Plasma Technology for Advanced Devices
Capacitive and Inductive Coupling
Most low and medium plasma density reactors utilize capacitive coupling while high density plasmas can be generated by inductively coupled, electron cyclotron resonance (ECR) and some high frequency capacitively coupled reactors (slide 1). The capacitive coupling requires a high capacitance between the electrode and the plasma (large amplitude RF voltages). The inductive coupling requires a high inductance between a coil and the plasma (large RF currents). Capacitive coupling results in a high energy ion bombardment while the ion bombardment energy is much lower in inductively couples discharges. In a capacitive discharge, the periodic electron current flow to the electrode causes a modulation of the plasma potential. In an inductive discharge, the time varying current induces a time-varying magnetic field which induces a time varying electric field that can sustain the plasma.
An ideal capacitively coupled discharge is a vacuum chamber with two flat electrodes one of which connected to a rf power supply (typically the bottom electrode which supports the wafer (cathode) (slide 2). The ion density is weak, between 1E9 and 1E10 ion/cm-3. The discharge works in a pressure range between 10 and 100 mTorr. The self bias voltage (Vdc) can reach several hundreds of volts. Major drawback of this design is that it is impossible to control independently the ion density and ion energy.
Slide 3 illustrates the effect of the frequency of the RF signal on the ion energy distribution. Generally, the ion energy distribution function (IEDF) for very high frequencies is monoenergetic. For lower frequencies, the IEDF splits into two peaks with one low energy and one high energy component. The IEDF is ion mass dependent. The IEDF become distorted at higher pressures for which collisions can take place in the plasma sheath.
In addition to the cathode, one of the chamber surfaces, typically the lid, can be RF powered (slide 4). The frequency of the top electrode is usually higher than the frequency of the bias electrode. The higher the delta of the two frequencies, the better the decoupling. The high frequency contributes to the plasma density (anywhere between 1010 and 1012 ion/cm-3 depending on frequency and power) and the low frequency is used to tune the ion energy. The discharge works in a pressure range between below 10 and several 100 mTorr. The high frequency RF power can also be applied to the bottom electrode.
Slide 5 explains the differences between ohmic and stochastic heating in capacitively coupled reactors.
Generally, the plasma density increases when the excitation frequency is increased (slide 6). The exact correlation between plasma density and excitation frequency is however still subject of theoretical and experimental investigations. Nonlinearities have been reported repeatedly (see H. Goto et al., JVST A 10 (1992) 3048
In Magnetically Enhanced Reactive Ion Etching (MERIE), a magnetic field around the source suppresses electron neutralization on the chamber walls and increase the plasma density. The plasma generated is non-uniform due to the drift imposed by the magnetic field ( v x B where v is the electron velocity and B the local magnetic field). Electrons get accumulated on one side of the wafer leading to a strong plasma density and Vdc non-uniformity across the wafer (slide 7).
The plasma uniformity can be improved by introducing a magnetic field gradient close to the wafer or by using a rotating magnetic field. The self bias voltage (Vdc) across the wafer becomes therefore uniform allowing plasma induce damage to be strongly reduced (slide 8). In MERIE sources, ion density and energy cannot be independently controlled unless a second high frequency electrode is introduced.
The effect of the magnetic field in inductively coupled plasmas ICP) is described in slide 9. In ICP sources, a time varying current circulates in the coil and induces time varying magnetic and electric fields in the plasma which sustain the plasma.
Electron Cyclotron Resonance (ECR) plasmas are based on the coupling of an AC electric field, E, with a frequency which matches the frequency at which the electrons rotate in the constant magnetic field, the so called Larmor frequency (slide 10).