Excimer Laser Exposure Research Articles

Self-organized submicron structures in photoresist films by UV-laser irradiation at water-confined conditions

A new kind of self-organized pattern formation process has been found during laser irradiation of polymer films in water confinement just below the laser ablation threshold, resulting in a randomly oriented pattern with a period of about 475 nm. The morphology, orientation, period, and amplitude of these patterns are inconsistent with both laser-induced periodic surface structures that typically consist of linear grooves with periods smaller the laser wavelength and wrinkling patterns that feature a much larger period and appear at layered systems. Excimer laser (λ = 248 nm, tp = 25 ns) exposure of 650 nm thick photoresist films on silicon wafers cause the growth of irregular submicron patterns. The pattern morphology that is examined with imaging techniques is correlated to processing parameters. The amplitude of these laser-induced self-organized (LISE) submicron structures are strengthened with pulse number and laser fluence. The experimental results are discussed together with simulations of laser heating the photoresist film in water confinement. The proposed pattern formation mechanism of such laser-induced self-organized submicron structures at temporal excitation of a confined polymer surface comprises the formation of an oriented roughness based on LIPSS that are developed to wrinkled structures due to the transient formation of a soft subsurface area that provides conditions for wrinkling of the water cooled, stiff polymer surface by laser-induced stress fields. Size, amplitude, and morphology of the LISE pattern provide good properties for applications in such fields as wetting, friction, optics, and bioactivity.

Analysis of Surface Texturing of Silicon with Surface Regular Microstructure Using C Method

Silicon, as the most abundant element in the earth’s crust and the most common material used in electronic and optical equipment, has attracted the attention of many individuals to change the properties of this material, improving its electronic and optical properties. One of these efforts relies on the reduction of surface reflection by making use of different methods. However, among them, the use of lasers in creating surface microstructures has been of special importance because there is no need for masks and other additional materials. In this work, a theoretical method is utilized to analyze these textures with the theorem of diffraction grating on a micrometer scale. The surface reflection of the microstructure created by excimer laser exposure on a silicon surface is simulated. The theoretical Coordinate transformation method (C method) gives out notable results against the experimental records by approximating triangular and trapezoidal microstructures. The model is useful for predicting the reflective response of the modified microstructural morphology. One of the main applications is the texturing of the solar cell front faces to enhance their efficiency, mainly due to photon trapping.

Antibacterial Properties of Silver Nanoclusters with Carbon Support on Flexible Polymer.

Here, we aimed at the preparation of an antibacterial surface on a flexible polydimethylsiloxane substrate. The polydimethylsiloxane surface was sputtered with silver, deposited with carbon, heat treated and exposed to excimer laser, and the combinations of these steps were studied. Our main aim was to find the combination of techniques applicable both against Gram-positive and Gram-negative bacteria. The surface morphology of the structures was determined by atomic force microscopy and scanning electron microscopy. Changes in surface chemistry were conducted by application of X-ray photoelectron spectroscopy and energy dispersive spectroscopy. The changes in surface wettability were characterized by surface free energy determination. The heat treatment was also applied to selected samples to study the influence of the process on layer stability and formation of PDMS-Ag or PDMS-C-Ag composite layer. Plasmon resonance effect was determined for as-sputtered and heat-treated Ag on polydimethylsiloxane. The heating of such structures may induce formation of a pattern with a surface plasmon resonance effect, which may also significantly affect the antibacterial activity. We have implemented sputtering of the carbon base layer in combination with excimer laser exposure of PDMS/C/Ag to modify its properties. We have confirmed that deposition of primary carbon layer on PDMS, followed by sputtering of silver combined with subsequent heat treatment and activation of such surface with excimer laser, led to the formation of a surface with strong antibacterial properties against two bacterial strains of S. epidermidis and E. coli.

Bi-metallic Pd/Au nanoparticles prepared by UV-excimer laser exposure

Multi-metallic nanoparticles are useful for applications in biophotonics or catalysis. Metal compounds alter the properties of pure metallic nanoparticles and thus expand the particle performance compared to particles consisting of the pure phase of metals. We investigated the formation of bi-metallic Pd/Au nanoparticle systems fabricated by UV excimer laser exposure at 248 nm in the ns range and the structural, the morphological as well as the optical properties of these alloy nanoparticles. Laser parameters like energy and pulse number or the thickness proportions of the individual layers to the total layer thickness were varied. Due to the volume percentage of the metals, the total layer thickness and the used laser parameters, the particle size and size distribution could be tuned. The corresponding nanoparticle formation is able to influence the plasmon resonance in position and in shape. Based on x-ray diffraction measurements and energy dispersive x-ray analysis an alloy formation was observed. Using x-ray diffraction, the formation of additional reflexes of palladium-gold alloys between the reflexes of the pure metals could be detected. Furthermore, the position and intensity was adjustable by the percentage composition of metals. The energy dispersive x-ray analysis showed the spatial presence of both elements in each particle.

Effect of fiber stretch on quasi-phase-matching for second-harmonic generation in thermally poled twin-hole silica-glass fiber

Optimization of the quasi-phase-matching (QPM) for second-harmonic generation in thermally-poled twin-hole fiber was performed by stretch of the fiber. The periodic structure for QPM was fabricated by erasure of the second-order nonlinearity by exposure with ultraviolet light through a mask. Exposures with a KrF excimer laser and a low-pressure mercury lamp were compared. The fundamental-wave source was a Q-switched Nd:YAG laser. The dependence of the second-harmonic output on the strain induced by the stretch of the fiber was measured. For periodic depoling by excimer-laser exposure, the second-harmonic power increased by a factor of up to 4 by the stretch. For periodic depoling by mercury-lamp exposure, the maximum increase by the stretch was also by a factor of up to 4. The fiber stretch was effective for fine tuning of QPM for poled fibers.

Controlled biopolymer roughness induced by plasma and excimer laser treatment

A new method for biopolymer poly-L-lactic-acid (PLLA) surface nanostructuring with surface plasmon reso- nance appearance is proposed. The motivation of this work is to determine optimal conditions for rough or flat biopolymer surface which may find broad application in tissue engineering as biocompatible carrier for various types of cell lines. A combination of plasma pre-treatment with consequent excimer laser exposure is proposed as a method for increasing the roughness of PLLA surface and changing its morphology. The focus of this paper is to determine morphology changes in combination with mass loss changes. The ablation loss and morphology of PLLA was induced by excimer laser exposure of plasma pre-treated PLLA with different laser fluencies and number of pulses. The combination of a certain input parameters of plasma pre-treatment together with laser exposure induces extensive physico-chemical changes (morphology, contact angle, optical properties) on polymer surface with dramatic roughness increase. Gravimetric studies have revealed an extensive polymer ablation after excimer laser application. The effect of surface plasmon resonance was observed in laser modified PLLA. Conditions for PLLA surface flattening are also proposed.

Fabrication of Flexible TFTs on a Heat-Resistant Transparent Polymer using the Standard a-Si TFT Process above 300ºC

A method for fabricating thin-film transistors (TFTs) on flexible substrates using a spin-coated heat-resistant transparent polyimide analog polymer, polybenzoxazole (PBO), was developed. Since this material is heat-resistant up to 350ºC, it can withstand the conventional above-300ºC a-Si TFT process. With XeCl excimer laser exposure, the PBO layer with a-Si TFTs could be easily separated from the glass substrate without any damage to the TFTs.

Behavior of 157 nm excimer-laser-induced refractive index changes in silica

This study describes the observation of large induced refractive index changes produced by 157nm excimer laser exposure in high-purity synthetic silica glasses. With 157nm exposure, large induced changes are observed within a few hundred thousand pulses of exposure. Similar to 193 nm exposures, exposure with polarized 157 nm light yields polarization-induced birefringence (PIB). However, the 157 nm exposure also exhibits a behavior not observed with 193 nm exposures; namely, the initial response of the glass is a decrease in refractive index, followed by an increase with continued exposure. An explanation of the behaviors for both wavelength results is proposed where the induced refractive index is considered to arise from two different concurrent phenomena. One produces a decreased refractive index and also accounts for the PIB. The other, which accounts for the increased refractive index, is associated with an isotropic laser-induced volume change.

Reduction of laser-induced roughness in a-Si:H surfaces for vacuum compatible lithography

A vacuum compatible lithography technique has recently been demonstrated, whereby amorphous hydrogenated silicon (a-Si:H) films are used as a resist. Following plasma deposition of the a-Si:H film, poly-Si patterns are generated on the surface by excimer laser exposure through a projection mask. Development is then carried out by hydrogen plasma etching for which etch selectivities of over 1000:1 have been achieved between a-Si:H and poly-Si regions. However, the roughness induced by excimer laser irradiation can be well over ten times that of the as-deposited a-Si:H surface. This is problematic because the roughness may be transferred to underlying device layers during subsequent pattern transfer plasma etching. We have developed a stepwise laser irradiation procedure that reduces the surface roughness by an order of magnitude to a level of ∼1nm. This value is approximately equal to the surface roughness for the as-deposited a-Si:H film. The irradiation procedure uses multiple pulses with progressive increases in the energy density, in contrast to the single high energy density pulses previously used. Transmission electron microscopy and Fourier transform infrared spectroscopy are used to understand and confirm the mechanism behind this process. Our data suggest that the observed reduction in roughness is due both to smaller grain sizes and to a slow rate of H removal from the film surface. The results may also hold significance for other applications of laser processed a-Si:H, even where different film properties are desired.

Characteristics of lead zirconate titanate ferroelectric thick films from a screen-printing laser transfer method

Microstructures and electrical properties of lead zirconate titanate (PZT) thick films made by using a screen-printing laser transfer method have been studied. The PZT thick-film elements were first screen printed on a sapphire substrate and sintered at 1150to1250°C. Then they were bonded to a target substrate such as silicon and released from the sapphire by means of an excimer laser exposure from the backside of the sapphire substrate. This approach makes it possible to obtain highly densified films because there is less limitation on sintering conditions, and allows integrating patterned PZT thick films onto many kinds of substrates. The thick films have dielectric constants of 1397 to 1675, remnant polarization of 32to35μC∕cm2, a piezoelectric constant d31 of about −124pm∕V, and Young’s modulus Y11E(59.4GPa) of almost the same as the corresponding bulk ceramic.

Etching microscopic defects in polycarbonate due to high dose ArF or KrF laser exposure

The ArF or KrF excimer laser exposure on the polycarbonate (PC) with corresponding doses higher than ϕ th 5.2 J / cm 2 , at 32 mJ / cm 2 fluence per pulse and 5 Hz pulse repetition rate (PRR), induces regular defects leading to self assembled defect structure following electrochemical etching (ECE). We have observed the conical-like structure for ϕ > ϕ th , whereas the polymer experiences hardening effect due to crosslinking when ϕ < ϕ th . Subsequently, conical-like, structure turns into track-like pits developing under ECE multiple treeing. Self assembled defect structure may be seen by naked eye as white spots, despite SEM illustrates a type of periodic pit formation-morphology. The exact explanation of the effect is not well understood yet. It looks like alpha tracks in the polymer surface, however the PC pieces were simply treated by excimer lasers at high doses, and they have not been exposed to the nuclear particles afterwards. We could not observe those effects at 308 nm (XeCl laser) or longer wavelengths at 351 nm (XeF laser) where UV photoablation does not occur. It indicates that UV ablation establishes surface degradation at shorter wavelengths, leading to laser micro etching. The mean track (defect) density is about one order of magnitude greater than the normal alpha tracks. Increasing UV doses, polymer undergoes a plateau, corresponding to etched defect saturation on PC.

Processing and characterization of a-Si:H photoresists for a vacuum-compatible photolithography process

The vision of achieving a completely in-vacuum process for fabricating HgCdTe detector arrays is contingent on the availability of a vacuumcompatible photolithography technology. One such technology for vacuum photolithography involves the use of amorphous-hydrogenated Si (a-Si:H) as a photoresist. In this work, we deposit a-Si:H resists via plasma-enhanced chemical-vapor deposition (PECVD) using an Ar-diluted silane precursor. The resists are then patterned via excimer laser exposure and development etched in a hydrogen plasma where etch selectivities between unexposed and exposed regions exceed 600:1. To determine the best conditions for the technique, we investigate the effects of different exposure environments and carry out an analysis of the a-Si:H surfaces before and after development etching. Analysis via transmission electron microscopy (TEM) reveals that the excimer-exposed surfaces are polycrystalline in nature, indicating that the mechanism for pattern generation in this study is based on melting and crystallization. To demonstrate pattern transfer, underlying CdTe films were etched (after development of the resist) in an electron cyclotron resonance (ECR) plasma, where etch selectivities of approximately 8:1 have been achieved. The significance of this work is the demonstration of laser-induced poly-Si as an etching mask for vacuum-compatible photolithography.

Characterization of a-Si:H resists for a vacuum-compatible photolithography process

We report the characterization of amorphous hydrogenated silicon (a-Si:H) films for use as a dry photoresist, in a vacuum-compatible photolithography process. a-Si:H films are deposited, patterned via excimer laser exposure, and developed in a hydrogen plasma. Experiments on the effects of exposure environment and laser pulse energy density were carried out to determine the optimum conditions for the technique. Transmission electron microscopy (TEM) shows the excimer-exposed a-Si:H surfaces to be polycrystalline in nature. The polycrystalline surfaces serve very well as an etching mask during plasma development, exhibiting etch selectivities over 600:1. Preliminary methods for reducing undesirable surface roughness at the exposed surfaces are also discussed.

Design and synthesis of new photoresist materials for ArF lithography

AbstractA new class of photoresist matrix polymers based on vinyl ether–maleic anhydride (VEMA) alternating copolymers was developed for ArF single‐layer lithography. These polymers were synthesized by copolymerization of alkyl vinyl ether and maleic anhydride alternating copolymers with acrylate derivatives containing bulky alicyclic acid‐labile protecting groups. The resulting polymers showed good control of polymerization and high transmittance. Also, these resists exhibited good adhesion to the substrate, high dry‐etching resistance against CF4 mixture gas (1.02 times the etching rate of deep UV resist), and high selectivity to silicon oxide etching. Using an ArF excimer laser exposure system with 0.6 NA, 120‐nm L/S patterns were resolved under conventional illumination. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 165–170, 2004

Nanofabrication for micropatterned cell arrays by combining electron beam-irradiated polymer grafting and localized laser ablation.

Most methods reported for cell-surface patterning are generally based on photolithography and use of silicon or glass substrates with processing analogous to semiconductor manufacturing. Herein, we report a novel method to prepare patterned plastic surfaces to achieve cell arrays by combining homogeneous polymer grafting by electron beam irradiation and localized laser ablation of the grafted polymer. Poly(N-isopropylacrylamide) (PIPAAm) was covalently grafted to surfaces of tissue culture-grade polystyrene dishes. Subsequent ultraviolet ArF excimer laser exposure to limited square areas (sides of 30 or 50 microm) produced patterned ablative photodecomposition of only the surface region (approximately 100-nm depth). Three-dimensional surface profiles showed that these ablated surfaces were as smooth and flat as the original tissue culture-grade polystyrene surfaces. Time-of-flight secondary ion mass spectrometry analysis revealed that the ablated domains exposed basal polystyrene and were surrounded with PIPAAm-grafted chemistry. Before cell seeding, fibronectin was adsorbed selectively onto ablated domains at 20 degrees C, a condition in which the non-ablated grafted PIPAAm matrix remains highly hydrated. Hepatocytes seeded specifically adhered onto the ablated domains adsorbed with fibronectin. Because PIPAAm, inhibits cell adhesion and migration even at 37 degrees C when the grafted density is > 3 microg/cm2, all the cells were confined within the ablated domains. A 100-cell domain array was achieved by this method. This surface modification technique can be utilized for fabrication of cell-based biosensors as well as tissue-engineered constructs.

Low optical loss germanosilicate planar waveguides by low-pressure inductively coupled plasma-enhanced chemical vapor deposition

This Letter reports on the preparation and properties of germanium-doped silica thin films by a new technique namely inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD). 13.56 MHz ICP was generated inside the chamber upon the supplying of 1 KW r.f. power at 2.5 Pa. The Films have been deposited on Si-wafers from the tetraethoxysilane, tetramethoxygermane and oxygen system at 400 °C. The behaviors of ∼200 and ∼240 nm UV absorptions under different treatment conditions comprising annealing temperatures and excimer laser exposure have been systematically investigated. A low propagation loss of ∼0.1 dB/cm at 1550 nm has been achieved.

Improvement of the weak-link in BSCCO by controlled overlapping of excimer laser exposure

Excimer laser annealing is a novel scheme to reduce the surface weak-link of the high- T c superconductors. In the present work, we studied the optimal energy density of excimer laser and the effect of the overlapped area of the laser spots on the BSCCO(2223) samples using Kr–F excimer laser. The superconductor pellets were exposed to the laser with step-and-repeat process, and were then sintered. To form tightly linked intergrain boundaries, such optimal condition was experimentally determined as to have neither overlapping nor separation of the adjacent laser spots on the pellets, for an optimized laser energy density of 0.3–0.4 J/cm 2. When the exposure condition was deviated from the optimal one, the superconductors showed much decrease in critical temperature T c.

Parameter extraction for 193 nm chemically amplified resist from refractive index change

Some of the important areas to be improved for lithography simulation are: obtaining correct exposure parameters and determining the change of refractive index. It is known that the real and imaginary refractive indices are changed during exposure. We obtained these refractive index changes during exposure for 193 nm chemically amplified resists. The variations of the transmittance as well as the resist thickness were measured during ArF excimer laser exposure. We found that the refractive index change is directly related to the concentration of the photo acid generator and deprotected resin. It is important to know the exact values of acid concentration from the exposure parameters since a small difference in acid concentration magnifies the variation in the amplified deprotection during postexposure bake. We developed and used a method to extract Dill ABC exposure parameters for 193 nm chemically amplified resist from the refractive index change upon exposure.

AlGaN-based ultraviolet light detectors with integrated optical filters

AlGaN-based ultraviolet light detectors with high responsivities in a narrow range of photon energy (≈0.3 eV) were grown by plasma induced molecular beam epitaxy on sapphire substrates. A thin Al0.4Ga0.6N film acting as an optical filter blocking high energy light was separated by an insulating AlGaN barrier from a third light sensitive Al0.3Ga0.7N layer. By optimizing the alloy compositions in the heterostructure and thickness of the filter layer, a peak responsivity of up to 35 A/W was achieved over a narrow range of wavelength with a peak position at 305 nm. The rejection of visible light response with respect to the peak responsivity was 5 orders of magnitude for a photoconductor device with a filter thickness of 1.4 μm. Decay times of the photoresponse after excimer laser exposure were determined to be between 40 and 330 ns.

Poly (.ALPHA.-methyl-p-hydroxystyrene-co-methacrylonitrile) Based Single-Layer Resists for VUV Lithography: (2) F2 Excimer Laser Exposure Characteristics.

We have developed a novel single-layer resist for VUV lithography using a terpolymer of methacrylonitrile, p-tert-butoxycarbonyloxy-α-methylstyrene and p-hydroxy-α-methylstyrene. The transmittance at 157nm of this resist was pretty high (-35% per 1000 A-thickness) due to methacrylonitrile units with low absorption. This resist indicated the positive-resist behavior without crosslinking. The 220nm line at 1500A-thick resist and 300nm line at 2000A-thick resist were obtained by F2 excimer laser contact exposure.