Pad Asperities Research Articles

(Dielectric Science and Technology Division Poster Award, 1st Place) The Influence of Pad Micro-Features on Chemical Mechanical Polishing (CMP) Performance: Insights from Computational Fluid Dynamics (CFD)

With semiconductor chip dimensions shrinking below 7 nanometers, Chemical Mechanical Planarization (CMP) becomes more critical [1], [2]. To meet the increasing demand for precision and uniformity in the fabrication of wafers and chips, enhancements in CMP consumables and operating conditions are necessary. To address these requirements, novel pad designs have been developed, particularly those that do not require conditioning [3]. It is known that the CMP polishing rate depends on pressure, pad wafer relative velocity, and the distribution of pad asperities [4]. As conventional pads provide small and random contact areas, the external pressure applied by the head determines the material removal rate (MRR) [5] Advanced pads with micro-features provide more contact areas and help deliver consistent pressure and slurry distribution during Chemical Mechanical Polishing (CMP) [6], [7]. This study uses computational fluid dynamics (CFD) to simulate the flow between pad and wafer surfaces to determine how pad surface micro-features affect the CMP process. The abrasive residence time, total fluid pressure, and slurry flow velocity are evaluated for four distinct pad micro-feature geometries: square, square-circle, circle, and ellipse. Increasing the number of edges in the pad micro-feature patterns increases pressure drop, leading to more interaction between pad and slurry and enhancement of MRR. The findings of this study confirm the effectiveness of micro-featured pads in optimizing CMP processes and provide valuable insights into the influence of microfeature geometries on critical parameters for improving the semiconductor fabrication process.[1] J. Seo, Journal of Materials Research, 36, 235–257, Jan, 2021.[2] B. Suryadevara, Advances in Chemical Mechanical Planarization (CMP). Woodhead Publishing, 2021.[3] S. Lee and E. Hwang, “Smart pad fabrication for CMP,” LMA Res Rep, vol. 10, pp. 100–104, 2002.[4] G. Ahmadi and X. Xia, J Electrochem Soc, 148, G99, 2001.[5] X. Xia and G. Ahmadi, Particulate Science and Technology, 20, 187–196, 2002.[6] S. Lee, G. Kim, M. Mcgahay, H. Kim, and H. Cho, “Pad Designs-To navigate the fundamentals of CMP.”[7] M. Mcgahay, G. Kim, S. Lee, H. Cho, and H. Kim, “Critical Feature Size of Polishing Pad and Its Performance.”

Novel Probability Density Function of Pad Asperity by Wear Effect over Time in Chemical Mechanical Planarization.

Chemical mechanical planarization (CMP) reduces film thickness, eliminates step height, and achieves high levels of planarity in semiconductor manufacturing. However, research into its mechanisms is still in progress, and there are many issues to be resolved. To solve problems in CMP, it is necessary to understand the contact phenomenon that occurs at the pad-wafer interface, especially pad asperity. Moreover, understanding the non-uniform distribution of pad asperity, such as height and radius, is essential for predicting the material removal rate (MRR). In this study, based on the existing Greenwood-Williamson (GW) theory and probability density function (PDF), a modified mathematical model that includes changes in asperity distribution was developed and validated experimentally. The contact model proposed in this study included functions that calculated the time-dependent height and radius wear of the pad asperities. Specifically, the experimentally obtained values were compared with the values obtained by the model, and the comparison results were analyzed. Thereby, it was found that the contact model and MRR model considering the change in asperity wear and distribution due to CMP proposed in this study are in better agreement with the experimental results than the existing model, which shows that the MRR can be predicted by a mathematical model using the change in asperity distribution.

Effect of Viscoelastic Characteristics on the Real Contact Area of Polishing Pad Surface

Real contact area (RCA) between polishing pad and workpiece surface is one of the most important parameters indicating the mechanical action strength of chemical mechanical polishing (CMP), which has a dominated effect on the material removal. However, the effect of pad viscoelastic characteristics on RCA is not clear. In this study, a contact status measurement device that can apply cyclic load and record contact images is developed to study the viscoelastic behavior of the pad and its influence on RCA. The results show that when the pad undergoes cyclic compression load during CMP, the pad asperity layer gradually accumulates viscoelastic deformation and the RCA increases obviously, which can be mostly recovered after a long time. In particular, the accumulation and recovery of viscoelastic deformation lead to a significant change of the relationship between RCA and pressure, from linear to nonlinear and then to linear. Furthermore, an RCA model is established based on the viscoelastic constitutive model of pad and the mathematical relationship between RCA and pad deformation to explain the influence of pad viscoelastic behavior on RCA. This study is expected to provide new insights into RCA, and to give support for predictive control of the material removal during CMP.

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.

Experimental In-Situ Observatory on Brownian Motion Behavior of 105 nm Sized Silica Particles During Chemical Mechanical Polishing of 4H-SiC by an Evanescent Field

The experimentally observing optical systems for on-machine measurement have been developed to study on nano-polishing phenomena during the chemical mechanical polishing process, which is a wet process in semiconductor manufacturing. The developed optical system employs an evanescent field to selectively enhance exclusively the observatory of phenomena occurring on the surface being polished, offering a lateral resolving power of approximately 400 nm, in the slurry concentration of up to 5 wt% based on the numerical aperture of the objective lens. In addition, there is also the observability of 105 nm and down to 55 nm-sized silica particles without requiring additive fluorescence agents in or around the nano-particles, even when these particles are moving on surfaces such as silica glass or hard materials (silicon carbide: 4H-SiC). Consequently, the motion behavior of nano-particles disjoining with polishing pad asperity was explored and discussed, in this paper. Experimental results revealed that the polishing pad spatially constrains the movement of particles between the pad and the substrate surface, guiding them toward the surface being polished. During pad sliding, fluidically dragged nano-particles exhibit slower movement than the polishing pad sliding speed while retaining the Brownian motion. Furthermore, 105 nm-sized silica particles did not continuously approach to attach onto the SiC surface; the nano-particles approached in steps with reduced Brownian motion in all directions before attaching. This behavior can be attributed to the effects of van der Waals attraction and electrostatic repulsion forces between the particle and the substrate surfaces.

Planarization profile modeling using pad bulk deformation and real contact area analysis of macro-scale device pattern

In general, the shape of the polished pattern is not flat but has a rounded curved profile. Unlike micro-patterns that have similar scales to pad asperities, macro-patterns have a very large scale compared to asperities, so bulk deformation must also be considered. To derive the pad bulk deformation, real contact area (RCA) measurements were performed in this study. Based on the preceding contact model, a semi-empirical model for RCA and bulk deformation was derived. After including the constructed pad bulk deformation function in the existing Greenwood–Williamson model, a new model that can derive the material removal rate profile according to the position in the pattern was presented. Unlike the general upper and lower removal rate behavior, this model shows a unique behavior. At the beginning of polishing, the edge shows a higher removal rate than the center, but after a specific step height, the center has a higher removal rate due to the curved shape. Analysis was performed for comparison between the proposed model equation and the experimental value. When the polishing profile in the 10 mm pattern was compared with the previous model and the proposed model, respectively, the existing model did not predict the removal rate distribution according to the position in the pattern. However, this model has the advantage of predicting both over-polishing at the edge of the pattern and high removal rate at the bottom. In addition, it was confirmed through simulation that the 4 mm and 2 mm patterns had excellent matching properties.

Environmentally friendly buff cleaning of ceria nanoparticles using bubbles in gas-dissolved water

Buff cleaning serves as a crucial preparatory step following chemical mechanical polishing, aimed at removing bulk particles from wafer surfaces present in slurries. Most cleaning solutions comprise a variety of complex chemical compounds. Conventional cleaning solutions typically consist of complex chemical compounds, including toxic or carcinogenic substances, which hinder wastewater recycling and exacerbate environmental impacts. Here, an eco-friendly buff-cleaning solution, oversaturated gas-dissolved water, is proposed for effective elimination of colloidal ceria nanoparticles. The growth of inherent bubbles that accompanies heating and shear in buff cleaning is associated with the particle-cleaning process and contributes to particle-removal efficiency (PRE). Various optical methods were employed to examine the size and quantity of bubbles, revealing nitrogen (N2) bubbles to be approximately 1 µm in diameter, while carbon dioxide (CO2) bubbles varied from 1 to 10 µm. Gas-dissolved water demonstrated improved PRE, with CO2 exhibiting superior performance in comparison to N2. Negatively charged bubbles were found to effectively capture positively charged ceria particles from oxide film surfaces, cleaning solutions, and pad asperity surfaces. Additionally, bubbles enhanced particle dispersion, as evidenced by a colloidal ceria slurry displaying an average size of 162.5 nm in water, 158.1 nm in N2 water, and 149.8 nm in CO2 water. Well-dispersed particles can easily be flushed from the pad-wafer gap to minimize the risk of recontamination.

Chemically-induced active micro-nano bubbles assisting chemical mechanical polishing: Modeling and experiments

The material loss caused by bubble collapse during the micro-nano bubbles auxiliary chemical mechanical polishing (CMP) process cannot be ignored. In this study, the material removal mechanism of cavitation in the polishing process was investigated in detail. Based on the mixed lubrication or thin film lubrication, bubble-wafer plastic deformation, spherical indentation theory, Johnson-Cook (J-C) constitutive model, and the assumption of periodic distribution of pad asperities, a new model suitable for micro-nano bubble auxiliary material removal in CMP was developed. The model integrates many parameters, including the reactant concentration, wafer hardness, polishing pad roughness, strain hardening, strain rate, micro-jet radius, and bubble radius. The model reflects the influence of active bubbles on material removal. A new and simple chemical reaction method was used to form a controllable number of micro-nano bubbles during the polishing process to assist in polishing silicon oxide wafers. The experimental results show that micro-nano bubbles can greatly increase the material removal rate (MRR) by about 400% and result in a lower surface roughness of 0.17 nm. The experimental results are consistent with the established model. In the process of verifying the model, a better understanding of the material removal mechanism involved in micro-nano bubbles in CMP was obtained.

The mechanical effect of soft pad on copper chemical mechanical polishing

Smaller transistor size challenges traditional chemical mechanical polishing (CMP) in semiconductor fabrication. Novel consumables and in-depth mechanism research can support defect mitigation and improve yield. This study focused on soft pad polishing of copper film from the perspective of mechanical effects. To reduce the defects, a soft pad is replacing the hard pad in the barrier layer polishing and over-polishing steps of copper CMP. When the contact behavior of pad asperity and wafer was in the elastic-plastic regime, the exponential influence factors of pressure, abrasive concentration, and sliding velocity on the material removal rate (MRR) were around 0.64, 0.22, and 0.64, respectively. Stick-slip friction with squeal noise scratched the copper film at the beginning and end of the CMP process. The pad conditioning removed polishing residues but deflected the pad asperity. Proper conditioning frequency is necessary to optimize the polishing process.

Recent Advances In Silicon Carbide Chemical Mechanical Polishing Technologies.

Chemical mechanical polishing (CMP) is a well-known technology that can produce surfaces with outstanding global planarization without subsurface damage. A good CMP process for Silicon Carbide (SiC) requires a balanced interaction between SiC surface oxidation and the oxide layer removal. The oxidants in the CMP slurry control the surface oxidation efficiency, while the polishing mechanical force comes from the abrasive particles in the CMP slurry and the pad asperity, which is attributed to the unique pad structure and diamond conditioning. To date, to obtain a high-quality as-CMP SiC wafer, the material removal rate (MRR) of SiC is only a few micrometers per hour, which leads to significantly high operation costs. In comparison, conventional Si CMP has the MRR of a few micrometers per minute. To increase the MRR, improving the oxidation efficiency of SiC is essential. The higher oxidation efficiency enables the higher mechanical forces, leading to a higher MRR with better surface quality. However, the disparity on the Si-face and C-face surfaces of 4H- or 6H-SiC wafers greatly increases the CMP design complexity. On the other hand, integrating hybrid energies into the CMP system has proven to be an effective approach to enhance oxidation efficiency. In this review paper, the SiC wafering steps and their purposes are discussed. A comparison among the three configurations of SiC CMP currently used in the industry is made. Moreover, recent advances in CMP and hybrid CMP technologies, such as Tribo-CMP, electro-CMP (ECMP), Fenton-ECMP, ultrasonic-ECMP, photocatalytic CMP (PCMP), sulfate-PCMP, gas-PCMP and Fenton-PCMP are reviewed, with emphasis on their oxidation behaviors and polishing performance. Finally, we raise the importance of post-CMP cleaning and make a summary of the various SiC CMP technologies discussed in this work.

Modeling the microscale contact status in chemical mechanical polishing process

The micro-scale contact status between pad and workpiece surface plays one of the most significant roles in determining the material removal rate (MRR) during the chemical mechanical polishing (CMP) process. To evaluate the contribution of random contact status to the material removal process, the paper proposed a novel concept of “effective contact spots”. It represents the probability of randomly distributed pad asperities sliding across a given coordinate point on the workpiece surface. Furthermore, mechanical removal capacity parameter γ and chemical reaction capacity parameter β were defined to mathematically model the material removal process from a micro-scale chemical-mechanical synergistic perspective. To validate the model, a series of CMP tests (with in-situ conditioning) were conducted on three types of materials (BK7 glass, fused silica, and Si). The real contact status was also measured and analyzed statistically after being conditioned with two types of conditioners. Experimental observations and theoretical analysis results indicated that the chemical-mechanical synergism was the fundamental mechanism for the contribution of the micro-scale contact status to the material removal process. Resulting in the fact that the real contact ratio, radius, and spacing of the real contact spots play different dominant roles on MRR for different materials. Moreover, solution strategies in this study can provide the knowledge needed for improving the material removal stability, and further enable predictive control on the polishing or conditioning recipe before a non-ideal contact status result in an unexpected material removal rate.

Analyzing the effects of pad asperity on chemical mechanical polishing of copper thin film wafer

Copper thin film has been identified as a promising material for multi-level interconnecting materials in IC fabrication due to its unique electrical properties and high electrical conductivity. However, it is very challenging to manufacture a superior planar surface with low dishing and minimal erosion of copper thin film in the chemical mechanical polishing (CMP) process. In this study, the micro-topography model of a soft polishing pad is performed using an X-ray micro-computed tomography scan combined with finite element method simulation to evaluate the contact area between the CMP pad and wafer during the CMP process. Additionally, the material removal rate (MRR) model for the copper wafer is calibrated based on the wear of the material between abrasive particles and the wafer surface. The results of this study not only investigate the effect of CMP pad asperities during the CMP process but also provide a new method for fully calibrating MRR in comparison with CMP experiments for the verification of model parameters.

Investigation of abrasive behavior between pad asperity and oxide thin film in chemical mechanical planarization

In this paper, tribological effects on the material removal rate (MRR) are investigated using two types of slurry abrasives (i.e., ceria and silica) and three types of pads. From a micro to macro viewpoint, the physico-chemical interaction between the pad-wafer interface and abrasive was studied based on multi-asperity contact. First, the relative contact pressure in high or low proportions among all asperities was calculated using a contact mechanics model. The results indicate that for the commercial slurry used in the semiconductor industry, relatively high contact pressure due to the asperities can be transferred to the ceria abrasives, and only a low contact pressure can be supported by these silica abrasives. Furthermore, it was confirmed that the two different slurry abrasives showed opposite behaviors according to the real contact area of the pad asperities. That is, as the pad texture changed from ‘Desired pad’ to ‘Smoothed pad’, the MRR of the ceria increased by 47%, while the MRR of the silica decreased 59%. Lastly, the Stribeck curve, which is suitable for the CMP process, was re-analyzed to clarify the abrasion mechanism under the pad asperity scale. The results showed that two-body abrasion may be maintained for the ceria CMP, and transition is possible from two-body to three-body abrasion at the silica CMP as the contact area increases. The present paper is expected to clarify the tribological mechanism of the oxide CMP and can provide further insight into optimizing and selecting consumables such as conditioner, pad, and slurry.

Analysis of Correlation between Pad Temperature and Asperity Angle in Chemical Mechanical Planarization

Chemical mechanical planarization (CMP) is a technology widely employed in device integration and planarization processes used in semiconductor fabrication. In CMP, the polishing pad plays a key role both mechanically and chemically. The surface of the pad, consisting of asperities and pores, undergoes repeated cycles of glazing induced by polishing followed by the recovery of roughness by a conditioning process applied during CMP. As a polymer material, the pad also experiences thermal expansion from changes in temperature. Such changes can be expressed in terms of surface roughness values, but these do not fully capture the actual changes to the pad surface. In this study, the change in pad temperature occurring during CMP was analyzed with regard to its effect on the asperity angle, and the influence on CMP outcome was assessed. The changes in the surface asperities according to the steady-state pad temperature were evaluated using various measurement methods. The change in pad roughness was characterized in terms of the asperity angle, and the contact state predicted according to temperature were validated by measuring the contact perimeter, the number of contact points, and related values. Through Scanning Electron Microscope (SEM) and micro-CT analysis, it was confirmed that in the continuous polishing process and the conditioning process, the changes in asperity angle due to changes in pad temperature affect the polishing outcome.

Mechanical removal of SiC by multi-abrasive particles in fixed abrasive polishing using molecular dynamics simulation

Molecular dynamics (MD) as a powerful simulation tool revealed the mechanistic removal behaviors of SiC substrates using a fixed abrasive polishing (FAP) with randomly distributed multi-abrasive particles in a single cycle scratching at nano-scales. This study presents features of surface morphology, subsurface damage and temperature distribution of SiC substrates in nano-abrading. It is demonstrated that the exposed height and abrasive distribution of multi abrasives in a single pad asperity (SPA) dominates the removal behaviors of SiC substrates. MD simulation reveals that the random distribution of diamond abrasives in FAP pad would worsen the processing quality. Our investigation sheds new insights into the mechanical removal mechanisms of SiC in FAP at an asperity-scale.

CMP 패드의 돌기 변형에 의한 브레이크 인 메커니즘 규명

Chemical Mechanical Planarization (CMP) is an essential process for flattening the surface of the wafer to produce a fine structure. The CMP process is performed after a break-in step prior to optimizing the polishing pad. Break-in consists of the conditioning step and warming-up step. In the conditioning step, a conditioner embedded with diamonds is used to remove residues from the pad surface and manages the directionality and height deviation of asperities on the surface. The warming-up step serves to increase the temperature of the pad surface by polishing multiple wafers. The temperature in the warming-up step is raised due to friction between the wafer and pad, and the pad state is divided into a partly warmed up section, a transition section, and a fully warmed up section of the pad. In this study, as the wafer pressure increased in the warm-up stage, the time for the pad to reach the stable section was confirmed, and the break-in mechanism was analyzed in terms of surface characteristics and mechanical properties, such as surface photograph, surface roughness of the pad, and elastic modulus of pad asperities. Based on these results, the break-in mechanism that increases the material removal rate was analyzed.

Mechanical Abrasion by Bi-layered Pad Micro-Asperity in Chemical Mechanical Polishing

Pad asperities in chemical-mechanical polishing (CMP) provide necessary forces for mechanical abrasion. This article investigates the abrasive behaviour of polishing pads at the asperity contact scale. A contact mechanics model predicts that compliant and soft asperities or rigid and hard asperities may solely achieve either large contact area or high indentation depth respectively, whereas bi-layered asperities can enable both the enlarged contact and deep abrasion. Hemispherical pad micro-asperities with precise dimensions, including the new bi-layered design, were fabricated using thermal reflow and micro-replica molding techniques and their polishing behaviours were experimentally compared using a pin-on-disk polishing setup.

Micro-scale contact behavior and its effect on the material removal process during chemical mechanical polishing

A real-time coefficient of friction (COF) analysis method, in conjunction with an in-situ contact area measurement method, is used to determine the frictional and material removal behaviors induced by the micro-scale pad asperity. The results indicate that the material removal rate (MRR) shows no correlation with the average COF, but shows a strong relationship with the fluctuation of the friction force, i.e. the stick-slip phenomenon. Moreover, a waviness topography is observed on the polished surface, which is further verified to be raised from the stick-slip phenomenon. Futhermore, a micro-asperity scale model is established to explain the underlying mechanism of stick-slip phenomenon. The results provide some new insights into the material removal mechanisms and the real-time monitoring technology for the CMP process.

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.

Synergistic Effect of Pad “Macroporous-Reactors” on Passivation Mechanisms to Modulate Cu Chemical Mechanical Planarization (CMP) Performance

Decoupling the key interfacial mechanisms (chemical and mechanical) present during Cu CMP is critical to the development of slurry/pad consumable sets to reduce defectivity at advanced technology nodes. Understanding the Prestonian relationship, or lack thereof, can give rise to correlations between film density as a result of passivation film kinetics and thermodynamics as they relate to Cu oxidation/electrochemistry under dynamic conditions. The efficiency of film removability is strongly correlated to the molecular structure of the passivating agent and its synergistic relationship with the macroporous-reactor sites presented in this work. Results indicate that passivation film activation energy (Ea) is altered by the transport of fresh and waste slurry chemistry to the Cu interface via pad asperity contact. Furthermore, this work employs inhibitors with varying structural attributes to probe how the density of film formation is impacted by the efficiency of complexation and non-covalent interactions at the Cu surface. When comparing the best-in-class benzotriazole (BTA) with salicylhydroxamic acid (SHA), the triazole film formation is driven by a traditional complexation/π-stacking mechanism, while the hydroxamic acid film is the result of a colloidal supramolecular complex and soft surface-adsorption requiring reduced downforce for Cu removal.