Ivan Ghanashev

High-density flat plasma production based on surface waves

Recent development of large-diameter (>30 cm) high-density microwave plasma production at low pressures (<20 mTorr) without an external DC magnetic field is reviewed in view of application to the next generation ULSI devices and flat panel displays. Understanding the discharge physics - excitation, propagation and absorption of the surface wave in a flat plasma geometry under overdense conditions - is indispensable for controlling the plasma. Experimental evidence of discrete surface-wave modes is clearly found in optical emission and microwave field measurements. The analysis of the full-wave electromagnetic dispersion successfully identified the observed eigenmodes. Stability analysis of the wave-plasma interaction resulted in a stability criterion predicting hysteresis loops in the power-density dependence, which were found in the experiment. A possibility of collisionless absorption of surface waves, i.e. mode conversion to electron plasma waves at the resonant layer, is discussed with the recent experimental results taken into account. From the plasma technology point of view, examples of surface-wave plasma tools (some of them commercially available) are introduced and the significance of the antenna structure is emphasized. Finally, the advantages of the surface-wave plasma source in comparison with other high-density sources are summarized.

Surface wave eigenmodes in a finite-area plane microwave plasma

The resonance frequencies of electromagnetic surface modes propagating along a plane dielectric-plasma interface are computed, taking into account the finite area of the latter. The analysis results in simple analytical formulae for estimating the plasma density at which a given mode can be expected to occur for given geometry and wave frequency. Comparison with measurements in large-area circular plasmas is made.

Mode identification of surface waves excited in a planar microwave discharge

A planar high-density plasma, 22 cm in diameter and 9 cm in length, is produced by a 2.45 GHz microwave radiation of 500 W through small slot antennas in argon at 20 - 350 Pa without a magnetic field. Several types of azimuthal and radial standing wave mode pattern are observed in the optical emission from the plasma depending on the discharge conditions. The microwave field in the plasma measured by a movable antenna decreases exponentially in the axial direction from the quartz wall adjacent to the slot antennas, thus suggesting the propagation of surface waves in the r, directions. The measured azimuthal microwave field distributions and the optical emission pattern clearly show a mode transition of the standing surface wave from a mode to a mode when the pressure is decreased from 140 to 44 Pa at the constant power of 400 W. Here denotes the transverse magnetic mode of azimuthal mode number m and radial mode number n. A wave dispersion analysis based on a one-interface uniform-density model predicts these modes in a range of electron densities corresponding to those measured by a Langmuir probe in the experiment.

Electron energy distribution functions and the influence on fluorocarbon plasma chemistry

Two different modes of electron heating are found in microwave discharges: the bulk heating mode characterized with low electron density ne and high electron temperature Te (~10 eV), and the surface heating mode with high ne and low Te (~3 eV). The correlation between the heating mode and the electron energy distribution function (EEDF) is qualitatively interpreted in terms of non-local kinetic theory, taking account of the ambipolar potential well. A biased optical probe diagnostics of a surface wave plasma (SWP) reveals that the surface heating mode gives a bi-Maxwellian type EEDF, that is, a sum of two Maxwellian distributions of bulk temperature Tb and tail temperature Tt>Tb. On the other hand, the EEDF of inductively coupled plasma (ICP) is close to a single-Maxwellian distribution with electron temperature higher than the bulk temperature Tb of the SWP. Such differences in the EEDFs make the composition of the reactive species of the two plasmas different; namely, ion and radical measurements at the same electron density show that the ICP contains more F radicals and less CF3 and CF2 radicals in comparison with the SWP. In addition, a simplified model based on the bi-Maxwellian EEDF shows how the EEDF determines the ion and radical compositions, supporting the major experimental results. These observations and calculations suggest that plasma chemistry is controllable by tailoring the EEDF with proper adjustment of bulk heating and/or surface heating of electrons.

Mode Jumps and Hysteresis in Surface-Wave Sustained Microwave Discharges

Various electromagnetic surface modes along a finite area dielectric interface can sustain large area overdense plasmas. Mode jumps between these modes have been reported when changing the gas pressure. In this paper we report similar mode jumps caused by changing the wave power at fixed gas pressure. A simple theoretical analysis is proposed to explain this phenomenon on the basis of the resonance behavior of the chamber impedance and a stability criterion is formulated. The theory predicts a hysteresis in the power-density dependence, which was also observed experimentally.

Production and control of large diameter surface wave plasmas

A planar cylindrical plasma has been produced using small slot antennas in an aluminum discharge chamber with a cylindrical section with a diameter of 22 cm. The 2.45 GHz microwave with a power of 0.2-2.0 kW was fed through a quartz window into the discharge chamber filled with Ar or at a pressure of a few mTorr to 1 Torr. At relatively high pressures, interesting mode patterns of optical emission and microwave field intensity, that is, the TM mode at 1 Torr and TM mode at 0.3 Torr, were observed just below a quartz window. From theoretical analysis, these modes are considered to be attributable to the surface waves excited in a large planar cylindrical plasma. Surface wave plasmas filled with a low-pressure Ar or gas, say 10 mTorr, were also studied for the etching application. Furthermore, we have carried out time- and space-resolved measurements of plasma parameters and microwave field intensity using a pulse-modulated microwave source to study the power absorption mechanism in the surface wave plasma and to demonstrate the pulse-modulated surface wave plasma. Preliminary results also show that short-wavelength fluctuations in the electric field intensity, which might be considered as mode-converted electron plasma waves, exist near the quartz window mm) for 5-10 s after the microwave pulse was applied. Lastly, a recent result of minimizing a dielectric window is briefly presented.

Diagnosis for advanced plasma control of materials processing

Large-diameter high-density plasmas such as electron-cyclotron-resonance (ECR), helicon wave, inductively coupled, and surface-wave plasmas are currently being developed for plasma-assisted thin-film processes in the next generation. Actual applications of such high-density plasmas require a deeper understanding of discharge physics as well as advanced techniques for plasma control. In this paper, new findings on antenna - plasma couplings are reported. One is resonant directional excitation of helicon waves and a mechanism of density jump in a helicon RF discharge. The other is the identification of long-wavelength surface-wave modes with observation of the short-wavelength mode in a planar microwave discharge. In addition, comprehensive diagnostics of a pulsed inductively coupled plasma in chlorine is presented, which explains the pronounced effect of pulsed power discharges on charge-up suppression.

Effect of slot antenna structures on production of large-area planar surface wave plasmas excited at 2.45 GHz

The effect of slot antenna structures on plasma production was investigated in the large-area planar surface wave plasmas (SWPs) excited with 2.45 GHz microwave energy. Plasma production characteristics were measured for various types of slot antennae (inclined, transverse, longitudinal and diverging slots) in Ar discharges. In all the slot antennae, we clearly observed the density jumps when the incident power and pressure were varied. These density jumps correspond to the mode jumps between transverse magnetic (TM) eigenmodes in the SWP. In spite of different slot antenna structures, the same optical emission patterns of the TM62 mode were observed at the almost same electron density, except for the diverging slots, in which the TM92 mode was selectively excited under the same operating conditions, where the incident power was 0.8-1.2 kW at a pressure of 100-280 mTorr. At lower pressures, say 10 mTorr, the plasmas entirely broadened over the chamber cross section for all the slot antennae. Among the four slot antennae, the transverse slot antenna was more efficient for plasma production under the same incident microwave power at a fixed pressure.

LARGE-AREA HIGH-DENSITY PLASMA EXCITATION USING STANDING PURE AND HYBRID SURFACE WAVES

Plasma processing of large flat surfaces requires low pressure high density (ne=1011–1012 cm−3) plasmas with uniform plasma density distribution near to the processed surface. Microwave discharges may provide a valuable alternative to the inductively coupled plasmas applied widely now for this purpose. In a recent article [Jpn. J. Appl. Phys., Part 1 35, L341 (1996)] we proposed a plasma source in which the plasma is sustained by a standing surface wave propagating radially and azimuthally along the interface between the plasma and a dielectric plate located at the top wall of a large-diameter cylindrical metal chamber, the wave being launched by a pair of slot antennas cut in the top chamber wall above the dielectric plate. Here we present new experimental results at lower pressures (down to 3 mTorr) and in a non-noble reactive gas (CF4) demonstrating the applicability of the new source for dry etching. The electron density was about one order of magnitude lower than the one observed by previous experime...

Local resonant excitation of plasma oscillations in a planar surface-wave plasma device

The paper presents space-resolved microwave intensity measurements in a surface-wave plasma device clearly demonstrating the existence of a local resonance in a resonance layer where the local electron plasma frequency is equal to the surface-wave frequency. It has already been suggested in the literature that Landau damping of electron plasma waves excited in such a resonance layer and/or stochastic electron heating there might contribute to the surface-wave plasma energy balance. Since this does not involve collisions, it may become an important energy channel at low gas pressures. In order to avoid having the resonance layer too close to the plasma boundary, the measurements were performed not in the original surface-wave plasma, but using a weak non-ionizing 2.4 GHz microwave propagating in an inductively coupled plasma created by an internal loop antenna fed by a high-power 13.56 MHz generator. Still the original surface-wave plasma source geometry, including the microwave input port, was preserved. The resonance layer was identified by the microwave intensity peak, which was found to shift on changing the plasma density profile and/or the wave driving frequency in compliance with theoretical expectations. Accompanying space-resolved plasma density measurements confirmed this interpretation.