report
8.5.4 State of R&D

The chapter address nanotechnology-based manufacturing technologies with a primary application to create integrated circuits or other electronic devices. More general manufacturing methods, such as Molecular Beam Epitaxy (MBE), and Chemical Vapour Deposition (CVD) are covered in other ObservatoryNano reports.

1. Nanoimprint Lithography

This technology was first described by Stephen Chou et al. in 1995. Nanoimprint Lithography is conceptually a rather simple technology, relying on mechanical deformation of the media which is  patterned.

A number of techniques can be used to pattern features onto a stamp. This may include electron beam lithography; because one stamp is created which can then be used multiple times, the low speed of e-beam lithography is outweighed by it’s accuracy.

The stamp is then applied to a substrate. This may involve heating the substrate (thermal nanoimprint) so that the surface deforms when the stamp is applied. Another technique, UV nanoimprint, applies both UV-light and pressure to the stamp and substrate. This technique appears to meet high-volume throughput requirements.  It is also possible to directly pattern some materials, including aluminium and silicon.
The printing of the stamp onto the substrate creates a contrast thickness pattern. The part of the pattern which is compressed can then be removed using normal etching process.

Nanoimprint technology has a number of advantages. It is relatively inexpensive when compared to deep UV methods, which has meant that it is particularly suitable for experimental and low volume production. The technique has also been applied to create nanoscale devices other than electronic circuitry, such as sensors and micro-fluidic devices.

Nanoimprint lithography is an active field with a number of companies and research groups working in this space. However, nanoimprint is not yet considered to be a viable candidate to supplant UV lithography in semiconductor fabrication, due to concerns over the scalability of the technology – a combination of the time required to pattern stamps, the area that can be patterned by the stamps, and the durability of the stamp itself, given the mechanical nature of the manufacturing process.

2. Electron Beam Write Direct Lithography

Electron Beam Write Direct Lithography involves scanning an electron beam across a surface, exposing areas of this resist. This is a maskless technique, as the electron beams are directly applied to the surface. Industrial scale equipment for e-beam lithography is very costly, but it is possible to adapt at Scanning Electron Microscope (SEM) for e-beam lithography – this has made it possible for more research groups to use the technology.

The advantage of e-beam lithography is that the beam itself can be directed very precisely, leading to the creation of very exact feature sizes. This means that limitations of the resolution of this technique are caused not by the beam itself, but by its behaviour when it interacts with the photoresist. Electron scattering can reduce the pitch of feature sizes that is achievable, though this in turn can be reduced by using lower energy electron beams.

The serious limitation of e-beam lithography is that it is very slow, given that it is essentially drawing every feature onto a wafer. Throughput when patterning a whole 300mm wafer (the largest size currently in production) would be measured in years rather than minutes.

3. Scanning Beam Interference Lithography

An optical lithography technique, scanning beam interference lithography employs two lasers which interfere with each other.
In practice it is necessary to move the wafer under the scanning beam in order to pattern larger areas. This in turn creates a problem of Doppler shifts caused by the movement of the wafer relative to the lasers. A scanning beam process developed by Mark Schattenburg at MIT uses sound waves to accurately gauge the position of the wafer and to adjust the scanning beam accordingly.

4. Dip Pen Nanolithography

Dip Pen Nanolithography (DPN) was developed by Chad Mirkin at Northwestern University. The technique uses the tip of an atomic force microscope (AFM) to transfer a substance to a surface. The process is analogous to a pen being dipped in ink and then drawn across paper.

In order for the ‘ink’ to transfer from the AFM tip to the substance to be patterned, the whole process occurs in humid atmosphere. This causes moisture to gather on the end of the AFM tip, which in turn transfers the ‘ink’ to the substrate with a capillary action, leaving the tip itself clean.
This technique has been used to generate patterns with line widths of as little as 10-15nm. Despite its low speed, the precision of the technique has made it suitable for low volume and experimental use. Indeed, flexibility in the choice of ‘ink’ has also led to the use of DPN in biological applications.

5. Reactive Ion Etching

Reactive Ion Etching (RIE) can take place as a component of normal photolithography processes. It is an etching technique, which is used to remove excess material after a patterning process.

The process itself involves using an electromagnetic field to create a plasma, which delivers ions vertically, showering the surface of a wafer. Positive ions interact with the surface features, transferring kinetic energy which removes some of the material. The vertical delivery of ions leads to very sharp vertical (anisotropic) etching of features.

6. Scanning Probe Lithography

Scanning probe lithography really describes a number of techniques which employ an STM or AFM tips to pattern a substrate (though this is a distinct field from DPN). These include directly patterning the substrate by scratching, indenting or local heating.

The Quate group at Stanford is developing methods which use at STM to generate an intense, local electrical field. This could be used in field enhanced oxidation, in which the electrical field causes a material, such as silicon, to oxidise. With the same starting point, the SPM tip can be used as a field emitter, applying a stream of electrons to a thin resist, causing changes in the resist material.