Kwang-Salk Kim

Effects of Organic Contaminants during Metal Oxide Semiconductor Processes

Adverse influences of organic contaminants on electronic devices have been studied and the results are reported. Contamination of silicon wafers by organic compounds during their manufacturing processes has been clearly demonstrated by a few surface analytical techniques. Silicon wafers were intentionally contaminated by one of the major contaminants, bis(2-ethylhexyl) phthalate; its incorporation into the silicon oxide layer during the thermal oxidation of silicon and its influences on device performances have been evaluated in detail by monitoring the breakdown voltages. During thermal oxidation of the contaminated silicon surface, the atomized carbon species produced from the pyrolysis of organic contaminants help grow the oxide thicker, expand the silicon oxide lattice, and degrade the silicon oxide, which was shown by transmission electron microscopy and secondary ion mass spectroscopy, and finally exert adverse influences on the device performance.

Lateral force microscopy study of functionalized self-assembled monolayer surfaces

Patterned self-assembly of functionalized alkanethiols on the gold surface on a submicron scale was achieved by microcontact printing with elastomeric poly(dimethylsiloxane) (PDMS) stamps. Atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to examine the topography and properties of the self-assembled monolayer (SAM) surface containing Br and OH groups. Au-coated probe tips were modified with alkanethiolates of such functional groups as CH 3 , NHCOCH 3 and NHCOCF 3 . LFM images were examined to elucidate the adhesive interaction between modified probe tips and SAM surfaces. AFM images scanned by normal and modified probe tips clearly show that the height difference of the alternating lines on the exposed surface is attributed to the size differences of Br and OH groups. With an NHCOCH 3 probe the image contrast of the lines due to the friction is almost indistinguishable or inverted in the LFM image. LFM image by a normal and NHCOCF 3 probes exhibit the same contrast as topography in AFM images. Therefore, the specific functional group in this system could be recognized by the comparison of the adhesive interaction between tip and sample surfaces.

Quantitative Evaluation of Gettering Efficiencies Below 1 × 1012 Atoms ∕ cm3 in p-Type Silicon Using a #2#1 Tracer

Gettering efficiencies of copper, whose bulk concentrations are lower than 1 X 10 12 atoms/cm 3 in p-type silicon, have been evaluated quantitatively and the results are reported. Bulk copper introduced by intentional spiking and subsequent heat-treatment was shown to be gettered by bulk microdefects (BMDs), which had been introduced by heat-treatment prior to intentional contamination using a 65 Cu isotope tracer as a probe. For evaluation of gettering efficiencies, we found the trace analysis of the 65 Cu isotope to be critical and, thus, developed a procedure for trace analysis of bulk copper in the silicon bulk by modifying the published analytical technique, which allowed gettering efficiencies to be quantitatively evaluated for copper levels of below 10 12 atoms/cm 3 . We also describe a few other parameters important to the evaluation of gettering efficiencies, including out-diffusion of copper through the silicon matrix, formation of BMDs, and low-temperature out-diffusion.

A new PLS-II in-vacuum undulator and characterization of undulator radiation

This paper describes the result of overall studies from development to characterization of undulator radiation. After three years of upgrading, PLS-II has been operating successfully since 21st March 2012. During the upgrade, we developed and installed an in-vacuum undulator (IVU) that generates brilliant X-ray beam. The IVU with a 3 GeV electron beam generates undulator radiation up to ~ 21 keV by using 11th higher harmonic. The characterizations of the undulator radiation at an X-ray beam line in PLS-II agreed well with the simulation. Based on this performance demonstration, the in-vacuum undulator is successfully operating at PLS-II.

Effects of Bulk Microdefects and Metallic Impurities on p–n Junction Leakage Currents in Silicon

The effects of bulk microdefects and metallic impurities on leakage currents at p–n junctions have been evaluated quantitatively by relating leakage currents with bulk defects and metallic impurities, and the results are reported. Bulk defects and metallic impurities, which were introduced by appropriate thermal treatment and intentional contamination by spin-coating metal ion solutions onto the silicon surfaces, were shown to induce heavy leakage currents at p–n junctions, which had been manufactured by boron implantation and phosphorus diffusion. We found the bulk microdefects to be critical in causing leakage currents to flow and propose that their measurements be used as a means for the determination of the bulk defect densities. Die failure rates were also used for the evaluation of the effects of metallic impurities such as Cu, Ni, and Fe on the leakage currents.

Failure Mechanism by Organic Contaminants in Si Device Fabrication

Organic contaminants are particularly more difficult to classify or control than the others such as metal impurities or particulate matters because of their ubiquitous contamination from all sources of manufacturing facilities. Especially, bis(2-ethylhexyl) phthalate (DOP) used as plasticizer is shown to be an indicator of organic contamination because of its nonor low volatility and strong adsorption onto the silicon wafer. 2 We used DOP for quantitatively contaminating the wafer surface and studied its effects. Contaminated DOP spots were almost completely removed from the wafer surface during thermal oxidation of silicon. However, a very small portion of the spots remained on the surface and captured in the interface between polysilicon and silicon oxide layers during the polysilicon deposition process as schematically shown in Fig. 1. Fig. 2 shows the results of current vs. electric field (I/EF) measurements after the gate oxidation processing on a wafer. In the reference sample, large leakage currents of about 1x10 A/cm are observed only at the electric field of higher than 12 MV/cm and the increases in current begin to level off beyond about 7 MV/cm. On the other hand, the threshold EF values, at which currents begin to increase, shift to increasingly smaller values and rapid increases in current upon small increase in the electric field start to show up as the currents get close to the saturated values when the initial DOP concentration is increased. The decrease in slope just before breakdown voltage (BV) indicates that the resistance increases due to the thicker silicon oxide layers. However, the decrease in the BV suggests that the interface breaks down at a lower electric field resulting in an increase in leakage current due most likely to the presence of more conducting substance than the silicon oxide at the interface. To address how these contaminants induce the device failures, we show the cross-sectional views of the TEM images on the failed cells for the varied initial DOP concentrations in Fig. 2. The thickness of the silicon oxide layer, which was supposed to have grown to 80 A (= 8 nm), was increased from 8 nm to 13, 31 and 32 nm upon contaminating the surface by increasing the initial DOP surface concentration from 0 to 5x10 molecules/cm although the thermal oxidation of the silicon surface was carried out for the same duration in a furnace under identical experimental conditions. It is conceivable that the organic compounds may be reduced to elemental carbon in the presence of large amounts of silicon oxide at high temperature of 800C. The elemental carbon would then serve as a current leakage site, through which the current begins to flow and facilitates the breakdown of the interface. We have shown clearly that the organic contaminants are trapped during the thermal oxidation processing of silicon, which provide various routes to device failures; a portion of the organic contaminants remaining as the carbon compounds induces the expansion of the silicon oxide lattice, deteriorating the silicon oxide layer, resulting in the device failure. Results of our study clearly provide reasons for why the facilities for device fabrication must be kept extremely clean and the polymers using high concentrations of plasticizers should be avoided as much as possible from the clean room environments.

Operational Experience of Cooling Water Systems for Accelerator Components at PLS

The PLS cooling water system has been utilized for absorbing thermal load generated by a multitude of electromagnetic rf power delivering networks at PLS. The low conductivity cooling water for heat removal from the accelerator components is deionised and filtered to more than 2 MΩ·cm of specific resistance. The operation temperature of dedicated components in the accelerator is sustained as tight as ±0.1°C to minimize the influence of temperature fluctuation on the beam energy and stability. Although the PLS cooling systems were initially installed with a high degree of flexibility to allow for the conditioned operations for high beam gain, the system improvements and repairs have been employed to enhance the operational performance and reliability, and to incorporate the newly developed operating interfaces such as EPICS accelerator control systems.

Surface Modification with Functionalized Self-Assembly and Surface Characterization by Lateral Force Microscopy

Abstract Elastomeric poly(dimethylsiloxane) (PDMS) stamps were used for micro-contact printing (μ-CP) to pattern the adsorption of alkanethiolates on surfaces of gold on a scale of submicron. With this printing, organic surfaces patterned with well-defined regions exhibiting different chemical and physical properties have been produced. Au-coated mica substrates were treated with functionalized thiols to produce self-assembled monolayer (SAM) surfaces terminating with Br and OH groups by μ-CP method. Lateral force microscopy (LFM) has been utilized to characterize the adhesive interactions between substrates and probe tips that had been functionalized with SAMs which terminate with Br and COOH groups, respectively. LFM data exhibit that friction force between the probe tip and sample surfaces strongly depends on the functionality of both surfaces. This approach can be served as a method for mapping more complex and chemically heterogeneous surfaces.