In nanotechnology patterning (using photolithography) and etching away material, as in building integrated circuits is called top-down approach. Top-down is very difficult for manufacturing at nanoscale level. Self-assembly of atoms and molecules, as in chemical and biological systems is the bottom-up approach. Either top-down is more common and bottom-up is generally done on thin film substrates like silicon wafers and usually together with some sort of top-down technique. Even when doing things at micron scale or larger (fabrication MEMS structures, doing thin film adhesion studies, etc.), vertical dimension is nanometer, and/or assumed that these can be applied or extended to lateral nanoscale regime in many cases.
Limitations of top-down fabrication:
• Due to diffraction effects, the practical limit for optical lithography is around 100 nm.
• Electron beams, of smaller wavelengths can be used to define smaller features and
feature sizes smaller than 20 nm can be patterned.
• But e-beam projection systems using masks have not been fully developed yet – instead,
“direct-write” e-beam lithography has been used.
• While optical lithography works in parallel over the wafer (with high throughput),
direct-write e-beam lithography works as a series process (with low throughput).
Limitations of bottom-up fabrication
• Getting the structures to grow exactly how and where it is wanted to be
• Making complicated patterns
• Fabricating robust structures
Some common strategies are:
• Use catalysts, stress fields, diffraction gratings to achieve selective growth in specific
locations
• Use top-down processes in conjunction with bottom-up processes, and build on silicon
substrates
Requirements for nanomaterials building
- Tools are needed to analyze, visualize, and manipulate very small features
- In addition to standard integrated circuit processing tools, others such as atomic force
microscopes (AFM) are utilized
- Requires very clean environment: “clean room”