Paper PS1+NS-WeM13
Comparison between NF₃ and CF₄ Chemistries for the Selective Etching of SiGe Sacrificial Layers in a 300mm Chemical Dry Etching Reactor

Wednesday, October 17, 2007, 12:00 pm, Room 606

The selective removal of a SiGe sacrificial layer is a key step in the realization of several architectures that are based on the SON technology. Such a process has been developed and studied in 200mm, showing very good results in terms of selectivity by using CF₄ as an etching gas. This competence has been transferred in 300mm by using a new generation chemical dry etching tool that offers an additional and alternative source of fluorine which consists in NF₃. A process based on this environment-friendly molecule has been developed and compared with the CF₄ reference in terms of SiGe:Si selectivity while keeping a comparable etch rate. In that aim, a strong dilution (1:10) was necessary because of the high dissociation level of NF₃ that results in high etch rates. Morphological analyses clearly show that the selectivity obtained by using NF₃ is prohibitive for the realization of advanced devices. Indeed, the selectivity value is around 3, which means that the thinning of the Si cap is only 3 times lower than the SiGe tunnel depth whereas it needs to be higher than 30 (it reaches 70 by using CF₄). Surface analyses by XPS reveal that a Si substrate exposed to the CF₄ process presents some carbon, which suggests a passivation mechanism that may be involved in the selectivity. With the carbon-free NF₃ molecule, this phenomenon does not occur, hence the lack of selectivity we observe. As a conclusion, even if it is possible to etch SiGe faster than Si by using NF₃, the reference process made of CF₄ remains the best solution for the tunnel etching of advanced devices where a controlled etch rate is necessary and a high SiGe:Si selectivity is mandatory.