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Delayed atomic layer deposition (ALD) of ZnO, i.e., area selective (AS)-ALD, was successfully achieved on silicon wafers (SiSiO2) terminated with tris(dimethylamino)methylsilane (TDMAMS). This resist molecule was deposited in a home-built, near-atmospheric pressure, flow-through, gas-phase reactor. TDMAMS had previously been shown to react with SiSiO2 in a single cycle/reaction and to drastically reduce the number of silanols that remain at the surface. ZnO was deposited in a commercial ALD system using dimethylzinc (DMZ) as the zinc precursor and H2O as the coreactant. Deposition of TDMAMS was confirmed by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and wetting. ALD of ZnO, including its selectivity on TDMAMS-terminated SiSiO2 (SiSiO2TDMAMS), was confirmed by in situ multi-wavelength ellipsometry, ex situ SE, XPS, and/or high-sensitivity/low-energy ion scattering (HS-LEIS). The thermal stability of the TDMAMS resist layer, which is an important parameter for AS-ALD, was investigated by heating SiSiO2TDMAMS in air and nitrogen at 330 °C. ALD of ZnO takes place more readily on SiSiO2TDMAMS heated in the air than in N2, suggesting greater damage to the surface heated in the air. To better understand the in situ ALD of ZnO on SiSiO2TDMAMS and modified (thermally stressed) forms of it, the ellipsometry results were plotted as the normalized growth per cycle. Even one short pulse of TDMAMS effectively passivates SiSiO2. TDMAMS can be a useful, small-molecule inhibitor of ALD of ZnO on SiSiO2 surfaces.

Delayed atomic layer deposition (ALD) of ZnO, i.e., area selective (AS)-ALD, was successfully achieved on silicon wafers (Si/SiO2) terminated with tris(dimethylamino)methylsilane (TDMAMS). This resist molecule was deposited in a home-built, near atmospheric pressure, flow-through , gas-phase reactor. TDMAMS has been shown to react with Si/SiO2 in a single cycle/reaction and to drastically reduce the number of silanols that remain at the surface. ZnO was deposited in a commercial ALD system using dimethylzinc (DMZ) as the zinc precursor and H2O as the coreactant. Deposition of TDMAMS was confirmed by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and wetting. ALD of ZnO, including its selectivity on TDMAMS – terminated Si/SiO2 (Si/SiO2/TDMAMS), was confirmed by in situ ellipsometry, ex situ SE, XPS, and/or high sensitivity – low energy ion scattering (HS-LEIS). The thermal stability of the TDMAMS resist layer, which is an important parameter for AS-ALD, was investigated by heating Si/SiO2/TDMAMS in air and nitrogen at 330 ºC. ALD of ZnO takes place more readily on Si/SiO2/TDMAMS heated in the air than in N2, suggesting greater damage to the surface heated in the air. To better understand the in situ ALD of ZnO, the data were also plotted as the normalized growth per cycle and to show the selectivity. Even one, short pulse of TDMAMS effectively passivates Si/SiO2. TDMAMS can be an effective small molecule inhibitor of ALD of ZnO on Si/SiO2 surfaces.

Thermal atomic layer etching (ALE) of cobalt was developed using sulfuryl chloride (SO2Cl2) for chlorination and either tetramethylethylenediamine (TMEDA) or trimethylphosphine (PMe3) for ligand addition. In situ quartz crystal microbalance (QCM) measurements were used to monitor the thermal ALE of cobalt using the SO2Cl2/TMEDA and SO2Cl2/PMe3 processes. For every SO2Cl2 exposure, there was a mass gain during chlorination. For every TMEDA or PMe3 exposure, there was a mass loss during ligand addition. The result was a net removal of cobalt during each chlorination/ligand-addition reaction cycle. Average etch rates determined from QCM measurements for the SO2Cl2/TMEDA process at 175, 200, 225, 250, 275, and 300 °C were 0.62 ± 0.41, 1.35 ± 0.64, 2.31 ± 0.91, 6.43 ± 1.31, 10.56 ± 2.94, and 7.62 ± 4.87 Å/cycle, respectively. These etch rates were corroborated using x-ray reflectivity (XRR) studies on cobalt thin films on silicon coupons. Quadrupole mass spectroscopy analysis also revealed that the cobalt etch product from TMEDA exposures on CoCl2 powder was CoCl2(TMEDA). The SO2Cl2/TMEDA process could remove the surface chloride layer formed by each SO2Cl2 exposure with one TMEDA exposure. In contrast, the SO2Cl2/PMe3 process required 20–40 individual PMe3 exposures to remove the surface chloride layer formed from each SO2Cl2 exposure at 130–200 °C. An increasing number of PMe3 exposures were needed as the temperature decreased below 130 °C. The etch rates for the SO2Cl2/PMe3 process with multiple PMe3 exposures were 2–4 Å/cycle as determined by the QCM and XRR studies. For both the SO2Cl2/TMEDA and SO2Cl2/PMe3 processes, the etch rate was determined by the amount of CoCl2 created during the SO2Cl2 exposure. Thicker surface CoCl2 layers from larger SO2Cl2 exposures resulted in higher Co etch rates that could exceed one crystalline unit cell length. Atomic force microscopy measurements determined that the cobalt surface roughness decreased after Co ALE with the SO2Cl2/TMEDA process. In contrast, the cobalt surface roughness increased after Co ALE with the SO2Cl2/PMe3 process. The chlorination and ligand-addition mechanism should be generally applicable for metal ALE for metals that form stable chlorides.

The thermal atomic layer etching (ALE) of nickel was demonstrated using sequential chlorination and ligand-addition reactions. Nickel chlorination was achieved using SO2Cl2 (sulfuryl chloride) as t...

Funding Information: The FTIR studies were funded by Intel through a member specific research grant through the Semiconductor Research Corporation (SRC). Support for the new QMS reactor and the QMS investigations was provided by Lam Research. S.K.N. and S.D.E. acknowledge the Irish Centre for High-End Computing (Project Code No. tiche077c) and the Science Foundation Ireland funded computing cluster at Tyndall for computer time. Publisher Copyright: © 2022 Author(s). The spontaneous etching of boron oxide (B2O3) by hydrogen fluoride (HF) gas is important during thermal atomic layer etching after BCl3 converts the surface of various metal oxides to a B2O3 layer. In this study, the chemical vapor etching (CVE) of B2O3 by HF was experimentally monitored using Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS). The spontaneous etching of B2O3 by HF gas was also analyzed using density functional theory (DFT). B2O3 films were grown using B2O3 atomic layer deposition with BCl3 and H2O as the reactants at 40 °C. FTIR spectroscopy then observed the CVE of B2O3 by HF at 150 °C. B2O3 etching was monitored by the loss of absorbance for B-O stretching vibration in B2O3 films. FTIR spectroscopy studies also observed B-F stretching vibrations from BFx species on the B2O3 surface after HF exposures. In addition, the QMS analysis was able to identify the etch products during the spontaneous etching of B2O3 by HF gas at 150 °C. The QMS studies observed the main volatile etch products as BF3, BF2(OH), and H2O. Additional volatile etch products were also detected including B3O3F3 and other boroxine ring compounds. The DFT predictions were consistent with the spontaneous etching of B2O3 by HF gas. DFT confirmed that CVE was likely because the energetics of the spontaneous etching reaction B2O3(s) + 6HF(g) → 2BF3(g) + 3H2O(g) were more favorable than the self-limiting reaction B2O3(s) + 6HF(g) → 2BF3(s) + 3H2O(g). The spontaneous etching of B2O3 was predicted at temperatures above-163 °C for an HF reactant pressure of 0.2 Torr and BF3 and H2O product pressure of 0.01 Torr.

A ZrO2 monolayer was demonstrated as a removable etch stop layer (ESL) for thermal Al2O3 atomic layer etching (ALE) using HF and Al(CH3)3 (trimethylaluminum (TMA)) as the reactants. The ZrO2 ESL wa...

Gallium oxide (Ga2O3) is a transparent semiconducting oxide with a large bandgap that has applications for power electronics and optoelectronics. Ga2O3 device fabrication requires etching for many ...

ConspectusAtomic layer control of semiconductor processing is needed as critical dimensions are progressively reduced below the 10 nm scale. Atomic layer deposition (ALD) methods are meeting this challenge and produce conformal thin film growth on high aspect ratio features. Atomic layer etching (ALE) techniques are also required that can remove material with atomic layer precision. ALE processes are defined using sequential, self-limiting reactions based on surface modification and volatile release. Plasma ALE methods employ energetic ion or neutral species to release the modified material anisotropically using sputtering. In contrast, thermal ALE processes utilize gas species to release the modified material isotropically using thermal reactions. Thermal ALE can be viewed as the "reverse of ALD".There are a number of mechanisms for thermal ALE that have developed over the last five years. This Account will first examine the fluorination and ligand-exchange mechanism for thermal ALE. This mechanism is applicable for many metal oxide and metal nitride materials. Subsequently, the "conversion etch" mechanisms will be explored that are derived from the conversion of the surface of the substrate to a new material. The "conversion etch" mechanisms are needed when the initial material does not have a viable etching pathway via fluorination and ligand-exchange or when the material has a volatile fluoride. The thermal ALE mechanisms founded on either oxidation or halogenation of the initial substrate will then be examined with an emphasis on metal thermal ALE. Lastly, thermal ALE mechanisms will be considered that are based on self-limiting surface ligands or temperature modulation mechanisms. These various mechanisms offer a wide range of pathways to remove material isotropically with atomic layer control.Thermal ALE will be required to fabricate advanced semiconductor devices. This fabrication will increasingly occur beyond the limits of lithography and will extend into the third dimension. The situation is like Manhattan during the advent of skyscrapers. When there was no more room on the ground, building started to move to the third dimension. Three-dimensional devices require a sequential series of deposition and etching steps to build the skyscraper structures. Some etching needs to be vertical and anisotropic to make the elevator shafts. Other etching needs to be horizontal and isotropic to form the hallways. The mechanisms of thermal ALE will be critical for the definition of isotropic ALE processes.Reaching beyond the limits of lithography will also increase the need for maskless processing. The mechanisms of thermal ALE lead to strategies for selective etching of one material in the presence of many materials. In addition, area-selective deposition can benefit from the ability of thermal ALE to enhance deposition on the desired growth surfaces by removing deposition from other surrounding surfaces. Looking ahead, thermal ALE will continue to provide unique capabilities and will grow in importance as a nanofabrication processing technique.

The thermal atomic layer etching (ALE) of VO2 was demonstrated using sequential exposures of BCl3 and SF4. The VO2 etch rate measured by quartz crystal microbalance investigations at 250 °C was 2.3 Å/cycle. The mass losses during individual BCl3 and SF4 reactions were nearly self-limiting versus BCl3 and SF4 exposures. The VO2 etch rates were also dependent on temperature and varied from 0.05 Å/cycle at 150 °C to 2.3 Å/cycle at 250 °C. Fourier transform infrared (FTIR) spectroscopy studies observed VO2 etching by monitoring the decrease in absorbance from V—O stretching vibrations in the VO2 film. The FTIR spectra during the initial BCl3 exposures on the VO2 film observed the growth of absorbance from B—O stretching vibrations from B2O3 and the concurrent loss of V=O vibrational features. These changes were consistent with BCl3 converting VO2 to B2O3. The FTIR difference spectra during subsequent SF4 and BCl3 reactions also observed the growth and loss of absorbance features that were attributed to F3V=O and V—F stretching vibrations, respectively. These changes indicate that SF4 fluorinates VO2 to form a VOF3 surface layer and then BCl3 undergoes ligand-exchange with VOF3 to volatilize the VOF3 surface layer as VOCl3. There was also evidence for conversion of VO2 to B2O3 during BCl3 exposures and then removal of B2O3 by SF4 exposures. In addition, quadrupole mass spectrometry (QMS) measurements observed that the SF4 exposures produced ion intensities for SOxFyClz products in oxidation states greater than 4+. These SOxFyClz products indicate that SF4 is being oxidized and acting as a deoxyfluorination reactant. Concurrently, the QMS analysis also monitored ion intensity for S8+, S7+, S6+, S5+, and S4+. These S8 electron impact ionization products argue that SF4 oxidation occurs concurrently with SF4 reduction. The QMS also observed ion intensities corresponding to VCl4+ and VOCl3+. The presence of VOCl3+ indicates that the oxidation state of vanadium has increased to 5+ in some of the volatile etch products. The QMS also detected trichloroboroxin (B3O3Cl3) during BCl3 exposures. B3O3Cl3 is a known etch product of B2O3 during BCl3 exposures. BCl3 can convert VO2 to B2O3 and then proceed to etch the converted B2O3. Thermal VO2 ALE using BCl3 and SF4 reveals the rich complexity of surface etching reactions that can proceed by multiple pathways including conversion, ligand-exchange, and oxidation state changes.

Thermal etching of metal oxides can occur spontaneously or by sequential surface reactions during atomic layer etching (ALE). This thermal etching does not require the use of plasmas or energetic species to remove material. Thermal etching can be studied by measuring the decrease of film thickness. However, the mechanisms of thermal etching still require the identification of volatile etch products. To address this issue, we have used quadrupole mass spectrometry (QMS) to detect volatile etch products during thermal etching. A custom reactor was built to measure the etch products with high sensitivity. This reactor uses molecular beam techniques to direct the etch products with line-of-sight to the QMS ionizer. Figure 1 shows a schematic of the QMS reactor. Nested inlet lines allow two reactants to be transported to the sample holder. This talk will explore the thermal etching of metal oxides using three different approaches: (1) chlorination followed by ligand addition; (2) exposure to acetylacetone; and (3) fluorination followed by ligand exchange or conversion. For the first approach, chlorination followed by ligand addition will be utilized for CoO thermal ALE with SO2Cl2 as the chlorination reactant and tetramethylethylenediamine (TMEDA) as the ligand-addition reactant at 250°C. The proposed mechanism for CoO thermal ALE is shown in Figure 2. SO2Cl2 converts the CoO surface to a CoCl2 layer and yields SO3. Then the TMEDA adds to CoCl2 and forms volatile CoCl2(TMEDA) etch products. QMS studies were able to confirm the formation of CoCl2(TMEDA) etch products. Figure 3 displays the CoCl2(TMEDA) etch products observed for the 5th CoO ALE cycle during the TMEDA exposure at 250°C. The natural abundance of the Cl isotopes affirmed that CoCl2(TMEDA) was the volatile etch product. Additional time-resolved experiments were able to monitor the reactants and volatile etch products. Figure 4 displays the ion intensities of C3N8N+ (largest crack of TMEDA) and CoCl(TMEDA)+ (largest crack of CoCl2TMEDA) during the TMEDA exposure at 250°C. The self-limiting nature of the ligand-addition reaction is revealed by the decrease of the CoCl(TMEDA)+ ion intensity with time while the TMEDA ion intensity remains constant. Chlorination followed by ligand addition was also utilized for the thermal ALE of other first row transition metal oxides (Fe2O3, NiO, and ZnO). M(TMEDA)Cl2 was observed as the volatile etch product for M= Fe and Ni. Zn(TMEDA)Cl was observed for ZnO. For the second approach, acetylacetone (Hacac) and hexafluoroacetylacetone (Hhfac) have been known to spontaneously etch a variety of metal oxides including ZnO, CuOx and Fe2O3. In addition, some thermal ALE procedures have been developed using Hacac together with an oxidation reactant to clean the metal oxide surface after the Hacac exposure. This talk will highlight results for ZnO etching using sequential Hacac and O3 exposures and also consecutive Hacac exposures. Zn(acac)2 was observed by the QMS studies during the alternating Hacac and O3 exposures on ZnO throughout the Hacac exposures at 250°C. Additional experiments using Hacac exposures with no O3 exposures also observed the continuous yield of Zn(acac)2. These experiments provide evidence for the spontaneous etching of ZnO by Hacac according to the reaction: ZnO + 2Hacac -> Zn(acac)2 +H2O. This reaction is believed to proceed spontaneously through a ligand substitution/hydrogen transfer mechanism. Other metal oxides were also etched using sequential Hacac and O3 exposures including V2O5, Fe2O3, CoO and CuO at 250°C. M(acac)2 species were observed for M=Co, Fe and Cu. VO(acac)2 was observed for V2O5. Lastly, for the third approach, Al2O3 thermal ALE was explored using sequential BCl3 and HF exposures at 300°C. This thermal ALE process could occur by (A) conversion of Al2O3 to B2O3 by BCl3 followed by the spontaneous etching of B2O3 by HF or (B) fluorination of Al2O3 to AlF3 followed by the ligand exchange of AlCl3 with BCl3 to produce volatile AlCl3. QMS analysis revealed that initial exposures of BCl3 on Al2O3 led to a conversion reaction forming B2O3 and volatile AlCl3. Subsequently, the HF exposure on B2O3 released BF3 and H2O. After removing the B2O3 conversion layer, the HF proceeded to fluorinate Al2O3 to form a AlF3 layer and volatile H2O. Additional BCl3 exposures then underwent ligand exchange with the AlF3 layer to form BFCl2 and AlCl3. The BCl3 continued to convert the underlying Al2O3 to B2O3. Moreover, the BCl3 exposures were also observed to spontaneously etch the B2O3 conversion layer. QMS studies revealed that the reaction of BCl3 with the B2O3 conversion layer or initial B2O3 substrates produced B3O3Cl3, a boroxine ring species. BCl3 has the unusual ability to convert Al2O3 to B2O3 and then spontaneously etch the B2O3. Figure 1

Thermal atomic layer etching (ALE) is typically defined by sequential surface reactions. Thermal ALE can be viewed as the reverse of atomic layer deposition (ALD). Thermal ALE is usually performed with two steps: modification of the surface and removal of the modified surface. The modification can be oxidation, fluorination, or chlorination. The removal step is often a ligand-exchange reaction, creating stable volatile species that leave the surface, leading to a net removal of material. For example, aluminum oxide can be etched using thermal ALE with HF to fluorinate the surface and trimethylaluminum (TMA) to exchange methyl ligands for surface fluoride ligands, resulting in volatile Al(CH3)xFy species. Thermal ALE of metals is particularly challenging because the oxidation state of the metal must be changed to match the oxidation state of the volatile metal etch product. This oxidation is performed in the modification step of thermal ALE. After changing the oxidation state, the metal then produces a stable complex during the volatile release reaction. In this work, Ni and Co ALE are developed by modifying the metal via chlorination using SO2Cl2. Subsequently, the metal is removed by addition of an L ligand to the metal center to create a stable, volatile etch product, MClxLy (Figure 1). The L ligands explored in this work were PMe3 and tetramethyl ethylenediamine (TMEDA). The chlorination and ligand-addition approach for metal ALE is based on the Covalent Bond Classification (CBC) method. X ligands are one-electron donors like Cl and other halogens. L ligands are two-electron donors like PMe3. TMEDA is a bidentate ligand. One TMEDA ligand acts as two L ligands since TMEDA binds to the metal center through the lone electron pairs on the two nitrogen groups. According to the CBC method, nickel complexes typically have NiX2L3 or NiX2L2 configurations. Likewise, cobalt complexes are typically found in the CoX2L4, CoX2L2, or CoX3L3 configurations. The goal for Ni or Co ALE is to create these volatile metal compounds using sequential SO2Cl2 and L ligand surface reactions. Both Ni and Co ALE have been studied with in situ quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) to determine the etch rates. Quadrupole mass spectrometry (QMS) was also employed to determine the volatile metal-containing etch species. Using SO2Cl2 and PMe3, Ni etch rates varied from 0.1-3 Å/cycle at temperatures from 75-200 °C. QMS measurements also identified NiCl2(PMe3)2 (NiX2L2) as the stable volatile etch product. Cobalt ALE using SO2Cl2 and TMEDA displayed etch rates from 1-12 Å/cycle at temperatures from 175-300 °C. QMS measurements also detected CoCl2(TMEDA) (CoX2L2) as the stable volatile etch product. In addition, cobalt can be etched using SO2Cl2 and PMe3 from 70-200 °C, with etch rates between 2-4 Å/cycle. These two Co ALE pathways with TMEDA and PMe3 are complementary with TMEDA useful at higher temperatures and PMe3 useful at lower temperatures. Many other X and L ligands could be used for the halogenation and ligand-addition reactions during metal thermal ALE. The CBC method can be used as a guide to develop the X ligands for surface modification and the L ligands for volatile release. This approach should lead to various other metal ALE processes. Selective metal thermal ALE should also be possible using different X and L ligands. Figure 1

The thermal atomic layer etching (ALE) of Al2O3 can be achieved using sequential fluorination and ligand-exchange reactions. Although previous investigations have characterized the etch rates and s...

The particulate properties of a material after primary manufacturing have a large impact on the secondary manufacturing processes. Especially, powder characteristics leading to poor flowability are...

Atomic layer etching (ALE) of Al2O3, HfO2, and ZrO2 was accomplished using sequential exposures with hydrogen fluoride (HF) as the fluorination reagent and dimethylaluminum chloride (DMAC, AlCl(CH3...

Cycling stability is central to implementing silicon (Si) anodes in next-generation high-energy lithium-ion batteries. However, challenges remain due to the lack of effective strategies to enhance ...

Thermal atomic layer etching (ALE) is an important technique for the precise isotropic etching of nanostructures. Thermal ALE of many materials can be achieved using a two-step fluorination and lig...

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...

Atomic layer control of microelectronic processing is required as critical dimensions are reduced below the 10 nm scale. Thermal atomic layer etching (ALE) has developed over the last 5 years to meet etching challenges. This paper will define thermal ALE in terms of sequential, self-limiting surface reactions. Thermal ALE can be viewed as the reverse of atomic layer deposition (ALD). Various microelectronic materials will be used to demonstrate thermal ALE including Ga 2 O 3 , Si, and Si 3 N 4 .

The thermal atomic layer etching (ALE) of germanium-rich SiGe was demonstrated using an oxidation and "conversion-etch" mechanism with oxygen (O-2) or ozone (O-3), hydrofluoric acid (HF), and trimethylaluminum [TMA, Al(CH3)(3)] as the reactants. The crystalline germanium-rich SiGe film was prepared using physical vapor deposition and had a composition of Si0.15Ge0.85. In situ spectroscopic ellipsometry was employed to monitor the thickness of both the SiGe film and the surface oxide layer on the SiGe film during thermal ALE. Using a reactant sequence of O-2-HF-TMA, the etch rate of the SiGe film increased progressively with temperatures from 225 to 290 degrees C. At 290 degrees C, the SiGe film thickness decreased linearly at a rate of 0.57 angstrom /cycle with a surface oxide thickness of 18-19 angstrom. This etch rate was obtained using reactant pressures of 25, 0.2, and 0.4Torr and doses of 1.5, 1.0, and 1.0s for O-2, HF, and TMA, respectively. The TMA and HF reactions were self-limiting and the O-2 reaction was reasonably self-limiting at 290 degrees C. Using an O-3-HF-TMA reaction sequence, the SiGe ALE etch rate was 0.42 angstrom /cycle at 290 degrees C. This etch rate was obtained using reactant pressures of 15, 0.2, and 0.4Torr and dose times of 0.5, 1.0, and 1.0s for O-3, HF, and TMA, respectively. The O-3, TMA, and HF reactions were all self-limiting at 290 degrees C. Atomic force microscopy images revealed that thermal ALE with the O-2-HF-TMA or O-3-HF-TMA reaction sequences did not roughen the surface of the SiGe film. The SiGe film was etched selectively compared with Si or Si3N4 at 290 degrees C using an O-2-HF-TMA reaction sequence. The etch rate for the SiGe film was >10 times faster than Si(100) or Si3N4 that was prepared using low-pressure chemical vapor deposition. This selectivity for the SiGe film will be useful to fabricate Si nanowires and nanosheets using SiGe as the sacrificial layer.

Electron-enhanced atomic layer deposition (EE-ALD) was used to deposit boron nitride (BN) thin films at room temperature and 100 °C using sequential exposures of borazine (B(3)N(3)H(6)) and electrons. Electron-stimulated desorption (ESD) of hydrogen surface species and the corresponding creation of reactive dangling bonds are believed to facilitate borazine adsorption and reduce the temperature required for BN film deposition. In situ ellipsometry measurements showed that the BN film thickness increased linearly versus the number of EE-ALD cycles at room temperature. Maximum growth rates of ~3.2 Å/cycle were measured at electron energies of 80–160 eV. BN film growth was self-limiting versus borazine and electron exposures, as expected for an ALD process. The calculated average hydrogen ESD cross section was σ = 4.2 × 10(−17) cm(2). Ex situ spectroscopic ellipsometry measurements across the ~1 cm(2) area of the BN film defined by the electron beam displayed good uniformity in thickness. Ex situ X-ray photoelectron spectroscopy and in situ Auger spectroscopy revealed high purity, slightly boron-rich BN films with C and O impurity levels

Atomic layer deposition (ALD) is a well-established technique for depositing nanoscale coatings with pristine control of film thickness and composition. The trimethylaluminum (TMA) and water (H2O) ALD chemistry is inarguably the most widely used and yet to date, we have little information about the atomic-scale structure of the amorphous aluminum oxide (AlOx) formed by this chemistry. This lack of understanding hinders our ability to establish process-structure-property relationships and ultimately limits technological advancements employing AlOx made via ALD. In this work, we employ synchrotron high-energy X-ray diffraction (HE-XRD) coupled with pair distribution function (PDF) analysis to characterize the atomic structure of amorphous AlOx ALD coatings. We combine ex situ and in operando HE-XRD measurements on ALD AlOx and fit these experimental data using stochastic structural modeling to reveal variations in the Al-O bond length, Al and O coordination environment, and extent of Al vacancies as a function of growth conditions. In particular, the local atomic structure of ALD AlOx is found to change with the substrate and number of ALD cycles. The observed trends are consistent with the formation of bulk Al2O3 surrounded by an O-rich surface layer. We deconvolute these data to reveal atomic-scale structural information for both the bulk and surface phases. Overall, this work demonstrates the usefulness of HE-XRD and PDF analysis in improving our understanding of the structure of amorphous ALD thin films and provides a pathway to evaluate how process changes impact the structure and properties of ALD films.

Thermal atomic layer etching (ALE) of silicon was performed using O2, HF, and Al(CH3)3 as the reactants at temperatures from 225 to 290 °C. This thermal etching process is based on Si oxidation usi...

Electron-enhanced atomic layer deposition (EE-ALD) was used to deposit boron nitride (BN) thin films at room temperature and 100 °C using sequential exposures of borazine (B3N3H6) and electrons. Electron-stimulated desorption (ESD) of hydrogen surface species and the corresponding creation of reactive dangling bonds are believed to facilitate borazine adsorption and reduce the temperature required for BN film deposition. In situ ellipsometry measurements showed that the BN film thickness increased linearly versus the number of EE-ALD cycles at room temperature. Maximum growth rates of ∼3.2 A/cycle were measured at electron energies of 80–160 eV. BN film growth was self-limiting versus borazine and electron exposures, as expected for an ALD process. The calculated average hydrogen ESD cross section was σ = 4.2 × 10–17 cm2. Ex situ spectroscopic ellipsometry measurements across the ∼1 cm2 area of the BN film defined by the electron beam displayed good uniformity in thickness. Ex situ X-ray photoelectron spe...

The recent advancement of semiconductor devices to the near-atomic scale necessitated the development of atomic layer processing methods, including molecular layer deposition (MLD). This gas-phase deposition technique creates semipermeable polymer films with precise control of composition and thickness. Herein, MLD was used to produce thin-film composite reverse osmosis membranes. Aromatic polyamide films as thin as 0.5 nm were applied to NF270 nanofiltration membranes using m-phenylenediamine and trimesoyl chloride. Within two molecular layers, desalination performance was affected. As film thickness increased to 15 nm (48 MLD cycles), performance progressed from nanofiltration to reverse osmosis metrics in terms of salt rejection and water permeance. With film thickness > 5 nm, rejection values exceeded a small sampling of commercial membranes. In all cases, a tradeoff between rejection and permeance was observed. Atomic force microscopy measurements indicate that MLD enhancement led to removal of small-scale roughness features and resulted in a root mean square roughness difference of

Al2O3 atomic layer deposition (ALD) was used to smooth the roughness on silicon wafers obtained prior to chemical mechanical polishing (CMP). The initial silicon wafers had an average RMS surface roughness of 3.3 nm as determined by atomic force microscopy (AFM) measurements. AFM line scans also measured an average lateral spacing of ≈490 nm between the surface asperities. The RMS roughness decreased and the average lateral spacing increased progressively with number of Al2O3 ALD cycles. After 3000 Al2O3 ALD cycles that deposit an Al2O3 film thickness of 370 nm, the RMS roughness reduced to 1.5 nm and the average lateral spacing between the surface asperities increased to ≈890 nm. Additional Al2O3 ALD cycles produced little change in the RMS roughness or average lateral spacing. The efficiency of the smoothing decreased when the lateral distance between the surface asperities was much larger than the Al2O3 ALD film thickness. Power spectral density (PSD) analysis revealed that the ALD smoothing was most effective for surface topographical features with lateral spacings in the range of 10s to 100s of nanometers. Reflectivity studies of silver films deposited on the silicon wafers also demonstrated that Al2O3 ALD smoothing improved the optical performance of reflective mirrors.

­Ultrathin Al2O3 atomic layer deposition (ALD) films with low pinhole density were fabricated using a deposit and etchback approach. This strategy was able to avoid the pinholes that originated during nonuniform nucleation of Al2O3 ALD films. In this method, an Al2O3 ALD film was deposited to a thickness greater than the desired thickness to reduce the number of pinholes and form a more continuous Al2O3 ALD film. Subsequently, the Al2O3 ALD film was etched back to a smaller thickness using thermal Al2O3 atomic layer etching (ALE). The number of pinholes in the resulting Al2O3 ALD film was measured by the percentage yield of metal-insulator-metal (MIM) capacitors based on an Ag/Al2O3/Al structure that did not have an electrical short. The device yield was improved using the deposition and etchback approach. For example, using device areas of 0.01 mm2, Al2O3 ALD films that were grown to 5 nm in the MIM capacitor gave a yield of 30%–40%. In contrast, Al2O3 ALD films that were grown to 24 nm and then etched back to 5 nm to form the MIM capacitor provided a yield of 65%–75%. This increase in yield of approximately 100% indicates that the deposit and etchback approach can improve the yield of MIM devices based on ultrathin Al2O3 ALD films. Although this method has been previously suggested to improve the quality of ultrathin films, this report is believed to be the first demonstrated application of using the deposit and etchback approach for device fabrication. Additional experiments revealed that a portion of the yield improvement can be attributed to the fluorination of the Al2O3 ALD films that produced a volume expansion when forming AlF3. This expansion may produce a compressive stress that helps to close the pinholes. The dielectric constant was also measured for Al2O3 ALD films versus Al2O3 film thickness. The dielectric constant was the same for as-deposited Al2O3 ALD films and Al2O3 ALD films that were first grown to 24 nm and then etched back to smaller thicknesses. This agreement indicates that the dielectric constant can be understood in terms of a series capacitor model and that Al2O3 ALE does not affect the electrical properties of the Al2O3 films.

Polyamide thin films, designated Nylon 2,6, were grown on flat and particle substrates using molecular layer deposition (MLD) in a custom-built isothermal enclosure containing a rotary reactor. The polyamide films were grown using sequential exposures of ethylene diamine and adipoyl chloride. The reactor and precursors were contained in a fiberglass oven to keep all reactor components at the same temperature. A growth rate of 4.0 A/cycle at 67 °C was determined on flat substrates with ex situ x-ray reflectivity and spectroscopic ellipsometry. The temperature dependence of the Nylon 2,6 displayed a peak growth rate at 67 °C with decreasing growth rates above and below this temperature. X-ray photoelectron spectroscopy of the polyamide film on flat substrates also revealed an elemental composition consistent with the Nylon 2,6 polymer with a small amount of chlorine in the film. The isothermal reactor allowed MLD to be performed consistently on high surface area particles at low temperatures. Transmission electron microscopy (TEM) images showed growth of the Nylon 2,6 films on ZrO2, cellulose, and metformin particles that was consistent with the growth on witness wafers. The growth of the Nylon 2,6 films was also linear versus the number of MLD cycles. The TEM images displayed reproducible MLD growth on particles of varying size and composition. Fourier transform infrared spectroscopy and energy dispersive spectroscopy were consistent with the expected characteristics of the Nylon 2,6 polyamide film. Nylon 2,6 MLD should find application when low-temperature MLD is needed to coat thermally sensitive substrates such as organic films or pharmaceutical powders.

Thermal atomic layer etching (ALE) can be achieved with sequential, self-limiting surface reactions. One mechanism for thermal ALE is based on fluorination and ligand-exchange reactions. For metal oxide ALE, fluorination converts the metal oxide to a metal fluoride. The ligand-exchange reaction then removes the metal fluoride by forming volatile products. Previous studies have demonstrated the thermal ALE of amorphous Al2O3 films. However, no previous investigations have explored the differences between the thermal ALE of amorphous and crystalline Al2O3 films. This study explored the thermal ALE of amorphous and crystalline Al2O3 films. HF, SF4, or XeF2 were used as the fluorination reactants. Trimethylaluminum (TMA) or dimethylaluminum chloride (DMAC) were used as the metal precursors for ligand-exchange. Spectroscopic ellipsometry measurements revealed that the amorphous Al2O3 films had much higher etch rates than the crystalline Al2O3 films. When using HF and TMA at 300 °C, the amorphous Al2O3 film was removed at an etch rate of 0.78 A/cycle. For the crystalline Al2O3 film, an etch rate of 0.06 A/cycle was initially observed prior to the stoppage of etching after removing about 10 A of the film. Thermal ALE with HF and DMAC resulted in similar results. Etch rates of 0.60 and 0.03 A/cycle were measured for amorphous and crystalline Al2O3 films at 300 °C, respectively. Other fluorination agents, such as SF4 or XeF2, were also used together with TMA or DMAC for Al2O3 ALE. These reactants for fluorination and ligand-exchange were able to etch amorphous Al2O3 films at 300 °C. However, they were unable to etch crystalline Al2O3 film at 300 °C beyond the initial 10–20 A surface layer. The investigations also examined the effect of annealing temperature on the etch rate per cycle using HF and TMA as the reactants at 300 °C. Amorphous Al2O3 films were etched at approximately the same etch rate of 0.78 A/cycle until the crystallization of amorphous Al2O3 films at ≥ 880 °C. The differences between amorphous and crystalline Al2O3 thermal ALE could be used to obtain selective thermal ALE of amorphous Al2O3 in the presence of crystalline Al2O3.

The development of a hollow cathode plasma electron source (HC-PES) facilitated the rapid nucleation and low temperature deposition of thin cobalt films using electron-enhanced atomic layer deposition (EE-ALD). The Co EE-ALD was performed near room temperature (30–60 °C) using sequential exposures of cobalt tricarbonyl nitrosyl and low energy (100–200 eV) electrons. Electron-stimulated desorption of CO and NO surface species creates open sites for precursor adsorption to facilitate the low temperature film growth. The HC-PES displayed high electron currents, rapid ALD cycling, and low susceptibility to chemical interference. Electron steering optics were also used to mitigate the effects of sputtering in the HC-PES. The high electron currents from the HC-PES yielded rapid nucleation of cobalt films in as few as four EE-ALD cycles with Co growth rates over 2 A/cycle on areas >4 cm2. In high aspect ratio structures, transmission electron microscopy and energy dispersive spectroscopy analyses revealed a 4:1 topographical selectivity in favor of horizontal compared with vertical surfaces. This selectivity was attributed to the directional electron flux from the HC-PES. This topographical area selective deposition suggests that Co EE-ALD may be successful in achieving bottom-up fill of trenches and vias.

The thermal atomic layer etching (ALE) of TiN was demonstrated using a new etching mechanism based on sequential, self-limiting oxidation and fluorination reactions. The oxidation reactant was either O3 or H2O2, and the fluorination reactant was hydrogen fluoride (HF) derived from HF-pyridine. In the proposed reaction mechanism, the O3 reaction oxidizes the surface of the TiN substrate to a TiO2 layer and gaseous NO. HF exposure to the TiO2 layer then produces TiF4 and H2O as volatile reaction products. The overall reaction can be written as TiN + 3O3 + 4HF → TiF4 + 3O2 + NO + 2H2O. Quartz crystal microbalance studies showed that HF can spontaneously etch TiO2 films. Spectroscopic ellipsometry and X-ray reflectivity analysis showed that TiN films were etched linearly versus the number of ALE cycles using O3 and HF as the reactants. The TiN etching also occurred selectively in the presence of Al2O3, HfO2, ZrO2, SiO2, and Si3N4. The etch rate for TiN ALE was determined at temperatures from 150 to 350 °C. Th...

We introduce atomic layer deposition (ALD) as a novel method for the ultrathin coating (nanolayering) of minitablets. The effects of ALD coating on the tablet characteristics and taste masking were investigated and compared with the established coating method. Minitablets containing bitter tasting denatonium benzoate were coated by ALD using three different TiO2 nanolayer thicknesses (number of deposition cycles). The established coating of minitablets was performed in a laboratory-scale fluidized-bed apparatus using four concentration levels of aqueous Eudragit (R) E coating polymer. The coated minitablets were studied with respect to the surface morphology, taste masking capacity, in vitro disintegration and dissolution, mechanical properties, and uniformity of content. The ALD thin coating resulted in minimal increase in the dimensions and weight of minitablets in comparison to original tablet cores. Surprisingly, ALD coating with TiO2 nanolayers decreased the mechanical strength, and accelerated the in vitro disintegration of minitablets. Unlike previous studies, the studied levels of TiO2 nanolayers on tablets were also inadequate for effective taste masking. In summary, ALD permits a simple and rapid method for the ultrathin coating (nanolayering) of minitablets, and provides nanoscale-range TiO2 coatings on porous minitablets. More research, however, is needed to clarify its potential in tablet taste masking applications. (C) 2017 Elsevier B.V. All rights reserved.

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 A/cycle at 128 °C to 4.19 A/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 fir...

Organic solid pharmaceutical powders are used for the preparation of various drug dosage forms. Primary particles in powder form undergo several processing steps first in pharmaceutical formulations followed by pharmaceutical manufacturing to final dosage form of a drug. These unit operations involve both handling of powders in aqueous or solvent solutions and drying. There will be a probable rise for a demand for the different unit operations in the requirements of protecting the active pharmaceutical ingredient or challenges in powder handling. Besides pharmaceutical manufacturing, there are many biological interfaces where control of surface characteristics of pharmaceutical powders can improve the therapeutic response and bioavailability. In this work, we have modified acetaminophen particles with atomic layer deposition (ALD) by conformal nanometer scale coatings in a one-step coating process. According to the results, ALD comprising common chemistries for Al2O3, TiO2 and ZnO is shown to be a promising coating method for solid pharmaceutical powders. Acetaminophen does not undergo degradation during the ALD coating and maintains its stable polymorphic structure. The nanometer scale ALD coating can sustain the drug release. ALD oxide coated acetaminophen particles show different cytocompatibility assessed in in vitro intestinal Caco-2 cells.

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 A/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...

Freestanding LiCoO2/multiwall carbon nanotube/nanocellulose fibril (LCO-MWCNT-NCF) electrodes are fabricated by a vacuum filtration technique. The electrode has a high LCO loading of 20 mg/cm2 with excellent flexibility, uniform material distribution, and low surface resistivity. When coated with 2 ALD cycles of AlF3, LCO-MWCNT-NCF has a high specific capacity of 216 mAh/g at 4.7 V. The freestanding AlF3-coated electrode preserves 75.7% of its initial capacity after 100 cycles and 70% after 160 cycles of charge discharge. In contrast, electrodes coated with 2 ALD cycles of Al2O3 cannot be cycled above 4.5 V. By elimination of the unnecessary weight of current collector, and increasing in the working voltage simultaneously, this freestanding LCO-MWCNT-NCF electrode can significantly improve the gravimetric and volumetric energy density of lithium ion batteries.

The atomic layer etching (ALE) of ZnO thin films was demonstrated using sequential, self-limiting thermal reactions with hydrogen fluoride (HF) and trimethylaluminum (TMA) as the reactants. The initial polycrystalline ZnO films were grown by atomic layer deposition (ALD) using diethylzinc (DEZ) and H2O at 150 °C. The thermal ZnO ALE process was then studied between 205 and 295 °C using various techniques. In situ quartz crystal microbalance (QCM) measurements monitored ZnO ALE at 265 °C. The ZnO etching was linear versus number of ALE cycles. The HF exposure caused a mass gain of ΔMHF = +53 ng/cm2 from fluorination. The subsequent TMA exposure caused a large mass loss of ΔMTMA = −172 ng/cm2. This mass loss was much larger than expected from metal fluoride removal resulting from ligand-exchange transmetalation. The large mass loss suggested that there is a “conversion-etch” mechanism where TMA also converts the ZnO surface to an Al2O3 surface layer. This conversion reaction is believed to occur according t...

Thermal atomic layer etching (ALE) was demonstrated on ternary III-V compound semiconductors. In particular, thermal ALE on InGaAs and InAlAs was achieved with sequential, self-limiting fluorination and ligand-exchange reactions using hydrogen fluoride (HF) as the fluorination reactant and dimethylaluminum chloride (DMAC) as the ligand-exchange reactant. Thermal ALE was investigated on planar surfaces and three-dimensional nanostructures. The measured radial etch rates on In

This paper presents the first study of the effect of thermal atomic layer etching (ALE) on the residual stress in suspended atomic layer deposition (ALD) structures. Optical curvature measurements on silicon wafers with varying thicknesses of Al 2 O 3 ALD on polyimide were collected ex situ to obtain the residual stress evolution of the film. Brownian motion of micromachined resonators fabricated with an ALD-on-polyimide process [1] was measured in a range of 1-10 MHz using an optical interferometer before and after ALE processing. Experimental resonant frequencies were compared with FEM simulations to validate residual stress in suspended structures. Results show good agreements between the residual stress obtained using resonant frequency and wafer curvature measurements. Furthermore, suspended beams with an initial thickness of 30 nm were thinned down to 10 nm using ALE. TEM imaging was used to confirm beam thickness after etching. Experimental data with FEM validation shows that ALE can be used to remove highly stressed nucleation and coalescence regions in free-standing structures.