Polyurethane Pad Research Articles

Towards Digital Twin Standards in Chemical Mechanical Planarization (CMP) for Advanced Packaging

A digital twin is an integrated multi-physics, multiscale, probabilistic simulation of a system that uses the best available physical models, sensor updates, process history, etc., to mimic and to predict the performance of the system. In the semiconductor industry, there is a need to establish such fundamental, mechanism-based, correlations between process conditions and observed process performance. Such understanding can lead to the creation of industry-wide technical standards, and digital twins (i.e., real time digital replicas of unit processes), that will enable inter-operability, ease process selection and integration, as well as help reduced cost of ownership.In this paper, we revisit our decades old, but still relevant, quest for appropriate mechanism-based metrology to support the digitization of Chemical-Mechanical Polishing (CMP) for the back-end-of line (BEOL), with advanced packaging, specifically hybrid bonding for 3D interconnects, in mind. Here, we consider the digitization of the nanoscale tribological aspects of CMP. For example, many CMP processes use chemically unstable polyurethane pads in a variety of slurry formulations which make the process difficult to understand and to model. We had previously described how the chemical instability of the PU-pads affects the CMP process performance. Here we contextualize the historical performance data, identify data gaps, and propose some next steps towards digital twin CMP modules.

Effect of rotation of abrasives on material removal in chemical mechanical polishing using a proposed three-body model: Molecular dynamics simulation

A three-body wear model, comprising a soft polyurethane pad, silica abrasive, and crystalline silicon substrate, was established to analyze the impact of abrasive rotation on material removal process during chemical mechanical polishing (CMP) through molecular dynamics simulation. The results revealed that abrasive rotation would reduce the local temperature in the contact area between the abrasive and the substrate due to the cooling effect, thereby enhancing the surface quality of the substrate and decreasing the material removal volume (MRV). However, another effect of abrasive rotation, termed the "hedgehog effect", resulting in an increase in MRV. Through the trade-off between the cooling effect and the porcupine effect, the abrasive rotation can not only improve surface quality but also increase the MRV.

Development of Novel Conditioning Method Using Thermal Shape Memory Characteristics of Polyurethane CMP Pad

Traditionally, the pad roughness has been maintained by wearing down the polyurethane pad with diamond disk. However, that method generates debris and reduces pad lifetime. This study propose a new approach to pad surface recovery by synthesizing a polyurethane-based raw material that exhibits shape memory behavior and can recover its shape upon heating. The findings suggest that the pad’s surface can be maintained by utilizing its shape memory trait and designing a system to heat the pad. The pad recovery tests were conducted using universal test machine (UTM) samples and found that, in terms of heat recovery, increasing the temperature had a greater effect than increasing the exposure time. CMP test was performed by using three conditioning potions: diamond disk conditioning, heat conditioning, and no conditioning. The results showed that pad asperity was recovered more efficiently with heat conditioning than with no conditioning (demonstrated by a 19% higher removal rate). The experimental results can be expected that combines diamond disk conditioning with heat conditioning could be a superior alternative for pad surface refreshment. Shape memory pads can return to their original form, leading to better chemical mechanical planarization (CMP) performance and an extended pad lifetime.

Influence of plasticity and vibration isolators on an underground floating slab track using finite element analysis

The floating slab track (FST) is largely viewed as the most efficient structure for mitigating the vibrations caused by underground railway systems. This paper presents a study on the dynamic behavior of the FST subjected to train-induced vibrations using a finite element approach. A three-dimensional simulation model of an FST was analyzed under a moving load. The effects of material plasticity of underground strata and concrete damaged plasticity of the floating slab were analyzed. A comparison between the vibration attenuation of the steel-spring isolator (SSI) and polyurethane pad (PUP) was performed. The load was moved at constant speeds ranging from 90 to 324 km/h using a pressure-loaded moving block. The results were analyzed in the form of four general output parameters. A slight vibration attenuation was observed when plastic material properties were included in the simulation model. The PUP was more effective in reducing vibrations at FST than SSI by a vibration transfer mechanism.

Development of a floating element photoelastic force balance

We present a floating element force balance design that uses an optical measurement of the force using photoelastic stress analysis. The force sensing element consists of pins embedded in photoelastic polyurethane pads, which generate an internal stress when the floating element is loaded that is observed via a transmission polariscope. A series of known loads and their corresponding fringe patterns allow a calibration matrix to be derived using a polynomial model solved by least squares regression. Finite element analysis (FEA) simulation is carried out to validate the proposed method. The balance then measured a lift curve of the NACA0015 wing at low speed. A comparison of the photoelastic balance and a commercial, 6-axis strain-gauge load cell showed typical differences of less than 6%. This optical approach enables accurate measurements with inexpensive and simple components inside the sensor. This work demonstrates that a photoelastic balance is a simple, inexpensive, and sensitive force transducer.

Effect of elastohydrodynamic characteristics on surface roughness in cylindrical shear thickening polishing process

The shear thickening fluid exhibits a non-Newtonian characteristic, i.e. the viscosity follows a power law with the increasing shear rate. The shear thickening polishing technology has been proposed and developed to realize the precision finishing, utilizing the slurry with free abrasives prepared by the shear thickening fluid. In this paper, the formation mechanism and evolution of surface roughness in the cylindrical shear thickening polishing process is investigated in the elastohydrodynamics theoretically and experimentally. The elastohydrodynamic properties and the surface roughness are modeled by a novel solution method, based on the integration of polishing pad’s elastic deformation and slurry’s hydrodynamics. The Taguchi orthogonal tests of polishing the cylindrical AISI 52100 steel with the foam polyurethane pad were conducted, and the experimental surface roughness Ra of 9.689 nm was achieved under conditions of loading force 10 N, α-Al2O3 powder with grain size 1 μm and concentration 20 wt%, starch powder with concentration 25 wt%. According to the comprehensive analysis of variance and level average response, it is revealed that the slurry’s viscosity has a great influence on the surface roughness. The effecting mechanisms of processing factors on the surface roughness are interpreted from the perspective of effects transition in aspects of viscosity, pressure distribution and single grain’s cutting depth, interestingly exhibiting significant differences from that of traditional polishing methods.

An investigation of the Vickers hardness of polishing pads using simulations based on microtomography model

Porous pads used during the chemical mechanical polishing are critical in IC fabrication for achieving consistent throughput. The hardness of thermoplastic polyurethane pads is identified as a crucial parameter in calibrating the amount of material removed during the polishing process. Through micro-CT scanning technology, a three-dimensional micro-topography model of polyurethane IC1000 pad was developed. Nanoindentation measurements were conducted to evaluate the physical properties of IC1000 pad in the manufacture process. In order to investigate the Vickers hardness of the polyurethane IC1000 pad, which widely use in the fabrication of semiconductor manufacturing, the finite element method was employed to simulate the indentation between Vickers indenter diamond tip and the model of IC1000 pad. This study is not only successfully calibrating the Vickers hardness of polyurethane IC1000 pad with a 3 percent error compared to experimental verification, but also developed a novel methodology for investigating the micro-topography structure of polymer foam materials.

Smoothing tool design and performance during subaperture glass polishing.

During subaperture tool grinding and polishing, overlaps of the tool influence function can result in undesirable mid-spatial frequency (MSF) errors in the form of surface ripples, which are often corrected using a smoothing polishing step. In this study, flat multi-layer smoothing polishing tools are designed and tested to simultaneously (1)reduce or remove MSF errors, (2)minimize surface figure degradation, and (3)maximize the material removal rate. A time-dependent convergence model in which spatial material removal varies with a workpiece-tool height mismatch, combined with a finite element mechanical analysis to determine the interface contact pressure distribution, was developed to evaluate various smoothing tool designs as a function of tool material properties, thicknesses, pad textures, and displacements. An improvement in smoothing tool performance is achieved when the gap pressure constant, h¯ (which describes the inverse rate at which the pressure drops with a workpiece-tool height mismatch), is minimized for smaller spatial scale length surface features (namely, MSF errors) and maximized for large spatial scale length features (i.e.,surface figure). Five specific smoothing tool designs were experimentally evaluated. A two-layer smoothing tool using a thin, grooved IC1000 polyurethane pad (with a high elastic modulus, E p a d =360M P a), thicker blue foam (with an intermediate modulus, E f o a m =5.3M P a) underlayer, and an optimized displacement (d t=1m m) provided the best overall performance (namely, high MSF error convergence, minimal surface figure degradation, and high material removal rate).

Development of cubic freeform optical surface for wavefront coding application for extended depth of field Infrared camera

Freeform surfaces are being developed for number of applications like illumination systems, compact projection systems, head up display, ophthalmic and automotive industries. One of the important use of such surfaces in optical imaging application is wavefront coding for large depth of focus camera. The thermal induced defocus is the well-known problem in infrared (IR) imaging systems. The efficient technique to mitigate the effect of defocus is wavefront coding, The phase profile for wavefront coding application with IR camera is a cubic freeform profile, that enables the focus invariant across the region near the focal plane. This work presents the development of a high precision cubic freeform profile on Germanium (Ge) substrate and its application for extended depth of focus for IR imaging camera. The cubic freeform profile on the surface is generated using single point diamond turning (SPDT) in slow tool servo (STS) configuration. It is further polished by using bonnet polisher with polyurethane and uninap-13 pads in seven axis polishing machine. The precise work piece alignment during the generation and characterization of the surface is very important aspect which is taken care by introducing fiducials from the beginning of the process of realization of the component. Full field measurement of grinded surface is carried out by using long wavelength interferometer. Computer generated hologram (CGH) in Fizeau interferometer is used for corrective polishing metrology and final testing of the surface profile. Precise work piece alignment and full field measurement starting from the grinding stage helped to achieve surface with residual error of 0.2 µm PV and 1.4 nm RMS roughness (Rq). The developed cubic profile is used in infrared camera for wavefront coding in an experimental setup for the verification of the intended results.

Accounting for shape factor effects in Ogden-Hill elastomeric foam model

Abstract A framework for the determination of shape-factor dependent parameters in the constitutive models of elastomeric foams, to account for the shape factor effect in quasi-static pressure-strain response of elastomeric foam isolators, is presented. The methodology is an empirical-based method which is applied to the Ogden-Hill foam model. There is no information or model in the literature to account for the shape factor effect on mechanical behaviour of elastomeric foams. Also, none of the existing models of elastomeric foam including Ogden-Hill model are able to predict accurately the pressure-strain response of vibrational isolation systems designed with elastomeric foam isolator pads/strips of different sizes or shape factors. The reason is that all the parameters in such models are a function of material intrinsic property alone which have constant values irrespective of the size of the pads/strips. Therefore, the aim of this study is to derive a set of empirical shape factor dependent parametric functions μ ( s ) and β ( s ) , and to apply them instead of constant parameters μ and β in Ogden-Hill model. This will enable the model to take the shape factor (s) of the pads/strips into account when predicting the pressure-strain response of an elastomeric foam-based system. The study involved testing of polyurethane (PU) pads/strip samples of different sizes and densities (160, 270 and 385 kg/m 3 ) at a displacement rate of 5 mm/min up to 30% compressive strain. Analysis of the measured data showed that the static stiffness of an elastomeric foam-based system increases under a constant compressive load when the shape factor of the pads/strips increases. This implies that the compression modulus of such systems should be a function of both intrinsic material property and shape factor. The test results show that μ ( s ) and β ( s ) are nonlinear functions of the shape factor. The predictions using the derived shape factor-dependent parameters in Ogden-Hill model show good agreement with the measured pressure-strain response of the tested pads/strips. They show that the accuracy of the predicted pressure at, for example, 15% strain is higher than 90% for most of the samples. • Shape factor independent parameters μ and β in Ogden-Hill model give large errors. • Errors minimised by use of shape factor dependent material parameters μ ( s ) and β ( s ) • Derivation of μ ( s ) and β ( s ) from compression stress-strain data of different samples. • Polyurethane pads of different shape factors and densities tested up to 30% strain. • Prediction errors of Ogden-Hill model using μ ( s ) and β ( s ) mostly less than 10%.

Study on rheological properties and polishing performance of viscoelastic material for dilatancy pad

Shear dilatancy polishing (SDP) is an emerging flat polishing method for hard and brittle materials, which can achieve high-efficiency and high-precision polishing of workpieces under high-pressure and high-speed conditions. In our research, a novel viscoelastic material (VM) with non-Newtonian fluid properties was prepared. The dilatancy pad was subsequently obtained based on VM, abrasives, and a polyurethane pad. The effects of abrasive size and abrasive concentration on the rheological properties of VM were analyzed. The mechanism of abrasives in VM was investigated, and the principle of SDP was revealed. VM-30 wt%Diamond (0.5 μm) with excellent shear hardening performance was selected in the polishing experiment to study the effects of polishing time, polishing speed, and polishing pressure on polishing performance, and the optimal polishing parameters of SDP for tungsten were determined. Compared with traditional abrasive free polishing (AFP), SDP can realize a higher material removal rate (MRR increased by 33.4%) and better surface quality (Sa reduced by 32.2%) due to the shear dilatancy effect under high-pressure and high-speed conditions. After 90 min of SDP, the surface roughness (Sa) of tungsten workpiece was reduced from 307.9 nm to 12.1 nm. Results show that SDP is a promising and feasible polishing method for hard and brittle materials.

Development of modeling to investigate polyurethane pad hardness in chemical mechanical planarization/polishing (CMP) process

In chemical mechanical polishing (CMP) for IC fabrication, porous pads are critical for achieving extremely consistent throughput in CMP process. The pad hardness, porosity of pad, and pore sizes in the thermoplastic polyurethanes foam fabrication procedure determine the properties of CMP pads. This paper aims to develop a methodology for modeling macro and micro pad hardness. The IC1000 CMP pad has been analyzed with a developed image processing method to determine porosity and pores size. In addition, nano-indentation tests are used to determine physical properties of micro-solid components. For macro hardness analysis, five different sizes of three-dimensional representative volume elements (RVE) have been investigated using the random sequential adsorption technique. Furthermore, simulation of macro pad hardness of RVE models are completed with finite element method. Results of this study not only achieves macro hardness of CMP pad simulation as 3% errors as compared with developed micro-indentation test, but also complete three dimensional porous pad modeling with RVE.

Study on Improving the Precise Machinability of Single Crystal SiC by an Ultrasonic-Assisted Hybrid Process.

Silicon carbide (SiC) devices have become one of the key research directions in the field of power electronics. However, due to the limitation of the SiC wafer growth process and processing capacity, SiC devices, such as SiC MOSFET (Metal-oxide-semiconductor Field-effect Transistor), are facing the problems of high cost and unsatisfied performance. To improve the precise machinability of single-crystal SiC wafer, this paper proposed a new hybrid process. Firstly, we developed an ultrasonic vibration-assisted device, by which ultrasonic-assisted lapping and ultrasonic-assisted CMP (chemical mechanical polishing) for SiC wafer were fulfilled. Secondly, a novel three-step ultrasonic-assisted precise machining route was proposed. In the first step, ultrasonic lapping using a cast iron disc was conducted, which quickly removed large surface damages with a high MRR (material removal rate) of 10.93 μm/min. In the second step, ultrasonic lapping using a copper disc was conducted, which reduced the residual surface defects with a high MRR of 6.11 μm/min. In the third step, ultrasonic CMP using a polyurethane pad was conducted, which achieved a smooth and less damaged surface with an MRR of 1.44 μm/h. These results suggest that the ultrasonic-assisted hybrid process can improve the precise machinability of SiC, which will hopefully achieve high-efficiency and ultra-precision machining.

Single-Wafer Chemical Mechanical Planarization (CMP) on 150mm Silicon Carbide Substrate with Superior Surface Finish and Removal Efficiency

Silicon carbide (SiC) has excellent performances such as superior thermal conductivity, good carrier mobility and high chemical stability in comparison with those of silicon (Si). SiC has a great potential in the area of the next generation power controlling devices, high performance communication, and LED lighting. Chemical mechanical polishing (CMP) techniques combine mechanical polishing with a chemical etching action and can achieve truly defect-free surface with wafer flatness control capability, which is important for obtaining good epitaxial layer. However, Silicon carbide (SiC) presents many challenges for wafer surface treatment because of its high hardness and remarkable chemical inertness. Currently, low material removal rate (MRR) of CMP technique is a fundamental limit, it extends the CMP process time and effects the yield in mass production of substrate.In this study, we proposed an ultra-high MRR CMP process on 150mm n-doped, 4° off-axis, single crystal, 4H-SiC wafers. The process was developed at Applied Materials Mirra Durum® CMP configured with the membrane polishing head using an acidic slurry with strong potassium permanganate oxidant and thermoplastic polyurethane (TPU) pad. The effect of polishing process parameters (speed of the head and platen rotating, head sweeping distance and applied pressure) on MRR are analyzed in JMP® (SAS institute) while a design of experiments (DOE) is employed. The chemical composition of slurry, which is also critical to MRR, has also been evaluated via DOE. Potassium permanganate with certain concentration has been selected as oxidant and aluminum oxide micron particle has been used as abrasive. We have also conduct studies on the SiC wafer flatness control, which is proven to depend on structural design of polishing head. The head has been optimized to offer a flexible zone pressure control, therefore the impact on wafer flatness during the CMP process has also been minimized. In addition, accepted industry measurements of surface roughness, surface defect and wafer deformation are also taken into consideration.The successful development of CMP process provided an optimized MRR result, the reported MRR is 22.5 um/hr from C face and 6.4um/hr from Si face. The MRR have achieved highest level within literature and industry. The MRR results are analyzed by least–square modelling, which reveals that Prob >|t| is less than 0.05 for platen rotation speed and membrane pressure. Prediction profile demonstrates that MRR typically follows Preston’s law, which is a linear function of the platen speed and the pneumatic pressure applied to substrate. MRR is highly stable throughout a 2000-wafer repeat run with wafer-to-wafer uniformity of 2.4-3.7%. Scratch-free wafer surfaces are observed for the ultra-smooth wafers via atomic force microscopy (AFM) and defect inspection (Candela) tool. Roughness on Si face has been reduced from 2.0-2.5nm to less than 0.15nm with acceptable repeatability. Surface defects including scratches, pits have been greatly reduced. Total scratch length of 10mm has been achieved. Total thickness variation (TTV) is measured to characterize wafer flatness by Tropel tool, which indicates that the total amount of removed thickness on faces has a profound influence on TTV. With focused process control, post-CMP TTV can be improved by 0.6 ~2.1 um from pre-CMP TTV.

Quantification of Carbonic Contamination of Fused Silica Surfaces at Different Stages of Classical Optics Manufacturing.

The chemical composition of ground and polished fused silica glass surfaces plays a decisive role in different applications of optics. In particular, a high level of carbon impurities is often undesirable for further processing and especially for gluing or cementing where adhesion failure may be attributed to carbonic surface-adherent contaminants. In this study, the surface carbon content at different stages of classical optics manufacturing was thus investigated. Two different standard processes—grinding and lapping with two final polishing processes using both polyurethane and pitch pads—were considered. After each process step, the chemical composition and roughness of the surface were analysed using X-ray photoelectron spectroscopy and atomic force microscopy. An obvious correlation between surface roughness and effective surface area, respectively, and the proportion of carbon contamination was observed. The lowest carbon contamination was found in case of lapped and pitch polished surfaces.

Novel three-body nano-abrasive wear mechanism

Current three-body abrasive wear theories are based on a macroscale abrasive indentation process, and these theories claim that material wear cannot be achieved without damaging the hard mating surface. In this study, the process of three-body nano-abrasive wear of a system including a single crystalline silicon substrate, an amorphous silica cluster, and a polyurethane pad, based on a chemical mechanical polishing (CMP) process, is investigated via molecular dynamics simulations. The cluster slid in a suspended state in smooth regions and underwent rolling impact in the asperity regions of the silicon surface, realizing non-damaging monoatomic material removal. This proves that indentation-plowing is not necessary when performing CMP material removal. Therefore, a non-indentation rolling-sliding adhesion theory for three-body nano-abrasive wear between ultrasoft/hard mating surfaces is proposed. This wear theory not only unifies current mainstream CMP material removal theories, but also clarifies that monoatomic material wear without damage can be realized when the indentation depth is less than zero, thereby perfecting the relationship between material wear and surface damage. These results provide new understanding regarding the CMP microscopic material removal mechanism as well as new research avenues for three-body abrasive wear theory at the monoatomic scale.

Asymmetric Flexible Electroadhesive Clutches

SummaryFlexible electroadhesive clutches with high shear stress working at low voltage and fast response are much desired in wearable electronics and robotic systems. Dielectric materials of opposite electron affinity may enhance the clutch performance by boosting the electrostatic attraction between the two surfaces in an asymmetric clutch. In this paper, the first batch of asymmetric electroadhesive clutches is proposed, fabricated and evaluated. The effect of the charge characteristics of the dielectric materials on the clutch performance is investigated. A shear stress of 197.30 ± 15.44 kPa is generated at 300 V in around 20 ms by the asymmetric clutch made from polyurethane and polyimide active dielectric pads. The shear stress is more than eight times of other reported symmetrical clutches working at a similar voltage range. In addition, the hydrogen bonds at the interface are found to contribute to the high electroadhesive stress. This work demonstrates an effective asymmetric and multi-mechanism strategy to develop high performance electroadhesive clutches with diverse materials selection.

Development of hip protectors for snowboarding utilizing 3D modeling and 3D printing

The purpose of this study was to develop a highly comfortable 3D male hip protector using 3D modeling and printing technologies. The hip protector pads and patterns were devised using 3D human body shapes, and three types of pads were chosen in consideration of snowboarding motions. The three types of pads were as follows: first, the original type with no hole; second, an inner open type with an incision on the inside; and third, an outer open type, with an incision on the outside. Another variable of the protective pads was the material: 3D printed thermoplastic polyurethane (TPU) pad + ethylene–vinyl acetate (EVA) foam or only EVA foam. Six types of pad prototypes were 3D printed and evaluated for subjective wearing comfort. Subjective comfort, fit, activity comfort, and shock absorption were evaluated on an 11-point Likert scale. The study results showed that protectors printed using TPU material were not different from the results of 3D modeling. The evaluation results revealed that comfort, fit, and motion comfort were all negatively evaluated by subjects when wearing the original pad. While fit, comfort, and motion comfort were all positively evaluated by subjects when wearing the outer open-type pad, and comfort and motion comfort were positively evaluated by subjects when wearing the inner open-type pad. With respect to materials, pads made with the 3D printing (TPU) and EVA foam combination provided the best results in terms of overall comfort, buttocks comfort, and activity comfort.

Modified method of vacuum therapy in the treatment of infected poststernotomy wounds

Sternal wound infections are a terrible complication that require long and complex treatment. The aim of the study was to evaluate the results of using the modified method of vacuum therapy to treat purulentseptic complications of post-sternotomy wounds in clinical practice. Materials and methods. According to the applied method of vacuum therapy, all patients with infectious complications of post-sternotomy wounds were divided into two groups (n = 25, average age 56.6 years) . The classical vacuum therapy was used in the first group consisting of 12 patients. In the second group, 13 patients were treated with the help of the modified method of vacuum therapy. Results. In the first group, 1 patient (8.3%) experienced osteomyelitis of the sternum, following a partial resection of bone plates; 1 patient (8.3%) developed sternal fistulas, which required long-term treatment; 1 patient (8.3%) had bleeding due to the injury of the left brachiocephalic venous trunk because of the direct contact of the polyurethane pad with the blood vessel wall. The bleeding was eliminated by fixing the damaged area of the vascular wall with U-shaped sutures using polytetrafluoroethylene pads. In the second group, no complications of this nature were observed. The modified method of vacuum therapy allows for the effective evacuation of the hemorrhagic discharge of the wound surface, the reduction of the degree of pathogen contamination in the adjacent tissues, and the elimination of bleeding risk . Conclusion . The modified method of vacuum therapy in combination with effective algorithms for treating purulent-septic complications of post-sternotomy wounds allows physicians to avoid fatal complications and achieve good clinical results.

Basement vibration isolation efficiency investigation

The article deals with experimental investigation of insertion loss estimation for a building elastic shielding vibration isolation system. Precision mechatronics production facility is located inside it with VC-D requirements for their foundation vibration level. A road with double tram lines lays 23 m off the exterior building’s walls producing reasonable vibration background noise. In order to reduce surface wave amplitudes elastic layer of Sylomer® polyurethane pads are glued to the underground exterior walls of the building. The vibration measurements were conducted before and after the elastic shield installation and the insertion loss was estimated using these results. Moreover, simultaneous dual channel analysis of vibration transfer through the wall structure was evaluated. Measured vibration isolation efficiency reaches 14 dB in octave band 31.5 Hz in vertical and from 9 to 15 dB in horizontal direction. For higher frequencies vibration isolation efficiency reduces due to wave effects in the isolators.