The energy distribution of ions (IED) bombarding a substrate during plasma etching has demonstrated effects on etch selectivity for integrated circuit fabrication. Accurate control of the IED is desired to better understand the nature of plasma-surface interaction and to control process outcomes. IED control can be achieved by tailoring the wave form shape of an rf bias applied to the substrate, using a programmable wave form generator in combination with a power amplifier. Due to the frequency dependence of the amplifier gain and the impedance of the plasma in contact with the substrate, it is not practical to predict the shape of the input wave form needed to produce a desired result at the substrate. Introduced here is an iterative approach using feedback control in the frequency domain to produce arbitrary wave form shapes at the substrate. A fast Fourier transform (FFT) of the substrate wave form is compared, one frequency at a time, with the FFT of a desired target wave form, to determine adjustments needed at the generator. This iterative procedure, which is fully automated and tested for several target wave form shapes, is repeated until the substrate wave form converges to the targeted shape.

Since the mean, standard deviation, and modality of nanoparticle size distributions can vary greatly between similar input conditions (e.g., power and gas flow rate), plasma diagnostics were carried out in situ using a double-sided, planar Langmuir probe to determine the effect the plasma has on the heating of clusters and their final size distributions. The formation of Cu nanoparticles was analyzed using cluster-plasma physics, which relates the processes of condensation and evaporation to internal plasma properties (e.g., electron temperature and density). Monitoring these plasma properties while depositing Cu nanoparticles with different size distributions revealed a negative correlation between average particle size and electron temperature. Furthermore, the modality of the size distributions also correlated with the modality of the electron energy distributions. It was found that the maximum cluster temperature reached during plasma heating and the material's evaporation point regulates the growth process inside the plasma. In the case of Cu, size distributions with average sizes of 8.2, 17.3, and 24.9 nm in diameter were monitored with the Langmuir probe, and from the measurements made, the cluster temperatures for each deposition were calculated to be 1028, 1009, and 863 K. These values are then compared with the onset evaporation temperature of particles of this size, which was estimated to be 1059, 1068, and 1071 K. Thus, when the cluster temperature is too close to the evaporation temperature, less particle growth occurs, resulting in the formation of smaller particles. V C 2016 AIP Publishing LLC.

Patterson, M. M.; Cochran, A.; Ferina, J.; Rui, X.; Zimmerman, T. A.; Sun, Zhiguang; Kramer, Matthew J.; Sellmyer, David J.; and Shield, Jeffrey E., "Early stages of direct L1 0 FePt nanocluster formation: The effects of plasma characteristics" (2010 The formation of FePt nanoclusters via gas condensation has attracted a great deal of attention. The clusters normally form with the magnetically soft A1 structure rather than the desired L1 0 structure with high magnetocrystalline anisotropy. This work has examined the effects of plasma characteristics on the early stages of order in the formation L1 0 FePt nanoclusters via inert gas condensation. The plasma characteristics have been modified to control ion density in the nanocluster condensation region. Increased ion density results in more cluster-ion collisions. The energy imparted to the clusters as a result of these collisions allows atomic rearrangements to form the ordered structure. The results indicate that controlled ion density directly impacts the early stages of FePt nanocluster ordering, according to high-resolution electron microscopy structure observations and coercivity measurements.

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