Loading Effects and Aspect Ratio Dependent Etching

Loading effects in plasma etching can be classified into macroscopic loading, microscopic loading and etch rate dependent etching, ARDE (slide 1).

Macroscopic loading causes an Etch rate decreases when the total area of the material to be etched decreases. The effect is caused by the consumption of reactive species during the etch process.

In addition, identical features are etched with different rates depending on their position with respect to open area features (dense areas, semi dense areas, open areas). This effect is called microloading.

ARDE is an effect where features with a high aspect ration (depth/width) have a higher etch rate then those with a small aspect ratio. Under certain conditions this effect can be reversed and is then called reverse ARDE.

Macroscopic loading (slide 2) is most common for isotropic etches with spontaneous etch mechanism, for instance removal of poly-Si with SF6 based plasmas. Certain silicon gate overetch processes also show significant macroloading. Macroloading is related to a change in plasma composition when wafers with different open areas are being etched, i.e. a change in the loading of the plasma with reactants and reaction products. Simply increasing the concentration of reactive species by increasing the source power is not practical because of the simultaneous production of reaction products. Possible solutions are:
- Very high pump speeds (overall very low concentration of reaction products)
- Dilution with inert gases (He, Ar)
- Change of the process properties such that the limiting step is not the reactant supply for instance by adding passivating gases.
- Find a way to favor reactive species consumption or recombination on the chamber walls
- Plasma pulsing to allow reaction products to be removed.

Microloading is caused by a localized depletion of the reactive species or accumulation of reaction by-products as a result of the local pattern density on the wafer (slide 3). Processes with strong macroloading typically also show strong microloading unless the mean free path of the species is much larger than the wafer diameter (which is not the case for normal operating pressures between 1 and 100 mTorr).

ARDE is caused by the effect of the wafer topography on the etch rate of a certain feature. Possible mechanisms have been described on a review paper by Gottscho at el. (JVST 10 (1992) 2133). These mechanisms include Knudsen transport of neutrals, ion shadowing, neutral shadowing, differential charging of insulating microstructures, charging of a polymer sidewall and the interaction of etch and deposition (slide 4).

ARDE has been shown to be dependent on the composition of the oxide for a SAC (selfaligned contact) etch process (slide 5).

SAC etch is a very polymerizing process and the question arises as to how ARDE depends on the interaction between deposition and etch (slide 6). From the experimental data it is clear, that polymerizing chemistries show a different ARDE than pure etching chemistries. In certain cases, heavily polymerizing processes show reverse ARDE (SAC and Al interconnect etch, for instance). To understand the effect of the polymer formation on ARDE, one has to consider the relative importance of etch and deposition and their transport to the etch front within the high aspect ratio microstructure.

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