Laser-induced Surface Melting Research Articles

Nanosecond laser debonding of strong adhesives

ABSTRACT The goal of this paper is to determine whether laser-induced surface melting can generate adhesive debonding. Commercial cyanoacrylate and acrylic adhesives are used to attach an aluminum (Al) cylinder to a transparent polymethyl(methacrylate) (PMMA) plate, and then a variable force is applied to create an axially loaded butt joint. High energy nanosecond laser pulses at 1064 and 532 nm are directed through the transparent PMMA to be absorbed at the Al surface, causing transient localized heating that leads to joint failure. The dependence of this debonding on both laser fluence (energy per area) and applied force are investigated. Single shot debonding occurs at fluences on the order of 0.47 J/cm2 for 1064 nm and 0.29 J/cm2 for 532 nm pulses with an applied pressure of 0.22 MPa. Characterization of the Al surface before and after laser impact confirms that the debonding arises from surface melting and causes only slight changes to the Al surface. A simple model of the debonding process is developed to explain the dependence of the debonding on the applied load. Single laser pulses can generate instantaneous, relatively clean separation of bonded joints, suggesting that laser debonding may be a promising strategy to initiate deadhesion.

Rapid, deep dopant diffusion in crystalline silicon by laser-induced surface melting

As solar cell back contact schemes have improved in recent years, the non-ideal emitter region has become a dominant contributor to overall losses in solar cell efficiency. Our analysis shows that these losses can be greatly reduced by creating a lightly (< 1018 cm−3), uniformly doped emitter that is about 10 µm deep. However, the constraints imposed by current manufacturing techniques, namely using a tube-furnace for solid state dopant diffusion, limits the emitter to a much more highly doped (~ 1020 cm−3) and shallow (< 500 nm) region. A laser-based process by which dopants are diffused in a liquid silicon state can overcome these limitations. In this work, we provide semiconductor device simulations that compare the effects of these two types of emitters on device performance. Then, we used a heat transport model to demonstrate that a focused laser line beam can create a molten region at the surface of a silicon wafer that would allow dopants to diffuse deeply within a short amount of time. We then performed the laser processing with a scanned laser line beam on preheated silicon samples. The resulting junction depth (> 10 µm) is measured by cross-sectional etching and electron beam-induced current measurements. Electrochemical capacitance-voltage measurements are used to measure both the depth as well as the dopant profile resulting from the laser-driven process. We also analyse dislocation defects induced by the large thermal gradients inherent to the laser-based process which would greatly limit the device performance.

Probing the potential of structured and surface functionalized tools for dry cold forging of aluminium

In order to reduce friction and wear in cold forging of aluminium and its alloys a tremendous amount of lubricants is used. The main task of the liquid lubricants is the separation of tool and workpiece surfaces to avoid adhesion and abrasion. From an ecological and economical point of view, a reduction of the consumption of lubricants is meaningful. Novel surface modification techniques have been investigated to enable dry cold forming. In previous work, a reduced wear by adhesion of aluminium to an AISI H11 ESR tool steel surface was detected for a combination of laser polishing and a subsequent surface functionalization. Here, structured surfaces combined with surface functionalization are investigated regarding their wear behaviour. Therefore, different structures are realized on AISI H11 ESR tool steel surfaces by laser structuring. These structures are aligned parallel and perpendicular to the material flow direction. Furthermore, different structure depths are generated. It is evident that the laser induced surface melting causes both, a reduced micro roughness as well as a homogenized elementary distribution. Moreover, the surfaces are functionalized with an octadecylphosphonic acid selfassembled monolayer (C18PA-SAM). The SAM’s distal alkyl termini functionalize the tool steel surface und thus only weak van-der-Waals interactions are present in the aluminium-tool steel interface rendering an interaction, which is expected to be similar to the interaction in conventionally lubricated aluminium forming. Finally, the surfaces are appraised regarding their wear behaviour in a compression-torsion-wear tribometer. The different experimental results document the potential of the here investigated combined tool surface modification. Although, the performance of a lubricated process is not completely reached a reduction in friction and wear can be achieved compared to unmodified tools.

Laser polishing of niobium for application to superconducting radio frequency cavities

Superconducting radio frequency niobium cavities are at the heart of an increasing number of particle accelerators. Their performance is dominated by a several nanometer thick layer at the interior surface. Maximizing the smoothness of this surface is critical, and aggressive chemical treatments are now employed to this end. The authors describe laser-induced surface melting as an alternative “greener” approach. Selection of laser parameters guided by modeling achieved melting that reduced the surface roughness from the fabrication process. The resulting topography was examined by scanning electron microscope and atomic force microscope (AFM). Plots of power spectral density computed from the AFM data give further insight into the effect of laser melting on the topography of the mechanically polished (only) niobium.

Molecular dynamics simulations of laser induced surface melting in orthorhombic Al13Co4

Laser induced surface melting of the aluminum–cobalt alloy Al13Co4 is investigated. For the simulations of the lattice ions we use molecular dynamics, while for the time evolution of the electron temperature a generalized heat-conduction equation is solved. Energy transfer between the sub-systems is allowed by an electron–phonon coupling term. This combined treatment of the electronic and atomic systems is an extension of the well-known two-temperature model [Anisimov et al. in JETP Lett. 39(2), 1974]. The alloy shows large structural affinity to decagonal quasicrystals, which have an in-plane five-fold symmetry,while in perpendicular direction the planes are stacked periodically. As a consequence we observe slight anisotropic melting behavior.

Effect of laser annealing on delta-doped boron for super-steep-retrograded well formation using selective Si epitaxy

The effect of laser thermal annealing (LTA) on δ-doped B has been investigated for the applications of super-steep-retrograde (SSR) 70 nm n-type metal–oxide–semiconductor field-effect transistors using undoped selective silicon epitaxy. Shallow (⩽50 Å) melting by LTA was found to suppress the B loss, causing an anomalous lowering and fluctuation of threshold voltage (Vt), upon epitaxial channel growth and rapid thermal annealing (RTA). Under the laser fluence of 0.42 J/cm2, the B profile was also observed to freeze without further diffusion upon RTA at 900 °C for 20 s. Significant B loss observed in conventional δ-doped SSR devices stemmed from the hydrogen ambient at 800 °C required for selective silicon epitaxy as well as the rapid B outdiffusion behavior. The laser-induced surface melting decreased the outdiffusion rate by increasing the portion of substitutional B, and also led to a surface roughening which made the B loss from the silicon surface difficult. As a result of LTA, much lower Vt fluctuation with reasonable Vt was obtained along with improved short channel characteristics.

Fast melting of amorphous silicon carbide induced by nanosecond laser pulse

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid–solid–liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.

Modeling of Nonequilibrium Surface Melting and Resolidification for Pure Metals and Binary Alloys

A model was developed for surface melting and resolidification of both pure metal and binary alloy substrates. Nonequilibrium kinetics are introduced in the model to account for the departure from thermodynamic equilibrium at the solid/liquid interface. The modeled problem involves a moving boundary with both heat and solute diffusion and is solved by an implicit control volume integral method with solid/liquid interface immobilization by coordinate transformation. To illustrate the model capabilities, we have analyzed laser-induced surface melting of pure metals (Al, Cu, Ni, Ti) and dilute Al–Cu alloys, and some typical results are presented. The computation results show some large solid overheating and melt undercooling effects, which result from the high heat flux and the slow kinetics. Large interface velocity variations are also seen during the process, depending on the substrate material and laser flux. Complex interface velocity variations during the earlier stages of resolidification were also predicted for the alloys, and result from interactions between the several physical mechanisms involved. Results on interface temperatures, solute concentrations, and nonequilibrium partition coefficients are also presented.

Kinetics of laser-induced surface melting and oxide removal in single-crystalline Ge

Kinetics of laser-induced surface melting and oxide removal in single-crystalline Ge

Kinetics of laser-induced surface melting and oxide removal in single-crystalline Ge

The process of oxide removal in crystalline Ge using a pulsed ultraviolet laser has been studied by means of real-time reflectivity measurements with nanosecond resolution. The interaction of laser radiation with a clean, oxide-free surface has been characterized and the inhomogeneous and homogeneous energy density melting thresholds of c-Ge for 193 nm radiation have been determined. The values are 180 and 370 mJ/cm2, respectively. We have demonstrated that it is possible to remove an oxide overlayer by irradiation in vacuum and to produce a surface that shows the same response to laser radiation as a smooth, oxide-free, chemically cleaned surface. Under certain specific irradiation conditions it is even possible, after removing the oxide overlayer, to produce an enhanced crystalline quality in the near-surface region compared to that obtained upon chemical cleaning as evidenced by Rutherford backscattering/channeling measurements.

Laser etching of silicon: Dopant desorption, diffusion and segregation during laser induced surface melting

AbstractThe laser induced redistribution and desorption of dopant atoms (B, As, Sb) is measured in presence and in absence of chlorine at the surface. We define a desorption efficacy which is found to depend strongly on the dopant atom, increasing from boron to arsenic and to antimony. The implications for etching, doping and annealing experiments are discussed.

Laser-induced chemical etching of silicon in chlorine atmosphere

Chemical etching of single-crystalline (100)Si induced by pulsed laser irradiation at 308, 423, and 583 nm has been investigated as a function of the laser fluence and C12 pressure. Without laser-induced surface melting, etching requires Cl radicals which are produced only at laser wavelengths below 500 nm. With low laser fluences (Φ(308 nm) 440 mJ/cm2) etching is thermally activated. In the intermediate region both thermal and non-thermal mechanisms contribute to the etch rate.

Low cost molecular ion implantation equipment

A low cost, high current ion implantation system is described which includes fast annealing by laser induced surface melting. The implant machine is very simple, 20 cm in length and consists of an ion source with glow discharge in a gas atmosphere containing the dopant. No mass separation is used and the high density beam (1 mA/cm 2) consists of both atomic and molecular ions. Applications of this system are in the field of large area silicon solar cells.

Electrical and Structural Properties of Grain Boundary in Polycrystalline Si

ABSTRACTA special type of high angle grain boundary with mirror symmetry of the crystallographic orientation between the two grains is examined electrically and structurally. Transmission electron microscopy investigations show the nonuniform nature of the grain boundary. Both coherent and incoherent regions are observed along the boundary plane, and the types of dislocations and their densities vary from place to place. Electrical measurements of the boundary properties indicate no measurable barrier height. Heat treatment of the sample in an oxygen atmosphere at 950°C for 30 min did not change the boundary resistance. However, recombination effects are enhanced after boron diffusion. When pulsed laser radiation was used to process this material, it was found that the grain boundary structures were modified by laser-induced surface melting, i.e., incoherent boundaries were converted to coherent boundaries in the melted region.