S.M. Morley, 'The Etching of Si and Al in CCl4 Plasmas', PhD Thesis, 1991.

This thesis addresses the plasma etching of Si and Al substrates in CCl₄ discharges. These processes are crucial in Si device fabrication. Studies were performed using a commercial plasma etcher (Plasma Technology, PE80) with various process gas flow rates and RF power densities, but at a constant RF frequency of 107 kHz and a constant gas pressure of 200 mtorr.

During etching a significant amount of 'glow-polymer' of form (CCl₂)_n, is formed on all surfaces in the plasma chamber. Surfaces subject to ion bombardment re- sputter this (CCl₂)_n, back into the gas phase.

A spatially resolved quadrupole mass spectrometric (QMS) study of the gas-phase species inside the plasma was made by inserting a movable capillary probe into the plasma. Little variation of species concentrations was found as a function of position across the chamber. A variable electron energy ion source was employed in the QMS to aid identification of ion parents. Gas phase species detected in a pure CCl₄ plasma were CO, CCl₄, Cl₂, C₂Cl₂, C₂Cl₄ and HCl. Products detected during Si etching were SiCl₄ and a form of polymeric Si: (SiCl₂)_n,. No product species were detectable during Al etching. For both Si and Al, it is established that Cl₂ is a reactant.

Optical emission spectra were recorded for CCl₄ plasmas in the range 200-750 nm. Excited states of C, Cl, Cl+, C1₂, C1₂+, CO, CO+, CCl, HCI+, Al and AlCl were observed. When etching Si(100) wafers product emission from only Si and SiCI₄+ was observed. Optical emission spectra of SiCl₄ plasmas were recorded: excited states of Si, SiCl, SiCl₂ and SiCl₄+ were observed. A spatially resolved optical study of the CCl₄ plasma was made using a fine probe, inserted into the discharge, to monitor the intensity of emission of various species. The spatial emission intensities of excited species varied widely with the gaseous species and the substrate being etched in the plasma. Our experiments show that the PE80 reactor has a permanent central volume where the plasma density is weaker than the surrounding plasma. This is due to non- uniform ion bombardment of the upper electrode, producing a non-uniform secondary electron emission distribution. Non-uniform etching of Al and Si is observed, due in part to this effect but mainly caused by the lack of available etchant (Cl₂) at the wafer surface.

An XPS investigation of plasma-etched Si was made. This revealed that Cl and C are only incorporated to a limited extent in the Si surface during etching. Post-etch exposure to the atmosphere caused Cl to be widely replaced by O. C is found both as interstitial atoms and also on Si lattice sites replacing Si that has been removed.

It is suggested that in this reactor the ion-assisted etching of Si by Cl proceeds via several simultaneous mechanisms producing SiCl_x, (x=1,2,4) as direct etch products.