Photo-Assisted Etching in Halogen Containing Plasmas



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Plasma etching is indispensable in microelectronics manufacturing, due to its ability to precisely pattern feature with lateral dimensions of <10 nm. To advance this capability to etch layers with atomic fidelity, while also achieving ultra-high material selectivity, low ion energies (10s eV) are required. However, at low ion energies, an alternative etching pathway catalyzed by vacuum ultraviolet (VUV) photons generated in the plasma has been shown to be very important.

In this thesis, photo-assisted etching (PAE) was studied in various halogen containing plasmas. Cl2, Br2, HBr, Br2/Cl2, and HBr/Cl2 feed gases diluted in Ar (50%–50% by volume) were used to study etching of p-type Si(100) in a radio frequency, inductively coupled, Faraday-shielded plasma. PAE was observed in all cases, with Br2/Ar and HBr/Cl2/Ar plasmas having the lowest and highest PAE rates, respectively. Etching rates measured under MgF2, quartz, and opaque windows showed that high energy VUV photons are more effective in inducing PAE compared to low energy photons. Characterization of etched surfaces using Scanning Electron and Atomic Force Microscopy (SEM and AFM) revealed that photo-etched surfaces were rough, quite likely due to the inability of the photo-assisted process to remove contaminants from the surface. PAE in Cl2/Ar plasmas resulted in the formation of 4-sided pyramidal features with bases that formed an angle of 45° with respect to <110> cleavage planes, suggesting that the PAE is sensitive to crystal orientation.

Various mechanisms have been proposed to explain photo-assisted etching of Si, including photo-generated carrier-mediated etching, and photon-induced damage (breaking of Si-Si bonds) caused by VUV photons irradiating the substrate. Optical Emission Spectroscopy was used to gain an insight into possible in-plasma PAE mechanisms. Emissions from Cl, Si, SiCl, and Ar were recorded as a function of power while etching p-Si in a 50%Cl2/50%Ar plasma at a pressure of 60 mTorr with no substrate bias. The Si:Ar optical emission intensity ratio, ISi/IAr (proportional to the etching rate of Si), increased substantially with power. Accounting for the contribution to this signal from the dissociation of SiClx (x=1-3) etch products, the residual increase in the emission indicated that the photo-assisted etching rate also increased with power. Time resolved emissions were also recorded in a pulsed plasma where power was modulated between 500W and 300W. ISi/IAr was found to modulate with the instantaneous power. This rules out the photon-induced damage mechanism since, if this mechanism was dominant, the ISi/IAr signal would not be modulated.



Photo-assisted etching, Plasma


Portions of this document appear in: Zhu, Weiye, Shyam Sridhar, Lei Liu, Eduardo Hernandez, Vincent M. Donnelly, and Demetre J. Economou. "Photo-assisted etching of silicon in chlorine-and bromine-containing plasmas." Journal of Applied Physics 115, no. 20 (2014): 203303. DOI: 10.1063/1.4878895.