8.3.5 Alternative plasma chamber designs: MERIE and ICP
There are several variants of the parallel-plate RIE chamber. For example:
The ‘magnetically enhanced’ MERIE, where magnetic fields are used to slow the leakage of plasma to the chamber walls, reducing the operating voltage and improving the power efficiency.
‘Plasma mode’ operation, where the RF voltage is applied to the chamber ceiling and the platen is grounded. This reduces the ion energy at the wafer from hundreds of volts to tens of volts, giving a physically much gentler etch for low-damage processing.
Parallel-plate chambers, however, have two significant limitations
It is not possible to control the current (ion flux), voltage (ion energy) and power (chemical dissociation) independently of one another.
They will operate only over a limited range of pressure, from ~20 Pa to ~200 Pa.
To achieve independent ion flux and energy control requires two power sources – one to create the plasma and a second to accelerate the ions to the wafer. Plasma creation in this case uses a very different technology, in the form of an inductively coupled plasma (ICP) chamber – see Figure 39. The chamber has an insulating wall (or sometimes ceiling), with an antenna coil wrapped around the outside (or occasionally inside) through which an RF current flows. The oscillating magnetic field induces currents in the plasma, heating its electrons and driving the electron-molecule collisions that sustain it.
The choice of RIE (high pressure, low ion flux, high ion energy, and low radical density) against ICP (low pressure, high ion flux, controllable ion energy and high radical density) depends on the process result required – each has its advantages and there is not always agreement on the best choice.