G. C. Lim

Process study for laser-induced surface coloration

A KrF excimer laser beam is used to induce controllable and high-contrast colors on a stainless-steel surface under different processing conditions. The laser beam acts as a localized heating source that promotes the formation of semitransparent oxide films of varying thicknesses and morphologies on the sample surface. Ellipsometry and atomic force microscopy were employed to understand the laser-induced colors by studying the surface morphology and optical properties of the oxide films and their thickness. It is found that the laser-induced colors vary with the oxide layer thickness. The laser-treated areas have granular structures, which affect oxide growth and light scattering.

Laser-induced Ni(Ti) silicide formation

Effects of Ti alloying during laser-induced Ni silicide formation is studied. Unique triple layer microstructures were found with the presence of supercell in the NiSi2 grains formed at the interface. This supercell formation was caused by a local ordering of Ni and Si atoms that favor lower free energy during rapid solidification. Ti rapidly segregates from the alloy melt and forms a protective TiOx overlayer on the surface during solidification. Melt front progressing towards the Ni-rich region leads to quenching of an amorphous layer sandwiched between NiSi2 grains and the TiOx overlayer.

Silicide formation from laser thermal processing of Ti/Co bilayers

A bilayered CoTi silicide structure consisting of an amorphous CoTi silicide and a highly textured CoTi silicide was found after pulsed excimer laser annealing of titanium/cobalt/silicon stack at high fluence of 0.6 J/cm 2 . The highly textured CoTi silicide is monocrystalline and fully coherent with the Si(111) plane of the substrate but has a large amount of microstructural defects. The constitutional supercooling phenomenon is the solidification mechanism responsible for the highly textured CoTi silicide. The incomplete crystallization shown by the presence of the amorphous CoTi silicide is attributed to a high concentration of titanium impurity.

Nickel silicide formation using multiple-pulsed laser annealing

The effect of multiple-pulsed laser irradiation on Ni silicide formation in Ni(Ti)∕Si system was studied. A layered structure consisting of both crystalline NiSi2 and Ni-rich Ni–Si amorphous phases with a protective TiOx overlayer was formed after five-pulsed laser annealing at 0.4Jcm−2. Different solidification velocities caused by a variation in the atomic concentration across the melt have led to the formation of this layered structure. On the other hand, by increasing the number of laser pulses, a continuous layer of polycrystalline NiSi was obtained after a 20-pulsed laser annealing at 0.3Jcm−2 laser fluence. Its formation is attributed to a better elemental mixing which occurred during subsequent pulses. Enhancement of surface absorption and remelting of the phases formed is proposed as the mechanism governing the continuous NiSi layer formation.

Laser-induced Ni(Pt) germanosilicide formation through a self-limiting melting phenomenon on Si1−xGex∕Si heterostructure

Laser-induced Ni(Pt) germanosilicide formation on Si1−xGex∕Si substrate has resulted in the formation of smooth Ni(Pt) germanosilicide/Si interface with minimum interface roughness which is preferred as a contact material. A confined (self-limiting) melting phenomenon occurred during the laser-induced silicidation process at laser fluence of 0.4Jcm−2 (just at the melting threshold of the sample). This phenomenon is caused by significant differences in material properties of Si1−xGex alloy and Si substrates. Formation of highly textured [Ni1−v(Pt)v](Si1−yGey) phase was detected in the sample after 20-pulsed laser thermal annealing at 0.4Jcm−2. The formation mechanism of the Ni(Pt) monogermanosilicide is discussed.

Multiple-pulse laser thermal annealing for the formation of Co-silicided junction

Formation of Co-silicide contact layers on narrow silicon regions using multiple-pulse excimer laser annealing is demonstrated. Excellent performance of junction leakage behavior can be attained on narrow-width n/sup +//p and p/sup +//n junction as compared with standard rapid thermal annealed samples. Liquid-phase epitaxial Co-silicide regrowth has been found to occur and create a smooth and abrupt silicide/Si interface with high junction integrity using multiple-pulse laser annealing. Heat confinement created by the shallow trench isolation surrounding the narrow-width n/sup +//p and p/sup +//n junctions has minimized rapid quenching that might result in an amorphous structure. This has facilitated the crystallization of Co-silicide with multiple-pulse laser annealing.

Pulsed laser-induced silicidation on TiN-capped Co∕Si bilayers

This paper studies the effects of pulsed laser-induced annealing of TiN-capped Co∕Si bilayers with and without preamorphized Si substrate. For a low fluence of 0.2J∕cm2, nonstoichiometry Co silicide with triple-layered structure is formed. On the other hand, highly textured CoSi2 grains in (111) direction are formed for a high fluence of 0.7J∕cm2. The highly textured CoSi2 layer is monocrystalline and fully coherent with the (111) plane of the Si substrate. However, it has a large amount of microstructural defects throughout the layer. Competitive growth mechanisms between crystallization of homogenous intermixed layer and the nucleation from the melt boundary are discussed.

Formation of Silicided Hyper-Shallow p+/n- Junctions by Pulsed Laser Annealing

In this invited talk, we first review various methods for the formation of p + /n junctions of depth of less than 40nm using excimer pulsed laser annealing technique of 248nm wavelength. The characteristics of junctions formed using single and multiple-pulsed shallow melt and nonmelt laser annealing on amorphous and single crystalline Si substrate would be compared. To overcome the defect-related enhanced diffusion during pulsed laser annealing, novel junction engineering methodology using implantation generated defects such vacancies is illustrated. Methods employing ultra-low energy implants with laser annealing for junctions less than 25 nm are proposed. In addition, we will also talk briefly about the potential application of the laser annealing to be used for an ultra-low thermal budget silicide formation.

Laser direct writing of copper on polyimide, FR4, and Al203 substrates from solid-metalorganic film

Laser induced copper deposition from solid copper formate precursor films has been studied on polyimide, FR4 and Al2O3 substrates. Unlike most work reported in the literatures, we used 532 nm Nd:YAG laser beam instead of CW Ar+ laser for the process. A writing speed of 10 mm/s was achieved for deposition of micron-thick copper lines with a typical electrical resistivity of around 85 (mu) ohm-cm. To further increase the electrical conductivity and copper thickness, selective electroless copper plating was performed on the laser processed sample. This has reduced the electrical resistivity of the copper line to below 5 (mu) ohm-cm that is about 3 times the value of bulk copper (1.673 (mu) ohm-cm). A typical copper thickness exceeding 10 micrometers has been achieved after the electroless plating process. The surface morphology and chemical composition were analyzed by using SEM and EDS. The copper line was found to adhere well to the substrate. Besides circuit repair and customization, the reported technique is potentially useful for rapid prototyping of PCB circuits.