The research presented here focuses on the study of phenomena associated with the rf power absorption, electron heating and transport in local or non-local regimes. To investigate the study rf current-voltage monitoring system and rf-compensated Langmuir probe system are developed. Experiment has performed in wide range external parameters, various gas pressures, frequencies, gases, magnetic fields, powers, grid-biases, and gap sizes.
From the monitoring of the discharge current-voltage, we found the capacitive discharge dissipate most of the rf power through the ion motion in the sheath rather than electron motion in the bulk at low frequency, while the capacitive discharge dissipate most of the rf power through the electron motion in bulk at high frequency. As a result, the mode transition for rf power dissipation from ion dominated dissipation to electron dominated dissipation takes place while increasing the driving frequency. This is due to the fact that sheath resistance corresponding the power dissipation by ion in the sheath decreases greatly with driving frequency. To conform the argument, we theoretically investigate transition with a simple circuit model and PIC simulation. Both theoretical result is in a good agreement with the experimental result.
The transverse magnetic field also induced the similar transition mentioned above. As the magnetic field increases, the electron motion to the surface well which is perpendicular to the field line is strongly reduced, so that self-bias (dc-sheath voltage) in the sheath decreases. Because the self-bias is directly related to the rf dissipation by the ion ($P_{ion} ∝ V_{self}ㆍI_i$), the rf power dissipation in the sheath decreases strongly with the magnetic field. As result, rf power dissipation mode transition from the ion dominated dissipation to electron dominated dissipation is induced by increasing the magnetic field.
Gas species effect on the rf power dissipation mode transition is investigated. When the discharge gas changes to bigger one having large collision cross-section and heavy atom, ion dissipation in the sheath decrease due to its low mobility, thus the mode from the ion dominated dissipation to electron dissipation take plasma when the gas species change to heavy atom gas at constant current.
The paradoxical sheath width variation in magnetized CCP is found with increasing the magnetic field at fixed discharge voltage. Although the electron density decreases with the magnetic field, the sheath width paradoxically decreases with the magnetic field ($s=\sqrt{1/n}$). We solve the problem by measuring the spatial electron density profile under the different magnetic field. Spatially resolved langmuir probe measurement reveals that although the electron density at the center decreases, the electron density near the sheath edge increase as increase the magnetic field. Therefore, the paradoxical sheath width variation stem from overlooking the axial redistribution of plasma density while increasing the magnetic field.
The inhomogeneous radial density profile under the transverse magnetic field has reported indirectly in many other studies by measuring the non-uniform charging distribution and etching result on the wafer. The $E \times B$ drift has long been believed the main effect to make the non-uniformity without any direct measurement. We measure the spatially non-uniform density profile at different magnetic field, and from the measurement, we can solidify the long believed argument without direct verification, the E×B drift is the origin of radial non-uniformity of electron density and with simple fluid model we can reproduce the density drift in the plasma.
While increasing the transverse magnetic field at high pressure Ar discharge (300 mTorr), a low energy electron cooling induced by the transverse magnetic field is observed in measured Electron Energy Distribution Functions (EEDFS). This is due to the fact that electron heating of low energy electrons near the Ramsauer minimum from the rf electric field is strongly prevented by the electron gyro-averaging effect. A theoretical calculation of electron distribution based on the electron kinetics agrees well with the experimental result.
The measurement of EEDFs in the low pressure capacitive discharge under the collisionless electron heating regime, where the electron mean free path is comparable to or larger than the system length, reveals that there is a new feature of electron energy distribution with a plateau in the low energy electron range, indicating the strong electron heating in that energy range. This observed result can be explained in terms of collisionless heating from the interaction between the electron bouncing motion and the oscillating sheath. A simple calculation of the electron energy distribution with the energy diffusion coefficient including the electron bounce effect is in good agreement with the experiment.
From the spatially resolved measurements of electron energy distribution functions (EEDFs) in a magnetized capacitive discharge, we found that the non-local electron kinetic property, the coincident property of the EEDFs of the total energy (kinetic energy(u) + potential energy(φ)) in different spatial positions, disappears as the magnetic field increases. This result can be understood as a transition of electron kinetic property from a non-local to a local regime induced by the magnetic field. This transition results from the fact that the magnetic field decreases the electron diffusion in the coordinates space but increases the electron diffusion in the energy space.
The electron density and temperature which can not control independently in conventional low-pressure capacitive discharge can control independently in modified capacitive discharge installing the mesh grid at the discharge center. Because electron density and temperature are related to th radical denities and heir composition, respectively, the independent control of them may be important work to find the processing window for nano-scale etch and deposition. Normally in the low pressure discharge, the electron density increases with the rf discharge current keeping a almost same electron temperature, so that just one electron temperature is possible for one electron density. As the grid bias increases, we found that the electron temperature greatly decreases. Therefore, while varying the grid bias and the discharge current, various electron temperatures are possible for a given electron density, and the electron density and temperature can be controlled from $4 × 10^8 cm^{-3}$ to $1×10^{10} cm^{-3}$ and from 1 eV to 4 eV, respectively. This control mechanism of electron temperature results from non-local kinetic property of the electrons passing through the mesh grid.
With the biased grid, a method to control negative ion density in $SF_6/Ar$ capacitive discharge is proposed. By changing the grid bias, the negative ion density ($n_-$) and the ratio between the negative and positive ions ($α\equiv n_-/n_+$) can be controlled within a wide range from $2.8 × 10^7 cm^{-3}$ to $4 × 10^9 cm^{-3}$ and from 0.18 to 0.86, respectively. This ability to control the negative ion density is due to the fact that the fraction of low energy electrons in the Electron Energy Distribution Functions (EEDFs), which is important for generation of the negative ions in $SF_6$ plasma, can be changed by the grid bias.
The evolution of the EEDF over the gap size range from 2.5 to 7 cm in 65 mTorr Ar discharges is investigated both experimentally and theoretically. The measured EEDFs exhibit typical bi-Maxwellian forms with low-energy electron groups. A significant depletion in the low-energy portion of the bi-Mawellian is found with decreasing gap size. Results are shown that electron heating by bulk electric fields, which is the main heating process of the low-energy electrons, is greatly enhanced as the gap size becomes small, resulting in the abrupt change of the EEDF. The calculated EEDFs based on non-local kinetic theory are in good agreement with the experiments.
