Ikuo Sawada

Relationship between center-peaked plasma density profiles and harmonic electromagnetic waves in very high frequency capacitively coupled plasma reactors

An understanding of the factors that control radial plasma uniformity in very high frequency (VHF) capacitively coupled plasma (CCP) sources is important for many plasma processes in semiconductor device manufacturing. Here, we report experimental measurements and high-resolution self-consistent numerical simulations that illustrate the plasma density profile and the higher harmonic wave content in two types of VHF-CCP test-bench reactors. A distinct sharp center peak superimposed on a broad center peak in argon plasma was observed for driving frequencies of 60 and 106 MHz. Experimental measurements and numerical simulations of the electric field power spectrum reveal the presence of UHF waves when the electron density is over 5 × 1016 (#/m3). The presence of UHF waves closely correlates with the occurrence of a distinct and sharp-center-peaked electron density. The numerical simulations show that specific frequency bands in the UHF spectrum are amplified in the plasma and lead to the evolution of the sharp-center-peaked electron density.

Plasma Chemistry of Octafluorocyclopentene/Argon/Oxygen Mixtures

An octafluorocyclopentene (c-C5F8)/Ar/O2 plasma chemistry mechanism has been developed using a combination of quantum chemistry methods, a zero-dimensional plasma kinetics model, and results from quadrupole mass spectrometer (QMS) and actinometry experiments on a 200 mm capacitively coupled plasma source driven at 60 MHz. Quantum chemistry methods reveal that the degradation process of c-C5F8 under electron impact is sensitive to the characteristics of the isomeric structure of the products. We find that the primary loss process for c-C5F8 as a feed gas is electron impact dissociation into isomers of C5F7 via excitation to the triplet state of c-C5F8. Electron impact dissociation of C5F7 isomers leads finally to the production of C5F5 (an isomer with two conjugate π bonds) and C5F6 (an isomer with two π bonds and a folded ring structure). Through dissociative ionization and subsequent wall recombination C5F8 produces C4F6 less with less probability than the triplet state. C4F6 in turn produces C2F4, C2F2, and C4F4 (two double bonds) and is a gas phase pathway for the production of CF2. CF2 is also produced via electron impact dissociation of an isomer of C5F6. C5F5 is not readily dissociated by electron impact because of the existence of extra π bonds that absorb electron energy in these species. The specific isomers encountered in c-C5F8 plasmas, C4F4, C5F5, and C5F6, have additional stability because of their structure. The etch precursor, atomic fluorine, is primarily produced from electron impact dissociation of the feed-gas and its degradation products. CF is produced from dissociation of CF2. CF3 is produced primarily from the walls. Because of their stability in the gas phase, C4F4, C5F6, and C5F5 are important polymer deposition species. The rate-limiting step for fluorine production is the electron impact dissociation of the triplet state of c-C5F8. Predicted etch rates are in good agreement with experimental data examining large substrate RF bias and low pressure.

Measurement of spatial and temporal evolution of electromagnetic fields in a 100 MHz plasma source using B dot and double dipole probes

Very high frequency plasma sources are often accompanied by plasma density nonuniformities associated with a standing-wave effect. Electron density measurements using a plasma absorption probe show density nonuniformities that can be larger than predicted by a standing wave model. These structures have been associated with harmonics of the electric fields in the plasma. The authors present the first time and phase-resolved measurements of the spatial structure of the electromagnetic waves in a 100 MHz plasma source using argon at 40 mTorr employing a B dot probe. The authors show that the harmonic structure is related to a current reversal and subsequent circulation that appears when the sheath collapses during the radio frequency cycle. The circulation is driven by inward traveling waves that are electromagnetic in nature, not plasma waves traveling at the electron thermal velocity. Double dipole probe measurements were used to validate the B dot probe electric field measurements derived from the time de...

Segmented Electrode Plasma Source with Anti-Phase Discharge

A new plasma source concept, consisting of segmented electrodes and an economical 13.56 MHz power generator, has been developed for the plasma-enhanced chemical vapor deposition (PECVD) of microcrystalline silicon thin films on large-area substrates. Using a plurality of rectangular column-type electrodes fed with RF (13.56 MHz) at various relative phases, a two-stage plasma, which consists of an upper and lower plasma, was generated. In the case of supplying the anti-phase RF, we could control the ion incident energy on a substrate to be lower, so as to keep the high crystallinity and high deposition rate during the film growth. The agreement between the observed light emission profiles from Ar or Ar/H2 plasma and the numerically calculated ones was good, confirming the accuracy of the simulation model, and we performed the thin-film growth of microcrystal silicon with this new plasma source and H2/SiH4 gas. Consequently, we could achieve a fast deposition rate of more than 1.5 nm/s with good crystallinity over 60%.