Patterning processes for nanofabrication

The creation of patterns within layers of material is in many cases an essential step for the nanofabrication of useful structures. One common method of pattern creation involves coating substrates with thin films of material which can be chemically altered by selectively exposing some regions of the film to light or to a beam of electrons. The exposure-sensitive material is known as a photoresist, and often contains one or more polymers as part of its composition. This type of pattern transfer process is generally referred to as lithography.

After exposure, the substrate is treated with a chemical developer. Depending on the chemistries involved, either the exposed or the unexposed regions or the photoresist film are dissolved away by the developer, resulting in the formation of a useful pattern within the remaining polymer film. The patterned film can be used for further definition of the desired structures, for example by serving as an etch mask for a subractive process, a template for an additive process, or in some cases serving as the actual structures.

The three main methods used to expose the photoresist are outlined below. They are based on either exposure by an electron beam, using a digital pattern file to direct the beam and write structures into the photoresist, or on exposure to UV light, using a previously prepared photomask to selectively expose only certain regions of the photoresist. The latter process is known as optical lithography; it is further classified by whether the mask is in contact with the substrate, or whether the image of the mask is projected onto the substrate.

In addition to optical and e-beam patterning tools, WCNT provides all support equipment needed to apply, develop, and remove photoresist.

Contact Lithography (Optical Lithography)

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What contact lithography does

Optical lithography is used to transfer a pattern from a previously prepared photomask into a photoresit, to form structures which can be used as the basis for further processing. In contact lithography, the mask and substrate are in contact during exposure.

How it works

The photomask and the photoresist-coated substrate are brought into contact to minimize diffraction effects. A controlled amount of UV light energy is applied to the mask. This produces chemical alterations in the photoresist, selectively in regions of the mask that allow UV light to pass through, resulting in pattern transfer from the mask to the substrate.

Instrumentation Available

Strengths

  • Fast exposure time: Since the process is parallel, exposing all parts of the mask at once, all structures are transferred at the same time; in e-beam lithography, pattern transfer is a serial process
  • Reproducibility: The mask does not change between exposures, so many identical exposed samples can be prepared from a single mask
  • Relative ease of use
  • Good resolution: Feature sizes down to one micron or less can be fabricated
  • Suitable for large structures as well as for structures of mixed sizes
  • Contact lithography tools have mostly manual operation, allowing researchers detailed control over the process

Limitations

  • Lack of flexibility: Once the photomask has been designed and manufactured, it cannot be changed
  • Limits on high resolution: nanometer-scale structures cannot be produced by optical lithography tools available to researchers
  • Manual tool operation means that throughput can be low

Electron Beam Lithography

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What it is used to do

Electron beam lithography (EBL) is used to write nanometer scale patterns onto materials.  EBL is capable of achieving the smallest line dimensions (~6nm) available at the NFC.

How it works

A highly focused electron beam is used to expose a photoresist, causing a chemical alteration in the exposed regions. After exposure, the substrate is immersed in a liquid developer. Depending on the tone of the photoresist, either the exposed or the unexposed regions are removed by the developer, resulting in a patterned substrate that is available for further processing.

Instrumentation Available

Strengths

  • Capable of extremely high resolution, on the order of nanometers
  • Does not require a photomask, so it is possible to modify the pattern between processing interations

Limitations

  • Serial patterning process (as opposed to optical lithography, which is a parallel process), so patterning is slow
  • A considerable learning curve is involved: Since there is no fixed photomask, the number exposure of options researchers must intelligently modify is much larger than for optical lithography
  • Since this is a serial process, throughput will be low

Additional Reading

Projection Lithography (Optical lithography)

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What it is used to do

Optical lithography is used to transfer a pattern from a previously prepared photomask into a photoresit, to form structures which can be used as the basis for further processing. In projection lithography, the mask and substrate are not in contact during exposure.

How it works

The photomask and the photoresist-coated substrate are mounted in the tool. A controlled amount of UV light energy is applied to the mask. This produces chemical alterations in the photoresist, selectively in regions of the mask that allow UV light to pass through, resulting in pattern transfer from the mask to the substrate.

Instrumentation Available

Strengths

  • Because this is a projection process, lenses can be used to reduce the image of the mask before it reaches the substrate; therefore, the mask can be manufactured with larger features, making it perhaps easier to create
  • Also due to the reduction process, projection lithography may produce smaller features that contact lithography, under certain circumstances
  • Handling of mask and wafer is automated, so throughput can be high

Limitations

Additional Reading

Lithography Support

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What it is used to do

These are methods used to support the patterning process.  This includes resist spinners to put down the photoresist, solvent benches to remove exposed resist, develop benches for developing the exposed resist, and a wet bench for cleaning the photomasks.

How it works

Instrumentation Available

Key Attributes

Strengths

Limitations

Applications

Additional Reading