Selective Material Removal for Nanostructure Formation

As the industry continues to fabricate structures with smaller dimensions — referred to as nanostructures — conventional wet etching can cause problems. The removal of silicon oxide to form freestanding nanostructures presents several challenges to process integration, including residue formation and stiction. Structure deformation and stiction are caused by the surface tension of liquids as they are removed from the surface.

There are many methods being used to counteract these problems, such as the deposition of self-assembled monolayers (SAMs), the use of release agents, and the use of surfactants or a final dry in supercritical CO₂ (SCCO₂). These methods use liquid chemicals and water to condition the surfaces and make them less likely to bond. Another approach is the use of gaseous anhydrous HF (AHF), which removes the sacrificial oxide layer in a gaseous process and avoids the liquid phase altogether.

FSI International Inc. (Chaska, Minn.) optimized the AHF process for nano-structure formation and presented its findings at this year's SEMICON Europa . The experiments showed that process conditions with AHF could be adjusted to achieve highly selective removal of doped oxide films relative to undoped oxide films. The process also removed undoped oxide films without generating the liquid residues that cause stiction and collapse of polysilicon structures. The company found that aluminum and copper formed a fluorinated surface layer and required protection during AHF processing, yet other metals such as gold, platinum and chromium were not affected by exposure to AHF.

One challenge in forming large planar structures was that a large lateral under-etch was required. This can cause the etching rate to slow or a liquid layer to form. To successfully remove the undoped oxide layer and avoid stiction, the AHF and water vapor process was optimized to provide a sustainable etch rate. The process also was stopped periodically in order to purge water vapor from the system and restart the etching reaction.

Shown is an example of a polysilicon beam structure that was formed by removing a sacrificial oxide layer in the AHF and water vapor process. (Source: FSI)

In the formation of high-aspect-ratio cylindrical capacitors, lateral etching under structures was not required. However, a local loading effect was found under some conditions where the undoped oxide etches faster in dense arrays of capacitors than it does in open field areas. This loading effect was evident in a low-pressure (100 Torr) AHF process, but was not seen at a near-atmospheric (800 Torr) AHF process.

Another issue with cylindrical capacitor formation is selectivity to the underlying nitride layer. Silicon nitride tends to form an oxidized surface layer that etches in AHF. The etching of the "oxynitride" layer can leave a residue with low volatility depending on process conditions. Heating the wafer to 150°C for 120 sec easily removes this residue.

While the AHF process has ultimate selectivity between oxide and silicon for structure formation, there are often other materials exposed on the wafer surface that must be preserved for the final device to operate properly. Metal films that are compatible with the AHF process include gold, platinum, chromium and nickel. Metals that are not compatible with AHF include titanium, aluminum and copper.

One way to avoid problems with exposed aluminum or copper is to protect those surfaces with a material that can be removed by a subsequent gaseous ashing process. Organic films like photoresist or polyimide are not suitable because AHF quickly diffuses through them and then reacts at the interface.

With proper optimization, reliable AHF processes are achievable for nanostructures. The AHF process is also an option that avoids the use of liquids during the formation of nanostructures.

For additional information on clean processing, go to www.semiconductor.net/clean