Kyung-Ryul Kim

Effect of film formation process on residual stress of poly(p-phenylene biphenyltetracarboximide) in thin films

Abstract Soluble poly(p-phenylene biphenyltetracarboxamine acid) (BPDA-PDA PAA) precursor, which was synthesized from biphenyltetracarboxylic dianhydride and p-phenylene diamine in N-methyl-2-pyrrolidone (NMP), was spin-cast on silicon substrates, followed by softbake at various conditions over 80–185°C. Softbaked films were converted in nitrogen atmosphere to be the polyimide films of ca. 10 μm thickness through various imidizations over 120–400°C. Residual stress, which is generated at the polymer/substrate interface by volume shrinkage, polymer chain ordering, thermal history, and differences between properties of the polymer film and the substrate, was measured in situ during softbake and subsequent imidization processes. Polymer films imidized were further characterized in the aspect of polymer chain orientation by prism coupling and X-ray diffraction. Residual stress in the polyimide film was very sensitive to all the film formation process parameters, such as softbake temperature and time, imidization temperature, imidization step, heating rate, and film thickness, but insensitive to the cooling process. Softbaked precursor films revealed 9–42 MPa at room temperature, depending on the softbake temperature and time. That is, residual stress in the precursor film was affected by the amount of residual solvent and by partial imidization possibly occurring during softbake above the onset of imidization temperature, ca. 130°C. A lower amount of residual solvent caused higher stress in the precursor film, whereas a higher degree of imidization led to lower stress. Partially imidized precursor films were converted to polyimide films revealing relatively high stresses. After imidization, polyimide films exhibited a wide range of residual stress, 4–43 MPa at room temperature, depending on the histories of softbake and imidization. Relatively high stresses were observed in the polyimide films which were prepared from softbaked films partially imidized and by rapid imidization process with a high heating rate. The residual stress in films is an in-plane characteristic so that it is sensitive to the degree of in-plane chain orientation in addition to the thermal history term. Low stress films exhibited higher degree of in-plane chain orientation. Thus, residual stress in the film would be controlled by the alignment of polyimide chains via the film formation process with varying process parameters. Conclusively, in order to minimize residual stress and to maximize in-plane chain orientation, precursor films should be softbaked for 30 min-2 h below the onset imidization temperature, ca. 130°C, and subsequently imidized over the range of 300–400°C for 1–4 h by a two-step or multi-step process with a heating rate of ⩽ 5.0 K min−1, including a step to cover the boiling point, 202°C, of NMP. In addition, the final thickness of the imidized films should be

Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies

We have investigated compositional changes on GaN surfaces under Ar-ion bombardment using synchrotron-based high-resolution x-ray photoemission (PES) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-ray absorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaN surfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.

Photoemission study of monolayer Co on Si(111) surface

Abstract The interaction of monolayer Co with the Si(111) surface is investigated by means of core-level and valence-band photoemission spectroscopy in the range of room temperature to 700°C. Upon increasing the annealing temperature, several formations of silicides are observed. This ultra-thin layer upon annealing at 300–400°C has electronic structure closely related to the metastable CsCl-type CoSi which was discovered by molecular beam epitaxy (MBE) grown onto the CoSi2 template to a thickness of ∼100 A. Further annealing up to 500°C leads to the e-CoSi-type electronic structure. Finally, the formation of CaF2-type CoSi2 is observed after annealing above 600°C. They lead also to an island formation above 700°C.

Synchrotron Radiation Facilities in Korea: Pohang Light Source and Future XFEL Project

The Pohang Light Source (PLS) at the Pohang Accelerator Laboratory (PAL) is a third-generation light source, the only synchrotron radiation facility in Korea, and the fifth machine of its kind in the world (see Figure 1). In 1988, PAL was organized for the construction of the PLS. Ground-breaking was celebrated in 1991, and PLS construction was completed in 1994. In 1995, the PLS opened two beamlines to public users. The PLS was initially operated at 2.0 GeV in 1995. Since 2002, the energy of the electron beam has been upgraded to 2.5 GeV (see Table 1 for the principal parameters of PLS). Remarkable increases in the number of beamlines, users, and scientific results have been achieved since the opening of the PLS in 1995. Two or three beamlines have been added each year for the past 15 years, and as of February 2009 we have in total 27 beamlines in operation and 3 beamlines under construction, which will be completed by the end of 2009 (Figures 2 and 3).

Interaction of low-energy nitrogen ions with GaAs surfaces

We have studied the interaction of low-energy nitrogen ions (0.3–2 keV N2+) with GaAs (100) surfaces by photoemission spectroscopy (PES) around N 1s and Ga 3d core levels and near-edge x-ray absorption fine structure (NEXAFS) around the N K edge. At the lowest bombardment energy, nitrogen forms bonds with both Ga and As, while Ga-N bonds form preferentially at higher energies. Thermal annealing at temperatures above 350 °C promotes the formation of GaN on the surface, but it is insufficient to remove the disorder introduced by ion implantation. Nitrogen interstitials and antisites have been identified in NEXAFS spectra, while interstitial molecular nitrogen provides a clear signature in both PES and NEXAFS.

An NMR study on polycondensation reactions of 3,5‐diamino‐1,2,4‐triazole with dianhydrides and diacyl chlorides

The reactivity of 3,5-1H-diamino-1,2,4-triazole (DAT) in condensation reactions with acetic anhydride, aromatic dianhydrides and diacyl chlorides was investigated. In comparison to ordinary aromatic diamines. the primary amino groups of DAT were found to have relatively low reactivities so that they are partially involved in the reactions. In contrast, the secondary amino group of DAT was determined to be so reactive that it is always involved completely in the reactions. The reactivities of the amino groups in DAT were found to decrease in the order =NH(1) » -NH 2 (3) » -NH 2 (5). The low reactivites of the primary amino groups in DAT might be attributed to the relatively high electronegativity of the heterocyclic five-membered nitrogens, the imino characteristics of the primary amino groups via tautomerization, and the amidization of the reactive secondary amino group and the resultant steric hinderance. Because of the limited reactivities, polymers with high molecular weight could not be synthesized from DAT with diacyl chlorides and dianhydrides.

Status of PLS‐II Upgrade Program

The Pohang Light Source (PLS) at the Pohang Accelerator Laboratory has been operated first at 2.0 GeV since 1995, and later was upgraded to 2.5 GeV. During this time, 6 insertion devices like undulators and multipole wigglers have been put into operation to produce special photon beams, with a total of 27 beamlines installed and 3 beamlines under construction. Recently, Korea synchrotron user’s community is demanding high beam stability, higher photon energies as well as more straight sections for insertion devices in the PLS. To meet the user requirements, the PLS‐II upgrade program has been launched in January, 2009, incorporating a modified chromatic version of Double Bend Achromat (DBA) to achieve almost twice as many straight sections as the current PLS with a design goal of the relatively low emittance, e, of 5.9 nm⋅rad. In the PLS‐II, the top‐up injection using full energy linac is planned for much higher stable beam as well and thus the production of hard x‐ray undulator radiation of 8 to 13 keV i...