Sidewall Taper Research Articles

Fabrication of silicon electrodes used for micro electrochemical machining

In micro electrochemical machining (ECM), stray corrosion always causes undesired material dissolution and deteriorates the machining localization. Adopting sidewall insulating films on the electrode is proven to be an effective approach for reducing stray corrosion. Sidewall films with characteristics of good insulation, thin thickness, and excellent durability are required. In previous work, we proposed a heavily-doped silicon electrode, on which silicon-based films were prepared and acted as the insulating films. To fabricate silicon electrodes, this research investigates wet etching behavior of heavily doped Si (100) and then presents a fabrication process. Firstly, shapes of micro electrodes are designed on photomasks of lithography. Aiming at patterned profiles, effects of etchants on etch rate, dimension accuracy and surface integrity are investigated. Experimental results indicate that tetramethylammonium hydroxide (TMAH) leads to smoother electrode surfaces and fewer defects compared with potassium hydroxide. Then, insulation and durability performances of silicon-based films are investigated. 500 nm SiO2 and 300 nm Si3N4 composite film achieves more durable insulation and excellent compactness. Consequently, silicon electrodes with the dimension of 86 × 54 μm are fabricated on the Si (100) wafer with 25 wt% TMAH etchant, on which SiO2 and Si3N4 film is deposited by low-pressure chemical vapor deposition. In ECM experiments, stray corrosion of the non-machined surface is significantly reduced, and the sidewall taper of the machined structure is reduced to 5.5°. The validity and durability of silicon electrodes for micro ECM processes are verified.

Analysis of the flow field in counter-rotating electrochemical machining

In electrochemical machining (ECM), the electrolyte flow field has marked influence on the processing stability, efficiency and surface quality. This paper presented a new electrolyte flow mode to improve the flow field distribution in counter-rotating electrochemical machining (CRECM), in which the electrolyte flows into the processing zone from both sides and top of the fixture, and flows out from the bottom of the fixture. The flow field distributions were analyzed and simulated via computational fluid dynamics (CFD) software. The simulation results showed that the low-velocity area in CRECM was eliminated and the velocity distribution was more uniform by using the optimized flow mode. Experiments were also conducted using a specific fixture. The experimental results indicated that a stable machining process with a small inter-electrode gap could be achieved, and the ECM accuracy was remarkably improved. The sidewall taper angles and rounded corners of the machined convex structure were reduced to be only about -0.8° and 0.6 mm.

Hybrid lithography based fabrication of 3D patterns by deep reactive ion etching

Electron-beam lithography (EBL) possesses tremendous capabilities in terms of patterning ultra-small features, with very low line edge roughness and high overlay accuracy. In order to reduce write times and processing steps, hybrid lithography is an interesting alternative. Here, an approach combining EBL and ultraviolet lithography (UVL) was pursued in order to pattern multi-level 3D-structures by inductively coupled plasma deep reactive ion etching (ICP-DRIE). The substrate etched hereby consisted of a silicon nitride thin film and the underlying silicon. The correlation of UVL under- and overexposures with the resist layer height and sidewall taper after development, in combination with the optimization of the ICP-DRIE selectivity in dependence of the biasing conditions and the aspect ratio dependent etch lag for small features, allowed for the accurate prediction and therefore programming of the desired structures. Further, the selectivity was tailored by introducing an oxygen pulse after the breakthrough step. This facilitated the simultaneous etching of structures with an in-plane resolution of down to 250 nm (aspect ratio 1:22) and trench depths ranging from 200 nm to 25 μm.

CFD Modeling of Melt Spreading Behavior on Spinning Discs and Cups for Centrifugal Granulation of Molten Slag

The spreading behaviors of molten slag on spinning discs and cups were studied through performing free surface flow numerical simulations by means of computational fluid dynamics (CFD) modeling technique. In this work, liquid slag film thickness at the edge of the spinning discs and cups was predicted by using the CFD model, since the slag film thickness has a predominant influence on the size of the slag granules produced after the slag film breakup. The effects of the shape of discs and cups and the operating conditions (slag flowrate and spinning speed) on the slag film thickness were examined. Flat surface disc, disc with curved surface, and cups with different sidewall height and taper angle were investigated. It was found from the modeling results that, under the same slag flowrate and spinning speed, the larger the wetting area of the slag on the discs and cups, the smaller the slag film thickness. For the same size (radius) discs and cups, the slag film thickness on the flat surface disc is larger than those on the cup and the curved surface disc. Furthermore, the film thickness on the cup is larger than that on the curved surface disc. The reason is that the cup has a sharp corner and a sidewall that impose a larger resistance to the slag flow, whereas the curved surface disc has a smooth surface that has a smaller resistance to the slag flow.

Energy absorption analysis for tapered multi-cell tubes improved by foams: Theoretical development and numerical simulation

In this paper, Energy absorption characteristics of tapered multi-cell tubes improved by foams are studied. For the energy absorption process, a theoretical formulation is developed to represent the mean crush load of the foam-filled tubes considering a combination of mean crush loads due to non-filled tube, the foam-filler, and a frictional resistance between them (interaction effects). The formulation is based on the assumptions including equivalent segmented non-tapered single-cell tubes as well as a parallel resisting force of the contributors (non-filled tubes, foam-filler, and interaction effects). Moreover, an extensive numerical analysis on the crushing of the foam-filled tapered multi-cell tubes, with a taper angle ranging from 0 to 7 degrees, a wall thickness ranging from 0.25 to 3 mm, and a cell number ranging from 11 to 10 × 10, is conducted to evaluate the effects of side wall tapering, cross section’s division into multiple cells and wall thickness enlargement on the crashworthiness characteristics. Analyses indicate that there are significant enhancements in the energy absorption behavior of the foam-filled tubes with respect to the non-filled ones. As a special case, the force-displacement results obtained in the numerical simulation are verified against those experimentally observed and reported in the literature for a foam-filled non-tapered single-cell tube.

Layer-by-layer combination of laser powder bed fusion (LPBF) and femtosecond laser surface machining of fabricated stainless steel components

Post-processing techniques for surface machining of laser powder bed fusion (LPBF) fabricated parts offer limited access to internal surfaces in complex shapes, precision surfacing, side-wall morphology, and material removal rates, thereby considerably limiting their applicability. In this work, a two-step fabrication involving the initial fusion of a powder layer in a LPBF set-up followed by the selective ablation of layer contours by rastering normally incident femtosecond laser pulses, is repeated for every layer of the built. For 1 kHz repetition rate, pulse fluence of 6.7 J/cm2 and super-Gaussian beam of shape factor 16, an experimentally obtained groove-free surface on laser machined walls was correlated with scan speeds when the pulse accumulated fluence distribution profile was horizontally flat or constant. Increasing the line overlap in raster scans did not yield any noticeable change in the machined wall surface. Groove-free surfaces, uniformly textured with sub-micrometer spaced laser-induced periodic surface structures (LIPSS), were successfully extended over side-walls of several unidirectionally machined consecutive layers. Ultimately, laser machined multi-layered LPBF parts possessing average side-wall roughness of Rt=4.7±1μm and side-wall taper angle of as low as ∼3°±1° were fabricated.

Silicon nanostructures with very large negatively tapered profile by inductively coupled plasma-RIE

Dry etching of silicon has been extensively studied, mostly with a goal of obtaining perfectly vertical sidewalls with high aspect ratio. Yet, sloped sidewall with a negative taper angle (i.e., diameter/width decreases linearly with depth) may find various applications. However, the systematic study on the etching process development to obtain such a profile is rather scarce. In this work, the authors present a controlled and reproducible fabrication process to achieve silicon nanostructures with negatively tapered sidewall profile using inductively coupled plasma-reactive ion etching with C4F8 and SF6 gas. The plasma etching parameters have been thoroughly optimized in order to avoid the undercut or curved reentrant profile due to isotropic etching, so as to achieve a negatively tapered profile. The influence of the plasma etching parameters, especially the radio freguency power and C4F8/SF6 gas flow ratio, on the etching rate and the sidewall taper angle has been analyzed. With an optimal etching recipe, the silicon nanostructures with an unprecedented large 10° negative taper angle were achieved. These results were demonstrated on different structure sizes of 500 nm, 700 nm, and 1.2 μm diameters.

Novel Manufacturing Route for Scale Up Production of Terahertz Technology Devices

The advances in the Terahertz (THz) technology drive the needs for the design and manufacture of waveguide devices that integrate complex three-dimensional (3D) miniaturized components with meso- and micro-scale functional features and structures. Typical dimensions of the waveguide functional structures are in the range from 200 μm to 50 μm and dimensions decrease with the increase in the operating frequency of the waveguide devices. Technological requirements that are critical for achieving the desired microwave filtering performance of the waveguides include geometrical accuracy, alignment between functional features and surface integrity. In this context, this paper presents a novel manufacturing route for the scaled-up production of THz components that integrate computer numerical control (CNC) milling and laser micromachining. A solution to overcome the resulting tapering of the laser-machined structures while achieving a high accuracy and surface integrity of the machined features is applied in this research. In addition, an approach for two-side processing of waveguide structures within one laser machining setup is described. The capabilities of the proposed manufacturing process chain are demonstrated on two THz waveguide components that are functionally tested to assess the effects of the achieved machining results on devices' performance. Experimental results show that the proposed process chain can address the manufacturing requirements of THz waveguide filters, in particular the process chain is capable of producing filters with geometrical accuracy better than 10 μm, side wall taper angle deviation of less than 1 deg from vertical (90 deg), waveguide cavities corner radius better than 15 μm, and surface roughness (Sa) better than 1.5 μm. The manufacturing efficiency demonstrated in this feasibility study also provides sufficient evidences to argue that the proposed multistage manufacturing technique is a very promising solution for the serial production of small to medium batches of THz waveguide components. Finally, analyses of the manufacturing capabilities of the proposed process chain and the photoresist-based technologies were performed to clearly demonstrate the advantages of the proposed process chain over current waveguide fabrication solutions.

Theoretical development and numerical investigation on energy absorption behavior of tapered multi-cell tubes

In this study, the crashworthiness behavior of the tapered multi-cell tubes is theoretically and numerically investigated. The side wall tapering and the cross section dividing into the cells are used in order to improve the energy absorption of the thin walled structure. In order to estimate the non-constant mean crush load of the tapered multi-cell tubes, incorporating the theoretical relations on the energy absorption of the simple multi-cell and tapered single-cell tubes, in conjunction with numerical results, would lead to the development of the analytical formula. The analytical formula is based on the mean crush load calculation of the equivalent segmented simple multi-cell tubes. In order to prepare a realistic finite element model, the elastic–plastic-damage material assumption is introduced into the numerical simulation. The numerical predictions of the mean crush load are consisted with experimental data in literature. The results reveal that the increase of the taper angle, the wall thickness and the number of cells in the cross section would enrich the crashworthiness capacity of the structure. The results indicate that the crashworthiness improvement rate of the structure with number of cells in the cross section is decreased in such a way that the variation of the energy absorption efficiency in the thin walled structure with the 8×8, 9×9 and 10×10 cells in the cross section would be negligible. The transition wall thickness in which the crashworthiness of the tapered multi-cell structure transits from thin to thick walled behavior are determined.

Investigating the characteristics of TM-pass/TE-stop polarizer designed using plasmonic nanostructures.

Plasmonics-based polarizers are important for many photonic devices and applications. In this paper, we design and investigate the characteristics of a new TM-pass/TE-stop polarizer using silver nanograting of exponentially tapered slit sidewalls. Performance of the designed polarizer is determined through monitoring the modification of its insertion loss, return loss, extinction ratio, and far-field transform due to changing its structural parameters. We find that the structural parameters of the reported polarizer such as a slit sidewall tapering coefficient and slit opening widths have a significant impact on tuning the polarizer characteristics.

Charge Transport in Uniform Metal-Assisted Chemical Etching for 3D High-Aspect-Ratio Micro- and Nanofabrication on Silicon

Recently, metal-assisted chemical etching (MaCE) has been proposed as a promising method for micro- and nanostructures fabrication on silicon (Si) with high aspect ratio, high geometric uniformity and low cost. In MaCE, electron holes (h+) are injected into Si through catalytic reduction of H2O2 on metal catalyst thin film patterns. Si beneath the metal is etched through a redox reaction where h+ are involved. This work investigated a fundamental electrochemical process during MaCE: the transport of h+, and revealed its unique correlation with the 3D profile of the etching results. It is discovered that under the uniform etching condition, etching occurs both in the Si beneath the catalysts as well as on the sidewall of the etched space. On N-type Si, the sidewall etching is intrinsically depressed, and highly vertical HAR structures are formed; on P-type Si and undoped Si, the sidewall tapering becomes significant as the pattern number and density increase. The variation of the 3D profile can be explained by the CT during etching using a Schottky junction model, which show dependence on the intrinsic properties of the Si. CT involved in the two etching process can further be correlated by diffusion or drift of h+, which explains the influence of catalysts geometry.

Uniform Metal-assisted Chemical Etching and the Stability of Catalysts

ABSTRACTRecently, metal-assisted chemical etching (MaCE) has been demonstrated as a promising technology in fabrication of uniform high-aspect-ratio (HAR) micro- and nanostructures on silicon substrates. In this work, MaCE experiments on 2 μm-wide line patterns were conducted using Au or Ag as catalysts. The performance of the two catalysts show sharp contrast. In MaCE with Au, a HAR trench was formed with uniform geometry and vertical sidewall. In MaCE with Ag, shallow and tapered etching profiles were observed, which resembled the results from isotropic etching. The sidewall tapering phenomena can be explained by the dissolution and re-deposition of the Ag catalyst in the etchant solution. The existence of Ag that was redeposited on the sidewall was further confirmed by energy dispersive spectrum. Also, etchant composition is found to play a profound role in influencing the etching profile by the Ag catalysts.

Chemical Control of Semiconductor Nanowire Kinking and Superstructure

We show that methylgermane (GeH(3)CH(3)) can induce a transition from 111 to 110 oriented growth during the vapor-liquid-solid synthesis of Ge nanowires. This hydride-based chemistry is subsequently leveraged to rationally fabricate kinking superstructures based on combinations of 111 and 110 segments. The addition of GeH(3)CH(3) also eliminates sidewall tapering and enables Ge nanowire growth at temperatures exceeding 475 °C, which greatly expands the process window and opens new avenues to create Si/Ge heterostructures.

Dimensional metrology of bipolar fuel cell plates using laser spot triangulation probes

As in any engineering component, manufacturing a bipolar fuel cell plate for a polymer electrolyte membrane (PEM) hydrogen fuel cell power stack to within its stated design tolerances is critical in achieving the intended function. In a bipolar fuel cell plate, the dimensional features of interest include channel width, channel height, channel parallelism, side wall taper, straightness of the bottom or side walls, plate parallelism, etc. Such measurements can be performed on coordinate measuring machines (CMMs) with micro-probes that can access the narrow channels. While CMM measurements provide high accuracy (less than 1 µm), they are often very slow (taking several hours to measure a single plate) and unsuitable for the manufacturing environment. In this context, we describe a system for rapid dimensional measurement of bipolar fuel cell plates using two laser spot triangulation probes that can achieve comparable accuracies to those of a touch probe CMM, while offering manufacturers the possibility for 100% part inspection. We discuss the design of the system, present our approach to calibrating system parameters, present validation data, compare bipolar fuel cell plate measurement results with those obtained using a Mitutoyo UMAP (see footnote 1) fiber probe CMM, and finally describe the uncertainty in channel height and width measurements.

Growth of high quality InP layers in STI trenches on miscut Si (0 0 1) substrates

In this work, we report the selective area epitaxial growth of high quality InP in shallow trench isolation (STI) structures on Si (0 0 1) substrates 6° miscut toward (1 1 1) using a thin Ge buffer layer. We studied the impact of growth rates and steric hindrance effects on the nano-twin formation at the STI side walls. It was found that a too high growth rate induces more nano-twins in the layer and results in InP crystal distortion. The STI side wall tapering angle and the substrate miscut angle induced streric hindrance between the InP facets and the STI side walls also contribute to defect formation. In the [1̄ 1 0] orientated trenches, when the STI side wall tapering angle is larger than 10°, crystal distortion was observed while the substrate miscut angle has no significant impact on the InP defect formation. In the [1̄ 1 0] trenches, both the increased STI tapering angle and the substrate miscut angle induce high density of defects. With a small STI tapering angle and a thin Ge layer, we obtained extended defect free InP in the top region of the [1 1 0] trenches with aspect ratio larger than 2.

300-mm Production-Worthy Magnetically Enhanced Non-Bosch Through-Si-Via Etch for 3-D Logic Integration

We report a process development route toward 300-mm production-worthy non-Bosch through-silicon-via (TSV) etch with critical dimensions between 1-5 ?m and aspect ratios up to 20:1 for 3-D logic applications. The etch development was performed on an experimental alpha-tool: a magnetically enhanced capacitively coupled plasma etcher with a dipole ring magnet that aims to capture the strengths (anisotropicity, profile uniformity) while eliminating the weaknesses (scalloping, undercut, residues) of a nominal Bosch process. Key factors contributing to the control of sidewall taper and roughness, etched TSV volume and depth, mask undercut, local bowing effects, and within wafer (WIW) center-to-edge depth and profile uniformity were evaluated. TSVs with nominal sizes of 5 × 25 ?m, 5 × 40 ?m and 1 × 20 ?m with less than 1% WIW nonuniformity, negligible silicon scalloping/mask undercut, and good profile anisotropicity were developed. Up to 3 × 20 ?m and 5 × 25 ?m void-free Cu-filled TSVs were demonstrated with both vertical TSVs and tapered TSVs.

Sidewall slopes of SU-8 HARMST using deep x-ray lithography

We describe a model that predicts sidewall slope in SU-8 high aspect ratio microstructures made using x-ray lithography. Our experimental results are in accordance with our model and we demonstrate that essentially zero sidewall taper can be obtained under certain conditions. We use our results to explore the feasibility of optimizing a structure simultaneously for slope and surface roughness.

Sidewall slopes and roughness of SU-8 HARMST

In recent years we have investigated the feasibility of deep X-ray lithography in SU-8 as a means of fabricating very high aspect ratio and densely packed arrays of square channels. We have demonstrated how to overcome the problems in adhesion and stiction accompanying the fabrication of such high aspect ratio structures. We have also examined how to calculate the development time of these structures and how this can be used to optimize the dimensional fidelity of the resulting structures. More recently we have considered the effect of the non-uniform dose deposition and beam hardening as a function of depth in the resist on factors such as the surface roughness and the sidewall taper. In this paper we describe our experimental program, review these results and discuss their implications for the operation of our target device—the lobster-eye telescope.

The role of plug design in determining wall thickness distribution in thermoforming

AbstractAn experimental investigation has been carried out into the effects of changes in plug design on the wall thickness distribution of thermoformed products. Plugs were machined with a series of geometrical variations and their effects on the process were measured. The overall results show that the plug has a crucial role in controlling the wall thickness distribution in thermoforming. Larger plugs tend to distribute more material to the base of the product, but the introduction of a small sidewall taper, base radius, or a reduction in plug diameter tend to lead to more balanced distributions. However, larger changes in any of the variables tend to destroy these benefits. It has also been demonstrated that the frictional and thermal properties of the plug are important in determining the deformation response of the sheet material. There is a clear evidence of slip in the sheet during plug contact and, although the cooling effect of the plug appears to be minimal, cooling in the highly deformed regions away from the plug appears to be a significant factor. POLYM. ENG. SCI., 47:804–813, 2007. © 2007 Society of Plastics Engineers

A Model Analysis of Feature Profile Evolution and Microscopic Uniformity during Polysilicon Gate Etching in Cl2/O2 Plasmas

A phenomenological model has been developed to simulate the feature profile evolution of polycrystalline silicon (poly-Si) gate etching in Cl2/O2 plasmas. The model takes into account the deposition of etch products, surface oxidation, and the forward reflection of energetic ions on feature sidewalls. To describe the formation of multilayer SiClx or SiClxOy on feature surfaces during etching, the substrates consist of a number of small cells or lattices of atomic size in the computational domain; this model provides a nanometer-scale representation of the feature geometry and the chemical constituents therein. The inelastic or nonspecular reflection of incoming ions from feature surfaces and the penetration of ions into substrates are incorporated into the model by calculating the trajectory of ions through successive binary collisions with substrate atoms. Etching experiments were performed to evaluate and improve the accuracy of the model. To analyze the effects of the control variables of a plasma reactor on profile evolution, the simulated profiles for different gas flow ratios and incident ion energies were compared with the etched profiles obtained in the experiments. The numerical results reproduced the behaviors of profile anomalies such as sidewall tapering and microtrenches at the corner of the feature bottom, upon varying the incident fluxes of O neutrals and etch by-products, and the incident energy of ions. Moreover, the simulated profiles exhibited passivation layers deposited on feature sidewalls, which is a similar geometry to those obtained in the experiments.