Two plasma process-induced damage mechanisms are investigated in detail: charging damage by plasma conduction current through two different gate dielectrics (SiO2 and HfAlOx/SiO2 (high-k)) in metal-oxide semiconductor (MOS) devices and ion-bombardment damage in Si surfaces. The charging damage was identified as the changes in tunnelling currents and the decrease in the reliability lifetime. The behaviours were found to depend on the dielectric thickness as well as dielectric materials (SiO2 and high-k). For p-ch MOS transistors, the threshold voltage of devices with high-k exposed to Ar-based plasma shifted towards the positive direction, while with SiO2, towards the negative direction. This may be attributed to the difference in the trap site generation process and trapping phenomenon. With regard to the ion-bombardment damage, the changes in optical structures (surface layer thicknesses, dielectric function and mechanical strain within the interfacial damaged layers) by direct current and electron cyclotron resonance plasma exposures were investigated by spectroscopic ellipsometry and photoreflectance spectroscopy. The plasma-induced stressing current and the defect density were estimated to quantify the damage. The experimental findings provide key implications that a plasma-induced damage is one of the limiting factors for future-scaled device designs.
Bias frequency effects on damaged-layer formation during plasma processing were investigated. High-energy ion bombardment on Si substrates and subsequent damaged-layer formation are modeled on the basis of range theory. We propose a simplified model introducing a stopping power S
d(E
ion) with a power-law dependence on the energy of incident ions (E
ion). We applied this model to damaged-layer formation in plasma with an rf bias, where various energies of incident ions are expected. The ion energy distribution function (IEDF) was considered, and the distribution profile of defect sites was estimated. We found that, owing to the characteristic ion-energy-dependent stopping power S
d(E
ion) and the straggling, the bias frequency effect was subject to suppression, i.e., the thickness of the damaged layer is a weak function of bias frequency. These predicted features were compared with experimental data on the damage created using an inductively coupled plasma reactor with two different bias frequencies; 13.56 MHz and 400 kHz. The model prediction showed good agreement with experimental observations of the samples exposed to plasmas with various bias configurations.
Performance degradation of n-MOSFETs with plasma-induced recess structure was investigated. The depth of Si recess (d R ) was estimated from the experiments by using Ar gas plasmas. We propose an analytical model by assuming that the damage layer was formed during an offset spacer etch. A linear relationship between threshold voltage shift (ΔV th ) and d R was found. Device simulations were also performed for n-MOSFETs with various (d R ). Both |ΔV th | and OFF-state leakage current increased with an increase in d R . The increase in |ΔV th | becomes larger for smaller gate length. The results from device simulations are consistent with the analytical model. These findings imply that the Si recess structure induced by plasma damage enhances V th -variability in future devices.
Si surface damage induced by H 2 plasmas was studied in detail by optical and electrical analyses. Spectroscopic ellipsometry (SE) revealed a decrease in the pseudo-extinction coefficient hi in the region of photon energy higher than 3:4 eV upon H 2 -plasma exposure, which is attributed to the disordering of crystalline silicon (c-Si). The increase in hi in the lower energy region indicates the presence of trap sites for photogenerated carriers in the energy band gap in the E -k space of Si. The current-voltage (I-V ) measurement of metal-contacted structures was performed, revealing the following characteristic structures: thinner surface (SiO 2 ) and thicker interface (SiO 2 :c-Si) layers on the Si substrate in the case of H 2 -plasma exposure than those with Ar-and/or O 2 -plasma exposure. The structure assigned on the basis of both SE and I-V was further analyzed by a layer-by-layer wet-etching technique focusing on the removability of SiO 2 and its etch rate. The residual damage-layer thickness for the H 2 -plasma process was thicker (10 nm) than those for other plasma processes (<2 nm). Since the interface layer plays an important role in the optical assessment of the plasma-damage layer, the present findings imply that a conventional two-layer (SiO 2 /Si) optical model should be revised for in-line monitoring of H 2 -plasma damage. #
We have developed a three-dimensional particle model for a miniature microwave discharge ion thruster to elucidate the mechanism of ECR discharges confined in a small space. The model consists of a particle-in-cell simulation with a Monte Carlo collision algorithm (PIC-MCC) for the kinetics of charged particles, a finite-difference time-domain method for the electromagnetic fields of 4.2-GHz microwaves, and a finite element analysis for the
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