Non-uniform Material Removal Research Articles

Investigation on form-preserving polishing of side-wall surfaces via an active fluid jet.

In recent years, extensive research and development have been conducted on an active fluid jet (AFJ) polishing-based post-polishing process aimed at removing periodic marks from diamond-turned surfaces. This cost-effective method demonstrates its machining capability across a wide range of materials. Notably, it excels in preserving the form accuracy during the post-polishing process for traditional optics, allowing for the attainment of high-precision shape and an ultra-smooth texture. However, the challenge arises when attempting to maintain form preservation on diamond-cut surfaces located on the side-walls of structures due to non-uniform material removal. This limitation significantly restricts its application in advanced opto-mechanical systems, including monolithic multi-surface workpieces. Therefore, this paper systematically investigates the form-preserving capability of AFJ polishing for side-wall surfaces through multi-scale analysis. The micromachining characteristics of the diamond-cut surface are elaborated upon using elastic-plastic theory, and the impact of polishing parameters on form preservation is studied at the microscopic scale. Furthermore, at the macroscopic scale, a simulation model of the AFJ polishing process is established based on fluid-structure interaction (FSI) analysis and rigid dynamic analysis. To validate the proposed theory, a series of tests are conducted. Theoretical and experimental results indicate that non-uniform material removal occurs in the contact area between the tool and the workpiece due to the influence of gravity, thereby hindering the form-preservation polishing process. Building upon the simulation model, a new AFJ tool is designed and optimized to enhance the form-preserving capability of AFJ post-polishing for side-wall surfaces. Experimental results confirm that the innovative AFJ tool uniformly eliminates the periodic marks on diamond-cut surfaces. The theory and methodology presented in this work have broad applicability to various form-preservation post-polishing processes on diamond-cut surfaces.

Formation and reduction of taper in electrochemical single-pass milling grooves

Grooves are used widely in aeroengines, die and moulds and other industrial contexts. Electrochemical single-pass milling (ECSPM) is a flexible method to manufacture high-aspect-ratio grooves using a simple tube tool. However, there is always a taper on the sidewall of the groove due to the dominant flow field and electric field effects in the inter-electrode gap (IEG), which reduces the machining quality. This paper develops a coupled multi-physics three-dimensional model (including turbulent flow field, electric field, heat transfer of fluid and moving mesh) to reveal the formation mechanism for these tapered sidewalls of grooves. Furthermore, a novel tube tool with a wedged end face and partial insulated layer on the outer wall (W-tube) is designed to reduce the sidewall taper by controlling the coupling of the pulsating flow and electric fields. Simulation results show that the low electrolyte flow velocity through the side IEG weakens the removal of electrolytic by-products and leads to non-uniform material removal along the groove depth, thereby forming tapered sidewalls. By using the W-tube tool, the flow and electric fields are coupled in the pulsating state, which eliminates the zones of persistent low flow velocity and disperses the material dissolution process. Also, the coupling phase difference can be controlled by adjusting the position of the insulated layer. Compared with other coupling processes, ECSPM with a coupling phase difference of 0° (type A) is more suitable for enhanced mass transfer, and the difference in dissolution overcut along the groove depth decreases to 0.015 μm in one rotation of the tube tool, thereby reducing the sidewall taper. Experimental results verify the theoretical analysis: the least-tapered groove was that manufactured with pulsating fields coupled exactly in phase, which was realized with the type-A W-tube tool; the taper was reduced to 1.21±0.10° compared to that of 2.21±0.37° with traditional ECSPM, and the surface quality was also improved.

Curvature effect-based modeling and experimentation of the material removal in polishing optical surfaces using a flexible ball-end tool.

Optical surfaces with high quality have been widely applied in high-tech industries for their excellent performances. To precision manufacture those surfaces efficiently and effectively, various machining technologies involved become extremely crucial. As one of the promising ultra-precision machining technologies, inflated or solid elastic tool polishing has attracted more attention for its own superiority. However, there is still lack of understanding on material removal mechanisms especially with the consideration of curvature effect, and it is of great importance to determine the surface quality and form control in ultra-precision polishing process. In this paper, originating from the famous macro-scale Preston equation, the curvature effect-based material removal model in polishing using a flexible ball-end tool has been developed successfully on the basis of two key sub-models, one is the generic model of effective relative velocity and the other refers to the semi-experimental contact pressure model. A series of spot polishing experiments subsequently are conducted on concave surfaces with a curvature radius range from 75 mm to 225 mm. The experimentally measured section profiles of polishing spots do match well with the predicted data, which verifies the effectiveness of the proposed material removal model. On the measured polishing spots, it is also observed that there have two nonuniform material removal phenomena, one is analyzed along the central axis and the other is discussed by two regions symmetrical about the central axis. Compared with the effective relative velocity, it is found that, the contact pressure is more sensitive to curvature effect by investigating the variation of maximum removal depth within a broader curvature radius range from 75 mm to 1000 mm. This study can provide a valuable foundation for polishing optical surfaces with deterministic removal.

Theoretical investigation and implementation of nonlinear material removal depth strategy for robot automatic grinding aviation blade

Due to the requirements of manufacturing accuracy and surface quality consistency, which will influence the dynamic properties and service cycle of aircraft engines, the high-precision material removal depth model is urgently needed for robot automatic grinding aviation blade system. This research developed a novel material removal depth (MRD) model to ensure quantitative grinding depth point by point for robot automatic grinding aviation blade. Firstly, the relationship between contact stress and contact force is developed based on simulation and theoretical derivation, and the contact contour is further investigated by the experiments. Then, the nonlinear material removal depth model is established to predict grinding depth according to Preston equation. Meanwhile, multiple linear regression method is introduced to analyze parameters of material removal depth model. The previous researches show that grinding parameters have strong correlation with MRD, especially for the non-negligible contact force consideration. Therefore, the non-uniform material removal with variable contact force strategy is developed according to the proposed model, and the proposed strategy is applied to the robot automatic grinding aviation blade system to achieve the quantitative grinding depth point by point on the aviation blade surface. The practice experiments further prove that the developed strategy is feasible and effective. Comparing the predicted and experimental MRD, the maximum errors and average relative errors are respectively 9.21% and 4.68%, when the aviation blade is ground by the proposed material removal strategy. Simultaneously, the experimental results tend to produce much better surface roughness with uniform surface texture which satisfies grinding requirements.

Predictive topography model for shape adaptive grinding of metal matrix composites

Compliant finishing (e.g. shape adaptive grinding) can routinely achieve nanoscale roughness on diverse metal and ceramic materials. However, when processing multiphase materials consisting of distinct phases with different mechanical properties, non-uniform material removal often occurs and the inherent mechanism remains unclear. Here, a deterministic and stochastic model to predict processed surface topography is derived from the different material removal behaviours across phases. Experimental topographies, profiles and roughness on three selected Si-SiC samples show high consistency with theoretical predictions. Both model and experiments indicate that improved and stable surface finish can be achieved on metal matrix composites with small grain size.

Quantifying hole quality through geometry accuracy and surface qualities in rotary ultrasonic machining of carbon fiber–reinforced plastic composites

Rotary ultrasonic machining has been approved as an effective and efficient hole making process for carbon fiber–reinforced plastic composites. Hole quality plays an important role in assembling carbon fiber–reinforced plastic components and can be affected by the carbon fiber reinforcement structures. In this study, experiments are conducted to assess hole quality in carbon fiber–reinforced plastic composites with three carbon fiber reinforcement structures under different combinations of machining variables. Hole quality is quantified through geometrical accuracy (perpendicularity, cylindricity, and hole diameter) and surface qualities (delamination and surface roughness). Results show that the highest level of interlacement among yarn of plain woven structure induce the highest level of compression to the workpiece and the largest amount of additional material removal, leading to the largest perpendicularity and hole diameter. The worst fabric integrity of unidirectional structure generates the largest amount of non-uniform material removal on the machined surface, resulting in the largest cylindricity. It is also found that compared with woven structures, unidirectional structure is more likely to induce push-out delamination due to its smaller critical energy release rate. The lowest constancy of the fabric in twill woven structure leads to the largest surface roughness.

The effect of process vibrations under different rotational rates on the non-uniform material removal during continuous polishing

Abstract This research illustrates the effect of process vibrations under different rotational rates on the non-uniform material removal, which is characterized by the MRR/v and removed material distribution (RMD) during the continuous polishing. To quantitively calculate the removed material thickness of points on the workpiece, a novel RMD model considering the kinematic parameters, dynamic pressure and the process vibrations is proposed. Several polishing experiments are conducted under different rotational rates. The process vibrations are enhanced by higher rotational rate, resulting in higher MRR/v and fluctuations in the profile of measured RMD. Furthermore, the percentage of error in radial removed thickness (ERRT) to the average removed thickness on the whole workpiece surface is defined to evaluate the accuracy of the RMD model.

Theoretical model and experimental analysis of non-uniform material removal during full-aperture polishing

Full-aperture polishing is a key process in the fabrication of large flat optical elements with a high-precision surface figure. Controlling of the surface figure, which is primarily dependent on the material removal distribution, during the polishing process is challenging. In this study, a novel model is proposed to calculate the material removal distribution and the resultant surface figure. The model determines the material removal amount of points on the workpiece by considering the kinematic parameters and pressure distribution along the sliding trajectory relative to the pad. Moreover, the pressure distribution during the polishing process is acquired from the mechanical and morphological characteristic of polishing pad. With this model, the final surface figures under several polishing conditions were simulated and were found to be in close agreement with the experimental results.

Chemical Mechanical Planarization: Advanced Material and Consumable Challenges

The unprecedented and continuing increase in the computational power and functionality of microelectronic devices, extending beyond more-than-Moore, are driven by the ever shrinking dimensions of logic and memory circuits, now numbering over a billion and half, all packed into an area of ∼15 cm2. As the costs of these devices drop, they have become ubiquitous across the world, from high performance computing to smart phones to driverless cars to children’s toys. The most common substrate for these devices continues to be Si and the process sequence used to fabricate these devices is frequently divided into front-end-of-the-line (FEOL) and back-end-of-the-line (BEOL) processes, each with its own peculiar technological and manufacturing yield challenges. Once all the active circuit elements are fabricated in the Si substrate using FEOL processes, they need to be interconnected and also connected to an external power source to become functional. While new materials continue to be incorporated at various levels into the processing sequence, copper remains the metal of choice for the interconnect wiring which is created using BEOL processes. The Cu wiring itself is electrically insulated by a low-k dielectric and protected by a diffusion barrier layer that can also act as an adhesion promoter and, depending on its electrical conductivity, enable direct electroplating of Cu. The characteristics of the low-k dielectric and the barrier layer have an immense influence on the manufacturing process sequence as well as the performance and long term reliability of the devices. Significant advances continue to be made in identifying newer sets of materials to fill these roles, especially for the narrower Cu lines where the space for the liner is limited, along with the associated processing steps. While advances in lithographic techniques facilitated the feature size reduction that has already reached 14 nm in high volume commercial production and seemingly headed lower, the same advances in lithography also demand ever more stringent surface planarity requirements, both locally across the chip and more globally across the entire wafer. Chemical mechanical planarization (CMP) is the only technique that has so far been capable of achieving these demanding requirements at the large volume manufacturing level in spite of it being a counter intuitive process that is prone to non-uniform material removal, defects and contamination caused by pad debris, by the billions of abrasive particles contacting the wafer surface and by the host of reactive and nonreactive chemicals used in the planarization process. Mitigating such yield-diminishing factors is one of the major challenges driving the advances in CMP technology and led to improvements in the polisher components like wafer carrier, slurry delivery system, retainer ring, backing film, etc., as well as all the other consumables including polishing pads, conditioners, and polishing slurries. Given the heterogeneity and pattern density and feature size variations of the films being planarized on the wafer surface, process

Convergent polishing: a simple, rapid, full aperture polishing process of high quality optical flats & spheres.

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., surface figure), will converge to final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. In contrast, conventional full aperture polishing methods require multiple, often long, iterative cycles involving polishing, metrology and process changes to achieve the desired surface figure. The Convergent Polishing process is based on the concept of workpiece-lap height mismatch resulting in pressure differential that decreases with removal and results in the workpiece converging to the shape of the lap. The successful implementation of the Convergent Polishing process is a result of the combination of a number of technologies to remove all sources of non-uniform spatial material removal (except for workpiece-lap mismatch) for surface figure convergence and to reduce the number of rogue particles in the system for low scratch densities and low roughness. The Convergent Polishing process has been demonstrated for the fabrication of both flats and spheres of various shapes, sizes, and aspect ratios on various glass materials. The practical impact is that high quality optical components can be fabricated more rapidly, more repeatedly, with less metrology, and with less labor, resulting in lower unit costs. In this study, the Convergent Polishing protocol is specifically described for fabricating 26.5 cm square fused silica flats from a fine ground surface to a polished ~λ/2 surface figure after polishing 4 hr per surface on a 81 cm diameter polisher.

Origin, modeling and suppression of grinding marks in ultra precision grinding of silicon wafers

A phenomenon commonly encountered in grinding of silicon wafers is the grinding marks, which are difficult to remove by subsequent polishing process, and have been a great obstacle to the manufacture of silicon wafers with higher flatness. In this paper, the grinding marks formation mechanism was clarified, a grinding marks formation model and an angular wavelength model were developed, and a grinding marks suppression method was proposed. A series of grinding experiments were carried out to verify the developed models and investigate the effect of the wafer rotational speed, the wheel rotational speed, the infeed rate, the axial run out of the cup wheel and the spark out time. The results show that: (1) grinding marks are waviness generated on silicon wafers caused by non-uniform material removal circumferentially due to the axial run out of the cup wheel; (2) grinding marks present multiple angular wavelengths characteristics; (3) the angular wavelength of grinding marks is a one-variable function of the rotational speed ratio of the wheel to the wafer; and (4) grinding marks could be suppressed significantly by properly selecting the rotational speed ratio.

Impact of Wafer Geometry on CMP for Advanced Nodes

Material removal uniformity during chemical mechanical polishing (CMP) for IC fabrication processes such as shallow trench isolation has previously been shown to be affected by nanotopography (NT) of the wafer frontside (pattern surface). NT is the high frequency height variations of the wafer surface within spatial wavelengths from 0.2 to 20 mm. However, the effect of other topography information such as wafer backside NT and the relatively lower frequency wafer shape on CMP have not been addressed sufficiently. In the present work, the effect of wafer geometry of current and advanced generation wafers on CMP material removal uniformity are investigated using numerical simulations of the CMP process. Specifically, a finite-element based mechanics model was developed and used in conjunction with a wear model based on Preston’s equation to simulate the material removal process during CMP. The results demonstrate that the impact of backside NT depends on the stiffness of the CMP backplate (carrier). Simulation results also suggest that higher order wafer shape components can affect CMP results. As technology advances it may be important to control frontside and backside NT as well as higher-order wafer shape in order to reduce their contributions to non-uniform material removal during CMP and the associated yield impact on the integrated circuits that will be manufactured on these substrates.

Coffee Stain Ring Effect and Nonuniform Material Removal in Chemical Mechanical Polishing

When a drop of coffee dries on the counter-top, it leaves a dense, ringlike stain along its perimeter. Solids immersed in a drying drop will migrate toward the edge of the drop and form a solid ring. Such phenomena create ringlike stains and happen for a wide variety of surfaces, solvents, and solutes. It is referred to as the coffee stain ring effect. The phenomenon is caused by the outward microfluidic flow of the solute within the drop, which is driven by the evaporation of solvent. We show that the mechanism for the ring effect contributes to the nonuniform material removal in chemical mechanical polishing (CMP), specifically, at edges of blanket wafers causing the edge effect or at edges and corners of protrusive features on patterned wafers inducing the doming effect; metal dishing and dielectric erosion. By controlling the evaporation profile of the solvent in the slurry layer between the wafer surface and the polishing pad, such as making grooves or embedding the abrasive particles on the pad, or delivering the slurry from the bottom of the pad, one can improve the uniformity of material removal during the CMP process.

Research on the With-In Wafer Non-Uniformity (WIWNU) of the Large Quadrate Optic in the Fast Polishing Process

In this paper, the with-in-wafer non-uniformity (WIWNU) of the lager quadrate optic in the fast polishing process (FPP) is discussed from the machine side. At the machine side, the non-uniform stress on the wafer surface is the major reason for the non-uniform material removal rate which results in the WIWNU. Stresses arise mainly from two sources, namely the pressure exerted by the polishing pad and shear stress due to the relative motion between the wafer and pad. Based on the special chemical mechanical polishing FPP, the kinematic motion of the wafer is analyzed and the non-uniform stress is analyzed by a 3D axisymmetric quasi-static model. The WIWNU can be reduced by adjusting the stress.

A fundamental model proposed for material removal in chemical–mechanical polishing

So far there is no consensus on the fundamental mechanism of material removal in chemical–mechanical polishing (CMP). Some researchers model the CMP process based on the mechanism proposed by Kaufman et al. [1], who attribute the material removal from the wafer surface to chemistry-aided mechanical abrasions. Other researchers develop models for the CMP process based on a chemical tooth mechanism conjectured by Cook [2], who emphasizes the role of chemical reactions on the wafer surface. However, there is much evidence that both mechanisms are flawed. To clarify these issues, and reconcile the apparently contradictory conjectures [1,2], and provide a framework for further detailed modeling, a unified mechanism of material removal is proposed: the material at the wafer surface is removed by the strong shear stress from the non-Newtonian slurry flow in-between the wafer surface and the pad. Based on the proposed mechanism, a brief mathematical formulation is introduced to qualitatively describe the material removal rate. The proposed mechanism can be readily applied to explain the edge or the doming effect associated with non-uniform material removal for polishing blanket or patterned wafers.

Kinematical Modeling of Pad Profile Variation during Conditioning in Chemical Mechanical Polishing

Conditioning is the process of removing the glazing area from a polishing pad surface and restoring the quality of the surface to maintain a stable polishing performance. However, the conditioning process can induce a non-uniform profile variation of the pad, which can result in nonuniform material removal rates across the wafer. In this paper, a kinematical model based on Preston's equation is proposed to examine the pad profile variation (PPV) induced by swing arm conditioning with a diamond disk. The proposed model was simulated with various swing arm velocity profiles (SAVPs), and the results were compared with experimental results. The results showed the relationship between kinematical parameters and the PPV. The PPV was proportional to sliding distance based on the kinematical model, and then the sliding distance distribution across the pad was dependent on the SAVP. This study has proven the effectiveness of the kinematical model on the PPV during conditioning in chemical mechanical polishing (CMP).

Optimization of Wafer-Back Pressure Profile in Chemical Mechanical Planarization

In chemical mechanical planarization, a rotating wafer is pressed facedown against a rotating pad, while a slurry is dragged into the pad–wafer interface to assist in planarizing the wafer surface. Due to stress concentration, the interfacial contact stress near the wafer edge generally is much higher than that near the wafer center, resulting in a spatially nonuniform material removal rate and hence an imperfect planarity of the wafer surface. Here, integrating theories of fluid film lubrication and two-dimensional contact mechanics, we calculate the interfacial contact stress and slurry pressure distributions. In particular, the possibility of using a multizone (i.e., piecewise constant) wafer-back pressure profile to improve the contact stress uniformity is examined by studying a particular case with realistic parameter settings. The numerical results indicate that using a two-zone wafer-back pressure profile with optimized zonal sizes and pressures can increase the “usable” wafer surface area (within which the average contact stress nonuniformity is below 0.1%) by as much as 12%. Using an optimized three-zone wafer-back pressure profile, however, does not increase the usable wafer surface area much further.

Modeling and Analyzing on Nonuniformity of Material Removal in Chemical Mechanical Polishing of Silicon Wafer

Chemical mechanical polishing (CMP) has already become a mainstream technology in global planarization of wafer, but the mechanism of nonuniform material removal has not been revealed. In this paper, the calculation of particle movement tracks on wafer surface was conducted by the motion relationship between the wafer and the polishing pad on a large-sized single head CMP machine. Based on the distribution of particle tracks on wafer surface, the model for the within-wafer-nonuniformity (WIWNU) of material removal was put forward. By the calculation and analysis, the relationship between the motion variables of the CMP machine and the WIWNU of material removal on wafer surface had been derived. This model can be used not only for predicting the WIWNU, but also for providing theoretical guide to the design of CMP equipment, selecting the motion variables of CMP and further understanding the material removal mechanism in wafer CMP.

Fluid pressure and its effects on chemical mechanical polishing

The experimental results of interfacial fluid pressure and friction measurements during polishing are presented, as well as their dependence on some major process variables. Under simulated conditions, a sub-ambient fluid pressure was observed, and its magnitude was of the same order of magnitude as the applied polishing load. Since this fluid pressure is non-uniformly distributed, the contact stress, obtained by combining the effects of both applied load and the fluid pressure, is not uniform across the wafer and will result in non-uniform material removal. The mechanism of the presence of the fluid pressure was investigated, and an analytical model was developed to predict the magnitude and distribution of this fluid pressure. The effects of the sub-ambient fluid pressure on material removal rate and profile were tested with thermally grown silicon dioxide on 100 mm diameter, p-type (1 0 0), single-crystal silicon wafers. The polishing experiments show the effect of sub-ambient fluid pressure on polishing rate and profile.

Strained-Si-on-Insulator (SSOI) and SiGe-on-Insulator (SGOI): Fabrication Obstacles and Solutions

ABSTRACTAdvanced CMOS substrates composed of ultra-thin strained-Si and SiGe-on-insulator were fabricated, combining both the benefits of high-mobility strained-Si and SOI. Our pioneering method employed wafer bonding of SiGe virtual substrates (with strained-Si layers) to oxidized handle wafers. Layer transfer onto insulating handle wafers can be accomplished using grind-etchback or delamination via implantation. Both methods were found to produce a rough transferred layer, but polishing is unacceptable due to non-uniform material removal across the wafer and the lack of precise control over the final layer thickness. To solve these problems, a strained-Si stop layer was incorporated into the bonding structure. After layer transfer, excess SiGe was removed using a selective etch process, stopping on the strained-Si. Within the context of ultra-thin SSOI and SGOI fabrication, this paper describes recent improvements including metastable stop layers, low temperature wafer bonding, and improved selective SiGe removal.