The term nanolithography is derived from the Greek words “nanos”, meaning Dwarf; “lithos”, meaning rock; and “grapho” meaning to write. Therefore the literal translation is "tiny writing on rocks". Nanolithography is the science of etching, writing or printing to modify a material surface with structures under 100nm.
A typical commercial use of nanolithography is in the manufacture of semiconductor chips for computers.
Several lithographic techniques are used for patterning in the nanoscale region. These techniques include:
• Electron beam lithography (EBL)
• X-ray lithography
• Extreme ultraviolet lithography (EUVL)
• Light coupling nanolithography (LCM)
• Scanning probe microscope lithography (SPM)
• Nanoimprint lithography
• Dip-Pen nanolithography
Photolithography techniques as applied to nanolithography are very similar to conventional lithography for the production of images and printing. Photolithography is commonly used to produce computer chips. The silicon chip substrate is coated with a chemical known as a photoresist. By using a mask to selectively expose the resist to light, the exposed areas harden. The soft areas are then chemically etched away and the process repeated to build up a series of layers on the chip. This process allows hundreds of chips to be simultaneously built on a single silicon wafer. Photolithography can be used to produce features as small as 50nm.
Electron Beam Lithography (EBL)
Electron Beam Lithography (EBL) uses a tightly focussed beam of electrons scanned over the surface of a substrate to etch a pattern of nano sized features. EBL can produce features as small as 20nm but is very expensive and time consuming. For example a lithographic process that would take 5 minutes using photolithography would take approximately 5 hours using EBL.
The X-ray lithography process is almost identical to photolithography and extreme ultraviolet lithography but uses a mask is made from an X-ray transparent material with a pattern of high Z material either etched or deposited on it. Exposure of the substrate resist is done using a parallel beam of X-rays. The resolution of the etched pattern depends upon the mask features and exposure control.
Extreme Ultraviolet Lithography (EUVL)
Extreme Ultraviolet Lithography (EUVL) is very similar to standard photolithography but uses intense beams of ultraviolet light reflected from a circuit design pattern to burn the pattern into a silicon wafer. Unlike photolithography that uses a system of lenses to focus light, in EUVL extreme ultraviolet light is reflected through a series of mirrors. Building mirrors with sufficient precision is the current limiting factor in EUVL but once this problem is overcome resolutions below 30nm are expected.
Light Coupling Nanolithography (LCM)
Light coupling nanolithography (LCM) is another variation on photolithography. In LCM a polymer mask is placed in contact with the photoresist. Transparent regions protrude through the topographically patterned mask where exposure is required. The contrast of masked areas can be further increased with the application of a thin gold layer.
Scanning Probe Microscope Lithography (SPM)
Scanning Probe Microscope Lithography (SPM) uses the electric field at the tip of a scanning probe microscope to oxidise material in a specific pattern. The oxidised material can then be removed by preferential etching.
Nanoimprint lithography is a simple process that uses a mould to emboss the resist with the required pattern. After embossing the resist, compressed resist material is removed using anisotropic etching and the substrate exposed. Nanoimprint lithography can give resolutions lower than 10nm with high throughput and low cost. The current barrier to production at these resolutions is the development of the mould itself. Nanomanufacturing using self assembly may hold the solution to this hurdle.
Dip-Pen Nanolithography (DPN) is a direct write lithographic technique that uses an atomic force microscope (AFM) to build a pattern on the substrate material rather than etching it away. In the same way that an old fashioned dip pen picks up ink from an ink well and is then used to write on paper, molecules are picked up from a reservoir on the end of the AFM tip and deposited to the surface of the substrate via a solvent or water. DPN can be used to create patterns with resolutions down to 40nm.