8.3.3 Reactive ion etching: chlorine/argon plasma etching of aluminium
In a reactive ion etch (RIE), a chemical reaction is used to weaken the bonding of the surface of the material and assist the sputtering process. This combines the high rate and selectivity of a gas-phase etch with the directionality of a sputter etch.
For example, consider aluminium etched anisotropically by a Cl2/Ar mixed-gas plasma, which etches at up to 1 μm min−1:
Power pumped into the plasma breaks the gases up, releasing argon and chlorine ions and an equal number of free electrons.
These electrons are accelerated in the electric field, and bombard the gas molecules to create more ions and break Cl–Cl bonds to release free chlorine atoms (radicals). (Radicals are fragments of molecules. They are chemically important because they are highly reactive: they want to recombine with something to make a stable molecule again.)
Although Cl2 gas does not react strongly with aluminium, the more aggressive chlorine radicals can rapidly convert the aluminium surface into AlCl3.
At 100 Pa, AlCl3 is still a solid but its molecules are not joined to the aluminium surface by primary chemical bonds, and it would boil off at about 100 °C. This makes it much easier than the aluminium to sputter away.
Bare aluminium exposed at the base of the etch is continually chlorinated and sputtered away, but the etch sidewalls are barely exposed to the ions, so the AlCl3 is removed from them only by the much slower process of evaporation. Any AlCl3 that does come off the walls tends to redeposit on the bottom corners of the etch floor, and the net effect is that the etch proceeds mostly vertically, tapering inwards and with slow undercutting of the mask.
Since the vertical and sideways etch processes act by different mechanisms, there are several ways to control the profile of the sidewalls:
More electrode voltage increases the ions' energy, and therefore their sputtering prowess, but makes them more likely to attack the lithographic mask.
More gas pressure has an opposite effect. It increases the likelihood of chemical etchants attacking the sidewalls, while slowing the ions down by collisions with gas molecules, so increasing the relative importance of chemistry compared to physics in the process. Similar effects come from changing the proportions of Cl2 and Ar in the gas mixture.
Lower platen temperature suppresses evaporation, resulting in less undercutting and tapering.
Adding HBr to the gas mixture introduces a competing chemical reaction whereby bromine radicals form AlBr3, which is a slower reaction but with a product that is easier to remove than AlCl3 – Figure 37 shows the effect of adding HBr to the mix.
Process engineers therefore have a good choice of control knobs, but they are never satisfied. They now demand that they should be able to:
eliminate undercutting and tapering
etch materials that don't form a convenient, easily sputtered compound – such as silicon.