Chang-seung Lee

Clean transfer of graphene and its effect on contact resistance

We demonstrate herein an effective method of forming a high-quality contact between metal and graphene on a wafer as large as 6 in. This gold-assisted transfer method producing no polymer residue on the graphene surface is introduced, and then the gold film is used directly as an electrode to form the transfer length method pattern for calculating the contact resistance. The graphene surface obtained using the gold-assisted transfer method is clean and uniform without residue or contamination, and its contact resistance is at least 60% lower than that obtained using the conventional poly(methyl methacrylate) assisted transfer method.

Crack-Release Transfer Method of Wafer-Scale Grown Graphene Onto Large-Area Substrates

We developed a crack-release graphene transfer technique for opening up possibilities for the fabrication of graphene-based devices. Graphene film grown on metal catalysts/SiO2/Si wafer should be scathelessly peeled for sequent transferring to a target substrate. However, when the graphene is grown on the metal catalyst on a silicon substrate, there is a large tensile stress resulting from the difference of the coefficient of thermal expansion in the catalyst and silicon. The conventional methods of detaching graphene from metal catalysts were found to induce considerable mechanical damage on graphene films during separation processes including metal wet etching. Here we report a new technique wherein bubbles generated by electrolysis reaction separate thin metal catalysts from the SiO2/Si wafer. The dry attachment of graphene to the target wafer was processed utilizing a wafer to wafer bonding technique in a vacuum. We measured the microscopic image, Raman spectra, and electrical properties of the transferred graphene. The optical and electrical properties of the graphene transferred by the bubbles/dry method are better than those of the graphene obtained by mechanical/wet transfer.

Fabrication and characterization of PZT/TiOx/SiO2/SiNx/SiO2/Si structure for acousto-optic device applications

Abstract In this study, we fabricated the PZT/TiOx/SiO2/SiNx/SiO2/Si structure for acousto-optic (AO) device applications using the interaction of surface acoustic wave (SAW) and optical beam. SAW was generated using the inter-digital transducer (IDT) on PZT film, and optical beam (He-Ne laser) was guided in SiNx film using prism-coupling method. Two SiO2 layers were used for optical cladding layer, and TiOx for the buffer layer to prevent the inter-diffusion between PZT and SiO2 layers. In this structure, we measured insertion loss of - 7dB at minimum point, resonance frequency of 17.6 MHz and sound wave velocity of 8800 m/s under impedance matching condition. We also measured optical propagation loss of the SiNx-film in both SiNx/SiO2/Si and PZT/TiOx/SiO2/SiNx/SiO2/Si structure for confirming the applicability of SiNx film to the waveguide layer.

Catalytic chemical vapor deposition of large-area uniform two-dimensional molybdenum disulfide using sodium chloride

The effective synthesis of atomically thin molybdenum disulfides (MoS2) of high quality and uniformity over a large area is essential for their use in electronic and optical devices. In this work, we synthesize MoS2 that exhibit a high quality and large area uniformity using chemical vapor deposition (CVD) with volatile S organic compound and NaCl catalysts. In the latter process, the NaCl enhances the growth rate (5 min for synthesis of monolayer MoS2) and purity of the synthesized MoS2. The optical microscopy, Raman spectroscopy, X-ray photoemission spectroscopy, photoluminescence, and transmission electron microscopy measurements indicate that the NaCl-CVD MoS2 has a large grain size, clear Raman shift, strong photoluminescence, good stoichiometry, and 6-fold coordination symmetry. Moreover, we demonstrate that the electron mobility (10.4 cm2/V•s) and on/off current ratio (3 × 107) of monolayer MoS2 measured using a field-effect transistor are comparable to those of previously reported MoS2 synthesized using CVD.

The Effect of Iridium Bottom Electrode on the Characteristics of Pb(Zr,Ti)O3 Films Grown by MOCVD Method

A novel method of metal-organic chemical vapor deposition (MOCVD) Pb(Zr,Ti)O3 (PZT) has been developed for use in high-density ferroelectric memory device. Well-aligned polycrystalline PZT films were grown onto Iridium bottom electrode by the MOCVD method with tmhd-family precursors in octane-based solvent under oxygen atmosphere at 550°C. Moreover, crystallinity of the PZT films on Ir bottom electrode was improved dramatically by inserting TiAlN barrier layer. It is also investigated the Iridium bottom electrode effect on the PZT in the ways of roughness, grain size, remnant polarization, fatigue and retention properties. Resultantly, highly reliable (111)-oriented PZT capacitors were obtained with 2Pr of 45 μC/cm2 and Vc of 0.8 V through MOCVD method and the interface engineering of the Iridium bottom electrode.

Effects of Ag addition and Ag3Sn formation on the mechanical reliability of Ni/Sn solder joints

Abstract Formation of intermetallic compounds (IMCs) in solder joints is closely associated with the mechanical reliability of the system. Though internal voids formed in Ni/Sn solder joints are known to be related to the formation of Ni 3 Sn 4 IMC, a detailed study on the mechanical reliability has not yet been reported. In this study, the mechanical reliability of Ni/Sn joints was investigated using two different soldering systems: Ni/Ag-Ag/Sn/Ni bilayers and Ni/Sn/Ag-Ag/Sn/Ni sandwich structures. The failure mode was found to be closely related to the formation and growth of an Ag 3 Sn phase. Filling of the voids with Ag 3 Sn IMC resulted in maximum shear strength, with a failure locus through Ni 3 Sn 4 and Ag 3 Sn. However, formation of a large amount of Ag 3 Sn decreased the shear strength once again.

Enhancement of optical performance of the light emitting diode packages with advanced thermal design of die-attaching layers

To guarantee long lifetime of light emitting diodes (LEDs), thermal reliability of LED packages should be secured with suitable die-attaching materials. Die-attaching materials are important for the interconnection between optical devices and substrates, as they not only provide electrical transmission but also the emission of thermal budget generated under the operation of the devices. In this study, the joints with different die-attaching materials were closely investigated in conjunction with the optical performance of LED packages. Thermal reliability of the joints was quantitatively analyzed by estimating the thermal resistances of the metallic and intermetallic layers incorporating the joints. The experimental results revealed that insertion of nano-Ag paste between eutectic Au-Sn and Ag finish significantly improved heat emission by effectively suppressing thermal resistance of eutectic Au-Sn layer. However, to ensure long-term reliability, complete removal of numerous nano-voids among Ag nanoparticles should be accomplished to prevent accumulation of thermal budget.

Effect of multiple flip-chip assembly on the mechanical reliability of eutectic Au–Sn solder joint

Eutectic Au–Sn solder has widely been used for high temperature bonding since it enables fluxless soldering and provides mechanically stable solder joint. However, effect of multiple bonding process on the mechanical reliability has not been studied in detail. In this study, microstructure evolution of Au–Sn solder joint and its effect on mechanical reliability were systematically investigated. During the multiple reflow process, the eutectic phase gradually transformed into ζ phase, which resulted in loss of solderable area. The phenomenon turned out to be responsible for degradation of bonding strength of the solder joint.