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In this paper hydrodynamic and kinetic approaches to model low pressure capacitively coupled radio-frequency discharges are discussed. In particular approaches and results for power modulated discharges in a mixture of silane and hydrogen and for discharges containing a considerable amount of dust particles will be presented. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A 2D particle -in-cell and Monte Carlo code has been developed to study the behaviour of VHF discharges in a mixture of SiH4/H-2 at pressures below 300 mTorr and frequencies from 13.56 to 65 MHz. The aim of our study is to explain the increase of the deposition rate of thin amorphous films with the applied frequency which has been observed in various experiments. The frequency dependence of the electron energy distribution in the discharge area, the energy distribution of the ion fluxes to the substrate and the power dissipation are investigated at a constant power density. The experimentally observed increase of the deposition rate is explained by the fact that more ions are in an energy range which effectively influences the deposition process.

The operation of a mass spectrometer system with an electrostatic energy analyser, designed for measurements of mass-resolved ion-energy distributions, is discussed. We show how the electric fields in the different electrostatic lenses present in the system can be optimized. These lenses direct the ions entering the system into the energy filter and the quadrupole mass filter. These lenses can exhibit chromatic aberration. The conditions without chromatic aberration have been found by simulating the ion trajectories in the part of the system up to the energy filter. Also, an experimental method is presented to find these settings. We show that the energy-dependent transmission of ions through the system is mainly determined by its acceptance angle. Ionenergy spectra from an argon plasma have been measured and corrected for the transmission of the ions through the system. Published by Elsevier Science B.V.

A one-dimensional fluid model for radio-frequency glow discharges is presented which describes silane/hydrogen discharges that are used for the deposition of amorphous silicon (a-Si:H). The model is used to investigate the relation between the external settings (such as pressure, gas inlet, applied power, and frequency) and the resulting composition of the gas and the deposition rate. In the model, discharge quantities such as the electric field, densities, and fluxes of the panicles are calculated self-consistently. Look-up tables of the rates of the electron impact collisions as a function of the average electron energy are obtained by solving the Boltzmann equation in a two term approximation for a sequence of values of the reduced electric field. These tables are updated as the composition of the background neutral gas evolves under the In fluence of chemical reactions and pumping, Pumping configuration and gas inlet are taken into account by adding source terms in the density balance equations, The effect of pumping is represented by an average residence time. The gas inlet is represented by uniformly distributed particle sources. Also the radial transport of neutrals from the discharge volume into the discharge-free volume is important. As the fluid model is one dimensional, this radial transport is taken into account by an additional source term in the density balance equations. Plasma-wall interaction of the radicals (i.e., the growth of a-Si:H) is included through the use of sticking coefficients, A sensitivity study has been used to find a minimum see of different particles and reactions needed to describe the discharge adequately and to reduce the computational effort. This study has also been used to identify the most important plasma-chemical processes and resulted in a minimum set of 24 species, 15 electron-neutral reactions, and 22 chemical reactions. Tn order to verify the model, including the chemistry used, the results are compared with data from experiments. The partial pressures of silane, hydrogen, disilane, and the growth rate of amorphous silicon are compared for various combinations of the operating pressure (10-50 Pa), the power (2.5-10 W), and the frequency (13.56-65 MHz). The model shows good agreement with the experimental data in the dust free alpha regime. Discharges in the gamma' regime, where dust has a significant influence, could not be used to validate the model. (C) 1997 American Institute of Physics.

A fluid model for an argon rf discharge in a cylindrical discharge chamber is presented. The model contains the particle balances for electrons and ions and the electron energy balance. A nonzero autobias voltage is obtained by imposing the condition that the time‐averaged current toward the powered and grounded electrode is zero. Particle densities and ionization profiles peak strongly in front of the smaller, powered electrode. There electric fields are stronger and the electron current density is higher, resulting in more ohmic heating and therefore higher ionization rates. The radial uniformity of the plasma in front of the powered electrode gives a homogeneous ion flux toward this electrode. The asymmetric character of the profiles of the cylindrical geometry is in clear contrast with the essentially one‐dimensional infinite parallel‐plate geometry, which is fully symmetric with respect to the center of the discharge and has a zero dc autobias voltage. A comparison with results of a one‐dimensional model shows that the average ion density, the average ion flux, and the average ionization rate in the cylindrical reactor are comparable to those in a parallel‐plate reactor. The numerical treatment of the time evolution of the transport equations and Poisson’s equation needs an implicit method to avoid numerical instabilities. The resulting system of discretized equations is solved by a multigrid technique. The spatial discretization uses the Sharfetter–Gummel scheme.

In this paper, we present kinetic models for radio-frequency glow discharges. Discussed are models for bulk electrons and positive ions based on the solution of the Boltzmann equation in a one-dimensional geometry. The resulting spatiotemporal behaviour of the electrons is used to compute the plasma emission, which is compared with experimental data. The model for the ions is used to solve the coupled dynamics of the electric field and the ion density in the whole discharge as well as the ion velocity distribution at the electrodes. The results are compared with experimental data and with the results of a Monte Carlo code.

The response of a radio-frequency discharge in an electronegative gas to laser-induced photodetachment is considered. The discharge is described by a numerical fluid model, including the electron energy balance. The detachment event is simulated by transforming instantaneously all or part of the negative ions into electrons in the region of the discharge passed by the laser beam. In order to avoid severe restrictions on the computational time step, the evolution of the electron density and of the electric field is solved with a fully implicit numerical method. It turns out that the relaxation towards the periodic quasisteady state is mainly governed by a simultaneous increase of the production rate of negative ions and decrease of the ionization rate. The qualitative behavior of the calculated electron decay rate is in accordance with experimental observations.

Ion and neutral parameters are determined in the high electron density, magnetized, hydrogen plasma beam of an ITER divertor relevant plasma via measurements of the n=2 excited neutrals. Ion rotation velocity (up to 7 km/s) and temperature (2-3 eV~T_{e}) are obtained from analysis of Hα spectra measured close to the plasma source. The methodology for neutral density determination is explained whereby measurements in the linear plasma beam of Pilot-PSI are compared to modeling. Ground-state atomic densities are obtained via the production rate of n=2 and the optical thickness of the Lyman-α transition (escape factor ~0.6) and yield an ionization degree85% and dissociation degree in the residual gas of ~4%. A 30% proportion of molecules with a rovibrational excitation of more than 2 eV is deduced from the production rate of n=2 atoms. This proportion increases by more than a factor of 4 for a doubling of the electron density in the transition to ITER divertor relevant electron densities, probably because of a large increase in the production and confinement of ground-state neutrals. Measurements are made using laser-induced fluorescence (LIF) and absorption, the suitability of which are evaluated as diagnostics for this plasma regime. Absorption is found to have a much better sensitivity than LIF, mainly owing to competition with background emission.

The behavior of an rf discharge can be modeled by using a fluid approach. For this approach, the values of the mobility and diffusion coefficients as well as the ionization rate are necessary. These values are often obtained by extrapolating the data of dc Townsend discharges. To check whether this is justified we computed the coefficients for electrons in an rf discharge by using a kinetic model based on a two-term approximation of the electron energy distribution function. The calculated electron mobility and electron diffusion coefficients agree reasonably well with the extrapolated Townsend values. Significant deviations were found between the extrapolated Townsend ionization rate and the computed rf ionization rate as a function of the reduced electric field.

The dynamical onset of lane formation is studied in experiments with binary complex plasmas under microgravity conditions. Small microparticles are driven and penetrate into a cloud of big particles, revealing a strong tendency towards lane formation. The observed time-resolved lane formation process is in good agreement with computer simulations of a binary Yukawa model with Langevin dynamics. The laning is quantified in terms of the anisotropic scaling index, leading to a universal order parameter for driven systems., Comment: 4 pages, 3 figures, movies available at http://www.mpe.mpg.de/pke/lane-formation/

A numerical study on the ion and neutral angular impact energy distribution at the rf-driven electrode of a reactive ion etcher is presented. The calculations for the ions are performed using a Monte Carlo method that includes charge exchange and elastic scattering. The contribution of both collision processes to the angular ion impact energy distribution is studied. For the case that charge exchange is the only collision process, the Monte Carlo results can be checked against those of a method based on a spatially uniform and time independent collision rate. In that case, both methods yield the same ion impact energy distribution. The position, velocity, and propagation angle of the energetic neutrals created in collisions of ions with the background gas are stored. These are used as input data for a separate code that follows the evolution of the angular neutral energy distribution, taking into account (multiple) neutral elastic scattering. From the ion and neutral distributions, the number of neutrals per ion, the average impact energy, and the energy-weighted average impact angle have been derived. It is shown that these parameters are well described by simple expressions. Finally, the sputter yield is calculated. The results show that the contribution of the angular distributions of both ions and neutrals to the yield can be neglected.

Collections of micrometer sized solid particles immersed in plamsa are used to mimic many systems from solid state and fluid physics, due to their strong electrostatic interaction, their large inertia, and the fact that they are large enough to be visualized with ordinary optics. On Earth, gravity restricts the so called dusty plasma systems to thin, two-dimensional layers, unless special experimental geometries are used, involving heated or cooled electrons, and/or the use of dielectric materials.In micro-gravity experiments, the formation of a dust-free void breaks the isotropy of three-dimensional dusty plasma systems. In order to do real three-dimensional experiments, this void has somehow to be closed. In this paper, we use a fully self-consistent fluid model to study the closure of a void in a micro-gravity experiment, by lowering the driving potential. The analysis goes beyond the simple description of the virtual void, which describes the formation of a void without taking the dust into account. We show that self-organization plays an important role in void formation and void closure, which also allows a reversed scheme, where a discharge is run at low driving potentials and small batches of dust are added. No hysteresis is found this way. Finally, we compare our results to recent experiments and find good agreement,but only when we do not take charge-exchange collisions into account.

The FOM Institute for Plasma Physics Rijnhuizen (FOM = Foundation for Fundamental Research on Matter) has started a new line of research to study the interaction of intense plasma fluxes with a material surface. An important experimental tool for this programme will be the Magnum-PSI high-flux linear plasma generator operating in the ITER- relevant regime of plasma surface interaction (PSI). ITER is the next step in fusion reactor research (ITER is Latin for 'the way'). In Magnum-PSI a plasma beam is guided from the source to a target by a strong axial magnetic field. Besides ionized particles, the plasma source produces hot neutral gas which has to be prevented from reaching the target region. In this paper it is investigated to what extent a differential pumping scheme can prevent the influx of neutral gas from the source in the target region. The results of neutral gas simulations using the Direct Simulation Monte Carlo (DSMC) method will be discussed. We will focus on the supersonic expansion as a function of background pressure and determine the influence of the skimmers on the expansion. We will demonstrate that differential pumping can be used in Magnum-PSI to reach low enough pressures in the target region. Finally, we give the optimum position of the skimmer.

When dust particles are introduced in a radio‐frequency discharge under micro‐gravity conditions, usually a dust free void is formed due to the ion drag force pushing the particles away from the center. Experiments have shown that it is possible to close the void by reducing the power supplied to the discharge. This reduces the ion density and with that the ratio between the ion drag force and the opposing electric force. We have studied the behavior of a discharge with a large amount of dust particles (radius 3.4 micron) with our hydrodynamic model, and simulated the closure of the void for conditions similar to the experiment. We also approached the formation of a Yukawa ball from the other side, starting with a discharge at low power and injecting batches of dust, while increasing the power to prevent extinction of the discharge. Eventually the same situation could be reached.

A Particle-in-Cell Monte Carlo model is used to simulate extreme ultraviolet driven plasma. In an extreme ultraviolet lithography tool, photons of a pulsed discharge source will ionize a low pressure argon gas by photoionization. Together with the photoelectric effect, this results in a strongly time dependent and low density plasma, which is potentially dangerous to the optical elements, the collector in particular. Plasma sheaths will develop and ions are accelerated towards the collector, which might lead to sputtering. A spherical geometry is used to study the plasma between the point source and collector. Simulations are performed to study the in.uence of background pressure and source intensity on the damage to the collector by sputtering.

When dust particles are created in a radio‐frequency processing discharge or when they are introduced from outside, they obtain a negative charge due to the large mobility of the electrons. A high dust density therefore changes the discharge characteristics, not only by the presence of an almost immobile charge, but also because there is a continuous loss of ions and electrons by recombination at the surface of the dust particles. In this paper we use a 1D Particle‐In‐Cell plus Monte Carlo model to investigate the interaction between the electron energy distribution and the dust charging plus recombination process, its influence on the plasma impedance and on the ionization rate. The background gas is argon and a spatial distribution of immobile mono‐disperse spherical dust particles is prescribed.

A dusty radio-frequency argon discharge is simulated with the use of a two-dimensional fluid model. In the model, discharge quantities, such as the fluxes, densities, and electric field are calculated self-consistently. The charge and density of the dust are calculated with an iterative method. During the transport of the dust, its charge is kept constant in time. The dust influences the electric potential distribution through its charge and the density of the plasma through recombination of positive ions and electrons on its surface. Results are presented for situations in which the dust significantly changes the discharge characteristics, both by a strong reduction of the electron density and by altering the electric potential by its charge. Simulations for dust particles having a radius of 7.5 microm show that a double space charge layer is created around the sharp boundary of the dust crystal. A central dust-free region (void) is created by the ion drag force. Inside this void a strong increase of the production of argon metastables is found. This phenomenon is in agreement with experimental observations, where an enhanced light emission is seen inside the void.

A dusty radio-frequency silane/hydrogen discharge is simulated, with the use of a one-dimensional fluid model. In the model, discharge quantities like the fluxes, densities, and electric field are calculated self consistently. A radius and an initial density profile for the spherical dust particles are given and the charge and the density of the dust are calculated with an iterative method. During the transport of the dust, its charge is kept constant in time. The dust influences the electric field distribution through its charge and the density of the plasma through recombination of positive ions and electrons at its surface. In the model this process gives an extra production of silane radicals, since the growth of dust is not included. Results are presented for situations in which the dust significantly changes the discharge characteristics, both by a strong reduction of the electron density and by altering the electric field by its charge. Simulations for dust with a radius of 2 μm show that the stationary solution of the dust density and the average electric field depend on the total amount of the dust. The presence of dust enhances the deposition rate of amorphous silicon at the electrodes because of the rise in the average electron energy associated with the decrease of the electron density and the constraint of a constant power input.

Self-consistent dusty sheaths are simulated by the PIC/MC method in both plasmas with bi-thermal electrons and low-pressure RF discharges with a secondary electron emission from electrodes. The simulations are carried out by using plasma evolution without special boundary conditions for the sheaths that allows getting a self-consistent description of the sheaths. Simulation results show that dust particles can essentially influence spatial distributions of plasma parameters in the sheaths due to the space electric charge of the dust particles and a non-equilibrium of the electron energy distribution function. In particular, the spatial distributions of a self-consistent electric potential can be non-monotonic so that charged dust particles can protect electrodes from an intensive ion sputtering by decreasing the energy of ions arriving to the electrodes.

The results of a two‐dimensional fluid model for argon rf discharges in a closed cylindrical vacuum chamber are compared with experimental data from an amorphous silicon deposition reactor. Good agreement is obtained for the relation between the dc autobias voltage and the dissipated power in the frequency range 50–100 MHz at pressures between 12 and 90 Pa. A scaling law is presented for the relation between the power, the dc bias voltage, the rf excitation frequency, and the background pressure. The model yields a linear relation between the applied rf voltage and the dc bias voltage. This relation depends only on the geometry of the discharge chamber and shows an offset.

A 1D PIC/MCC method has been developed for computer simulations of low-pressure RF discharges with dust particles using the method for dust-free discharges. A RF discharge in argon with dust particles distributed uniformly in the interelectrode gap is simulated at parameters providing a possibility to consider the discharge as a physical model of processing plasmas. The spatial distribution of discharge parameters, including a charge of dust particles, across the discharge interelectrode gap has been obtained at various values for the density of the dust particles and for the electric current in the external discharge circuit. The results show that the RF discharge with dust particles has a quasi-neutral central part with a low electric field and non-stationary sheaths with a strong electric field separating the electrodes from the central part. The dust particles essentially influence the spatial distribution of the discharge parameters. In particular, an increase of a dust particle density causes an expansion of sheaths. The electric current in the external electric circuit changes at a sufficiently high density of dust particles. The dust particle charge changes non-monotonously across the interelectrode gap and has a maximum at a sheath edge, due to the spatial distribution of the plasma parameters and a peculiarity of the electron energy distribution function in the quasi-neutral central part of the RF discharge.

For the first time ion energy distributions (IED) of different ions from silane-hydrogen (SiH4-H2) RF plasmas are presented, i.e. the distributions of % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaae4uaiaabM % gacaqGibWaa0baaSqaaiaaiodaaeaacqGHRaWkaaGccaGGSaGaae4u % aiaabMgacaqGibWaa0baaSqaaiaaikdaaeaacqGHRaWkaaaaaa!3F61! $${∈xt{SiH}}_3^ +,{∈xt{SiH}}_2^ + $$ and % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaae4uaiaabM % gadaWgaaWcbaGaaGOmaaqabaGccaqGibWaa0baaSqaaiaaisdaaeaa % cqGHRaWkaaaaaa!3B42! $${∈xt{S}}{{∈xt{i}}_2}{∈xt{H}}_4^ + $$. The energy distributions of % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaae4uaiaabM % gacaqGibWaa0baaSqaaiaaiodaaeaacqGHRaWkaaaaaa!3A4F! $${∈xt{SiH}}_3^ + $$ and % MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaae4uaiaabM % gacaqGibWaa0baaSqaaiaaikdaaeaacqGHRaWkaaaaaa!3A4E! $${∈xt{SiH}}_2^ + $$ ions show peaks, which are caused by a charge exchange process in the sheath. A method is presented by which the net charge density in the sheath is determined from the plasma potential and the energy positions of the charge exchange peaks. Knowing the net charge density in the sheath and the plasma potential, the sheath thickness can be determined and an estimation of the absolute ion fluxes can be made. The flux of ions can, at maximum, account for 10% of the observed deposition rate.

The results of a 2-D fluid model for argon radiofrequency (RF) discharges in a closed cylindrical vacuum chamber are compared with experimental data from an amorphous silicon deposition reactor operated in argon. Good agreement is obtained for the relation between the DC autobias voltage and the dissipated power in the frequency range 40-100 MHz at pressures between 10 and 60 Pa. Scaling laws are presented for the dissipated power and for the ion fluxes toward the electrodes. These quantities are expressed in the DC bias voltage, the RF excitation frequency and the background pressure. Also the uniformity of the ion fluxes is studied. The model yields a linear relation between the applied RF voltage and the DC bias voltage. This relation depends only on the geometry of the discharge chamber and shows an offset. >

Recently it was shown that, by using the analysis of electrostatic waves entering the plasma-sheath edge, the direct-current (dc) Bohm criterion also holds for discharges under radio-frequency (rf) conditions. In this paper, the influence of Bohm's criterion on the sheath characteristics for generator frequencies much higher than the ion plasma frequency has been examined by means of an analysis which is based directly on the numerical solution of the Poisson equation. The present calculations indicate that for high rf voltages the time-dependent potential profile does not always increase monotonically, even if Bohm's criterion is fulfilled. However, as this nonmonotonic behavior does not lead to a more stringent Bohm criterion, the statement that the original dc Bohm criterion also holds in the high-frequency regime is confirmed. The calculations show further that the time-dependent sheath potential is almost completely modulated in the major part of the sheath and that the modulation drops abruptly to zero close to the plasma-sheath edge.

Complex plasma being the 'plasma state of soft matter' is especially suitable for investigations of non-equilibrium phase transitions. Non-equilibrium phase transitions can manifest in dissipative structures or self-organization. Two specific examples are lane formation and phase separation. Using the permanent microgravity laboratory PK-3 Plus, operating onboard the International Space Station, we performed unique experiments with binary mixtures of complex plasmas that showed both lane formation and phase separation. These observations have been augmented by comprehensive numerical and theoretical studies. In this paper we present an overview of our most important results. In addition we put our results in context with research of complex plasmas, binary systems and non-equilibrium phase transitions. Necessary and promising future complex plasma experiments on phase separation and lane formation are briefly discussed.

In this study we investigate how the hydrogen flux administered to amorphous hydrogenated carbon (a-C : H) samples influences the sticking probability of hydrocarbon radicals on these samples. We bombard a-C : H samples that were previously subjected to different hydrogen fluxes with hydrocarbon radicals. The energy of the radicals is 10 eV. The sample temperatures are 700 and 1000 K. The results show that the sticking probability is larger on samples pre-irradiated with a higher hydrogen flux caused by the higher sp(2) fraction in the sample. This suggests that redeposition can contribute to the observed flux dependency of the carbon erosion yield.

We study the flux dependence of the carbon erosion yield and the hydrogen enrichment of the surface in the high flux regime at 10(28) ions per m(2) s and higher by using molecular dynamics (MD). We simulate an amorphous hydrogenated carbon sample exposed to high flux hydrogen bombardment with a hydrogen energy of 10 eV at surface temperatures of 700 and 1000 K. As interaction potential the reactive empirical bond order potential of Brenner-Beardmore is taken and energy dissipation is simulated with the Berendsen thermostat. The simulation results show that the carbon erosion yield is higher for higher sample temperatures but does not show a strong dependence on the hydrogen flux. Hence, the hydrogen enrichment in the upper surface layer observed in the simulations most likely does not contribute to the erosion yield reduction in the experiments. Furthermore, the composition of the eroded material shows a slight increase in CH, C(2)H and C(2)H(2) for higher fluxes, whereas species with more hydrogen, C atoms and C(2) are decreased. However, the H : C ratio in the eroded material shows no flux dependence.

The well‐known dc Bohm criterion is extended to rf discharges. Both low‐ (ωrf≪ωpi) and high‐(ωpi ≪ ωrf) frequency regimes are considered. For low frequencies, the dc Bohm criterion holds. This criterion states that the initial energy of the ions entering the sheath must exceed a limit in order to obtain a stable sheath. For high frequencies, a modified limit is derived, which is somewhat lower than that of the dc Bohm criterion. The resulting ion current density in a high‐frequency sheath is only a few percent lower than that for the dc case.

Transport code simulations of m=2, n=1 mode control experiments by local electron cyclotron heating (ECH) in TFR are presented. The mode activity is calculated by means of the quasi-linear theory for tearing modes. Starting from an ohmic target plasma which is unstable to the m=2, n=1 tearing mode, the evolution of the m=2, n=1 magnetic island during a 100 ms ECH pulse is calculated for various positions of the electron cyclotron resonance and levels of injected power. The best results, i.e. a complete suppression of the island, are obtained for heating almost exactly on the q=2 surface. In contrast to experimental results, the suppression of the island is found to be only temporary. It is concluded that the suppression of the MHD activity obtained in the experiment cannot be explained by profile tailoring alone.