Alex Paterson

Frequency dependent plasma characteristics in a capacitively coupled 300 mm wafer plasma processing chamber

Argon plasma characteristics in a dual-frequency, capacitively coupled, 300 mm-wafer plasma processing system were investigated for rf drive frequencies between 10 and 190 MHz. We report spatial and frequency dependent changes in plasma parameters such as line-integrated electron density, ion saturation current, optical emission and argon metastable density. For the conditions investigated, the line-integrated electron density was a nonlinear function of drive frequency at constant rf power. In addition, the spatial distribution of the positive ions changed from uniform to peaked in the centre as the frequency was increased. Spatially resolved optical emission increased with frequency and the relative optical emission at several spectral lines depended on frequency. Argon metastable density and spatial distribution were not a strong function of drive frequency. Metastable temperature was approximately 400 K.

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.

Spatial and frequency dependence of plasma currents in a 300 mm capacitively coupled plasma reactor

There is much interest in scaling rf-excited capacitively coupled plasma reactors to larger sizes and to higher frequencies. As the size approaches operating wavelength, concerns arise about non-uniformity across the work piece, particularly in light of the well-documented slow-surface-wave phenomenon. We present measurements and calculations of spatial and frequency dependence of rf magnetic fields inside argon plasma in an industrially relevant, 300 mm plasma-processing chamber. The results show distinct differences in the spatial distributions and harmonic content of rf fields in the plasma at the three frequencies studied (13.56, 60 and 176 MHz). Evidence of a slow-wave structure was not apparent. The results suggest that interaction between the plasma and the rf excitation circuit may strongly influence the structures of these magnetic fields and that this interaction is frequency dependent. At the higher frequencies, wave propagation becomes extremely complex; it is controlled by the strong electrical nonlinearity of the sheath and is not explained simply by previous models.

Control of ion energy and angular distributions in dual-frequency capacitively coupled plasmas through power ratios and phase: Consequences on etch profiles

Anisotropic etching, enabled by energetic ion bombardment, is one of the primary roles of plasma–assisted materials processing for microelectronics fabrication. One challenge in plasma etching is being able to control the ion energy-angular distributions (IEADs) from the presheath to the surface of the wafer which is necessary for maintaining the critical dimension of features. Dual frequency capacitive coupled plasmas (DF-CCPs) potentially provide flexible control of IEADs, providing high selectivity while etching different materials and improved uniformity across the wafer. In this paper, the authors present a computational investigation of customizing and controlling IEADs in a DF-CCP resembling those industrially employed with both biases applied to the substrate holding the wafer. The authors found that the ratio of the low-frequency to high-frequency power can be used to control the plasma density, provide extra control for the angular width and energy of the IEADs, and to optimize etch profiles. If the phases between the low frequency and its higher harmonics are changed, the sheath dynamics are modulated, which in turn produces modulation in the ion energy distribution. With these trends, continuously varying the phases between the dual-frequencies can smooth the high frequency modulation in the time averaged IEADs. For validation, results from the simulation are compared with Langmuir probe measurements of ion saturation current densities in a DF-CCP.

Measured radial dependence of the peak sheath voltages present in very high frequency capacitive discharges

The radial distribution of the measured voltage drop across a sheath formed between a 300mm electrode and an argon plasma discharge is shown to depend on the excitation radio frequency, under constant power and pressure conditions. At a lower frequency of 13.56MHz, the voltage drop across the sheath is uniform across the 300mm electrode, while at higher frequencies of 60 and 162MHz the voltage drop becomes radially nonuniform. The magnitude and spatial extent of the nonuniformity become greater with increasing frequency.

RF discharge under the influence of a transverse magnetic field

We examine the effects of an externally applied magnetic field (0–150 G) on an argon discharge generated capacitively at 13.56 MHz, in a Gaseous Electronics Conference reference cell. Dependence of the electrical characteristics of the discharge are measured as functions of applied magnetic field, rf power and argon pressure. At fixed power the rf voltage decreases with increasing magnetic field. Likewise, the impedance of the discharge is capacitive but becomes more resistive as the electron mobility becomes limited by the magnetic field. The impact of the magnetic field is found to diminish as the cyclotron frequency of the electron becomes smaller than that of the collision frequency of the electron. We also measure the impact the magnetic field has on the distribution of the plasma in vertical planes parallel and perpendicular to the magnetic field using Langmuir probes, optical emission and laser-induced fluorescence. It is found that the distribution of the plasma remains symmetric in the plane parallel to the magnetic field and becomes skewed in the plane perpendicular to the magnetic field. The degree of skew depends on the optical state probed. Finally, we examine the spatial distribution and the temporal evolution of the electric fields in the plasma. It is shown that with the presence of the magnetic field, the thickness of the sheath is reduced and that most of the voltage drop is contained within the sheath. Consistent with dc voltage trends, there was no significant sheath reversal observed at higher magnetic fields. Comparisons of the results presented here are made with trends predicted by models and simulations found in the literature.

Electric fields in the sheath formed in a 300 mm, dual frequency capacitive argon discharge

The spatial structure and temporal evolution of the electric fields in a sheath formed in a dual frequency, 300 mm capacitive argon discharge are measured as functions of relative mixing between a low frequency current and a high frequency current. It is found that the overall structure of the sheath (potential across the sheath and the thickness of the sheath) are dominated by the lower frequency component while (smaller) oscillations in these quantities are dictated by the higher frequency component. Comparisons of the measured spatial and temporal profiles are made for Liebermans and Robiche et al sheath model and with a particle in a cell calculation.

Investigation of feature orientation and consequences of ion tilting during plasma etching with a three-dimensional feature profile simulator

Pattern transfer in microelectronics fabrication using plasma-assisted etching processes is being challenged by the three-dimensional (3d) structures of devices such as fin field effect transistors. Etching of 3d structures typically requires a longer over-etch time to clear material in corners, introducing additional selectivity challenges to maintain feature scale critical dimensions. Feature open area, orientation, aspect ratio, and proximity to other nearby structures can influence the outcome of the etch process. In this paper, the authors report on the development and application of a 3d profile simulator, the Monte Carlo feature profile model in the investigation of aspect ratio, and feature orientation dependent etching. In these studies, energy and angularly resolved reactant fluxes were provided by the hybrid plasma equipment model. Results from the model were validated with trends from experimental data. Using reactant fluxes from He/Cl2 and Ar/Cl2 inductively coupled plasmas, etching of two di...

Role of neutral transport in aspect ratio dependent plasma etching of three-dimensional features

Fabrication of semiconductor devices having three-dimensional (3D) structures places unprecedented demands on plasma etching processes. Among these demands is the frequent need to simultaneously etch features with a wide variety of aspect ratios (AR) on the same wafer. Many plasma etching processes exhibit aspect ratio dependent etching (ARDE)—different etch rates for features that have different aspect ratios, usually slower for larger AR. Processes subject to ARDE require over-etch to clear the larger AR features, which increases the need for high selectivity and low damage. Despite these issues, the physical processes which contribute to ARDE are not well understood. In this paper, results are discussed from a computational investigation on the root causes of ARDE during Ar/Cl2 plasma etching of Si, and, in particular, the role which neutral transport plays in this process. Parametric studies were performed varying neutral-to-ion flux ratios, surface recombination rates of atomic Cl, and neutral and io...

Ion temperature and velocity in a 300?mm diameter capacitively coupled plasma reactor driven at 13, 60 and 162?MHz

Spatially resolved temperature, radial drift velocity and relative density of an argon ion metastable state were measured in a capacitively coupled, parallel-plate reactor. The argon plasma was generated using single-frequency excitation of 13, 60, 162 MHz and dual frequency excitation of 13 and 60 MHz. For the conditions investigated, the ion temperatures were between 400 and 750 K. At the lower excitation frequencies of 13 and 60 MHz, the radial ion velocity increased monotonically from the center of the plasma to the edge of the electrode while ion temperature did not depend strongly on radial position or frequency. At 162 MHz excitation, the radial ion drift velocity peaked off center and the ion temperature showed large variations with radial position. For all cases, the radial drift velocity was less than the average thermal velocity. At all drive frequencies, the ion temperature was only a weak function of pressure and rf power. Comparison of the ion temperature and the drift velocity suggests that for 162 MHz excitation, the ion heating mechanism may depend on the radial position.