Si Etch Rate Research Articles

ON MECHANISMS OF OXYGEN INFLUENCE ON GAS-PHASE PARAMETERS AND SILICON REACTIVE-ION ETCHING KINETICS IN HBr + Cl2 + O2 PLASMA

The effects of both HBr/O2 and Cl2/O2 mixing ratios in HBr+Cl2+O2 gas mixture on plasma parameters, steady-state densities of active species and Si etching kinetics were studied under the typical conditions of reactive ion etching process: total gas pressure (p = 10 mTorr), input power (W = 500 W), bias power (Wdc = 200 W). The data on internal plasma parameters and plasma chemistry were obtained using a combination of Langmuir probe diagnostics and 0-dimensional (global) plasma modeling. It was found that the variation in HBr/O2 mixing ratio at constant Cl2 fraction in a feed gas is characterized by the stronger impact on the steady-state plasma composition through both electron-impact and atom-molecular reaction kinetics as well as allows one to obtain the wider change in the total halogen atom density. It was shown that changes in both HBr/O2 and Cl2/O2 mixing ratios toward O2-rich plasmas lowers the Si etching rate that exhibits no evident correlations with total halogen atom flux and ion energy flux. The model-based analysis of Si etching kinetics allowed one to conclude that the effective reaction probability for Si + Cl/Br heterogeneous reaction depends on the flux of oxidative species – oxygen atoms and OH radicals. The reasons may be 1) the oxidation of silicon resulting in higher reaction threshold energy; and 2) the decreasing fraction of free adsorption sites for Cl/Br atoms due to the oxidation of reaction products into the lower volatile SiBrxOy and SiClxOy compounds.

Small Plasma Space with a Small Plasma Source and Its Advantage in Minimal Fab

We have developed an inductively-coupled plasma-reactive ion etching system (ICP-RIE) as a human-size tool of Minimal Fab for processing a half-inch wafer. The etching system has performed a Bosch etching process with a short switching cycle in a small chamber with a volume of 1/4 L. For the small chamber, we use a frequency of 100 MHz that is higher than the typical frequency of 13.56 MHz. The power consumption at 100 MHz is only ～ 40W. The Si etching rate of the Bosch process is ～ 2.5 μm/min. Moreover, residence times of deposition gas (C4F8) and etching gas (SF6) are estimated to be ～ 0.2 second, and actually waiting times until feeding one of the gases into the chamber after the stop feeding the other gas are nominally set to be zero. The resultant Bosch cycle time of the alternative feeds of the two gases is only 2 sec. For the high-speed Bosch cycle of 2 sec, the resultant etching sidewall of Si structure becomes a scallop-less straight wall.

Sub-20 nm Si fins with high aspect ratio via pattern transfer using fullerene-based spin-on-carbon hard masks

We report on a novel and simple pattern transfer process into Si via fullerene-based spin-on-carbon (SOC) hard masks in this work. Electron beam lithography and extreme ultraviolet interference lithography techniques are used to pattern high-resolution and dense lines on a resist/SOC bilayer. The patterns are subsequently transferred by a low-pressure O2 plasma etching (SOC) and reactive ion etching process with a gas mixture of SF6 and C4F8 (Si). Si sidewall trimming can be controlled by modifying the Si etching rate, achieving Si fins with dimension down to 15 nm half-pitch with aspect ratio as high as of 7:1.

Plasma parameters, gas-phase chemistry and Si/SiO2 etching mechanisms in HBr+Cl2+O2 gas mixture: Effect of HBr/O2 mixing ratio

Features of plasma chemistry as well as peculiarities of etching mechanisms for Si and SiO2 in the HBr + Cl2 + O2 gas mixture with variable HBr/O2 mixing ratio were investigated using a combination of experimental and theoretical approaches. Plasma diagnostics by Langmuir probe and 0-dimensional (global) plasma modeling provided the information on gas-phase plasma parameters, formation/decay kinetics of plasma active species and the steady-state plasma composition. It was found that the substitution of HBr for O2 at constant Cl2 fraction in a feed gas 1) leads to a weak increase in electron density, ion density and the ion energy flux; 2) influences the kinetics of neutral species through reactions involving O atoms and their reaction products; and 3) results in increasing total halogen atom density. It was shown also that the change in HBr/O2 mixing ratio causes the opposite behaviors of Si and SiO2 etching rates and lifts the SiO2/Si etching selectivity up to ∼10 in highly oxygenated plasmas. The analysis of Si and SiO2 etching kinetics with model-predicted fluxes of plasma active species indicated the different etching mechanisms for these materials.

Peculiarities of Si and SiO2 Etching Kinetics in HBr + Cl2 + O2 Inductively Coupled Plasma

Peculiarities of the etching kinetics and mechanisms for Si and SiO2 in the HBr + Cl2 + O2 inductively coupled plasma were investigated by analyzing the relationships between etching rates and fluxes of active species. The data on plasma parameters, plasma chemistry, and the steady-state plasma composition were obtained using both Langmuir probe diagnostics and 0-dimensional plasma modeling. It was found that an increase in the Cl2 mixing ratio and input power causes similar trends in the changes in ion energy flux and halogen atom flux but results in different tendencies for both Si and SiO2 etching rates. It was shown that the influence of input process parameters (HBr/Cl2 mixing ratio, input power, and bias power) on the Si and SiO2 etching kinetics may be adequately described in terms of the oxygen atom flux-sensitive reaction probability. The latter directly correlates with the oxygen atom flux/ion energy flux ratio.

A model for etching of three-dimensional high aspect ratio silicon structures in pulsed inductively coupled plasmas

A coupled plasma—feature scale model is employed to investigate the etching of high aspect ratio (HAR) silicon (Si) structures using a cyclic multi-step pulsed plasma process in an inductively coupled plasma (ICP) reactor. This process sequence includes an oxidation step to help protect the Si sidewalls, a main Si etch step where the ion energy and angular distribution (IEAD) and ion/neutral flux ratio are controlled through power pulsing, and a clean step prior to repeating the multi-step process. Two-dimensional plasma models are used to compute the IEAD as well as the fluxes of relevant ions and neutral radicals at the wafer. These plasma models are coupled to a three-dimensional feature scale model, where multiple cycles of the three-step etch sequence are simulated. The paper focuses on evaluation of several pulsing modes during the main Si etch step including separate pulsing of the ICP source or RF bias power, and their synchronized pulsing (with phase control). Process performance has been quantitatively evaluated by examining etch rates for Si and the SiO2-like mask, Si/mask etch selectivity, and CDs within the HAR features. When only the RF bias power is pulsed, Si and mask etch rates scale with pulse duty cycle (DC). As a result, if Si is etched to the same depth, the HAR trenches are wider at higher DCs due to less total oxidation time and less protection of the sidewalls. ICP source power pulsing provides higher Si etch rate because of RF bias power being on continuously, but suffers from poor mask selectivity. Synchronized pulsing of both the ICP source and RF bias powers in conjunction with phase control provides additional flexibility in modulating the IEAD and the ion/neutral flux ratio. RF bias pulsing and in-phase synchronized pulsing yield the best selectivity for the conditions explored.

Facile morphology control of high aspect ratio patterned Si nanowires by metal-assisted chemical etching

Facile and effective method to fabricate highly ordered silicon nanowires (SiNWs) using metal-assisted chemical etching (MACE) was demonstrated. MACE solutions with various concentrations were studied to understand the etching mechanism for patterned Si substrates with different doping concentrations. MACE rate of Si (100) at different time periods was studied with different doping concentrations (p, p+, n, and n+) at a MACE solution concentration of 5:1:1 for an accurate morphology control and reproducibility of the SiNWs. Based on a four-step model, the SiNW formation mechanism was proposed involving anisotropic etching of SiNWs based on hole transfer between Au/Si interfaces exposed when subjected to MACE solution. Time dependent variation in etch rate of Si to fabricate SiNWs was observed with different doping concentration. The effect of the doping concentration on the etching was revealed based on band diagrams. However, agglomeration of p+-SiNWs was observed, which was attributed to their doping and ability to act against various forces like surface tension during drying. Different aspect ratios of SiNWs were observed for different time periods; n+-SiNWs exhibited the maximum aspect ratio of approximately 81. A visible-light absorbance analysis revealed the potential of the synthesized SiNWs can be good base and host materials for various light harvesting and energy storage devices.

Effective improvement in the etching characteristics of Si{110} in low concentration TMAH solution

An aqueous tetramethylammonium hydroxide (TMAH) solution is widely used for silicon wet anisotropic etching to perform bulk micromachining for the fabrication of microstructures on a silicon wafer. To reduce the etching time to increase the productivity, etchant must provide high etch rate. In the present work, the etching characteristics of Si{110} in low concentration TMAH (5 wt%) with the addition of various concentrations (5–20%) of reducing agent hydroxylamine (NH2OH) have been studied to increase the etch rate of Si{110} to reduce the etch time for the fabrication of microstructures. Moreover, it is aimed to enhance the undercutting at convex corner for the fast release of the structure. The etch rate of Si{110} and the undercutting at convex corners with the addition of NH2OH increases by more than three times that in pure TMAH. In addition to the etch rate and undercutting, the effect of NH2OH on etched surface morphology is investigated systematically. The present study is focused to enhance the application of wet etching in silicon micromachining for the fabrication of various kinds of microstructures for applications in microelectromechanical systems.

Behavior Analysis of Si Etching Process with HF/HNO<sub>3</sub> Mixture in Single-Spin Wafer Process

The HF/HNO3mixture Si etching process is widely used to remove stress and damaged layers after Si wafer back grinding. Although there have been many reports on the dip process, there have been few detailed reports on the single-spin process. In a single spin process, Si etch rate distributions largely differ with different HF/HNO3concentrations. On the other hand, thermal SiO2etch rate distributions are similar even with different HF/HNO3concentrations. In this work, we analyzed Si surfaces with XPS (X-ray Photoelectron Spectroscopy) after processing various HF/HNO3mixture concentrations. SiOxstays steady in any wafer position and HNO3concentration, whereas SiO2thickens depending on HNO3concentration at the center. We assumed that Si etch rate distributions were caused by HF or HNO3consumption and confirmed this assumption was correct in a wafer center SiN cover experiment.

On-site SiH4 generator using hydrogen plasma generated in slit-type narrow gap

We have been developing an on-site silane (SiH4) generator based on use of the chemical etching reaction between solid silicon (Si) and the high-density H atoms that are generated in high-pressure H2 plasma. In this study, we have developed a slit-type plasma source for high-efficiency SiH4 generation. High-density H2 plasma was generated in a narrow slit-type discharge gap using a 2.45 GHz microwave power supply. The plasma’s optical emission intensity distribution along the slit was measured and the resulting distribution was reflected by both the electric power distribution and the hydrogen gas flow. Because the Si etching rate strongly affects the SiH4 generation rate, the Si etching behavior was investigated with respect to variations in the experimental parameters. The weight etch rate increased monotonically with increasing input microwave power. However, the weight etch rate decreased with increasing H2 pressure and an increasing plasma gap. This reduction in the etch rate appears to be related to shrinkage of the plasma generation area because increased input power is required to maintain a constant plasma area with increasing H2 pressure and the increasing plasma gap. Additionally, the weight etch rate also increases with increasing H2 flow rate. The SiH4 generation rate of the slit-type plasma source was also evaluated using gas-phase Fourier transform infrared absorption spectroscopy and the material utilization efficiencies of both Si and the H2 gas for SiH4 gas formation were discussed. The main etch product was determined to be SiH4 and the developed plasma source achieved a SiH4 generation rate of 10 sccm (standard cubic centimeters per minute) at an input power of 900 W. In addition, the Si utilization efficiency exceeded 60%.

Anisotropic selective etching between SiGe and Si

In Si/SiGe dual-channel FinFETs, it is necessary to simultaneously control the etched amounts of SiGe and Si. However, the SiGe etch rate is higher than the Si etch rate in not only halogen plasmas but also physical sputtering. In this study, we found that hydrogen plasma selectively etches Si over SiGe. The result shows that the selectivity of Si over SiGe can be up to 38 with increasing Ge concentration in SiGe. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) results indicate that hydrogen selectively bonds with Si rather than with Ge in SiGe. During the etching, hydrogen-induced Si surface segregation is also observed. It is also observed that the difference in etched amount between SiGe and Si can be controlled from positive to negative values even in Si/SiGe dual-channel fin patterning while maintaining the vertical profiles. Furthermore, no plasma-induced lattice damage was observed by transmission electron microscopy for both Si and SiGe fin sidewalls.

Difference in anisotropic etching characteristics of alkaline and copper based acid solutions for single-crystalline Si

The so called inverted pyramid arrays, outperforming conventional upright pyramid textures, have been successfully achieved by one-step Cu assisted chemical etching (CACE) for light reflection minimization in silicon solar cells. Due to the lower reduction potential of Cu2+/Cu and different electronic properties of different Si planes, the etching of Si substrate shows orientation-dependent. Different from the upright pyramid obtained by alkaline solutions, the formation of inverted pyramid results from the coexistence of anisotropic etching and localized etching process. The obtained structure is bounded by Si {111} planes which have the lowest etching rate, no matter what orientation of Si substrate is. The Si etching rate and (100)/(111) etching ratio are quantitatively analyzed. The different behaviors of anisotropic etching of Si by alkaline and Cu based acid etchant have been systematically investigated.

On the relationships between plasma chemistry, etching kinetics and etching residues in CF4+C4F8+Ar and CF4+CH2F2+Ar plasmas with various CF4/C4F8 and CF4/CH2F2 mixing ratios

In this work, we investigated how the CF4/C4F8 and CF4/CH2F2 mixing ratios in CF4+C4F8+Ar and CF4+CH2F2+Ar inductively coupled plasmas influence plasma parameters, densities and fluxes of plasma active species and etching characteristics (process kinetics, etching rates and selectivities, etching residues) for both Si and SiO2. For this purpose, we employed surface diagnostics by x-ray photoelectron spectroscopy (XPS), plasma diagnostics by Langmuir probes and 0-dimensional plasma model. It was found that the substitution of CF4 for CH2F2 causes the stronger decreases in both F atom flux and ion energy flux compared with the effect of C4F8. Accordingly, the CF4+CH2F2+Ar mixture provides the deeper fall of both Si and SiO2 etching rates, leaves the higher amount of the fluorocarbon polymer on the etched surface (especially in the case of Si) and results in a bit higher SiO2/Si etching selectivity. It was shown also that, in both gas systems, the etching process appears in the steady-state regime. The mechanisms influencing the etching/polymerization balance were discussed based on the correlation between Si and SiO2 etching rates with fluxes of plasma active species.

Atmospheric-pressure plasma jet system for silicon etching without fluorocarbon gas feed

We developed an atmospheric-pressure plasma jet (APPJ) system with a tungsten rod electrode coated with C2F4 particles of approximately 0.3 µm diameter for the surface treatment of a silicon wafer. The APPJ was generated by dielectric barrier discharge with a driving frequency of 22 kHz using a He gas flow. The characteristics of the APPJ were examined under various experimental conditions. The plasma jet length increased proportionally to the electric field. It was found that the treatment area of the silicon wafer was approximately 1 mm in diameter. By atomic force microscopy analysis, minute irregularities with a maximum length of about 600 nm and part of a ring-shaped trench were observed. A Si etching rate of approximately 400 nm/min was attained at a low power of 6 W and a He flow rate of 1 L/min without introducing molecular gas including F atoms.

Role of NO in highly selective SiN/SiO2 and SiN/Si etching with NF3/O2 remote plasma: Experiment and simulation

This paper describes the study of mechanisms of highly selective silicon nitride etching, in particular, the role of NO in silicon nitride etching by atomic fluorine. This paper presents experimental and simulation data about SiN, SiO2, and Si etching with NF3/O2 remote plasma. Quantum chemistry simulations show that NO reacts with the F–N bond in a SiN cluster with lower activation energy than the F-atom. Thus, NO increases the rate of fluorine migration on the silicon nitride surface from the nitrogen atom to the silicon atom during the etching process. In the absence of NO, such migration proceeds with relatively high activation energy, which limits the etching rate. The analytical model based on the SiN etching mechanism taking into account the fluorine migration is proposed. The results of calculations with the analytical model show a good agreement with the experimental SiN, SiO2, and Si etch rates.

Influence of block copolymer feature size on reactive ion etching pattern transfer into silicon

A successful realisation of sub-20 nm features on silicon (Si) is becoming the focus of many technological studies, strongly influencing the future performance of modern integrated circuits. Although reactive ion etching (RIE), at both micrometric and nanometric scale has already been the target of many studies, a better understanding of the different mechanisms involved at sub-20 nm size etching is still required. In this work, we investigated the influence of the feature size on the etch rate of Si, performed by a cryogenic RIE process through cylinder-forming polystyrene-block-polymethylmethacrylate (PS-b-PMMA) diblock copolymer (DBC) masks with diameter ranging between 19–13 nm. A sensible decrease of the etch depth and etch rate was observed in the mask with the smallest feature size. For all the DBCs under investigation, we determined the process window useful for the correct transfer of the nanometric cylindrical pattern into a Si substrate. A structural and physicochemical investigation of the resulting nanostructured Si is reported in order to delineate the influence of various RIE pattern effects. Feature-size-dependent etch, or RIE-lag, is proved to significantly affect the obtained results.

Modification of DHF for Removal of Metals from Silicon Wafer

With continued advances in logic devices below 10 nm design rule, the required specification for ultraclean wafer surface becomes very challenging to achieve zero defects in particles below 20 nm and lower than 0.5X1010 atoms/cm2 metal ions on the final substrate surface. Excluding crystal originated defects such as pits, stacking faults and dislocation, an ultraclean wafer surface is characterized as a surface which is particle, organic, metallic contamination, and surface roughness free. Traditional batch type RCA based wet chemical processing has been used as a major cleaning method in silicon wafer manufacturing. The biggest problem of this RCA based wet cleaning method is its side effect; a target contaminant is being removed, whereas another contaminant gets redeposited on the substrate surface. For example, APM (Ammonia Peroxide Mixture) cleaning is extremely effective in removing particle and organic contaminant, but it allows metallic contaminant to redeposit on substrate surface since the APM solution contains traces of metal ions. HPM (Hydrochloric Acid Peroxide Mixture) cleaning can remove the metal contaminant. However, it causes particle contaminant to redeposit on substrate surface because of the electrostatic attraction. The zeta potential of particles is almost positive, whereas the zeta potential of Si surface is negative in acidic solution. Since the level of a contaminant in particles and metallic concentration on the surface requires an extreme control, these side effects in conventional RCA chemistry could not be tolerated in 10 nm below device substrates. In this study, the possible chemistry modification was attempted to control both particle and metallic contaminants in DHF chemistry at final silicon wafer cleaning step. We have optimized the metal contamination procedure for Al and Cu with their analysis method with AFM by depositing nano metal particles on a silicon wafer. Al was chemically deposited on Si, whereas Cu was electrochemically deposited. Various additives such as ozone and chelating agents were chosen to add in DHF solutions to control metallic and particulate contaminants. The metal contaminants were removed by Si etching in HF and ozone mixture. The etching rate of Si increased with ozone and HF concentrations increasing. The chelating agent and HF mixture solution did not etch the silicon but oxide. The removal rates of silicon and oxide were measured in HF-based chemicals. The metal removal efficiency (MRE) has measured in various HF based solutions for Al and Cu. HF and ozone mixture was very effective in removing both metallic contaminants, but it has a certain limitation in controlling particles on silicon. The chelating agents were selected to control both metal and particles. CA 1 (chelating agent 1) and CA2 have chosen for experiments. The concentration was optimized to increase the contaminant removal efficiency. MRE higher than 95% was achieved by adding chelating agents in DHF solutions without causing a particle redeposition.

Effect of Surfactant and Alcohol Additives on Etching Characteristics in Aqueous Potassium Hydroxide Solutions

Anisotropic wet etching is a simple and low cost technique for silicon bulk micromachining to fabricate microelectromechanical systems (MEMS) components. In this work, we have systematically studied the etching characteristics of Si{100}, Si{110}, and Si{111}) in various concentration of KOH (5-40 wt% in step of 5 wt%) without and with addition of surfactant Triton-X-100 and isopropyl alcohol (IPA). KOH solution with addition of IPA and Triton is termed as ternary solution. The addition of Triton and IPA affects the etching characteristics dramatically, especially the undercutting at convex corners on Si{100} surface and the etch rate of Si{110}. The undercutting at convex corners and the etch rate of Si{110} in ternary solution suppress to a significantly low level. Low undercutting is very useful for the formation of microstructure with protected corners such as mesa, while significantly low etch rate of Si{110} can be exploited to fabricate microstructure with 45º sidewall (or 45º micromirror). In addition, ternary solution with low concentration KOH provides smooth etched surface morphology.

Silicon Anisotropic Etching in Ternary Solution Composed of TΜΑΗ+Triton+NH2ΟΗ

Present work reports the anisotropic etching characteristics of silicon in various concentrations of tetramethylammonium hydroxide (TMAH) without and with addition of 10% hydroxylamine (NH2OH) and Triton X-100 ranging from ppb to ppm. The etch rate, undercutting at convex corners, and etched surface morphology are investigated in pure TMAH, TMAH+NH2OH, and different compositions of TMAH+NH2OH+Triton (i.e. ternary solution). High etch rate and undercutting are achieved in NH2OH-added 15 wt% TMAH, while 25 wt% TMAH + 10% NH2OH + 1 ppm Triton provided significantly less corner undercutting with reasonably good etch rate of Si{100}. This work is very useful for engineering applications for the fabrication of microstructures using silicon wet bulk micromachining for MEMS-based devices.

Nonlinear Etch Rate of Au-Assisted Chemical Etching of Silicon.

We demonstrated time-dependent mass transport mechanisms of Au-assisted chemical etching of Si substrates. Variations in the etch rate and surface topology were correlated with catalyst features and etching duration. Nonlinear etching characteristics were associated with the formation of pinholes and whiskers. Variable rates of mass transport as a function of whisker density accounted for the nonlinear etch rates of Si. Nanopinholes on Au catalysts facilitated the vertical mass transport of reactants and byproducts, which dramatically changed the etch rate, surface topology, and porosity of Si. The suggested transport models describe the transient mass transport and the corresponding chemical reactions.