Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session QR1: Plasma Sources |
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Chair: Yohei Yamazawa, Tokyo Electron Room: Doubletree Crystal City Crystal Ballroom A |
Thursday, October 4, 2007 10:00AM - 10:15AM |
QR1.00001: Two-dimensional Collisionless Weakly-ionized Plasma in Fluid Approximation Valery Godyak, Natalia Sternberg A finite cylinder is a common plasma shape in many research experiments and plasma processing reactors. In the diffusion limit (the Schottky model), the two-dimensional plasma density profile for a finite cylinder of length $2L$ and diameter $2R$ is the product of the corresponding one-dimensional solutions, namely, $n(x,r)/n_0 = cos(\backslash pix/2L)J_0(2.4r/R)$. This representation of the plasma spatial distribution is commonly used at low gas pressures, even when the Schottky model is not applicable (such as in the cases of collisionless ions, or variable ion mobility). In this presentation, we will analyze, for a wide range of the aspect ratio $L/R$, the behavior of the ionization frequency, plasma densities at the radial and axial boundaries, the spatial plasma profile, the plasma flux to the wall, as well as the entering angle of ions at the plasma boundary. We will demonstrate that for cylindrical collisionless plasma, the spatial plasma profile cannot be represented by the product of the corresponding one-dimensional solutions. Moreover, in the limiting cases of small and large aspect ratios, the plasma distribution along the longer length ($L$ or $R$) approaches the diffusion distribution, which corresponds to the highly collisional ion motion, although the ion motion in this direction is collisionless. [Preview Abstract] |
Thursday, October 4, 2007 10:15AM - 10:30AM |
QR1.00002: Capacitive Discharges Driven by Combined DC/RF Sources M.A. Lieberman, Emi Kawamura, A.J. Lichtenberg, E.A. Hudson The performance of low pressure rf capacitive discharge reactors can be modified by applying an auxiliary dc power source to a reactor electrode. The dc source induces a dc current flow through the plasma and alters the sheath voltages and widths. This can increase the plasma density and etch rate. These effects may be ascribed to an enhanced density of high energy secondary electrons in the discharge due to the alteration of the sheath voltages. We have obtained analytic expressions for the sheath voltages and sheath widths for both collisional and collisionless sheaths driven by a combination of dc and rf voltage sources. The analysis is done for both symmetric (equal area) and asymmetric diode discharges, as well as a triode configuration. The analytical results for the symmetric and asymmetric diode discharges are compared to the results of numerical simulations using plane-parallel and cylindrical 1d3v (one-dimensional displacement, three velocity components) particle-in-cell (PIC) codes over a wide range of pressures and rf frequencies, finding good agreement. Secondary electron dynamics and energy distributions are also examined; these yield increased discharge efficiency. The uniformity of the secondary electron and ion fluxes at the target electrode are also examined with a series of two-dimensional (2D3v) PIC simulations. [Preview Abstract] |
Thursday, October 4, 2007 10:30AM - 10:45AM |
QR1.00003: RF discharge under the influence of a magnetic field Ed Barnat, Paul Miller, Alex Paterson We examined the effects of an externally applied magnetic field (0 to 150 Gauss) on a capacitive 13.56 MHz argon discharge in a Gaseous Electronics Conference (GEC) reference cell. The electrical characteristics of the discharge were measured as functions of applied magnetic field and rf power. At fixed power the rf voltage decreased with increasing magnetic field. The discharge impedance was predominantly capacitive and became more resistive as the electron mobility decreased with increasing magnetic field. We also measured the effect that the magnetic field had on the spatial distribution of the plasma in vertical planes parallel and perpendicular to the direction of the magnetic field using Langmuir probes, optical emission, and laser induced fluorescence. Due to ExB forces, the distribution of excited states in the plasma remained radially symmetric in the plane parallel to the magnetic field and became skewed in the plane perpendicular to the magnetic field. The degree of skew depended on the state probed. Finally, we examined the temporal evolution of the electric fields in the plasma. In the presence of magnetic field, the sheath thickness decreased and most of the voltage drop was contained within the sheath. Consistent with dc voltage trends, there was no significant sheath reversal observed at higher magnetic fields. [Preview Abstract] |
Thursday, October 4, 2007 10:45AM - 11:00AM |
QR1.00004: The effect of radio-frequency bias on electron density in an inductively coupled plasma reactor, measured by a wave cutoff probe Mark Sobolewski, Jung-Hyung Kim Rf-biased, inductively coupled plasma reactors allow ion energy and ion flux to be varied separately, but it is unlikely that perfectly independent control can be achieved. Although rf bias is intended to only affect ion energies, it may also produce changes in the plasma electron and ion densities and the total ion flux. To provide a better understanding of such changes, we performed a detailed study in Ar, CF$_{4}$, and Ar/CF$_{4}$ plasmas. We measured the electron density with a wave cutoff probe, which avoids problems with deposition and rf compensation that may affect Langmuir probes. The effect of rf bias on electron density was measured as a function of source power, position, pressure, bias frequency, bias amplitude, and time. Cutoff probe results were also compared to Langmuir probe measurements, and both showed the same effects. Two types of bias-induced changes in electron density were observed. One was a gas composition effect caused by etch or sputter products liberated from the wafer surface. The other was an electron heating effect caused by absorption of bias power by plasma electrons. Simple models of each effect were derived and shown to yield quantitative predictions in agreement with the observations. [Preview Abstract] |
Thursday, October 4, 2007 11:00AM - 11:15AM |
QR1.00005: Seasoning of Plasma Reactors: Feedback Control Strategies to Counter Wafer-to-Wafer Drifts Ankur Agarwal, Mark J. Kushner Seasoning of plasma etching reactors is the deposition of materials on wafers and surfaces of the chamber resulting in process or wafer-to-wafer drift in etch rates or uniformity. Feedback control with \textit{in situ} diagnostics is being investigated to combat this drift. The Virtual Plasma Equipment Model, an implementation of sensors, actuators and control algorithms in the HPEM, was used to investigate real-time and wafer-to-wafer control strategies. The model system is Ar/Cl$_{2}$ etching of Si in an inductively coupled plasma reactor. The passivation of surfaces in contact with the plasma, including the deposition of etch products, change reactive sticking coefficients and produce etch blocks which in turn affect etch rate. Sputtering of dielectrics may introduce additional etch-block capable species. A PID controller was used to vary the bias voltage in response to an etch rate monitor to enable control of etch rate. We found that control is problematic at high bias voltages where the flux of etch products from the wafer is sufficiently large that plasma properties are affected and redeposition increases etch blocks on the wafer. Multiple sensors-and-actuators may be necessary when sputtering of dielectrics produce additional etch-block species. [Preview Abstract] |
Thursday, October 4, 2007 11:15AM - 11:30AM |
QR1.00006: Diagnostics of continuous electron beam-generated plasma for material processing Evgeniya Lock, Scott Walton, Richard Fernsler Electron beam generated plasmas have several unique features that distinguish them from discharges. The latter have electron temperatures in the range of 1-10 eV and a large spread in the electron energy distribution function. The high temperature leads to large plasma potentials and large incident ion energies. The electron beam generated plasma has intrinsically lower electron temperature ($<$ 1 eV), resulting in smaller plasma potentials and in lower incident ion energies. This enables the electron beam generated plasmas to be used in processing of sensitive materials, where excessive ion energies can be problematic. In this work, spatial distributions of electron temperature and plasma density in argon, nitrogen and their mixtures were measured in magnetically confined, continuous electron beam-generated plasmas. The influence of the process parameters including beam energy, magnetic field strength and pressure was analyzed. The only factor that significantly influenced the electron temperature was the gas composition. On the other hand, the plasma density could be adjusted by modifying any of the process variables. Complementary studies on ion energy distributions were performed by Walton, S.G. et al. [Preview Abstract] |
Thursday, October 4, 2007 11:30AM - 11:45AM |
QR1.00007: Student Excellence Award Finalist: Double layers at anode spots in low-pressure plasma Scott Baalrud, Noah Hershkowitz The evolution of the potential profile near an electrode with positive applied potential has three distinct modes. For bias less than the ionization potential of the neutral gas, a monotonic electron sheath is present. When the applied bias is increased beyond the ionization potential, a thin region of increased ionization and a corresponding double layer (DL) form near the electrode. This regime is referred to as anode glow. When the ion density in the anode glow becomes large enough that there are many ions in a Debye sphere, a quasineutral anode spot (AS) forms and the DL moves many Debye lengths away from the electrode. The distance between electrode and DL can be calculated using current balance conditions, Bohm's criterion, and quasineutrality in the AS. For small electrodes, the AS diameter is typically larger than the electrode and the AS appears approximately spherical. However, for large electrodes the AS diameter can be much shorter than the electrode diameter and the AS is more akin to a cylinder with length longer than diameter. AS and electrode diameters are correlated. Data were taken in a multidipole chamber with mTorr range argon plasma generated by hot filaments. [Preview Abstract] |
Thursday, October 4, 2007 11:45AM - 12:00PM |
QR1.00008: Study of discharge characteristics on transition from metallic to reactive mode in radio frequency magnetron plasma Joyanti Chutia, Hemen Kakati, Arup Ratan Pal, Heremba Bailung The technique of reactive magnetron sputtering with argon as buffer gas and oxygen as reactive gas is widely used for deposition of different types of metal-oxide films. In this work, the influence of oxygen flow rate on the discharge characteristics at fixed argon pressure and applied power in a radio frequency magnetron plasma using aluminum as target is investigated. The reactive sputtering of aluminum target in argon and oxygen environment is generally operated in two stable modes namely metallic and reactive mode. The variation of discharge voltage and self-bias with oxygen flow rate is measured with the help of a high voltage probe and the transition of the discharge from metallic to reactive mode is observed. The sheath structure determined from the potential profile measured by the emissive probe, is found to be affected by the oxygen flow rate. From the optical emission spectroscopic measurement, the line intensities for aluminum and aluminum oxide are measured for different flows of oxygen and are found to be highly affected by the transition of the discharge from metallic to reactive mode. From these investigations, optimum value of the oxygen flow rate at fixed power and argon pressure is determined for the formation of aluminum oxide. [Preview Abstract] |
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