Gerhard Heise

Laser lift-off initiated by direct induced ablation of different metal thin films with ultra-short laser pulses

Molybdenum thin films on glass substrates play an important role as contact layer for thin film solar cells. They can be ablated by picosecond laser pulses irradiated from the substrate side at low laser fluences of less than 1 J cm−2, while structured trenches remain free from thermal damage and residues. The fluence for that so-called direct induced ablation from the substrate side is in contrast to metal side ablation reduced by approximately one order of magnitude and is far below the thermodynamic limit for heating, melting and evaporating the complete layer. For an extended investigation of the direct induced laser ablation and the underlying mechanism, further thin film materials, chromium, titanium and platinum, with thicknesses between 200 nm and 1 µm were examined. Finally, a simple thermo-dynamical model is able to connect the observed ablation energetics with the mechanical ductility and stress limit of the metal thin films.

Picosecond laser structuring of thin film platinum layers covered with tantalum pentoxide isolation

A thin film layer system consisting of platinum (Pt) as conductive layer on a glass substrate and tantalum pentoxide as isolating layer on top of the platinum is attractive for designing biocompatible conductor paths and contact pads for bio sensor chips. For the flexible and rapid patterning of the conductive and the isolating layers, both, the complete removal and the selective ablation of the individual thin films were investigated using ultra-short laser pulses with about 10 ps pulse duration and 1064 nm wavelength at low laser fluences. A platinum film covered with tantalum pentoxide shows a significantly lower ablation threshold than a single Pt film on glass alone when illuminated from the front side. Furthermore, we explored that the tantalum pentoxide film can be removed by glass side illumination from the Pt film, without affecting the Pt film and leaving the Pt film on the glass substrate intact. Those ablation phenomena occur at laser fluences of about 0.2 J/cm2, far below the evaporation limi...

Directly induced ablation of metal thin films by ultrashort laser pulses

Molybdenum films on a glass substrate are ablated from the glass side by picosecond laser pulses at fluences below 1 J/cm2, without damage. Thin films of chromium, titanium and platinum with thicknesses between 200 nm and 1 μm were examined to investigate the underlying ablation mechanisms. For molybdenum an influence of the intermediate buffer layer was observed. Ablation from the glass side clearly has higher ablation efficiency and a better structural quality in contrast to metal side patterning. A model will be presented, in which the ablation characteristics are connected with the mechanical ductility of the metal.

Monolithic interconnection of CIGSSe solar cells by picosecond laser structuring

We report on the selective structuring of CIS (Cu(In,Ga)(S,Se)2) thin film solar cells applying picosecond lasers at 1064 nm. For a monolithic serial interconnection the thin layers are selectively separated by so called laser patterns 1, 2 and 3 (P1, P2 and P3). We demonstrate that the half micron thick molybdenum back electrode can be structured with a P1 process speed of more than 4 m/s without detectable residues and damages by direct induced laser ablation from the back side. A CIS layer (~2 μm thickness) is structured by standard direct laser ablation at higher energy densities and a process speed up to 200 mm/s. A 1.5 μm thick ZnO front electrode layer can be line separated with P3 speed up to several 1000 mm/s by indirect induced laser ablation. We demonstrate that direct induced (P1) and indirect induced (P3) picosecond laser ablation are not purely thermal processes working at energy densities far below the evaporation enthalpy. To increase the scribing speed elliptical and rectangular beam profiles were investigated. Validation of the processes for functionality within a CIS solar cell will be presented.

Rapid prototyping of biocompatible sensor chips by picoseconds laser structuring of a platinum/ tantalum pentoxide thin film layer system

Sensor chips to measure electrical signals of living cells are patterned with lasers from thin films of platinum as the conductive layer and tantalum pentoxide as the isolating layer. The selective ablations of the transparent tantalum pentoxide alone as well as the complete removal of both layers were investigated using picosecond laser pulses at different fluences. Ablation threshold values were measured for the irradiation either from the layer (front) side or from the glass (back) side. We observed complete and selective laser ablation of the films at low laser fluences.

Micro-structuring of thin titanium films with ultrashort laser pulses

Electron beam guns with approximately 10 kW power are used for drying printing colors. As exit window for the electrons, 15 μm thin Titanium films are used, their thickness is at the current limit for industrial rolling processes. Thinner exit windows would increase the electron’s transmission and therefore reduce the required acceleration voltage, power consumption, shielding against X-rays and in the end machine and processing costs. The Titanium films should locally be thinned to about 5 μm, in the ranges of 3 mm diameter. Ultra-short laser pulses are well known for high precision micro structuring, as they offer small heat effect zones. We optimized the processing parameters and the ultra-short laser ablation of thin Titanium foils to achieve high manufacturing velocity and quality of the surface structure. Experiments with single pulse laser ablation and different spot diameter were conducted to find a connection between spot diameter and ablation threshold. The experiments show no dependency of the thresholds on the laser spot diameter.. First Experiments with different parameters were conducted to structure a three dimensional geometry in thin Titanium foils.

Time and space resolved investigations of confined fs ablation of Ta2O5/Pt thin film systems

Maskless patterning of biocompatible sensor chips consisting of a Ta2O5/Pt/glass layer system can be realized by ultrashort laser pulse ablation allowing fast and precise structuring. Here, a 650 fs laser at a center wavelength of 1053 nm is used at a peak fluence of 5 J/cm2. It was observed, that a greater diameter of the Pt film (400 nm) is ablated when it’s coated with Ta2O5 (200 nm) compared to the uncoated Pt. One reason was found in the anti-reflective effect of the Ta2O5 layer causing an increase of energy deposition in the material. The underlying physical effects of the ablation reaction are investigated over the whole reaction time ranging temporally from fs to μs by ultrafast pump-probe microscopy. For the direct ablation of the uncoated Pt, results show ultrafast heating and melting after 2 ps, the creation of a gas-liquid mixture and plasma at 10 ps. At around 100 ns the actual ablation takes place indicated by the ejection of small particles. The results for the Ta2O5/Pt layer system reveal heating and electron excitation in the Ta2O5 layer during the first 2 ps. In the following the spot center behaves identical to the direct ablation of Pt. Here, the Ta2O5 is ablated with the Pt. A confined ablation where an additional amount of laser energy is deposited in the layer system or at the layer interface is assumed to take place. In the rim of the spot only the Ta2O5 is removed by indirectly-induced ablation at around 35 ns.

Time and space resolved microscopy of induced ablation with ultra-short laser pulses

Laser lift-off processes have been observed during structuring CIS thin film solar cells with ultra-short laser pulses, if a Mo film on glass is irradiated from the glass substrate side. To investigate the underlying physical effects, ultrafast pump-probe microscopy is used for time- and space resolved investigations. The setup utilizes a 660 fs-laser pulse at a wavelength of 1053 nm that is split up into a pump and a probe pulse. The pump pulse ablates the thin film, while the frequency doubled probe pulse illuminates the ablation area after an optically defined delay time of up to 4 ns. For longer delay times, a second electronically triggered 600 ps-laser is used for probing. Thus, the complete ultra fast pulse initiated ablation process can be observed in a delay time range from femtoseconds to microseconds. First experiments on the directly induced ablation of molybdenum films from the glass substrate side show that mechanical deformation is initiated at about 400 ps after the impact of the pump laser pulse. The deformation continues until approximately 15 ns, then a Mo disk shears and lifts-off with a velocity of above 70 m/s free from thermal effects.

Ultrafast movies of thin metal film ablation with ultra-short laser pulses

Laser lift-off processes have been observed during structuring CIS (copper-indium-diselenide) thin film solar cells with ultra-short laser pulses, if a thin molybdenum film is irradiated from the glass substrate side.To investigate the underlying physical effects, ultrafast pump-probe microscopy was used to record the transient behavior of the single pulse ablation process. The ablation itself is initiated by a 660 fs pump pulse at a wavelength of 1053 nm. The ultrafast dynamic of the ablation process in the femtosecond and picosecond range is captured by illuminating the laser-material interaction region with an optically delayed 510 fs probe pulse up to 4 ns. Delay times can be extended by a second electronically delayed 600 ps probe laser pulse to record events in the nanosecond and microsecond range, which reveal mechanical deformation.Time-resolved investigations show that glass side ablation generates a confined gas-liquid mixture after 10 ps. Expansion of the gas and a generated shock wave delaminate the film at about 400 ps. The film then bulges to a maximum at 20 ns, if the fluence is below 0.6 J/cm². At higher fluences an intact Mo disk shears and lifts-off, while at fluences above 0.75 J/cm² a steep reflectivity decrease occurs due to nonlinear absorption in the glass substrate after 1 ps.Laser lift-off processes have been observed during structuring CIS (copper-indium-diselenide) thin film solar cells with ultra-short laser pulses, if a thin molybdenum film is irradiated from the glass substrate side.To investigate the underlying physical effects, ultrafast pump-probe microscopy was used to record the transient behavior of the single pulse ablation process. The ablation itself is initiated by a 660 fs pump pulse at a wavelength of 1053 nm. The ultrafast dynamic of the ablation process in the femtosecond and picosecond range is captured by illuminating the laser-material interaction region with an optically delayed 510 fs probe pulse up to 4 ns. Delay times can be extended by a second electronically delayed 600 ps probe laser pulse to record events in the nanosecond and microsecond range, which reveal mechanical deformation.Time-resolved investigations show that glass side ablation generates a confined gas-liquid mixture after 10 ps. Expansion of the gas and a generated shock wave delamina...

Selective femtosecond laser structuring of a platinum/tantalum pentoxide thin film layer system by induced laser ablation investigated with pump-probe microscopy

A biocompatible sensor chip consists of a 200 nm conducting platinum layer between a glass substrate and a 200 nm tantalum pentoxide layer as insulator on top. The irradiation of the biochip from the Ta2O5 side with ultra-short laser pulses (660 fs) at a wavelength of 1053 nm leads to a selective ablation of the Ta2O5 layer from the platinum. The ablation threshold for the removal of the Ta2O5 layer is about 0.08 J/cm². Moreover, the Ta2O5 lifts-off in form of an intact disk at a fluence of 0.14 J/cm². For deeper understanding of the ablation mechanisms, a pump-probe microscopy setup was utilized to record a stop-motion movie of the lift-off process. The setup utilizes an optically combined with an electronically delayed probe pulse. For the first time to our knowledge the laser lift-off of a transparent oxide could be recorded over the complete temporal dynamic range. The movies reveal that an intact disk leaves the hole after a few 10 ns. At higher fluences of about 0.20 J/cm² the disk disrupts in several particles at about 40 ns.A biocompatible sensor chip consists of a 200 nm conducting platinum layer between a glass substrate and a 200 nm tantalum pentoxide layer as insulator on top. The irradiation of the biochip from the Ta2O5 side with ultra-short laser pulses (660 fs) at a wavelength of 1053 nm leads to a selective ablation of the Ta2O5 layer from the platinum. The ablation threshold for the removal of the Ta2O5 layer is about 0.08 J/cm². Moreover, the Ta2O5 lifts-off in form of an intact disk at a fluence of 0.14 J/cm². For deeper understanding of the ablation mechanisms, a pump-probe microscopy setup was utilized to record a stop-motion movie of the lift-off process. The setup utilizes an optically combined with an electronically delayed probe pulse. For the first time to our knowledge the laser lift-off of a transparent oxide could be recorded over the complete temporal dynamic range. The movies reveal that an intact disk leaves the hole after a few 10 ns. At higher fluences of about 0.20 J/cm² the disk disrupts in sever...