Jihoon Jeong

A Comprehensive Study of the Sintering of Copper Nanoparticles Using Femtosecond, Nanosecond, and Continuous Wave Lasers

A high electrical and thermal conductivity coupled with low costs make copper (Cu) an enticing alternative to aluminum for the fabrication of interconnects in packaging applications. To tap into the benefits of the ever-reducing size of transistors, it is required to increase the input/output pin count on electronic chips, and thus, minimize the size of chip to board interconnects. Laser sintering of Cu nanoparticle (NP) inks can serve as a promising process for developing these micron sized, 3D interconnect structures. However, the exact processing windows for Cu NP sintering are not well known. Therefore, this paper presents an extensive experimental investigation of the sintering processing window with different lasers including femtosecond (fs), nanosecond (ns), and continuous-wave (CW) lasers. The dependence of the processing window on Cu layer thicknesses and laser exposure durations has also been investigated. A simplified model to estimate optimum laser sintering windows for Cu NPs using pulsed lasers is presented and the predicted estimates are compared against the experimental results. Given the simplicity of the model, it is shown to provide good estimates for fluence required for the onset of sintering and the processing window for good sintering of Cu NPs. [DOI: 10.1115/1.4038455]

Comparison between Grating Imaging and Transient Grating Techniques on Measuring Carrier Diffusion in Semiconductor

ABSTRACT Optical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change ∆T/T0 obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) by adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1).

In-plane Thermal Conductivity Measurement with Nanosecond Grating Imaging Technique

ABSTRACT We develop a nanosecond grating imaging (NGI) technique to measure in-plane thermal transport properties in bulk and thin-film samples. Based on nanosecond time-domain thermoreflectance (ns-TDTR), NGI incorporates a photomask with periodic metal strips patterned on a transparent dielectric substrate to generate grating images of pump and probe lasers on the sample surface, which induces heat conduction along both cross- and in-plane directions. Analytical and numerical models have been developed to extract thermal conductivities in both bulk and thin-film samples from NGI measurements. This newly developed technique is used to determine thickness-dependent in-plane thermal conductivities (κx) in Cu nano-films, which agree well with the electron thermal conductivity values converted from four-point electrical conductivity measurements using the Wiedemamn–Franz law, as well as previously reported experimental values. The κx measured with NGI in an 8 nm x 8 nm GaAs/AlAs superlattice (SL) is about 10.2 W/m⋅K, larger than the cross-plane thermal conductivity (8.8 W/m⋅K), indicating the anisotropic thermal transport in the SL structure. The uncertainty of the measured κx is about 25% in the Cu film and less than 5% in SL. Sensitivity analysis suggests that, with the careful selection of proper substrate and interface resistance, the uncertainty of κx in Cu nano-films can be as low as 5%, showing the potential of the NGI technique to determine κx in thin films with improved accuracy. By simply installing a photomask into ns-TDTR, NGI provides a convenient, fast, and cost-effective method to measure the in-plane thermal conductivities in a wide range of structures and materials.