Nanofabrication on engineered silicon (100) surfaces using scanning probe microscopy

Abstract

Fabricating and measuring sub-5 nanometer features brings to light several pressing issues in future semiconductor industry manufacturing and dimensional metrology. This dissertation presents a feasible process to create nanostructures using scanning probes with applications in dimensional metrology and nanomanufacturing processes. Using the lattice spacing of a crystal as the fundamental "ruler" or scale, sub 5 nm critical dimension reference standards can be created with atomic scale dimensional control.

This technique relies on atomically sharp tips to provide robust imaging and patterning (nanolithography) capabilities. We have developed a comprehensive process to routinely produce high quality scanning tunneling microscope (STM) tips. The quality of STM tips are a critical factor in achieving reproducible patterning. A modified electrochemical etching method has been used to create sharp tips with preferred apex geometry. By using a field ion microscope (FIM), tip surfaces have been cleaned by field evaporation. Finally, a thermal ultra-high-vacuum (UHV) process is implemented to stabilize the atoms on the tip apex for improved performance. The process is also found to be capable of restructuring the apex to regain atomic resolution when tips fail during imaging or patterning.

Silicon (100) samples with pre-patterned micrometer-size fiducial marks are used as templates in this technique. The fiducial marks are used as 2D references to relocate the tip scanned area and the lithographic patterns. Large atomically-flat reconstructed (100) surfaces are obtained after a wet chemical cleaning process and a high temperature annealing process. After the high temperature annealing process, we observed reproducible step-terrace patterns formed on surfaces due to the fiducial marks. A kinetic Monte-Carlo simulation was used to study quantitatively the evolution of surface morphology under the influence of fiducial marks. Some of the key aspects, such as the electromigration effect and step permeability have been extensively studied.

Hydrogen-passivated silicon (100) reconstructed surfaces are used to create nanopatterns by selective depassivation lithography. Optimized depassivation procedures enable us to fabricate patterns from the microscale to the atomic scale consistently using an UHV STM. To preserve and later enhance the nanopatterns, SiO2 hard etch mask marks are formed by oxidizing the patterns using ambient humidity or gaseous oxygen. A reactive ion etching (RIE) process is used to further enhance the aspect ratio of oxidized nanopatterns so that they can be served as 3D nanostructures on silicon surfaces.