Chemical Mechanical Polishing Process Research Articles

Oxidation anisotropy of 4H-SiC wafers during chemical-mechanical polishing

4H silicon carbide (4H-SiC) wafers are widely used in high-power, high-temperature, and high-frequency electronics owing to their excellent physical and electrical properties. In the processing of 4H-SiC wafers, chemical-mechanical polishing (CMP) is commonly employed to realize atomic-scale smoothness, damage-free surface, and global planarization. The CMP process is the synergy of wet oxidation and mechanical grinding, whose efficiency significantly relies on the oxidation process. Therefore, understanding the wet oxidation mechanism of Si face and C face of 4H-SiC wafers is critical to high-efficiency double-side CMP. In this work, the anisotropic CMP performance of Si face and C face of 4H-SiC wafers are investigated. It has been found that the material removal rate of the C face is 2–3 times that of the Si face. The thicknesses of the transient oxide layer on the C face and Si face are 8.71 nm and 3.50 nm, respectively. X-ray photoelectron spectroscopy analysis reveals that the oxygen content on the C face is higher than that on the Si face after wet KMnO4 oxidation. This indicates that the C face of 4H-SiC is easier to oxidize, which results in more oxides that that on the Si face of 4H-SiC. Our work shows that accelerating the oxidation of the Si face of 4H-SiC wafers during the simultaneous double-sided CMP process is crucial for the efficient polishing of 4H-SiC wafers.

A design of green slurry for copper/cobalt barrier-step chemical mechanical polishing with controlled removal selectivity and dynamic galvanic corrosion inhibition

Chemical mechanical polishing (CMP) is one critical process in chip fabrication while also regarded as highly polluting due to the toxic CMP slurries. Though green slurries were recently proposed and developed, few of them could satisfy the stringent requirements of the simultaneous planarization of copper/cobalt heterogeneous structure, especially in technology node below 14 nm. In this contribution, a green CMP slurry with a biodegradable chelator methylglycinediacetic acid (MGDA) and a plant-extract inhibitor disproportionated rosin (DR) was designed. Nanometer-scale smooth surface, removal selectivity, galvanic corrosion inhibition and residual control was realized with the developed slurry. Through comprehensive experiments and theoretical calculations, the roles of the ingredients played and their synergistic material removal mechanism were elucidated. MGDA partially dissolved the oxide film, especially Cu(OH)2 formed by H2O2 and the porous film could be more easily removed by the mechanical friction of abrasive particles. DR ensured post-polished surface quality by inhibiting over-corrosion through adsorption on copper surface. Density-functional-theory calculations analyzed DR's molecular reactivity and determined the most stable parallel adsorption configuration. The work provides useful insights into the fundamental interactions of components in CMP process. In addition, both MGDA and DR are commercially available with low cost, showing great practical application prospect in low-technology-node chip fabrication.

Preparations of Polyurethane Foam Composite (PUFC) Pads Containing Micro-/Nano-Crystalline Cellulose (MCC/NCC) toward the Chemical Mechanical Polishing Process.

Polyurethane foam (PUF) pads are widely used in semiconductor manufacturing, particularly for chemical mechanical polishing (CMP). This study prepares PUF composites with microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC) to improve CMP performance. MCC and NCC were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD), showing average diameters of 129.7 ± 30.9 nm for MCC and 22.2 ± 6.7 nm for NCC, both with high crystallinity (ca. 89%). Prior to preparing composites, the study on the influence of the postbaked step on the PUF was monitored through Fourier-transform infrared spectroscopy (FTIR). After that, PUF was incorporated with MCC/NCC to afford two catalogs of polyurethane foam composites (i.e., PUFC-M and PUFC-N). These PUFCs were examined for their thermal and surface properties using a differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), dynamic mechanical analyzer (DMA), and water contact angle (WCA) measurements. Tgs showed only slight changes but a notable increase in the 10% weight loss temperature (Td10%) for PUFCs, rising from 277 °C for PUF to about 298 °C for PUFCs. The value of Tan δ dropped by up to 11%, indicating improved elasticity. Afterward, tensile and abrasion tests were conducted, and we acquired significant enhancements in the abrasion performance (e.g., from 1.04 mm/h for the PUF to 0.76 mm/h for a PUFC-N) of the PUFCs. Eventually, we prepared high-performance PUFCs and demonstrated their capability toward the practical CMP process.

Effects of organic compounds rich in phosphate functional groups as multifunctional inhibitors on copper film chemical-mechanical polishing properties: Combined experiment and theoretical calculation

For copper (Cu) interconnection chemical mechanical polishing (CMP) process, the Cu film slurry follows the principles of high efficiency, environmentally friendly and low cost. In this study, hydroxyethylene diphosphonic acid (HEDP), amino-trimethylene phosphonic acid (ATMP) and ethylenediamine etramethylenephosphonic acid (EDTMP) were used as green inhibitors to study the effects of organic compounds rich in two, three and four phosphate functional groups on the corrosion inhibition properties of Cu films. The results indicated that the Cu film material removal rate (MRR) of HEDP reaches 5879 A/min and the surface roughness (Sq) is 0.85 nm, superior to the other two inhibitors. Through mechanical action analysis, electrochemical experiment, UV-Visible, X-ray photoelectron spectroscopy (XPS) test analysis and theoretical calculation, it was observed that three inhibitors could not only form stable complexes with Cu, which have the effect of corrosion inhibition, but also play a lubricating role of surfactant, and simplify the composition of slurry, and the inhibition efficiency order of them is HEDP > ATMP > EDTMP.

A novel process of chemical-mechanical fluid-solid coupling polishing with high-quality and high-efficiency for turbine blisk

Turbine blisk is the core part of an aero-engine, which is characterized by its complex blade profile, resulting in the difficulty of its polishing. In this study, a novel process of chemical-mechanical fluid-solid coupling polishing was proposed to achieve high-quality and high-efficiency polishing for the blisk. Analyzed the principle of chemical-mechanical fluid-solid coupling polishing process of the blisk with the material of K418 nickel-based alloy. Then, established the material removal theoretical model of the process, and investigated the influence law of process parameters on the blisk surface force state to obtain the optimal parameters by using FLUENT fluid simulation. Moreover, simulated the abrasive particle trajectory to reveal the formation mechanism of the blisk surface with high-quality and high-efficiency. Furtherly, experiment shows that the novel process can effectively improve the blisk surface quality, and the surface roughness Ra decreased from 3.151 μm to 0.779 μm. This method provides a novel solution for precision polishing of the complex structural parts.

Enhanced chemical mechanical polishing (CMP) performance of porous self-assembled spherical cerium oxide via RE(La/Pr/Nd) doping

With the linewidth of integrated circuits has progressively narrowed, imposing higher demands on the chemical mechanical polishing (CMP) process. Cerium oxide nanoparticles are widely used as abrasives in CMP, with their physical structure and the surface content of Ce3+ ions and oxygen vacancies directly affecting their CMP performance. Here in, we synthesized La/Pr/Nd doped CeO2 with the porous self-assembled spherical structures. Doping with La/Pr/Nd reduced the grain size of cerium oxide, increased its specific surface area, decreased the pore size, and increased the pore volume. HR-TEM analysis shows that the porous self-assembled spherical CeO2 primarily exposes the (111) crystal facet. Additionally, the density functional theory (DFT) calculation, XPS, H2-TPR and XANES analyses shown that doping of La/Pr/Nd facilitated the formation of oxygen vacancies and increased the Ce3+ content on the (111) crystal facet of the CeO2. La atoms are more likely to stabilize on the CeO2 (111) surface compared to Pr and Nd atoms. The CMP results indicated that, The La-CeO2 exhibited a higher MRR of 338 ± 7.54 nm/min, which is a 35 % increase compared to p-CeO2, and achieved superior surface roughness (Ra = 0.201 ± 0.010 nm).

Modeling of surface microtopography evolution in chemical mechanical polishing considering chemical-mechanical synergy

Surface microtopography is the most concerned topic in chemical mechanical polishing, yet no method currently exists to predict its evolution process. This paper introduces an approach to predicting the surface microtopography evolution considering chemical-mechanical synergy and polishing pad-workpiece random contact. Experiments confirm the effectiveness of the model through height probability density, roughness Ra, and power spectral density curves. Both experimental and simulation results indicate that under stronger chemical and mechanical actions, the surface micromorphology smooths faster, but the stabilized surface roughness ends up being larger. The presented model serves as a significant tool for studying the wear mechanisms under chemical-mechanical synergy and provides guidance for optimizing chemical mechanical polishing processes.

Evaluation of chemical mechanical polishing characteristics using mixed abrasive slurry: A study on polishing behavior and material removal mechanism

Chemical mechanical polishing (CMP) is currently the most widely used method for material removal and surface planarization of glass. An environmentally friendly method of improving polishing performance by using a mixed abrasive slurry of ceria and diamond was proposed in this study. The results of polishing experiments showed that compared with single ceria abrasive, the materials removal rate (MRR) of the mixed abrasive slurry increased from 82.7 nm/min to 109.6 nm/min, while the surface roughness (Ra) decreased from 26.4 nm to 0.6 nm after polishing. An amorphous reaction layer formed during the CMP process was observed directly using transmission electron microscopy. Thermodynamic analysis was conducted to investigate the interfacial chemical reactions during the polishing process. A model was proposed to describe the mechanism of the materials removal.

Experimental and theoretical computational study of corrosion inhibitors in the cobalt bulk chemical mechanical polishing (CMP) process

Cobalt is a potential substitute for copper as local interconnects in sub-10 nm interconnects, the chemical mechanical polishing (CMP) of which is quite challenging due to the high chemical reactivity of Co. By using a combination of experiments and theoretical calculations, the optimum corrosion inhibitor of Co is identified and the corrosion inhibition mechanism of TTLYK on Co is further investigated. It investigates the effects of various corrosion inhibitors, including potassium oleate, dodecyl benzene sulphonic acid, octyl hydroxamic acid, and 2,2′-[[(methyl-1H-benzotriazol-1yl) methyl] imino] bis-ethanol (TTLYK) on Co through chemical mechanical polishing and static etching experiments. The results show that among various corrosion inhibitors, TTLYK presents the best corrosion inhibition effect. When the basic slurry contains 10 mM TTLYK, the corrosion inhibition efficiency could reach 96.23 %, the material removal rates of Co is 161.79 nm/min, the static etching rates is 0.85 nm/min, and the material removal selectivity ratio of Co and Ti is 39:1. The results fully meet the requirements of the Co bulk CMP process. It is revealed TTLYK could form a protective layer with a synergistic physical and chemical adsorption on Co, in which the chemical adsorption occurs through the formation of CoN bonds. The adsorption of TTLYK could decelerate the transformation of CoO and Co(OH)2 to Co3O4, and the as formed Co-TTLYK complex provides the main corrosion inhibition.

Roughness control in the processing of 2-inch polycrystalline diamond films on 4H-SiC wafers

Diamond is an essential material for the manufacture of semiconductors devices, infrared optical windows, heat dissipation and other fields. The applications have put forward a practical demand that the diamond substrates have an ultra-smooth surface and are free of structural damage. However, precision polishing of diamond is challenging due to the extreme hardness and chemical inertness. The focus of this study is to prepare nanoscale smooth polycrystalline diamond films with roughness Ra down to 0.1 nm using chemical and mechanical synergies. After processing, the surface roughness and quality of CVD diamond film were characterized, and a material removal mechanism was proposed. The graphitization process is accelerated when diamond contacts the transition metal Fe in the mechanical polishing process. In chemical mechanical polishing process, the polished surface of diamond C is transformed into deformed diamond C* under friction. A relatively soft polishing pad is used to increase the contact area with the diamond, which is transformed into a softer, easy-to-remove structure under the action of potassium permanganate oxidant, and then the reaction layer is removed again by mechanical friction.

The effect of amino acid addition in CeO2-based slurry on SiO2/Si3N4 CMP: Removal rate selectivity, morphology, and mechanism research

The continuous reduction in feature size of integrated circuits has raised stricter material removal selectivity and surface quality requirements for the chemical mechanical polishing (CMP) process of shallow trench isolation (STI). To reduce surface defects, small-sized CeO2 abrasives are introduced into the CMP of STI. We investigated the impacts of three amino acids and their composites added to a 50 nm-sized CeO2 dispersion (pH maintained at 5) on the polishing of silicon dioxide and silicon nitride. The promoting or inhibitory effects of amino acids first increased and then decreased with increasing concentration. When 0.02 M lysine and 0.02 M glutamic acid were added to 0.5 wt% CeO2 dispersion, the best effect was achieved. At this time, the material removal rate (MRR) of SiO2 and Si3N4 were 2993.4 Å/min and 137.2 Å/min, respectively, with surface roughness of 0.108 nm and 0.142 nm in the 5 μm × 5 μm region. This study shows that amino acids promote the MRR of SiO2 primarily by enhancing the reactivity of Ce4+ by complexing with carboxyl groups. In contrast, amino acids inhibit the MRR of Si3N4 primarily by forming strong hydrogen bonds between the amino group and its surface, resulting in surface adsorption and suppressing the hydrolysis reaction. The CMP mechanisms of the three amino acids are similar, and the differences in their effects are mainly due to the varying numbers of amino and carboxyl groups they carry. The adsorption of three amino acids on SiO2 and Si3N4 surfaces was simulated by Materials Studio software, and the results were consistent with the experimental results. Finally, this paper explained the mechanism of action of amino acids at the microscopic level and provided some guidance on improving the performance of CeO2 slurry.

Electroless silver plating on through-glass via (TGV) as an adhesive and conducting layer

The proposed method presents a scheme to modify the surface of the glass substrate using the piranha solution and to fabricate the conducting layer of the Through-glass Via (TGV) by the glucose-Ag electroless plating to achieve TGV metallization. The arithmetic mean deviation of the surface contours (Ra) for the substrates increases from 0.001 to 0.135. The contact angle of the substrates (unmodified and modified) is reduced to 6.4° and 12.2°, respectively. The sensitization process with SnCl2 and the activation process with silver ammonia solution are conducted using the glucose as the reducing agent to prepare the silver seed layer covered in the TGV through-vias with a high aspect ratio (9:1). The adhesion between the metal coating and the surface is measured by the 3M tape and chemical mechanical polishing (CMP). The metal coating adheres well to the surface-modified substrate without peeling off, which can satisfy the demands for the electroplating and CMP processes.

Development of Ceria Slurry for Enhanced Chemical Mechanical Polishing (CMP) Performance

As the semiconductor device is shrinking and gets complicated, the chemical mechanical polishing (CMP) process has become important. Various types of abrasive particles are used as CMP slurry, and among them, ceria slurry is in the spotlight due to its high material removal rate (MRR) and selectivity on oxide CMP process. It is known that the ceria abrasives form Ce-O-Si chemical bonds with oxide film, which makes it possible to achieve a high CMP performance.To further advance the performance of ceria slurry, we conducted a series of researches on ceria. A study was conducted on scanning mobility particle sizer (SMPS) measurement to accurately evaluate particle size distribution along with the basic polishing mechanism of monodisperse ceria nanoparticles. In addition, an optimization study was conducted considering the CMP performance according to the abrasive concentration of ceria slurry and the influence of ceria nanoparticles remaining on the wafer surface before the post-CMP cleaning process. Furthermore, research on synthesis method and surface modification of ceria was also conducted to improve the polishing performance. Ce3+ ions on the surface of ceria form Ce-O-Si chemical bond with the silicon dioxide film. Therefore, an important key point is to increase the concentration of Ce3+ ions to improve polishing performance. To achieve high Ce3+ ion concentration, ceria nanoparticles were synthesized through a hydrothermal method, and this performance was improved through lanthanide doping. On the other hand, methods to enhance Ce3+ ion concentration by inducing a reduction reaction on the surface of ceria nanoparticles were also proposed. First, the polishing performance was improved by adding trace metals (FeCl2, CrCl2) to induce a reduction reaction of ceria nanoparticles. Next, the reduction reaction of ceria was induced by applying an eco-friendly method which did not add chemical agents. Ultraviolet (UV) irradiation and hydrogen-gas reduction methods showed sufficient reduction reaction to improve the polishing performance of ceria slurry, and maintained physical properties without causing CMP defects such as poor surface roughness and micro-scratches.As with previous polishing studies, post-CMP cleaning is also important to decease defects. While ceria nanoparticles exhibit great polishing performance for silicon dioxide due to their ability to form chemical bonds, however, they also have the disadvantage of being difficult to remove during the post-CMP cleaning process. To efficiently remove ceria nanoparticles from the wafer surface during the post-CMP cleaning process, various methods have been studied. First, the buff cleaning process was studied in which a deionized water-based cleaning solution is injected instead of slurry after the CMP process. Since the buff cleaning process must simultaneously minimize damage of wafer surface and maximize cleaning performance, optimization research on the properties of the physical factors and cleaning solution is important. For an environmentally friendly cleaning method, we proposed controlling the viscosity of deionized water or using gas-dissolved water. It has been proven that physical factors of cleaning solutions (viscosity, micro bubbles, etc.) can be effective in removing ceria nanoparticles. Especially, hydrogen gas in gas-dissolved water induced a reduction reaction, breaking Ce-O-Si bonds and improving the cleaning performance. Limited to the bulk ceria CMP process, we also studied the improvement of cleaning performance of remained ceria nanoparticles using the tangential flow filtration (TFF) method before CMP process. We focused that small contaminant particles are difficult to remove during the cleaning process, and through the TFF method, small abrasive particles in the slurry are filtered in advance while maintaining the polishing performance.

(Invited) Novel Brush Design for Post CMP Cleans

Chemical Mechanical Polishing (CMP) process is one of the key processes in semiconductor wafer processing. A sub 10nm wafer fabrication process may require more than 40 dielectric and metal CMP steps. As CMP process requirements get more stringent, CMP slurries are formulated with smaller and smaller metal oxide abrasive particles. The abrasive particles may also be surface modified or doped with other metal oxides. Post CMP cleaning chemistry formulations are designed to remove polish byproducts, pad residues and abrasive particles from wafer surface. Chemical composition and PH are tuned for modulating wafer surface-particle interactions so brushes can mechanically “sweep” them off the surface. PVA brushes, used of post CMP cleans, are a highly porous network of crosslinked polyvinyl acetate with pore size in the range of 30 – 100 um and 80% - 90% porosity, depending on the type of brush being used. While PVA brushes have largely met the process capability requirements, they have limitations for advanced cleans such as nano-particle removal. As CMP process continues to expand to new dielectric and metal films for different process steps in wafer fabrication, there is a need for customized brush material and surface design for optimal cleaning efficiency.We discuss a new engineered post CMP brush design that shows high efficiency in nano-particle cleans and allows high level of material tunability. New brush design is an engineered composite with an open cell microporous core, overlaid with engineered fibers in form of loops. Brush design is compatible with existing post CMP cleans. Figure 1(a) shows the new design, where in DIW enters the brush through the center of microporous core and flows outwards to the surface. As water exits the surface, it flows over and across the loops and contacts the wafer surface. Flow resistance through core and loops can be independently tuned, which allows better fluid distribution and more effective cleans. Figure 1(b) show a close-up schematic of interaction between loop and particle. Flexing of the loops on brush surface provides improved contact as well as cleaning ability Brush design can be optimized for chemical and mechanical properties with choice of fiber material, fiber diameter and loop height. Figure 2 shows the Zeta potential of PVA and different loop materials. The fiber material maybe selected such that zeta potential of the loops can enhance mechanical action in removing particle. Mechanical action of the Loops can be modeled as a cantilever and force on the particle is exerted by flexing of the loops.Deflection of loops can be approximated with flexure stiffness of rods with point load at free end. For initial purposed loop is approximated as single element. The equation can be rewritten to calculate cleaning force P at the free end for given loop deflection (Y), P = 3EIY/L3 (1)Where E is elastic modulus, I = moment, L = length of fiber and for rod shapes; I = Pi*(r4/4)The adhesion of particles on the surface is due to the effect of van der Waals and electrostatic forces. The total adhesion force on the surface can be written as: F=F vdw + F edl (2) where Ft is total adhesion force, Fvdw is the van der Waals force, and Fedl is the electrostatic force.Fvdw = AR/6r2 (3)Where A is the Hamaker constant, R is particle radius, r is the separation distanceFor effective removal of particles, P >/= F (4)Flex force and adhesion force were modeled based on equation (1) and (3) and are shown in Figure 3. New brush design was used for post CMP cleaning of silicon dioxide wafers polished with ceria slurry. Two different brush designs were tested for cleaning efficiency. Modeling data and comparison with experimental results is presented. Figure 1

(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.”

Total Holographic Characterization of Large Particle Contaminants in Chemical Mechanical Polishing (CMP) Slurries

Nanoparticle slurries are widely used in semiconductor CMP processing. Accurately evaluating quality attributes of CMP slurries can be challenging for standard optical methods. Ceria slurries can be too optically dense and inhomogeneous for most optical measurements. Diluting the slurry can induce agglomeration or break up the agglomerates already present in the slurry. Previous studies of silica CMP slurries demonstrated that Total Holographic Characterization (THC) is effective for monitoring agglomeration in slurries. THC uses particle holograms to determine particle size and composition. Ceria nanoparticles have higher refractive indexes than silica nanoparticles, and produce more scattered light, which can reduce the signal of measurements of agglomerates, making accurate, reproducible measurements more challenging. Ceria slurry was successfully analyzed at concentrations typically used in semiconductor processing. Effective-medium theory is used to understand the variation in refractive index of large particle contaminants as compared to native slurry nanoparticles and other contaminants such as pad debris.

Optimization of surface filtration to enhance wafer uniformity by removing large particle in ceria slurry

Owing to the limitations of scaling down, the development of complex structures for chip design is progressing in the semiconductor industry. As a result, the chemical mechanical polishing (CMP) process has become essential for the precise control of micro-level steps and for maintaining surface uniformity between layers during the stacking process. Structural management and improved performance of the consumables used in CMP processes are required. In particular, controlling the abrasive particles in the slurry that directly contact the wafer surface is crucial. Large particles within the slurry can occur scratches and cause non-uniformity in the polishing process, thus the particle size distribution of the slurry must be regulated. In this study, two types of filtration systems were constructed to determine the optimal conditions for CMP of ceria slurry. Depth filtration effectively removes particles through a cake layer formation mechanism but has limitations in achieving a consistent pore size within the fiber layer. Consequently, separation efficiency for particle sizes is relatively low. In contrast, surface filtration consisting of a single membrane demonstrates a consistent correlation between pore size and the size of the filtered particles, resulting in high particle removal efficiency. The pore size capable of removing large particles without active particle loss was 200 nm. After evaluating slurry productivity and filter-induced agglomeration, 0.8 L per minute (LPM) was identified as the optimal flow rate. Additionally, after filtration at 0.8 LPM, the maximum reduction in removal rates was relatively small at 272.54 Å/min due to decreased edge removal rates. However, by removing large particles to implement a monodisperse state, an improvement in uniformity of 17.11 % was achieved. Therefore, the CMP performance can be enhanced using surface filtration to control the particle size distribution (PSD).

Biologically inspired piezoresistive MEMS acoustic vector sensor for underwater applications

Underwater acoustics has assumed a pivotal role in the detection, navigation, communication, and target identification of ships and submarines. In recent years, advancements in noise reduction from marine vessels have underscored the significance of underwater acoustic vector sensors. This paper presents a novel approach to the design, fabrication, and characterization of a two-dimensional Micro-Electro Mechanical Systems (MEMS) vector acoustic sensor tailored for underwater environments. Traditional underwater acoustic vector sensors often employ doped polysilicon or silicon/germanium sensing films on rigid silicon substrates. The inflexible nature of silicon substrates necessitates labor-intensive and costly chemical-mechanical polishing processes to achieve submicron thickness, often requiring extended clean-room usage. In contrast, this work proposes a simplified, cost-effective, and scalable alternative. The sensor design utilizes reduced graphene oxide (rGO) as the sensing material and Kapton as the substrate, chosen for their unique properties. rGO exhibits high carrier mobility and low intrinsic noise, ensuring clear signals with minimal external interference, thereby improving measurement accuracy in challenging environments. Kapton provides excellent thermal stability, mechanical strength, and chemical resistance, along with superior insulating properties that reduce electrical noise, enhancing the signal-to-noise ratio. The designed vector sensor leverages the piezoresistive transduction principle, incorporating a thin piezoresistive film of rGO on a flexible Kapton substrate via a straightforward drop-casting method. Inspired by the sensory mechanisms of aquatic life, the two-dimensional vector sensor not only detects pressure associated with acoustic signals but also discerns the direction from which these low and ultralow frequency signals originate. The device's performance characteristics, sensitivity, and directivity are comprehensively evaluated through static, dynamic, and acoustic tests conducted in both air and water mediums. Static tests reveal the piezoresistive behavior of rGO under applied displacement, yielding displacement sensitivities of 0.0279 V/mm and 0.0276 V/mm for axial and radial displacements, respectively. Dynamic and acoustic tests demonstrate the sensor's capability to detect low-frequency signals spanning from 0.25 Hz to 200 Hz, exhibiting a smooth and favorable frequency response. In air, the sensor exhibits receiving sensitivities ranging from −15.18 dB to −19.75 dB (X-channel) and −13.67 dB to −16.67 dB (Y-channel). In the underwater environment, these values range from −134.40 dB to −138.93 dB (X-channel) and −131.83 dB to −136.93 dB (Y-channel). Notably, the variation in receiving sensitivity across channels in both air and water is limited to within 6 dB. Furthermore, the device showcases a symmetrical directivity pattern resembling the shape of an "8," with an approximate sensitivity of −136.66 dB, a significant improvement compared to the −165 dB sensitivity of conventional polysilicon underwater vector sensors. These experimental findings affirm the feasibility of the proposed underwater vector sensor, based on the piezoresistive effect and MEMS technology, highlighting its two-dimensional directivity capabilities.

Low-threshold green lasing in heterogeneously integrated InGaN-based micro-rings covered by distributed Bragg reflectors on Si (100).

In this work, combining a series of wafer bonding, laser lift-off and chemical mechanical polishing processes, submicron-thick wafer-scale GaN-based thin-film epilayers are successfully transferred on Si (100), which provides a heterogeneous platform for fabricating microcavities for nitride-based integrated photonics. Low-threshold lasing via optical pumping from these transferred dry-etched green micro-ring cavities on Si is demonstrated by covering the whole micro-rings with dielectric distributed Bragg reflectors (DBRs), which greatly reduces the lasing threshold upon a better optical confinement at the ring rim. A high quality-factor of ∼3800 can be observed from the micro-rings beyond the lasing threshold under pulsed excitation conditions. Furthermore, room-temperature continuous-wave (CW) lasing at a wavelength of 521.7 nm with an ultralow threshold of 0.35 kW/cm2 is achieved. Our results suggest the use of a burying DBR layer notably improves the WGM microcavity confinement, providing insights for the design of low-threshold micro-lasers and low-loss waveguides for potential integrated photonic applications in the visible light range on the Si platform.

Analysis of Subsurface Damage Structures of Gallium Nitride Substrates Induced by Mechanical Polishing with Diamond Abrasives

Subsurface damage (SSD) structures induced by mechanical polishing of gallium nitride (GaN) substrates are comprehensively investigated using atomic force microscopy, cathodoluminescence (CL) imaging, and cross‐sectional transmittance electron microscopy. The low removal rate of the CMP process is a barrier to high productivity of a GaN wafering process; therefore, reducing the chemical mechanical polishing (CMP) process time by reducing the depth of SSD induced by mechanical processing is an active research area. To better understand the SSD structures, the surface roughness, SSD depth, and SSD distributions induced by mechanical polishing are quantitatively evaluated in this study. The SSD structures induced by mechanical polishing can be quantitatively exhibited as the SSD distribution with the damage strength at the outermost surface and the damage propagation, which are obtained by CMP process time‐resolved CL imaging method. On the basis of the analysis results, a schematic model of the SSD structures for mechanically polished GaN substrates is proposed.