Recent Advances In Silicon Carbide Chemical Mechanical Polishing Technologies.

Chemical mechanical polishing (CMP) is a well-known technology that can produce surfaces with outstanding global planarization without subsurface damage. A good CMP process for Silicon Carbide (SiC) requires a balanced interaction between SiC surface oxidation and the oxide layer removal. The oxidants in the CMP slurry control the surface oxidation efficiency, while the polishing mechanical force comes from the abrasive particles in the CMP slurry and the pad asperity, which is attributed to the unique pad structure and diamond conditioning. To date, to obtain a high-quality as-CMP SiC wafer, the material removal rate (MRR) of SiC is only a few micrometers per hour, which leads to significantly high operation costs. In comparison, conventional Si CMP has the MRR of a few micrometers per minute. To increase the MRR, improving the oxidation efficiency of SiC is essential. The higher oxidation efficiency enables the higher mechanical forces, leading to a higher MRR with better surface quality. However, the disparity on the Si-face and C-face surfaces of 4H- or 6H-SiC wafers greatly increases the CMP design complexity. On the other hand, integrating hybrid energies into the CMP system has proven to be an effective approach to enhance oxidation efficiency. In this review paper, the SiC wafering steps and their purposes are discussed. A comparison among the three configurations of SiC CMP currently used in the industry is made. Moreover, recent advances in CMP and hybrid CMP technologies, such as Tribo-CMP, electro-CMP (ECMP), Fenton-ECMP, ultrasonic-ECMP, photocatalytic CMP (PCMP), sulfate-PCMP, gas-PCMP and Fenton-PCMP are reviewed, with emphasis on their oxidation behaviors and polishing performance. Finally, we raise the importance of post-CMP cleaning and make a summary of the various SiC CMP technologies discussed in this work.

Comprehensive design method of a soft multifocal contact lens with NURBS.

This study presents a comprehensive method for designing a multifocal contact lens (MCL) with Snell's law and non-uniform rational B-spline (NURBS) curves. Instead of using thin lens approximation, general mathematical formulas have been developed to achieve the accurate coordinates of points on the anterior lens surface profile of the MCL to meet various given optical power distributions. Then the NURBS curve is adjusted to fit these data points to obtain the smooth front lens surface profile. This method not only improves the accuracy of the optical power profiles of MCLs but also reduces the spherical aberration in near/distance optical zones. The experimental results show that the power profiles of soft MCLs agree with those of the simulation results and original design requirements. The proposed method has been proven for the MCL design, and it can be feasibly applied in complex optical lens designs.

Copper-to-copper direct bonding on highly (111)-oriented nanotwinned copper in no-vacuum ambient.

A vacuum-free Cu-to-Cu direct bonding by using (111)-oriented and nanotwinned Cu has been achieved. A fast bonding process occurs in 5 min under a temperature gradient between 450 and 100 °C. It is verified by grain growth across the bonded interface. To investigate the grain growth behavior, further annealing in the temperature gradient, as well as in a reversed temperature gradient, was performed. They showed similar recrystallization behavior with de-twinning. To analyze the de-twinning, we recall the classic model of annealing twin formation by Fullman and Fisher as comparison. Our case is opposite to the model of Fullman and Fisher. A mechanism of direct bonding by surface diffusion creep is proposed.

Compensating additional optical power in the central zone of a multifocal contact lens forminimization of the shrinkage error of the shell mold in the injection molding process.

This study aims to develop a compensating method to minimize the shrinkage error of the shell mold (SM) in the injection molding (IM) process to obtain uniform optical power in the central optical zone of soft axial symmetric multifocal contact lenses (CL). The Z-shrinkage error along the Z axis or axial axis of the anterior SM corresponding to the anterior surface of a dry contact lens in the IM process can be minimized by optimizing IM process parameters and then by compensating for additional (Add) powers in the central zone of the original lens design. First, the shrinkage error is minimized by optimizing three levels of four IM parameters, including mold temperature, injection velocity, packing pressure, and cooling time in 18 IM simulations based on an orthogonal array L18 (21×34). Then, based on the Z-shrinkage error from IM simulation, three new contact lens designs are obtained by increasing the Add power in the central zone of the original multifocal CL design to compensate for the optical power errors. Results obtained from IM process simulations and the optical simulations show that the new CL design with 0.1 D increasing in Add power has the closest shrinkage profile to the original anterior SM profile with percentage of reduction in absolute Z-shrinkage error of 55% and more uniform power in the central zone than in the other two cases. Moreover, actual experiments of IM of SM for casting soft multifocal CLs have been performed. The final product of wet CLs has been completed for the original design and the new design. Results of the optical performance have verified the improvement of the compensated design of CLs. The feasibility of this compensating method has been proven based on the measurement results of the produced soft multifocal CLs of the new design. Results of this study can be further applied to predict or compensate for the total optical power errors of the soft multifocal CLs.

Biomechanical Comparison between Preloaded Position Screw and Lag Screw Fixations for Their Compressive Effects in a Porcine Rib Fracture Model.

OBJECTIVES: The aim of this study was to compare the compressive effect between preloaded position screws and lag screws in fracture fixation. METHODS: Pairs of semi-cylindrical bone fragments were created on a porcine rib model to simulate fractured bones. The compressive forces of fracture fixation generated by preloaded position screws (P group) and conventional lag screws (L group) were recorded by a film stress sensor. In the P group, a pair of Angus bone holding forceps was used to compress the interfragmentary interface until reaching the preloading force of 1.3 MPa. Similar preloading procedure was applied on lag screw fixations to explore its potential of additional compressive force. RESULTS: With 1.3 MPa preloading force and 0.2 Nm screw insertion torque force, the interfragmentary compressive force of P group was similar to that of L group. When the insertion torque force was increased to 0.4 Nm, all screws in the L group failed due to thread stripping. When screws in the P group were installed under 0.4 Nm torque and in the L group under 0.2 Nm torque, the P group generated significantly greater compressive force. With preloading device applied on lag screw installation, it did not further increase the interfragmentary compressive force. CLINICAL SIGNIFICANCE: Compared with lag screws, preloaded position screws tolerated greater torque in screw installation and provided greater interfragmentary compressive force. The study suggests the applicability of preloaded position screws in fracture fixation requiring interfragmentary compression.

Optical design of soft multifocal contact lens with uniform optical power in center-distance zone with optimized NURBS.

This study aims to develop a new optical design method of soft multifocal contact lens (CLs) to obtain uniform optical power in large center-distance zone with optimized Non-Uniform Rational B-spline (NURBS). For the anterior surface profiles of CLs, the NURBS design curves are optimized to match given optical power distributions. Then, the NURBS in the center-distance zones are fitted in the corresponding spherical/aspheric curves for both data points and their centers of curvature to achieve the uniform power. Four cases of soft CLs have been manufactured by casting in shell molds by injection molding and then measured to verify the design specifications. Results of power profiles of these CLs are concord with the given clinical requirements of uniform powers in larger center-distance zone. The developed optical design method has been verified for multifocal CLs design and can be further applied for production of soft multifocal CLs.

Analysis on multifocal contact lens design based on optical power distribution with NURBS.

This paper aims to develop and analyze the design method of multifocal contact lenses to obtain curvature continuity in the optical surfaces with the high addition (Add) powers by adjusting non-uniform rational B-spline (NURBS) curves. The paper has developed mathematical formulae to generate the optical power distributions in which the powers continuously change from either near or distant center to the opposite focal length in the periphery of the optical region with different change rates and Add power values. This developed method can efficiently adjust and optimize three parameters, including control points, weight, and knots of the NURBS, to be anterior optical lens surface profiles to adapt for these given power profiles. The result shows that the proposed contact lenses not only achieve smooth and continuous anterior optical surfaces, but also satisfy various optical power distributions with high Add power values for different pupil diameters. Then, these designs of contact lenses can be feasibly converted to the computer-aided design format for analysis and manufacture for molding or single-point diamond turning. Experimental results of this method have been tested and proven when both the power distributions of simulation of lenses and the actual machined samples match the original specified powers provided by clinical demands of a multifocal contact lens. Future integration with variant clinical demands and optimization rules of lens design can be explored for a progressive contact lens.

Nanopattern fabrication by tip plowing technology on 55 nm grating with stitching image method.

An appropriate calibration positioning method is imperative to examine localized tip on nanoscale patterns for scanning probe microscopy (SPM). This paper is to develop a new nanofabrication processes for AFM tip positioning with image stitching method in tip plowing technology. Moreover, this paper adjusts the set-point amplitude (A(sp)) to develop the tip plowing technology for fabricating nanopattern on 55 nm grating gage of a silicon substrate. The developed image stitching program is based on an iterative closet point (ICP) algorithm which has six degrees of freedom alignment. A closed-loop piezo motor is used to tip approach and plow in Z-axis. Experimental result of fabricating nanobagua on 55 nm grating of silicon substrate show that the developed positioning processes with image stitching method verify the feasibility of repeatability for the tip plowing technology successfully. This developed method can be further performed by a commercial atomic force microscope (AFM) with CAD/CAM. This technology can also be applied in dip pen nanolithography (DPN), SPM oxidation lithography and related fabrication technology with AFM tips.