|
Perfluorocompounds like SF6,, widely used in the semiconductor industries for plasma etching applications, are among the strongest greenhouse gases, with global warming potentials 3/4 orders of magnitude higher than CO2 and can be considered as responsible of the huge recent climate changes. For this reason they are in the basket of the Kyoto-Protocol and there are several national proposals to ban SF6. In dry etching, SF6 is used to etch polysilicon layers due to its easy dissociation into radicals F and ions SF5+. In high vacuum conditions, radicals react with Si to form highly volatile molecule while ions bombard the substrate. SF6 is also heavily used to clean chamber walls from organic polymer deposition coming from etch processes: waferless autoclean (WAC). It consists of 2 steps: the 1st one, based on SF6 and O2, allows to remove Si and C residues, while in the 2nd one high O2 flow removes all the F compounds. The aim of this job is to replace SF6 in the WAC with a lower global warming potential gas like as NF3. The activity was done on in the R&D department 8 " fab of STMicrolectronics in Agrate Brianza (Milan - Italy) on a Lam polysilicon etching reactor, where turbo molecular pump allows vacuum as few mTorr and plasma is generated by planar inductive coil and bottom electrode powered at 13.56 MHz. Etch rates on different substrates (polysilicon, oxide and photo resist) showed that NF3_WAC is much aggressive towards photo resist (Carbon residues) respect to the banned chemistry. WAC efficiency is properly tested by the traces emission at fixed wavelength: λ=703 nm is able to monitor F compounds, while λ=516 nm checks along the time C compounds. On 703nm trace, NF3_WAC is faster to reach the saturation value than SF6_WAC, while during the 2nd step the traces of the 2 different cleanings are overlapped, showing a good efficiency of the NF3 for the chamber cleaning. On production, critical dimensions, profile by SEM cross sections and defectivity were tested on processes for Flash NOR on 0.090 nm technology node. The obtained results guarantee that production is stable along the time and not affected by NF3_WAC, as well SF6_WAC. NF3 flow reduction demonstrated that same equipment and process performances can be obtained even with half cleaning time. |