Softlanding During Gate Etching

As gate dielectrics are being scaled down to a few atomic layers, the plasma conditions used to form the bulk of the poly-Si gate profile can not be used anymore to land on the gate dielectric. Predictive endpoint techniques have to be used to stop the main etch before the gate dielectric is being exposed. Slide 1 illustrates the need for an intermediate step which has better profile formation properties than the overetch and better selectivity than the main etch.

Slide 2 shows that the gate oxide shows pitting when the main etch touches the gate dielectric while no pitting is observed when a softlanding step is introduced. The softlanding step is therefore a very crucial step for advanced gate etching.

The process trends for a typical HBr / Cl2 / O2 softlanding step are different for gate structured patterned with photoresist or hardmask. Slide 3 and slide 4 show that the increase of the oxygen flow has opposite effects for both hardmask materials.

Two potential mechanisms for the oxygen effect on the softlanding profiles are given in slide 5 and 6. Slide 5 gives an explanation based on the assumption that an Increased ion flux leads to stronger mask charging and sidewall attack, i.e. foot removal. In the hardmask case, charging is not significant. A higher ion flux and the availability of oxygen species for SiOxBry formation would lead to more reaction products which can be deposited onto sidewall. This would lead to a more tapered profile and a profile foot.

An alternative mechanism is proposed in slide 6 based on the role of carbon in the passivation process. In the case of the resist mask, carbon released from the mask could participate in the passivation of the bottom sidewall. An increase of O2 flow either removes the source of carbon via formation of carbon oxides or potentially encapsulates mask with SiOxBry. In the case of a dielectric hardmask, there is no source of carbon to begin with. The enhanced availability of O for SiOxBry formation leads to more reaction products which are deposited onto sidewall. This leads to a more tapered profile and the formation of a profile foot.

More plasma etch applications ...