Increase In Etch Rate Research Articles

Low-damage etching of poly-Si and SiO2 via a low-energy electron beam in inductively coupled CF4 plasma

Abstract Electron-assisted etching of poly-Si and SiO2 is performed via a grid system in inductively coupled CF4 plasma. The feasibility of electron-assisted etching is discussed with a focus on the low-surface damage of the etching. The etch rate increases with electron beam energy, which indicates that the electrons assist the surface etching process. To verify this, etching of poly-Si and SiO2 is performed in several plasma conditions, which leads to differences in etch rate that depend on the presence or absence of radicals and electron beams. Poly-Si and SiO2 are not etched without radicals of CF4 plasma, but they are etched when such radicals are present. When the electron beam and radicals exist simultaneously, the etch rate increases more dramatically than in the case of a CF4 plasma without an electron beam, demonstrating that the electron beam assists the etching process. Optical emission spectroscopy is employed to verify the F radical does not affect the etch rate increase. The surface roughness is measured after electron-assisted etching and compared with the surface roughness after ion-assisted etching.

(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

Thickness and porosity characterization in porous silicon photonic crystals: The etch-stop effect

One-dimensional photonic crystals and microcavities were fabricated using porous silicon via anodization. The study investigated the effect of an etch-stop layer, which is included between two adjacent porous layers to resolve the issue of HF diffusion during pore formation. Although proposed as a solution, no systematic investigation had been conducted on this parameter until now. Our study found that the etch-stop layer induces a redshift in the photonic bandgap, regardless of the electrolyte solution used. The fitting analysis of both single and multilayer structures revealed that the redshift resulted from an increase in the silicon etch rate and a decrease in porosity due to the enhancement of HF diffusion through the porous structure. Furthermore, an analysis of oxidation as a function of temperature demonstrated that the inclusion of the etch-stop layer led to the formation of porous layers with varying microstructures. This effect was particularly pronounced in devices where the first upper layer had high porosity. The thermal treatment caused a photonic bandgap inhibition, which was linked to the contraction-expansion phenomena of the low and high porosity layers, respectively. This resulted in the partial violation of Bragg's law. Overall, our study provides insights into the influence of etch-stop layers on the optical properties of porous silicon-based photonic structures.

Electron-assisted PR etching in oxygen inductively coupled plasma via a low-energy electron beam

In this study, electron-assisted photoresist (PR) etching is conducted using oxygen inductively coupled plasma at a pressure of 3 mTorr. During the PR etching, a low-energy electron beam is generated and is controlled by varying the acceleration voltage (0–40 V) on the grid to assist with the PR etching. When a low acceleration voltage (<20 V) is applied, no electron beam is generated, and PR etching is assisted by the accelerated ions. However, the acceleration voltage is increased (about 20–25 V), an electron beam is generated, and PR etching is assisted by the electron beam. At high acceleration voltages (>25 V), the etch rate increases, and the ion bombardment energy decreases with increasing electron beam energy. The electron energy probability function is measured to verify the relation between the etch rate and acceleration voltage with respect to the sheath thickness on the grid. Furthermore, low contribution of the O radical to the etch rate increment is observed via optical emission spectroscopy measurements.

(Invited) Chemical Approaches to Etch and Pattern Copper Films

Microelectronic device fabrication has traditionally been performed by selective removal or etching of material from deposited films to create patterns. Plasma etching has been the mainstay of this process since the early 1970s. The introduction of copper (Cu) metallization (~1997) by IBM resulted in the development of additive approaches (e.g., damascene process) due to the inability to plasma etch Cu. Despite attempts to plasma etch Cu as early as 1980, the only viable plasma-based methods in this time frame were inert gas sputtering, where selectivity to underlying layers and low etch rate were problematic, or chlorinated vapors that required elevated temperatures (>200 C) or significant ion bombardment energies to volatilize Cu chlorides. Both approaches required a hard mask rather than photoresist layers to ensure mask stability and precise pattern transfer. Due to the inability to form high volatility Cu halides that could be removed readily by plasma processes, other non-plasma or partial plasma approaches to Cu etching at lower temperatures were investigated. Addition of a UV lamp to facilitate excitation and desorption of volatile Cu chlorides in plasma etching was invoked. Formation and volatilization of copper by oxidation followed by complexation with organic ligands at lower (~150 C) temperatures was first reported in the mid-1990s, but this processes generated isotropic etch patterns; more recent approaches have reported anisotropy. Plasma chlorination followed by a liquid etch to remove the chlorinated Cu was also reported; due to the directionality of the plasma chlorination step, anisotropic profiles resulted. Hydrogen-based etching of Cu films was reported in 2010, initially with a two-step process consisting of plasma chlorination followed by a hydrogen plasma step to remove volatile products. This effort subsequently demonstrated that etching by pure hydrogen in a one-step process was possible albeit with low etch rates; etch rate increases were observed with vapor additives. Pure hydrogen plasmas etched photoresist layers, thereby requiring hard masks, but additives allowed employment of photoresist as a mask layer. Anisotropic etch profiles from these hydrogen-based processes were possible, in part due to additives protecting sidewalls and photoresist from etching and to ion bombardment during etching.This presentation will describe the development of Cu etching and patterning methods beginning in the 1970s. Mechanistic considerations of the various processes reported will be discussed and the current needs and limitations of these approaches indicated.

Elucidating the Role of Halides and Iron during Radiolysis-Driven Oxidative Etching of Gold Nanocrystals Using Liquid Cell Transmission Electron Microscopy and Pulse Radiolysis.

Graphene liquid cell transmission electron microscopy (TEM) has enabled the observation of a variety of nanoscale transformations. Yet understanding the chemistry of the liquid cell solution and its impact on the observed transformations remains an important step toward translating insights from liquid cell TEM to benchtop chemistry. Gold nanocrystal etching can be used as a model system to probe the reactivity of the solution. FeCl3 has been widely used to promote gold oxidation in bulk and liquid cell TEM studies, but the roles of the halide and iron species have not been fully elucidated. In this work, we observed the etching trajectories of gold nanocrystals in different iron halide solutions. We observed an increase in gold nanocrystal etch rate going from Cl-- to Br-- to I--containing solutions. This is consistent with a mechanism in which the dominant role of halides is as complexation agents for oxidized gold species. Additionally, the mechanism through which FeCl3 induces etching in liquid cell TEM remains unclear. Ground-state bleaching of the Fe(III) absorption band observed through pulse radiolysis indicates that iron may react with Cl2·- radicals to form an oxidized transient species under irradiation. Complete active space self-consistent field (CASSCF) calculations indicate that the FeCl3 complex is oxidized to an Fe species with an OH radical ligand. Together our data indicate that an oxidized Fe species may be the active oxidant, while halides modulate the etch rate by tuning the reduction potential of gold nanocrystals.

A Study of the Photoelectrochemical Etching of n-GaN in H3PO4 and KOH Electrolytes

We investigated the photoelectrochemical etching of n-GaN in H3PO4 and KOH as a function of electrolyte concentration, potential and light intensity. Etch rates measured by stylus profilometry were compared with coulometric and amperometric values. In both electrolytes, etch rates increased with concentration, reaching a maximum at 3.0 mol dm−3 and decreasing at higher concentrations. The increase in etch rate with concentration of either H3PO4 or KOH reflects the amphoteric nature of gallium and the decrease above 3.0 mol dm−3 is attributed to common-ion effects. Profilometric etch rates were lower than coulometric and amperometric etch rates reflecting formation of a surface film. SEM and profilometry demonstrated that thick surface films are formed at lower concentrations. Etch rates increased linearly with light intensity indicating a carrier-limited etching regime: a quantum efficiency of 57.6% was obtained. At light intensities greater than ∼35 mW cm−2 the etch rates showed evidence of saturation. AFM and SEM images of the etched GaN surfaces showed a distinctive ridge-trench structure with a hexagonal appearance. Photoluminescence spectra of the etched GaN show a significant increase in the defect-related yellow luminescence peak suggesting correlation to the formation of the ridge structures, which may represent dislocations terminating at the surface.

Silicon Nanohole Arrays Fabricated by Electron Beam Lithography and Reactive Ion Etching

The fabrication of silicon nanohole (SiNH) using a combination of electron beam lithography (EBL) and reactive ion etching (RIE) processes is reported. The optimum exposure dose of EBL process was found to be in the range of 210-240 µC/cm2 due to small enlargement of hole diameter after pattern development process. The anisotropic etching and isotropic etching was achieved at low and high reaction pressures, respectively. As expected, the etching rate increase with time and RF power. A relatively smooth and well-defined NH has been obtained at RF power of 100 W and reaction pressure of 0.08 Torr, which is suitable to be applied for optical waveguide.

Optimization of laser osteotomy at 1064 nm using a graphite topical absorber and a nitrogen assist gas jet.

Laser ablation of bone for the purposes of osteotomy is not as well understood as ablation of homogeneous, non-biological materials such as metals and plastics. Ignition times and etch rate can vary during ablation of cortical bone. In this study, we propose the use of two techniques to optimize bone ablation at 1064nm using a coaxial nitrogen jet as an assist gas and topical application of graphite as a highly absorbing chromophore. We show a two order of magnitude reduction in mean time to ignition and variance by using the graphite topical chromophore. We also show that an increase in volumetric flow rate of the assist gas jet does show an initial increase in etch rate, but increased pressure beyond a certain point shows decreased return. This study also demonstrates a 2 nd order relationship between exposure time, volumetric flow rate of nitrogen, and etch rate of cortical bone. The results of this study can be used to optimize the performance of laser ablation systems for osteotomy. This is a companion study to an earlier one carried out by Wong et al. [Biomedical Opt. Express6, 1 (2015)].

69‐1: Etch Properties of Silicon Nitride Films Using a New In‐Line Equipment with Atmospheric Glow Plasma for the OLED Flexible Display

DBD non‐thermal atmospheric pressure plasma (NAPP) is suitable for the OLED display manufacturing process using flexible substrate because it is low‐temperature atmospheric pressure process, low cost process and does not need a vacuum system. High dynamic etch rate of 220À·cm/s in Si3N4 thin films was obtained in flow rate ratio (O2/CF4) of 0.13, rf 480W, the speed of 10mm/s in a 200mm new inline NAPP system. About 5% increase of dynamic etch rate by N2 gas injection was conformed. Etch profile of Si3N4 film with patterned PR shows perfectly isotropic without undercut because the species such as CF3, COF2, F and O have low energy and chemical reaction.

Etched ion tracks in amorphous SiO2 characterized by small angle x-ray scattering: influence of ion energy and etching conditions

Small angle x-ray scattering was used to study the morphology of conical structures formed in thin films of amorphous SiO2. Samples were irradiated with 1.1 GeV Au ions at the GSI UNILAC in Darmstadt, Germany, and with 185, 89 and 54 MeV Au ions at the Heavy Ion Accelerator Facility at ANU in Canberra, Australia. The irradiated material was subsequently etched in HF using two different etchant concentrations over a series of etch times to reveal conically shaped etched channels of various sizes. Synchrotron based SAXS measurements were used to characterize both the radial and axial ion track etch rates with unprecedented precision. The results show that the ion energy has a significant effect on the morphology of the etched channels, and that at short etch times resulting in very small cones, the increased etching rate of the damaged region in the radial direction with respect to the ion trajectory is significant.

Electrochemical characterization of ruthenium dissolution and chemical mechanical polishing in hydrogen peroxide based slurries

Abstract Chemical mechanical planarization of ruthenium (Ru) in a slurry formulation containing silica abrasive and hydrogen peroxide oxidizer was investigated. The removal rate and the etch rate increase with increase in oxidizer concentration due to increased oxide formation on Ru surface. The removal rate selectivity and the galvanic corrosion of Ru/Cu couple were also studied. In the presence of oxidizer, formation of RuO2, Ru(OH)3, Ru(OH)4, RuO4─ and RuO42─ takes place in alkaline pH. The removal mechanism of the metals was studied using electrochemical techniques in the presence of a supporting electrolyte i.e., sodium perchlorate (NaClO4) and varying concentration of H2O2 solution. The electrochemical characterization was carried out at natural pH varying the hydrogen peroxide concentration. The Icorr values calculated using the extrapolation of potentiodynamic polarization plots follow the similar trend as the CMP removal rate. The electrochemical impedance spectroscopy (EIS) results of Ru and Cu show decreased impedance values with increase in H2O2 concentration which matches well with the etch rate and CMP experiments. Kramer-Kronig Transformation (KKT) was used to check the stability, linearity and causality of the system.

Metal-assisted photoenhanced wet chemical etching of GaN epitaxial layers

In our work we successfully achieved selective removal of GaN epitaxial layer from the Si(111) substrate by photoenhanced wet chemical etching in K2S2O8:KOH solution. Ti, Ni, Ti/Au, Cr/Au metal films were tested as etch masks. In case of thin Ti and Ni films the etch rate was quite low and metals were slightly dissolved. The use of mask containing Au film (noble metal) increased etch rate for 2-4 times. As a result, GaN was selectively removed from the substrate and highly anisotropic etch profile was formed.

Rapid atomic layer etching of Al2O3 using sequential exposures of hydrogen fluoride and trimethylaluminum with no purging

A dramatic increase in the Al2O3 atomic layer etching (ALE) rate versus time was demonstrated using sequential, self-limiting exposures of hydrogen fluoride (HF) and trimethylaluminum (TMA) as the reactants with no purging. The normal purging expected to be required to prevent chemical vapor etching or chemical vapor deposition (CVD) is not necessary during the Al2O3 ALE. This purgeless, rapid atomic layer etching (R-ALE) was studied from 250 to 325 °C using various techniques. In situ quartz crystal microbalance (QCM) measurements monitored Al2O3 R-ALE at 300 °C. The Al2O3 R-ALE process produced linear etching versus number of R-ALE cycles. Each HF exposure fluorinates the Al2O3 substrate to produce an AlF3 surface layer. Each subsequent dose of TMA then undergoes a ligand-exchange transmetalation reaction with the AlF3 surface layer to yield volatile products. Using reactant partial pressures of HF = 320 mTorr and TMA = 160 mTorr, the fluorination and ligand-exchange reactions produced a mass change per cycle (MCPC) of −32.1 ng/(cm2 cycle) using sequential, 1 s exposures for both HF and TMA with no purging. This MCPC equates to a thickness loss of 0.99 Å/cycle or 0.49 Å/s. Comparison experiments using the same reactant exposures and purge times of 30 s yielded nearly identical MCPC values. These results indicate that the etch rates for Al2O3 R-ALE are much faster than for normal Al2O3 ALE because of shorter cycle times with no purging. Smaller MCPC values were also observed at lower reactant pressures for both Al2O3 R-ALE and Al2O3 ALE. The QCM studies showed that the Al2O3 R-ALE process was self-limiting versus reactant exposure. Ex situ spectroscopic ellipsometry and x-ray reflectivity (XRR) measurements revealed temperature-dependent etch rates from 0.02 Å/cycle at 270 °C to 1.12 Å/cycle at 325 °C. At lower temperatures, AlF3 growth was the dominant mechanism and led to an AlF3 atomic layer deposition (ALD) growth rate of 0.33 Å/cycle at 250 °C. The transition temperature between AlF3 growth and Al2O3 etching occurred at ∼270 °C. XRR scans showed that the Al2O3 ALD films were smoothed by Al2O3 R-ALE at temperatures ≥270 °C. Additionally, patterned wafers were used to compare Al2O3 R-ALE and normal Al2O3 ALE in high aspect ratio structures. Scanning electron microscope images revealed that the etching was uniform for both processes and yielded comparable etch rates per cycle in the high aspect ratio structures and on flat wafers. The HF and TMA precursors were also intentionally overlapped to explore the behavior when both precursors were present at the same time. Similar to ALD, where precursor overlap produces CVD, precursor overlap during Al2O3 ALE leads to AlF3 CVD. However, any AlF3 CVD growth that occurs during precursor overlap is removed by spontaneous AlF3 etching during the subsequent TMA exposure. This spontaneous AlF3 etching explains why no purging is necessary during R-ALE. R-ALE represents an important advancement in the field of thermal ALE by producing rapid etching speeds that will facilitate many ALE applications.

Refined isolation techniques for GaN-based high electron mobility transistors

This work investigates the Ar+/N+ based ion implantation and Ar based reactive ion etching (RIE) techniques for device isolation. A comparison of ion implantation technique with three ion energies (20/35/65 keV) and 4 energies (20/35/65/160 keV) of Ar+ and N+ with different ion doses for isolation was reported. Use of 4 energy ion implantation provided better isolation. Ar based single and dual step reactive ion etching process was also explored for the mesa isolation of GaN HEMTs. The etch rate increases (44.7%) significantly after mixing of Ar gas directly with BCl3: Cl2 combination. Hence, the Ar addition in the single step etching proved to be more beneficial as compared to the double step etching technique.

Low Temperature Thermal ALD of Silicon Nitride Utilizing a Novel High Purity Hydrazine Source

The demand for faster, smaller and more energy efficient logic devices as well as higher density, higher speed, and increased reliability for advanced memory devices has led to numerous challenges in Semiconductor device manufacturing. Novel metal materials, 3D architecture and increasing High-aspect-ratio (HAR) structures are being used to address these challenges, however this has placed additional constraints on film deposition methods. CVD and ALD of SiN are used in several applications including, gates, spacers, etch stops, liners, encapsulation layers as well as passivation layers.1 Recently PEALD of SiN is taking on an increasingly important role due to new temperature constraints of <400°C. However, several challenges remain on HAR and 3D structures in applications where plasma approaches may not meet conformality requirements. In addition, thermal ALD with NH3 may not be feasible due to the high temperature requirement (500°C-700°C) of these reactions.2 Our approach involves the development and use of a novel hydrazine delivery system in order to develop methods for thermal ALD of SiN at <400°C. The resulting films must meet requirements of high growth rate, high density, and low wet etch rate similar to materials grown by thermal ammonia ALD at 600°C, or Ammonia PEALD at 400°C or less. In addition, the methods developed must show promise for highly uniform growth on 3D and HAR structures where PEALD methods currently have difficulties. A hydrazine delivery system was developed to provide a stable flow of ultra-dry hydrazine gas from a liquid source in a sealed vaporizer. The liquid source combines anhydrous hydrazine and a proprietary solvent that acts as a stabilizer. The solvent is highly non-volatile, where high purity hydrazine gas may be generated in-situ and delivered to the deposition chamber while the solvent remains in the vaporizer. Testing confirms that hydrazine vapor pressure is maintained at levels viable for ALD (12-14 torr) even in the presence of the solvent used to dilute the liquid source to safe handling levels. Previous studies with hydrazine were plagued by oxygen contamination.3 This study demonstrates high purity hydrazine delivery at <800ppb water contamination in the gas phase. Delivery system safety and optimization versus conventional hydrazine will be addressed. A study of silicon nitride deposition was conducted using hexacholorodisilane (HCDS) and hydrazine on a Si-H substrate. A custom made thermal ALD reactor was used to deposit silicon nitride films from 250-400°C. Film growth per cycle (GPC) with hydrazine was 0.4-0.5 Å/cycle at 400°C with refractive index of 1.813. Film stoichiometry was confirmed with X-ray photoelectron spectroscopy. SiN films with low impurities were achieved for oxygen (<2%) and chlorine (<1%). Highly uniform films were obtained across a 4 inch wafer for 200 as well as 400 cycles. Results were similar to films deposited using PEALD at 360°C with HCDS and NH3. Film growth and resulting properties at 350°C closely resemble those grown thermally at 400°C for hexachlorodisilane and hydrazine. Some discrete differences in film density and composition start to become apparent at a growth temperature of 325°C. Specifically, film density decreases and wet etch rate increases. The presentation will compare growth rates, film density, refractive index and wet etch rate results at different temperatures for hydrogen terminated silicon. Initial growth studies with the use of Hydrazine and Organosilicon Amide precursors will also be discussed.

Cu-assisted chemical etching of bulk c-Si: A rapid and novel method to obtain 45 μm ultrathin flexible c-Si solar cells with asymmetric front and back light trapping structures

Ultrathin c-Si solar cells (≤50 μm) are believed to be applied in military, aerospace and other special circumstances in the future due to their flexibility and high specific power density, and thus has attracted a great deal of research interest. However, until now, lacking fabrication means of high-quality ultrathin c-Si materials accompanied with their inefficient absorption of near-infrared light greatly limits their further application. In this work, we present a simple and novel method to realize rapid thinning and texturing of bulk c-Si at room temperature by varying the ρ ([HF]/([HF] + [H2O2])) values during the one-step Cu-assisted chemical etching process, followed by a systematic investigation of the formation mechanism of the surface structures. It is found that the sizes of surface structures accompanied with the etching rate increase with increasing the ρ values from 40% to 95% during the double sided etching process, and a high etching rate of 29.6 µm/min is obtained under the ρ value of 95%. For rapid thinning and efficient absorption of near-infrared light, 45 µm c-Si solar cell with asymmetric front and back light trapping structures is rapidly fabricated by directly immersing as-sawn bulk c-Si substrate into the thinning (ρ = 95%) and texturing (ρ = 60%) solution successively for only a few minutes. A high short-current density (Jsc) (36.12 mA/cm2) and energy-conversion efficiency (17.3%) are achieved, which are 1.09 mA/cm2 and 0.4% higher respectively than that in 45 µm c-Si with flat back surface. Based on the absorption spectra, it is demonstrated that the 45 µm c-Si cell with our asymmetric structures yields a high theoretical Jsc of 42.47 mA/cm2, which nearly approaches the Yablonovitch limit of 42.56 mA/cm2. All the findings offer additional insight into the structure formation mechanism and pave a rapid and novel way for exploration of next-generation flexible photovoltaics.

Holes generation in glass using large spot femtosecond laser pulses

We demonstrate high-throughput, symmetrical, holes generation in fused silica glass using a large spot size, femtosecond IR-laser irradiation which modifies the glass properties and yields an enhanced chemical etching rate. The process relies on a balanced interplay between the nonlinear Kerr effect and multiphoton absorption in the glass which translates into symmetrical glass modification and increased etching rate. The use of a large laser spot size makes it possible to process thick glasses at high speeds over a large area. We have demonstrated such fabricated holes with an aspect ratio of 1:10 in a 1 mm thick glass samples.

Fabrication of polygonal nanoholes by localized mask-free wet anisotropic etching

Nanoholes integrated into microfluidic systems have been widely researched, due to their practical applications in biosensing fields. This paper is devoted to report a strategy for fabricating polygonal nanoholes by localized mask-free anisotropic etching. Underetching occurs at the pore mouth, causing shape modification of the original square nanohole prepared by wet etching. The influence of the etching under different etching temperatures, KOH concentrations, as well as KOH with isopropanol (IPA) addition, on the shape formation of nanoholes are carefully analyzed and verified by experiments. Under low etching temperature or low KOH concentration, the shape of nanohole turns to be dodecagonal. Under high etching temperature and high KOH concentration, the increase of etching rate of (331) planes promotes transition of the nanohole to an octagonal shaped. By adding IPA into KOH solution, the pore shape is limited to be dodecagonal, and it is irrelative to the etching temperature and KOH concentration.

Effect of temperature on deposition layer formation in HBr/N2/fluorocarbon-based plasma

The effects of wafer temperature on etching rate and surface composition were investigated to clarify the surface reaction mechanism under HBr/N2/fluorocarbon-based gas plasma for developing a process for three-dimensional NAND flash devices. The etching rates of both polycrystalline silicon (poly-Si) and SiO2 were found to increase at a wafer temperature of 20 °C as compared with those at 60 °C. Comparing the gas combination of fluorocarbon/N2 and HBr/N2 mixtures, the temperature dependence of SiO2 etching rates was considered to relevant to the sticking probability of fluorocarbon polymers. To determine the cause of the temperature dependence of the poly-Si etching rate, surface composition was evaluated by thermal-desorption-spectroscopy and laser-sputtered-neutral-mass-spectrometry analyses. Ammonium bromide was confirmed in the deposition film at a wafer temperature of 20 °C. The observed increase in poly-Si etching rate at lower temperatures was possibly caused by increased amounts of nitrogen, hydrogen, and bromine fixed to the surface with the formation of ammonium bromide.