Lithography, Photolithography, Electron Beam Lithography and X-Ray Lithography – Nano Fabrication Techniques
Research and development is being increasingly concentrated on using controlled chemical techniques to build materials and devices with new attributes and characteristics. This is what nature does, and scientists are trying to emulate nature by finding ways of making atoms and molecules self-assemble, in order to build not just new features such as organic thin films or coated nanoparticles, but large structures.
Today manufacturing methods are very crude at the molecular level. Casting, grinding, milling and even lithography move atoms in great quantities, the top-down approach, using tools to cut out or ‘carve’ tinier and tinier components from a larger whole. If we are to continue the miniaturization trends it is necessary to develop new ‘post-lithographic’ manufacturing technology, utilizing nanotechnology. While the semiconductor industry’s roadmap seriously considers 30nm dimensions to be pursued with extensions of present photon based lithographies and possibly electron or ion beam technology, approaching the 10nm regime will require novel approaches. The bottom-up method involves self- assembly processes and molecular fabrication, assembling a larger whole starting with very small building blocks such as atoms and molecules. The ‘bottom-up’ approach is seen as the path to future products and processes, combining physics, chemistry, information technology, novel engineering, metrology and characterization techniques and biomimetics.
The ultimate manufacturing solution may involve a combination of both approaches i.e., firstly to fabricate building blocks through directed self- assembling to generate supramolecules (material goes bottom-up), and then to assemble them into more complex nano system by smaller and smaller nanomanipulator (tool goes top-down).
Lithography is the key technology to realize very small feature size for nano components. Optical lithography, the main technology used today is predicted to be applicable beyond 100nm and 70nm with the use of respectively 193nm wavelength and 157nm wavelength tools. The reduction of feature sizes down to 50nm and below will require more advanced lithography tools. As the candidate for the next generation for the microelectronics industry, Extreme Ultraviolet Lithography is being strongly supported. EUV lithography, at the wavelength of 13nm, will achieve feature size at 45nm and below.
Photolithography is the selective process that allows the patterning of a desired design onto the material we want to fabricate with (the wafer in the semiconductor industry). Photo resist is applied as the first step in applying a pattern in a uniform film. The mask is a metal sheet that holds the actual pattern that will be etched into the photo resist. The mask is cut so that when a UV light is shined from behind the exposed parts of the photo resist will be the actual pattern. These exposed parts can then be cleaned away (positive resist) or will stay on to the fabricated device (negative resist). As a result of photolithography being the number one limiting factor on the size of wafer production this is the field where most of the research has gone. Contact printing was the very first form of photolithography. In this form the mask was placed directly on top of the photo resist during the exposure process. This process gave a good resolution but sometimes resulted in slight damage to the wafer and the mask. In order to defeat the problems the next innovation, projection printing, separated the mask from the photo resist.
Today, Electron-Beam Lithography (EBL) is employed to make the smallest components on silicon substrates and is the most effective method of creating patterns on substrates such as photo masks and x-ray masks. Electrons are used to directly etch onto the resist. Through a series of coils and lenses a computer to expose the correct sections of the photo resist controls the electrons path. Effort for advanced e-beam lithography is focused on the elaboration of a matrix of a micro fabricated e-gun for e-beam masker. The objective is to parallelize electron beam lithography.
This uses the same procedure as above except that instead of using UV an X-Ray source is utilized.
Primary author: Institute of Nanotechnology
Source: Introduction To Nanotechnology CD ROM
For more information on this source please visit Institute of Nanotechnology
Uploaded March 2004