Development Of Plasma Processes Research Articles

The detachment-induced mode in electronegative capacitively coupled radio-frequency plasmas

Insights into the spatio-temporally resolved electron power absorption dynamics in capacitively coupled radio-frequency plasmas are essential for understanding the fundamentals of their operation and as a basis for knowledge-based plasma process development. Similar to the γ-mode, an ionization maximum is observed at the sheath edge around the time of maximum sheath voltage in electronegative oxygen discharges at a pressure of 300 Pa. Based on Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations, we demonstrate that this maximum is not only caused by secondary electrons emitted at the electrode and collisionally multiplied inside the sheath. In fact, it also occurs in the complete absence of secondary electrons in the simulation, and is caused by the generation of O− ions by electron attachment close to the electrode during the local sheath collapse. These negative ions are accelerated towards the plasma bulk by the sheath electric field during sheath expansion. By electron detachment from these negative ions, electrons are generated inside the sheath and are accelerated towards the plasma bulk by the instantaneous sheath electric field—similarly to secondary electrons. Ionization is also observed in the plasma bulk and caused by electrons generated by detachment and accelerated by the high drift-and ambipolar electric fields. This detachment-induced electron power absorption is found to have significant effects on the discharge in the presence and absence of secondary electron emission. Its fundamentals are understood based on an analysis of the spatio-temporal electron and O− power absorption dynamics as well as the trajectory of selected O− ions close to the electrode.

Ab Initio Group Additive Values for Thermodynamic Carbenium Ion Property Prediction

Carbenium ions are important intermediates in both zeolite and plasma chemistry. The construction of kinetic models for zeolite and plasma chemistry requires the incorporation of thermodynamic properties of these carbenium ions. In this way, thermodynamic equilibrium is incorporated resulting in more accurate and general kinetic models, which facilitate rational zeolite design and plasma process development. In this work, a consistent set of 46 group additive values (GAVs) and non-nearest neighbor interactions (NNIs) is determined for the standard enthalpy of formation, standard molar entropy, and heat capacity. The GAVs are regressed based on 165 quantum chemistry calculations with the CBS-QB3 composite method for carbenium ions as CBS-QB3 was proven a valid method for carbenium property prediction. The presented group additive approach gives the first GAVs and NNIs for entropy and heat capacity and 33 novel values for enthalpy prediction. The determined non-nearest neighbor interactions account for the effect of longer range hyperconjugation, resonance stabilization, and inductive stabilization, which are shown to be crucial for the carbenium stability. The presented group additivity scheme ensures an accurate prediction of all properties with a mean absolute error of 4.82 kJ/mol, 4.92 J/(mol·K), and 1.95 J/(mol·K) for the standard enthalpy of formation, standard molar entropy, and heat capacity, respectively. The reported GAVs and NNIs ensure the thermodynamic property prediction of paraffinic, olefinic, alkynic, aromatic, and five- and six-ring naphthenic cations.

Relative calibration of a retarding field energy analyzer sensor array for spatially resolved measurements of the ion flux and ion energy in low temperature plasmas.

A calibration routine is presented for an array of retarding field energy analyzer (RFEA) sensors distributed across a planar electrode surface with a diameter of 450mm that is exposed to a low temperature plasma. Such an array is used to measure the ion velocity distribution function at the electrode with radial and azimuthal resolutions as a basis for knowledge-based plasma process development. The presented calibration procedure is tested by exposing such an RFEA array to a large-area capacitively coupled argon plasma driven by two frequencies (13.56 and 27.12MHz) at a gas pressure of 0.5Pa. Up to 12 sensors are calibrated with respect to the 13th sensor, called the global reference sensor, by systematically varying the sensor positions across the array. The results show that the uncalibrated radial and azimuthal ion flux profiles are incorrect. The obtained profiles are different depending on the sensor arrangement and exhibit different radial and azimuthal behaviors. Based on the proposed calibration routine, the ion flux profiles can be corrected and a meaningful interpretation of the measured data is possible. The calibration factors are almost independent of the external process parameters, namely, input power, gas pressure, and gas mixture, investigated under large-area single-frequency capacitively coupled plasma conditions (27.12MHz). Thus, mean calibration factors are determined based on 45 different process conditions and can be used independent of the plasma conditions. The temporal stability of the calibration factors is found to be limited, i.e., the calibration must be repeated periodically.

Hydrogen Plasmas Processing of Graphene Surfaces

To assist the development of plasma processes to pattern graphene in a controlled way, interactions between hydrogen plasma species (H, H+, H2 +) and various types of graphene surfaces (monolayer, nanoribbons, multilayer) are investigated using atomic-scale simulations. It is shown that only “hot” H particles (i.e., with a kinetic energy greater than ~0.4 eV at 300 K) can adsorb on the basal plane of surface-clean graphene while adsorption is barrierless on free edges or vacancies. Surface reaction probabilities (reflection, adsorption, penetration) are found to strongly vary with the incident species energy, which allows to determine specific energy ranges (or process windows) for different types of H2 plasma treatment: lateral etching of graphene nanoribbons (GNRs), cleaning of graphene surfaces or vertical etching of multilayer graphene (MLG) stacks. Molecular dynamics simulations of GNRs trimming in downstream H2 plasmas allow to understand the mechanism which governs the anisotropic etching of ribbons and explains the absence of line-edge roughness on their edges. Interactions between low-energy (5–25 eV) H x + (x = 1, 2) ions with MLG are also investigated. Ion-induced damage (hydrogenation of successive graphene sheets, creation of vacancies) and etching of the MLG stack are found to vary with the ion energy, the ion fluence and the ion composition.

Challenges of Tailoring Surface Chemistry and Plasma/Surface Interactions to Advance Atomic Layer Etching

The ability to achieve atomic layer etch precision is reviewed in detail for a variety of material sets and implementation methods. For a cyclic approach most similar to a reverse ALD scheme, the process window to achieve a truly self-limited atomic layer etch (ALE) process is identified and the limitations as a function of controlling the adsorption step, the irradiation energy, and the reaction process are examined. Alternative approaches, namely processes to enable pseudo-ALE precision, are then introduced and results from their application investigated. Most of the recent work in plasma process development can be characterized by three fundamental approaches to atomic layer etching. Lastly, recent developments employing reactant flux control are briefly introduced, which have shown to provide a self-limited process that is able to exhibit high selectivity and pattern fidelity. The key feature of this novel method may be the ability to combine advances from the other atomic layer etch approaches.

Plasma deposition of optical films and coatings: A review

Plasma enhanced chemical vapor deposition (PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin film systems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasma systems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Thermal plasma process development in Norway

Abstract

Study of the surface properties and stability of polymer films treated by NH3 plasma and its mixtures.

This paper deals with the plasma surface treatment of polymers in a low frequency bell jar reactor with non-symmetrical configuration of electrodes. This special configuration of electrodes simulates the industrial surface treatment of polymers i.e. very short treatment times (milliseconds to a few seconds). The physicochemical properties of different polymers modified in such a reactor using ammonia and its mixtures have been investigated. The industrial aim of such treatments were for adhesion improvement in applications such as metallization and immobilization of proteins for the fabrication of immunosensors. The last part of this paper was devoted to the study of the stability of the plasma modified polymers which is an important issue for the industrial development of plasma processes. A pretreatment in a noble gas such as helium was proposed in order to stabilize the extreme surface layer via crosslinking before the functionalization treatment in ammonia plasma. In order to carry out the crosslinking and functionalization process in a one step process, mixtures of He+NH3 plasmas were studied.

Microscopic uniformity in plasma etching

As we enter the era of ultra-large-scale integrated circuit manufacture, plasma etching grows more important for fabricating structures with unprecedented dimensions. For feature sizes below 1 μm and aspect ratios (depth/width) much larger than one, etching rates have been observed to depend on aspect ratio and pattern density. Such dependencies tend to increase the cost of manufacturing because even small changes in device design rules, cell design, or wafer layout can result in time-consuming, new plasma process development. In addition, microscopically nonuniform etching affects the trade-off between chips lost from failure to clear and chips lost by damage from overetching. Although aspect ratio and pattern dependent etching have been observed for a large variety of material systems and processing conditions, the fundamental causes underlying these effects are poorly understood. Partly, this results from use of confusing and conflicting nomenclature and a lack of careful, quantitative comparisons between experiment and theory. In this article we review recent literature on microscopic uniformity in plasma etching and carefully define terminology to distinguish between aspect ratio dependent etching (ARDE) and the pattern dependent effect known as microloading. For ARDE, we use dimensional analysis to narrow the range of proposed mechanisms to four which involve ion transport, neutral transport, and surface charging. For microloading, we show that it is formally equivalent to the usual loading effect, where the reactant concentration is depeleted as a result of an excessive substrate load.

Mass spectrometric studies of positive ions in r.f. glow discharges

Mass spectrometric analysis of positive ions extracted from glow discharges is a common experimental procedure most often motivated by basic research considerations. An exception is the fairly recent application of this technique to the elemental analysis of solids. The experimental aspects of mass spectrometric analysis of plasma ions are discussed, and it is concluded that the insertion of an electrostatic deflection energy analyzer between the sampling orifice and the quadrupole mass filter is worthy of serious consideration in many research projects of this type. Several examples of the value of energy analysis are given. A brief discussion of the use of this technique in plasma process development is presented, and it is concluded that mass spectrometric ion sampling is not an obvious first choice in an applied environment but holds considerable promise in a basic research program.

Plasma process development and monitoring via mass spectrometry : G. B. Bunyard and B. A. Raby. Solid St. Technol. p. 53 (December 1977)

Plasma process development and monitoring via mass spectrometry : G. B. Bunyard and B. A. Raby. Solid St. Technol. p. 53 (December 1977)