Etching Threshold Research Articles

Laser sputtering–part IV: Laser interferometry and laser-induced fluorescence studies of insulators

Abstract Ultra-violet laser etching (sputtering) of solids has been explained in various terms, including thermal, electronic, hydrodynamic, exfoliational, and even collisional processes. To distinguish between these possibilities, two laser techniques are used here to probe the plume of material expanding away from the etched surface of certain insulators due to 248 nm pulses. The first set of experiments measures the free-electron density in the plume using a Michelson interferometer. The results indicate that a plasma is formed for laser fluences greater than some threshold value (which depends on the material). With Al2O3, the plasma threshold is, moreover, similar to but slightly higher than the etching threshold, suggesting that collisional sputtering due to fast ions from this plasma does not contribute substantially to the etching. The second set of experiments, using laser-induced fluorescence, measures the number densities and energy distributions of AI and AIO in the plume when etching Al2O3. Time-of-flight distributions indicate Boltzmann-like behavior with kT equal to ∼3.5–17eV for A1 and ∼1.1–0.3 eV for AlO. In contrast, the rotational and vibrational energies of A10 correspond to only 0.04–0.16 eV, suggestive of a low surface temperature during etching. These observations are consistent with an electronic desorption mechanism similar to that found with incident fission fragments, electron pulses, or small accelerated dust particles.

Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies

The formation of C2 in the ablation of polymethyl methacrylate (PMMA) by 193- and 248-nm laser pulses was studied by laser-induced fluorescence. The formation of C2 is observable at a fluence at 248 nm that is well below the fluence threshold where significant etching takes place. The opposite is true at 193 nm. The velocity distribution of the diatomic form was also measured and found to approach but not exactly fit a Maxwell–Boltzmann equation. Average translational energies as high as 6 eV were recorded even at the fluence threshold for this product. The other products of the laser ablation of PMMA at 193 or 248 nm are methyl methacrylate (MMA) and a solid that is a low-molecular-weight fraction of PMMA. Although the products are the same at both wavelengths, the mix is quite different. At 193 nm, 18% of the ablated polymer is MMA, whereas at 248 nm less than 1% of the polymer appeared as MMA. A semilog plot of the mass of material removed versus the fluence shows three distinct regions. The central portion at each wavelength (80–300 mJ/cm2 at 193 nm; 600–2000 mJ/cm2 at 248 nm), which corresponds to rapid etching, is identified as ablative photodecomposition. At lower fluences, the etching efficiency falls off rapidly, indicating that there is a threshold fluence. At fluences above the range for ablative photodecomposition, the etching levels off, probably because of the secondary absorption of the incoming photons by the products. The mass of material ablated per joule of energy absorbed in the ablated volume was remarkably similar at both wavelengths. It is suggested that ablative photodecomposition involves both a one-photon process that produces MMA and low-molecular-weight polymeric fragments and a many-photon process that gives rise to products such as C2 with high translational energy. In order to establish the role of the many-photon process, samples of PMMA doped with acridine were exposed to laser pulses. The presence of as little as 2% of acridine lowers the threshold for significant etching at 248 nm from 400 to 90 mJ/cm2. The absorption characteristics of the doped polymer at this wavelength show that the acridine molecules absorb >10 photons each, whereas the absorption of the polymer changes little. The drop in the threshold energy for etching should therefore be attributed to the many-photon decomposition of the acridine and the consequent ablation of the polymer as a whole.

Quartz crystal microbalance simulation of the directionality of Si etching in CF4 glow discharges

Two quartz crystal microbalances have been mounted in a planar rf discharge system in such a way that the potential of the microbalances with respect to the glow discharge can be varied. This apparatus allows a rapid simulation of the etching directionality that can be expected in real pattern transfer situations in that operating one microbalance at ground and one at a negative potential gives a measure of the sidewall and vertical etch rates, respectively. The voltage threshold for ion-assisted etching has been determined to be 20 V which is the approximate value of the plasma potential in this asymmetric system.

X-ray small angle scattering investigation of high energy Ar-tracks in mica

Abstract Latent nuclear tracks were observed below and above the track etch threshold using small angle scattering. In contradiction to previously accepted models, an agglomeration of point defects along the ion path can be ruled out. The latent track is a long uninterrupted cylinder with a Gaussian radial density variation.