Reactive Ion Etching (RIE) and Falcon RIE

Dry Etching and Plasma

Dry etching uses gas-phase species — ions, radicals, or a combination of both — to remove material from a substrate inside a low-pressure vacuum chamber. Unlike wet etching, no liquid etchant is involved, which eliminates capillary-force undercut beneath resist masks and enables highly anisotropic, well-controlled profiles.

The most widely used dry etching technique is plasma etching, where a radiofrequency (RF) electric field ionises a gas, generating a plasma consisting of ions, free electrons, and neutral radicals. Because electron mobility is far higher than ionic mobility, the electrons respond to the RF field much faster than the ions. The consequence is charge accumulation: on the electrode connected to the RF source, electrons deposit faster than ions can neutralise them. Since a blocking capacitor prevents DC current from flowing to ground, a steady negative self-bias DC voltage builds up on that electrode. This bias accelerates positive ions perpendicularly toward the electrode surface, producing highly directional (anisotropic) bombardment.

Near each electrode, the bulk plasma transitions into a thin layer, a few tens of micrometres to about one millimetre thick, called the sheath (or dark space). In this region the electron density drops sharply, leaving a net positive charge. The strong electric field inside the sheath accelerates ions normal to the electrode surface, which means the etching direction is always perpendicular to the sample — a key advantage for defining vertical sidewalls.

Physical vs. chemical mechanisms

Plasma etching combines two distinct mechanisms:

Physical etching (sputtering): heavy ions are accelerated toward the surface and knock out atoms by momentum transfer. This process is highly anisotropic (directional) but not selective — it attacks most materials at similar rates.
Chemical etching: reactive radicals (e.g. fluorine or oxygen species) diffuse to the surface and react chemically with the material to form volatile by-products that are pumped away. This is faster and more selective but tends to be isotropic, etching laterally as well as vertically.

In practice, most processes combine both mechanisms. Tuning the balance — via RF power, gas chemistry, pressure, and substrate temperature — allows control over anisotropy, selectivity, and etch rate.

Capacitively coupled plasma (CCP) and RIE

In a capacitively coupled system, plasma is sustained between two parallel plate electrodes. When the substrate is placed on the RF-powered electrode (cathode), the self-bias is maximised because the cathode develops a DC potential roughly ten times larger than the grounded anode. The ratio of ion acceleration voltages between cathode and anode scales with electrode areas:

\[\frac{V_C}{V_A} = \left(\frac{A_A}{A_C}\right)^4\]

Making the anode area much larger than the cathode area therefore concentrates the ion acceleration at the cathode, increasing the etch rate and anisotropy at the substrate. This is the Reactive Ion Etching (RIE) configuration.

RIE combines physical bombardment (from ion acceleration) with chemical reactivity (from radical species in the plasma), delivering higher etch rates and better directionality than purely chemical plasma etching while retaining some selectivity from the chemistry. Typical electron densities are \(10^9\)–\(10^{10}\ \mathrm{cm^{-3}}\) and operating pressures range from 50 to 500 mTorr.

Key process parameters

Parameter	Effect
RF power	Higher power → more ion energy → faster, more anisotropic etching; risk of resist damage
Gas flow (sccm)	Sets the supply of reactive species; affects pressure and chemistry
Pressure	Lower pressure → longer mean free path → more anisotropic; higher pressure → more chemical, more isotropic
Substrate temperature	Higher temperature → favours chemical etching (more isotropic); lower → more anisotropic
DC self-bias	Diagnostic of ion energy; monitor to ensure process reproducibility

Impedance matching

The RF generator has an output impedance of \(50\ \Omega\), but the plasma load impedance is highly reactive and variable. A matching network (with tunable tune and load capacitors) transforms the plasma impedance to match the generator, minimising reflected power. It is good practice to start with low initial values (e.g. tune 30, load 20) so the network has room to move upward while searching for the impedance match, rather than starting near its limits.

Falcon RIE — Step-by-Step Protocol

A standard RIE system consists of:

a stainless-steel vacuum chamber with two parallel plate electrodes;
a rotary (roughing) pump** for initial pump-down;
a turbomolecular pump backed by a foreline connecting it to the roughing pump;
a gate valve between the turbo and the chamber, used to throttle conductance and regulate pressure;
an RF generator and matching network;
mass flow controllers (MFCs) for process gases (e.g. O₂, CF₄, Ar);
pressure gauges covering the full range from atmosphere to high vacuum;
a vent valve to return the chamber to atmospheric pressure with dry nitrogen.

Pump-down and reaching base vacuum

Open the primary pump and the roughing valve to begin pumping the chamber from atmosphere,
monitor the chamber pressure, wait until it reaches at least \(10^{-1}\ \mathrm{mbar}\),
once the target roughing pressure is reached, close the roughing valve and open the foreline valve (connecting the roughing pump to the back of the turbo),
switch on the turbo pump and wait until it reaches 100 Hz,
at 100 Hz, open the gate valve between the turbo and the chamber and set the turbo to speed-regulated mode.

Starting the process

Set the RF power (5 W for descum) and the process time (5 min for descum),
wait until the turbo reaches 500–600 Hz,
open the gas cylinders, then open the gas line valves and them set the mass flow controllers (O₂ flow to 30 sccm for descum),
open the process gas valve to introduce the gasses into the chamber, the pressure will rise to approximately \(10^{-1}\ \mathrm{mbar}\),
press Start, the system will run for the set time and then cut the gas flow automatically, the pressure will drop as the gas is pumped away.

Venting the chamber and retrieving the sample

Once the process is complete and the gas flow has stopped:

Close the gate valve,
open the vent valve to admit dry nitrogen, wait for pressure to rise,
close the vent valve, close the foreline valve, open the roughing valve,
close the roughing valve, open the vent valve again and wait,
close the vent valve, open the roughing valve again and wait,
close the roughing valve, open the vent valve a final time until the chamber reaches atmospheric pressure,
open the chamber and retrieve the sample.

The repeated alternation of vent and roughing steps purges residual reactive gas from the chamber before opening, protecting both the operator and the sample.

Leaving the machine in standby

With the sample removed and the chamber closed, open the roughing valve and pump down until the pressure reaches \(10^{-1}\ \mathrm{mbar}\),
close the roughing valve,
log out of the system software,
record all process parameters in the logbook.