Pad Surface Temperature Research Articles

Thermo-hydrodynamic lubrication and energy dissipation mechanism of a pump-turbine thrust bearing in load-rejection process

The dynamic behavior of the pump-turbine thrust bearing is important to the safety operation of the unit. This paper analyzed the lubrication and energy dissipation mechanism of pump-turbine thrust bearing during load-rejection based on the thermo-hydrodynamic model. The results show that the variation of the axial force and the maximum pad surface pressure is basically consistent with that of the inclination of the pad surface. The change of the friction loss is consistent with the change of rotational speed, while the change of pad surface temperature is affected by the combination of pad inclination and rotational speed. The chaotic flow in the oil tank is accompanied by different forms of vortices, such as Taylor vortices, vortex pairs, and Karman vortices, and results in a significant asymmetry in the pressure distribution. The flow in the bearing pad groove has an effect on the energy dissipation in the oil film. This paper provides a theoretical basis for the design and optimization of thrust bearings, and provides a reference for solving the problems of wear, oil mist, and other related problems of thrust bearings in engineering.

Investigation of braking performance and thermal effect of sintering of brake pads with Al2O3 additive

Nowadays the improvement researches of pad materials, which are widely used in braking mechanisms of land vehicles, still maintain their popularity. In general, it has been proven by the studies that the material characteristics of the brake pads in composite structure vary greatly depending on the manufacturing parameters. In this study, the effects of sintering of Al2O3 added composite pads on braking performance via surface temperature were investigated. In this context, 50, 75, and 125 mm Al2O3 particles were added to the lining composition and the results were evaluated comparatively. The braking performances of the tested samples were evaluated in terms of pad wear, pad surface temperature and friction coefficient. As a result, it was observed that the braking performance of sintered Al2O3 additives in all three dimensions was better.

A Model for Tilting Pad Thrust Bearings Operating With Reduced Flow Rate—Do Benefits Outweigh Risks?

Abstract The literature on tilting pad thrust bearings (TPTB) calls for flow reduction as an effective means to reduce drag power losses as well as oil pumping costs. However, the highest level of flow reduction a bearing can undergo while maintaining reliable operation is a key question that demands comprehensive analysis. This paper implements a model into an existing thermo-elasto-hydrodynamic (TEHD) computational analysis tool to deliver load performance predictions for TPTBs operating with reduced flow rates. For bearings supplied with either a reduced flow or an overflow conditions, a sound model for the flow and thermal energy mixing in a feed groove determines the temperature of the lubricant entering a thrust pad. Under a reduced flow condition, the analysis reduces the effective arc length of a wetted pad until matching the available flow. Predicted discharge flow temperature rise and pad subsurface temperature rise from the present model match measurements in the archival literature for an eight-pad bearing supplied with 150% to 25% of the nominal flow rate, i.e., the minimum flow that fully lubricates the bearing pads. A supply flow above nominal rate increases the bearing drag power because the lubricant enters a pad at a lower temperature, and yet has little effect on a thrust pad peak temperature rise or its minimum film thickness. A reduced flow below nominal produces areas lubricant starvation zones, and thus the minimum film thickness substantially decreases while the film and pad's surface temperature rapidly increase to produce significant thermal crowning of the pad surface. Compared to the bearing lubricated with a nominal rate, a starved flow bearing produces a larger axial stiffness and a lesser damping coefficient. A reduction in drag power with less lubricant supplied brings an immediate energy efficiency improvement to bearing operation. However, sustained long-term operation with overly warm pad temperatures could reduce the reliability of the mechanical element and its ultimate failure.

Theoretical and Experimental Investigations on Directly Lubricated Tilting Pad Journal Bearing under Starved Lubrication

In this study, a comprehensive thermoelastohydrodynamic model considering turbulence and cavitation is developed. After comparing the Reynolds-Rayleigh-Plesset and Elrod and Adams models, the Elrod and Adams model is adopted to consider cavitation effects. To validate the developed model, a series of novel two-phase flow or mixed bubble experiments is designed by extracting the characteristics of actual operating conditions. Experiments are performed on a high-speed test rig. The oil film pressure and pad surface temperature of a loaded pad under various preset gas volume fractions are measured. The experimental and simulation results show reasonable agreement, and the developed model can successfully predict the bearing performance under starved conditions.

Tribological, Thermal, Kinetic, and Pad Micro-Textural Studies Using Polyphenylene Sulfide Retaining Rings in Interlayer Dielectric Chemical Mechanical Planarization

We compared the long-term wear performance of 2 different PPS retaining ring materials (E-PPS and O-PPS) provided by 2 different suppliers. After 4 h of wear, O-PPS resulted in higher values of mean pad summit height as compared to E-PPS (26.4 ± 1 vs 23.9 ± 1 μm, respectively). We believed this was due to excessive pad fragment generation (confirmed by its associated higher level of pore obscuration) which caused the asperities to artificially appear taller. Regarding mean summit curvature, O-PPS did not sharpen the asperity tips beyond what had already been achieved through conditioning (341 ± 50 vs 352 ± 50 per μm2, respectively). On the other hand, E-PPS caused the tips to sharpen significantly (508 ± 50 per μm2). The pad-slurry-ring coefficient of friction (COF) was higher for E-PPS (0.69 vs 0.65). This was consistent with the higher observed pad surface temperature for E-PPS and its associated sharper pad asperities. Both rings showed similar tribological behaviors, and the contact mechanism was one of “boundary lubrication.” Both rings caused the pad to wear at comparable rates, but regarding ring wear rate, E-PPS seemed to wear about 30 percent faster than O-PPS. This was deemed to be inconsequential in IC manufacturing due to the estimated excessive life of the ring (regardless of which type of PPS material was used) compared to the useful life of the pad and other consumables such as filters and discs.

Effect of spray nozzle position on pad temperature distribution and wafer non-uniformity

During chemical mechanical planarization (CMP), the pad surface temperature generally increases due to the friction between the wafer and the pad. A pad with a large sliding distance generates more heat compared to pad with small sliding distance. A nonuniform temperature distribution on the pad causes its mechanical properties to become uneven, which are affected by within-wafer non-uniformity (WIWNU). This study aims to improve the polishing uniformity by controlling the temperature profile of the pad surface for each spray nozzle position. When the slurry was sprayed on an area for which a large slide distance was measured, a uniform temperature distribution and improved WIWNU were obtained.

Insights into the Tribological and Kinetic Attributes of Retaining Rings in Chemical Mechanical Planarization

We have successfully applied the Stribeck+ curve to determine the tribological mechanism involved in the ring-slurry-pad interface during CMP at various pressures and velocities. Using two types of slurries (one for copper and another for inter-layer dielectric polishing) and two differently designed rings made of polyether ether ketone (PEEK), one with sharp slots and the other with rounded slots. We find “boundary lubrication” as the initial dominant lubrication mechanism followed by a transition to “mixed lubrication” as pseudo-Sommerfeld numbers increase. Coefficient of friction (COF) values for the ring with sharp slots are larger than their rounded counterparts. COF values are also higher for inter-layer dielectric (ILD) processes as compared to copper. In all cases, COF values are directly correlated with average pad surface temperature. The trend in Stribeck+ curves are explained by plotting a new parameter, referred to as directivity, which shows the cluster shape evolution to be a real phenomenon (and not an optical illusion due to the log-log nature of the curve, nor the inherent dependence of the two axes on one another) caused by force fluctuations at the extreme ranges of velocity and pressure. Raw data from the Stribeck+ curves are used to construct a new “kinetic” plot to help infer the relative wear rate of the ring thus allowing one to choose process parameters that balance it against wafer removal rate. Validation studies involving extended wear of eight differently formulated retaining rings show a correlation between wear rate and the “inferred” wear rate which renders credibility to our approach regarding the above-mentioned “kinetic” plots.

Effect of Various Slurry Injection System Configurations on Removal Rates of Silicon Dioxide Using a Ceria-Based Chemical Mechanical Planarization Slurry

In this work, the position of the novel slurry injection system (SIS) was optimized to achieve a more cost-effective and environmentally benign chemical mechanical planarization process using a widely-adopted ceria-based “reverse slurry”. SIS was configured with different angles in order to investigate slurry dilution characteristics caused by residual pad rinsing with ultrapure water (UPW) that affected silicon dioxide removal rates. UPW dilution effect on removal rate, coefficient of friction and pad surface temperature was explained by maintaining a constant dilution ratio for each of the SIS configuration tests. Results indicated that the negative rotation angle of the SIS increased the actual slurry dilution ratio on top of the polishing pad. This generated more Ce3+ which boosted the removal rate. Application of negatively rotated SIS allowed significantly lower slurry flow rate and/or shorter polishing time leading to achieve a more environmental friendly semiconductor manufacturing process. Finally, it was confirmed that variations in SIS configuration had no impact on silicon dioxide to silicon nitride removal rate selectivity.

Improvements in Stribeck Curves for Copper and Tungsten Chemical Mechanical Planarization on Soft Pads

Understanding the tribology of polishing processes using an improved Stribeck curve (i.e. referred as Stribeck+ curve) elucidates the essential factors governing the mechanical aspects of chemical mechanical planarization. This study investigated tribological and thermal aspects of copper and tungsten blanket wafer polishing on the Politex soft pad with a multitude of gradual yet continuous changes in polishing pressure and sliding velocity. This work also attempted to understand the frictional and thermal transients associated with such dynamic processes. Results showed that polishing with the Politex pad produced stable COF values that indicated a “boundary lubrication” regime throughout. In contrast, copper polishing on hard pads produced tremendous spread of data clusters having a trend in COF that started with “boundary lubrication” and then transitioned to the “mixed lubrication” mode. Regarding temperature effects, the plots between average pad surface temperature and the product of COF, velocity and downward pressure showed a linear relationship between the two parameters for all copper and tungsten polishing used in conjunction with Politex and hard pads.

Relationship between polishing pad surface temperature and removal rate in sapphire-chemical mechanical polishing

Relationship between polishing pad surface temperature and removal rate in sapphire-chemical mechanical polishing

Pad Surface Thermal Management during Copper Chemical Mechanical Planarization

A pad surface thermal management system was developed to improve copper removal rate within wafer non-uniformity by locally adjusting the pad surface temperature. The system consisted of one or more thermal transfer modules, which contacted the pad surface during polishing. Hot or cold water circulated between the thermal transfer module and an external heater or cooler. With the module placed on the pad surface, heat conduction occurred between the module and the pad surface, producing a localized pad surface with higher or lower temperatures. As such, it was expected that local removal rates would change accordingly due to the temperature-sensitive nature of copper chemical mechanical planarization (CMP). In this study, the system was used to adjust the “center-fast” removal rate profile to illustrate its effect during the process. Results showed that, when two thermal transfer modules were employed, local removal rates in the wafer center region decreased significantly while the removal rates near the wafer edge were maintained thereby significantly improving within wafer removal rate non-uniformity.

Mechanisms of Local Planarization Improvement Using Solo Pad in Chemical Mechanical Polishing

The mechanism of local planarization improvement using a solo pad in chemical mechanical polishing (CMP) was investigated, and the pad surface temperature was found to be the key factor. The use of a solo pad results in better planarity than that of a stacked pad under the same process conditions. When Cu CMP evaluation was conducted at various platen temperatures, a good correlation of local planarity to pad surface temperature was confirmed regardless of the pad type. Planarity improved when the pad surface temperature was lowered, and the solo pad had a lower temperature than the stacked pad at the same platen temperature. It is considered that the solo pad has a higher heat conductance than the stacked pad, so that heat generated during polishing is transferred to the platen more easily through the solo pad than through the stacked pad. The reason for the better planarity with the lower pad surface temperature was explained by the change in pad elasticity by the temperature.

Tilting Pad Journal Bearings—A Discussion on Stability Calculation, Frequency Dependence, and Pad and Pivot

For several years, researchers have presented predictions showing that using a full tilting-pad journal bearing (TPJB) model (retaining all of the pad degrees of freedom) is necessary to accurately perform stability calculations for a shaft operating on TPJBs. This paper will discuss this issue, discuss the importance of pad and pivot flexibility in predicting impedance coefficients for the tilting-pad journal bearing, present measured changes in bearing clearance with operating temperature, and summarize the differences between measured and predicted frequency dependence of dynamic impedance coefficients. The current work presents recent test data for a 100 mm (4 in.) five-pad TPJB tested in load on pad (LOP) configuration. Measured results include bearing clearance as a function of operating temperature, pad clearance and radial displacement of the loaded pad (the pad having the static load vector directed through its pivot), and frequency-dependent stiffness and damping. Measured hot-bearing clearances are approximately 30% smaller than measured cold-bearing clearances and are inversely proportional to pad surface temperature; predicting bearing impedances with a rigid pad and pivot model using these reduced clearances results in overpredicted stiffness and damping coefficients that are several times larger than previous comparisons. The effect of employing a full bearing model versus a reduced bearing model (where only journal degrees of freedom are retained) in a stability calculation for a realistic rotor-bearing system is assessed. For the bearing tested, the bearing coefficients reduced at the frequency of the unstable eigenvalue (subsynchronously reduced) predicted a destabilizing cross-coupled stiffness coefficient at the onset of instability within 1% of the full model, while synchronously reduced coefficients for the lightly loaded bearing required 25% more destabilizing cross-coupled stiffness than the full model to cause system instability. The same stability calculation was performed using measured stiffness and damping coefficients at synchronous and subsynchronous frequencies. These predictions showed that both the synchronously measured stiffness and damping and predictions using the full bearing model were more conservative than the model using subsynchronously measured stiffness and damping, an outcome that is completely opposite from conclusions reached by comparing different prediction models. This contrasting outcome results from a predicted increase in damping with increasing excitation frequency at all speeds and loads; however, this increase in damping with increasing excitation frequency was only measured at the most heavily loaded conditions.

Effect of Temperature on Pad Surface Contact Area in Chemical Mechanical Planarization

In this study, pad surface contact area measurement was performed using laser confocal microscopy at elevated temperatures under dry and static condition to illustrate the effect of temperature on the mechanical contacts in chemical mechanical planarization. Pad surface contact area and contact density were measured for a Cabot Microelectronics Corporation D100 pad sample at 27.6 kPa with a custom-made sample holder that was designed to heat the pad sample during the measurement. When the pad surface temperature increased from 25 to 45°C, pad asperities became much softer and resulted in a significant increase (from 0.029% to 0.092%) in the pad contact area percentage while the contact density remained in the same range.

Accidental renal injury by an external heating device during surgery in rats

Hypothermia can be caused by anaesthesia and/or surgery and represents a daily challenge in the operating room. Experimental animal surgery settings typically use heating pads or warming blankets to maintain the rodent's body temperature during long-lasting experiments. Warming is crucial in small animal experiments because these animals quickly lose temperature due to their large body surface to body weight ratio. While establishing a left ventricular infarction model in rats, we inserted a rectal temperature probe. The heating pad's set point was 37°C. Although a dual set point control circuit should prevent overheating, we observed a maximum heating pad's surface temperature of 43°C between the animal's back and the surface of the heating pad. At the end of the experiments, which lasted up to 8 h, the animals showed severe haematuria and segmental kidney damage. We hypothesized that overheating of the heating pad and uneven distribution of temperature led to kidney damage. Therefore, the maximal temperature of commonly used heating pads must be tightly controlled to avoid overheating, which may cause kidney or tissue injury, may falsify the experimental data and could influence the study results.

Correlation between break-in characteristics and pad surface conditions in silicon wafer polishing

The surface properties of a polishing pad play a significant role in chemical mechanical polishing (CMP). The buffing process is able to control the pad surface conditions and ultimately has an effect on polishing characteristics including removal rate and break-in time in the CMP process. In this paper the polishing pad was controlled by the roughening process, which was then investigated for its effect on the material removal characteristics during the silicon wafer polishing process. As a result of this monitoring, the removal rate and wafer to wafer non-uniformity (WTWNU) were influenced by the polishing pad surface conditions, such as roughness. In addition, abnormal signals from friction force and the polishing pad surface temperature were found during polishing process. This phenomenon was named “Break-in delay.” This means that the material on the wafer could not be removed uniformly due to the instability of the polishing consumables. The break-in time was reduced by controlling the pad surface conditions. Therefore, the surface conditions of the polishing pad have a strong correlation with the break-in characteristic in the silicon polishing process.

Effects of Slurry Flow Rate and Pad Conditioning Temperature on Dishing, Erosion, and Metal Loss during Copper CMP

The effect of slurry flow rate, pad surface temperature, and temperature during the pad conditioning process on surface tribology and pattern-related defects like dishing, erosion, and metal loss was studied. Experimental results suggest that dishing and erosion levels decreased with increase in slurry flow rate. Conditioning experiments at various temperatures revealed a significant impact of temperature on the effectiveness of the conditioning process and also on subsequent polishing performance. The polishing pad was conditioned more aggressively at lower temperatures compared to conditioning at elevated temperatures. The removal rate and coefficient of friction were found to be significantly affected by the pad surface temperature. The amount of dishing increased with increase in pad surface temperature and the uniformity of polishing. The study ascertains a correlation between process parameters and the extent of planarity defects. This study also demonstrates the use of a modified bench-top chemical mechanical polish (CMP) tester and large-stage atomic force microscope with automatic multiscan imaging procedure.

High thermal diffusivity materials for railway brakediscs

Two high thermal diffusivity brake discs have been worn againsta metallic pad, a composite pad and a new ceramic pad. Thesediscs are composed of a friction track made of molybdenum orCu-2.5% Be alloy. Molybdenum was used in the form of a coatingsprayed on an aluminum-based disc or as a thin solid disc mechanically clamped onto an aluminum-based stand disc. Coatingadherence, friction coefficient, surface temperature and wearthickness loss were determined for a set of reduced-scale steelbrake discs during a continuous braking simulation. It is shownthat the solid molybdenum disc tested against the composite padinduces significant surface temperature decreases as opposed tosliding against the bare steel disc. The friction coefficient isstable and wear damage acceptable. However, the molydenumcoating performs unsatisfactorily as the large difference inthermal expansion coefficient between molybdenum and aluminuminduces disbonding and crazing of the coating. The aluminumtitanate pad/Cu-2.5Be disc brake system has demonstrated remarkable tribological performance, with a steady frictioncoefficient, a pad surface temperature about 60°C lower thanthat of a steel disc and low pad wear.

The effect of the polishing pad treatments on the chemical-mechanical polishing of SiO 2 films

Polishing pads play an important role in chemical-mechanical polishing (CMP) which has recently been recognized as the most effective method to achieve global planarization in very large scale integrated circuit multi-level interconnects. In this work, we have investigated CMP performances of SiO 2 as a function of the time of soaking the pad in water, pad conditioning, and pad surface temperature. It has been observed that the dynamic shear modulus G' of an IC1000 pad decreases to two thirds of its dry value during 5 h water soaking, while its oxide removal rate remains practically unchanged. The shear modulus of a polishing pad also decreases with the increase of pad temperature. For example, G' of an IC1000 pad drops approximately by a factor of 2 when temperature increases from 40 °C to 80 °C. Correspondingly, increases of both oxide removal rate and planarization efficiency, which is the step-height reduction per unit oxide removed on the “up feature”, have been observed when the slurry temperature increases (which raises the pad surface temperature). Pad conditioning is a crucial step in CMP. The oxide removal rate decreases with time during a CMP process without pad conditioning until reaching a low steady-state value. On the other hand, the planarization efficiency increases with time during a CMP process without pad conditioning. Possible mechanisms which may lead to these observations are discussed.

Prediction of Temperatures in Tilting Pad Journal Bearings

A solution technique for tilting pad journal bearings is presented that includes two dimensional heat conduction in the individual pads and applies a heat convection boundary condition to the pad ends. The method makes use of approximations to the axial pressure profile and the temperature profiles and includes heat transfer between the film and its boundaries. A second order profile is assumed to represent the temperature distribution across the film. The classical Reynolds equation is applied, using the viscosity based on the cross-film average of temperature. The journal surface temperature is given by a circumferential average of the film temperatures. Results of calculations are compared with published experimental results to determine appropriate values for the heat convection coefficient at different conditions. The coefficient is found to be speed dependent. When the variation with speed is approximated as linear, calculated pad surface temperature rises agree with experimentally measured values w...