Masaaki Nagatsu

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.

Diagnostic first mirrors for burning plasma experiments (invited)

The current state of investigations of the problem of providing first mirrors (FMs) for diagnostic systems in a reactor-grade fusion device is summarized. Results obtained in simulation experiments that have been conducted during recent years in several laboratories are presented. Attention is concentrated on two processes that can have an opposite effect but both can lead to degradation of mirror optical properties, namely: sputtering by charge exchange atoms which leads to erosion, and deposition which leads to surface contamination. It is shown in the analysis that when sputtering dominates, mirrors of monocrystalline refractory metals (Mo, W) can have a sufficiently long lifetime even for FMs that have to be located close to the first wall. Similarly, films of low sputtering yield metals on high thermal conductivity substrates (e.g., Rh on Cu) can be used for FMs in locations where the charge exchange flux is reduced to about a tenth of that at the first wall. However, deposition poses a serious threa...

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.

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.

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...

Reflectivity measurements of graphite in the infrared and submillimeter wave regions

Abstract This paper presents first experimental results of the power reflectivity of graphite in the synchrotron radiation regime for the future fusion experiment. The reflectivities are measured for the C/C composite materials (CC-312 and PCC-2S) and carbon materials (PD-330S and B 4 C/PD-330S) used in the present JT-60U experiment. The submillimeter (SMM) wave laser and CO 2 laser sources were used to measure the wavelength and angular dependencies of reflectivity in the wavelength region from 10 to 790 μm. Theoretical analysis considerably agreed with experimental result.

Production of Low-Pressure Planar Non-Magnetized Plasmas Sustained under a Dielectric-Free Metal-Plasma Interface

Large-area Ar and CF4 non-magnetized plasmas were produced in an entirely metal (stainless steel) plasma chamber with a diameter of 220 mm by 2.45 GHz electromagnetic wave launched by slot antennas cut in the top circular metal lid. Dielectric (quartz) was used only for local vacuum sealing over the slot antennas, occupying less than 20% of the top metal area. At low pressure of 10 mTorr, overdense (>1011 cm-3) plasma was produced filling the whole chamber cross-section similarly to the known case of surface-wave plasmas produced below a large dielectric window covering 100% of the top chamber lid. The absence of this large dielectric suggests that this approach can be used for developing large-area non-magnetized plasma source with less impurities for thin-film processing.