From the results of ex situ analysis of the complete CuIn1−xGaxSe2 (CIGS) and CdTe PV devices, the deduced layer thicknesses in combination with the parameters describing ε(E) can be employed in further studies that simulate the external quantum efficiency (EQE) spectra of the devices. These simulations have been performed here by assuming that all electron-hole pairs generated within the active layers, i.e. layers incorporating a dominant absorber component (either CIGS or CdTe), are separated and collected. The active layers may include not only the bulk absorber but also window and back contact interface layers, and individual current contributions from these layers have been determined in the simulations. In addition, the ex situ SE analysis results enable calculation of the absorbance spectra for the inactive layers and the overall reflectance spectra, which lead to quantification of all optical losses in terms of a current density deficit. Mapping SE can be performed given the high speed of multichannel ellipsometers employing array detection, and the resulting EQE simulation capability has wide applications in predicting large area PV module output. The ultimate goal is an on-line capability that enables prediction of PV sub-cell current output as early as possible in the production process.

Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry

We report the design and testing of a small, high vacuum chamber that allows real-time, in situ spectroscopic ellipsometry (SE) measurements; the chamber was designed to be easily inserted within the arms of a commercial ellipsometer. As a test application, we investigated the temperature-induced solid-state dewetting of thin (20 to 8nm) Au layers on Si wafers. In situ SE measurements acquired in real time during the heating of the samples reveal features that can be related to the birth of a localized surface plasmon resonance (LSPR), and demonstrate the presence of a temperature threshold for the solid-state dewetting.

Thermal atomic layer etching of crystalline aluminum nitride using sequential, self-limiting hydrogen fluoride and Sn(acac)2 reactions and enhancement by H2 and Ar plasmas

Thermal atomic layer etching (ALE) of crystalline aluminum nitride (AlN) films was demonstrated using sequential, self-limiting reactions with hydrogen fluoride (HF) and tin(II) acetylacetonate [Sn(acac)2] as the reactants. Film thicknesses were monitored versus number of ALE reaction cycles at 275 °C using in situ spectroscopic ellipsometry (SE). A low etch rate of ∼0.07 Å/cycle was measured during etching of the first 40 Å of the film. This small etch rate corresponded with the AlOxNy layer on the AlN film. The etch rate then increased to ∼0.36 Å/cycle for the pure AlN films. In situ SE experiments established the HF and Sn(acac)2 exposures that were necessary for self-limiting surface reactions. In the proposed reaction mechanism for thermal AlN ALE, HF fluorinates the AlN film and produces an AlF3 layer on the surface. The metal precursor, Sn(acac)2, then accepts fluorine from the AlF3 layer and transfers an acac ligand to the AlF3 layer in a ligand-exchange reaction. The possible volatile etch products are SnF(acac) and either Al(acac)3 or AlF(acac)2. Adding a H2 plasma exposure after each Sn(acac)2 exposure dramatically increased the AlN etch rate from 0.36 to 1.96 Å/cycle. This enhanced etch rate is believed to result from the ability of the H2 plasma to remove acac surface species that may limit the AlN etch rate. The active agent from the H2 plasma is either hydrogen radicals or radiation. Adding an Ar plasma exposure after each Sn(acac)2 exposure increased the AlN etch rate from 0.36 to 0.66 Å/cycle. This enhanced etch rate is attributed to either ions or radiation from the Ar plasma that may also lead to the desorption of acac surface species.