Etching of Si through a thick condensed XeF2 layer

Etching of silicon by XeF2 is studied in a multiple-beam setup. Below 150 K XeF2 condenses and forms a layer on the silicon, which blocks the etching. Upon ion bombardment, this layer is removed and etching will resume. As a function of the layer thickness, the various removal mechanisms of the layer are studied. For a thick condensed layer it is found that 1 keV Ar+ ions sputter the condensed layer with a yield of 160 XeF2 molecules per ion for 1 keV Ar+ ions and 280 for 2 keV ions. For thinner layers (below 9 nm for 1 keV ions), this sputter rate by ions decreases significantly. Here, the removal is mainly due to consumption of XeF2 by etching at the bottom of the layer. This consumption rate reaches a maximum for a layer thickness of about 5 nm. In the steady-state situation, the layer thickness is further decreased, resulting in a smaller consumption and etch rate. Here, sputtering is the most important removal mechanism for the deposited XeF2 layer. From this, it is concluded that a pulsed ion beam should be used in cryogenic etching to obtain the highest etch rate.