Editorial Feature

Nanofabrication Techniques - Variations of Lithography

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Today, research and development are increasingly focused on utilizing controlled chemical methods to develop devices and materials with new properties and attributes.

Nature does the same, and researchers are attempting to mimic nature by looking for ways to make molecules and atoms to assemble on their own. This is to build not just novel features, like coated nanoparticles or organic thin films, but also massive structures.

Bottom-Up Manufacturing Techniques

Manufacturing techniques that are used today are highly unsophisticated at the molecular level. Milling, grinding, casting, and even lithography shift atoms in great proportions; by contrast, the top-down approach involves utilizing tools to “carve” or cut out increasingly smaller components from a larger whole. If these miniaturization trends have to be continued, it becomes important to develop innovative “post-lithographic” manufacturing technology using nanotechnology.

While the roadmap of the semiconductor sector seriously contemplates that 30-nm dimensions should be followed with extensions of current photon-based lithographies and possibly ion or electron beam technology, reaching the 10-nm regime will need new methods.

The bottom-up technique involves molecular fabrication and self-assembly processes—assembling a larger whole beginning with very minute building blocks like molecules and atoms. This “bottom-up” method is regarded as the path to upcoming processes and products, integrating chemistry, physics, biomimetics, novel engineering, information technology, and metrology and characterization methods.

The Ultimate Manufacturing Solution

A combination of both methods is involved in the ultimate manufacturing solution—that is, firstly to create building blocks via directed self-assembly to produce supramolecules (material goes bottom-up), and subsequently to arrange them into a more intricate nanosystem by increasingly smaller nanomanipulator (tool goes top-down).

Lithography - The Key Application

The main technology to realize a very tiny feature size for nano-components is lithography. Optical lithography is the key technology to be utilized today and it is expected to be relevant beyond 70 and 100 nm with the use of 157-nm wavelength and 193-nm wavelength tools, respectively.

The reduction of feature sizes as low as 50 nm and below will need more sophisticated lithography tools. Extreme ultraviolet (EUV) lithography is the candidate for the futuristic microelectronics sector, and it is now robustly supported. At the 13-nm wavelength, EUV lithography will attain feature size at 45 nm and below.

Photolithography Applications and Properties

Photolithography can be defined as a selective process that is used for patterning a required design onto the material that is used to fabricate with (the wafer in the semiconductor sector). As the first step, a photoresist is applied while applying a pattern in an even film. The mask is a metal sheet holding the real pattern that will be later etched into the photoresist.

This mask is subsequently cut so that the exposed parts of the photoresist become the real pattern, upon illuminating a UV light from behind. These exposed parts will continue to remain on to the fabricated device (negative resist) or can be subsequently cleaned away (positive resist).

With photolithography being the leading constraining factor on the size of wafer fabrication, this is the domain where most of the studies have been devoted. The very first form of photolithography is contact printing. In this form, the mask was directly positioned on top of the photoresist at the time of the exposure process. While this process provided an excellent resolution, at times, it also caused slight damage to the mask and the wafer.

The next breakthrough, projection printing, was able to separate the mask from the photoresist, and overcame the problems.

Electron Beam (e-beam) Lithography

Currently, electron-beam lithography, or EBL, is used for making the tiniest parts on silicon substrates and is the most effective technique for producing patterns on substrates, like X-ray masks and photomasks. Electrons are utilized to directly etch onto the photoresist. Through a range of lenses and coils, the electrons’ path is controlled by a computer to expose the right sections of the photoresist.

Efforts for next-generation e-beam lithography are targeted on elaborating a matrix of a micro-fabricated e-gun for e-beam masker. The main aim is to parallelize the electron beam lithography.

X-Ray Lithography

X-ray lithography employs the same process as above, except that an X-ray source is used instead of UV.

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