8.3.4 Etchants and protectants: sulphur hexafluoride/oxygen plasma etching of siliconL
A high etch rate requires a highly reactive etchant, forming a gaseous reaction product that we don't have to remove in a separate process. We have considered chlorine and bromine as etchants, but the reactivity series for simple radicals is F > O > Cl > N > Br > H, so we would prefer to use fluorine or oxygen.
Oxides are almost always solids, with the notable exception of carbon dioxide. This makes O2 the plasma etchant of choice for carbon compounds, as the rapid etch selectively removes only organic material, just forming a surface oxide layer elsewhere.
Fluorides, in contrast, are quite commonly gaseous, except for group III of the Periodic Table (Al, Ga, In) – where we must settle for chlorine when seeking the best etch rates – and most transition metals, where hardly any compounds are volatile. A fluorine-containing plasma gives the highest etch rates for most other materials. Several gases are available as the source of fluorine, in order of increasing bond strength: XeF2, F2, ClF3, NF3, SF6, HF, CF4. The most generous with fluorine is SF6. Moreover, it is easily ionised and is non-toxic, so it is a common favourite – although unfortunately it is also a very potent greenhouse gas.
Since SiF4 is a gas, there is no need for ion bombardment to remove it, and an SF6 plasma etch of silicon, although fast, is isotropic. To protect sidewalls during the etch, a small bleed of oxygen is added to the gas mixture, and a one-off reaction at a newly etched surface forms a solid passivation layer involving atoms of silicon, oxygen and fluorine. This passivation layer resists chemical attack by the fluorine, stopping the etch dead. To keep the etch moving downwards, the oxide is sputtered from the etch floor by ion bombardment. Figure 38 shows the effect of adding O2 to the SF6 etch.
This passivated plasma etch can be thought of as three processes, operating simultaneously:
passivation of exposed silicon by oxygen radicals, forming SiO2
removal of oxide, from the etch floor only, by ion bombardment
chemical etching by fluorine radicals, acting only where the passivation has been removed.
There are now even more knobs for the engineer, as each process can be controlled separately. For example, if we want a vertical etch, but rate is at less of a premium than undercut or selectivity to other materials, we might now deliberately weaken the etch chemistry. We could do this by substituting Cl2 for SF6, or by adding HBr, which soaks up fluorine from the plasma to form HF. Note that the role of HBr in this case is very different from its action in the aluminium etch above. The choices made when selecting etch chemistries can be very specific to the materials under consideration and the constraints of the desired result, and several variants of this process might be used at different stages in forming the same device.