High Aspect Ratio Holes Research Articles

Characterization of the ion angle distribution function in low-pressure plasmas using a micro-electromechanical system

In recent years, micro-electromechanical systems (MEMSs) have found broad applications in various sensors. However, aside from quartz crystal microbalances, they have not yet been utilized in plasma analysis. Building on previous work with piezoelectric MEMS, the functionality of a MEMS-based sensor system capable of measuring the ion angular distribution function on the wafer holder surface is demonstrated. To enable this functionality, an array of high aspect ratio holes was added to the tiltable silicon plate of a piezoelectric MEMS. These holes allow for the filtering of incoming ions based on their angle perpendicular to the surface of the tiltable element. An algorithm was developed to fit the width and mean of the ion angular distribution function (IADF) based on the RMS ion current for various MEMS amplitudes. Compared to previously used methods for measuring the IADF, the MEMS presented in this paper represents a significant miniaturization. This work is the first to successfully characterize the angular distribution function of ions using a MEMS.

Bessel Beam Femtosecond Laser Interaction with Fused Silica Before and After Chemical Etching: Comparison of Single Pulse, MHz-Burst, and GHz-Burst.

We investigate the elongated modifications resulting from a Bessel beam-shaped femtosecond laser in fused silica under three different operation modes, i.e., the single-pulse, MHz-burst, and GHz-burst regimes. The single-pulse and MHz-burst regimes show rather similar behavior in glass, featuring elongated and slightly tapered modifications. Subsequent etching with Potassium Hydroxide exhibits an etching rate and selectivity of up to 606 μm/h and 2103:1 in single-pulse operation and up to 322 μm/h and 2230:1 in the MHz-burst regime, respectively. Interestingly, in the GHz-burst mode, modification by a single burst of 50 pulses forms a taper-free hole without any etching. This constitutes a significant result paving the way for chemical-free, on-the-fly drilling of high aspect-ratio holes in glass.

In situ x-ray imaging to understand subsurface behavior during continuous wave laser drilling

A limited understanding regarding the underlying dynamics and mechanisms of material removal during continuous wave laser drilling has presented significant challenges in achieving precision and process control. To address this, we employed high-fidelity, in situ synchrotron x-ray imaging to reveal previously unknown material behaviors during continuous wave laser drilling with power modulation. Our findings highlight that high-aspect ratio drill holes are achieved when the laser modulation frequency falls within the range of 8–12 kHz, provided that the laser average power and modulation amplitude levels meet the specified limits. Under these conditions, we identified a material removal mechanism driven by incremental accumulation of recoil pressure that gradually pushes material upward from deep within the substrate to the surface. This mechanism manifested as a low-frequency fluctuation in the vapor depression depth, resulting in periodic instances of material ejection. Furthermore, our study underscores that rapid expansion of the melt pool and the widening of the drill hole opening can impede effective material removal by redirecting energy from material ejection to increasing the melt pool size. This investigation contributes essential insights into the subsurface dynamics involved in the drilling of high-aspect ratio holes, furthering our fundamental understanding of this intricate process.

Precise 3D profile determination of high aspect ratio hole patterns by transmission small-angle X-ray scattering

The etching process of high aspect ratio (HAR) hole patterns on a wafer surface is a key step for fabricating new-generation semiconductor memory devices with vertically stacked structures. As the stacking number of these memory devices increases, it is getting more challenging to maintain the ideal etching profile of HAR holes. Therefore, detailed profile evaluation of these HAR holes is increasingly important. In this study, we have measured 4.2 μm deep holes by transmission small-angle X-ray scattering (T-SAXS) to determine the precise three-dimensional (3D) hole profile. By applying an improved 3D shape model for a hole, we successfully determined a hole profile whose cross-section parallel to the sample surface changes from elliptical to rectangular along its depth. This 3D profile measurement demonstrated that T-SAXS has sufficient sensitivity to evaluate a cross-sectional shape change along the depth of HAR holes.

Waveguide mode of drilling high-aspect ratio holes in PMMA by CO laser beam

Experimental results on drilling of PMMA by mid-IR lasers are presented and discussed. Proper choice of a CO laser beam focusing conditions made it possible to produce high aspect ratio (∼100) holes. During the laser drilling a waveguide-like structure of the hole was formed in PMMA with almost a constant sub-millimeter diameter along its whole length. Characteristics of the holes drilled by CO laser (wavelength 5.0–5.5 µm) and CO2 laser (10.6 µm) beams are compared.

A High Energy Density Pulsed Power Supply for Micro Electrical Discharge Machining of High Aspect Ratio Holes

Abstract In micro-electrical discharge machining (EDM), it is difficult to guarantee machining efficiency and accuracy when further increasing the aspect ratio of microholes due to poor circulation of working fluid and removal of debris. In order to increase the ratio of material vaporization erosion and reduce the ejection of large debris caused by material melting, this study proposes a high energy density pulsed power supply that increases the peak discharge current by adding energy-storage inductors with designed charging and discharging control. Machining microholes of diameters from 100 μm to 200 μm with aspect ratios of 10:1, 20:1, and 30:1 show that when the inductance increases from 4.7 μH to 20 μH, the peak current increases by more than two times and the machining efficiency improves by approximately 30%, and the accuracy of the aperture consistency is enhanced, while the occurrence of microcracks on the machined surface is significantly reduced.

Inline metrology of high aspect ratio hole tilt and center line shift using small-angle x-ray scattering

High aspect ratio (HAR) structures found in three-dimensional nand memory structures have unique process control challenges. The etch used to fabricate channel holes several microns deep with aspect ratios beyond 50:1 is a particularly challenging process that requires exquisitely accurate and precise control. It is critical to carefully analyze multiple aspects of the etch process, such as hole profile, tilt, uniformity, and quality during development and production. X-ray critical dimension (XCD) metrology, which is also known as critical dimension small-angle x-ray scattering, is a powerful technique that can provide valuable insights on the arrangement, shape, and size of periodic arrays of HAR features. XCD is capable of fast, non-destructive measurements in the cell-area of production wafers, making XCD ideal for in-line metrology. Through several case studies, we will show that XCD can be used to accurately and precisely determine key properties of holes etched into hard mask, multilayer oxide/nitride film stacks and slit trenches. We show that the measurement of hole and slit tilt can be achieved without the aid of a structural model using a Fast Tilt methodology that provides sub-nanometer precision. Measurements were performed across several production wafers to determine the etch uniformity and quality. Particular attention was given at the edge of the wafers to account for large variations observed. In addition, we used a detailed physical model to characterize the HAR structures beyond linear tilt. This approach provides a more complete picture of the etch quality.

Crack-free high-aspect ratio holes in glasses by top–down percussion drilling with infrared femtosecond laser GHz-bursts

We report novel results on top-down percussion drilling in different glasses with femtosecond laser GHz-bursts. Thanks to this particular regime of light–matter interaction, combining non-linear absorption and thermal cumulative effects, we obtained crack-free holes of aspect ratios exceeding 30 in sodalime and 70 in fused silica. The results are discussed in terms of inner wall morphology, aspect ratio and drilling speed.

Surface reaction modelling of thermal atomic layer etching on blanket hafnium oxide and its application on high aspect ratio structures

Thermal atomic layer etching is rapidly becoming an important complementary processing technology in the manufacturing of 5 and 3 nm devices in the semiconductor industry. Critically, architectures such as 3D NAND and 3D DRAM require conformal isotropic etching to remove material such as HfO2 in hard-to-reach locations with aspect ratios that can be greater than 50:1. To achieve repeatable device performance throughout a 3D stack, the removal rate (etch per cycle) of the etched material during an etch process needs to be controlled such that the overall etch amount is the same from top to bottom of the device stack. In this work, the reaction kinetics of reactants and byproducts during a cyclical ligand exchange-based atomic layer etching (ALE) process have been modelled. This ALE process consists of two steps: a fluorination step followed by a fluorine-to-chlorine ligand exchange-based removal step. Modeling was performed for each of those steps separately. Experimental data revealed that the fluorine dosing during the fluorination step was predominantly responsible for controlling the etch rate of the ALE process but had only a minimal impact on the etch profile inside high aspect ratio holes. The ligand exchange dosing, on the other hand, predominantly controlled the etch profile (depth loading) with equal etch rates from top-to-bottom, obtained when the step was operated close to saturation. The model predicts that the chemical reaction rate of dimethylaluminum chloride (DMAC) on a fluorinated surface during the ligand exchange step is 9.1 s−1, about 46 times greater than the reaction rate of hydrogen fluoride (HF) on the hafnium oxide surface during the fluorination step (only 0.2 s−1). Furthermore, modeling results revealed that the sticking coefficient for DMAC on a hafnium fluoride surface far exceeded that of HF on a hafnium oxide surface in the conditions modelled (0.94 s−1 for DMAC vs 0.0058 s−1 for HF). With these modeling results, the different roles fluorination and ligand exchange steps have regarding the control of etch rate per cycle and profile inside high aspect ratio holes can be explained.

Area selective deposition of ruthenium on 3D structures

Trends in device miniaturization have driven the adoption of new materials that, in turn, have enabled significant advancements in the field of process engineering and integration for semiconductor technology. Continued progress for device scaling is necessary and can be enabled by advances in lithographic techniques and deposition schemes. Thin-film deposition for spacers and etch stop layers has become a mainstay to enable and extend traditional 2D scaling into the 3D realm for fabricating advanced semiconductor devices. For processing 3D structures, controlled film deposition with subnanometer resolution in high aspect ratio features is desired. Area selective deposition (ASD) can be a powerful response to such a challenge. ASD is a type of thin-film deposition technique scheme that can be used to eliminate the need for several expensive and time-consuming lithography steps with fewer performance penalties. In this work, we show ASD of ruthenium (Ru) on 3D molybdenum (Mo)–silicon oxide (SiO2) stacks by utilizing the inherent substrate preference of the Ru precursor to a H-terminated surface. In the best selectivity condition, our results show growth of ∼5 nm Ru on Mo, with no film growth on SiO2. Changes in Ru growth kinetics were observed after dilute hydrofluoric acid (DHF) treatment for both surfaces. Post-DHF treatment, the Ru growth rate on Mo was reduced by 5%. However, on SiO2 (after incubation delay), the growth rate was reduced by 94% compared to untreated surfaces. This translates to a very high difference in the growth rate of Ru on Mo vs SiO2, even after considering the incubation delay. Finally, by using 3D topologies with high aspect ratio holes, we have highlighted that it is important to deconvolute the effects of precursor depletion and selectivity. To the best of our knowledge, this is the first demonstration of ASD of Ru on 3D structures without the use of any blocking layers. Therefore, these results demonstrate a new paradigm for ASD in 3D features.

Deep learning model for 3D profiling of high-aspect-ratio features using high-voltage CD-SEM

High-aspect-ratio (HAR) channel holes were developed for competitive cost-per-bit 3D-NAND memory. High-throughput and in-line monitoring solutions for 3D profiling of the HAR features are the key to improving yields. We previously proposed an exponential model to identify the cross-sectional profile of the HAR features using backscattered electron (BSE) images of a high-voltage critical dimension scanning electron microscopy (CD-SEM). However, the 3D profiling accuracy was insufficient when the depth of the HAR features was far greater than the focus depth of the electron beam. To address this issue, we developed a deep learning (DL) model, which takes account of the aperture angle and the aberration of the electron beam, to predict the 3D profile from BSE images. The predicted cross-sections of the HAR holes with different bowing geometries were compared with field-emission SEM measurements. The results show that the DL model provides higher sensitivity than the exponential model does.

Control of etch profiles in high aspect ratio holes via precise reactant dosing in thermal atomic layer etching

Thermal atomic layer etching (ALE) was studied in HfO2-based 3D NAND test structures with an aspect ratio of more than 50:1. Etching was performed via ligand exchange with dimethyl-aluminum chloride (DMAC) after surfaces had been fluorinated with hydrogen fluoride (HF). In these 3D NAND structures, we found that the horizontal etch rate of HfO2 as a function of depth (depth loading) depended on the DMAC dosing but was nearly independent of the HF dose. The HF dose and the process pressure were keys to increasing the overall etch amount per cycle. With the highest tested HF dose of 192 Torr s and a total process pressure of 8 Torr, we achieved a uniform etch amount of 0.6 nm per cycle. In addition, we investigated the impact of film quality and film coating conformality in these structures on the depth loading in the succeeding ALE processes. The type of precursor, precursor dosing, deposition rate, and substrate temperature played a fundamental role in controlling the film quality and conformality of the deposited HfO2 layers inside high aspect ratio holes. Fluorination studies on blanket films revealed that fluorination efficiency is improving for pressures in the Torr range compared to previous milliTorr experiments and that only temperatures above 250 °C increased the fluorine concentration in HfO2 significantly, whereas fluorine levels were unchanged between 150 and 250 °C.

Bottom surface smoothing of high aspect ratio hole by guiding large-area electron beam with magnet

Bottom surface smoothing of high aspect ratio hole by guiding large-area electron beam with magnet

Investigation of hole roundness-error using different electrolytes in STED process

ABSTRACT Shaped-Tube Electrolytic Drilling (STED) is an electrochemical dissolution-based process able to fabricate high aspect-ratio (AR) holes rapidly on the superalloys. The electrolyte flowing in the side gap between the tool’s lateral surface and hole wall in the presence of an electric field during the entire process duration leads to non-uniform dissolution and stray-cutting. The localization of anodic dissolution mitigates the stray cutting, which contributes to the non-uniformity of micro-holes. The enhanced localized anodic dissolution is attainable by using neutral electrolytes mixed with the alkaline electrolytes that promote passivation on the surface of Inconel 718. The hole roundness error is crucial for the dimensional accuracy of micro-holes due to the overcutting constraints. Therefore, the present research investigates the feasibility of using electrolyte solutions of dissimilar anionic activity mixed with alkaline electrolytes to mitigate hole roundness error in high AR micro-holes. The effects of process parameters, such as applied voltage, tool feed rate, supplementary electrolyte concentration, and electrolyte entry temperature on the hole roundness error were studied to understand the dissolution behavior with different electrolyte solutions. The investigation results revealed that adding alkaline electrolytes in NaCl and NaNO3 based electrolyte solutions mitigate hole roundness error, whereas the KBr-based electrolyte solution performs unsatisfactorily.

Drilling high aspect ratio holes by femtosecond laser filament with aberrations.

A near-infrared femtosecond laser is focused by a 100 mm-focal-length plano-convex lens to form a laser filament, which is employed to drill holes on copper targets. By shifting or rotating the focusing lens, additional aberration is imposed on the focused laser beam, and significant influence is produced on the aspect ratio and cross-sectional shape of the micro-holes. Experimental results show that when proper aberration is introduced, the copper plate with a thickness of 3 mm can be drilled through with an aspect ratio of 30, while no through-holes can be drilled on 3-mm-thickness copper plates by femtosecond laser with minimized aberration. In addition, when femtosecond laser filament with large astigmatism is used, micro-holes that had a length to width ratio up to 3.3 on the cross-section are obtained. Therefore, the method proposed here can be used to fabricate long oval holes with high aspect ratios.

Effect of micro double helical grooved tools on performance of electric discharge drilling of Ti-6Al-4V

Machining of complex features and holes on superalloys is a topic of both industrial and academic interest. Though the demand for high aspect ratio holes on Ti-6Al-4V is high in aerospace, biomedical and chemical industries, machining the same is very challenging due to the low thermal conductivity and debris accumulation in the machining zone. The present-day methods of hole drilling by unconventional methods like Electric Discharge Machining (EDM) make use of electrodes with poor flushing capabilities. Hence the motivation of the present work is to develop a method to modify the geometry of Electric Discharge Drilling (EDD) tools in order to enhance debris removal. For the first time an attempt has been made to machine micro double-helical grooves on EDD tools in a single pass. Solid, single-helical and double-helical electrodes are developed and through-holes are drilled on a Ti-6Al-4V workpiece. A comparative analysis of the performance of these three types of tools with respect to the machining time, electrode wear rate and hole quality is carried out. From Computational Fluid Dynamics (CFD) simulations it is seen that the double-helical grooved tool is superior in debris removal. The best values of the machining time of 183.33 s, overcut of 2.19 mm, hole taper angle of 7.93° and area of recast layer of 6.66 mm2 are obtained on the holes machined using double-helical electrodes. The mentioned tool has the potential to overcome the problem of debris removal in high aspect ratio hole drilling and surpass the present-day electric discharge drilling tools.

Assessing the performance of STED process for fabricating high aspect ratio holes on Inconel 718 alloy

ABSTRACT Shaped-Tube Electrolytic Drilling (STED), being an electrochemical-based process, undergoes issues related to machining inaccuracies, e.g., stray cutting and non-uniform dissolution, and they still need attention. Along with other process-related factors, the instantaneous state of the inter-electrode gap (IEG) also affects the process behavior, as it can lead to electrolyte boiling and short-circuiting. Thereby, it is a challenge to have precise control over the state of IEG. While reviewing the available literature and performing experiments, it was observed that the initial temperature of electrolyte may be decisive for the performance of STED process. Therefore, the present research was aimed at investigating the effect, of varying electrolyte entry temperature (EET) along with other factors such as applied voltage, electrolyte concentration, tool feed rate, and electrolyte flow rate, on the average diametral overcut (ADOC), material removal rate (MRR), roundness error, and surface roughness (Ra). The outcomes of using two different EET were compared to elucidate the improvement in the localization of anodic dissolution. The relatively cooler electrolyte (EET 5°C) supplied into the machining zone curtailed extent of stray cutting, and hence, the ADOC, MRR, roundness error, and surface roughness (Ra) were decreased by about 1–21%, 1–5%, 4–19%, and 1–6%, respectively.

Time-dependent measurement of charge density on the bottom of high aspect capillary hole in pulse-modulated VHF capacitively coupled Ar plasma

Charging and discharging behavior of high aspect-ratio (AR) hole capillary plate (CP) exposed to a pulse-modulated very high frequency (VHF) capacitively-coupled plasma is investigated. From an equivalent circuit model, time-dependent charge density on the bottom of the CP is quantitatively evaluated. AR of the CP plays very important role for the charging current, although the discharge current is dominated by the leakage current of the CP. Importance of electron current flowing into the CP bottom during the VHF pulse-on phase is suggested at higher self-bias voltages.

Minimum-variance self-tuning regulator in EDM drilling processes for ultra-high-aspect-ratio small holes

High speed small hole electrical discharge machining (EDM) drilling processes can easily become unstable, due to changing flushing conditions, inappropriate preset electrical parameters, and machining debris residues. Although there are already successful attempts in controlling EDM processes, cost-effective adaptive control of high speed small hole EDM drilling has not been reported. Process stability can contribute to the improvement of machining efficiency and surface finish. To maintain a higher process stability, a minimum-variance self-tuning regulator (MVSTR) is designed based on an auto-regressive moving average exogenous (ARMAX) model. To determine specifications of the system, the time and frequency responses are analyzed, and the validation is executed for the offline system model. A series of high aspect ratio holes drilling experiments with different electrode diameters of 0.3 mm, 1.0 mm, and 3.0 mm show that, with the use of the MVSTR control scheme, a higher machining efficiency and a lower tool wear can be achieved as compared with the original controller. Microscopic pictures of drilled holes and electrodes show better surface quality.

Machining and Characterization of Double-Helical Grooves on Cylindrical Copper Parts by Wire Electric Discharge Turning

In the work, the design and development of a novel Wire-EDM setup with double-wire guide discs is presented. It facilitates sparks to be generated between the workpiece and wire at two locations separated by the helical pitch distance. This sparking causes two helical grooves to be generated simultaneously on the surface of the workpiece when it is given suitable rotational speed and table feed. In this work, machining is carried out on rods of 1.5 mm diameter. Helical groves with helix angles ranging from 35 to 500 were generated and characterized. This method of machining the double helical grooves with a single pass reduces the machining time and eliminates the complexities involved in machining one groove at a time. It was observed that the proposed method is suitable for machining double helical grooves with helix angles in the range of 40 - 50°. The parts produced by the mentioned method can be used as EDM tools for generation of high aspect ratio holes in turbine blades and injection nozzles.