[en] Optical emission spectroscopy measurements were performed with added trace probe gases in an atmospheric pressure direct current (DC) helium microplasma. Spatially resolved measurements (resolution ∼ 6 (micro)m) were taken across a 200 (micro)m slot-type discharge. Stark splitting of the hydrogen Balmer-line was used to investigate the electric field distribution in the cathode sheath region. Electron densities were evaluated from the analysis of the spectral line broadenings of H-β emission. The electron density in the bulk plasma was in the range 3-8 x 1013 cm-3. The electric field peaked at the cathode (∼60 kV/cm) and decayed to small values over a distance of ∼ 50 (micro)m (sheath edge) from the cathode. These experimental data were in good agreement with a self-consistent one-dimensional model of the discharge. The dependence of gas temperature on gas flow through the slot-type, atmospheric pressure microplasma in helium or argon was investigated by a combination of experiments and modeling. Spatially-resolved gas temperature profiles across the gap between the two electrodes were obtained from rotational analysis of N₂ (C³II_u → B³ II_g) emission spectra, with small amounts of N₂ added as actinometer gas. Under the same input power of 20 kW/cm³, the peak gas temperature in helium (∼650 K) was significantly lower than that in argon (over 1200 K). This reflects the much higher thermal conductivity of helium gas. The gas temperature decreased with increasing gas flow rate, more so in argon compared to helium. This was consistent with the fact that conductive heat losses dominate in helium microplasmas, while convective heat losses play a major role in argon microplasmas. A plasma-gas flow simulation of the microdischarge, including a chemistry set, a compressible Navier-Stokes (and mass continuity) equation, and a convective heat transport equation, was also performed. Experimental measurements were in good agreement with simulation predictions. Finally, laser scattering experiments were performed at pressures of 100s of Torr in argon or nitrogen. Laser Thomson Scattering (LTS) and Rotational Raman Scattering were employed in a novel, backscattering, confocal configuration. LTS allows direct and simultaneous measurement of both electron density (ne) and electron temperature (Te). For 50 mA current and over the pressure range of 300-700 Torr, LTS yielded Te = 0.9 ± 0.3 eV and ne = (6 ± 3) 1013 cm-3, in reasonable agreement with the predictions of a mathematical model. Rotational Raman spectroscopy (RRS) was employed for absolute calibration of the LTS signal. RRS was also applied to measure the 3D gas temperature (Tg) in nitrogen DC microdischarges. In addition, diode laser absorption spectroscopy was employed to measure the density of argon metastables (1s5 in Paschen notations) in argon microdischarges. The gas temperature, extracted from the width of the absorption profile, was compared with Tg values obtained by optical emission spectroscopy.

[en] Plasma and surface diagnostics of Cl₂/O₂ mixed-gas inductively coupled plasmas are reported. Using trace rare gas optical emission spectroscopy and Langmuir probe analysis, electron temperatures (T_e) and number densities for Cl atoms (n_Cl), electrons (n_e), and positive ions were measured as a function of percent O₂ in the feed gas and position in the plasma chamber. Adsorbates on and products desorbing from a rotating anodized aluminum substrate exposed to the plasma were detected with an Auger electron spectrometer and a quadrupole mass spectrometer. T_e and n_e increased with increasing percent O₂ in the plasma, while n_Cl fell off with O₂ addition in a manner reflecting simple dilution. Cl atom recombination probabilities (γ_Cl) were measured and were found to be a nearly constant 0.036±0.007 over the range of Cl₂/O₂ mixing ratios and Cl coverage. Large yields of ClO and ClO₂ were found to desorb from the surface during exposure to the plasma, ascribed predominantly to Langmuir-Hinshelwood reactions between adsorbed O and Cl.

[en] The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current silicon integrated circuit fabrication. Finally, some predictions are offered for future needs and advances in plasma etching for silicon and nonsilicon-based devices

[en] In plasma etching processes, the reactor wall conditions can change over time due to a number of intentional and unintentional reasons, leading to a variability in the radical number densities in the plasma, caused by changes in the probabilities for reactions such as recombination at the walls. This leads to loss of reproducibility in the etching process. Here the authors isolated one such effect in which the feed gas was changed in the absence of a substrate. The transient surface composition of an anodized aluminum surface was determined for inductively coupled plasmas as the gas was switched from Cl₂ to O₂ and vice versa. The study was carried out with the spinning wall method and Auger electron spectroscopy. When the surface was first conditioned in an O₂ plasma and then exposed to Cl₂ plasmas, a rapid uptake of Cl was found in the first tens of seconds, followed by a slow approach to a steady-state value within ∼5 min of plasma exposure. Conversely, when the surface was exposed to a Cl₂ plasma for a long time and then switched to an O₂ plasma, the anodized aluminum surface underwent a rapid dechlorination in the first few seconds and then a slow approach to steady state over ∼3 min. Throughout these treatments, the coverages of Si (from erosion of the quartz discharge tube) and O were nearly constant.

[en] The interplay between chlorine inductively coupled plasmas (ICP) and reactor walls coated with silicon etching products has been studied in situ by Auger electron spectroscopy and line-of-sight mass spectrometry using the spinning wall method. A bare silicon wafer mounted on a radio frequency powered electrode (−108 V dc self-bias) was etched in a 13.56 MHz, 400 W ICP. Etching products, along with some oxygen due to erosion of the discharge tube, deposit a Si-oxychloride layer on the plasma reactor walls, including the rotating substrate surface. Without Si-substrate bias, the layer that was previously deposited on the walls with Si-substrate bias reacts with Cl-atoms in the chlorine plasma, forming products that desorb, fragment in the plasma, stick on the spinning wall and sometimes react, and then desorb and are detected by the mass spectrometer. In addition to mass-to-charge (m/e) signals at 63, 98, 133, and 168, corresponding to SiCl_x (x = 1 – 4), many Si-oxychloride fragments with m/e = 107, 177, 196, 212, 231, 247, 275, 291, 294, 307, 329, 345, 361, and 392 were also observed from what appear to be major products desorbing from the spinning wall. It is shown that the evolution of etching products is a complex “recycling” process in which these species deposit and desorb from the walls many times, and repeatedly fragment in the plasma before being detected by the mass spectrometer. SiCl₃ sticks on the walls and appears to desorb for at least milliseconds after exposure to the chlorine plasma. Notably absent are signals at m/e = 70 and 72, indicating little or no Langmuir-Hinshelwood recombination of Cl on this surface, in contrast to previous studies done in the absence of Si etching.

[en] The authors have investigated the effects of elevated substrate temperature (T_s) on cleaning of boron residues from silicon substrates in 1%H₂-Ar plasmas, following etching of HfO₂ in BCl₃ plasmas. Vacuum-transfer x-ray photoelectron spectroscopy (XPS) provided a measure of total B removal rates, as well as information on individual BCl_xO_y moities. B cleaning rates increased with T_s in an Arrhenius manner, with an apparent activation energy of 1.7 kcal/mol. Conversely, the Si etching rate decreased with increasing substrate temperature with an apparent activation energy of -0.8 kcal/mol. Therefore, when considering selectivity with respect to Si etching, it is advantageous to remove B at higher T_s. For example, at T_s=235 deg. C, ∼90% of B is cleaned from Si in 10 s, while <1.5 nm of Si is removed. An apparent diffusion of H into the near-surface region of Si at higher temperatures, detected indirectly by a shift and broadening of the Si(2p) XPS peak, may limit the maximum optimum substrate temperature, however. It was also found that Si does not etch in 1%H₂/Ar plasmas if an oxide layer is present

[en] Chlorine atom recombination coefficients were measured on silicon oxy-chloride surfaces deposited in a chlorine inductively coupled plasma (ICP) with varying oxygen concentrations, using the spinning wall technique. A small cylinder embedded in the walls of the plasma reactor chamber was rapidly rotated, repetitively exposing its surface to the plasma chamber and a differentially pumped analysis chamber housing a quadruple mass spectrometer for line-of-sight desorbing species detection, or an Auger electron spectrometer for in situ surface analysis. The spinning wall frequency was varied from 800 to 30 000 rpm resulting in a detection time, t (the time a point on the surface takes to rotate from plasma chamber to the position facing the mass or Auger spectrometer), of ∼1–40 ms. Desorbing Cl₂, due to Langmuir–Hinshelwood (LH) Cl atom recombination on the reactor wall surfaces, was detected by the mass spectrometer and also by a pressure rise in one of the differentially pumped chambers. LH Cl recombination coefficients were calculated by extrapolating time-resolved desorption decay curves to t = 0. A silicon-covered electrode immersed in the plasma was either powered at 13 MHz, creating a dc bias of −119 V, or allowed to electrically float with no bias power. After long exposure to a Cl₂ ICP without substrate bias, slow etching of the Si wafer coats the chamber and spinning wall surfaces with an Si-chloride layer with a relatively small amount of oxygen (due to a slow erosion of the quartz discharge tube) with a stoichiometry of Si:O:Cl = 1:0.38:0.38. On this low-oxygen-coverage surface, any Cl₂ desorption after LH recombination of Cl was below the detection limit. Adding 5% O₂ to the Cl₂ feed gas stopped etching of the Si wafer (with no rf bias) and increased the oxygen content of the wall deposits, while decreasing the Cl content (Si:O:Cl = 1:1.09:0.08). Cl₂ desorption was detectable for Cl recombination on the spinning wall surface coated with this layer, and a recombination probability of γ_Cl = 0.03 was obtained. After this surface was conditioned with a pure oxygen plasma for ∼60 min, γ_Cl increased to 0.044 and the surface layer was slightly enriched in oxygen fraction (Si:O:Cl = 1:1.09:0.04). This behavior is attributed to a mechanism whereby Cl LH recombination occurs mainly on chlorinated oxygen sites on the silicon oxy-chloride surface, because of the weak Cl–O bond compared to the Cl–Si bond.

[en] The authors have investigated the influence of plasma exposure time (t) on the Langmuir-Hinshelwood (i.e., delayed) recombination of O atoms on electropolished stainless steel surfaces using the spinning-wall method. They found a recombination probability (γ_O) of 0.13±0.01 after about 60 min of plasma exposure. γ_O decreased to 0.09±0.01 for t≥12 h and was independent of the O flux impinging onto the surface. These recombination probabilities are much lower than those obtained in plasma chambers exclusively made of stainless steel, but similar to values recorded in stainless steel reactors with large silica surfaces exposed to the plasma. Near real-time elemental analysis by in situ Auger electron spectroscopy showed that the stainless steel surface became rapidly coated with a Si-oxide-based layer (Fe:[Si+Al]:O≅2:1:9 for t=60 min and 1:2:9 for t=12 h), due to the slow erosion of the silica discharge tube and anodized Al chamber walls. Thus, the recombination probability of oxygen atoms on stainless steel in plasma reactors with large amounts of exposed silica is largely determined by the amount of sputtered silica coating the chamber walls

[en] The authors have investigated plasma etching of HfO₂, a high dielectric constant material, and poly-Si in BCl₃ plasmas. Etching rates were measured as a function of substrate temperature (T_s) at several source powers. Activation energies range from 0.2 to 1.0 kcal/mol for HfO₂ and from 0.8 to 1.8 kcal/mol for Si, with little or no dependence on source power (20-200 W). These low activation energies suggest that product removal is limited by chemical sputtering of the chemisorbed Hf or Si-containing layer, with a higher T_s only modestly increasing the chemical sputtering rate. The slightly lower activation energy for HfO₂ results in a small improvement in selectivity over Si at low temperature. The surface layers formed on HfO₂ and Si after etching in BCl₃ plasmas were also investigated by vacuum-transfer x-ray photoelectron spectroscopy. A thin boron-containing layer was observed on partially etched HfO₂ and on poly-Si after etching through HfO₂ films. For HfO₂, a single B(1s) feature at 194 eV was ascribed to a heavily oxidized species with bonding similar to B₂O₃. B(1s) features were observed for poly-Si surfaces at 187.6 eV (B bound to Si), 189.8 eV, and 193 eV (both ascribed to BO_xCl_y). In the presence of a deliberately added 0.5% air, the B-containing layer on HfO₂ is largely unaffected, while that on Si converts to a thick layer with a single B(1s) peak at 194 eV and an approximate stoichiometry of B₃O₄Cl

[en] A particle-in-cell simulation of beam extraction through a hole in contact with plasma was developed. Particular emphasis was placed on plasma moulding over the hole, ion neutralization in high aspect ratio holes and the energy and angular distributions of the residual ions and fast neutrals in the beam downstream of the hole. When the sheath thickness is much smaller than the diameter of the hole, plasma moulding is severe, and the resulting ion and neutral beams are highly divergent. When the sheath thickness is much larger than the hole diameter, plasma moulding is weak, and collimated beams may be extracted. The angular distribution of fast neutrals peaks off axis, less so for smaller diameters and deep holes. Larger diameters and shallow holes (more plasma moulding) yield narrower ion energy distributions. The fast neutral energy distribution is predicted to be similar to that of ions, but it is shifted to lower energies