Sidewall Profile Research Articles

Simulation and analysis of the over-expanded flow field in asymmetric nozzles with lateral expansion

Abstract Numerical simulations of a three-dimensional asymmetric nozzle with lateral expansion were carried out, focusing on the characteristics of the three-dimensional flow field in the over-expanded state and the influence of the sidewall surface profiles on the over-expanded flow field. The results show that the sidewall profiles cause the separation zones inside the nozzle to exhibit different shapes. The center separation bubble and the corner zone separation bubble are independent of each other in the lower wall surface confined shock wave separation mode. In the upper wall-constrained shock wave separation mode, the center separation bubble, the corner zone separation bubble, and the sidewall surface separation bubble are interconnected. Under the same drop pressure ratio, changes in the sidewall profile affect the shape and size of the separation zones, and the convex wall can slow the occurrence of separation to a certain extent.

Stacked hydrogel-based brain-on-chips utilizing capillary force flow pinning

Cellular processes in brain tissue such as migration, proliferation, morphology, and differentiation are influenced by mechanical cues, demonstrating the interplay between the structure and function. Given the complexity, it remains a substantial challenge to establish a reliable in vitro model mimicking structural properties as brain tissue. To address this challenge, we propose an innovative approach to create vertical hydrogel stacks based on microfluidic technology. 3D-printed microfluidic features in the sidewall profile of our chip designs allowed us to faithfully replicate these capillary force flow pinning structures in polydimethylsiloxane. After the successful application of hydrogel of a defined height, thanks to the pinning process and plating of stem-cell-derived neurons, the results demonstrated the potential of our BoC platform, providing a valuable tool for neuroscience research.

Fabrication of Bulk Tungsten Microstructure Arrays for Hydrophobic Metallic Surfaces Using Inductively Coupled Plasma Deep Etching.

Hydrophobic surfaces have attracted great attention due to their ability to repel water, and metallic surfaces are particularly significant as they have several benefits, for example they self-clean and do not corrode in marine environments, but also have several applications in the aircraft, building and automobile industries. Tungsten is an ideal material for metallic surfaces due to its remarkable mechanical properties. However, conventional micromachining methods of micro- or nanostructures, including mechanical fabrication and laser and wet etching are incapable of balancing functionality, consistency and cost. Inspired by the etching process of silicon, deep etching of bulk tungsten has been developed to achieve versatile microstructures with the advantages of high efficiency, large scale and low cost. In this article, fabrication methods of tungsten-based hydrophobic surfaces using an ICP deep etching process were proposed. Micro- or hierarchical structure arrays with controllable sidewall profiles were fabricated by optimizing etching parameters, which then exhibited hydrophobicity with contact angles of up to 131.8°.

Ordered silicon nanocone fabrication by using pseudo-Bosch process and maskless etching

Nanocone arrays are widely employed for applications such as antireflection structures and field emission devices. Silicon nanocones are typically obtained by an etching process, but the profile is hard to attain because anisotropic dry etching generally gives vertical or only slightly tapered sidewall profiles, and isotropic dry plasma etching gives curved sidewalls. In this work, we report the fabrication of cone structures by using masked etching followed by maskless etching techniques. The silicon structure is first etched using fluorine-based plasma under the protection of a hard metal mask, with a tapered or vertical sidewall profile. The mask is then removed, and maskless etching with an optimized nonswitching pseudo-Bosch recipe is applied to achieve the cone structure with a sharp apex. The gas flow ratio of C4F8 and SF6 is significantly increased from 38:22 (which creates a vertical profile) to 56:4, creating a taper angle of approximately 80°. After subsequent maskless etching, the sidewall taper angle is decreased to 74°, and the structure is sharpened to give a pointed apex. The effect of an oxygen cleaning step is also studied. With the introduction of periodic oxygen plasma cleaning steps, both the etch rate and surface smoothness are greatly improved. Lastly, it was found that the aspect ratio-dependent etching effect becomes prominent for dense patterns of cone arrays, with a greatly reduced etch depth at a 600 nm pitch array compared to a 1200 nm pitch array.

2D Analysis and simulation of quartz crystal etch penetration by revisiting a previous geometric method

This paper presents a quartz wet etching simulation procedure tailored for microelectromechanical systems (MEMS) fabrication. It utilizes geometric methods and is primarily founded on modifying some aspects of the traditional slowness vector approach. Through a theoretical analysis of the crystal etching trajectory, we provide a macroscopic elucidation for the formation of 2D etch profiles and optimize wafer etch penetration during double-sided wet etching convergence of upper and lower surfaces. Based on the proposed theoretical model, we develop a CAD-like quartz crystal etch simulator that allows for simulations of various quartz cut types and etch penetration. To validate the accuracy of the simulator, we quantitatively compare the simulation with experimental results using the angle between adjacent etched sidewall profiles as a metric. The comparison reveals a minimum error of only 2.2° and a maximum error of 11.7° between the simulation and experimental results. The developed simulator is able to predict the profiles of different etching profiles well, and it has certain engineering application capability.

Experimental study on the characteristics of a novel aerodynamic wake width flameholder

In this study, a novel flameholder named aerodynamic wake width flameholder (AWF) was proposed. The AWF optimized the sidewall profile of conventional bluff body flameholders and widened the downstream recirculation zone through aerodynamic force, while maintaining the same trailing edge width as conventional flameholders. Consequently, the combustion properties were impacted. To investigate the flow field structure, flame stability limits, and flame evolution of the novel flameholder, an aerodynamic wake width evaporating flame holder (AWEF) was designed and compared with a normal evaporating flameholder (EF). Comparative analysis of the downstream flow field structure revealed that the AWEF exhibited a wider zero-velocity contour and a higher reflux rate. Furthermore, the AWEF demonstrated a larger velocity gradient near the trailing edge, indicating the presence of a thicker shear layer. Within the inlet Ma range of 0.1 to 0.17, the AWEF exhibited wider flame stability limits compared to the EF. The ignition fuel-air ratio (FAR) of the AWEF remained relatively constant within this range, suggesting that the novel flameholder could withstand the deterioration of ignition performance caused by an increase in the inlet Ma. Moreover, the AWEF achieved a larger flame projection area and flame width downstream compared to the EF. Finally, by combining the flow field characteristics and flame stability properties of the AWEF, the mechanisms underlying the changes in flame stability characteristics were analyzed. The results of this study can serve as valuable references for enhancing flame stability in current and future advanced augmentor systems.

Exploring oxide-nitride-oxide scalloping behavior with small gap structure and chemical analysis after fluorocarbon or hydrofluorocarbon plasma processing

The scalloping of oxide-nitride-oxide (ONO) stacked layers on vertical sidewalls during high-aspect-ratio contact etch is commonly seen and characterized by the horizontal etching of oxide and nitride layers at different etch rates. To understand the mechanisms of ONO scalloping in complex plasma chemistry, it is crucial to examine the surface chemistry of silicon dioxide and silicon nitride processed with single fluorocarbon (FC) or hydrofluorocarbon (HFC) gases. To simulate the isotropic etching of SiO2 and Si3N4 sidewalls, we use a horizontal trench structure to study the effect of neutral radicals produced by FC (Ar/C4F8), HFC (Ar/CH3F, CH2F2, or CH3F), FC/HFC (Ar/C4F8/CH2F2), or FC/H2 (Ar/C4F8/H2), plasma for aspect-ratio (AR) up to 25. To eliminate the effect of ions, oxide and nitride trench structures were treated by inductively coupled plasma. The changes in the film thickness as a function of AR were probed by ellipsometry. Additionally, x-ray photoelectron spectroscopy (XPS) measurements on oxide and nitride substrates processed by Ar/C4F8 and Ar/CH2F2 plasma were performed at various locations: outside of the trench structure, near the trench entrance (AR = 4.3), and deeper in the trench (AR = 12.9). We find a variety of responses of the trench sidewalls including both FC deposition and spontaneous etching which reflect (1) the nature of the FC and HFC gases, (2) the nature of the surfaces being exposed, and (3) the position relative to the trench entrance. Overall, both the etching and deposition patterns varied systematically depending on the precursor gas. We found that the ONO scalloping at different ARs is plasma chemistry dependent. Oxide showed a binary sidewall profile, with either all deposition inside of the trench (with FC and FC/H2 processing) or etching (HFC and FC/HFC). Both profiles showed a steady attenuation of either the deposition or etching at higher AR. On the nitride substrate, etching was observed near the entrance for HFC precursors, and maximum net etching occurred at higher AR for high F:C ratio HFC precursors like CHF3. XPS measurements performed with Ar/C4F8 and Ar/CH2F2 treated surfaces showed that Ar/C4F8 overall deposited a fluorine-rich film outside and inside of the trench, while Ar/CH2F2 mostly deposited a cross-linked film (except near the trench entrance) with an especially thin graphitic-like film deep inside the trench.

Positive-tone chemically amplified photosensitive polyimides with controllable taper angles

Current methods for tuning the taper angle of photosensitive polyimide (PSPI) often require additional equipment or materials, and has limited achievable angle tuning range while maintaining resolution. To overcome these limitations, a positive tone chemically amplified photosensitive polyimide (p-CA-PSPI) was prepared from tert-butylcarbonate-protected polyimide (t-Boc-PI), photoacid generator (PAG), other additives, and solvent. By optimizing the protection ratios of t-Boc-PI resins, the p-CA-PSPI achieved a high film retention rate and wide processing window. The photo-patterning conditions were able to manipulate the sidewall profile and taper angle within the range of 11–53°, maintaining a resolution of 3 μm for patterns with taper angles higher than 28° at 1.5 μm film thickness, by controlling the acid diffusion. The thermally cured polyimide films demonstrated excellent mechanical properties, including a tensile strength of 95.7 MPa, tensile modulus of 2.9 GPa, and elongation at breakage of 24.2%, high thermal stability with a glass transition temperature of 343 °C, decomposition temperature of 409 °C, and good chemical resistance. This novel taper angle tuning method has the potential to optimize the internal stress of semiconductor devices and advance the development of display devices, MEMs, and multifunctional electronics that require a single package containing multiple or unique taper profiles.

Localized sinking electrochemical machining of pre-shaped convex structure on revolution surface using specifically designed cathode tool

Counter-rotating electrochemical machining (CRECM) is an effective method for machining thin-walled revolving parts with complex structures. However, it is difficult to achieve a desirable uniform straight sidewall during the CRECM of isolated convex structures with variable cross-sectional profiles, so a subsequent local machining process is necessary. In this paper, a localized sinking ECM method is proposed to locally machine a pre-shaped convex structure by CRECM. A mathematical model is established to analyze the anode shaping process. In addition, a special cathode tool is designed to control the junction mark between the subsequent dissolution regions and the surrounded non-machined areas. The simulation and experimental results indicate that the sidewall profiles of the convex structure can be improved remarkably. In lateral view, convex side wall taper is reduced from 21.81° to 3.05°, In front view, convex side wall taper is reduced from 6.45° to 0.28°, Furthermore, the junction mark can be effectively controlled to within 0.07 mm.

Sidewall Modification Process for Trench Silicon Power Devices

In this study, trench sidewall modification processes were designed to improve profile uniformity and thereby enhance the electrical performance of silicon power devices in large-scale production. The effects of trench sidewall modification on the morphology, structure and electrical properties were studied. Plasma-induced damage in etching processes was also observed and briefly explained. Straight and smooth sidewall profiles were achieved through adjusting the SF6/CHF3 proportion in a combined etchant gas flow in the main etching procedure. By comparing HRSEM images from different etching protocols, it was evident that an enhanced CHF3 flow formed a proper passivation of the sidewall, eliminating the ion damages that are common in current main etch steps. To address the impurities introduced from the etchant gas and improve the gate oxide uniformity, further steps of depolymerization were applied in a plasma asher chamber, followed by wet clean steps. In the meantime, the plasma-induced charge accumulation effect was reduced by UV curing. Improved trench sidewall profiles and the gate oxide uniformity contributed to a lower leakage current between the gate and source terminals, leading to an overall yield enhancement of device properties in large-scale silicon wafer fabrication.

Metamaterials for light extraction and shaping of micro-scale light-emitting diodes: from the perspective of one-dimensional and two-dimensional photonic crystals.

Metamaterials have attracted broad attention owing to their unique versatile micro- and nano-structures. As a kind of typical metamaterial, photonic crystals (PhCs) are capable of controlling light propagation and constraining spatial light distribution from the chip level. However, introducing metamaterial into micro-scale light-emitting diodes (µLED) still exists many unknowns to explore. This paper, from the perspective of one-dimensional and two-dimensional PhCs, studies the influence of metamaterials on the light extraction and shaping of µLEDs. The µLEDs with six different kinds of PhCs and the sidewall treatment are analyzed based on finite difference time domain (FDTD) method, in which the optimal match between the PhCs type and the sidewall profile is recommended respectively. The simulation results show that the light extraction efficiency (LEE) of the µLEDs with 1D PhCs increases to 85.3% after optimizing the PhCs, and is further improved to reach 99.8% by the sidewall treatment, which is the highest design record so far. It is also found that the 2D air ring PhCs, as a kind of left-handed metamaterials, can highly concentrate the light distribution into 30° with the LEE of 65.4%, without help of any light shaping device. The surprising light extraction and shaping capability of metamaterials provides a new direction and strategy for the future design and application of µLED devices.

Reactive ion Etching of SiO2 and Mo Sidewall Profile Process

This paper investigates the process parameters affecting the sidewall angle of Mo and SiO2 films based on the reactive ion etching (RIE) system, firstly, the photoresist mask with a certain sidewall angle was obtained by adjusting the hardening temperature and hardening time of the photoresist during the lithography process, and their effects on the sidewall angle of the photoresist mask were investigated. In the RIE etching of SiO2 and Mo, the etching gas type (CHF3, CF4, SF6), O2 flow rate, and their effects on the sidewall angle of SiO2 were adjusted to obtain the sidewall angle of SiO2 in the range of 15°~53°, During the RIE etching of Mo, the RF power, SF6 flow rate, and O2 flow rate were adjusted and their effects on the Mo sidewall angle were investigated, resulting in a range of 17.3° to 30.2° for the Mo sidewall angle. This is useful for the study of the thin film body acoustic resonator (FBAR) process.

Development of high uniformity Al1-xScxN piezoelectric film stack dry etching process on 8-inch silicon wafers

This paper have investigated the basic properties of inductively coupled plasma reactive ion etching (ICP-RIE) of scandium doped aluminum nitride (Al1-xScxN) thin films on 8-inch silicon wafers, and presented a pretreatment scheme which can effectively improve the etching uniformity and wafer yield in the mass fabrication of Al1-xScxN-MEMS (micro-electromechanical systems) devices. Based on BCl3/Ar pretreatment and Cl2/Ar main etching scheme, an etching rate of 128 nm/min is obtained, with the low nonuniformity, and selectivity of 1.8 with respect to the loss of molybdenum (Mo) electrode is achieved, respectively. The relationship between etching parameters and etching uniformity is revealed in detail by the 5-step Al1-xScxN etching model. Meanwhile, to optimize the etching compatibility in the fabrication of Al1-xScxN-piezoelectric MEMS devices, a variety of etching masks (e.g. photoresist and silicon dioxide) are respectively used to obtain the consistent sidewall profile at different locations, with photoresist of 24° and silicon dioxide of 57°.

Enhancing the precision of 3D sidewall measurements of photoresist using atomic force microscopy with a tip-tilting technique

A key issue associated with advanced lithography techniques for semiconductor-device manufacturing is the reduction in the sidewall roughness of photoresist line patterns, known as line-edge roughness (LER). We have developed a technique for measuring the sidewall of the resist pattern using atomic force microscopy (AFM) that enables three-dimensional (3D), high-resolution, low-noise, and nondestructive measurements. Conventional LER measurement technology using scanning electron microscopy (SEM) causes shrinkage of the resist pattern due to electron-beam (EB) exposure, whereas our new AFM technique can in principle avoid EB-induced shrinkage. This AFM technology is capable of 3D measurements because it employs a tip-tilting mechanism that enables the sharp AFM tip to scan the vertical sidewalls, which is difficult for a conventional AFM technique. In addition, laser interferometers are equipped for the measurement of the AFM tip displacement, which yields high-resolution, high-accuracy, and low-noise results. This technology overcomes issues such as low resolution, noise, and destructive measurements that afflict conventional SEM measurements. In addition, it enables observations and quantitative analyses of the 3D sidewall roughness. For example, in the present experiment, we observed that grain shapes (several tens of nm in size) were formed randomly on the resist sidewall and that there were almost no footing shapes. By analyzing the sidewall profiles with a height resolution of 1 nm, we obtain the roughness (self-affine fractal) parameters at each height. This AFM-based resist sidewall measurement technique can, thus, provide important insights into resist patterning and related process technologies for next-generation semiconductor-device manufacturing.

A sleeve and bulk method for fabrication of photonic structures with features on multiple length scales

Traditional photonic structures such as photonic crystals utilize (a) large arrays of small features with the same size and pitch and (b) a small number of larger features such as diffraction outcouplers. In conventional nanofabrication, separate lithography and etch steps are used for small and large features in order to employ process parameters that lead to optimal pattern transfer and side-wall profiles for each feature-size category, thereby overcoming challenges associated with reactive ion etching lag. This approach cannot be scaled to more complex photonic structures such as those emerging from inverse design protocols. Those structures include features with a large range of sizes such that no distinction between small and large can be made. We develop a sleeve and bulk etch protocol that can be employed to simultaneously pattern features over a wide range of sizes while preserving the desired pattern transfer fidelity and sidewall profiles. This approach reduces the time required to develop a robust process flow, simplifies the fabrication of devices with wider ranges of feature sizes, and enables the fabrication of devices with increasingly complex structure.

Method for Keyhole-Free High-Aspect-Ratio Trench Refill by LPCVD.

In micro-machined micro-electromechanical systems (MEMS), refilled high-aspect-ratio trench structures are used for different applications. However, these trenches often show keyholes, which have an impact on the performance of the devices. In this paper, explanations are given on keyhole formation, and a method is presented for etching positively-tapered high-aspect ratio trenches with an optimised trench entrance to prevent keyhole formation. The trench etch is performed by a two-step Bosch-based process, in which the cycle time, platen power, and process pressure during the etch step of the Bosch cycle are studied to adjust the dimensions of the scallops and their location in the trench sidewall, which control the taper of the trench sidewall. It is demonstrated that the amount of chemical flux, being adjusted by the cycle time of the etch step in the Bosch cycle, relates the scallop height to the sidewall profile angle. The required positive tapering of 88° to 89° for a keyhole-free structure after a trench refill by low-pressure chemical vapour deposition is achieved by lowering the time of the etch step.

Vertical sidewall of silicon nitride mask and smooth surface of etched-silicon simultaneously obtained using CHF3/O2 inductively coupled plasma

Silicon nitride (SiNx) has been playing a vital role in the fabrication of micro- and nano-scale structures in numerous applications for several decades. Optimal etch recipes and a mechanistic understanding of the SiNx plasma-etching process are necessary for its adoption as a hard mask. We present a systematic investigation on SiNx films and underlying silicon (Si) plasma etching via an inductively coupled plasma (ICP) system using a CHF3/O2 mixture. Effects of ICP power, RF power, CHF3 flow rate on SiNx etching rate, CHF3/O2 gas ratio on SiNx sidewall profile, Si surface roughness, and SiNx/Si etching selectivity have been methodically investigated. We paid particular attention to studying the effect of O2 concentration on SiNx and Si etching mechanisms. It was found that SiNx sidewall profile, Si surface roughness, and SiNx/Si etching selectivity depend strongly on the O2 concentration. A nearly vertical SiNx sidewall of 89°, a low Si surface roughness of 0.2 nm, and a high SiNx/Si etching selectivity of 2 were simultaneously achieved by adjusting O2 concentration to 10% with CHF3+O2 total flow rate of 50 sccm. Finally, the composition of the residue on the surface of the processed sample was examined by X-ray photoelectron spectroscopy (XPS).

Study on the Transformation of Si Trench Profile With Low Pressure of SF₆/O₂ Containing Plasmas

The reason for the change of trench sidewall profile under low pressure of SF6/O2 containing plasma was explored by studying the trench morphology under different etching conditions. The result shows that the Si undercut decreases with the decreasing of pressure and the uniformity will become better. However, the sidewall of the trench presents a negatively tapered under low pressure. The reasons for different profile of trench are explored by the thickness of sidewall protective layer for the first time. The TEM results show that the trench sidewall forms a protective film with a thickness of about 100Å. The thickness of the oxide layer decreases from top to bottom. And the EDX results show that the main elements of the protective film are Si and O. Due to the decrease of oxide thickness, the width of trench bottom will become larger and larger, resulting in negatively tapered morphology. It is worth noting that the profile of trench changes to positively tapered again under the condition of low ratio of SF6/O2. Based on this, an optimized trench etching process is proposed and the etch rate is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.15{\mu }\text{m}$ </tex-math></inline-formula> /min and the selectivity to the oxide mask is 14.

A three-dimensional (3D) analytical solution for the ultimate side shear resistance of rock-socketed shafts

This paper presents a new analytical solution to the ultimate side shear resistance τsf of rock socketed shafts with consideration of three-dimensional (3D) strength and shaft sidewall roughness. Specifically, the shaft sidewall profile is simplified as triangular asperities and micro-mechanical analysis is performed on the shear behavior of one representative asperity. A 3D version of the Hoek-Brown (HB) criterion for rock mass and the equilibrium equations in a cylindrical coordinate system are first utilized to derive the governing equations. Then the governing equations are solved by using the method of characteristics. Finally, the τsf is obtained by considering the constant normal stiffness condition and the relative displacement of the rock mass due to sliding along the shaft-rock mass interface. Twenty-four (24) model and field test shafts with measured τsf are collected for analysis to verify the proposed solution. The results indicate that the predicted τsf values are in good agreement with the measured τsf values. Extensive parametric studies are also conducted to investigate the effects of rock mass properties, shaft dimensions, and sidewall roughness parameters on the ultimate side shear resistance factor αf (the ratio of τsf to the unconfined compressive strength of intact rock σc) of rock socketed shafts. The results indicate that the αf increases with rock constant mi, geological strength index (GSI) and asperity half chord length λ, but decreases with shaft diameter B, σc and disturbance D. Furthermore, the αf first increases and then decreases with the asperity inclination angle δ, resulting in an optimum asperity inclination angle δOp. Therefore, it is important to reduce the disturbance of rock mass from construction and select a proper asperity inclination angle when designing a rock socketed shaft.

Low-Temperature Deposition of High-Quality SiO2 Films with a Sloped Sidewall Profile for Vertical Step Coverage

SiO2 is one of the most widely used dielectric materials in optical and electronic devices. The Josephson voltage standard (JVS) chip fabrication process has rigorous requirements for the deposition temperature and step-coverage profiles of the SiO2 insulation layer. In this study, we deposited high-quality SiO2 insulation films at 60 °C using inductively coupled plasma-chemical vapor deposition (ICP-CVD) to fulfill these requirements and fabricate JVS chips simultaneously. SiO2 films have a high density, low compressive stress, and a sloped sidewall profile over the vertical junction steps. The sidewall profiles over the vertical junction steps can be adjusted by changing the radio frequency (RF) power, ICP power, and chamber pressure. The effects of sputtering etch and sloped step coverage were enhanced when the RF power was increased. The anisotropy ratio of the deposition rate between the sidewall and the bottom of the film was lower, and the sloped step coverage effect was enhanced when the ICP power was increased, or the deposition pressure was decreased. The effects of the RF power on the stress, density, roughness, and breakdown voltage of the SiO2 films were also investigated. Despite increased compressive stress with increasing RF power, the film stress was still low and within acceptable limits in the device. The films deposited under optimized conditions exhibited improved densities in the Fourier transform infrared spectra, buffered oxide etch rate, and breakdown voltage measurements compared with the films deposited without RF power. The roughness of the film also decreased. The step-coverage profile of the insulation layer prepared under optimized conditions was enhanced in the junction and bottom electrode regions; additionally, the performance of the device was optimized. This study holds immense significance for increasing the number of junctions in future devices.