The Effect of Metal Masks on the Plasma Etch Rate of Silicon

Abstract

The etch rate of silicon in CF4/O2 plasmas is a strong function of the masking material coated on the wafer to define etch patterns. Samples coated with several different metals (silver, copper, chromium, and aluminum) all exhibit a higher silicon etch rate compared to samples coated with a photoresist mask. For silver-masked samples, the silicon etch rate can be enhanced by as much as a factor of five relative to a photoresist mask. These results can be explained in terms of a catalytic reaction occurring on the mask surface which gives rise to a local enhancement of the fluorine radical concentration leading to an increased etch rate of silicon. The validity of this explanation was confirmed by actinometry.We recently reported on the observation that etch rates of silicon and silicon dioxide in CF4/O2 plasmas are dependent upon the material used as an etch mask (1). It was shown that aluminum-masked silicon etched about twice as fast as silicon masked by photoresist. Conversely, aluminum-masked oxide etched about 10% more slowly than silicon dioxide that was masked by photoresist. We subsequently extended this work to other fluorine containing etch gases, and showed that a similar effect was observed in the silicon etch rate with a variety of fluorocarbon and nonfluorocarbon gases (2). We have demonstrated that this enhancement is not restricted to aluminum, but that other metal masks, such as silver and copper, enhance the silicon etch rate, some even more dramatically than aluminum (3, 4). Most recently, we have shown that this effect is not restricted to silicon and silicon dioxide, but is also observed with four other important substrates used in microelectronics fabrication; specifically plasma-deposited and thermally grown polysilicon and plasma and thermal silicon nitride (5). A possibly related effect, arising from metal residues on silicon surfaces, has also been reported to enhance the plasma etch rate in F2 plasmas (6).We have suggested that the reason for the increase in the silicon etch rate was a catalytic production of fluorine radicals on the metal surface which had the result of locally increasing the fluorine radical concentration and hence the silicon etch rate. We have previously presented evidence for metal masks increasing the fluorine radical concentration in fluorine-based plasmas (4).We now wish to report a detailed study on the use of chromium, copper, and silver etch masks, and their effect on the plasma etch rate of silicon. We show that the effect of mask dependent etch rates is not restricted to aluminum, but is observed to different degrees with other metal masks. We also show that the metals can increase the relative concentration of fluorine radicals in the plasma, and that this increase in fluorine radical concentration is responsible for the increase in plasma etch rates. ExperimentalFive different wafers were used for these experiments: silicon masked with photoresist, silicon masked with aluminum, silicon masked with chromium, silicon masked with copper, and silicon masked w...

Surface-science aspects of plasma-assisted etching

“…However, it is not clear whether this effect is caused by an increase in the oxygen dissociation rate (i.e., an increase in the electron density and/or energy distribution) or by a decrease in the wall-catalysed recombination of the O atoms [34]. It has also been noted that the nature of the mask material influences the etch rate in many cases [35,36]. Again, whether or not these effects are caused by changes in mask-induced recombination processes is not clear.…”

Interfacial Contact is Required for Metal‐Assisted Plasma Etching of Silicon

“…Interestingly, some masking metals have demonstrated a generalized enhancement of etching in plasmas. Mask‐enhanced etching of Si and SiO 2 using Al, Cr, Cu, and Ag masks is known in fluorine‐containing etch chemistries . The increased etch rates for Si with metal masking are attributed to increases in the concentration of fluorine radicals in the plasma surrounding the metal due to the catalytic production of radicals on the surface of the metal mask, driving the formation of SiF 4 products in the dry etching reaction.…”

“…Mask-enhanced etching of Si and SiO 2 using Al, Cr, Cu, and Ag masks is known in fluorine-containing etch chemistries. [ 21 – 24 ] The increased etch rates for Si with metal masking are attributed to increases in the concentration of fluorine radicals in the plasma surrounding the metal due to the catalytic production of radicals on the surface of the metal mask, driving the formation of SiF 4 products in the dry etching reaction. In one report, Au placed upstream of the substrate was shown to increase downstream Si etch rate by 3.6 times in a CF 4 /O 2 plasma, with the increased etch rate attributed to gas phase transport of Au oxides and fluorine radicals to the substrate.…”

Metal-Assisted Plasma Etching of Silicon: A Liquid-Free Alternative to MACE

“…While a significant body of literature has explored the process and controllability of MACE, there is a relative lack of previous studies investigating how metals can be used to enhance etching in plasmas. Mask-enhanced etching of Si and SiO2 using Al, Cr, Cu, and Ag masks is known in fluorine-containing etch chemistries [43][44][45][46] ; the increased etch rates for Si with metal masking is attributed to local increases in fluorine concentration due to the catalytic production of fluorine radicals on the surface of the metal mask. In one report, Au placed upstream of the substrate was shown to increase downstream Si etch rate by 3.6 times in CF4/O2 plasma, with the increased etch rate attributed to gas phase transport of Au oxides and fluorine radicals to the substrate 47 .…”