K. Bali

Laser-assisted metal deposition from liquid-phase precursors on polymers

Abstract In this work, a short review is presented for results utilizing the technique of laser-assisted metallization of dielectrics. Experimental efforts and results related to the metal (palladium (Pd), copper (Cu) and silver (Ag)) deposition on polymeric materials (polyimide (PI), mylar) are reported. These polymers and metals are chosen due to their growing importance in the rapidly-developing microelectronics packaging industry. The method of laser-induced chemical liquid-phase deposition (LCLD) offers many advantages compared to other techniques such as laser-induced forward transfer (LIFT), pulsed-laser deposition (PLD) and laser-assisted chemical vapor-phase deposition (LCVD). The LCLD is time and cost effective because vacuum tools and special pre-treatments are not required. The consumed chemicals used in precursors are non-harmful and easy to handle due to the liquid phase. For the optimal physical and chemical properties of deposits, the laser and solution parameters are varied. XeCl and KrF excimer and Ar + lasers are employed for executing the palladium, Ag and/or Cu formation on the polymer substrates. Chemical and physical analyses of the formed metal patterns are performed by EDX, XRD, FESEM, SEM, resistance and adhesion measurements.

Laser direct writing of copper on polyimide surfaces from solution

Conductive copper patterns were deposited on polyimide (PI) substrates by using a focused, scanned continuous wave (cw) Ar+ laser beam at 488 nm wavelength. The deposition process was initiated by a photothermal reaction of a tartarate-complex solution of Cu2+ ions in an alkaline and reducing environment. Deposits were characterised by Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-ray (EDX) Spectrometry, DEKTAK profilometer and resistance measurements. The dependence of the characteristics of laser direct written patterns on the scanning speed of the laser beam, number of scans and laser power was examined. Uniform copper lines covered with copper-oxide (line width from 30 to 60 μm and height from 2 to 20 μm) with high conductivity and adhesion were achieved under optimised conditions.

Palladium thin film deposition on polyimide by CW Ar+ laser radiation for electroless copper plating

Ar+ laser radiation (CW, 488 nm) is used to induce deposition of palladium (Pd) thin films onto polyimide surfaces from palladium–amine {[Pd(NH3)4]2+} solutions. The chemical reaction is carried out by formaldehyde-assisted reduction of palladium–amine complex molecules to metallic Pd, and is localized on the polymer substrate by a focused and scanned Ar+ laser beam. The narrow (6–14 μm), homogeneous and thin (<50 nm) palladium patterns are found to be excellent catalysts for further copper autocatalytic electroless deposition. Both the Pd and Cu layers are characterized by scanning electron microscopy (equipped with EDAX), a DEKTAK profiler and resistivity measurements.

Reaction dynamics of CW Ar+ laser induced copper direct writing from liquid electrolyte on polyimide substrates

Abstract Conductive copper patterns were deposited on polyimide (PI) surfaces using a focused, scanned CW Ar + laser beam at 488-nm wavelength. The deposition process was initiated by a photothermal reaction of a tartarate–complex solution of Cu 2+ ions in an alkaline and reducing medium. Deposits were characterised by Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-ray Spectrometry (EDS) and resistance measurements. The mass of the deposited copper ( m Cu ) and also the rate of the deposition (d m Cu /d t ) were calculated from the resistance measurements. The dependence of the copper deposition rate on the scanning speed of the laser beam, number of scans, laser power and the temperature of the solution were examined. It was found that more chemical reactions were running parallel during the direct writing process yielding metallic copper and copper(II)-oxide on the PI surface.

Laser direct writing of palladium on polyimide surfaces from solution

Abstract Conductive palladium (Pd) patterns were generated by laser-induced chemical liquid-phase deposition (LCLD) on polyimide (PI) substrates using a continuous Ar + laser beam. The deposition process was executed from the amine complex solution of Pd 2+ ions. Dependence of the characteristics of the laser direct written patterns on scanning speed of the laser beam, number of scans and laser power was examined. Deposits were characterised by Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive Spectrometry (EDS), and profilometric measurements. The resistance of the patterns was also measured. It was found that good quality lines — narrow, homogeneous, and with high conductivity and adhesion — were achieved under optimized conditions.

Palladium thin film deposition from liquid precursors on polymers by projected excimer beams

Abstract Palladium thin films are deposited on polyimide (PI) and mylar (PET) surfaces from a solution by employing XeCl and KrF excimer lasers. The substrates are plunged in a [Pd(NH 3 ) 4 ] 2+ /HCOH system, and an illuminated diaphragm is projected onto the polymers through the precursor layer. Due to the incident laser beam, chemical reactions take place at the solution/polymer interface, yielding thin Pd films on the surface. The formation and thickness of the Pd layer depend on the numbers of pulses, laser fluence and wavelength of the laser. The formed metallic, adhesive, and homogeneous Pd deposits are found to be suitable for further electroless copper deposition. Techniques such as DEKTAK profilometry, optical microscopy, FESEM (equipped with EDX) and XRD are involved in the determination of morphological, structural and compositional characteristics of the deposited layers.

Quest for high quality local electroless laser deposition from the liquid phase: decomposition of ammonium molybdate

Abstract A systematic study on concentration (0.001–0.1M), laser power (up to 200 mW) and scanning speed (from 10 to 200 μm/s) dependence of pyrolytic laser decomposition of ammonium molybdate is reported. It is shown that there exists an optimal concentration range in which best quality deposition is observed and all the disadvantageous features typical for slow writing can significantly be improved by the increase of scanning speed. At speeds exceeding 0.1 mm/s well defined lines without macrostructure and of excellent morphology can be written and all the important characteristics of the lines remain virtually unchanged in a wide range of processing parameters.

Optical strength in UV region of amorphous carbon

Abstract Optical strength (damage properties) of amorphous carbon films was investigated under the influence of UV laser pulses in the nanosecond (ns) and femtosecond (fs) region on different laser wavelengths. The damaged area was found to depend linearly on the laser fluence in a narrow region around the damage threshold tested by raising the number of pulses and fluence. The optical damage strength was found to be 2–3 times higher for fs pulses on both wavelengths used. This can be explained by the different weights of the ablation and thermal processes in cases where pulse lengths differ strongly. The fs pulse heats the focal spot less effectively, and cools down before the arrival of the next pulse. The ns pulses are better heaters (proven by the lower threshold), they have smaller crater diameters and slower incubation periods.

UV-laser-induced etching and metal seeding on polymers; a surface characterization

Abstract Results of UV (308 nm) laser pulse induced dry etching with subsequent Pd deposition from a PdCl2 solution (acid base with pH=1) on polyimide surface are reported. The surface roughness has been determined before and after illumination. The fractal-based examination techniques based on the area–perimeter and the structure function methods. It could be concluded that the preetching of the polyimide surface significantly enhances the flux of Pd atoms deposited onto the surface, which is a power function of the number of shots (30 mJ/cm2 at 20 ns pulse duration) with a power coefficient close to 0.5. It has been observed that the seeding process started simultaneously at a number of places resulting in island-like deposits. Fractal characterisation of surfaces resulted in a fractal dimension changing between 2.28 and 2.71 carried out with the methods outlined earlier. It might be concluded that the seeding process occurs at atomic scale, but it exhibits a very strong trend towards the formation of aggregates and/or cluster-type structures.

Kinetic model for scanning laser-induced deposition from the liquid phase

Abstract The effect of precursor concentration (0.005–0.05M) and scanning speed (0.20–0.96 mm/s) on the volume of the deposited material, determined with a circumstantial procedure using the cross-sectional data measured by a DEKTAK profilometer, in pyrolytic laser writing from aqueous solutions of ammonium heptamolybdate has been studied. From the detailed investigation we have given evidences that the structures of the deposits and their concentration and scanning speed dependencies can be consequently interpreted on the basis of material supply speeds, i.e. short illumination times, minutes deposition rates are observed indicating that material transport takes place via short-range diffusion. On decreasing the writing speed, i.e. increasing the dwell time, the slope of the volume versus dwell time function is higher because of forced (convectional) material supply due to local stirring in the liquid phase in close proximity to the heated surface area. With a further increase of the dwell time a saturation effect was also observed due to material depletion from the vortex affected liquid volume which can only be eliminated by less effective long-range diffusion. The main result of this work is a semi-quantitative description of the kinetics of the deposition process which gives an explanation for the large variety of morphologies and changes in deposition rates in terms of material transport.