Micromachining of copper using Nd:YAG laser radiation at 1064, 532, and 355 nm wavelengths

The interaction phenomena of nanosecond time period Q-switched diode-pumped Nd:YAG laser pulses using 1064, 532 and 355 nm with 0.25 mm thick pure-copper foil was investigated at an incident laser intensity range of 0.5–57.9 GW/cm². For each sample, etch rate and surface structure were determined. Analysis of the results of the tests included scanning electron microscopy (SEM). A maximum etch rate of 13.3 μm per pulse was obtained for the etch rate tests carried out at 532 nm. The maximum etch rate obtainable for 1064 nm was 2.21 μm per pulse, and for 355 nm, 6.68 μm per pulse. The dramatic decrease in etch rate observed when processing at 1064 nm is thought to occur due the highly reflective nature of copper as the interaction wavelength is increased, plus the nature of the plasma formed above the material during the high-intensity laser–material interaction. This plasma then imparts energy to the surface of the processed area leading to surface melting of the area surrounding the hole as can be seen by the SEM photographs.     

Using IR laser radiation for backside etching of fused silica

Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about 3 J/cm² and 7 J/cm² for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom. A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm. The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Backside etching of fused silica with Nd:YAG laser

The laser-induced backside etching of fused silica with gallium as highly absorbing backside absorber using pulsed infrared Nd:YAG laser radiation is demonstrated for the first time. The influence of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography was studied. The comparable high threshold fluences of about 3 and 7 J/cm² for 18 and 73 ns pulses, respectively, are caused by the high reflectivity of the fused silica-gallium interface and the high thermal conductivity of gallium. For the 18 and 73 ns long pulses the etch rate rises almost linearly with the laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. Incubation processes are almost absent because etching is already observed with the first laser pulse at all etch conditions and the etch rate is constant up to 30 pulses.The etched grooves are Gaussian-curved and show well-defined edges and a smooth bottom. The roughness measured by interference microscopy was 1.5 nm rms at an etch depth of 0.6 μm. The laser-induced backside etching with gallium is a promising approach for the industrial application of the backside etching technique with IR Nd:YAG laser.     

Laser-induced etching of titanium by Br2 and CCl3Br at 248 nm

A quartz crystal microbalance (QCM) has been used to study the KrF* excimer laser-induced etching of titanium by bromine-containing compounds. The experiment consists of focusing the pulsed UV laser beam at normal incidence onto the surface of a quartz crystal coated with 1 m of polycrystalline titanium. The removal of titanium from the surface is monitored in real time by measuring the change in the frequency of the quartz crystal. The dependence of the etch rate on etchant pressure and laser fluence was measured and found to be consistent with a two-step etching mechanism. The initial step in the etching of titanium is reaction between the etchant and the surface to form the etch product between laser pulses. The etch product is subsequently removed from the surface during the laser pulse via a laser-induced thermal desorption process. The maximum etch rate obtained in this work was 6.2 Å-pulse^–1, indicating that between two and three atomic layers of Ti can be removed per laser pulse. The energy required for desorption of the etch product is calculated to be 172 kJ-mole^–1, which is consistent with the sublimation enthalpy of TiBr₂ (168 kJ-mole^–1). The proposed product in the etching of titanium by Br₂ and CCl₃Br is thus TiBr₂. In the etching of Ti by Br₂, formation of TiBr₂ proceeds predominantly through the dissociative chemisorption of Br₂. In the case of etching with CCl₃Br, TiBr₂ is formed via chemisorption of Br atoms produced in the gas-phase photodissociation of CCl₃Br.     

InGaAsP/InP 1.55-μm lasers with chemically assisted ion beam-etched facets

Chemically assisted ion beam etching (CAIBE) involving an Ar ion beam and a halogen ambient gas (Cl₂, IBr₃) has been used to etch high-quality laser facets for InGaAsP/InP bulk lasers (1.55 m). We achieved eich rates of 40.0–75.0 nm min^–1 at substrate temperatures between-5 and +10°C. These low temperatures have allowed us to utilize UV-baked photoresists as well as PMMA as etch masks, facilitating very simple process development. Higher substrate temperatures (50 to 120°C) yield still higher etch rates, but at the expense of severely degraded surface morphologies. Angle resolved x-ray photoelectron spectroscopy (XPS) was investigated for observing etched InP surfaces. A disproportioned surface has been detected after etching in the higher temperature range; low temperatures yield stoichiometric surfaces.     

Effects of halogenated organic solvents on laser-induced backside wet etching of fused silica

Laser-induced backside wet etching of fused silica using a solution of pyrene dissolved in halogenated and non-halogenated solvents is presented. A significant influence of the solvent used on the etch rate and the etched surface appearance was ascertained. The etching of uniform and smooth surfaces with rates of ∼0.1 nm/pulse for laser fluences below 500 mJ/cm² is observed only for halogenated solvents. Furthermore, reduced threshold fluences, only small incubation effects, and a constant etch rate in dependence on the pulse number were found. The experimental data suggest an additional etch process at low laser fluences characterized by the very low etch rate and the smooth etching observed only with halogen-containing solvents. The generation of halogen radicals/compounds close to the heated surface due to the decomposition of the solvent causing the attack of the surface seems the most probable mechanism. PACS 81.65.Cf; 81.05.Kf; 79.20.Ds; 61.80.Ba; 42.55.Lt; 68.45.Da     

Structural characterization and optoelectronic properties of GaN thin films on Si(111) substrates using pulsed laser deposition assisted by gas discharge

GaN films have been grown on Si(111) substrates with a thin AlN buffer layer using pulsed laser deposition (PLD) assisted by gas discharge. The crystalline quality, surface morphology and optoelectronic properties of the deposited films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and room-temperature Van der Pauw–Hall measurements. The influence of the deposition temperature in the range 637–1037 K on the crystallinity of GaN films, the laser incident energy in the range 150–250 mJ/pulse on the surface morphology and the optoelectronic properties were systematically studied. The XRD analysis shows that the crystalline quality of the GaN films improves with increasing deposition temperature to 937 K, but further increase of the deposition temperature to 1037 K leads to the degradation of the crystalline quality. AFM results show that the surface roughness of the GaN films can be decreased with increasing laser incident energy to 220 mJ/pulse. Further increase of the laser incident energy to 250 mJ/pulse leads to an increase in the surface roughness. The optoelectronic properties of GaN films were also improved by increasing the laser incident energy to 220 mJ/pulse. GaN films which have a n-type carrier concentration of 1.26×10¹⁷ cm^-3 and a mobility of 158.1 cm²/Vs can be deposited at a substrate temperature of 937 K, a deposition pressure of 20 Pa and a laser incident energy of 220 mJ/pulse. Their room-temperature PL spectra exhibit a strong band-edge emission at 365 nm. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 73.61.Ey; 78.66.Fd     

Laser-assisted etching of gallium arsenide in chlorine at 308 nm

Xenon chloride (308 nm) excimer laser-assisted etching of GaAs (100) in Cl₂ was demonstrated and characterized with respect to laser and gas parameters. The etch rate increased linearly with laser fluence from thresholds in the range of 50 to 75 mJ/cm² to the highest fluence studied, 650 mJ/cm². For a laser fluence of 370 mJ/cm², the etch rate varied with Cl₂ pressure reaching a maximum at a Cl₂ pressure of about 2 Torr. The etch rate decreased monotonically with Ar buffer gas pressure because of redeposition of GaCl₃ products into the etched channel. The redeposited GaCl₃ affected the etch rate and the etch morphology. The etch rate and morphology also varied with laser repetition rate. The mobility of chlorine on the surface also plays an important role in the etching mechanism.     

Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses

The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for structuring transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF₂, MgF₂), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond (t_p = 10 ps) lasers at UV wavelengths (λ₁ = 355 nm and λ₂ = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with different slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a λ₁ = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths (λ₁ = 266 nm and λ₂ = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm², respectively.     

Nanosecond and femtosecond excimer laser ablation of fused silica

Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 m/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm²) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.The results lead to the conclusion that two different ablation mechanisms are involved: the first is based on two photon bulk absorption, the second on controlled surface damage in relation with (partially laser induced) singularity conditions at the surface.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Laser etching of transparent materials at a backside surface adsorbed layer

The laser etching using a surface adsorbed layer (LESAL) is a new method for precise etching of transparent materials with pulsed UV-laser beams. The influence of the processing parameters to the etch rate and the surface roughness for etching of fused silica, quartz, sapphire, and magnesium fluoride (MgF₂) is investigated. Low etch rates of 1 nm/pulse and low roughness of about 1 nm rms were found for fused silica and quartz. This is an indication that different structural modifications of the material do not affect the etching significantly as long as the physical properties are not changed. MgF₂ and sapphire feature a principal different etch behavior with a higher etch rate and a higher roughness. Both incubation effects as well as the temperature dependence of the etch rate can be interpreted by the formation of a modified near surface region due to the laser irradiation. At repetition rates up to 100 Hz, no changes of the etch rate have been observed at moderate laser fluences.     

Laser projection-patterned etching of (100) GaAs by gaseous HCl and CH3Cl

Laser projection-patterned etching of GaAs in a HCl and CH₃Cl atmosphere performed using a pulsed KrF-excimer laser (=248 nm, =15 ns) and deep-UV projection optics (resolution 2 m) is reported. The etching process carried out in a vacuum system having a base pressure of 10^–6 mbar is shown to result from a purely thermochemical reaction. Etching takes place in two steps: (i) between the laser pulses, the etchant gas reacts with the GaAs surface-atomic layer to form chlorination products (mainly As and Ga monochlorides), (ii) local laser surface heating results in the desorption of these products allowing further reaction of the gas with the surface. The influence of the etching parameters (laser energy density, gas pressure and pulse repetition rate) on the etch rate and the morphology of the etched features was studied. Etch rates up to 0.15 nm per pulse, corresponding to the removal of 0.5 GaAs molecular layer, are achieved. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the projected features of the photomask are achieved depending on the laser energy density. Etched lines having a width of 1.3 m were obtained at low fluences by the projection of 2 m wide lines onto the GaAs surface.     

Laser-fluorescence diagnostics for condensation in laser-ablated copper plasmas

We are investigating the thermodynamic conditions under which condensation occurs in laser ablated copper plasma plumes. The plasma is created by XeCl excimer laser ablation (308 nm, 300 mJ/pulse) at power densities from 500–1000 MW/cm² into backing pressures of helium in the range 0–50 torr. We use laser-induced fluorescence (LIF) to probe velocity and relative density of both atomic copper and the copper dimer molecule, Cu₂, which is formed during condensation onset. At low pressure (10 mtorr), the atomic Cu velocity peaks at approximately 2×10⁶ cm/s. Copper dimer time-of-flight data suggest that condensation onset occurs after the Cu atoms have slowed very significantly. Excitation scans of the Cu₂A-X (0,0) and (1,1) bands yield a rotational and vibrational temperature in the neighborhood of 300 K for all conditions studied. Such low temperatures support the theory that Cu₂ is formed under thermally and translationally cold conditions. Direct laser beam absorption is used to determine the number density of atomic copper. Typical densities attained with 5 torr of helium backing gas are 6–8×10¹³ cm^–3. Rayleigh scattering from particulate is easily observable under conditions favorable to particulate production.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

Indirect laser etching of fused silica: Towards high etching rate processing

The indirect laser processing approach (LIBWE) laser-induced backside wet etching allows defined microstructuring of transparent materials at low laser fluences with high quality. The optical and the thermal properties of the solid/liquid interface determine the temperatures and therefore the etching mechanism in conjunction with the dynamic processes at the interface due to the fast heating/cooling rates. The exploration of organic liquid solvents and solutions such as 0.5 M pyrene/toluene results in low etch rates (∼20 nm/pulse). By means of liquid metals as absorber here, demonstrated for gallium (Ga), etch rates up to 600 nm/pulse can be achieved. Regardless of the high etch rates a still smooth surface similar to etching with organic liquid solutions can be observed. A comparative study of the two kinds of absorbing liquids, organic and metallic, investigates the etch rates regarding the fluence and pulse quantity. Thereby, the effect of incubation processes as result of surface modification on the etching is discussed. In contrast to pyrene/toluene solution the metallic absorber cannot decompose and consequently no decomposition products can alter the solid/liquid interface to enhance the absorption for the laser radiation. Hence, incubation can be neglected in the case of the silica/gallium interface so that this system is a suitable model to investigate the primary processes of LIBWE. To prove the proposed thermal etch mechanism an analytical temperature model based on a solution of the heat equation is derived for laser absorption at the silica/gallium interface.     

Reactive pulsed laser deposition and laser induced crystallization of SnO2 transparent conducting thin films

Transparent conducting SnO₂ thin films with a thickness between 1000–2000 Å were deposited on glass, quartz and silicon substrates using standard pulsed laser deposition techniques with two different targets (Sri and SnO₂) and with three different laser wavelengths (1.06, 0.532 and 0.266 ) from a Q-switched Nd: YAG laser. Tin dioxide films with optical transmission over most of the visible spectrum exceeding 80% were obtained using a Sn target and a background oxygen pressure of 20 Pa. The electrical resistivity () depended strongly on the substrate temperature during deposition, with the lowest values of of about 10^–2 -cm obtained when the substrate was maintained at 400°C during deposition. Using SnO₂ targets, predominantly amorphous phase SnO₂ films were deposited on Si substrates and then transformed into polycrystalline Sn₃O₄ by laser induced crystallization ( = 1.06 m). Whereas these later films were essentially non-conducting as deposited ( > 400 -cm), the electrical resistivity was permanently reduced after laser induced crystallization by a factor greater than 1000 to a value of approximately 4 × 10^–1 -cm.     

Laser-induced doping of GaAs

Thin layers of GaAs are heavily doped locally by laser induced Se or Zn diffusion. H₂Se or diethylzinc gases are used to provide Se or Zn dopant atoms. The surface is locally heated with 3 ns light pulses from a Q-switched frequency doubled Nd-YAG laser. The doping process is described in detail. Doping profiles and sheet carrier concentrations are measured as a function of substrate temperature, laser fluence and processing time. Dopant concentrations of more than 10²¹ cm^–3, with a thickness of the doped layer of less than 20 nm can be achieved.     

Laser etching of fused silica using an adsorbed toluene layer

A new method for laser etching of transparent materials with a low etch rate and a very good surface quality is demonstrated. It is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. The threshold fluence for etching of fused silica amounts to 0.7 J/cm². The constant etch rate of about 1.3 nm/pulse that has been observed in a fluence interval from 2 to 5 J/cm² is evidence of a saturated process. The limited thickness of the adsorbed layer causes the low etch rates and the rate saturation. The etched surface structures have well defined edges and low surface roughness values of down to 0.4 nm rms. PACS 81.65.Cf; 81.05.Kf; 79.20.Ds; 61.80.Ba; 42.55.Lt     

Tunable,sub-nanosecond KrF-Raman laser in the ultraviolet

A 0.5 cm^–1 bandwidth injection-locked KrF laser pumps a rare-gas Brillouin cell to produce a reflected pulse with a leading edge risetime of 1 ns, tunable from 248.1 to 248.7 nm. Consistent with Lamb theory of laser amplifiers, subsequent excimer amplification of this pulse produces an intense 500 ps spike on the pulse leading edge. Stimulated Raman scattering then separates the spike from the parent pulse, yielding a tunable short pulse at the first Stokes (S ₁) wavelength. Varying the Raman cell length results in a variable Raman threshold and an adjustable short pulse duration: 250 ps pulses at energies of 3–4 mJ at 268 nm with a 50 cm methane cell and 350 ps, 5 mJ pulses from a 100 cm cell are measured with a streak camera. First pass Raman conversion of the spike toS ₁ followed by second pass backward Raman amplification, where the parent 248 nm pulse serves as the pump beam for the reflectedS ₁ pulse, yields simultaneousS ₁ pulses of 20–25 mJ in the 800 ps range andS ₂ pulses of 550 ps at 5–6 mJ near 290 nm. This laser will avoid collision effects during laser excitation and enable quantitative, single pulse imaging of OH radicals in turbulent combustion because of its high pulse energy.     

Photokinetic etching of polyethylene terephthalate films by continuous wave ultraviolet laser radiation

Continuous wave laser radiation from an argonion laser in the wavelength range 275–330 nm can be used to etch polyethylene terephthalate (PET) films with as little thermal damage as from a pulsed, ultraviolet laser (248 nm or 308 nm) provided the beam is focussed to a spot of 10–100 kW/cm² of power density and is moved over the surface at speeds at which the transit time over its own diameter (which can be looked upon as a pulse width) is on the order of 10–200 s. In contrast to results which had been obtained previously on the photokinetic etching of polyimide and doped polymethyl methacrylate films under similar conditions, the sensitivity of PET to etching is >5-fold greater than either of these polymers and increases steadily with increasing pulse width. There is lateral thermal damage as the pulse widths increase to >200 s. The material that is removed is vaporized in part. More than 20% is probably ejected in a molten state and resolidifies at the edge of the cut. There is no acoustic report similar to that seen in ablative photodecomposition. The process appears to be largely thermal in nature.