Chemical Mechanical Polishing Research Articles

High precision polishing of aluminum alloy mirrors through a combination of magnetorheological finishing and chemical mechanical polishing

After the aluminum alloy mirror machined by single point diamond turning (SPDT), the residual tool marks and surface accuracy of the aluminum alloy mirror cannot meet the requirements of visible or ultraviolet light system. In this study, a processing method combining magnetorheological finishing (MRF) and chemical mechanical polishing (CMP) is proposed to realize the polishing of aluminum alloy mirrors with high efficiency, high precision and high-quality. Firstly, the properties and composition of passivation layer after MRF were analyzed and the polishing performance of acidic, neutral and alkaline alumina polishing fluid on passivation layer were investigated based on the computer numerical control (CNC) polishing equipment. Based on the experimental results, a new acidic nano-silica polishing fluid which is suitable for the efficient and high-quality removal of passivation layers on aluminum alloy surfaces was developed. Finally, a combined approach of MRF-CMP was used to the directly polishing of a rapidly solidified aluminum mirror (RSA-6061) with a diameter of 100 mm after SPDT. With two iterative of MRF-CMP polishing in 220 minutes, the surface accuracy of the aluminum alloy mirror was improved from 0.1λ (λ=632.8 nm) to 0.024λ, and the surface roughness (Ra) decreased from 3.6 nm to 1.38 nm. The experiment results manifest that high precision, and high-quality aluminum alloy mirror can be achieved by MRF-CMP method with the new developed acid nano-silica polishing fluid and suitable MR polishing fluid. The research results will provide a new strategy for ultra-precision direct polishing of aluminum alloy mirrors and will also give the important technical support for the extensive use of aluminum alloy mirror in visible light and ultraviolet optical systems.

Homoepitaxial Regrowth of AlGaN on AlGaN Templates Prepared via Chemical Mechanical Polishing and Its Application to UV‐B Laser Diodes

In this article, the homoepitaxial regrowth of AlGaN on AlGaN templates fabricated via chemical mechanical polishing (CMP) on periodically aligned AlN nanopillars is reported on. These templates are lattice relaxed according to X‐Ray diffraction reciprocal space mapping measurements, with a low dislocation density of ≈3 × 108 cm−2. However, the AlGaN surface has hillocks exceeding 104 cm−2. Contrastingly, no hillocks are observed on the homoepitaxially regrown AlGaN on the CMP–AlGaN template and after the UV‐B laser diode (LD) layer structure is crystallized and grown on the homoepitaxially regrown AlGaN. Furthermore, when this wafer is processed for LD fabrication, room‐temperature pulsed oscillation is confirmed in the UV‐B region at a wavelength of 296 nm. The turn‐on voltage and operating voltage also reduce by ≈2 V.

Influence of Cerium Oxide Abrasive Particle Morphologies on Polishing Performance

AbstractIn this study, two morphologies of cerium oxide (CeO2) abrasive particles, octahedral and spheroidal, are synthesized by solvothermal method using cerium nitrate hexahydrate (Ce(NO3)3‐6H2O) as raw material. Fourier infrared spectroscopy, X‐ray diffractometer (XRD) and scanning electron microscopy (SEM) are used to characterize the composition and morphology of CeO2 abrasive particles. The synthesized CeO2 is used for chemical mechanical polishing (CMP) of the Si surface of 6H‐SiC wafers, and the surface morphology of the polished wafers are observed using atomic force microscopy (AFM). After polishing with octahedral and spheroidal abrasive particles, the surface roughness of the wafers are 0.327 and 0.287 nm, and the material removal rates (MRR) are 870 and 742 nm h−1, respectively. Calculations comparing the ultraviolet absorption spectra and bandgap energies of the two types of abrasive particles as well as molecular dynamics (MD) simulations reveal that the synthesized octahedral CeO2 particles possessed stronger surface chemical activity and material removal performance.

Effect of the Post‐Annealing Temperature on the Interfacial Reaction Between a High TC Superconductor and Topological Insulator

AbstractThis work reports the formation of a heterojunction composed of Tl2Ba2CaCu2O8 (Tl‐2212) and Bi2Se3 as the high critical temperature superconductor and topological insulator, respectively. The Bi2Se3 film is deposited on the Tl‐2212 by a chemical mechanical polishing process. Due to the rough surface of Tl‐2212, the treated Tl‐2212 is then subjected to post‐annealing at various temperatures. This temperature does not affect the formation of the interfacial layer at the heterojunction; however, an improved layered crystal structure of the Bi2Se3 film is observed at 500 °C. The in‐depth profile of the relative atomic concentration indicates the diffusion of Cu and Tl into the Bi2Se3 film with the Ba diffusion being closer to the interfacial layer. At 500 °C, Cu atoms are more actively diffused into the Bi2Se3 film, thus blocking Tl diffusion, and improving the layered crystal structure of the film. The density functional theory calculations reveal that the lowest formation energy of the Cu vacancy is when Bi2Se3 is in contact with Tl‐2212, which results in the diffusion of Cu atoms from Tl‐2212, thus transforming the interstitial Cu atoms into the film. These atoms have an n‐type doping effect and prefer intercalating at a vdW gap deep inside the layered Bi2Se3.

Surface microtopography evolution of monocrystalline silicon in chemical mechanical polishing

Chemical mechanical polishing (CMP) is a widely utilized technique for achieving ultra-smooth surfaces, yet a comprehensive understanding of the surface microtopography evolution remains elusive due to the intricate chemical and mechanical interactions inherent in chemical mechanical polishing. Accordingly, this study explores the evolution law of different spatial frequency features of surface microtopography of monocrystalline silicon by analyzing the changes in the power spectral density (PSD) curve. More importantly, the evolution mechanism of the chemical mechanical polishing surface micromorphology is revealed through the analysis of the power spectral density under different mechanical and chemical conditions. The results emphasize the noticeable impact of both mechanical and chemical actions on the evolution of different spatial frequency features in surface microtopography. The mechanical action of a large particle, less likely to contact the valley region with a small curvature radius, enhances the smoothing speed of high-spatial frequency roughness. Moreover, the mechanical action of polishing pad demonstrates a greater efficiency in smoothing mid-spatial frequency roughness when utilizing a hard pad. This is attributed to the limited impact of pad hardness on the removal depth of single particles within the reaction layer, rendering it less significant for high-spatial frequency roughness. Upon elevating the chemical reaction rate, a notable acceleration in the reduction of roughness in both mid- and high-spatial frequency regions is observed. Furthermore, insights derived from molecular dynamics (MD) simulations of monocrystalline silicon and quartz glass show that the manner of surface atom removal, whether through single atom removal or cluster removal, exerts a significant effect on the final high-spatial frequency components of the power spectral density. This study contributes valuable insights into the intricate formation mechanism of a smooth surface in chemical mechanical polishing.

Cathodoluminescence investigations of dark-line defects in platelet-based InGaN nano-LED structures

We have investigated the optical properties of heterostructured InGaN platelets aiming at red emission, intended for use as nano-scaled light-emitting diodes. The focus is on the presence of non-radiative emission in the form of dark line defects. We have performed the study using hyperspectral cathodoluminescence imaging. The platelets were grown on a template consisting of InGaN pyramids, flattened by chemical mechanical polishing. These templates are defect free, whereas the dark line defects are introduced in the lower barrier and tend to propagate through all the subsequent layers, as revealed by the imaging of different layers in the structure. We conclude that the dark line defects are caused by stacking mismatch boundaries introduced by multiple seeding and step bunching at the edges of the as-polished, dome shaped templates. To avoid these defects, we suggest that the starting material must be flat rather than dome shaped.

Spectral stability of V2 centres in sub-micron 4H-SiC membranes

Colour centres in silicon carbide emerge as a promising semiconductor quantum technology platform with excellent spin-optical coherences. However, recent efforts towards maximising the photonic efficiency via integration into nanophotonic structures proved to be challenging due to reduced spectral stabilities. Here, we provide a large-scale systematic investigation on silicon vacancy centres in thin silicon carbide membranes with thicknesses down to 0.25 μm. Our membrane fabrication process involves a combination of chemical mechanical polishing, reactive ion etching, and subsequent annealing. This leads to highly reproducible membranes with roughness values of 3–4 Å, as well as negligible surface fluorescence. We find that silicon vacancy centres show close-to lifetime limited optical linewidths with almost no signs of spectral wandering down to membrane thicknesses of ~0.7 μm. For silicon vacancy centres in thinner membranes down to 0.25 μm, we observe spectral wandering, however, optical linewidths remain below 200 MHz, which is compatible with spin-selective excitation schemes. Our work clearly shows that silicon vacancy centres can be integrated into sub-micron silicon carbide membranes, which opens the avenue towards obtaining the necessary improvements in photon extraction efficiency based on nanophotonic structuring.

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.

Material removal rate model for chemical–mechanical polishing of single-crystal SiC substrates using agglomerated diamond abrasive

Material removal mechanisms are essential for the selection of polishing slurries and parameter optimization of chemical–mechanical polishing (CMP) processes. This study established a material removal rate (MRR) model for CMP of single-crystal SiC substrates using a fixed agglomerated diamond (AD) abrasive pad (FADAP). The distribution of the polishing pressure between the AD abrasive and FADAP matrix was examined, and the influence of the abrasive and polishing parameters on the MRR was revealed. Furthermore, single-factor experiments were conducted to validate the rationality of the MRR model. The experiments demonstrated that when the thickness of the surface-affected layer of the single-crystal SiC substrate in the MRR model was set to 5 nm, the error between the calculated and experimental values of the model could be controlled to within 20%. This result experimentally verified the validity of the MRR model and its associated assumptions. Moreover, the MRR of the CMP process of a single-crystal SiC substrate reached 36.26 μm/h with a polishing pressure of 27.6 kPa and an AD abrasive primary particle size range of 7–10 μm. Therefore, the feasibility of efficiently processing single-crystal SiC substrates using an FADAP was theoretically confirmed.

Damascene versus subtractive line CMP process for resistive memory crossbars BEOL integration

In recent years, resistive memories have emerged as a pivotal advancement in the realm of electronics, offering numerous advantages in terms of energy efficiency, scalability, and non-volatility [1]. Characterized by their unique resistive switching behavior, these memories are well-suited for a variety of applications, ranging from high-density data storage to neuromorphic computing [2]. Their potential is further enhanced by their compatibility with advanced semiconductor processes, enabling seamless integration into modern electronic circuits [3]. A particularly promising avenue for resistive memory lies in its integration at the Back-End-of-Line (BEOL) stage of semiconductor manufacturing [4]. BEOL integration involves processes that occur after the fabrication of the transistors, primarily focusing on creating interconnections that electrically link these transistors. Integrating resistive memories at this stage can lead to compact, efficient, and high-performance architectures, pivotal for in-memory computing applications where data storage and processing are co-located [5]. This paper studies three ways to integrate TiOx-based resistive memory into passive crossbar array structures, using chemical mechanical polishing (CMP) processes, focusing on identifying the optimal integration techniques.

A Systematic Study of the Factors Affecting the Surface Quality of Chemically Vapor-Deposited Diamond during Chemical and Mechanical Polishing

Diamond surfaces must be of high quality for potential use in semiconductors, optical windows, and heat conductivity applications. However, due to the material’s exceptional hardness and chemical stability, it can be difficult to obtain a smooth surface on diamond. This study examines the parameters that can potentially influence the surface quality of chemically vapor-deposited (CVD) diamonds during the chemical and mechanical polishing (CMP) process. Analysis and experimental findings show that the surface quality of polished CVD diamonds is significantly influenced by the crystal structure and the growth quality of the diamond. In particular, when the surface roughness is below Ra 20 nm, the pores and grain boundaries on CVD diamond obstruct surface roughness reduction during mechanical polishing. To obtain a smooth polished surface, careful consideration of the size of diamond abrasives and polishing methods is also a prerequisite. Chemical mechanical polishing is a novel method to achieve a surface quality with roughness below Ra 3 nm, as in this method, the anisotropy of the CVD diamond allows the uneven steps to be efficiently erased. However, the chemical actions of polishing slurry should be controlled to prevent the formation of chemical etching pits.

Chemical-mechanical synergies effects of multi-purpose pH regulators on C-, A-, and R-plane sapphire polishing

The development of sapphire slurry follows the requirements of environmental friendliness, low cost and high efficiency. Therefore, the effects of KOH, aminomethyl propanol (AMP) and arginine (ARG) as pH regulators on the chemical mechanical polishing (CMP) performance of C-, A- and R-plane sapphire were investigated. The results showed that ARG was the most effective, which could increase the removal rates of C-, A- and R-plane sapphire to 5.65 µm/h, 2.80 µm/h and 3.59 µm/h, and reduce the surface roughness Sq to 0.194 nm, 0.161 nm and 0.173 nm, respectively. Mechanical action analysis, XPS, UV-Vis, and theoretical calculations revealed that ARG could improve the mechanical action of CMP and chemical action by acting as a complexing agent to complex with AlOH4−.

Investigation on the effect of chemical mechanical polishing sapphire substrate on chemical vapor deposition growth of two-dimensional MoS2

Two-dimensional molybdenum disulfide (2D MoS2), with atomic-level thickness, whose crystal crystallinity and size are easily influenced by the surface morphology of the substrate during growth. This work systematically investigates the impact of chemical mechanical polishing (CMP) sapphire substrate on chemical vapor deposition (CVD) growth of 2D MoS2. A green and highly efficient CMP method for the sapphire substrate is first proposed, whose maximum material removal rate (MRR) of 4.27 µm/h with surface roughness (Ra) of 0.10 nm (AFM, scanning area of 5 × 5 μm2) can be attained. The size and thickness statistics of 2D MoS2 domains grown on different substrates indicate that the CMP sapphire substrate in this work can promote the size and monolayer uniformity of 2D MoS2 during CVD growth. Meanwhile, Raman and photoluminescence (PL) spectra of monolayer MoS2 grown on different substrates also indicates the monolayer MoS2 grown on the CMP sapphire substrate in this work has superior crystalline quality and optical properties. The above findings have provided new insights into the mechanisms underlying 2D MoS2 growth on surfaces in various states and are expected to accelerate the development of large size large-size and high-quality growth of two-dimensional material for further device applications.

Characterization and formation mechanism of short step-bunching defects on 4H-SiC thick homoepitaxial films

Short Step-Bunching (SSB) are a kind of linear defects consisting of convex and concave undulations on the surface of 4H-SiC homoepitaxial layer. In this work, we report the characterizations of SSBs on 12 μm, 50 μm and 85 μm thick SiC epilayers by using optical microscopy (OM), micro-photoluminescence spectra (PL), atomic force microscopy (AFM), chemical mechanical polishing (CMP), molten KOH etching (10 min, 500 ℃) and high resolution transmission electron microscope (HRTEM). It is found that these SSBs are 200 ∼ 250 μm long, perpendicular to the step-flow [11–20] direction, leading to local rugged surfaces of epilayer. No other poly-types are identified on and around these SSBs. H2 etching the substrate reveals the existence of short-line defects on the substrate with one or more dislocations (TSD or TED) or an amorphous bump locating in center of most of them. Presumably, the amorphous bump was a kind of carbon inclusion. Based on the above results, we proposed that the SSBs develop from the short-line defects generated when etching the substrates prior to epilayer growth. Elimination of short-line defects by proper etching process is an effective route to reducing the SSBs in 4H-SiC homoepitaxial layers.

First-principles insight into the pH-dependency of the corrosion inhibition ability of benzotriazole and 1,2,4-triazole for copper in chemical mechanical polishing slurry

A comprehensive set of first-principles calculations was conducted to elucidate the disparity and pH-dependency of the corrosion inhibition performance of benzotriazole and 1,2,4-triazole in copper chemical mechanical polishing (CMP) slurry. Unlike previous calculations adopted the simplified and rigid model with neutral molecules adsorbed on metallic copper surface, the models, including Cu and Cu2O surfaces covered with O or -OH species, as well as neutral and deprotonated forms of inhibitor molecules, were constructed to simulate the adsorption interface based on the experimental results. The calculated adsorption energy successfully explained the pH-dependency of the inhibition ability of the two inhibitors.

Impact of Atomic Defects on Ceria Surfaces on Chemical Mechanical Polishing of Silica Glass Surfaces.

This study investigates the impact of atomic defects, such as oxygen vacancies and Ce3+ ions, on cerium oxide (ceria) surfaces during chemical mechanical polishing (CMP) for silica glass finishing. Using density functional theory (DFT) and reactive molecular dynamics simulations, the interaction of orthosilicic molecules and silica glass with dry and wet ceria surfaces is explored. Defects alter the surface reactivity, leading to the dissociation of orthosilicic acid on oxygen vacancies, forming a strong Si-O-Ce bond. Hydroxylated surfaces exhibit easier oxygen vacancy formation and thermodynamically favored substitution of hydroxyl groups with orthosilicic acid. A new ReaxFF library for silica/ceria interfaces with defects is validated using DFT outcomes. Reactive MD simulations demonstrate that ceria surfaces with 30% Ce3+ ions on (111) planes exhibit higher polishing efficiency, attributed to increased Si-O-Ce bond formation. The simultaneous presence of oxygen vacancies and various acidic and basic sites on ceria surfaces enhances the polishing efficiency, involving acid-base reactions with silica. Defective surfaces show superior efficiency by removing silicate chains, contrasting with nondefective surfaces removing isolated orthosilicate units. This study provides insights into optimizing CMP processes for high-precision glass industry surface finishing.

Characterization of Ceria Nanoparticles as Abrasives Applied with Defoaming Polymers for CMP (Chemical Mechanical Polishing) Applications.

Chemical mechanical polishing/planarization (CMP) is an essential manufacturing process in semiconductor technologies. This method combines chemical and mechanical forces to smooth the surfaces of wafers. The effectiveness of CMP relies on a carefully chosen slurry, demanding a sophisticated manufacturing technology. This technology must seamlessly integrate both chemical composition and mechanical elements, highlighting the intricate synergy required for successful semiconductor fabrication. Particularly in milling processes, if agglomerated particles due to slurry particle corrosion are present during polishing, uneven polishing, numerous fine scratches occur, leading to an increase in roughness and a deterioration in the quality of the finished surface. In this study, to overcome the issue of particle agglomeration and uneven polishing in commonly used ceria nanoparticle slurries during CMP processes, we investigated the ceria nanoparticle behavior based on styrene-maleic acid (SMA) dispersant polymer applied with three types of defoaming polymers. The investigations are expected to open up the possibility of utilizing ceria nanoparticles with applied defoaming polymer as an abrasive for advanced CMP applications. All samples were characterized by DLS (dynamic light scattering), SEM-EDX (scanning electron microscopy-energy dispersive X-ray spectroscopy), pH, conductivity, viscosity, a 10-day stability test at 60 °C, the AF4 test, and the polishing rate efficiency test. Our research demonstrates a significant improvement achieved through the use of SMA dispersant polymer, resulting in a polishing selection ratio exceeding 80 for oxide and nitride films. The G-336 defoaming polymer utilized here is expected to serve as a viable alternative in CMP processes by providing stable uniformity.

Improving monocrystalline silicon surface quality with chemical mechanical polishing using the slurry with the additive of isopropanol

Enhancing the silicon surface smoothness with chemical mechanical polishing (CMP) is crucial to achieve better beam quality in X-ray systems. The additive in CMP slurry is the key factor in determining the resulting surface morphology. This paper exploits the effect of isopropanol (IPA) additive on regulating the material removal process and resulting microscopical surface morphology through varying the IPA concentration and polishing pressure. The improvement of surface quality and a dropping material removal rate (MRR) are obtained by increasing the IPA concentration from 0.1 to 2.5 mol/L. When the polishing pressure increases from 0.25 to 0.48 psi, a better surface quality and enhanced MRR are observed. By using the optimal polishing pressure of 0.48 psi and IPA concentration of 2.5 mol/L, the root-mean-square roughness can be reduced to 0.10 nm under a measurement area of 1 µm × 1 µm with atomic force microscopy, more than 58.3% lower compared to surfaces polished without IPA. This work suggests an approach to obtain the ultra-smooth silicon mirrors, which has promising applications in the field of X-ray, aerospace, and photovoltaics.

Polishing performance and mechanism of a novel Fe-based slurry for chemical mechanical polishing

The planarization of silicon carbide (SiC) achieved through chemical mechanical polishing (CMP) poses a significant challenge. In this work, the CMP performance of a novel slurry containing Fe, Al2O3, and graphene oxide (GO) on SiC substrate was systematically investigated. Morphological characterization revealed a significant enhancement in the dispersion of Fe due to the presence of GO. Furthermore, CMP results demonstrated that the material removal rate (MMR) using the novel slurry could reach 700 nm/h, resulting in an average surface roughness (Sa) reduction to 0.6175 nm. Through the measurement of hydroxyl radical (·OH) concentration, the novel slurry was found to generate sufficient ·OH for the oxidation of SiC surface. This study proposes a viable processing method for CMP of hard materials.

Three anionic surfactants for corrosion inhibition in cobalt CMP: Research on validity and mechanism

Inhibitors are introduced into the slurry to avoid excessive metal corrosion during the chemical mechanical polishing (CMP) process. The anti-corrosion effect of surfactants in CMP was not investigated systematically in past studies. In this paper, three eco-friendly anionic surfactants, potassium laurate, potassium oleate, and dodecyl benzenesulfonic acid, were studied in depth as cobalt CMP inhibitors. Electrochemical experiments demonstrated that the three inhibitors act as anodic inhibitors. Static etching tests and surface morphology analysis proved that they can effectively inhibit the corrosion rate of cobalt and improve surface morphology. XPS results demonstrated that the inhibitor reduces corrosion by inhibiting cobalt oxidation. DFT calculations demonstrated that inhibitors adsorb on the cobalt surface through the O atoms in hydrophilic groups, while the CC double bond and benzene ring can significantly enhance the adsorption. Molecular dynamics simulations showed that the inhibitors are adsorbed on the cobalt surface in parallel. Additionally, all three surfactants improved the solution's wettability on cobalt. This report provides critical insights into the corrosion inhibition specific to cobalt CMP and offers an approach to realize the multi-functionality of a single agent in slurries.