Layer Etching Research Articles

Investigation for Low-Rate Fenceless Al Etching Applied in Fabrication of Superconducting Circuits

Ion beam etching (IBE) is introduced in the fabrication process developed from Nb03 in SIMIT, for the etching of Al-AlOx layer. However, we found that IBE creates fences at the edges of etched profiles because of redeposition of the Al. To solve this problem, we systematically investigated the yield of fenceless patterns and the junction quality as a function of ion beam incidence angle. The yield increases as the incidence angle decreases, 0° causing the least fence formation. The yield dependence was explained by a fence formation model developed from a two-dimensional redeposition algorithm. Given that the un-etched foots due to the shadow effect become visible as angle decreases, the optimal condition is finally set to be 400 eV with incidence angle of 10°. Nb/Al-AlOx/Nb junctions fabricated by Al-AlOx layer etching at incidence angle of 10° showed good junction quality with a low subgap leakage current. We could also obtain a high yield of shunted and unshunted Nb/Al-AlOx/Nb junctions with the minimum size of 1.4 μm in diameter on 4-inch Si wafers. These results prove the feasibility to achieve fabrication without anodization process and patterning of Al to improve the reliability.

Monolithic Integration and Analysis of Vertical, Partially Encapsulated Nanoelectrode Arrays

This study reports on the development of vertical, partially encapsulated nanoelectrodes for electrically contacting the interior of electrogenic cells with microelectronics. Intracellular electrical stimulation and recording with single cell resolution enables new insights into the electrophysiology of cells embedded in a complex multicellular network, providing detailed understanding of fundamental processes affecting cell to cell communication and thereby paving the way for novel applications including pharmacological studies and other neuromodulation techniques like focused ultrasound and electroceuticals. In order to minimize the influence of the measurement system, an approach based on nano-sized hollow electrodes, achieving an adhesion based intracellular access, is used. The focus of the presented work is on the novel fabrication technology and the characterization of the resulting nanoelectrodes. In CMOS compatible processes, the hollow geometry is achieved using a sacrificial layer technique combining deep reactive ion etching and atomic layer deposition of Ru. For decoupling the extracellular milieu, a partial passivation of the nanoelectrodes by Ta2O5 is realized. The monolithic integration allows an application specific fine-tuning of geometry and placement of the nanoelectrodes. A discrete microelectrode array was designed to electrically and electrochemically characterize the nanoelectrodes. Resistance measurements, cyclic voltammetry and electrochemical impedance spectroscopy show the feasibility of the developed electrodes as an electronic interface to electrochemical fluids. Specifically, an electrode resistance of 2.92 $\text{k}\Omega $ and charge delivery capacitance of $748.13~\frac {\mu \text {C}}{\text {cm}^{2}}$ were observed. Confocal microscopy analyses of neural cells interfaced with the nanoelectrodes indicate an adhesion based intracellular access as well as biostability and biocompatibility. [2020-0224]

Self-limiting processes in thermal atomic layer etching of nickel by hexafluoroacetylacetone

In thermal atomic layer etching (ALE) of Ni, a thin oxidized Ni layer is removed by a hexafluoroacetylacetone (hfacH) etchant gas at an elevated surface temperature, and etching ceases when a metallic Ni surface appears (self-limiting step). However, atomistic details of the self-limiting step was not well understood. With periodic density-functional-theory calculations, it is found that hfacH molecules barrierlessly adsorb and tend to decompose on a metallic Ni surface, in contrast to the case of a NiO surface, where they can form volatile Ni(hfac)2. Our results clarify the origin of the self-limiting process in the thermal ALE.

Dry etching strategy of spin-transfer-torque magnetic random access memory: A review

The spin-based memory, spin transfer torque-magnetic random access memory (STT-MRAM), has the potential to enhance the power efficiency of high density memory systems. Its desirable characteristics include nonvolatility, fast operation, and long endurance. However, dry etching of MRAM structures remains a challenge as the industry is ramping up its production. In this paper, we explore the etching strategies that have been used to etch the MRAM structures. Several etching techniques have been developed to attain optimal device performance. These are reactive ion etching, time modulated plasma etching, atomic layer etching, and ion beam etching. Sidewall profile, sidewall contamination or damage, redeposition, selectivity, and noncorrosiveness are the main factors to consider while selecting the best etching methods. This paper starts with the fundamentals of MRAM reading, writing, and storing principles and finishes with the current approaches to solve the etch challenges. For etching, the most commonly used magnetic materials such as CoFeB, CoFe, and NiFe are covered in this article.

Effect of etching depth on the performance of InP-based hybrid plasmonic waveguides

Hybrid plasmonic (HP) waveguides have attracted a lot of attention in recent years for its ability to provide long propagation length along with subwavelength confinement. In this study, the effect of InGaAsP(Q) layer etch depth on an InP-based HP-waveguide was investigated. Then, the layer thicknesses were optimized to achieve a maximum propagation length of 80 µm and the minimum effective area of 0.03 µm2 indicating a significant improvement compared to previous studies. Meanwhile, the coupling length of a directional coupler for various etch depth was calculated. Finally, the effect of the etch depth on the optical power transmission of the 90° curved waveguide and coupling between the curved and straight waveguides were determined.

Formation and desorption of nickel hexafluoroacetylacetonate Ni(hfac)2 on a nickel oxide surface in atomic layer etching processes

Thermal atomic layer etching (ALE) of nickel (Ni) may be performed with a step of thin-layer oxidation of its surface and another step of its removal by gas-phase hexafluoroacetylacetone (hfacH) as an etchant. In this study, adsorption of hfacH and possible formation of volatile nickel hexafluoroacetylacetonate Ni(hfac)2 on a NiO surface were investigated based on the density functional theory (DFT) with more realistic surface material models than those used in the previous study [A. H. Basher et al., J. Vac. Sci. Technol. A 38, 022610 (2020)]. It has been confirmed that an hfacH molecule approaching a NiO surface deprotonates without a potential barrier and adsorbs on the surface exothermically. In addition, stable adsorption of two deprotonated hfacH molecules on a NiO (100) surface was found to occur not on a single Ni atom but over a few Ni atoms instead, which makes the formation of a Ni(hfac)2 complex on the flat surface very unlikely even at elevated temperature. However, if the surface is rough and a Ni atom protrudes from the surrounding atoms, two hexafluoroacetylacetonate anions (hfac−) can bond to the Ni atom stably, which suggests a possibility of desorption of a Ni(hfac)2 complex from the surface at elevated temperature. Given the experimentally observed fact that desorption of Ni(hfac)2 complexes typically takes place on a NiO surface at a temperature of ∼300–400 °C, our DFT calculations indicate that the surface roughness of an oxidized Ni surface facilitates the formation and desorption of organometallic complexes Ni(hfac)2, and therefore, the resulting Ni surface after ALE can be smoother than the initial surface.

Etch selectivity during plasma-assisted etching of SiO2 and SiNx: Transitioning from reactive ion etching to atomic layer etching

Continued downscaling of semiconductor devices has placed stringent constraints on all aspects of the fabrication process including plasma-assisted anisotropic etching. To address manufacturing challenges associated with atomic-scale control, material selectivity, etch fidelity, and increasingly complex device architectures, reactive ion etching (RIE) is transitioning to plasma-assisted atomic layer etching (ALE). Even though the number of elements used in the semiconductor devices has increased several-fold over the last four decades, SiO2 and SiNx remain the most commonly used dielectric materials. In fact, fluorocarbon based, plasma-assisted ALE processes for SiO2 and SiNx have already been integrated into semiconductor manufacturing, including etching of self-aligned contacts for advanced transistors. However, several challenges remain in achieving ultrahigh etch selectivity of SiO2 over SiNx and vice versa. In this article, first, the authors provide a focused review on selective RIE of SiO2 over SiNx and contrast this with ALE. A particular focus is given to the etching mechanism, including the role of the mixing layer composition and thickness at the fluorocarbon-SiO2 interface, the F-to-C ratio in the fluorocarbon parent gas, H2 dilution, surface composition on the nonetched SiNx, ion flux and energy, Ar plasma activation duration in ALE, and chamber memory effects. Second, we discuss the reverse case of selectively etching SiNx over SiO2 with careful attention given to the role of novel hydrofluorocarbon gases and dilution of the primary feed gas with other gases such as CH4 and NO. In the second part of this review, we also discuss how novel surface chemistries are enabled by the introduction of ALE, which include selective (NH4)2SiF6 formation on the SiNx surface and selective surface prefunctionalization of SiO2 to enable ultrahigh selectivity. Through this review, the authors hope to provide the readers with an exhaustive knowledge of the selectivity mechanisms for RIE of SiO2 over SiNx and vice versa, which provides a basis for developing future highly material-selective ALE processes.

Label-free detection of DNA by a dielectric modulated armchair-graphene nanoribbon FET based biosensor in a dual-nanogap setup.

A Double-Gate Armchair-Graphene Nanoribbon FET is proposed to realize a high-sensitive and small-size biosensor in order to detect DNA without high-cost and time-consuming labeling process. Two nanogap cavities open inside the top and bottom gate oxides by the method of sacrificed layer etching. When the DNA biomolecule is introduced to the nanogap cavity, the hybridization event which is actually the formation of a double-strand of DNA will occur thus electrically modulating the GNR channel leading to a change in the drain current. The important report of this research is about attained high sensitivity of the proposed biosensor for a vast spectrum of the DNA biological samples. It is worth noting that a DNA sequence by 23 nucleotides extracted from Neisseria gonorrhoeae can be detected as a special case. An extensive numerical approach has been applied in order to characterize the proposed biosensor. The suggested biosensor has been evaluated by solving Schrödinger equation )SE( with Non-Equilibrium Green Function (NEGF) method in the mode-space coupled into Poisson solver in a self-consistent manner assuming ballistic limit. Two different expressions of sensitivity in terms of the threshold voltage and current have been defined giving a good metric for the sensitivity analysis. The results revealed a relative sensitivity of 1mV/nm2 by a filled area by the DNA about 120nm2 showing the excellent superiority for the proposed biosensor as compared to other counterparts. The effective area of the proposed biosensor obtains 240nm2 which is very small in comparison with other reports highlighting high capability of the biosensor in the detection. It has been shown that the proposed biosensor can be implemented in ultra-scaling domain resulting in considerable increase in the sensitivity promising a potent and reliable candidate for high-sensitive and small-size biosensors. Also, the technical issues on designing the suggested biosensor have been investigated to achieve a useful guideline in detection and identification of the target DNAs.

Mechanism of Thermal Atomic Layer Etch of W Metal Using Sequential Oxidation and Chlorination: A First-Principles Study.

Thermal atomic layer etch (ALE) of W metal can be achieved by sequential self-limiting oxidation and chlorination reactions at elevated temperatures. In this paper, we analyze the reaction mechanisms of W ALE using the first-principles simulation. We show that oxidizing agents such as O2, O3, and N2O can be used to produce a WOx surface layer in the first step of an ALE process with ozone being the most reactive. While the oxidation pulse on clean W is very exergonic, our study suggests that runaway oxidation of W is not thermodynamically favorable. In the second ALE pulse, WCl6 and Cl2 remove the oxidized surface W atoms by the formation of volatile tungsten oxychloride (WxOyClz) species. In this pulse, each adsorbed WCl6 molecule was found to remove one surface W atom with a moderate energy cost. Our calculations further show that the desorption of the additional etch products is endothermic by up to 4.7 eV. Our findings are consistent with the high temperatures needed to produce ALE in experiments. In total, our quantum chemical calculations have identified the lowest energy pathways for ALE of tungsten metal along with the most likely etch products, and these findings may help guide the development of improved etch reagents.

Significance of plasma-photoresist interactions for atomic layer etching processes with extreme ultraviolet photoresist

Extreme ultraviolet (EUV) lithography has emerged as the next generational step in advancing the manufacturing of increasingly complex semiconductor devices. The commercial viability of this new lithographic technique requires compatible photoresist (PR) materials that satisfy both the lithographic and etch requirements of good feature resolution, chemical sensitivity, a low line edge roughness, and good critical dimension uniformity. Achieving the decreased feature pitches of modern processing nodes via EUV lithography places a limit on the available photoresist thickness for a pattern transfer process. Therefore, etch processes are required to maximize the etching selectivity of a hard mask material, such as SiO2, to an EUV photoresist. In this work, the authors evaluated the ability of an atomic layer etching (ALE) process to maximize the SiO2/EUV PR etching selectivity. Through the flexible parameter space available in an ALE process, the authors evaluated the etching behaviors as a function of the ALE parameters of ion energy, etch step length, fluorocarbon (FC) deposition thickness, and precursor gas type. The authors found that the interaction between the energetic argon ion bombardment and a deposited FC layer produces a modified surface layer on the PR material that can strongly control the PR etch rate and even produce an etch stop under some conditions. Under the same processing conditions, the etching behavior of SiO2 continues unimpeded, thus resulting in a high overall SiO2/PR etching selectivity. Secondary characterization using x-ray photoelectron spectroscopy and atomic force microscopy was used to support the conclusions derived from the ellipsometric modeling based on the surface chemistry evolution and determine the impact of the ALE process on the surface roughness of the EUV PR, respectively. Additionally, attenuated total reflection Fourier-transform infrared spectroscopy was used to track the impact on specific functional groups within the PR composition from both the argon ion bombardment and FC deposition components of the ALE process. The ALE-based PR etching concept established in this work serves as a foundation for both the understanding of the impacts of an ALE process on an EUV PR material and for future works, employing an ALE process for PR-based pattern transfer.

Patterning nickel for extreme ultraviolet lithography mask application. II. Hybrid reactive ion etch and atomic layer etch processing

A two-step plasma-thermal atomic layer etching (ALE) process that is capable of etching Ni with high selectivity with respect to the SiO2 hard mask and high anisotropy is evaluated in this work with a reactive ion etching (RIE) process to highlight the contrast between these two processes and the advantages of combining these two processes to tailor the sidewall profile with greater processing efficiency. The RIE chemistry leveraged the enhanced volatility of chlorinated nickel in the presence of hydrogen atoms. The hybrid RIE and ALE process achieved the desired sidewall profile, with no measurable residual halogen.

Formation of White Etching Layers by Deep Rolling of AISI 4140 Steel

Deep rolling is an effective mechanical surface treatment method; however, the induced deformation may eventually lead to changes that result in a structure known as a white etching layer (WEL). The WEL possesses a distinct constitution and properties compared with the surrounding bulk material. The presence of the WEL drastically affects the surface integrity of the part and impairs the performance of the metallic components. However, the influence of deep rolling on WEL formation has not been fully clarified. Therefore, the goal of this study was to investigate WEL formation after deep rolling AISI 4140 steel with various microstructures (obtained through four heat treatment routes) and employing distinct rolling pressure and feed values. The results show that larger lattice distortion (resulting from refined martensitic structures) and higher deep rolling feed (increased work hardening) lead to the formation of a thicker WEL. Moreover, the increase in rolling pressure does not affect the WEL within the tested range. Finally, the mechanisms involved in WEL formation are discussed. It is suggested that the shear energy associated with deep rolling and applied along the surface grains leads to WEL formation by grain refinement.

Controllable preparation of monolayer MoO3/MoOx by using plasma oxidation and atomic layer etching

In this letter, we propose two different methods to prepare monolayer MoO3 and MoOx by low-temperature inductively coupled plasma etching technology. The monolayer MoO3 is obtained by soft oxygen plasma oxidation of monolayer MoS2 due to the high specific surface area. The monolayer MoOx is prepared through plasma oxidation and atomic layer-by-layer etching of multilayer MoS2 due to the weak van der Waals interaction between the layers. It is proved that the prepared monolayer molybdenum oxide has good surface morphology and highly controllable components. The proposed process can precisely control the thickness of molybdenum oxide and has good uniformity and higher selectivity. This work could promote the application of atomically thin transition metal oxides in two-dimensional materials based electronic devices and its integration.

In situ surface analysis of an ion-energy-dependent chlorination layer on GaN during cyclic etching using Ar+ ions and Cl radicals

Gallium nitride (GaN) semiconductor devices must be fabricated using plasma etching with precise control of the etching depths and minimal plasma-induced damage on the atomic scale. A cyclic process comprising etchant adsorption and product removal may be suitable for this purpose but an understanding of the associated etching surface reactions is required. The present work examined the formation of a chlorinated layer based on Cl radical adsorption on a GaN surface in conjunction with Ar ion irradiation. This research employed beam experiments and in situ x-ray photoelectron spectroscopy. The results show that N atoms are preferentially desorbed during exposure to Ar ions to produce Ga-rich layers at depths of 0.8 and 1.1 nm at an Ar ion dosage on the order of 1016 cm−2 and ion energies of 116.0 and 212.6 eV, respectively. Subsequent exposure of the irradiated Ga-rich layer to Cl radicals removes some Ga atoms and produces a chlorinated layer over the GaN surface. This chlorinated layer has a thickness on the order of 1 nm following Cl radical dosages on the order of 1019 cm−2. This study of plasma-treated surfaces is expected to assist in developing means of controlling the etching depth during the atomic layer etching of GaN via Ar ion bombardment.

Patterning nickel for extreme ultraviolet lithography mask application I. Atomic layer etch processing

The stringent requirement for patterning highly absorbing metal thin films as a mask for the next-generation extreme ultra-violet lithography system dictates the development of an atomic layer etching process to tailor the etch rate and the etch profile. A “plasma-thermal” atomic layer etching process is developed where an oxygen plasma is used to convert the metallic Ni layer into NiO, followed by formic acid vapor reacting with NiO to form nickel formate [Ni(COOH)2], thereby removing nickel. The directionality of the oxygen ions is used to create a directional chemical conversion of Ni into NiO, resulting in an anisotropic etch profile. Using the SiO2 patterned Ni thin film, a high etching selectivity to the mask (virtually no etching of SiO2) and a high etching anisotropy (a sidewall angle up to 87°) are achieved.

Role of physisorption in atomic layer etching of silicon nitride

Atomic layer etching (ALE) techniques are growing in popularity due to their various benefits such as low damage, high selectivity, and precise and controlled etching. In ALE, surface modification is typically achieved by chemisorption of radicals generated in a plasma followed by ion assisted removal of the modified surface. A surface modification process assisted by a plasma may lead to unwanted side effects such as excessive polymerization or spontaneous etching. In this work, the authors demonstrate the feasibility of atomic layer etching of silicon nitride by physisorption of hydrofluorocarbon gas precursors [CHxF(4−x), x = 1−4] in the absence of a plasma. Self-limited SiN etching was observed for all hydrofluorocarbon gases, with the largest etch depth per cycle observed for CF4 and CHF3 adsorption (∼15 Å/cycle), and smallest etch depth per cycle observed using CH3F adsorption (∼6 Å/cycle). Etch precursor availability on the surface was manipulated by varying absolute pressure, partial pressure of the adsorbate, and adsorption time. The results of these experiments indicated that of physisorption of precursors is enough to modify the surface, leading to its removal in the subsequent plasma assisted desorption step.

Vacuum Ultraviolet-Enhanced Oxidation—A Route to the Atomic Layer Etching of Palladium Metal

Low-temperature, plasma-free atomic layer etching (ALE) of Pd0 is explored. A vacuum ultraviolet (VUV) light source (115 < λ < 400 nm) is used in conjunction with a controlled O2 gas exposure to pr...

Causes of anisotropy in thermal atomic layer etching of nanostructures

In this work, the authors have investigated the dependence of the anisotropy level in an atomic layer etching (ALE) process of Al2O3 on form factor constraints when the ALE process involves etching in non-line-of-sight locations beneath a silicon nitride mask. In the experiments described here, thermal etching of Al2O3 without the use of any direction-inducing plasma components was explored utilizing the well characterized hydrogen fluoride/dimethyl-aluminum-chloride atomic layer etching process. The degree of anisotropy was quantified by measuring the ratio of lateral etch rate of this process in comparison to the vertical etch rate as a function of process step time inside 60 nm holes of aluminum oxide. Inside these holes, the authors determined that the horizontal etch rates slowed to an amount of 19% compared to the vertical rate when short process times were used. For process times operating in the saturation mode of the ALE process, horizontal etch rates per cycle could be sped up to 71% of the vertical rate but never reached parity with the latter. The authors propose a simple mechanism for explaining the anisotropy dependence on process step time and applied a reduced-order algorithm to model it. In this model, the authors introduced fitting parameters for surface modification depths and reaction times to match the experimentally found etch results. Conclusions could be drawn regarding topological hindrance or tortuosity for reactants to reach surfaces in shaded areas under the mask and for reaction by-products to escape from these locations and the impact on etch rate. In addition, the authors recognize that this mechanism could explain the unwanted depth dependence of the etch rate per cycle in high aspect ratio structures.

A laboratory demonstration of rail grinding and analysis of running roughness and wear

Rail grinding has been widely used for rail maintenance to keep the performance of the rail track at satisfactory levels. However, there is a lack of knowledge on the relationship between different grinding parameters and the effect on the rails and the post-grinding evolution of roughness and wear. Rail undergoes significant levels of stress during grinding to remove any defects that appear during its life-cycle which can cause significant microstructural change. By developing a method of demonstrating rail grinding it will allow further investigation of the process and potentially the exploration of different grinding patterns. Moreover, an analysis of the roughness of wheel and rail is an input parameter in most modern simulation tools that could be associated with the coefficient of friction. This paper reports a laboratory demonstration of rail grinding followed by a series of tests to determine the wear of the rail and if there is a relationship between friction coefficient and roughness. The rail specimens that were ground have undergone a phase transformation and showed White Etching Layer (WEL) on the contact surface. This was found to initially act protectively for the rail disc reducing the wear rates, but then contributed to crack formation. No clear long-term correlation was identified between the coefficient of friction and roughness during the testing, however a relationship could be drawn when test periods were studied individually.

Thermal Atomic Layer Etching of Gallium Oxide Using Sequential Exposures of HF and Various Metal Precursors

Gallium oxide (Ga2O3) is a transparent semiconducting oxide with a large band gap that has applications for power electronics and optoelectronics. Ga2O3 device fabrication requires etching for many processing steps. In this work, the thermal atomic layer etching (ALE) of Ga2O3 was performed using hydrofluoric acid (HF) and a wide range of different metal precursors including BCl3, AlCl(CH3)2, Al(CH3)3, TiCl4, and Ga(N(CH3)2)3. Because Ga2O3 is not a particularly stable oxide, the B-, Al-, or Ti-containing metal precursors can possibly convert the surface of Ga2O3 to B2O3, Al2O3, or TiO2. These metal precursors can also provide Cl, CH3, and N(CH3)2 ligands for ligand-exchange reactions. Consequently, the thermal ALE of Ga2O3 can occur via “conversion-etch” or fluorination and ligand-exchange reaction pathways. Using sequential HF and BCl3 exposures and in situ spectroscopic ellipsometry techniques, Ga2O3 etch rates were observed to vary from 0.59 to 1.35 Å/cycle at temperatures from 150 to 200 °C, respectively. The Ga2O3 etch rates were also self-limiting versus HF and BCl3 exposure. The lack of BCl3 pressure dependence for the etch rates argued against the conversion-etch mechanism and in favor of a fluorination and ligand-exchange reaction pathway. In situ quartz crystal microbalance techniques also revealed that Ga2O3 could be etched using sequential exposures of HF and various other metal precursors. Ga2O3 etch rates at 250 °C were 1.2, 0.82, 0.85, and 0.23 Å/cycle for AlCl(CH3)2, Al(CH3)3, TiCl4, and Ga(N(CH3)2)3 as the metal precursors, respectively. The mass changes during the individual exposures of HF and the AlCl(CH3)2 and Al(CH3)3 metal precursors argued for a fluorination and ligand-exchange mechanism. The AlCl(CH3)2 and Al(CH3)3 exposures may also lead to some conversion of Ga2O3 to Al2O3. In contrast, the mass changes during the HF and TiCl4 exposures were consistent with the conversion of the surface of Ga2O3 to TiO2 and then the spontaneous removal of the TiO2 surface layer by HF. Distinctly different behavior was observed during the HF and Ga(N(CH3)2)3 exposures. The large mass gain during the Ga(N(CH3)2)3 exposures suggested that Ga(N(CH3)2)3 can adsorb on the fluorinated Ga2O3 surface prior to the ligand-exchange reaction. The wide range of metal precursors that can etch Ga2O3 argues that the ability of these precursors to convert Ga2O3 or to undergo ligand-exchange reactions provides multiple pathways for effective thermal Ga2O3 ALE.