Plasma-Enhanced CVD from Silane and N2O

Silicon dioxide can be deposited using parallel-plate showerhead reactors. N2O is employed as an oxidant; pure oxygen is typically found to be too reactive and can produce lots of powder. Conditions are typically from about 0.2 Torr to a few Torr, temperatures of 200-400 °C, gas flows from a few sccm to a liter or two, generally with a high ratio of oxidant:silane.

The film properties are fairly similar to those obtained using thermal CVD from silane: conformality is generally poor to mediocre. However, the addition of the plasma enables rather facile control of film stoichiometry, all the way from amorphous silicon to silicon-rich oxides to nearly pure silicon dioxide, by varying the gas mixture and flows. Silicon-rich oxides can be used as moisture barriers, due to the ability of the Si-H bond to react with water molecules to form silanol and release molecular hydrogen. Silicon-rich oxides may also be useful as etch stop layers.

The stress of plasma-deposited films is also adjustable, particularly when a dual-frequency reactor is employed: increasing relative ion bombardment energy can be used to adjust stress to more compressive levels. Higher RF power or smaller electrode gaps can achieve similar effects (if with less versatility) in single-frequency reactor configurations.

Plasma-deposited silicon dioxide layers from silane are widely employed when conformality is not critical. They can be used as thin bottom layers in sandwiches with TEOS/ozone or spin-on glass layers for intermetal dielectric gap fill, or as part of a final passivation layer. In Damascene processes, metal lines are inlaid into trenches formed in the intermetal dielectric and then polished flat, so the IMD is always deposited on a flat smooth surface and conformality is not significant. IMD for Damascene processes can use PECVD silane oxide films. By adjusting the deposition conditions (making the gas silane-rich) one can deposit silicon-rich films; these can be employed as moisture barriers to prevent water from TEOS-ozone oxides or spin-on glasses from damaging underlying transistors. However, many modern IC processes have replaced silicon dioxide with lower dielectric constant materials in these applications.

PSG films deposited using plasma from silane and phosphine may fail to completely oxidize the phosphine, and incorporate it as a hydride into the film. This can lead to the formation of bubbles and voids in the film during subsequent annealing, if high phosphorus contents are used.

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