Decrease In Etch Rate Research Articles

Effect of Cl2 Radical on Dry Development of Spin-Coated Metal Oxide Resist.

In this study, the effects of Cl2 radicals on dry development of spin-coated metal oxide resist (MOR) and changes in its surface binding states were investigated to verify the mechanism of dry development. Dry development characteristics of tin hydroxide (Tin OH), which is one of the MOR candidates for next generation lithography, were investigated as functions of process time and temperature using a Cl2 radicals source. Non-UV-exposed Tin OH film showed a linear etch rate (1.77 nm/min) from the initial thickness of ∼50 nm, while the UV-exposed film showed slower etch behavior (1.46 nm/min) in addition to the increase of film thickness for up to 3 min during the Cl2 radical dry development. UV-exposed photoresist (PR) contained more oxygen (Sn-O bonding) in the film due to the removal of butyl compounds from the clusters during the UV exposure process. Therefore, due to the lower reaction of chlorine radicals with Sn-O in the UV-exposed Tin OH than the other bindings, the non-UV-exposed PR was preferentially removed compared to the UV-exposed PR. As the temperature decreases, the overall etch rate decreases, but the difference in etch rate between exposed and unexposed Tin OH becomes larger. Finally, at a substrate temperature of -20 °C, the non-UV-exposed Tin OH with a thickness of 50 nm was completely removed, while ∼30 nm thick PR remained for UV-exposed Tin OH. Eventually, a negative tone development was possible with Cl2 radical plasma due to the difference in activation energy between the UV-exposed and non-UV-exposed films. It is believed that dry development using Cl2 radicals will be one of the most important process techniques for next-generation patterning to remove problems such as pattern leaning, line edge roughness, residue, etc., caused by wet development.

Pseudo-Wet Plasma Mechanism Enabling High-Throughput Dry Etching of SiO2 by Cryogenic-Assisted Surface Reactions.

Manufacturing semiconductor devices requires advanced patterning technologies, including reactive ion etching (RIE) based on the synergistic interactions between ions and etch gas. However, these interactions weaken as devices continuously scale down to sub-nanoscale, primarily attributed to the diminished transport of radicals and ions into the small features. This leads to a significant decrease in etch rate (ER). Here, a novel synergistic interaction involving ions, surface-adsorbed chemistries, and materials at cryogenic temperatures is found to exhibit a significant increase in the ER of SiO2 using CF4/H2 plasmas. The ER increases twofold when plasma with H2/(CF4 + H2) = 33% is used and the substrate temperature is lowered from 20 to -60°C. The adsorption of HF and H2O on the SiO2 surface at cryogenic temperatures is confirmed using in situ Fourier transform infrared spectroscopy. The synergistic interactions of the surface-adsorbed HF/H2O as etching catalysts and plasma species result in the ER enhancement. Therefore, a mechanism called "pseudo-wet plasma etching" is proposed to explain the cryogenic etching process. This synergy demonstrates that the enhanced etch process is determined by the surface interactions between ions, surface-adsorbed chemistry, and the material being etched, rather than interactions between ion and gas phase, as observed in the conventional RIE.

(Invited) Plasma-Enhanced Deposition Mechanism of Hydrogenated Amorphous Carbon Analyzed By Combining Reactive Species Measurement and Machine Learning

Hydrogenated amorphous carbon (a-C:H) thin films synthesized by plasma-enhanced chemical vapor deposition (PECVD)are widely used in various fields of science and technology. For example, they are essential as etching hard masks for the production of 3D-NAND flash memory, because of their high selectivity to Si-based materials and chemical stability. With the demand for higher aspect ratio etching, there is a strong need to improve etching resistance and reduce residual stress of a-C:H masks. They are also widely used in the field of tribology as an important coating for improving friction resistance and slidability. Analyzing the plasma process in which synergistic effects among reactive species and higher-order reactions contribute is generally difficult. Therefore, similar to other process technologies, machine learning-based condition optimization approaches have been actively investigated in recent years. However, machine learning models are generally black boxes, and it is difficult to understand their mechanism of action. Also, the versatility of the learning model is low between different devices. On the other hand, reactive species measurement is effective for understanding the reaction mechanism of the plasma process, but the synergy of multiple reactive species is still extremely complicated. In this study, the contribution of radical species to the etching resistance of a-C:H thin films were quantitatively analyzed using machine learning.The deposition of a-C:H by PECVD using H2/C3H6/CH4 plasma and etching with O2 plasma were alternately repeated at 120 conditions. The radicals generated during the film deposition were measured by a quadrupole mass spectrometer (QMS), and film thickness change during the etching was measured by spectroscopic ellipsometry (SE). A random forest model was learned to predict the etching rate from the mass spectra, and the contribution of each radical to the etching rate was quantitatively evaluated by SHapley additive exPlanation (SHAP).The distribution of SHAP values for the 10 most influential radicals is shown in Figure 1. The positive and negative SHAP values represent the correlation between QMS intensity and SHAP value. The radicals with high hydrogen ratios such as CH~CH4, C2H5, C4H9, C5H9, and C5H11 contributed to the increase of etching rate, while the radicals with low hydrogen ratios such as C3H3 and C5H5 contributed to the decrease of etching rate. The deposition of radicals with low hydrogen ratios and multiple bonds, and the effect of hydrogen extraction in the film are considered to have increased the density of a-C:H and reduced the etching rate. It was found that the control of C/H ratio of radicals as film precursors is important to improve etching resistance. Comparisons with different data set range or other contribution degrees such as LIME enable us to understand the mechanism of action of local active species. By using the contribution analysis methods, it is possible to quantitatively analyze the synergistic effect of reactive species in the process plasma. Figure 1

Effect of various pulse plasma techniques on TiO2 etching for metalens formation

TiO2 meta materials composed of high aspect ratio pillars with different widths can be used for a planar metalens with a thickness that are less than a sheet of paper and with no chromatic aberrations. In this study, TiO2 pillars with different sizes masked by ACL were etched with an inductively coupled plasma (ICP) etcher and a gas mixture of C4F8/SF6/Ar using various plasma techniques such as continuous wave (CW), bias pulsing, synchronous pulsing, and asynchronous pulsing and their TiO2 etch characteristics and plasma characteristics were compared. The results showed that the etch rates were decreased in the sequence of CW, bias pulse, synchronous pulse, and asynchronous pulse conditions. However, the etch selectivities over ACL and SiO2 and the aspect ratio dependent etching (ARDE) effect were improved in that sequence. The decrease in etch rates in that sequence was related to no power input during the processing such as no ICP source power and/or no bias power at those pulse conditions. And, the increase of etch selectivities was related to the increased CFx/F and CFx(x = 2,3)/CF in the plasmas due to less dissociation and/or more recombination of fluorocarbon gas during the pulse conditions. The improvement of the aspect ratio dependent etching (ARDE) effect was caused by the decreased compositional differences between wide TiO2 pattern area and narrow TiO2 pattern area. Especially, the ARDE effect was the lowest for the asynchronous pulse condition due to the cyclic etch characteristics composed of the reactant chemisorption step by supplying ICP power only and the etch step by supplying bias power only similar to atomic layer etching (ALE).

Investigation of a Macromolecular Additive on the Decrease of the Aluminum Horizontal Etching Rate in the Wet Etching Process

The effect of a macromolecular additive on the etching rate of aluminum (Al) horizontal etching in the wet process was investigated in this work. The horizontal etching in the Al wet etching process became more evident as the film Al becomes thicker. The proposed macromolecule additive, including polyethylene-polypropylene glycol, was added into the Al etchant solution to reduce the Al horizontal etching rate (ER). The undercut problem during metal patterning can then be improved. By using this method, the Al horizontal ER was reduced from 2.0 to 0.9 μm per minute and the selection ratio between the horizontal and vertical ER was effectively improved from 3 to 1.3 times. As well, a hypothesis of physical mechanism for the improvement was proposed. The dispersed particles from the additive emulsion inhibited the transport and exchange of liquid in a horizontal direction. This work provides an alternative reference to improve the selection ratio performance in the metal wet etching process compared with that when using traditional method.

Influences of substrate temperatures on etch rates of PECVD-SiN thin films with a CF4/H2 plasma

The dependence of substrate temperatures (50 to −20 °C) on etch rate in two kinds of PECVD SiN films were investigated by a CF4/H2 mixture plasma. The XRR and XPS results indicate that the chemical composition and film density were almost identical for the films. The FTIR shows that the ratio of NH and SiH groups were found to be significantly different in the SiN films. The NH rich films exhibited a lower etch rate at −20 °C than that observed at room temperature or higher, whereas the SiH rich films showed a higher etch rate at −20 °C. We found that the fluorocarbon thickness was thicker in the SiH rich samples than NH rich samples. The fact suggests that hydrogen originated from the broken SiH bonds enhanced the polymerization, which causes the decrease of etch rate. A thinner fluorocarbon thickness was found in the SiH rich samples at low temperature, which results in the higher etch rate. Angular-resolved XPS indicates that NH bonding formed easier on film surface at −20 °C. These results indicate that the bonding structure and substrate temperature affected the fluorocarbon thickness, fluorine reaction probability and hydrogen dissociation during the SiN etching.

Effect of water vapor on plasma processing at atmospheric pressure: Polymer etching and surface modification by an Ar/H2O plasma jet

The authors evaluate the effect of water vapor on the plasma processing of materials using a model system consisting of a well-characterized radio-frequency plasma jet, controlled gaseous environment, and polystyrene as target material. The authors find that the effluent of Ar/H2O plasma jet is capable of (1) etching polymers with relatively high etch rate and (2) weakly oxidizing the etched polymer surface by forming O containing moieties. When increasing the treatment distance between the polymer and the Ar/H2O plasma, the authors find that the polymer etch rate drops exponentially, whereas the O elemental composition of the etched surface shows a maximum at intermediate treatment distance. The OH density in the Ar/H2O jet was measured near the substrate surface by laser induced fluorescence, and the density change of the OH radicals with treatment distance is found to be consistent with the exponential decrease of polymer etch rate, which indicates that OH may play a dominant role in the polymer etching process. A control experiment of Ar/H2 plasma shows that the observed fast polymer etching by Ar/H2O plasma cannot be attributed to H atoms. By correlating the OH flux with the polymer etch rate, the authors estimated the etching reaction coefficient of OH radicals (number of C atoms removed per OH radical from the gas phase) as ∼10−2. The polymer etch rate of Ar/H2O plasma is enhanced as the substrate temperature is lowered, which can be explained by the enhanced surface adsorption of gas phase species. For the same molecular admixture concentration and plasma power, the authors find that Ar/H2O/O2 plasma has much reduced etching efficiency compared to either Ar/H2O or Ar/O2 plasma.

Etching of germanium-tin using ammonia peroxide mixture

The wet etching of germanium-tin (Ge1-xSnx) alloys (4.2% < x < 16.0%) in ammonia peroxide mixture (APM) is investigated. Empirical fitting of the data points indicates that the etch depth of Ge1-xSnx is proportional to the square root of the etch time t and decreases exponentially with increasing x for a given t. In addition, X-ray photoelectron spectroscopy results show that increasing t increases the intensity of the Sn oxide peak, whereas no obvious change is observed for the Ge oxide peak. This indicates that an accumulation of Sn oxide on the Ge1-xSnx surface decreases the amount of Ge atoms exposed to the etchant, which accounts for the decrease in etch rate with increasing etch time. Atomic force microscopy was used to examine the surface morphologies of the Ge0.918Sn0.082 samples. Both root-mean-square roughness and undulation periods of the Ge1-xSnx surface were observed to increase with increasing t. This work provides further understanding of the wet etching of Ge1-xSnx using APM and may be used for the fabrication of Ge1-xSnx-based electronic and photonic devices.

(Invited) Selective Etch of Si and SiGe for Gate All-Around Device Architecture

The next era in CMOS technology scaling seeks not only to introduce SiGe or Ge [1] but also aims at replacing the FINFET technology by a gate all around concept, requiring selective removal of Si1-xGex to Si1-yGey(with 0 ≤ x, y ≤ 1). Gate All-Around (GAA) integration schemes are investigated for improved short channel control. In this paper we will show how alkaline solutions can be used for selective etch of Si and SiGe up to 50% and that the alkaline etch behavior of SiGe is not merely an extension of Si, even at low to moderate Ge concentrations (Ge ≤ 50%). The exponential decrease in alkaline etch rate of SiGe with increasing Ge concentration enables the selective removal of Si to Si0.75Ge0.25. [2] Alkaline etching of Si has been studied extensively and is very well understood. [3] The oxidation of Si by water is catalyzed by the hydroxyl anion and results in anisotropic etch properties. The Si (001) and (110) directions etch much faster compared to the Si (111) crystallographic planes. The alkaline dissolution mechanism of SiGe is much less studied and understood. Figure 1 shows a Si-Si0.75Ge0.25 multilayer stack after alkaline selective etch in 5% TMAH at 60¢ªC. The stack was made by FIN patterning of an epitaxial Si0.75Ge0.25 / Si / Si0.75Ge0.25 / Si multilayer. The TEM picture shows the complete removal of the top Si layer. The selective etch of the Si layer sandwiched between two Si0.75Ge0.25 layers is limited by the slowly etching Si (111) planes, resulting in the formation of inverted pyramids. Figure (1) shows that the Si0.75Ge0.25 layer is also limited by the (111) crystallographic planes. The shape of convex corners results from the crystallographic planes with the highest etch rate. [4] The pyramidal shape of the Si0.75Ge0.25 layer indicates that the Si0.75Ge0.25(111) plane etches faster than (110). This is very different from Si where (111) planes etch much slower than the (110) planes. In this paper we will show how SiGe (Ge ≤ 50%) alkaline etching behaves differently compared to Si and its implications to Si or SiGe selective etch. In addition we will show how improper control of process conditions can change SiGe etching from anisotropic to isotropic. [1] M.L. Lee et al. Appl. Phys. Rev. 97 (2005) 011101. [2] Seidel et al. J. Electrochem. Soc. 137 (1990) 3626. [3] M.A. Hines, Annu Rev Phys Chem 54 (2003) 29-56. [4] P. Pal et al. J. Micromech. Microeng 17 (2007) R111-R133. Figure 1

Three-dimensional kinetic Monte Carlo simulations of diamond chemical vapor deposition.

A three-dimensional kinetic Monte Carlo model has been developed to simulate the chemical vapor deposition of a diamond (100) surface under conditions used to grow single-crystal diamond (SCD), microcrystalline diamond (MCD), nanocrystalline diamond (NCD), and ultrananocrystalline diamond (UNCD) films. The model includes adsorption of CHx (x = 0, 3) species, insertion of CHy (y = 0-2) into surface dimer bonds, etching/desorption of both transient adsorbed species and lattice sidewalls, lattice incorporation, and surface migration but not defect formation or renucleation processes. A value of ∼200 kJ mol(-1) for the activation Gibbs energy, ΔG(‡) etch, for etching an adsorbed CHx species reproduces the experimental growth rate accurately. SCD and MCD growths are dominated by migration and step-edge growth, whereas in NCD and UNCD growths, migration is less and species nucleate where they land. Etching of species from the lattice sidewalls has been modelled as a function of geometry and the number of bonded neighbors of each species. Choice of appropriate parameters for the relative decrease in etch rate as a function of number of neighbors allows flat-bottomed etch pits and/or sharp-pointed etch pits to be simulated, which resemble those seen when etching diamond in H2 or O2 atmospheres. Simulation of surface defects using unetchable, immobile species reproduces other observed growth phenomena, such as needles and hillocks. The critical nucleus for new layer growth is 2 adjacent surface carbons, irrespective of the growth regime. We conclude that twinning and formation of multiple grains rather than pristine single-crystals may be a result of misoriented growth islands merging, with each island forming a grain, rather than renucleation caused by an adsorbing defect species.

Application of the Level Set Method in Three-Dimensional Simulations of the Profile Evolution

In this paper, the angular dependence of the etching rate is applied in the three dimensional simulations of the profile evolution during etching process. Etching rates are determined from the silicon symmetryproperties by the interpolation technique using experimentally obtained values. The influence of the KOH concentration on the etch rate is also discussed. Asthe etchant concentration increases the etching rate decreases, while increasing etching temperat ure resulted in increased etching rate. Threedimensional simulation results were obtained using an extension of the sparse field method for solving level set equations that describe the time evolution of the surface during etching process. The obtainedsimulation results illustrate that relatively simple model with only four parameters describes convex corner undercutting correctly.

Abrasive waterjet micro-machining of channels in metals: Comparison between machining in air and submerged in water

Abrasive water jet technology can be used for micro-milling using recently developed miniaturized nozzles. Abrasive water jet (AWJ) machining is often used with both the nozzle tip and workpiece submerged in water to reduce noise and contain debris. This paper compares the performance of submerged and unsubmerged abrasive water jet micro-milling of channels in 316L stainless steel and 6061-T6 aluminum at various nozzle angles and standoff distances. The effect of submergence on the diameter and effective footprint of AWJ erosion footprints was measured and compared. It was found that the centerline erosion rate decreased with channel depth due to the spreading of the jet as the effective standoff distance increased, and because of the growing effect of stagnation as the channel became deeper. The erosive jet spread over a larger effective footprint in air than in water, since particles on the jet periphery were slowed much more quickly in water due to increased drag. As a result, the width of a channel machined in air was wider than that in water. Moreover, it was observed that the instantaneous erosion rate decreased with channel depth, and that this decrease was a function only of the channel cross-sectional geometry, being independent of the type of metal, the jet angle, the standoff distance, and regardless of whether the jet was submerged or in air, in either the forward or backward directions. It is shown that submerged AWJM results in narrower features than those produced while machining in air, without a decrease in centerline etch rate.

Impact of doping level on the metal assisted chemical etching of p-type silicon

The impact of doping level on the metal assisted chemical etching of p-type silicon using silver particles is investigated. 12 boron doped wafers with doping levels ranging from 1012 up to 1019cm−3 were used. Silver particles generated from a sputter-deposited thin film via a dewetting procedure act as catalyst in a HF:H2O2 etch mixture. The characteristics of the etch rate as a function of time was investigated, revealing in addition an enhanced material removal at the beginning (i.e. in the range of several seconds) and an undesired etching of silver free regions at longer etch times (i.e. >15min). Furthermore, a strong decrease in etch rate at doping levels exceeding 1016cm−3 is observed. Only at these highly doped samples, a porous sub-structure is detected located adjacent and in-between the pores which are generated by the Ag particles. It is assumed that the injected holes representing majority charge carriers in p-type silicon diffuse at higher doping levels faster and more efficiently away from the metal-semiconductor interface into the silicon sample resulting in a decreased etch rate.

Chemical resistance of thin film materials based on metal oxides grown by atomic layer deposition

Etching rate of technologically important metal oxide thin films in hot sulphuric acid was investigated. The films of Al-, Ti-, Cr-, and Ta-oxides studied were grown by atomic layer deposition (ALD) method on silicon substrates from different precursors in large ranges of growth temperatures (80–900°C) in order to reveal process parameters that allow deposition of coatings with higher chemical resistance. The results obtained demonstrate that application of processes that yield films with lower concentration of residual impurities as well as crystallization of films in thermal ALD processes leads to significant decrease of etching rate. Crystalline films of materials studied showed etching rates down to values of <5pm/s.

Effect of H2 Flow Rate on High-Rate Etching of Si by Narrow-Gap Microwave Hydrogen Plasma

For the purpose of realizing a low-cost production process of silane (SiH4) gas, we have proposed the high-rate etching of metallurgical-grade Si by narrow-gap microwave hydrogen plasma. In this paper, effect of hydrogen gas flow rate (0–10 L/min) on the etch rate has been investigated and correlated with the relative variation of hydrogen-atom density estimated by actinometry. By decreasing hydrogen gas flow rate, the etch rate gradually increases up to the maximum value of 11 μm/min at 2 L/min. This increase is well correlated with the increase of hydrogen-atom density due to the longer residence time of hydrogen molecules in the plasma. On the other hand, when the gas flow rate is lower than 2 L/min, the etch rate abruptly decreases with decreasing gas flow rate in spite of the increase of hydrogen-atom density. From the surface observations and Raman measurements, it is found that the decrease in etch rate in the lower flow rate range is attributed to the formation of microcrystalline Si particles due to the decomposition of generated-SiH4 molecules in the plasma.

Reactive ion etching of Si2Sb2Te5 in CF4/Ar plasma for a nonvolatile phase-change memory device

Phase change random access memory (PCRAM) is one of the best candidates for next generation nonvolatile memory, and phase change Si2Sb2Te5 material is expected to be a promising material for PCRAM. In the fabrication of phase change random access memories, the etching process is a critical step. In this paper, the etching characteristics of Si2Sb2Te5 films were studied with a CF4/Ar gas mixture using a reactive ion etching system. We observed a monotonic decrease in etch rate with decreasing CF4 concentration, meanwhile, Ar concentration went up and smoother etched surfaces were obtained. It proves that CF4 determines the etch rate while Ar plays an important role in defining the smoothness of the etched surface and sidewall edge acuity. Compared with Ge2Sb2Te5, it is found that Si2Sb2Te5 has a greater etch rate. Etching characteristics of Si2Sb2Te5 as a function of power and pressure were also studied. The smoothest surfaces and most vertical sidewalls were achieved using a CF4/Ar gas mixture ratio of 10/40, a background pressure of 40 mTorr, and power of 200 W.

Investigations on the Si/SiO2 interface defects of silicon nanowires

Silicon nanowires have been fabricated by the metal-assisted chemical etching method. The etching rate of silicon can be adjusted by changing the concentration of titanium in the solution. Investigations by electron spin resonance of silicon nanowires show that Pb0 center is the prominent defect at the Si/SiO2 interfaces, and its density decreases as the etching rate decreases. The red-shift of the Raman peak with decrease of the etching rate indicates a progressive increase of tensile strain in Si at the interface, which induces the shortening of the length of the Si–O bond confirmed by the Fourier transform infrared spectrometer investigations.

Effects of Surface Temperature on High-Rate Etching of Silicon by Narrow-Gap Microwave Hydrogen Plasma

Effects of surface temperature on high-rate etching of Si by narrow-gap microwave hydrogen plasma have been investigated. The etch rate strongly depended on the surface temperature. The optimum temperature for the etching at high rate was about 70 °C. With increasing the temperature higher than 70 °C, decrease in etch rate was observed, and activation energy for the reaction process was estimated to be about -1.8 kcal/mol. This value agreed well with the result of the previous studies using low-pressure plasma conditions. In addition, hydrogen concentration was also affected by the surface temperature. The reduction in hydrogen concentration near the surface was observed in the etched sample at higher temperature. From these results, reaction mechanism causing the decrease in etch rate with the temperature increase has been discussed in terms of both desorption and in-diffusion of hydrogen from the surface.

Absolute etch rates in alkaline etching of silicon (1 1 1)

The absolute etch rate of silicon (1 1 1) during wet chemical etching in aqueous KOH solution has been investigated with optical interferometry, using masked samples. The etch rate is constant at 0.62 ± 0.07 μm/h and independent of alkaline concentration for 1–5 M KOH solutions at 60 °C. Only at lower alkaline concentrations, the etch rate decreases. Adding isopropanol slightly lowers the absolute etch rate. The activation energy of the etching reaction is 0.61 ± 0.03 eV in standard KOH solutions and 0.62 ± 0.03 eV with 1 M isopropanol added to the solution. This indicates that the reaction is determined by reaction kinetics and not by transport limitations. In all cases the surfaces are covered by shallow etch pits, not related to defects in the crystal. This implies that the actual factor that determines the etch rate is the 2D nucleation of new vacancy islands at the bottom of these pits. This process is likely catalyzed by a local accumulation of reaction products, which preferentially occurs near the mask edges.

Neutral species in inductively coupled SF6/SiCl4 plasmas

Inductively coupled SF6/SiCl4 plasmas interacting with a bulk silicon substrate and a SiO2-coated substrate have been investigated. Mass spectrometry and optical emission spectroscopy diagnostics were used to characterize the neutral population in the diffusion chamber. SiF4 molecules were detected as the dominant species, and their formation has been attributed to the high reactivity of F radicals with SiClx species. In a complementary experiment, a silicon chloride layer was deposited on the reactor walls during a SiCl4 plasma step and subsequently etched by a SF6 plasma. Time-resolved measurements of the neutral densities during the SF6 plasma step showed the importance of heterogeneous reactions between impinging F radicals and SiClx species deposited on the reactor walls. In SF6/SiCl4 plasmas, these reactions lead to a depletion in F radicals, which results in a decrease in the silicon substrate etch rate. Furthermore, this impacts on the concentration of SFx species and on the creation of new species, such as ClF, SF5Cl and S2Cl2.