Pad Matrix Research Articles

Super hard and brittle material removal mechanism in fixed abrasive lapping: Theory and modeling

Currently, the problems of inefficient fixed abrasive lapping (FAL) for super hard and brittle material wafers (such as sapphire) with fixed abrasive pads (FAPs), restrict the development of the semiconductor industry. Most importantly, the mechanisms of FAL processes are still unclear, often analyzed based on the theoretical models of chemical mechanical polishing (CMP). However, the material removal models of CMP are specific to loose abrasives, do not effectively apply to the lapping processes for super hard and brittle material wafers with a FAP. In this paper, the spherical crown model modified by elastic-plastic mechanics and brittle-plastic transformation mechanism was applied to super hard and brittle material removal mechanism in FAL processes. In consideration of the influences of the yielding abrasives embedded in the FAP matrix and the matrix surface topography on the wafer-pad contact, a material removal model suitable for super hard and brittle material wafers lapped by FAPs was obtained. The loads applied on the fixed abrasives and the matrix asperities during the elastic-plastic-brittle deformation processes of wafers were analyzed, respectively. The model indicates that the back-off of the abrasives embedded in the FAP matrix intensifies along with the decrease of the matrix hardness and the increase of the abrasive size, respectively. The penetrating depth of the fixed abrasive can be raised by increasing matrix hardness, applied pressure, abrasive size, and decreasing abrasive number, as same as the elastic recovery of the super hard and brittle material slid by the abrasive after unloading. The pad matrix surface dressed by loose grains greatly affects the MRR of wafers. The predicted MRR of sapphire wafers based on the FAL model is consistent with experimental data. Finally, the keys to improving the processing efficiency of super hard and brittle material wafers are clarified in detail.

Electrotactile Communication via Matrix Electrode Placed on the Torso Using Fast Calibration, and Static vs. Dynamic Encoding.

Electrotactile stimulation is a technology that reproducibly elicits tactile sensations and can be used as an alternative channel to communicate information to the user. The presented work is a part of an effort to develop this technology into an unobtrusive communication tool for first responders. In this study, the aim was to compare the success rate (SR) between discriminating stimulation at six spatial locations (static encoding) and recognizing six spatio-temporal patterns where pads are activated sequentially in a predetermined order (dynamic encoding). Additionally, a procedure for a fast amplitude calibration, that includes a semi-automated initialization and an optional manual adjustment, was employed and evaluated. Twenty subjects, including twelve first responders, participated in the study. The electrode comprising the 3 × 2 matrix of pads was placed on the lateral torso. The results showed that high SRs could be achieved for both types of message encoding after a short learning phase; however, the dynamic approach led to a statistically significant improvement in messages recognition (SR of 93.3%), compared to static stimulation (SR of 83.3%). The proposed calibration procedure was also effective since in 83.8% of the cases the subjects did not need to adjust the stimulation amplitude manually.

Effects of fly ash introduction on friction and wear characteristics of brake pads

Fly ash is a waste matter generally emitted abundantly from chimneys of the production facilities and should mostly be recycled. In this context, this study reveals the tribological effects of fly ash on brake pad components by doping the fly ash in basic brake pad matrix with various weight fractions of 30% (S30), 35% (S35) and 40% (S40) by reducing aluminum powder in the pad matrix. According to the results, as the fly ash concentration increases in the matrix, density and hardness of the structure were prone to decrease to an extent. Water immersion technique was used to determine density values and specially modified pin-on-disc tribotester was utilized to measure coefficient of friction (CF) and specific wear rate (SWR) values between brake pad samples and the cast iron rotating disc. Among prepared samples, maximum average reduction in density and hardness were observed to be by 3.97% and 10.67%, respectively. S30 depicted the minimum CF of 0.32 and maximum CF of 0.43 was performed by S40. Maximum specific wear rate was observed for S40 subtending to an increase of 8.67% from that of S30 to S40. Results showed that, though higher escalation in CF as the fly ash fraction elevates in the matrix, wear rates did not show a dramatic increase which is an indication of effectiveness of fly ash in brake pads in terms of braking performance and long term durability.

Encoding of spatial patterns using electrotactile stimulation via a multi-pad electrode placed on the torso.

Tactile stimulation can be used to convey information to a user in different scenarios while avoiding overloading other senses. Tactile messages can be transmitted as spatial patterns, potentially allowing for a high information throughput. The aim of the present study was to design and test different encoding schemes to determine the best approach for conveying spatial patterns. Encoding schemes with simultaneous (SIM) and sequential pad activation (SEQ) were evaluated, including four SEQ variants designed to potentially facilitate the recognition. In SEQ-col and SEQ-row, the column and row of the activated pad were signified using different frequencies, while SEQ-all and SEQ-all-fast included the activation of all pads where those belonging to the pattern were indicated by changes in frequency (ON pads). The success rate (SR) of the pattern identification and the response time were quantified in 15 participants who recognized 20 patterns delivered through a 3 × 2 pad matrix placed on the lateral torso. SIM was not a feasible method to present the patterns (median, 15%; IQR, 5%). The SR improved with SEQ (median, 60%; IQR, 20%) and further increased with additional cues, particularly with SEQ-row (median, 78.3%; IQR, 23.3%) and SEQ-all (median, 96.7%; IQR, 5%). Importantly, the stimulation time of SEQ-all could be decreased without a substantial drop in accuracy (SEQ-all-fast: median, 89.2%; IQR, 19.2%). The spatiotemporal stimulation with sequential activation of all pads (SEQ-all) seems to be the method of choice when conveying tactile messages as spatial patterns. This is an important outcome for increasing the information bandwidth of communication through the tactile channel.

Research of Subsurface Damage Depth of Lithium Niobate Crystal by Fixed-Abrasive Lapping

The subsurface damage depth of optical material introduced by lapping determines the efficiency of lapping and polishing process. In order to explore the subsurface damage (SSD) of fixed-abrasive lapping process, angle polishing method was employed to evaluate the SSD depth and damage form of soft-brittle material lithium niobate (LN) crystal. The influences of the grit size, the applied load and the hardness of pad matrix on SSD were investigated experimentally. The results show that: with the grit size decreasing form W28 to W14, the damage depth decreases by 70%. With the applied load decreasing form 15 kPa to 7 kPa, the SSD decreases by 40%. The dense and deep subsurface micro-cracks are caused by the hard matrix pad during lapping processing, while the matrix with low hardness causes plastic scratches on the surface/subsurface of the workpiece without obvious micro-cracks around the scratches. This study affords effective method for SSD evaluation and control of soft brittle materials such as LN crystal.

Relationship between mechanical properties and processing performance of agglomerated diamond abrasive compared with single diamond abrasive

Fixed abrasive is taking the place of loose abrasive owing to its high efficiency and little pollution. The self-conditioning ability of fixed abrasive pads (FAPs) plays a favorable role in the lapping process. In this paper, agglomerated diamond (AD) abrasives were prepared for the sapphire lapping process. Material removal rate (MRR), material removal rate variation (MRRV) and surface topography of sapphire wafers lapped by fixed AD abrasive pad were investigated. Abrasive wear and wear debris were characterized. The material removal mechanisms of AD abrasives were discussed. Comparison of mechanical properties between AD and SD in abrasive wear process was analyzed. Influence of mechanical properties of AD abrasives on processing performance was explained. Comparing with fixed single diamond (SD) abrasive pads, the higher MRR, the lower MRRV and surface roughness of sapphire wafer were obtained in the fixed AD abrasive lapping process. Stable large protrusion height and consecutive sharp edges of AD abrasives ensure the excellent lapping performance. The protrusion height of abrasives is positive correlated with the self-conditioning ability of FAPs. The stable protrusion height of AD abrasives is attribute to two points: one is the rough surface of an AD abrasive with multiple sharp cutting edges improves the bonding strength between the abrasives and the pad matrix; the other is the detached diamond particles due to the micro-fracture phenomenon of AD abrasives accelerates the abrasion of matrix.

Comparison of lapping performance between fixed agglomerated diamond pad and fixed single crystal diamond pad

The ability for self-conditioning is the most important characteristic of hydrophilic Fixed Abrasive (FA) pads using in lapping. The failure mode of diamond abrasives embedded in a FA pad greatly impacts its self-conditioning process. In this investigation, three types of FA pads were prepared. One was embedded with W50 single crystal diamond (SCD) abrasives, one was embedded with W10 SCD abrasives, and the other was embedded with agglomerated diamond (AD) abrasives. The lapping performance was tested on fused quartz workpieces. The pad wear mechanisms and the wear debris of the three pads was evaluated using Scanning Electron Microscopy (SEM). The FA pad embedded with AD exhibited a higher material removal rate and a lower arithmetic average surface roughness (Ra) of the lapped quartz than those embedded with SCD. Pad loading and flattening of the of abrasive grit are the main factors causing the failure of FA pad with SCD. Compared with FA pads with SCD, the rough surfaces of AD grains improves the holding force of pad matrix to them, which in turn leads to a more stable cutting depth of abrasive grain. Size analysis of the debris suggests that AD grains are inclined to micro-fracture rather than to wear flat or pull out. The micro-fracture phenomenon of AD in FA lapping process improves the material removal rate and lowers the average surface roughness of lapped work-pieces. The adoption of AD grains was concluded to be important in improving the self-conditioning performance of FA pads.

Quantification of Water Diffusion in CMP Pad Polymers and Impact on Mechanical Properties

Due to the hydrophilic nature of polyurethanes used in CMP pads, water from polishing slurries and rinse solutions is prone to diffuse into the pad matrix. In this study, theoretical and experimental methods are established to calculate the water diffusion coefficient and water diffusion profile within pad polymers. More hydrophilic pads show higher water uptake (3 to 5 wt%) and higher water diffusion coefficient, while less hydrophilic pads show lower water uptake (<2 wt%) and lower diffusion rate. The storage modulus, hardness, and tensile properties of CMP pads are found to drop monotonically with increasing water uptake. More hydrophilic pads experience a greater reduction in mechanical strength than less hydrophilic pads. The shell of the hollow filler (used to impart porosity) is found to act as a relatively hydrophobic barrier that prevents water diffusion into the filler. At 25°C and 50°C—bracketing the bulk pad surface temperatures reached in typical CMP processes—pH shows no additional effects on pad mechanical properties beyond those of water alone. This finding indicates that acidic vs. basic slurries may be assessed for compatibility with pad materials solely in terms of water content.

A novel readout concept for multianode photomultiplier tubes with pad matrix anode layout

We have developed a new analog readout concept for multianode photomultiplier tubes with a pad matrix anode layout. This new readout technique is the result of a modification of a technique previously developed at the Detector Group of Jefferson Lab, (V. Popov, US patent No: 6,747,263 B1) [V. Popov, S. Majewski, A.G. Weisenberger, Readout Electronics for Multianode Photomultiplier Tubes with Pad Matrix Anode Layout, Thomas Jefferson National Accelerator Facility, IEEE 2003 Medical Imaging Conference Record, November 2003]. The new analog readout circuit provides the same analog conversion of matrix 2-D output into X and Y projective output with a significant reduction of analog outputs. The old readout network consists of resistors’ matrix and current collecting amplifiers, and it provides decoupling of each anode output into two directions (one to X and one to Y coordinates), but a decoupling function that is carried out independent of photomultiplier tube gains nonuniformity. A newly developed readout network (US patent pending) also consists of resistors’ matrix and current collecting amplifiers, but the new matrix includes an additional dumping resistor that provides an excess current from anode pad grounding. As a result, we subtract an extra current of high-gain pads in order to move the pads gain to an absolute minimum value for a given photomultiplier tube. This gain equalization procedure reduces image distortion related to gain nonuniformity. The new readout technique was used in several new radiation imaging detectors designed in the Detector Group of Jefferson Lab. It shows a visible readout uniformity and linearity improvement. The test results of an initial evaluation of this readout that is applied for data readout of four H8500 Hamamtsu PSPMT are presented.

Tribological properties of automotive disc brakes with solid lubricants

In automotive brake systems, high temperatures and pressures are generated at the contacting surfaces. This affects the pad and disc materials, creating a friction film between the surfaces, which consists of wear particles and volatile reactants from the pad and disc. To acquire suitable tribological properties, a pad matrix contains up to 20 different ingredients, mainly selected from experience. In the present report the effect of solid lubricants has been studied in three different pad matrices with relatively few components as compared to commercial brake pads. The components are Cu 2S, PbS and Sb 2S 3, which are known to modify and stabilise the friction coefficient. The friction coefficient and wear rates of the pads are examined on a dynamometer, which simulates series of real-life car brake events. Two different energy levels and two ambient temperatures are included. After these tests, the brake disc surfaces are analysed with energy dispersive X-ray (EDX) and Auger electron spectroscopy (AES) in combination with argon ion sputtering to study the microscopic lateral and in-depth distribution of elements on and below the surface, looking for traces of the friction film. These experiments are used to discuss the correlation between the tribological properties and the external variables — braking temperature, solid lubricants and pad matrix.

A Study on the Wear Properties of Cu-free Ecofriendly Vehicle Brake Pad

&lt;p&gt;The friction characteristics of Al-Fe alloy powders are investigated in order to develop an eco-friendly friction material to replace Cu fiber, a constituent of brake-pad friction materials. Irregularly shaped Al-Fe alloy powders, prepared by gas atomization, are more uniformly dispersed than conventional Cu fiber on the brake pad matrix. The wear rate of the friction material using Al-8Fe alloy powder is lower than that of the Cu fiber material. The change in friction coefficient according to the friction lap times is 7.2% for the Cu fiber, but within 3.8% for the Al-Fe alloy material, which also shows excellent judder characteristics. The Al-Fe alloy powders are uniformly distributed in the brake pad matrix and oxide films of Al and Fe are homogeneously formed at the friction interface between the disc and pad, thus exhibiting excellent friction and lubrication characteristics. The brake pad containing Al-Fe powders avoids contamination by Cu dust, which is generated during braking, by replacing the Cu fiber while maintaining the friction and lubrication performance.&lt;/p&gt;