The time multiplexed deep silicon etch (TMDSE) process is the etch process of choice to make MEMS devices and through wafer vias. It has been used to produce deep trenches and vias at reasonable throughputs. Significant issues remain for the TMDSE process as well as room for improvement even though it has been both experimentally studied and modeled by a wide variety of researchers. This is because it is a highly complex process. Aspect ratio dependencies, selectivity, and the ability to use photoresist masks (instead of SiO2) are examples of remaining issues. The presently obtainable etch rates do not indicate efficient use of the etchant species. In this article, the authors focus on the deposition step in the TMDSE process. While prior research has generally assumed that the deposition step can be adequately modeled as being controlled by a reactive sticking coefficient, they have experimentally examined the deposition step of the process and found that the film growth is dominantly ion-enhanced. The results shown here were obtained in C4F8 plasmas but are also consistent with results found in CHF3 and C4F6 plasmas. As a result, the deposited film thickness can be larger at the bottom of a high aspect ratio feature than at the top sidewall, which is exactly the opposite of the desired profile. The very nature of the deposition mechanism leads to mask undercut at the same time as feature closing/etch stop.