Will Allis Prize Talk: Plasma Etching of Silicon: 50 Years and Still Surprises

Anisotropic plasma etching has long been a cornerstone process in microelectronics manufacturing, enabling the continued advancement of silicon integrated circuits to ever-higher performance levels. The long-held mechanism for directional etching invokes a combination of chemisorption of reactants, for example Cl atoms in the case of silicon, and desorption of products, such as SiCl and SiCl₂, induced by energetic positive ions that are accelerated across the plasma sheath adjacent to the substrate surface. While this explanation is no doubt correct at higher ion energies (>100 eV), it has recently been found that vacuum ultraviolet (VUV) light generated by the plasma is surprisingly efficient at stimulating desorption of etching products. At lower ion energies (<30 eV), this photo-assisted etching process can dominate over ion-stimulated desorption. Using a tandem plasma system to generate calibrated fluxes of VUV light at mostly 104 and 106 nm, etching yields in a chlorine-containing plasma of more than 100 Si atoms-per-photon were found under conditions where the maximum ion energy was <15 eV. Such high yields cannot be a result of electron-hole formation; the maximum possible yield for this mechanism is 2. Instead, it is believed that etching is a result of a photo-catalytic chain reaction, initiated by VUV photo-desorption of a negative ion or electron, leaving behind a hole at the surface. This hole migrates to a nearby surface Si-Si bond, weakening it and lowering the energy barrier for desorption of SiCl and/or SiCl₂. The hole diffuses along the surface, where it causes this process to occur over and over again.