Pad Asperities Research Articles

Mathematical modeling based on contact mechanism due to elastic and plastic deformation of pad asperities during CMP

Technologies in semiconductor industry have been developed into a three-dimensional multilayer wiring for high integration of devices. Chemical mechanical planarization (CMP) process is one of the key technologies for achieving multilayer wiring, which enables global planarization. In addition, highly integrated devices can be realized by increasing the depth of focus in the photolithography process. However, in the inter-layer dielectric (ILD) CMP of the transistor, the uppermost oxide layer has the step due to the arrangement of the devices. The ideal material removal mechanism is to gradually remove materials from the top of the step height which allows for global planarization. However, in the CMP of the patterned wafers, simultaneous polishing of the upper and lower layers occurs when the step height reaches a certain height. This means that the polishing is strongly dependent on the structural characteristics of the pattern. Especially, the difference in the material removal rate depending on the pattern density acts as a constraint in terms of device layout. Therefore, it is essential to develop an accurate prediction model of material removal rate as a function of pattern density, size and arrangement. This study aims to define the mathematical planarization model according to contact mode between a polishing pad and patterned wafer. Considering that the real contact area between the actual polishing pad and the wafer is about 1 %, the mathematical model is derived based on the microscopic deformation of the pad asperities, not the macroscopic deformation of the bulk pad. Finally, we describe the verification between the theoretical material removal rate model and step height reduction and the actual CMP results. The root mean square error of the upper layer material rate, the lower material removal rate, and the step height reduction were 24.59 nm/min, 22.03 nm/min and 22.6 nm, respectively. Compared with the previous studies, the new model of this study improved the error by up to 50.9 %.

Investigation of critical material removal transitions in compliant machining of brittle ceramics

Compliant machining processes, such as bonnet polishing, can be used on hard and brittle ceramic materials such as alumina and silicon carbide, to produce ultra-precise surfaces with sub-micron form accuracy and nanometric surface roughness. However, a comprehensive understanding of the removal mechanism in such process is lacking. In this paper, an analytical model is proposed that is based on the existence of “three zones” in compliant machining process, namely elastic recovery, plastic removal and brittle fracture. The inherent relationships of the three critical pressures with actual pressure, due to compression of the elastic bonnet tool and asperity effect, are established and analyzed in association with different material removal behaviors. Analysis indicates that pad asperity plays an important role in material removal and that lower material hardness combined with higher tensile strength contributes to enlarged plastic removal zone, and thus higher manufacturability. Removal footprints and polishing tests were then generated to verify accurate prediction of the material removal rate under different conditions and demonstrate effectiveness of the proposed model.

Performance of Trizact™ Microreplicated Pad in W Buff Marathon Process

This work demonstrates the performance of a microreplicated pad used in a W buff polish process. 3M™ Trizact™ microreplicated pads consist of matrix designed asperitity and pore structures that can be tuned independently, into a wide variety of complex 3D structures. More uniform pad asperity diameter and height control allows for better wafer-pad contact interacions. Consequently, microreplicated pads have significantly higher contact area with the wafer surface compared to industry standard hard and soft pads. The unique heringbone groove design allows for efficient slurry transportation and uniform slurry flow distribution – enabling the potential for slurry saving and cost of ownership reduction. The CMP wafer polish results show stable oxide RR (se Figure 1) and consistent W:Ox film selectivity over 3000 wafers. Wafer removal rate recovey was repeatablly fast following periods of wet idle. Stable uniformity/edge profile control were also achieved. Total defect counts were controlled and consistent throughout the duration of the 3000 wafer marathon run. Upon completion of the marathon run, the loss in pad asperity height loss was less than 3 microns (14 %) compared to initial asperity height illustrating the potential to extend the pad life significantly beyond 3000 wafers (see Figure 2). The results illustrate the benefit of micro-replicated asperities in CMP pads, These results are important because of the potential to improve pad performance and reduce the cost of CMP. Figure 1

Effect of Conditioner Type and Downforce, and Pad Surface Micro-Texture on SiO2 Chemical Mechanical Planarization Performance.

Based on a previous work where we investigated the effect of conditioner type and downforce on the evolution of pad surface micro-texture during break-in, we have chosen certain break-in conditions to carry out subsequent blanket SiO2 wafer polishing studies. Two different conditioner discs were used in conjunction with up to two different conditioning downforces. For each disc-downforce combination, mini-marathons were run using SiO2 wafers. Prior to polishing, each pad was broken-in for 30 min with one of the conditioner-downforce combinations. The goal of this study was to polish wafers after this break-in to see how the polishing process behaved immediately after break-in. One of the discs used in this study produced similar micro-texture results at both downforces, which echoed the results seen in the mini-marathon. When comparing the different polishing results obtained from breaking-in the pad with the different discs used in this study, the coefficient of friction (COF) and SiO2 removal rate (RR) were uncorrelated in all cases. However, the use of different discs resulted in different COF and RR trends. The uncorrelated COF and RR, as well as the differing trends, were explained by pad micro-texture results (i.e. the differing amount of fractured, poorly supported pad asperity summits).

CMP에서 패드 돌기와 산화막 패턴의 접촉 모드에 따른 평탄화 모델링

CMP에서 패드 돌기와 산화막 패턴의 접촉 모드에 따른 평탄화 모델링

Model of Particle Contact Area for Friction in Oxide Chemical Mechanical Polishing

Friction from Chemical Mechanical Polishing (CMP) of integrated circuits can cause damage to surface and sub-surface layers. To understand potentially damaging CMP friction, a model is presented which separates the contributions of: 1) bare pad asperity to substrate junction contacts, 2) slurry nanoparticles in the pad asperity contacts, and 3) a proposed substrate area where slurry particles adhere to and are swept along with the moving asperity contact. The model is used to interpret data from pin-on-disk friction experiments performed with a fused silica wafer and various polymer polishing pad materials, flooded with 100 nm diameter silica nanoparticle slurries. The results showed that only 18% of the pad asperity junction contact area had active particles and contacts outside of the asperity area were 38 times of the asperity junction area. This suggests that 93% of CMP material removal activity results from particles outside of the asperity/substrate contact.

研磨パッド・アスペリティ内のミクロなスラリー循環流れ

Chemical mechanical polishing (CMP) is used in a wide range of processes in semiconductor production. However, the polishing mechanism of CMP is not completely understood. The behavior of the abrasive particles and chemical species in the slurry is believed to play a critical role in the polishing mechanism. Therefore, in this study, we observed the slurry flow occurring in the polishing pad asperities experimentally. Because in situ observation of the slurry flow is exceptionally difficult, an enlarged visualization model of the pad asperities was made, based on the scaling laws of fluid dynamics. A refractive index matching technique and a particle suspension method were applied to visualize the scaled-up slurry flow. As a result, the circulation flows in the vertical direction to the wafer surface have been observed at the openings of the pad pores, and at the valleys of the undulations formed by the pad conditioning. The size of the biggest circulation flow observed in this study is approximately 200 μm. This result suggests that, similar to the pad pores, the valleys of undulations of the pad have the function of aiding the slurry transport to the entire wafer surface.

Real-Time Shear and Normal Force Trends in Copper Chemical Mechanical Planarization with Different Conditioning Discs

The effect of two different commercially-available CVD-coated diamond conditioning discs on copper CMP was investigated tribologically and kinetically with the intent of correlating pad surface micro-texture to polish performance. Data analyses were particularly focused on in-situ shear and normal force measurements taken during polishing at several pressures (P) and sliding velocities (V), including Fast Fourier Transform (FFT) analysis of the data. One of the two discs consistently resulted in greater variances of shear force with increasing P and V indicative of increased pad-slurry-wafer stick-slip events. This trend correlated to higher values of coefficient of friction (COF) and removal rate (RR). By dividing the shear force variance by the normal force variance, a new parameter, termed directivity, was used to further emphasize the improved performance of the above-mentioned disc. Lastly, the tribological and kinetic data, combined with data regarding the pad's micro-texture and its extent of contact with the wafer, showed that at all values of P and V, higher values of directivity correlated to more contacting pad asperities, a greater asperity density and a thinner micro hydrodynamic lubrication layer, thus highlighting the potential importance of directivity in future studies.

Study on the Mechanism of Nano-Flake Defect during Tungsten Contact Chemical Mechanical Polishing

Tungsten chemical mechanical polishing (CMP) has been used for metal interconnect formation in semiconductor manufacturing for a long time. Its application continues to the advanced device manufacturing with transistor scaling. However, the defect concerns of the tungsten CMP process have become critical to manufacturing such as metal flake defects that can cause electrical shorts between contacts. Although there are many published studies on tungsten CMP fundamentals, process applications and CMP-induced defects, the mechanism of thin metal flake formation in nanoscale (nano-flake defect) has not been clearly known to industry. In this study, nano-flake defects are identified after tungsten contact CMP, and mechanism of its formation is explored. Significant CMP dishing at wide contact area removes all tungsten and open barrier metal. Friction force by 3-body interaction among wafer, abrasive particle and pad asperity breaks the adhesion of barrier material from intermetallic dielectric material. And it allows re-deposition of barrier metal to the wafer surface. Insufficient CMP in-situ cleaning results in residual nano-flake defects at the end of CMP process. Analysis of device macros, thickness of flake, cross sectional analysis by transmission electron microscope (TEM) and topographical analysis support this suggested mechanism.

Chemical/mechanical balance management through pad microstructure in CMP

CMP is a complex process that combines the synergistic effect of a chemical action performed by the slurry and a mechanical one done by the pad and the abrasive particles of the slurry. The pad plays a key role in the CMP process as it contributes to both chemical and mechanical actions. In this paper, we focused on the pad pores, which act on the two sides of the chemical/mechanical balance. Pads with different porosities were investigated. The effect of pad microstructure on CMP performance were studied. A 3D confocal microscope was used to analyze pad roughness parameters, height distributions and asperities properties. Silicon blanket wafers were polished using different pads and were then characterized in order to extract surface roughness, removal rate and defect level. The surface quality of the wafer post process presents a direct correlation with pad topography. A smaller pad pore size (i.e. smooth surface) shows a beneficial effect on the wafer roughness.We further discuss the effect of pore size on pad topography by studying asperities properties and volume available for slurry transportation. The consequences of pore characteristics are then correlated to CMP performance. This study allows us to understand how we can manage the chemical/mechanical actions through the pad microstructure.

Study on pad cutting rate and surface roughness in diamond dressing process

Diamond dressing process has an important role in the elimination of scrap materials on the pad surface and regeneration of the pad surface topography before and after Chemical Mechanical Planarization/Polishing (CMP) process for integrated circuit (IC) fabrication. During diamond dressing process, distributions of cutting locus and overlap cutting of diamond grits usually work as a datum surface for pad surface planarization and uniform pad asperities. This paper proposes a novel view to predicting the distribution of pad cutting rate (PCR) and surface roughness based on cutting length and overlap cutting locus. The geometrical model of diamond dressing on the pad has been built and CMP tool parameters are used for the simulation of cutting length and overlap distribution. The distribution of PCR and predict surface roughness is calculated on each pad zone. Simulation results presented a high PCR and roughness is observed in center zones. The verified experiment is in agreement with simulation results where high PCR and roughness is near the center zones, which cause the shape of the pad surface to be concave. The results of this study can be further applied for prediction and optimization of diamond dressing design and improvement of dressing process.

Chemical–mechanical wear of monocrystalline silicon by a single pad asperity

Chemical mechanical polishing (CMP) processes have been widely used in many fields with the ability to obtain an ultra-smooth surface. However, a comprehensive understanding of the material removal mechanisms at single pad asperity scale is still lacking, where a large number of abrasive particles are entrapped in the pad asperity/wafer microcontact area and then participate into polishing. In this study, two different pad asperity-scale material removal models are derived based on the indentation-sliding mechanism and chemical bond removal mechanism, respectively. Furthermore, series of pad asperity scale polishing tests are conducted on monocrystalline silicon wafer surface by using a polyoxymethylene (Pom) ball to mimic a single pad asperity. The results show that under the asperity-scale, material removal is highly related to the chemical reaction time between sequential asperity-wafer interactions, indicating the chemical control of the removal rate by controlling the reacted layer thickness. In particular, it is found that the reacted layer thickness follows the diffusion equation, and atoms within not only the topmost surface layer, but also the next or deeper layer can participate in the chemical reaction. Material removal behavior can be well explained by the dynamic formation and breakage of the interfacial chemical bonds between the Si atoms and SiO2 particles, rather than the indentation-sliding mechanism. It is further confirmed that no damage, such as lattice distortion or dislocation, is found in the subsurface of a wafer by the high-resolution transmission electron microscopy (HRTEM). This study provides new insights into the material removal mechanisms in CMP at an asperity-scale.

Editors' Choice—Efficiency of a CMP Pad at Removing Protective Material from Copper during CMP

The efficiency of a pad asperity and abrasives embedded between the asperity and wafer at removing the protective material on the surface of copper (removal efficiency) during chemical mechanical planarization (CMP) was determined experimentally using current densities from in situ electrochemical measurements while polishing with a slurry containing BTA. The removal efficiency was insentitive to the pressure and sliding velocity, but was dictated by the pad surface topography parameters and abrasive concentration in the slurry. An analytical estimate was derived by comparing the trajectories of a pad asperity and the abrasives embedded in the asperity. Comparison of the experimental and analytical estimate suggests that the asperities are deformed enough by the embedded abrasives to contact the surface of copper at abrasive concentrations up to 1 wt%. At higher concentrations up to 5 wt%, the asperities deflected to a lesser amount, making the force exerted on the copper increase. At these higher concentrations, some of the copper interacting with the squeezed abrasives was plastically deformed, yielding higher removal efficiency than when elastically deformed. The removal efficiency can be used as a standard metric for assessing the material removal ability of various consumables such as slurry, CMP pad and pad conditioner.

研磨パッド・アスペリティ内スラリー流れの可視化方法の検討

研磨パッド・アスペリティ内スラリー流れの可視化方法の検討

Effect of conditioner load on the polishing pad surface during chemical mechanical planarization process

During the Chemical mechanical planarization (CMP), the pad conditioning process can affect the pad surface characteristics. Among many CMP process parameters, the improper applied load on the conditioner arm may have adverse effects on the polyurethane pad. In this work, we evaluated the pad surface properties under the various conditioner arm applied during pad conditioning process. The conditioning pads were evaluated for surface topography, surface roughness parameters such as Rt and Rvk and Material removal rate (MRR) and within-wafer non-uniformity after wafer polishing. We observed that, the pad asperities were collapsed in the direction of conditioner rotation and blocks the pad pores applied conditioner load. The Rvk value and MRR were founded to be in relation with 4 > 1 > 7 kgF conditioner load. Hence, this study shows that, 4 kgF applied load by conditioner is most suitable for the pad conditioning during CMP.

Immersion Metrology for CMP Pad Monitoring

Selectivity between materials being polished over the lifetime of the CMP consumable set will typically change. Changes in selectivity overtime are varied and complex, but one main contributor identified in this study using Sensofar immersion metrology, is the changing state of the polishing pad asperities overtime (aka Glazing). This study (of a damascene process) demonstrated; --> Successful monitoring of the same “wet” polishing pad while on the platen. --> Identification of multiple points in usage when glazing occurred --> Characterization of successful interventions to reverse the glazing effect --> Successful return to target selectivity --> Longer consumable lifetime --> Reduced manufacturing downtime (MTTR). Figure 1

Mathematical modeling of material removal rate in roll-type linear CMP (roll-CMP) process: Effect of polishing pad

Recently, a roll-type linear chemical mechanical polishing (Roll-CMP) process was developed for fabricating large flexible substrates such as flexible printed circuit boards (FPCBs). The major difference between the Roll-CMP and the conventional CMP is the type of contact between the polishing pad and the substrate. Roll-CMP uses line contact for material removal of the Cu film on FPCBs. Many researchers have studied mathematical models to understand the conventional CMP process. In this paper, a mathematical model on the material removal rate (MRR) of Roll-CMP is proposed based on Hertzian contact theory and previously studied models on conventional CMP to understand the effect of the polishing pad. Two kinds of polishing pads were prepared to investigate their effect on the material removal of copper clad laminate (CCL). The increase in the average radius of pad asperities over the standard deviation of pad asperities increases MRR. The slurry loading capacity of the polishing pad impacts the MRR of Cu as well. The proposed model may offer a theoretical understanding for the Roll-CMP process.

Feasibility of a Real-Time Method in Determine the Extent of Pad Break-In During Copper Chemical Mechanical Planarization

Material removal rate, shear force and variance of shear force during copper polishing are studied as a function of pad break-in time. Results show that copper removal rate and variance of shear force increase within 30 min of pad break-in. Spectral analysis of raw shear force data is employed to help elucidate the fundamental physical phenomena during copper chemical mechanical planarization. Fast Fourier transform is performed to convert the shear force data from time domain to frequency domain. The energy distribution of copper polishing is quantified which sheds light on the effect of pad break-in time. Spectral analysis indicates that as pad break-in progresses, newly formed pad asperities dominate the interaction with the wafer and therefore shifting the spectral amplitudes to the higher frequency peaks (i.e. 101–108 Hz). Further spectral analysis also confirms that such increases in the high-frequency range dominate the distribution of energies. This study shows that a combination of unique spectral fingerprinting and analysis of force variance can be used to monitor the progress of pad break-in in real-time. This work underscores the importance of real-time detection and non-destructive method in chemical mechanical planarization process to quantify the extent of pad break-in.

Mechanism and Simulation of Removal Rate and Surface Roughness During Optical Polishing of Glasses

Glass optics with ultra‐low roughness surfaces (<2 Å rms) are strongly desired for high‐end optical applications (e.g., lasers, spectroscopy, etc.). The complex microscopic interactions that occur between slurry particles and the glass workpiece during optical polishing ultimately determine the removal rate and resulting surface roughness of the workpiece. In this study, a comprehensive set of 100 mm diameter glass samples (fused silica, phosphate, and borosilicate) were polished using various slurry particle size distributions (PSD), slurry concentrations, and pad treatments. The removal rate and surface roughness of the glasses were characterized using white light interferometry and atomic force microscopy. The material removal mechanism for a given slurry particle is proposed to occur via nano‐plastic deformation (plastic removal) or via chemical reaction (molecular removal) depending on the slurry particle load on the glass surface. Using an expanded Hertzian contact model, called the Ensemble Hertzian Multi‐gap (EHMG) model, a platform has been developed to understand the microscopic interface interactions and to predict trends of the removal rate and surface roughness for a variety of polishing parameters. The EHMG model is based on multiple Hertzian contacts of slurry particles at the workpiece–pad interface in which the pad deflection and the effective interface gap at each pad asperity height are determined. Using this, the interface contact area and each particle's penetration, load, and contact zone are determined which are used to calculate the material removal rate and simulate the surface roughness. Each of the key polishing variables investigated is shown to affect the material removal rate, whose changes are dominated by very different microscopic interactions. Slurry PSD impacts the load per particle distribution and the fraction of particles removing material by plastic removal. The slurry concentration impacts the areal number density of particles and fraction of load on particles versus pad. The pad topography impacts the fraction of pad area making contact with the workpiece. The glass composition predominantly impacts the depth of plastic removal. Also, the results show that the dominant factor controlling surface roughness is the slurry PSD followed by the glass material's removal function and the pad topography. The model compares well with the experimental data over a variety of polishing conditions for both removal rate and roughness and can be extended to provide insights and strategies to develop practical, economic processes for obtaining ultra‐low roughness surfaces while simultaneously maintaining high material removal rates.

A Chemical Mechanical Planarization Model Including Global Pressure Distribution and Feature Size Effects

In this paper, we propose a chip-scale Chemical mechanical polishing (CMP) model based on the Greenwood–Williamson theory combining the effects of both the pad bulk deformation and the feature size, which are mainly responsible for global pressure distribution and feature-scale removal rate variation, respectively. A reference camber is first constructed to simulate the pad bulk deformation, which can be directly introduced into the global pressure distribution computation for incorporating the long-range characteristics of chip profiles. Then, both the effective curvature and the modified distribution function of pad asperity height are involved in the material removal rate. The model is first verified through some experimental data from CMP test structures and found to be in good agreement with the test data. For further validating the rationality and accuracy of the present model, some simulations are compared with our experimental data and discussed in detail.