Improve Thickness Uniformity Research Articles

Wrinkling suppression in cryogenic forming of high-strength Al-alloy ultra-thin shells by controlling interface shear stress

Avoiding wrinkling defects is extremely difficult in the sheet metal forming of ultra-thin components made from high-strength aluminum alloys. A novel cryogenic forming is thus proposed for solving this very challenging problem, wherein a thicker cladding blank is stacked above the thinner target blank to establish interface shear stress for reducing the hoop compressive stress, so that the critical wrinkling stress is increased. The enlarged radial deformation is transferred and withstood by the increasing hardening and ductility at cryogenic temperatures. The wrinkling suppression mechanism is revealed through mechanical and numerical analyses. Systematic experiments were conducted for studying the feasibility of high-strength AA7075 in cryogenic forming with different stacking sequences of cladding and target blanks, cladding blanks including three material strength levels (AA1060-O, AA5052-O, and SUS-304), two thicknesses (3.0, and 2.0 mm), and blank-holder forces. The effects of the cladding blank and blank-holder forces were clarified as reflected by forming defects, thickness, and strain distributions. The mechanical and numerical analyses can indicate that contact pressure can be produced by the wrinkling tendency of thinner target blank and limitation of cladding blank to wrinkling, which is accompanied by a decrease in the hoop compressive stress and increase in the critical wrinkling stress. Therefore, applying a relatively thicker blank on the punch side can prevent the wrinkling of ultra-thin components. The wrinkling tendency decreases with increasing strength and thickness of the cladding blank, which results in an increase in deformation or even splitting, which can be solved by the cryogenic temperature. The thickness of the cladding blank can be reduced by increasing the blank-holder force, which further reduces the material cost. AA5052-O, which has a strength similar to that of the target blank, is more suitable as a cladding blank for AA7075-W because the balance preventing wrinkling, improving thickness uniformity, and controlling the forming force. A Φ200 mm hemispherical shell with an initial thickness of 0.3 mm was formed successfully, and the corresponding thickness-to-diameter ratio reached 0.8‰, which almost increased one time on the basis of direct cryogenic forming. This new approach can be used for fabricating ultra-thin components from high-strength aluminum alloys.

Improving uniformity and performance of electroformed copper using rare earth element additives

Electroforming can realize high precision and complex shape parts manufacturing, making it widely used in the field of micromachining and micromanufacturing. The drawback lies in the nonuniform thickness distribution, which can cause quality issues including voids, pitting, and particles. In order to enhance the microstructure uniformity and electroforming performance, lanthanum chloride and cerium chloride were employed as additives in the copper electroforming experiment. This study analyzes the effects of lanthanum chloride and cerium chloride on the surface morphology, microhardness, tensile strength, and uniformity of electroformed copper deposits. The results show that adding rare earth elements into the electrolyte solution can improve thickness uniformity, promote ion mass transfer, and refine the copper deposit’s grain. Compared to the microstructure without adding additives, the copper deposit’s nonuniformity prepared with 1.0 g/L lanthanum chloride is 76.9% lower, and with cerium chloride, it is 72% lower. Additionally, the copper deposit prepared with lanthanum chloride exhibits superior overall quality compared to that prepared with cerium chloride.

In Situ Stereo Digital Image Correlation with Thermal Imaging as a Process Monitoring Method in Vacuum-Assisted Thermoforming

This experimental study probes the dynamic behaviour of a 3 mm ABS sheet during positive mould vacuum-assisted thermoforming. In this process, the sheet undergoes large and fast deformations caused by the applied vacuum and mechanical stretching by the mould. The objective is to elucidate the complexities of these large, rapid, and non-isothermal deformations. The non-isothermal conditions are caused by the radiative heating of the sheet, convective heat loss to the surrounding air, and conductive heat transfer from the sheet to the mould. By utilizing stereo digital image correlation (DIC) in tandem with thermal imaging, the present study accurately maps the occurring displacement, strain, and strain rate field in relation to real-time temperature variation in the material. The study progresses to observe the ABS material from the moment it contacts the mould until it conforms to a positive 250 mm diameter semi-sphere cast aluminum mould. The DIC methods are validated by comparing thickness values derived from DIC’s principal strain directions to ultrasonic thickness gauge readings. This knowledge not only broadens the understanding of the thermo-mechanical behaviour of the material but also aids in optimizing process parameters for improved thickness uniformity in thermoformed products.

New precision electroforming process for the simultaneous improvement of thickness uniformity and microstructure homogeneity of wafer-scale nanotwinned copper arrays

Nanotwinned (nt) Cu has received much attention because of its superior mechanical and electrical properties, but only a few production processes can yield nt-Cu parts with uniform thickness and a homogeneous microstructure on the wafer scale. To solve this problem, a new precision electroforming process is proposed that combines auxiliary cathodes with pulse reverse current (PRC) electroforming, which provides a synergistic effect to increase the homogeneity of the thickness and a nanoscale twin structure. As a practical example of the proposed process, 4-inch nt-Cu lamina arrays were fabricated and numerically modeled to probe into the synergistic mechanisms. The intrinsic correlations among the array element spacing, current waveform, and main forms of thickness nonuniformity were determined. In addition, the effects of the processing parameters on the microstructural evolution and microhardness of the nt-Cu arrays were analyzed. The results indicated that such a significant improvement in thickness uniformity and microstructure homogeneity were due to the auxiliary-cathode/PRC combination, which enables maximization of the PRC leveling efficiency by inducing a uniform current distribution; this effectively ensures that the microstructures are uniform across all laminae on the wafer scale. Additionally, thick nt-Cu deposited on the current-crowding regions was preferentially stripped during the application of reverse current. This alleviates the adverse effects of the current redistribution resulting from the auxiliary cathode on the thickness uniformity of the laminae and offers additional possibilities for homogeneous growth of nt-Cu. The new precision electroforming process has significant potential to produce wafer-scale components with uniform thickness and specific microstructures.

Improving Thickness Uniformity of Amorphous Oxide Films Deposited on Large Substrates by Optical Flux Mapping

In this study, three amorphous oxide thin films are prepared by an electron beam evaporation combined with ion-assisted deposition technique. With the aid of optical flux mapping method, thin film thickness distribution with good uniformity can be obtained by appropriate coating masks. Three metal oxide single-layer thin films are SiO2, Ta2O5 and Nb2O5, respectively. These thin films were deposited on a substrate holder with a radius of 275 mm that was divided into five different segments. Based on the optical flux mapping method, we can effectively simulate the geometric dimensions of the coating mask and obtain the width of the coating mask at different segments. If the film thickness uniformity is a function of masking area and center angle, it is necessary to determine the thickness distribution of the different segments and use a surface profiler to accurately measure the film thickness. We analyzed the thickness uniformity of three oxide films deposited at five different segments. The experimental measurement results show that the deviation of thickness uniformity is 0.38% for SiO2, 0.36% for Ta2O5, and 0.15% for Nb2O5 thin films, respectively.

Twin-free, directly synthesized MFI nanosheets with improved thickness uniformity and their use in membrane fabrication.

Zeolite nanosheets can be used for the fabrication of low-defect-density, thin, and oriented zeolite separation membranes. However, methods for manipulating their morphology are limited, hindering progress toward improved performance. We report the direct synthesis (i.e., without using exfoliation, etching, or other top-down processing) of thin, flat MFI nanosheets and demonstrate their use as high-performance membranes for xylene isomer separations. Our MFI nanosheets were synthesized using nanosheet fragments as seeds instead of the previously used MFI nanoparticles. The obtained MFI nanosheets exhibit improved thickness uniformity and are free of rotational and MEL intergrowths as shown by transmission electron microscopy (TEM) imaging. The nanosheets can form well-packed nanosheet coatings. Upon gel-free secondary growth, the obtained zeolite MFI membranes show high separation performance for xylene isomers at elevated temperature (e.g., p-xylene flux up to 1.5 × 10−3 mol m−2 s−1 and p-/o-xylene separation factor of ~600 at 250°C).

Study on Controllable Thickness and Flatness of Wafer-Scale Nickel Shim in Precision Electroforming Process: Simulation and Validation

Abstract A new approach to precision electroforming of a wafer scale nickel shim using a rotating cathode with an auxiliary cathode mask is developed to improve thickness uniformity and flatness. The effects of critical process parameters, including cathode rotating speed, cathode mask size, and current density, on the thickness uniformity and flatness of electroformed nickel shim are systematically studied based on experiments and simulations. The results show that the thickness uniformity of the deposits is highly dependent on the current density distribution, where a cathode mask can effectively tune electrical field lines and induce a more uniform current density distribution. The simulations and experimental results consistently agree that a minimum thickness nonuniformity of 8% and below 1% on the wafer with a diameter of 80 mm and 40 mm, respectively, can be achieved using a mask with a 70 mm opening size. However, for flatness, the cathode rotating speed influences the surface warpage due to the intrinsic stress. It is also found that the gradient current density can significantly reduce the intrinsic stress with better flatness. The best flatness is controlled below 47 µm and 32 µm on the wafer with diameters of 80 mm and 40 mm, respectively, under the synergistic effect of optimal cathode rotating speed (30 rpm) and gradient current density.

Improving Thickness Uniformity of Mo/Si Multilayers on Curved Spherical Substrates by a Masking Technique

In this work, a masking technique was used to improve the thickness uniformity of a Mo/Si multilayer deposited on a curved spherical mirror by direct current (DC) magnetron sputtering with planetary rotation stages. The clear aperture of the mirror was 125 mm with a radius of curvature equal to 143.82 mm. Two different shadow masks were prepared; one was flat and the other was oblique. When using the flat mask, the non-uniformity considerably increased owing to the relatively large gap between the mask and substrate. The deviation between the designed and measured layer thickness and non-uniformity gradually reduced with a smaller gap. The second mask was designed with an oblique profile. Using the oblique mask, the deviation from multilayer thickness uniformity was substantially reduced to a magnitude below 0.8% on the curved spherical substrate over the clear aperture of 125 mm. Multilayers still achieved a smooth growth when deposited with obliquely incident particles. The facile masking technique proposed in this study can be used for depositing uniform coatings on curved spherical substrates with large numerical apertures for high-resolution microscopes, telescopes, and other related optical systems.

Topology optimization of blank holders in nonisothermal stamping of magnesium alloys based on discrete loads

The structure of blank holders has a great influence on heat transfer in nonisothermal stamping of magnesium alloys. In order to improve the quality of nonisothermal stamping parts, a topology optimization method based on discrete loads is proposed, and applied to optimize blank holders. Based on the finite element model (FEM) of a cross-shaped cup in NUMISHEET2011, the one-step method is used to inverse the optimization area of the blank holder. The optimization area is rediscretized into squares. The coupled thermo-mechanical analysis is carried out based on the discretized blank holder, and the reactive forces of the discrete squares are obtained, and taken as the constraint to carry out the topology optimization of the blank holder. Based on the optimized blank holder, the plasticity difference of the blank flange can be realized by controlling the temperature on the blank flange. The nonisothermal stamping is analyzed based on the optimized blank holder. Compared with the results based on the initial blank holder, the optimization results show that the optimized blank holder can improve thickness uniformity effectively in the nonisothermal stamping of AZ31B magnesium alloy.

Improving the thickness uniformity of micro electroforming layer by megasonic agitation and the application

Throwing power can be used to quantitatively characterize the uniform distribution ability of electroforming layer thickness. From the perspective of throwing power, the effect of megasonic agitation on improving the thickness uniformity was studied under the experiment system of nickel containing electrolyte. Firstly, throwing power experiments were performed with different megasonic power densities. Experiment results show that throwing power increases with the increase in power density. When power density is 2.4 W/cm2, throwing power has a maximum value of 37.1% and increases 50.2% compared with the value of 24.7% obtained without megasonic. Then, the effect factors of throwing power, including electrolyte conductivity and cathodic polarizability, were investigated. The results reveal that megasonic has little effect on the cathodic polarizability, but it can increase the conductivity, which is conducive to improving thickness uniformity. Finally, to prove the feasibility of this method on micro metal devices, metal microfluidic chip molds were fabricated by megasonic micro electroforming. The result indicates that the thickness uniformity of the electroformed micro mold is improved by 43.4%.

Pancake making and surface coating: Optimal control of a gravity-driven liquid film

This paper investigates the flow of a solidifying liquid film on a solid surface subject to a complex kinematics, a process relevant to pancake making and surface coating. The flow is modeled using the lubrication approximation, with a gravity force whose magnitude and direction depend on the time-dependent orientation of the surface. Solidification is modeled with a temperature-dependent viscosity. Because the flow eventually ceases as the liquid film becomes very viscous, the key question this study aims to address is: what is the optimal surface kinematics for spreading the liquid layer uniformly? Two methods are proposed to tackle this problem. In the first one, the surface kinematics is assumed a priori to be harmonic and parameterized. The optimal parameters are inferred using the Monte-Carlo method. This ``brute-force'' approach leads to a moderate improvement of the film uniformity compared to the reference case when no motion is imposed to the surface. The second method is formulated as an optimal control problem, constrained by the governing partial differential equation, and solved with an adjoint equation. Key benefits of this method are that no assumption is made on the form of the control, and that significant improvement in thickness uniformity are achieved with a comparatively smaller number of evaluations of the objective function.

Simulation and Optimization of Film Thickness Uniformity in Physical Vapor Deposition

Optimization of thin film uniformity is an important aspect for large-area coatings, particularly for optical coatings where error tolerances can be of the order of nanometers. Physical vapor deposition is a widely used technique for producing thin films. Applications include anti-reflection coatings, photovoltaics etc. This paper reviews the methods and simulations used for improving thin film uniformity in physical vapor deposition (both evaporation and sputtering), covering characteristic aspects of emission from material sources, projection/mask effects on film thickness distribution, as well as geometric and rotational influences from apparatus configurations. Following the review, a new program for modelling and simulating thin film uniformity for physical vapor deposition was developed using MathCAD. Results from the program were then compared with both known theoretical analytical equations of thickness distribution and experimental data, and found to be in good agreement. A mask for optimizing thin film thickness distribution designed using the program was shown to improve thickness uniformity from ±4% to ±0.56%.

TOPSIS based Taguchi design optimization for CVD growth of graphene using different carbon sources: Graphene thickness, defectiveness and homogeneity

Abstract Chemical inhomogeneity of chemical vapor deposition (CVD) grown graphene compromises its usage in high-performance devices. In this study, TOPSIS based Taguchi optimization was performed to improve thickness uniformity and defect density of CVD grown graphene. 1.56% decrease in the mean 2D/G intensity ratio, 87.96% improvement in the mean D/G intensity ratio, 56.07% improvement in the standard deviation D/G intensity ratio, 25.21% improvement in the standard deviation 2D/G intensity ratio, and 69.32% improvement in the surface roughness were achieved with TOPSIS based Taguchi optimization. The statistical differences between the copper and silicon substrates have been found significantly in terms of their impacts on the graphene's properties with the 0.000 p-value for the mean D/G intensity ratio and with the 0.009 p-value for the mean 2D/G intensity ratio, respectively. Graphene having 11% lower mean D/G intensity ratio (low defective graphene products) compared to the values given in the literature using single-response optimization was obtained using multi-response optimization.

Pre-annealing of metal stack precursors and its beneficial effect on kesterite absorber properties and device performance

A variety of approaches is being used to fabricate kesterite absorbers, such as sputtering, co-evaporation and solution based techniques. Annealing at high temperatures is a common processing step to stimulate elemental mixing and grain growth. This study investigates the effect of pre-annealing of metal stack precursors at 150 °C, 200 °C, 300 °C and 450 °C on kesterite absorber and solar cell properties. Structural, morphological and vibrational properties of the thin films were investigated. It was found that pre-annealing at 450 °C exhibited a structural mixture distinct from precursors annealed at lower temperatures. The absorber showed improved thickness uniformity, compositional surface homogeneity and absence of Sn-S phases. Lower temperatures resulted in Sn-S compounds in the absorber. In situ XRD is used to monitor phase transitions during pre-annealing. It is shown that implementing a pre-annealing step can improve the absorber properties and the device efficiency. The best performing solar cell had a 4.02% efficiency and was achieved by pre-annealing the precursor layer at 450 °C.

Formation process of graphite film on Ni substrate with improved thickness uniformity through precipitation control

Large-scale graphitic thin film with high thickness uniformity needs to be developed for industrial applications. Graphitic films with thicknesses ranging from 3 to 20 nm have rarely been reported, and achieving the thickness uniformity in that range is a challenging task. In this study, a process for growing 20 nm-thick graphite films on Ni with improved thickness uniformity is demonstrated and compared with the conventional growth process. In the film grown by the process, the surface roughness and coverage were improved and no wrinkles were observed. Observations of the film structure reveal the reasons for the improvements and growth mechanisms.

Inkjet printed polystyrene sulfuric acid-doped poly(3,4-ethylenedioxythiophene) (PEDOT) uniform thickness films in confined grooves through decreasing the surface tension of PEDOT inks

Films with improved thickness uniformity can be inkjet printed as a result of low surface tension modification of inks.

Multiscale modeling and run-to-run control of PECVD of thin film solar cells

In this work, we focus on the development of a multiscale modeling and run-to-run control framework with the purpose of improving thin film product quality in a batch-to-batch plasma-enhanced chemical vapor deposition (PECVD) manufacturing process. Specifically, at the macroscopic scale, gas-phase reaction and transport phenomena yield deposition rate profiles across the wafer surface which are then provided to the microscopic domain simulator in which the complex microscopic surface interactions that lead to film growth are described using a hybrid kinetic Monte Carlo algorithm. Batch-to-batch variability has prompted the development of an additional simulation layer in which an exponentially weighted moving average (EWMA) control algorithm operates between serial batch deposition sequences to adjust the operating temperature of the PECVD reactor to overcome drift in the electron density of the plasma. Application of the run-to-run (R2R) control system developed here is shown to reduce offset in the product thickness from 5% to less than 1% within 10 batches of reactor operation. Finally, we propose an extension of the EWMA algorithm to four independent, radial wafer zones in order to improve thickness uniformity in the presence of spatially non-uniform species concentrations. It is demonstrated that the produced thin films can be driven to the desired thickness set-point of 300 nm in less than 10 batches in the presence of both electron density drift and non-uniform deposition rate profiles.

Study on Improving Thickness Uniformity of Microfluidic Chip Mold in the Electroforming Process.

Electroformed microfluidic chip mold faces the problem of uneven thickness, which decreases the dimensional accuracy of the mold, and increases the production cost. To fabricate a mold with uniform thickness, two methods are investigated. Firstly, experiments are carried out to study how the ultrasonic agitation affects the thickness uniformity of the mold. It is found that the thickness uniformity is maximally improved by about 30% after 2 h electroforming under 200 kHz and 500 W ultrasonic agitation. Secondly, adding a second cathode, a method suitable for long-time electroforming is studied by numerical simulation. The simulation results show that with a 4 mm width second cathode used, the thickness uniformity is improved by about 30% after 2 h of electroforming, and that with electroforming time extended, the thickness uniformity is improved more obviously. After 22 h electroforming, the thickness uniformity is increased by about 45%. Finally, by comparing two methods, the method of adding a second cathode is chosen, and a microfluidic chip mold is made with the help of a specially designed second cathode. The result shows that the thickness uniformity of the mold is increased by about 50%, which is in good agreement with the simulation results.

Significantly improved thickness uniformity of graphene monolayers grown by chemical vapor deposition by texture and morphology control of the copper foil substrate

We demonstrate that the thickness uniformity of chemical vapor deposited graphene monolayers strongly depends on the pre-annealing conditions of the copper foils used as substrates. We show that pre-annealing of copper foils under specific conditions leads to (111)-oriented smooth copper surfaces, while other conditions lead to corrugated surface morphologies. The differences in the morphologies of copper foils pre-annealed under varying conditions can lead to substantially different adlayer number densities. An analysis of the correlation between the adlayer number density and the surface corrugation indicates that the adlayer number density increases with a decrease in the corrugation periodicity and with an increase in the roughness, suggesting that the Ehrlich-Schwoebel barrier on the copper surface promotes formation of the adlayer. The adlayer formation can be suppressed to form centimeter-scale adlayer-free graphene monolayers by obtaining (111)-oriented smooth copper surfaces with minimal corrugation. We explain the strong dependence of surface corrugation on the pre-annealing conditions based on the influence of oxygen adsorption or thermal fluctuation on surface instabilities, such as step bunching, faceting, and step meandering.

Integration of InP and InGaAs on 300 mm Si Wafers Using Chemical Mechanical Planarization

Integration of III-V high mobility channel materials in complementary metal oxide semiconductors (CMOS) and III-V photonic materials for integrated light sources on Si substrates requires low defect density III-V buffer layers in order to enable epitaxial growth of high crystal quality active layers. For the fabrication of In0.53Ga0.47As n-channel MOSFET on Si, a lattice matched InP buffer layer is one of the most effective approaches when used in combination with the aspect ratio trapping technique, an integration method known for reducing the density of defects formed during relaxation of strain induced by the lattice mismatch between InP and Si. The InP buffer should be planarized in order to improve thickness uniformity and roughness before subsequent deposition of active layers. In this work we discuss the development of InP planarization on 300 mm Si wafers and investigate slurry composition effects on the final oxide loss and condition of the InP surface. To further explore viability of this approach we deposited an epitaxial In0.53Ga0.47As n-MOS channel layer on top of the planarized InP buffer.