Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10⁸ to 10¹⁰ K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.     

XPS studies of metal/polymer interfaces — Thin films of Al on polyacrylic acid and polyethylene

A reactivity experimental study of Al with CH₂ and COOH functionalities in polymers is performed using polyacrylic acid (PAA) and low density polyethylene (LDPE). The Al metal is deposited on the polymer surfaces using in-situ sputter deposition. Using X-ray photoelectron spectroscopy (XPS), an Al oxide-carbide complex is identified at the PAA/Al interface while an Al carbide-like species is observed at the LDPE/Al interface. These conclusions are based on carefully referenced binding energy measurements of the C(1s), O(1s), and Al(2p) core electron levels. Near surface XPS studies involving solvent cast PAA films indicate that they are CH₂ rich, suggesting that a larger than statistical number of carboxylic acid groups are pointing away from the surface towards the bulk. PAA powder, however, is found to have near stoichiometric composition at the surface. The conclusions regarding Al/polymer reactivity agree well with recent literature for other metal/polymer interfaces.     

Pulsed laser deposition vs. matrix assisted pulsed laser evaporation for growth of biodegradable polymer thin films

Thin films of poly (lactide-co-glycolide) (PLGA), a biodegradable polymer, were deposited on Si wafers by both conventional pulsed laser deposition (PLD) and matrix assisted pulsed laser evaporation (MAPLE) using chloroform (CHCl₃) as a matrix solvent. This research represents an initial study to investigate the deposition characteristics of each technique at comparable conditions to gain insight into the transport and degradation mechanisms of each approach. The deposited materials were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR), and gel permeation chromatography (GPC) with refractive index (RI) detection. While FTIR and NMR results do not show a measurable departure from the native, in sharp contrast GPC results show a significant change (up to 95%) in molecular weight for both deposition methods. This result makes it clear that it is possible to overlook substantial degradation when incomplete chemical analysis is conducted.Optical transmission measurements of the starting MAPLE targets yielded laser penetration depths on the order of 0.362 cm and 0.209 cm for pure CHCl₃ and 1 wt. % PLGA in CHCl₃, respectively. Straightforward application of the Beer–Lambert law for laser energy deposition predicts a negligible temperature rise of less than 1 K at the target surface, which is in clear contradiction with ablation rates of 1.85 μm/pulse experimentally measured for polymer loaded samples. With an ablation process of this magnitude, the material ejection is likely due to contributions of nonlinear or non-homogeneous laser light absorption rather than evaporation. Severe non-uniformity of the final surface morphologies of the MAPLE films, similar to solvent wicking artifacts found in spin casting supports the spallation scenario in MAPLE. PACS 81.15.Fg; 79.20.Ds; 78.66.Qn; 42.70Jk     

Experimental studies and thermal modelling of 1064- and 532-nm Nd:YVO4 micro-laser ablation of polyimide

Experiments and thermal modelling of polyimide ablation using the fundamental 1064-nm emission from a 20-kHz nanosecond diode-pumped solid-state Nd:YVO₄ micro-laser are described and compared with findings for the 532-nm doubled output. For exposures restricted to short pulse trains, it is found that micron-scale-size ablation features can be defined with this laser, even though polyimide films have weak absorption at 1064 nm and relatively weak absorption at 532 nm. There is evidence at both wavelengths of an incubation effect, driven by thermal modification of the polymer and, with long-term exposure at 1064 nm, Raman micro-spectroscopy reveals a progressive growth of predominantly amorphous carbon in the ablation site. Calculations of the temperature rise produced in the polymer by exposure to a high-repetition-rate pulse train are described that aid an understanding of the thermal aspects of the interaction at the two wavelengths. PACS 42.55.Xi; 44.10.+i; 52.38.Mf     

Effect of surface property of polyimide substrate on the formation of pentacene thin-film

We investigated the effect of surface property of polyimide substrate on the formation of pentacene thin-film by using atomic force microscopy (AFM) and X-ray reflectivity (XRR) and diffuse scattering (XDS). Two types of polymer films were prepared: (1) polyimide (PAA-PI) from poly(amic acid) (PAA) (2) polyimide hybrid (PAA-PI-H) prepared by hybridizing the PAA and soluble polyimide (PI) with a octadecyl side chain. The hybridization ratio of PI to PAA was 2/98 in wt%. The water contact angle for PAA-PI-H and PAA-PI were around 80° and 64°, respectively. Morphology of pentacene with a ropelike structure and (1 1 0) peak around 1.4 Å in q_z was found when it was deposited on PAA-PI thin-film. Different pentacene morphology was observed when it was deposited on PAA-PI-H thin-film. The different morphology might be due to a 5-6 nm thick additional layer (∼0.95 ρ_film) at the interface between pentacene and PAA-PI-H thin-film caused by a long alkyl side chain introduced to the polymer main chain.     

The effect of the target structure and composition on the ejection and transport of polymer molecules and carbon nanotubes in matrix-assisted pulsed laser evaporation

Matrix-assisted pulsed laser evaporation (MAPLE) is a prominent member of a broad and expanding class of laser-driven deposition techniques where a matrix of volatile molecules absorbs laser irradiation and provides the driving force for the ejection and transport of the material to be deposited. The mechanisms of MAPLE are investigated in coarse-grained molecular dynamic simulations focused on establishing the physical regimes and limits of the molecular transfer from targets with different structures and compositions. The systems considered in the simulations include dilute solutions of polymer molecules and individual carbon nanotubes (CNTs), as well as continuous networks of carbon nanotubes impregnated with solvent. The polymer molecules and nanotubes are found to be ejected only in the ablation regime and are incorporated into matrix-polymer droplets generated in the process of the explosive disintegration of the overheated matrix. The ejection and deposition of droplets explain the experimental observations of complex surface morphologies in films deposited by MAPLE. In simulations performed for MAPLE targets loaded with CNTs, the ejection of individual nanotubes, CNT bundles, and tangles with sizes comparable or even exceeding the laser penetration depth is observed. The ejected CNTs align along the flow direction in the matrix plume and tend to agglomerate into bundles at the initial stage of the ablation plume expansion. In a large-scale simulation performed for a target containing a network of interconnected CNT bundles, a large tangle of CNT bundles with the total mass of 50 MDa is separated from the continuous network and entrained with the matrix plume. No significant splitting and thinning of CNT bundles in the ejection process is observed in the simulations, suggesting that fragile structural elements or molecular agglomerates with complex secondary structures may be transferred and deposited to the substrate with the MAPLE technique.     

Experimental investigations of laser-induced forward transfer process of organic thin films

This paper deals with transfer induced by laser of thin layers of a conducting polymer, the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), for applications in plastic electronics. This relatively simple technique of direct writing offers the ability to make surface micro-patterning by localized deposits of material. The study of the various mechanisms (ablation, transfer and deposit) has been carried out according to different conditions of irradiation: wavelength (from ultraviolet to infrared radiation), pulse duration (nanosecond and sub-nanosecond) and fluence. The morphology of the transferred patterns has been analyzed by optical microscopy and scanning electronic microscopy. Our objective is to understand the different mechanisms involved in the process in order to optimize it in terms of geometrical resolution while preserving the properties of the transferred material.     

Deposition conditions in tailoring the morphology of highly porous reticular films prepared by electrostatic spray deposition (ESD) technique

Spongy-like reticular structure is a unique morphology fabricated by electrostatic spray deposition (ESD) technique. The effects of solvent, substrate temperature, precursor feeding rate, static electric field strength, and deposition time on tailoring the reticular structure were investigated. Scanning electron microscopy was used to observe the film morphology. MnO_x or LiMn₂O₄ were selected as the model materials. It is found that in addition to the conventional solvent butyl carbitol, other kinds of solvents such as ethylene glycol and propylene glycol can also be used to obtain reticular films at a suitable substrate temperature. Porous films with a low cross-linking degree pore structure can be prepared by increasing precursor feeding rate or decreasing substrate temperature. Increasing the deposition time or the electric field strength helps to obtain reticular films with more homogeneous pore size distribution. In addition, the addition of a high boiling-point solvent in mixed alcohol solvent results in the increase of proper substrate temperature. It is concluded that the fluidity of the spray droplets on the surface of a hot substrate is an important factor to form a reticular film.     

Nanosecond surface interferometry measurements on designed and commercial polymers

The effect of the ablation mechanism on surface morphology changes during an ablation process was studied by comparing three different polymers: a triazene polymer, a polyimide and poly(methylmethacrylate) (PMMA) with nanosecond surface interferometry. The triazene polymer, for which only indications for a photochemical ablation mechanism had been detected in previous studies, revealed no surface swelling, which could be attributed to a thermal ablation mechanism. For polyimide, a photothermal ablation mechanism is usually used to describe the ablation process at irradiation wavelengths 248 nm. However, the interferometric measurements do not show any surface swelling, which would be a clear indication for a thermal ablation mechanism. A surface swelling was only detected for PMMA with irradiation at 248 nm and fluences below the threshold of permanent surface modification. The detected phase shift, which is proportional to the change of the film thickness and the refractive index, can be explained by the opposite signs of the thermal expansion coefficient and the thermal refractive-index coefficient. PACS 52.38.Mf; 42.87.Bg; 71.20.Rv     

低温生长Cu(InGa)Se2薄膜吸收层的掺钠工艺研究

在柔性聚酰亚胺衬底上低温制备Cu(In,Ga)Se₂薄膜太阳能电池, Na的掺入会改善电池特性, 但不同的掺Na工艺对Cu(In,Ga)Se₂薄膜和器件特性的改善机理不同. 本实验通过对比前掺NaF和后掺NaF工艺发现, 在前掺Na工艺下, 由于Na始终存在于Cu(In,Ga)Se₂薄膜生长过程中, Na存在于多晶 Cu(In,Ga)Se₂ 薄膜晶界处, 起到了扩散势垒的作用, 导致晶粒细碎、加剧两相分离, 同时减小了施主缺陷的形成概率; 而在后掺Na工艺下, 掺入的Na对薄膜的结构及生长不产生影响, 仅仅起到了钝化施主缺陷、改善薄膜缺陷态的作用. 同时, 研究表明, 后掺Na工艺中, NaF必须依靠外界能量辅助才能扩散进Cu(In,Ga)Se₂内部, 实验结果证实, 只有衬底温度达到350 ℃以上时, 掺入的NaF才能较好地改善薄膜特性. 最终经掺Na工艺的优化, 得到低温工艺制备的柔性聚酰亚胺衬底器件效率达10.4%.     

Imaging-XPS/Raman investigation on the carbonization of polyimide after irradiation at 308 nm

The laser ablation of polyimide was studied using 308 nm laser irradiation 𙜡 mJ cm^-2. Confocal Raman microscopy revealed the deposition of carbon surrounding the ablation crater, which consists of amorphous carbon with some crystalline features. Inside the crater, graphitic material was detected on top of the cones, very similar to the material from cw-Ar⁺ ion laser irradiation. FT-Raman measurements reveal the presence of intermediates of the polyimide decomposition. Imaging-X-ray photoelectron spectroscopy confirmed the deposition of carbon material surrounding the ablation crater and showed that the oxygen and nitrogen contents of the remaining material decrease.     

Microstructure and interfacial interaction of polyimide/silica hybrid nanocomposites prepared with 3-aminopropyltriethoxysilane

A series of polyimide (PI)-silica hybrid nanocomposites are prepared from 3,3′,4,4′biphenyltetracarboxylic dianhydride (BPDA)-4,4′-oxydianiline (ODA) polyamic acid (PAA) and tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) by the sol-gel process. 3-Aminopropyltriethoxysilane (3-APS) is used to enhance the interfacial interaction between polyimide and silica. The morphology, interfacial interaction, and properties of the hybrids are investigated using scanning electron microscope (SEM), UV-vis spectroscopy, atomic force microscope (AFM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). SEM and AFM images indicate that silica particles of ca. 45-55 nm size are uniformly distributed in polyimide matrices and that the interfacial interaction between PI and TEOS is better than that between PI and TMOS. The optical transparencies of the PI/TEOS hybrids are better than that of the PI/TMOS hybrids. FTIR spectra confirm the Si O Si bond as well as the conversion of PAA to polyimide and PI/silica hybrid films. The thermal stability is increased after incorporation of the silicas in the polyimide matrix.     

Fundamentals and applications of polymers designed for laser ablation

The ablation characteristics of various polymers were studied at low and high fluences for an irradiation wavelength of 308 nm. 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 thresholds 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 have 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 the laser pulse, indicating photochemical ablation. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for an application as the polymer fuel in laser plasma thrusters. Received: 21 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/3104-412, E-mail: thomas.lippert@psi.ch     

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic,biological and nanoparticle thin films: a review

The matrix-assisted pulsed laser evaporation (MAPLE) technique offers an efficient mechanism to transfer soft materials from the condensed to the vapor phase, preserving the versatility, ease of use and high deposition rates of the pulsed laser deposition (PLD) technique. The materials of interest (polymers, biological cells, proteins, …) are diluted in a volatile solvent. Then the solution is frozen and irradiated with a pulsed laser beam. Here, important results of MAPLE deposition of polymer, biomaterials and nanoparticle films are summarized. Finally, the MAPLE mechanism is discussed. A review of experimental and theoretical works points out that the simple model of individual molecule evaporation must be abandoned. Solute concentration, solubility, evaporation temperature of solvents, laser pulse power density and laser penetration depth emerge as important parameters to explain the morphology of the MAPLE-deposited films.     

A novel laser transfer process for direct writing of electronic and sensor materials

MAPLE direct write (MAPLE DW) is a new laser-based direct-write technique which combines the basic approach employed in laser-induced forward transfer (LIFT) with the unique advantages of matrix-assisted pulsed-laser evaporation (MAPLE). MAPLE DW utilizes an optically transparent substrate coated on one side with a matrix consisting of the material to be transferred mixed with a polymer or organic binder. As in LIFT, the laser is focused through the transparent substrate onto the matrix. When a laser pulse strikes the matrix, the binder decomposes and aids the transfer of the material of interest to an acceptor substrate placed parallel to the matrix surface. MAPLE DW is a maskless deposition process which operates in air and at room temperature. Powders of Ag, BaTiO₃, SrTiO₃, and Y₃Fe₅O₁₂ with average diameters of 1 7m were transferred onto the surfaces of alumina, glass, silicon, and printed circuit board substrates. Parallel-plate and interdigitated capacitors and flat inductors were produced by MAPLE DW over Rogers RO4003 substrates. MAPLE DW was also used to transfer polymer composites for the fabrication of gas sensor chemoresistors. One such composite chemoresistor fabricated with polyepichlorohydrin/graphite was used to detect organic vapors with a sensitivity of parts per million.     

Solvent-related effects in MAPLE mechanism

To study the role of the solvent and of the laser fluence in the matrix-assisted pulsed laser evaporation (MAPLE) process, we used a soft polymer (polydimethylsiloxane—PDMS) as “sensing surface” and toluene as solvent. Thin films of the PDMS polymer were placed in the position of the growing film, while a frozen toluene target was irradiated with an ArF laser at the conventional fluences used in MAPLE depositions (60–250 mJ/cm²). Apart the absence of solute, the MAPLE typical experimental conditions for the deposition of thin organic layers were tested. The effects on the PDMS films of the toluene target ablation, at different fluences, were studied using atomic force microscopy and contact angles measurements. The results were compared with the effects produced on similar PDMS films by four different treatments (exposure to a drop of the solvent, to saturated toluene vapors and to plasma sources of two different powers). From this comparative study, it appears that depending on the MAPLE experimental conditions: (1) the MAPLE process may be “semidry” rather than purely dry (namely the solvent is likely to be present in the deposition environment near the growing film), (2) the solvent, if sufficiently volatile, is in form of vapor molecules (neutral, ionized and probably dissociated) rather than in liquid phase near the substrate and (3) at relatively high laser fluences (>150 mJ/cm²), the formation of an intense plasma plume results which can damage/affect a soft substrate as well as a growing polymer film.     

掺杂红外染料聚叠氮缩水甘油醚工质激光烧蚀推进性能优化探索

选择含能聚合物聚叠氮缩水甘油醚(GAP)作为激光烧蚀微推力器的工质,分析了红外染料掺杂对激光烧蚀GAP工质推进性能的影响.通过对比掺杂红外染料GAP在不同激光功率密度、掺杂浓度、靶材厚度和激光烧蚀模式下的推进性能数据和烧蚀羽流,初步探索了掺杂红外染料GAP工质的推进性能优化方式.实验结果表明:透射式激光烧蚀模式下,激光能量的指数衰减特性和掺杂红外染料GAP的强黏性使得烧蚀羽流中易存在未充分烧蚀的工质; GAP的推进性能受红外染料掺杂浓度和靶材厚度的综合影响,当靶材厚度与激光吸收深度接近时,靶材充分吸收激光能量使中心烧蚀区达到化学能释放的温度阈值,同时沿激光传播方向未充分烧蚀的质量最少,此时推进性能达到最优值.反射式下掺杂红外染料的聚合物的激光烧蚀过程遵循"先吸收激光能量先喷射"的规律,工质分解充分,推进性能优于透射式.     

Infrared radiation thermometer using a silver halide optical fiber for thermal ablation

In this study, we have fabricated non-contact temperature sensor using an infrared optical fiber for measuring temperature distributions during radiofrequency ablation. We have measured an infrared radiation, which is transferred by a silver halide optical fiber from the multi-points on the water around inserted electrode, using a thermopile sensor and the output voltages of a thermopile sensor are compared with those of the thermocouple recorder. Also, the relationship between the temperatures and the output voltages of a thermopile sensor at the measuring points is determined to obtain the temperature distribution. The measurable temperature range of a fiber-optic non-contact temperature sensor is from 37 to 80 °C.     

Resonant infrared pulsed laser deposition of cyclic olefin copolymer films

Barrier materials on thin-film organic optoelectronic devices inhibit the uptake of water, oxygen, or environmental contaminants, and fabricating them is a major challenge. By definition, these barrier layers must be insoluble, so the usual routes to polymer- or organic-film deposition by spin coating are not problematic. In this paper, we report comparative studies of pulsed laser deposition of cyclic olefin copolymer (COC), an excellent moisture barrier and a model system for a larger class of protective materials that are potentially useful in organic electronic devices, such as organic light-emitting diodes (OLEDs). Thin films of COC were deposited by resonant and nonresonant infrared pulsed laser ablation of solid COC targets, using a free-electron laser tuned to the 3.43 μm C–H stretch of the COC, and a high-intensity nanosecond Q-switched laser operated at 1064 nm. The ablation craters and deposited films were characterized by scanning-electron microscopy, Fourier-transform infrared spectrometry, atomic-force microscopy, high-resolution optical microscopy, and surface profilometry. Thermal-diffusion calculations were performed to determine the temperature rise induced in the film at the C–H resonant wavelength. The results show that resonant infrared pulsed laser deposition (RIR-PLD) is an effective, low-temperature thin-film deposition technique that leads to evaporation and deposition of intact molecules in homogeneous, smooth films. Nonresonant PLD, on the other hand, leads to photothermal damage, degradation of the COC polymers, and to the deposition only of particulates.     

Properties of conductive polymer films deposited by infrared laser ablation

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser vapor deposition (RIR-LVD). The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (1.3% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 S/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.