Ablation of silicon suboxide thin layers

We investigate the ablation of SiO _x thin films on fused silica substrates using single-pulse exposures at 193 nm and 248 nm. Two ablation modes are considered: front side (the surface of a film is irradiated from above) and rear side (a film is irradiated through its supporting substrate). Fluence is varied from below 200 mJ/cm² to above 3 J/cm². SiO _x films of thickness 200 nm, 400 nm, and 600 nm are ablated. In the case of rear-side illumination, at moderate fluences (around 0.5 mJ/cm²) the ablation depth corresponds roughly to the film thickness, above 1 J/cm² part of the substrate is ablated as well. In the case of front-side ablation the single-pulse ablation depth is limited for all film thicknesses to less than 200 nm even at fluences up to 4 J/cm². Experimental results are discussed in relation to film thickness, fluence, and ablation mode. Simple numerical calculations are performed to clarify the influence of heat transport on the ablation process.     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

Selective ablation of thin SiO<Subscript>2</Subscript> layers on silicon substrates by femto- and picosecond laser pulses

The selective ablation of thin (∼100 nm) SiO₂ layers from silicon wafers has been investigated by applying ultra-short laser pulses at a wavelength of 800 nm with pulse durations in the range from 50 to 2000 fs. We found a strong, monotonic decrease of the laser fluence needed for complete ablation of the dielectric layer with decreasing pulse duration. The threshold fluence for 100% ablation probability decreased from 750 mJ/cm² at 2 ps to 480 mJ/cm² at 50 fs. Significant corruption of the opened Si surface has been observed above ∼1200 mJ/cm², independent of pulse duration. By a detailed analysis of the experimental series the values for melting and breaking thresholds are obtained; the physical mechanisms responsible for the significant dependence on the laser pulse duration are discussed.     

Direct observation of laser-induced crystallization of a-C:H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm². At fluences below 100 mJ/cm² an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm². The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.     

Single and multiple shot near-infrared femtosecond laser pulse ablation thresholds of copper

The single-shot ablation threshold and incubation coefficient of copper were investigated using an amplified near-infrared, femtosecond Ti:sapphire laser. To date, the near-infrared femtosecond ablation threshold of copper has been reported in the range of several hundred millijoules per cm² based primarily on multiple shot ablation studies. A careful study of the single shot ablation threshold for copper was carried out yielding an incident single-shot ablation threshold of (1.06±0.12) J/cm² for a clean copper foil surface. This was determined by measuring the diameters of the ablation spots as a function of the laser pulse energy using scanning electron microscopy for spatially Gaussian laser spots. When multiple shots were taken on the same spot, a reduction in ablation threshold was observed, consistent with a multiple shot incubation coefficient of 0.76±0.02. Similar experiments on 250 nm and 500 nm copper thin films sputtered on a silicon substrate demonstrated that scaling the threshold values with the absorbance of energy at the surface yields a consistent absorbed fluence threshold for copper of (59±10) mJ/cm². This absorbed threshold value is consistent with the expected value from a two-temperature model for the heating of copper with an electron-lattice coupling constant of g=10¹⁷ Wm^-3 K^-1. Single-shot rippling of the surface in the threshold ablation intensity regime was also observed for the foil target but not for the smooth thin film target. PACS 61.80.Ba; 61.82.Bg     

Sub-picosecond UV-laser ablation of Ni films

Laser ablation of thin Ni films on fused silica by 0.5 ps KrF-excimer-laser pulses at 248 nm is reported. The onset of material removal from different film thicknesses (0.1, 0.3, 0.6 and 1.0 m) was measured in a laser ionization time-of-flight mass spectrometer by the amount of Ni atoms vs laser fluence. Significant amounts of metal atoms are already evaporated at laser fluences around 20 mJ/cm², a threshold up to 100 times smaller compared to the one for 14 ns pulses. In contrast to ns laser pulses, the ablation threshold for 0.5 ps pulses is independent of the film thickness. These results reflect the importance of thermal diffusion in laser ablation of strongly absorbing and thermally good conducting materials and prove that for ablation with short pulses, energy loss to the bulk is minimized.     

Dependences of structural and electrical properties on thickness of polycrystalline Ga-doped ZnO thin films prepared by reactive plasma deposition

Polycrystalline Ga-doped (Ga content: 4 wt%) ZnO (GZO) thin films were deposited on glass substrates at 200 C by a reactive plasma deposition with DC arc discharge technique. The dependences of structural and electrical properties of GZO films on thickness, ranging from 30 to 560 nm, were investigated. Carrier concentration, n, and Hall mobility, μ, increases with increasing film thickness below 100 nm, and then the n remains nearly constant and the μ gradually increases until the thickness reaches 560 nm. The resistivity obtained of the order of 10⁻⁴ Ω cm for these films decreases with increasing film thickness: The highest resistivity achieved is 4.4×10⁻⁴ Ω cm with n of 7.6×10²⁰ cm⁻³ and μ of 18.5 cm²/V s for GZO films with a thickness of 30 nm and the lowest one is 1.8×10⁻⁴ Ω cm with n of 1.1×10²¹ cm⁻³ and μ of 31.7 cm²/V s for the GZO film with a thickness of 560 nm. X-ray diffraction pattern for all the films shows a hexagonal wurtzite structure with its strongly preferred orientation along the c-axis. Full width at half maximum of the (002) preferred orientation diffraction peak of the films decreases with increasing film thickness below 100 nm.     

Synthesis and field emission of patterned ZnO nanorods

A simple and self-catalytic method has been developed for synthesizing finely patterned ZnO nanorods on ITO-glass substrates under a low temperature of 500 °C. The patterned ZnO nanorod arrays, a unit area is of 400 × 100 μm², are synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized ZnO nanorods are characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The mechanism of formation of ZnO nanorods is also discussed. The measurement of field emission (FE) reveals that the as-synthesized ZnO nanorods arrays have a turn-on field of 3.3 V/μm at the current density of 0.1 μA/cm² and a low threshold field of 6.2 V/μm at the current density of 1 mA/cm². So this approach must have a potential application of fabricating micropatterned oxide thin films used in FE-based flat panel displays.     

UV laser-induced grating formation in PDMS thin films

Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxane (PDMS) thin films is demonstrated. Ellipsometric measurements provide values of the optical constants of the films as well as their thicknesses, which are below 1 m. At fluences above 160 mJ/cm² two pulses of UV light induce gratings with at minimum 1-m periods and crossed gratings with 4-m periods. The structure heights are between 10 nm and 20 nm with ridge widths of several hundred nanometres. The ablation occurs after a single incubation pulse with a threshold that increases logarithmically with the ablation wavelength increasing from 157 nm to 1064 nm. At 193 nm the ablation rate for 2 J/cm² is 127 nm/pulse. PACS 79.20.La; 34.50.Dy; 68.55.Jk     

Dependence of ablation threshold and LIPSS formation on copper thin films by accumulative UV picosecond laser shots

The ablation threshold and Laser-induced periodic surface structure (LIPSS) formation on copper thin film were investigated using a picosecond laser (Nd:YAG laser: 266 nm, 42 ps, 10 Hz). We show that the ablation threshold varies with respect to the number of laser shots (N) on two different substrates. The single-shot ablation threshold was estimated to be close to 170 ± 20 mJ/cm². The incubation coefficient was estimated to be 0.68 ± 0.03 for copper thin films on silicon and glass substrates. In addition, morphology changes of the ablated regions, in the same spot area, were studied as a function of fluence and number of laser shots. An intermediate structure occurred with a mix of low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and regular spikes at a fluence F < 250 mJ/cm² and 1,000 < N ≤ 10.000 shots. LSFL was observed with a spatial period close to the irradiation wavelength and an orientation perpendicular to the laser polarization, and HSFL with a spatial period of ～120 nm and a parallel orientation. Lastly, the global relationship between the laser parameters (i.e. fluence and number of shots) and LIPSS formation was established in the form of a 2D map.     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Development of a novel high speed (electron-mobility) epi-n-ZnO thin films by L-MBE for III–V opto-electronic devices

Intrinsic epitaxial zinc oxide (epi-ZnO) thin films were grown by laser-molecular beam epitaxy (L-MBE), i.e., pulsed laser deposition (PLD) technique using Johnson Matthey “specpure”-grade ZnO pellets. The effects of substrate temperatures on ZnO thin film growth, electrical conductivity (σ), mobility (μ) and carrier concentration (n) were studied. As well as the feasibility of developing high quality conducting oxide thin films was also studied simultaneously. The highest conductivity was found for optimized epi-ZnO thin films is σ=0.06×10³ ohm⁻¹ cm⁻¹ (n-type) (which is almost at the edge of semiconductivity range), carrier density n=0.316×10¹⁹ cm⁻³ and mobility μ=98 cm²/V s. The electrical studies further confirmed the semiconductor characteristics of epi-n-ZnO thin films. The relationship between the optical and electrical properties were also graphically enumerated. The electrical parameter values for the films were calculated, graphically enumerated and tabulated. As a novelty point of view, we have concluded that without doping and annealing, we have obtained optimum electrical conductivity with high optical transparency (95%) for as deposited ZnO thin films using PLD. Also, this is the first time that we have applied PLD made ZnO thin films to iso-, hetero-semiconductor–insulator–semiconductor (SIS) type solar cells as transparent conducting oxide (TCO) window layer. We hope that surely these data be helpful either as a scientific or technical basis in the semiconductor processing.     

Low-temperature laser processes for synthesizing (100)-textured Pb(Zr,Ti)O3 thin films on Si substrate

High-performance Pb(Zr,Ti)O₃, PZT, thin films were synthesized on Si substrates by using low-temperature laser-assisted processes, which combine pulsed laser deposition (PLD), laser lift-off (LLO) and laser-annealing (LA) processes. The PZT films were first grown on sapphire substrates at 400 °C, using Ba(Mg_1/3Ta_2/3)O₃, BMT, as seeding layer, by the PLD process, and were then transferred to Si substrates at room temperature by a LLO transferring process. Utilization of the BMT layer is of critical importance in those processes, since it acted as a nucleation layer for the synthesis of the PZT thin films on the sapphire substrates and, at the same time, served as a sacrificial layer during laser irradiation in the LLO process. After the LLO process, the surfaces of the PZT films were recovered by the LA process for removing the damage induced by the LLO process. A thin BMT (∼30 nm) layer is randomly oriented, resulting in non-textured PZT films with good ferroelectric properties, viz. P_r=20.6 μC/cm² and E_c=126 kV/cm, whereas a thick BMT (∼100 nm) layer is (100) preferentially oriented, leading to (100)-textured PZT films with markedly better ferroelectric properties, viz. P_r=34.4 μC/cm² and E_c=360 kV/cm. PACS 81.15.Fg; 77.84.-s     

Preparation of Y2O3:Eu thin films by excimer-laser-assisted metal organic deposition

Europium-doped yttrium oxide (Y₂O₃:Eu) thin films were successfully deposited on quartz and ITO/glass substrates by excimer-laser-assisted metal organic deposition (ELAMOD) at low temperatures. The effects of laser wavelength and thermal temperature on the films’ crystallinity and photoluminescence properties were investigated. Films irradiated by an ArF laser at 80 mJ/cm² and 400–500°C were highly crystallized compared with those prepared by thermal MOD. In contrast, when the film was irradiated by a KrF laser at 500°C, no crystalline Y₂O₃:Eu was formed. The Y₂O₃:Eu film irradiated by the ArF laser at 80 mJ/cm² and 500°C showed typical PL spectra of Eu³⁺ ions with cubic symmetry and a ⁵D₀→⁷F₂ transition at ∼612 nm. The PL intensity at 612 nm was much higher for the film prepared with ELAMOD than for that prepared by the thermal-assisted process, and the photoemission intensity of the film prepared with ELAMOD strongly depended on the substrate material.     

Direct observation of phase transitions by time-resolved pyro/reflectometry of KrF laser-irradiated metal oxides and metals

New experimental results are obtained by coupling both time-resolved reflectivity and rapid infrared pyrometry under a hemispherical reactor. The heating source KrF laser beam (28 ns, 248 nm) is homogenized and as for probing, a CW He-Ne laser beam (10 mW, 633 nm) is used.Using both methods infrared pyrometry with an IR detector cooled with liquid nitrogen and sensitive in the spectral range 1-12 μm, and time-resolved reflectivity with a rapid photodiode, we were able to study complex thermodynamic transitions with nanosecond time resolution. Three different materials are studied by varying the KrF fluence (energy/surface) from 100 to 2000 mJ/cm²: thin films melting (Au/Ni), the threshold of plasma formation (Ti), and complex liquid phase segregation under semi-conductor state (ZnO). The formation of a liquid Zn film induced by temperature gradient is well evidenced by our signals. Also melting of thin films irradiated by low laser fluences (less than 500 mJ/cm²) translates the typical thermodynamic behavior. Finally, wide fluence dynamic (400-2000 mJ/cm²) is analyzed in the case of Ti surface, and results show two distinguished regimes: first one bellow 1000 mJ/cm² corresponding to the early stage plasma initiation, and second one over 1000 mJ/cm² to the dynamics of plasma expansion.     

High-resolution electron microscopy study of SiGeC thin films grown on Si(1 0 0) by laser-assisted techniques

PLIE was used for rapid crystallisation of a-SiGeC films deposited by LCVD on Si(1 0 0) substrates. HRTEM study of thin films grown with several laser energies shows that the combination of the two laser techniques gives an almost completely crystallised alloy, even for the lowest laser fluence. Island formation is observed below a certain threshold of fluence (about 450 mJ/cm²). In the case of the lowest energy (100 mJ/cm²) the material was partially crystallised (with the crystalline material being the predominant state), to a nanocrystalline alloy with a considerable amount of epitaxialy grown grains and with grain sizes of several tens of nanometers. Above the threshold of 450 mJ/cm² a rather smooth thin film is grown. The crystallisation is almost complete and the alloy is grown in an almost perfect epitaxial way.     

Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

Mechanism of backside etching of transparent materials with nanosecond UV-lasers

In consequence of high interest in micro- and nanomachining of transparent materials by laser irradiation, studies on the mechanism of laser-induced backside wet etching (LIBWE) are presented. To reveal the role of the surface modification due to LIBWE the backside ablation (BSA) of LIBWE-modified fused silica (mFS) surfaces at 248 nm was investigated. The threshold fluence and the etch rate of BSA are similar to that of LIBWE and amount ∼250 mJ/cm² and 30 nm for 1 J/cm², respectively. The sample transmission after backside ablation of mFS increases and proves the decreasing thickness of the absorbing layer. Time-resolved reflection studies at LIBWE and BSA of mFS show similar patterns in the backside reflection that can be assigned an ablation process as the comparison to thin polymer films demonstrates. By fitting the BSA data to an exponential decay absorption model a modification depth and a surface absorption of about 38 nm and α ^S ∼1.3×10⁷ m⁻¹ were calculated, respectively. In conclusion of the results a new model for LIBWE is proposed.     

Hardening of smooth pulsed laser deposited PMMA films by heating

Smooth poly(methyl methacrylate) (PMMA) films without any droplets were pulsed laser deposited at a wavelength of 248 nm and a laser fluence of 125 mJ/cm². After deposition at room temperature, the films possess low universal hardness of only 3 N/mm². Thermal treatments up to 200°C, either during deposition or afterwards, lead to film hardening up to values of 200 N/mm². Using a combination of complementary methods, two main mechanisms could be made responsible for this temperature induced hardening effect well above the glass transition temperature of 102°C. The first process is induced by the evaporation of chain fragments and low molecular mass material, which are present in the film due to the ablation process, leading to an increase of the average molecular mass and thus to hardening. The second mechanism can be seen in partial cross-linking of the polymer film as soon as chain scission occurs at higher temperatures and the mobility and reactivity of the polymer material is high enough.     

Patterning of PLZT and PSZT thin films by excimer laser

The patterning of lanthanum-doped lead zirconate titanate (PLZT) and strontium-doped lead zirconate titanate (PSZT) thin films has been examined using a 5-ns pulsed excimer laser. Both types of film were deposited by rf magnetron sputtering with in situ heating and a controlled cooling rate in order to obtain the perovskite-structured films. The depth of laser ablation in both PSZT and PLZT films showed a logarithmic dependence on fluence. The ablation rate of PLZT films was slightly higher than that of PSZT films over the range of fluence (10–150 J/cm²) and increased linearly with number of pulses. The threshold fluence required to initiate ablation was ∼ 1.25 J/cm² for PLZT and ∼ 1.87 J/cm² for PSZT films. Individual squares were patterned with areas ranging from 10×10 μm² up to 30×30 μm² using single and multiple pulses. The morphology of the etched surfaces comprised globules which had diameters of 200–250 nm in PLZT and 1400 nm in PSZT films. The diameter of the globules has been shown to increase with fluence until reaching an approximately constant size at ≤ 20 J/cm² in both types of film. The composition of the films following ablation has been compared using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. PACS 79.20.Ds; 82.80.Pv; 82.80.Ej