AVS2013 Session PS1-TuA: Plasma Diagnostics, Sensors and Control

Inductively-coupled plasmas (ICP) containing hydrogen bromide are widely used for conductor-etch applications, often using mixtures with Cl₂ and O₂. However, very few scientific studies (whether theoretical, simulation or experimental) have been made of HBr plasmas [ 1, 2] . We have studied pure HBr plasmas in an industrial-scale ICP (diameter 550mm, height 100mm, excited at 13.56MHz by a 4-turn planar coil) adapted for advanced diagnostic techniques.

We have demonstrated the first detection of Br atoms by two-photon laser-induced fluorescence (TALIF). The relative variation of Br atoms was determined as a function of HBr pressure (5-90 mTorr) and RF power (20-500W). The Br density increases with pressure over this range, although the dissociation fraction (Br density divided by the total gas pressure) decreases with pressure. The Br density also increases with RF power up to about 100W, but then progressively saturates. This saturation could be attributed to complete dissociation of the HBr, or to the onset of gas heating leading to gas density reduction. Measurements of the Br decay rate in the afterglow of a pulsed plasma allow the surface reaction coefficient of Br to be estimated.

The electron density was determined using a microwave hairpin resonator[ 3] . With HBr pressure the electron density peaks at 10 mTorr, and it increases with RF power. This behaviour is very similar to that observed in pure Cl₂, although the densities are about a factor 2 lower in HBr. The radial profiles of electron density are centre-peaked at low pressure, but at pressures above 50 mTorr the maximum is off-axis, peaking under the RF antenna coils.

This work was supported by Agence Nationale de la Recherche project INCLINE (ANR-09 BLAN 0019) and by the Applied Materials University Research Partnership Program. NS acknowledges the Ecole Polytechnique for a post-doctoral grant.

[1] Cunge G., Fouchier M., Brihoum M., et al., J Phys D Appl Phys, 2011, 44,

[2] Sasic O., Dujko S., Makabe T., et al., Chemical Physics, 2012, 398, 154-159

[3] Piejak R., Al-Khuzee J. and Braithwaite N. S., Plasma Sources Sci. Technol., 2005, 14, 734

Low pressure plasmas are used extensively in modern industry to process substrates. Both etching and deposition processes are controlled by energetic ion bombardment of the substrate. As substrates become larger, feature sizes smaller and film properties more complex there is an increasing demand for accurate monitoring and control of the ion energy distribution. Ion energy measurements have been reported in the literature for many decades. The two most common instruments that have been used are the quadrupole mass spectrometer and the retarding field energy analyzer (RFEA). The simplicity of the RFEA has led to its widespread use in various applications. The focus of this talk will be on the recent advances in RFEA technology for ion energy measurements in different types of plasma processes. In most cases the substrate is processed using an electrical bias. The substrate holder may be powered with radio-frequency (RF), pulsed-RF, direct current (DC) or pulsed-DC electrical signals. This complicates the use of RFEAs for ion energy measurement at the substrate location. The presence of the RFEA must not disturb the electrical bias and the electrical bias must not impair the RFEA data acquisition. Here we present an electrically filtered RFEA technology that allows for deployment at the biased substrate location. The filtering allows the RFEA sensor to float at the electrical bias voltage. We will discuss a number of filter configurations that cover the many frequencies, waveform shapes and bias levels that can be encountered. We also discuss a special configuration that permits time resolved measurements of the ion energy at the biased substrates under certain conditions. A number of novel extensions to the standard technology will also be presented. Spatially resolved measurements of the ion energy at multiple locations (simultaneously) across 300mm and 450mm substrate holders, used for plasma etching, will be presented. This RFEA design gives important information about the spatial uniformity of the plasma process under various plasma conditions. An embodiment of the RFEA design which can be used to determine the ion angular distribution is presented. This technique uses an additional orifice with variable aspect ratio to scan the angular distribution of the in coming ions across the energy range. Measurement of the ion angular distribution is particularly important near the edge of substrates, where yield is often poor due to sheath curvature in this region. Finally, the first results of a novel RFEA configuration that enables discrimination of the different ion species in the bombarding energy distribution will be presented and discussed.

CH₃F plasmas, mostly with added O₂, are used in selective Si₃N₄ etching over Si or SiO₂. Despite their use, fundamental plasma studies in these gas mixtures are very scarce. In this work, optical and Langmuir probe diagnostics were employed to study inductively couple plasmas in CH₃F/O₂ gas mixtures. In 50% CH₃F/50% O₂ plasmas, the electron density increased linearly (0.7x10¹¹→2.7x10¹¹ cm^-3) as power was increased 150-400W at 10 mTorr, but only weakly (1.7x10¹¹→2.7x10¹¹ cm^-3) within the pressure range of 10-40 mTorr at 300W. The effective electron temperature representing the high energy tail of bi-Maxwellian EEPFs was nearly independent of power and pressure. The gas temperature increased from 400-900 K as a function of inductive mode power between 75 and 400 W at 10 mTorr. For a constant feed gas flow rate and composition, the absolute H, F and O atom densities, estimated by optical emission rare gas actinometry, increased linearly with power. The feedstock gas was highly dissociated and most of the fluorine and oxygen was contained in reaction products HF, CO, CO₂, H₂O and OH. Reaction mechanisms were proposed to explain the observed behavior of the relative density of F and HF vs. power and pressure. Measured relative densities as a function of O₂ addition to CH₃F/O₂ changed abruptly for H, O, and particularly F atoms (factor of 4) at 48% O₂. A corresponding transition was also observed in electron density, effective electron temperature and gas temperature, as well as in C, CF and CH optical emission. These abrupt transitions were attributed to the reactor wall reactivity, changing from a polymer-coated surface to a polymer-free surface, and vice-versa, as the O₂ content in the feed gas crossed 48%.

*Work supported by Lam Research Corp.

High-precision, rapid temperature measurements on wafers such as silicon, sapphire, etc. are required for plasma processing. A method is demonstrated here that uses a frequency-domain low-coherence interferometer (FD-LCI).

In order to realize the plasma processing with high precision, the temperature control of wafer or chamber-wall is one of important factors. Especially the wafer temperature influences on etched profile and etching rates. Moreover, a spatial distribution of radicals in gas phase or plasma-surface interactions on the surface are affected by temperatures of the chambar-wall. Therefore, the precisely non-contact temperature-measurement technology is needed for the fabrication of electric devices.

In this study, we have developed a highly precise and non-contact temperature monitor using a Fourier domain low-coherence interferometer (FD-LCI) and a super luminescent diode (SLD: center wavelength: 1330 nm, spectral width: 37.6 nm) as a low coherence light source. The FD-LCI can measure an optical path length of wafer on the basis of auto-correlation signals, which are generated by interferences between the light reflected from the front and back surfaces, and analyzed by the inverse Fourier transform of spectral interferogram. Since the optical path is changed by thermal expansion and refractive-index, the wafer-temperatures can be analyzed. By this method, we have successfully achieved to estimate a temperature of various commercial wafers such as silicon, sapphire during plasma processes.

The spectrum of light emitted by plasmas used in materials processing applications includes vacuum ultraviolet (VUV) photons, which are known to play a significant role in critical surface reactions under certain process conditions. Monitoring of the surface flux of VUV photons emitted from the inductively coupled plasma (ICP) and its dependence on discharge parameters is thus highly desirable. However, non-invasive direct detection of VUV photons is generally difficult, as few window materials transmit in the VUV. We thus examine the argon resonance level atom concentration as a prospective proxy for VUV emission, as 106.7 and 104.8 nm VUV photons are produced in the spontaneous radiative decay from Ar resonance levels to the ground state. Argon resonance level concentrations have been measured in the center of an ICP with a planar spiral induction antenna through “branching fraction” analysis of the visible optical emission (OES) spectrum.* Measured concentrations are subsequently used as inputs to a VUV radiation transport model developed to determine the corresponding axial VUV photon flux. Reabsorption of VUV photons by ground state atoms is significant even at pressures as low as 1 mTorr, and a proper accounting in the model thus requires accurate representation of the gas temperature. Model results based on the resonance level concentrations over a range of pressures (1-25 mTorr) and RF (13.56 MHz) power (up to 1000 W) compare favorably with the axial VUV photon flux sampled directly through a small hole at the center of an electrode located opposite the ICP antenna. Absolute VUV fluxes were measured with a windowless aluminum oxide photodiode sensitive to wavelengths below ~110 nm. Additionally, relative VUV fluxes were also obtained using a sodium salicylate coating on the inside of a side port vacuum window. The sodium salicylate converts VUV into a detectable visible light signal through fluorescence, and, unlike the photodiode, is sensitive in the wavelength range of H atom VUV emissions (122 nm). Preliminary results suggest that a combination of photodiode and sodium salicylate signals thus allows discrimination between hydrogen and argon contributions to the VUV flux in Ar/H₂ gas mixtures.

Support from NSF grant PHY-1068670 and the Applied Materials Corporation is gratefully acknowledged.

*Plasma Sources Sci. Technol. 18 (2009) 035017.

The detection of stable and transient species along with gas temperature measurements remains a challenge for the majority of molecular (complex) plasmas. Considering particularly plasmas at atmospheric pressure with inherently small discharge volumes, (optical) access to the active plasma is often hampered. On the other hand, phase- and time-resolved in-situ measurements are a valuable tool (i) to establish heavy particle temperatures, (ii) to identify excitation mechanisms in the plasma, (iii) to discriminate gas phase and surface reactions as they occur on significantly different time scales, and (iv) to unravel particularly temperature-dependent reaction mechanisms. Modern mid-infrared laser sources, known as quantum cascade lasers (QCLs), provide a means for highly time-resolved absorption spectroscopy in the molecular "fingerprint" region. The time-resolution can be thereby as good as a few tens of nanoseconds. Although continuous-wave QCLs are increasingly being applied for conventional monitoring purposes, pulsed distributed feedback QCLs are perfectly suited for diagnostic studies on transient plasmas.

CO₂ containing dielectric barrier discharges (DBDs) operated in the mid-frequency (kHz) range were studied by means of in-situ time-resolved QCL absorption spectroscopy. Special beam shaping optics was used to accommodate the laser beam diameter to typical gap widths of ~ 1 mm in single and multiple-pass configuration. Different synchronisation schemes were applied to achieve phase-resolved measurements during individual AC cycles as well as to monitor molecular absorption signals during pulsed discharge operation. Mixing ratios of CO in its electronic and vibrational ground state were of the order of a few percent and thus confirmed earlier ex-situ studies of the effluent. More importantly, the concentrations levels were changing only slowly in time, i.e. of the order of the residence time. A direct CO₂-to-CO dissociation through electron impact appears very unlikely under these conditions. The kinetics of low-lying ro-vibrational states of CO₂ along with the evolution of the CO concentration were studied on a sub-millisecond time-scale to (i) establish (rotational) gas temperatures, and (ii) to estimate the influence of vibrational-vibrational and vibrational-translational energy transfer processes.