Steven Shannon

Origin of radiation tolerance in 3C-SiC with nanolayered planar defects

We have recently found that the radiation tolerance of SiC is highly enhanced by introducing nanolayers of stacking faults and twins [Y. Zhang et al., Phys. Chem. Chem. Phys. 14, 13429 (2012)]. To reveal the origin of this radiation resistance, we used in situ transmission electron microscopy to examine structural changes induced by electron beam irradiation in 3C-SiC containing nanolayers of (111) planar defects. We found that preferential amorphization, when it does occur, takes place at grain boundaries and at (1¯11) and (11¯1) planar defects. Radiation-induced point defects, such as interstitials and vacancies, migrate two-dimensionally between the (111) planar defects, which probably enhances the damage recovery.

The impact of frequency mixing on sheath properties: Ion energy distribution and Vdc∕Vrf interaction

A dual frequency rf sheath is analyzed using a simple rf sheath model to study the interaction between the two driving rf currents and their effect on sheath parameters. A symmetric rf discharge with defined electron density and dc sheath potential is modeled using a sharp boundary sheath approximation. Three results of this study are reported: (1) reproduction of trends in ion energy distribution functions predicted and measured in previous studies, (2) a frequency-mixing-dependent relationship between the dc sheath potential and applied rf potential, and (3) an additional asymmetry in the ion energy distribution function generated by the intermodulation components resulting from the nonlinear sheath.

Ion Energy Distribution Skew Control Using Phase-Locked Harmonic RF Bias Drive

The energy distribution of ions accelerated through a radio frequency sheath and incident on a plasma-facing material significantly influences material interaction with the plasma and can impact manufacturing at the nanoscale. Ion energy distributions are controlled through appropriate mixing of drive frequencies, which has been shown to control distribution width. This paper presents a modification to multifrequency drive for ion energy control by exploiting a digital frequency and phase controller that enables modification of the higher order moments of the distribution, specifically, controlling the skew of the distribution. By modulating the sheath with two frequencies where one frequency is the harmonic of the other and controlling the relative phase of these two waveforms incident on the plasma-facing surface, skew control is achieved. A simple empirical model is presented to describe this method, as well as experimental validation of the model and demonstration of skew control in a parallel plate capacitively coupled reactor.

Spatially resolved fluorine actinometry

A method has been developed to obtain spatially resolved optical emission spectra. This method is used in a diagnostic known as actinometry, where the relative concentration of fluorine can be obtained by examining the ratio of two spectral lines having similar excitation thresholds and excitation cross sections. Generally, the etch rate of silicon is correlated to the concentration of fluorine. In this modified actinometry method, radial emissivity profiles of the discharge are obtained 1 cm above the wafer surface by using a rotating stage to capture small wedges of light from the etching discharge, and analyzing these wedges using a regularized reconstruction algorithm. The relative fluorine concentration is obtained by comparing the ratio of a fluorine (703.75 nm) to argon (750.39 nm) emission line. The atomic fluorine radial profiles correlate to hard masked silicon etch radial profiles processed in a Lam TCP 9400 SE inductively coupled plasma processing tool using an SF6/Ar chemistry. Fluorine loadi...

A spatially resolved optical emission sensor for plasma etch monitoring

A spatially resolved optical emission spectroscopy sensor has been developed, and the resulting reconstructed radial emission profiles from an ArI and ArII line compare well with Ar sputter etch uniformity profiles. The new sensor collects light from a wedge shaped field of view, and is rotated around a single collection point in order to observe the entire plasma through a relatively small viewpoint.

Control of ion energy distributions using phase shifting in multi-frequency capacitively coupled plasmas

Summary form only given. Anisotropic etching for microelectronics fabrication is accomplished by energetic ion bombardment in chemically enhanced sputtering. One challenge is being able to control the ion energy-angular distributions (IEADs) onto the surface of the wafer to selectively activate desired processes, which is advantageous for maintaining the critical dimension (CD) of features. Capacitive coupled plasmas (CCPs) powered by non-sinusoidal waveforms and or using multiple frequencies are strategies employed to provide flexible control of IEADs which produce high selectivity and uniformity. Varying relative voltages, powers and phases between multiple frequencies that differ by integer multiples have demonstrated potential control mechanisms for the IEADs and optimization of etching profiles. In this paper, we report on computational and experimental investigations of lEAD control in a dual-frequency CCP where the phase between the frequencies is used as a control variable. The rf frequency and its harmonic frequencies are both applied to the wafer substrate. Both symmetric and asymmetric CCPs are studied. The Hybrid Plasma Equipment Model (HPEM) was employed to predict plasma properties and obtain the harmonic contributions to the power applied to the same electrode. The ion and radical fluxes incident onto the surface are used as input to the Monte Carlo Feature Profile Module (MCFPM) with which profiles are predicted. The operating conditions are 5-100 mTorr in Ar and Ar/CF4/O2 gas under different frequency mixing and phase of integer multiple frequency drives. We find that by changing the phase between the applied rf frequency and its second harmonic, the Electrical Asymmetric Effects (EAE) is significant and can shift the dc self-bias.[I] When changing phases between the rf and its higher harmonics, the EAE becomes less effective and ion energy distributions spike at specific energies. Computed results for lEADs are compared with rf phase locked harmonic experimental results measured by Radio Frequency Ion Energy Analyzer.

Electron Energy Distribution Function Extraction Using Integrated Step Function Response and Regularization Methods

Recently, electron energy distribution function (EEDF) extraction techniques have been evaluated using regularized solutions to the integral problem. These techniques do not assume any mathematical representation of the EEDF and solve the integral problem for any function that best represents the EEDF. Also, unlike the more widely used point-by-point extraction of the second-derivative relationship, the integrated relationship between electron current and the EEDF is used, instead of a relatively small fraction of the integrated data in the point-by-point method. In this paper, the electron current for an arbitrary distribution function is derived, assuming that the distribution is a sum of step functions representing such a function. This technique for EEDF extraction is validated by adding noise to numerically generated data and using a regularized least squares (RLS) method to calculate the original function by solving for the individual step function contribution to the total electron current. Comparisons are then made between the expected and the reconstructed solution to evaluate its accuracy with respect to EEDF reconstruction and integrated normalization of the electron density.

The impact of Langmuir probe geometries on electron current collection and the integral relation for obtaining electron energy distribution functions

The Druyvesteyn relation for obtaining electron energy distribution functions (EEDFs) from Langmuir probes is derived based on a model that assumes spherical probe geometry and extends this formulation to arbitrary geometries including the more commonly used planar and cylindrical probes. In this paper, we revisit the formulation of the relationship between electron current, probe potential and EEDF for a cylindrical geometry that considers geometric differences between the spherical and cylindrical case and provides an identical integral relationship to that posed by Mott-Smith and Langmuir in 1926. Comparing the spherical and cylindrical integral relationships and EEDFs reconstructed from them, noticeable differences in EEDF shape are seen using the Druyvesteyn relation for cylindrical probes that becomes more pronounced for highly non-Maxwellian distributions. In order to minimize this geometry-induced distortion, a solution of the integral relation between EEDF and probe current may be needed in place of the more commonly used derivative formulation of Druyvesteyn.

Reduction of EEDF Measurement Distortion in Regularized Solutions of the Druyvesteyn Relation

Electron energy distribution function (EEDF) extraction from Langmuir probe data is an ill-posed problem due to the integral relationship between electron current and probe voltage. Both curve fitting of experimental data and reconstruction of the integral problem through methods, such as Tikhonov regularization, address this to some measure, with regularized solutions offering an advantage in overall EEDF accuracy over curve fitting. Although Tikhonov regularization provides a more accurate estimation of EEDF overall energy space, it typically also can distort the overall shape of the reconstructed distribution, particularly at high energies and energies below the distribution peak. This, combined with the relative ease of use that simple data smoothing algorithms provide, has limited the use of the more advanced reconstruction algorithms in EEDF analysis. In this paper, we will shed some light on these limitations and offer an alternative method to overcome these limitations.

Uniformity control with phase-locked RF source on a high density plasma system

Summary form only given. Semiconductor device manufacturing continues to achieve decreased feature sizes with a corresponding density increase along device area and volume. The consequence of manufacturing semiconductor devices with a higher level of integration remains a vexing challenge to achieving repeatable target yields, minimizing plasma induced damage, and optimizing process throughput all while gaining the technological advantage to reach the next high-performance node. We present control mechanisms to improve the fidelity of plasma density and the control of ion energies for a high-density plasma source. For a high-density plasma source, plasma generation is associated with the coupling of RF power to the plasma discharge through a coil antenna arrangement. The RF bias, coupled to a substrate, creates the ion energies utilized for material-etch processing associated with high-volume semiconductor manufacturing. Our fundamental technique is based on a frequency-and-phase locking controller. For the RF source, this frequency-and phase locking enables precise control of the electromagnetic field emissions from the antenna coil. By amplitude and relative phase manipulation of a dual-RF power supply scheme providing the excitation for the source coil arrangement, the constructive-deconstructive interaction of the coil fields enables the finest control of plasma density and uniformity along the wafer area. We further exploit the frequency-and-phase locking capability with the bias RF power delivery system to control the width and skew of the ion energy distribution function (IEDF). The coupling of these RF power delivery systems to a high-density plasma source formulates a systematic control of plasma parameters, ameliorating the state of thin-film manufacturing capability closer to the elusive atomic layer etch facility necessary to achieve future semiconductor nodes.