Christophe Cardinaud

Plasma etching : principles, mechanisms, application to micro- and nano-technologies

Abstract Nowadays, plasma-etching processes are asked to produce patterns from the nanometer to the micrometer range with the same efficiency. The very severe requirements in terms of etch rate, selectivity, profile control and surface damage plasma-etching processes lead to, have been at the origin of the development of mechanistic studies by means of plasma diagnostics and surface analysis, as well as the development of new etching devices. We review here the basic concepts of plasma etching, and using examples, we describe more in details important features. We recall, in particular, the important role of the surface layer, the ion bombardment and the substrate temperature.

Studies on structural properties of amorphous nitrogenated carbon films from electron energy loss, ellipsometry, Auger electron spectroscopy, and electron-spin resonance

We report the variation of the structure and the electronic properties of amorphous nitrogenated carbon films (a-CH:Nx) prepared in dual electron cyclotron resonance-radio-frequency plasma from different mixtures of methane and nitrogen. Electron energy-loss spectroscopy, Auger electron spectroscopy, spectroscopic ellipsometry, and electron-spin-resonance spectroscopy are used to characterize the films. Unlike previous reports, addition of a low percentage (2.3%) of nitrogen in the films induces a strong change in their structure. The variation of electronic properties is rather small for a high concentration of nitrogen. From these experimental studies it seems that the efficiency of nitrogen doping depends on the nitrogen concentration. Modification of structure of the carbon network by nondoping and doping configurations of carbon nitrogen bonds is also emphasized. Our analyses establish an inter-relationship between the structure and electronic properties of nitrogenated carbon films, which helps to u...

Global model and diagnostic of a low-pressure SF6/Ar inductively coupled plasma

A global model has been developed for low-pressure (3–20 mTorr), radio-frequency (rf) (13.56 MHz) inductively coupled plasmas (ICPs), produced in SF6/Ar mixtures. The model is based on a set of mass balance equations for all the species considered, coupled to the discharge power balance equation and the charge neutrality condition. Simulations are used to show the impact of operating conditions, such as the rf power, the pressure and the percentage of argon in the mixture, on the evolution of charged and neutral species. Langmuir probe and optical emission spectroscopy measurements are used to determine the electron temperature and the densities of electrons, ions and atomic fluorine in the SF6/Ar ICPs under study. These data are compared with simulation results obtained from the global model. A satisfactory agreement is found between the simulation results and the measured values of the electron density and temperature, for rf powers in the range 900–1700 W, regardless of the percentage of argon in the mixture. Predictions for the atomic fluorine density (~1014 cm−3) are in good agreement with experiment, for various rf powers.

RF sputtered amorphous chalcogenide thin films for surface enhanced infrared absorption spectroscopy

The primary objective of this study is the development of transparent thin film materials in the IR enabling strong infrared absorption of organic compounds in the vicinity of metal nanoparticles by the surface plasmon effect. For developing these optical micro-sensors, hetero-structures combining gold nanoparticles and chalcogenide planar waveguides are fabricated and adequately characterized. Single As2S3 and Ge25Sb10Se65 amorphous chalcogenide thin films are prepared by radio-frequency magnetron sputtering. For the fabrication of gold nanoparticles on a chalcogenide planar waveguide, direct current sputtering is employed. Fabricated single layers or hetero-structures are characterized using various techniques to investigate the influence of deposition parameters. The nanoparticles of gold are functionalized by a self-assembled monolayer of 4-nitrothiophenol. Finally, the surface enhanced infrared absorption spectra of 4-nitrothiophenol self-assembled on fabricated Au/Ge-Sb-Se thin films hetero-structures are measured and analyzed. This optical component presents a ~24 enhancement factor for the detection of NO2 symmetric stretching vibration band of 4-nitrothiophenol at 1336 cm−1.

In situ x-ray photoelectron spectroscopy analysis of SiOxFy passivation layer obtained in a SF6/O2 cryoetching process

The oxyfluorinated silicon passivation layer created during various cryoetching processes is of interest in order to improve high aspect ratio profiles. In this work, the desorption of a SiOxFy layer obtained in an overpassivating SF6/O2 regime was investigated during the wafer warm-up from the cryogenic temperature to room temperature. An in situ x-ray photoelectron spectroscopy (XPS) device is used in order to probe the top-surface layer and understand the desorption mechanism. A new mechanism can be proposed using the evolution of fluorine, oxygen, silicon, and carbon contributions evidenced by XPS.

Electronic and optical investigation of hydrogenated amorphous carbon (a-C:H) by X-ray photoemission spectroscopy and spectroscopic ellipsometry

Hydrogenated amorphous carbon (a-C:H) films were prepared by low-pressure PECVD of CH4 in a dual electron cyclotron resonance (ECR)–radio frequency (RF) discharge by applying an independently controlled RF substrate bias voltage from 0 to −200 V. The films were analyzed with core-level and valence band (VB) X-ray photoemission spectroscopy (XPS) as well as spectroscopic ellipsometry. A progressive change from soft transparent polymer-like to hard absorbing diamond-like was observed with increasing bias voltage in this range. The C 1s core-level spectra were fitted by using a Doniach–Sunjic photoemission line-shape. The C 1s spectra showed the enhanced asymmetric behavior with increasing bias voltage. In addition, the VB spectra exhibited distinct π density of states according to the bias. Such XPS features were analyzed and interpreted in terms of the π→π* electron transition model on amorphous carbon through spectroscopic ellipsometry.

Etching studies of silica glasses in SF6/Ar inductively coupled plasmas: Implications for microfluidic devices fabrication

To fabricate microlaboratories, commercially available silica glasses represent a good alternative to the expensive quartz or fused silica substrates. Therefore, the authors have here investigated the behavior of four of them—Vycor, Pyrex, D263, and AF45—in SF6 and SF6/Ar inductively coupled plasmas. Using Vycor, a material close to pure SiO2, as a reference, they demonstrated that the etch rate negatively correlates with the global content in metallic oxides. However, no such clear trend was found for the surface roughness and they hypothesize that the large asperities (>500 nm) sometimes observed might be due to local variation in the glass surface composition. Furthermore, investigations on the influence of the plasma conditions (i.e., source power, dc self-bias, gas mixture, and pressure) on the etch rate, surface chemistry, and surface morphology, as well as positive ion current and fluorine concentration measurements, enable them to unravel an ion enhanced chemical etching mechanism, where stronger ...

Etching of Si at low temperatures using a SF6 reactive ion beam: Effect of the ion energy and current density

Reactive ion beam etching of silicon is performed at low substrate temperature (153<T<300 K). The beam is extracted from a SF6 microwave plasma. The energy of the beam is 100 eV<E<400 eV, the ion current density is 0<J<5 mA cm−2. Energy distribution and composition of the beam have been studied using a mass spectrometer coupled with a cylindric mirror analyzer. The beam can be considered as monoenergetic with a full width at half-maximum about 8 eV. Using SF6 plasma, F+ is the predominant species, a significant proportion of SF+, SF2+, S+, and SF3+ ions is present in the beam. The active neutral flux of fluorine atoms coming from the ion source has been estimated between 6×1016 and 4×1017 cm−2 s−1. When no ion beam strikes the sample, the probability of reaction of neutral F atoms with Si is evaluated between 0.02 and 0.1 at 300 K. The decrease of the substrate temperature has revealed the role of the ion-induced reaction and the ion-stimulated desorption of SFx radicals in the etching process. At relativ...

Ion energy distributions in a pulsed plasma doping system

Discharge parameters in a pulsed dc plasma doping system have been studied using measurements of time-resolved ion energy distributions, relative ion density, plasma potential, and electron temperature in BF3 and Ar plasmas during active discharge and afterglow periods. Negative plasma potentials are observed when using a hollow cathode to create a plasma while implanting at ultralow energies (<500eV). The kinetics of ion generation and decay in BF3 during the pulse on and off periods have been discussed.

Roughening of porous SiCOH materials in fluorocarbon plasmas

Porous SiCOH materials integration for integrated circuits faces serious challenges such as roughening during the etch process. In this study, atomic force microscopy is used to investigate the kinetics of SiCOH materials roughening when they are etched in fluorocarbon plasmas. We show that the root mean square roughness and the correlation length linearly increase with the etched depth, after an initiation period. We propose that: (1) during the first few seconds of the etch process, the surface of porous SiCOH materials gets denser. (2) Cracks are formed, leading to the formation of deep and narrow pits. (3) Plasma radicals diffuse through those pits and the pore network and modify the porous material at the bottom of the pits. (4) The difference in material density and composition between the surface and the bottom of the pits leads to a difference in etch rate and an amplification of the roughness. In addition to this intrinsic roughening mechanism, the presence of a metallic mask (titanium nitride) c...