Ablation plume analysis showed that the most important problems of PLD, i.e. complex deviations of the film composition are clearly related to the properties of the ablation plume. The variation of the deposition conditions combined with plume analysis suggests also certain approaches how to overcome these problems, e.g. by varying background gases and pressures, but also when more drastic approaches must be used, e.g. an enrichment of certain elements in the target.
The development of organic electronic requires a non contact digital printing process. The European funded e-LIFT project investigated the possibility of using the Laser Induced Forward Transfer (LIFT) technique to address this field of applications. This process has been optimized for the deposition of functional organic and inorganic materials in liquid and solid phase, and a set of polymer dynamic release layer (DRL) has been developed to allow a safe transfer of a large range of thin films. Then, some specific applications related to the development of heterogeneous integration in organic electronics have been addressed. We demonstrated the ability of LIFT process to print thin film of organic semiconductor and to realize Organic Thin Film Transistors (OTFT) with mobilities as high as 4 10-2 cm2.V-1.s-1 and Ion/Ioff ratio of 2.8 105. Polymer Light Emitting Diodes (PLED) have been laser printed by transferring in a single step process a stack of thin films, leading to the fabrication of red, blue green PLEDs with luminance ranging from 145 cd.m-2 to 540 cd.m-2. Then, chemical sensors and biosensors have been fabricated by printing polymers and proteins on Surface Acoustic Wave (SAW) devices. The ability of LIFT to transfer several sensing elements on a same device with high resolution allows improving the selectivity of these sensors and biosensors. Gas sensors based on the deposition of semiconducting oxide (SnO2) and biosensors for the detection of herbicides relying on the printing of proteins have also been realized and their performances overcome those of commercial devices. At last, we successfully laser-printed thermoelectric materials and realized microgenerators for energy harvesting applications.
The application of energetic polymers has resulted in an increased thrust in micro laser plasma thrusters compared to
standard polymers. In this study we tested a novel concept for micro laser plasma thrusters, i.e. the application of liquid
polymeric fuels, by using polymer solutions of the energetic materials with different viscosity. Shadowgraphy
experiments suggest that for higher viscosity solutions ablation without splashing is possible, indicating that liquids are
applicable as fuels in laser plasma thrusters. First thrust measurements on a viscous polymer solution confirmed this by
yielding a specific impulse similar to a solid material.
The micro laser plasma thruster (μLPT) is a micro propulsion device, designed for the steering and propelling of small satellites (1 to 10 kg). A laser is focused onto a polymer layer on a substrate to form a plasma. The thrust produced by this plasma is used to control the satellite motion. To understand the influence of the specific properties of the polymers, three different "high"- and "low"-energetic polymers were tested: poly(vinyl chloride) (PVC) as a low-energetic reference polymer that showed the best properties among commercial polymers, a glycidyl azide polymer (GAP), and poly(vinyl nitrate) (PVN) as high-energetic polymers. It was necessary to dope the polymers with carbon nanoparticles or an IR-dye to achieve absorption at the irradiation wavelength in the near IR. Decomposition into smaller fragmentation was measured for the energetic polymers than for PVC corresponding well to the higher momentum coupling coefficient of the energetic polymers, which indicates that more thrust can be gained from a chosen incident laser power. The measurements of the kinetic energies of selected decomposition fragments revealed no significant difference between the different carbon doped polymers. Only for GAP with the IR-dye a change in the ratio between ions with different kinetic energy was observed with increasing fluence. More C+ ions with higher kinetic energy were detected at higher fluences. No correlation between the kinetic energies of the ablation products and the specific impulse could be established for the obtained data.
KEYWORDS: Etching, Quartz, Ultraviolet radiation, Excimer lasers, Chemical elements, Nd:YAG lasers, Temperature metrology, Liquids, High power lasers, Pulsed laser operation
The combination of a gray tone phase mask with a laser assisted wet etching process was applied to fabricate complex microstructures in UV transparent dielectric materials. This one-step method allows the generation of arrays of plano-convex and Fresnel micro-lenses using a conventional XeCl excimer laser and an absorbing liquid, which is in contact with the UV transparent material. An array of plano-convex micro-lenses was tested as beam homogenizer for a high power XeCl excimer and ps Nd:YAG laser. The roughness of the etched features varies from several μm to 10 nm, depending on the laser fluence and concentration of the dye in the organic liquid. The etching process can be divided into several etching mechanisms which vary with laser fluence.
For the first time, we have measured the momentum coupling coefficient and plasma expansion velocity (specific impulse) in the femtosecond region, over a laser intensity range from ablation threshold to thirty times threshold. These measurements extend the laser pulsewidth three orders of magnitude relative to previous reports. We studied several pure metals and three organic compounds as targets. The organic compounds were exothermic polymers specifically developed for the micro-laser plasma thruster, and two of these used "tuned absorbers" rather than carbon particles for the laser absorption function. The metals ranged from Li to W in atomic weight. We measured time of flight profiles for ions and found dramatic two-temperature distributions for some conditions. Specific
impulse reached record values for this type of measurement and ablation efficiency was near 100%.
The micro laser plasma thruster (μLPT) is a micropropulsion device, designed for the steering and propelling of small satellites (10 to 100 kg). A diode laser is focused on a two-layer polymer tape, where it forms a plasma. The thrust produced by this plasma is used to control the satellite motion. Three different polymers (GAP, PVN and PVC) doped with carbon and/or IR-dye were investigated for their performance as fuel polymer. The different dopants for GAP seem to have only little influence in the ablation properties. The most pronounced differences are observed in the fragment ejection detected in the shadowgraphy measurements and the crater appearance. For all carbon doped polymers, the ablation spots have a similar rough morphology. The shadowgraphy measurements of PVN reveal, that the shockwave and particle plume propagates faster as in the case of the other polymers. The particle plumes showed a very different expansion behavior for all polymers, whereas the plasma temperature and electron density measurements showed no significant difference. Only PVC displayed a slower almost linear drop of the plasma temperature over time. The thrust measurements showed the best results for GAP.
Nanosecond-interferometry and shadowgraphy is used to observe the dynamic behavior of the etching process during and after the irradiation pulse. Commercially available polymers exhibit quite often poor laser ablation properties for irradiation wavelengths >=248nm. At these wavelengths the absorption is due to the quite photostable aromatic groups. A photolabile triazene polymer was selected to compare the influence of a photolabile group on the laser ablation process. The photochemical active triazene reveals a strong absorption band at 332 nm and is responsible for the observed high etch rates and the low threshold for 308 nm irradiation. The absorption coefficients at 193 nm and at 308 nm are comparable, allowing to study the influence of the different absorption sites by ns-interferometry and shadowgraphy measures. The etching of the triazene polymer starts and ends with the laser beam. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of the ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than for 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer than for the polyimide.
We have developed a new type of miniature jet for pointing microsatellites. It is based on laser ablation produced by a multi-mode diode laser. The target is a specially prepared tape with a transparent layer through which the laser light passes and an absorbing layer which produces the thrust. We have achieved specific impulse up to 1000 seconds (greater than possible with chemistry), together with laser momentum coupling coefficients of order 6 dyne/W. The preprototype should achieve 100 dynes of thrust. We will discuss the target interaction physics, the materials science involved in creating the targets, and some of our measurements with the preprototype thruster.
The ablation characteristics of various polymers were studied at low and high fluences. The polymers can be divided into three groups, i.e. polymers containing triazene groups, designed ester groups, and reference polymers, such as polyimide. The polymers containing the photochemically most active group (triazene) exhibit the lowest threshold of ablation (as low as 25 mJ cm-2) and the highest etch rates (e.g. 250 nm/pulse at 100 mJ cm-2), followed by the designed polyesters and then polyimide. Neither the linear nor the effective absorption coefficients reveal a clear influence on the ablation characteristics. The different behavior of polyimide might be explained by a pronounced thermal part in the ablation mechanism. The laser-induced decomposition of the designed polymers was studied by nanosecond interferometry and shadowgraphy. The etching of the triazene polymer starts and ends with a laser pulse, clearly indicating photochemical etching. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for applications ranging from microoptical elements to polymer fuel for laser plasma thrusters.
Flat top shocks generated reproducibly by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of Gaussian spatial mode 120 - 400 fs duration incident laser pulses are reproducibly flattened via surface optical breakdown in dielectric substrates at fluences just about the breakdown threshold. These flat top laser profiles have been used to produce shocks flat to 0.7 nm RMS over a 75 - 100 micrometer diameter.
Si-containing polymers, which act as RIE-resists by generating a passivating SiO2 film on the polymer surfaces, are applied in microlithographic processes. New Si-containing polymers, i.e., Si-containing polycarbonates and polyestercarbonates have been synthesized. Various contents of Si (up to 30 wt%) were incorporated into the polymers. Thin films (about 100 micrometers thick) of the polymers were prepared for ablation and microstructuring at 308 nm. With a pinhole mask and a lens, a 10 X 10 matrix of circular craters was created on the polymer films. The fluence and number of pulses were varied for each crater. Ablation parameters (alpha) eff (effective absorption coefficient) and Fth (threshold fluence) were determined for each polymer. Using a reflecting objective (Schwarzschild type), microstructures with a sub-micron resolution can be produced on the polymer films. The quality of the ablated structures was evaluated by scanning electron microscopy. The incorporation of Si into polymers does not affect the ablation behavior of the polymers. Si-containing polyestercarbonates exhibited good ablation behavior, while surface swelling (incubation) was observed for polycarbonates films. The results from this work includes that Si-containing polyestercarbonates are suitable for laser ablation and microstructuring.
During the last decade laser processing of polymers has become an important field of applied and fundamental research. One of the most promising proposal, to use laser ablation as dry etching technique in photolithography, has not yet become an industrial application. Many disadvantages of laser ablation, compared to conventional photolithography, are the result of the use of standard polymers. These polymers are designed for totally different applications, but are compared to the highly specialized photoresist. A new approach to laser polymer ablation will be described; the development of polymers, specially designed for high resolution laser ablation. These polymers have photolabile groups in the polymer backbone, which decompose upon laser irradiation or standard polymers are modified for ablation at a specific irradiation wavelength. The absorption maximum can be tailored for specific laser emission lines, e.g. 351, 308 and 248 nm lines of excimer lasers. We will show that with this approach many problems associated with the application of laser ablation for photolithography can be solved. The mechanism of ablation for these photopolymers is photochemical, whereas for most of the standard polymers this mechanism is photothermal. The photochemical decomposition mechanism results in high resolution ablation with no thermal damage at the edges of the etched structures. In addition there are not redeposited ablation products or surface modifications of the polymer after ablation.
Photopolymers based on the triazeno chromophore group (-NequalsN-N<) have been developed. The absorption properties can be tailored for a specific irradiation wavelength (e.g. 308 cm XeCl laser). The photochemical exothermic decomposition yields high energetic gaseous products which are not contaminating the surface. The polymer can be structured with high resolution. No debris has been found around the etched corners. Maximum ablation rates of about 3 micrometers / pulse were achieved due to the dynamic absorption behavior (bleaching during the pulse). No physical or chemical modifications of the polymer surface could be detected after irradiation at the tailored absorption wavelength, whereas irradiation at different wavelengths resulted in modified (physical and chemical) surfaces. The etching of the polymer starts and ends with the laser pulse, shown by ns-interferometry, confirming that the acting mechanism is mainly photochemical. TOF-MS revealed fragments which are also totally compatible with a photochemical decomposition mechanism.
We have developed novel photopolymers based on the triazeno chromophore group. The absorption properties can be tailored for a specific irradiation wavelength. With the introduction of a photolabile group into the main chain of the polymer we expected a mechanisms which is mainly photochemical. This should result in high resolution etching with no thermal damage or chemical/physical modification to the material. The gaseous products of the photochemical decomposition were thought to assist the material removal, and to prevent the re-deposition of solid products which would contaminate the surface. We confirmed that the irradiation of the polymer at 308 nm resulted in high resolution etching. No debris has been found around the etched corners. Maximum ablation rates of about 3 (Mu) m/pulse were achieved due to the dynamic absorption behavior. No physical or chemical modifications of the polymer surface could be detected after irradiation at the tailored absorption wavelength, whereas irradiation at different wavelengths resulted in modified surfaces. The etching mechanism can be described as a laser induced microexplosion, revealed by ns-imaging. The etching of the polymer starts and ends with the laser pulse, shown by ns- interferometry, confirming that the acting mechanism is mainly photochemical at high fluences for our polymers, which can be used as high resolution laser dry etching resists.
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