Hojoong Kim

Rat pancreatitis produced by 13-week administration of zinc oxide nanoparticles: biopersistence of nanoparticles and possible solutions

Zinc oxide (ZnO) nanoparticles (NPs) are used in diverse applications ranging from paints and cosmetics to biomedicine and food. Although micron‐sized ZnO is a traditional food supplement, ZnO NPs are an unknown public health risk because of their unique physicochemical properties. Herein, we studied the 13‐week subchronic toxicity of ZnO NPs administered via the oral route according to Organization for Economic Cooperation and Development (OECD) test guideline 408. Well‐dispersed ZnO NPs were administered to Sprague–Dawley (SD) rats (11/sex/group) at doses of 67.1, 134.2, 268.4 or 536.8u2009mgu2009kg–1 per body weight over a 13‐week period. The mean body weight gain in males given 536.8u2009mgu2009kg–1 ZnO NPs was significantly lower than that of control male rats, whereas no significant differences were observed between the other treatment groups and the controls. Male and female rats dosed at 536.8u2009mgu2009kg–1 ZnO NPs had significant changes in anemia‐related hematologic parameters. Mild to moderate pancreatitis also developed in both sexes dosed at 536.8u2009mgu2009kg–1, whereas no histological changes were observed in the other treatment groups. To evaluate the mechanism of toxicity, we performed a bio‐persistence study and evaluated the effects of the ZnO NPs on cell proliferation. The treatment of a human gastric adenocarcinoma cell line with ZnO NPs resulted in a significant inhibition of cellular proliferation. The anti‐proliferative effect of ZnO NPs or Zn2+ was effectively blocked by treatment with chelators. These results indicate that the bio‐persistence of ZnO NPs after ingestion is key to their toxicity; the no‐observed‐adverse effect level (NOAEL) of ZnO NPs was found to be 268.4u2009mgu2009kg–1 per day for both sexes. Copyright

Study of Polishing Characteristics of Monodisperse Ceria Abrasive in Chemical Mechanical Planarization

In this study, the monodisperse abrasive deposition (MDAD) system was developed to investigate the polishing mechanism of ceria abrasives for the chemical mechanical planarization of a wafer. Using the MDAD system, groups of ceria particles with a specific size were collected from slurry with various particle size distributions, and each group was deposited on a blanket silicon wafer to study the size effect of the ceria particle on polishing results. The effects of the ceria particle size on surface roughness, contact area ratio, and subsurface damages on the surface and subsurface crystallographic structure were investigated using atomic force microscopy, transmission electron microscopy, and a surface inspection system. A similar set of experiments was performed with types of ceria abrasive particles with the same diameter but different morphologies to investigate the effect of the ceria abrasive geometry on polishing results. In addition, the effect of the ceria particle size on the pattern loading was investigated by polishing the shallow trench isolation and high aspect ratio patterned wafers.

Development of optical monitor of alpha radiations based on CR-39.

Fukushima accident has highlighted the need to intensify efforts to develop sensitive detectors to monitor the release of alpha emitting radionuclides in the environment caused by the meltdown of the discharged spent fuel. Conventionally, proportional counting, scintillation counting and alpha spectrometry are employed to assay the alpha emitting radionuclides but these techniques are difficult to be configured for online operations. Solid State Nuclear Track Detectors (SSNTDs) offer an alternative off line sensitive technique to measure alpha emitters as well as fissile radionuclides at ultra-trace level in the environment. Recently, our group has reported the first ever attempt to use reflectance based fiber optic sensor (FOS) to quantify the alpha radiations emitted from (232)Th. In the present work, an effort has been made to develop an online FOS to monitor alpha radiations emitted from (241)Am source employing CR-39 as detector. Here, we report the optical response of CR-39 (on exposure to alpha radiations) employing techniques such as Atomic Force Microscopy (AFM) and Reflectance Spectroscopy. In the present work GEANT4 simulation of transport of alpha particles in the detector has also been carried out. Simulation includes validation test wherein the projected ranges of alpha particles in the air, polystyrene and CR-39 were calculated and were found to agree with the literature values. An attempt has been further made to compute the fluence as a function of the incidence angle and incidence energy of alphas. There was an excellent correlation in experimentally observed track density with the simulated fluence. The present work offers a novel approach to design an online CR-39 based fiber optic sensor (CRFOS) to measure the release of nanogram quantity of (241)Am in the environment.

Measurement of CMP Slurry Abrasive Size Distribution by Scanning Mobility Particle Sizer

This article introduces an abrasive particle size measurement by a scanning mobility particle sizer (SMPS) to measure precise size distribution in the nanoregime. Static light scattering (SLS), which is a conventional method, is compared with the SMPS-based technique. Both measurement techniques differ in the measurement of SiO 2 and CeO 2 abrasive particle sizes. Results showed that SMPS is more sensitive than the SLS equipment due to a single-particle count, which is suitable for representing nanoparticle abrasives. To the best of our knowledge, there has been no article yet about the use of an SMPS for the measurement of abrasive size in chemical mechanical planarization (CMP) technology.

Effects of Ceria Abrasive Particle Size Distribution below Wafer Surface on In-Wafer Uniformity during Chemical Mechanical Polishing Processing

In this study, the abrasive size distribution of ceria-based slurry below wafer and its effect on in-wafer uniformity were examined. Based on our observation, the abrasive size varies depending on the location on the wafer. Hence process parameters such as pad surface morphology and slurry viscosity were thoroughly investigated to observe their effect on the distribution on the wafer surface. It was found that the small size particles were considerably reduced near the center location of the wafer surface during the pad lifetime with reduced slurry viscosity and high polishing pressure. Hence, the contact conditions and the characteristic of fluid should be simultaneously considered in order to obtain the stable in-wafer uniformity.

Optimization of CMP Pad Surface by Laser Induced Micro Hole

In this paper, the optimization of the chemical mechanical planarization (CMP) pad design is investigated to reduce defect generation during the CMP process. Recently, obtaining the optimum performance from the viewpoint of scratch generation has become the most challenging area in the CMP process. To achieve uniform performance of the process at the defect level, the surface texture and/or groove of the CMP pads should be designed effectively, and their original surface should be maintained throughout their lifetime. Because the transport of slurry and wear debris, which is the source of CMP scratches, is highly dependent on the surface geometry of the CMP pad, maintaining suitable process conditions is considered the most important design factor. In this work, the surface geometries of two CMP pads with different body structures are modified by a laser-induced micro hole; one is a porous-type pad and the other is a nonporous type. Based on the experimental and numerical simulation results, the micro-hole induced pads had a lower defect level than the conventional porous pads without holes because the micro holes acted as a defect source or coarse particle reservoir to prevent micro scratch during the process. In addition, the micro holes of the porous pad showed a lower defect generation than the holes of the nonporous pad as the process time increased.

Experimental Evaluation of the Effect of Pad Debris Size on Microscratches during CMP Process

Polishing debris generated by pad surface conditioning has been suspected as a major source of microscratches in the chemical–mechanical planarization (CMP) process. In this study, we investigated the pad debris generated by an inxa0situ conditioning process during oxide CMP as one of the major scratch sources. We evaluated the relationship between the size of pad debris and the shape of microscratches on a wafer to find the cause of scratches. Pad debris was gathered in real time during the polishing process. Then, by transmission electron microscopy we observed a mixed layer of pad material and abrasive particles on the surface of the pad debris and the pad surface, which hardened the pad debris and pad surface. The results reveal a size range of pad debris that led to a minimum scratch count. Pad debris size smaller or larger than the minimum scratch region seems to cause higher scratch count due to the hardened pad surface and pad debris.

A novel pad conditioner and pad roughness effects on tungsten CMP

In the BEOL (back end of line) process, tungsten (W) chemical mechanical planarization (CMP) has been commonly used to form contacts. As size of the metal line is shrunk, metal CMP will directly impacts resistance and reliability characteristic of final product. As examples, W contact recess and seam (or void) defects make resistance problems and block failure. In the W CMP Process, W is oxidized by etchant, and then removed by abrasives. Progressively, this process is cycled again until all bulk W is removed. Hydrogen peroxide (H2O2) and silica nanoparticles are commonly used as etchant and abrasive, respectively, to remove the material. To overcome W contact recess issue, we recently have used less selective slurry for W bulk and oxide films. To maintain such selectivity between these films during CMP, however, control of pad roughness evolution throughout the pad lifetime is the most important. Thus, proper selection of pad conditioner is demanded. Chemical etchant is essential to remove W but it also impacts on diamond grits on the pad conditioner in the manner if accelerating wear of the grits. As the pad roughness is diminishing by the wear of the grits, wear resistance of diamond is a key factor to keep desired W to oxide selectivity throughout the pad lifetime. In this experimental study, we used low selective slurry to decrease W recess and evaluated two different types of pad conditioners for keeping the process time variation as stable as possible throughout the pad lifetime. The conditioners have novel design of diamond grit pattern. Pad cut rate (PCR) of the each conditioner was evaluated as well. It was shown that minimum threshold PCR is exist to maintain the pad roughness to reduce process time variation. Finally, it is suggested that moderate surface roughness is required to form contact between pad and wafer to maintain moderate process time.

The experimental study on relationship between shape of diamond and micro scratches

Chemical Mechanical Polishing (CMP) is very important process in integrated circuit fabrication. The most advantage of CMP is the scalability and it will shrink the pattern size of all kinds of IC devices. As the design rule shrinks in Semiconductor Devices, the need of reducing micro scratch level in the CMP Process is unavoidable. In the process of mechanical polishing, micro scratch is one of several side effects. Since the number of CMP Process steps have been increased in IC devices, the needs of reducing micro scratch in polishing processes is getting higher. There are many kinds of reasons to generate micro scratches in the CMP Process. While a pad is worn away by diamond disk, pad debris is generated incidentally. The pad debris is critical source to make micro scratches. It is discovered as various sizes according to diamond disk. Experiments were composed of samples of several grade of diamond on disk. We found out that the shape of diamond on disk affect to the size of pad debris. It was also closely related to the cause to occur micro scratch in CMP process closely. In this work, we are going to introduce some characteristics of some grades of diamonds and the results of our fabrication test.

Investigation of Correlation between Polishing Characteristic and Pad Roughness during Chemical Mechanical Planarization Process

The Chemical mechanical planarization (CMP) process has become a primary planarization technique required for the manufacture of advanced integrated circuit (IC) devices. As the feature size of IC chips shrinks down to 65 nm and below, the role of CMP as a robust planarization process becomes increasingly important. In this work, we evaluated surface roughness of CMP pad to correlate the roughness with CMP performance such as material removal rate (MRR) and pad lifetime. Pad surface was analyzed by 3-dimensional profiler and scanning electron microscope (SEM). We found that MRR could be varied with the pad life time and roughness. We also found that suitable roughness range is exist to get stable CMP performance. Finally, we introduced ‘pre-conditioning’ method to manage the roughness of CMP pad to get stable CMP performance at the initial pad life time.