Constantine P. Grigoropoulos

Femtosecond laser aperturless near-field nanomachining of metals assisted by scanning probe microscopy

Ultrashort pulsed-laser radiation is an effective method for precision materials processing and surface nano-/micromodification because of minimal thermal and mechanical damage. This study demonstrates that controllable surface nanomachining can be achieved by femtosecond laser pulses through local field enhancement in the near-field of a sharp probe tip. Nanomachining of thin gold films was accomplished by coupling 800-nm femtosecond laser radiation with a silicon tip in ambient air. Finite-difference time-domain numerical predictions of the spatial distribution of the laser field intensity beneath the tip confirmed that the observed high spatial resolution is due to the enhancement of the local electric field. Possible structuring mechanisms and factors affecting this process are discussed. The present process provides an intriguing means for massive nanofabrication due to the flexibility in the substrate material selection, high spatial resolution of ∼10 nm (not possible with standard nanomachining techniques), and fast processing rates achievable through simultaneous irradiation of multiarray tips.

Melting and surface deformation in pulsed laser surface micromodification of Ni-P disks

The nanosecond pulsed laser-induced transient melting and miniature surface deformation of Ni-P hard disk substrates has been investigated experimentally. A photothermal displacement method has been developed to detect the transient melting and surface deformation process with nanosecond time resolution. The deflection signals show the variation of the feature shape in response to different pulse energies of the near-infrared pulsed nanosecond heating laser beam. A laser flash photography system is also developed to visualize the growth dynamics of the entire feature with nanosecond time resolution and submicron spatial resolution. The feature formation is explained as a result of surface tension driven flow. The surface tension depends not only on the surface temperature, but also on the surfactant concentration. Competition between the thermocapillarity and a surfactant concentration effect is revealed in the course of the bump formation process. Smaller features with diameters of 5 μm are obtained by using visible pulsed laser radiation. On-line monitoring of the transient growth process of such small features is achieved by a new laser flash deflection microscope

Modeling of pulsed laser irradiation of thin silicon layers

Abstract Interactions of pulsed laser irradiation with matter may lead to controlled phase change transformations and material structure modifications. During transient heating at the nanosecond scale, the thermal gradients across the heat affected zone are accompanied by changes in the material complex refractive index. These changes, coupled with wave interference, modify the energy absorption, and thus the temperature field in the target material. This work accounts for these effects in a rigorous manner using thin film optics theory. Results are presented for the induced temperature field in thin silicon films by pulsed ruby and Nd:YAG laser light.

Transient Temperature During the Vaporization of Liquid on a Pulsed Laser-Heated Solid Surface

The thermodynamics of the rapid vaporization of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The transient temperature field is measured by monitoring the photothermal reflectance of an embedded thin film in nanosecond time resolution. The transient reflectivity is calibrated by considering a temperature gradient across the sample based on the static measurements of the thin film optical properties at elevated temperatures. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The metastability behavior of the liquid and the criterion for the onset of liquid-vapor phase transition in nanosecond time scale are obtained quantitatively for the first time.

Heat transfer in laser processing of thin films

Melting and solidification of a silicon film by continuous wave laser beam irradiation has been studied. The silicon film melting and recrystallization is controlled by the temperature distribution in the semiconductor. Calculations have been carried out for a range of laser beam parameters and material translational speeds. The temperature field development also has been monitored with localized transient reflectivity measurements. During transient heating of semitransparent materials at the nanosecond scale, the thermal gradients across the heat affected zone are accompanied by changes in the material complex refractive index. These changes, coupled with wave interference, modify the energy absorption and thus the temperature field in the target material. These affects are taken into account in a rigorous manner using thin film optics theory.

Heat Transfer in Excimer Laser Melting of Thin Polysilicon Layers

A pulsed KrF excimer laser with nanosecond pulse duration is used for surface melting of thin polycrystalline silicon films. The velocity of the moving phase boundary during melting and solidification, the maximum melting depth, as well as the melting duration are experimentally determined by combined optical and electrical methods. A melting interface tracking model is used to calculate the melt front propagation and the transient temperature field in the semiconductor. A phase-change model, which allows the occurrence of melting and solidification at temperatures other than the equilibrium melting temperature, is employed in the numerical calculation. The effect of interfacial superheating/undercooling is discussed.

Transient reflectivity measurements and heat transfer modeling in laser annealing of semiconductor films

Abstract Melting and solidification of a silicon film by continuous wave laser beam irradiation has been studied. The silicon film melting and recrystallization is controlled by the temperature distribution in the semiconductor. Numerical calculations have been carried out for a range of laser beam parameters and material translational speeds. The results for the melt pool size have been compared with experimental data. The temperature field development has also been monitored with localized reflectivity measurements. Experimental and predicted transient reflectivity distributions have been compared.

Near-field scanning optical microscopy based nanostructuring of glass

Nanofabrication, at lateral resolutions beyond the capability of conventional optical lithography techniques, is demonstrated here. Femtosecond laser was used in conjunction with Near-field Scanning Optical Microscopes (NSOMs) to nanostructure thin metal films. Also, the possibility of using these nanostructured metal films as masks to effectively transfer the pattern to the underlying substrate by wet etching process is shown. Two different optical nearfiled processing schemes were studied for near-field nanostructuring. In the first scheme, local field enhancement in the near-field of a scanning probe microscope (SPM) probe tip irradiated with femtosecond laser pulses was utilized (apertureless NSOM mode) and as a second approach, femtosecond laser beam was spatially confined by cantilevered NSOM fiber tip (apertured NOSM mode). The minimized heat- and shock-affected areas introduced during ultrafast laser based machining process, allows processing of even high conductivity thin metal films with minimized formation of any interfacial compounds between the metal films and the underlying substrate. Potential applications of this method may be in the fields of nanolithography, nanofluidics, nanoscale chemical and gas sensors, high-density data storage, nano-opto-electronics, as well as biotechnology related applications.

Multilayer Direct-Writing of Electrical Conductors With Gold Nanoinks Using the Fountain-Pen Principle

Previous publications showed the potential of gold nanoparticle inks in microelectronic manufacturing. The main advantage of using nanoparticles for the production of microelectronic conductors is their low melting point. Indeed the melting point of gold nanoparticles decreases dramatically with decreasing size. This interesting property presents us with an uncomplicated way in which to produce electronic conductors on plastics, thus manufacture flexible electronics. Microelectronic applications which make use of materials other than silicon make their appearance ever more often. In this paper we present a method of manufacturing multilayered electronic circuits using a scanning-probe-inspired technology to deposit and anneal a gold nanoink on various substrates. We then tested the quality of this technology by applying it to a real complete electronic circuit.Copyright

Ultrafast laser-induced processing of materials: fundamentals and applications in micromachining

Fundamental questions arise regarding the possibility and nature of melting and the ensuring mechanism of ablation in femtosecond laser processing of materials. A comprehensive experimental study is presented to address these issues in depth and detail. The mechanisms of ultra-fast laser-induced phase-transformations during the laser interactions with materials have been investigated by time-resolved pump-and- probe imaging in both vacuum and ambient environment. The temporal delay between the pump and probe pulses is set by a precision translation stage up to about 500 ps and then extended to the nanosecond regime by an optical fiber assembly. Ejection of material in the form of nanoparticles is observed at several picoseconds after the main pulse. The ignition of surface-initiated plasma into the ambient air immediately following the pump pulse and the ejection of ablated material in the picosecond and nanosecond time scales have been proven by high-resolution, ultra-fast shadowgraphy. To further dissect the origin and evolution of the ablation process, a double pulse experiment has been implemented, whereby both the pump and probe pulses are split into two components each separated by variable temporal delays. A diffractive optical element is used to fabricate micro-channels in silicon wafers.