Développement et caractérisation de procédés de gravure des espaceurs Si3N4 pour les technologies FDSOI

摘要

In CMOS technologies on FDSOI substrate, the silicon recess in transistor's source/drain regions caused by multiple etch steps is a critical parameter. The plasma etching of Si3N4 spacers, which occurs after the gate etch step, must allow the fabrication of straight spacer profiles which will define the effective channel length under the gate, while minimizing the consumption of the underlying silicon thin film. Moreover, the silicon surface state generated by the spacers etching must not prevent the epitaxial silicon growth used for the realization of raised source/drain regions.The study of current spacers etch processes based on CHxFy/O2 chemistries shows that silicon is consummated by oxidation when the plasma lands on the silicon surface. Furthermore, the XPS analysis shows that carbon is implanted in the silicon substrate by plasma ions, and that it inhibits the silicon epitaxial regrowth. We are able to reduce the implanted carbon concentration without any additional silicon recess by using non-oxidizing plasma post-treatments based on hydrogen.After identifying the limitations of current etch processes, we developed and characterized Si3N4 spacers etch processes using synchronously pulsed CH3F/O2/He plasmas. The modulation in short pulses with low duty cycles decreases the dose of high energy ions bombarding the substrate, which allows to reduce the oxidized silicon thickness as well as the concentration of implanted carbon. The addition in the plasma of a Si-containing gas, SiCl4 or SiF4, also leads to a reduction of the silicon consumption thanks to the deposition of a SiOxFy layer by radicals from the gas phase. The best result is obtained with a CH3F/O2/He plasmas pulsed at 1 kHz and 10% duty cycle with the addition of 5 or 10 sccm of SiF4 : the silicon recess is then almost zero.We also developed and evaluated an alternative etching technique, based on the implantation of He+ and H+ ions followed by a HF wet etch, for the etching of Si3N4 spacers. This innovative etch process does not generate any silicon recess and shows some promising results.