Difference between revisions of "Chemical processing"

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==Further reading==
 
==Further reading==
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*[http://lnf-wiki.eecs.umich.edu/wiki/User_Resources#LNF_Tech_Talks_.28technology_seminar_series.29 LNF Tech Talk for Chemical processing is Coming Soon!]
 
*[https://docs.google.com/document/d/1OwW_EG5_kB060kTvhDwRPZoYymRMo1UDE-d6ejSxgII/preview LNF Training Manual] pages 23 - 30
 
*[https://docs.google.com/document/d/1OwW_EG5_kB060kTvhDwRPZoYymRMo1UDE-d6ejSxgII/preview LNF Training Manual] pages 23 - 30
  

Revision as of 08:57, 31 March 2020

Chemical processing is widely used in device fabrication, for cleaning, etching, electroplating, patterning or other types of processing. While a certain amount of chemistry is involved in all aspects of processing, this page does not address solid phase, gas phase, and plasma chemistry. All of the processes described here refer to wet chemical processes.

If you have questions as to whether a wet chemical process is available or compatible with your materials, please create a LNF Helpdesk Ticket. If you are interested in using a wet bench in the cleanroom, you can sign up for a General Cleanroom Wet Bench Training session.

Technologies

Wet chemical processing is used for a variety of purposes, from chemical removal of material (Wet etching) to deposition of material (Electroplating), to sample cleaning, to the creation of patterns on the surface using Optical lithography techniques. Many chemicals and materials can be handled in the LNF. To minimize the danger of incompatible materials mixing or cross-contaminating each other, different wet benches are designated for handling of different materials (see Segregation).

Overview of chemical capabilities

Describe the technology, particularly including why you might use it over another within the same group.

Equipment

Most of the wet chemical processing is done in dedicated wet benches. These are categorized by the type of etchant used in the benches (Acid, Base, and Solvent) and are further broken down to allow certain materials into the bench. The following benches are available at the LNF:

The following benches are available in the ROBIN lab:

Cleaning

Main article: Cleaning

As processing gets down into the micro- and nanoscale, microscopic contamination can affect the performance of the devices. Cleaning techniques are needed to remove unwanted contamination including particles from the air, processing-related particles, and organic residues. Cleaning is extremely important to obtain consistent and reproducible results.

Wet etching

Main article: Wet_etching

Wet etching is the removal of material from the surface using chemicals to dissolve the material. When there is a pattern (usually generated through lithography) it is called an etch; if a complete layer of material is being removed, it is often referred to as stripping, or a blanket etch. It is important to determine the selectivity [1] of the etchant to other materials exposed to the etchant.

The following table describes situations for which wet etching is likely to be suitable or not:

Recommended Not Recommended
  • Thin layers (<1µm thick)
  • Materials that can be easily or selectively etched
  • Larger features (>3um)
  • Materials that are damaged by heating or exposure to plasma
  • Thick layers (>1µm thick)
  • Smaller features (<3µm)
  • Straight sidewalls needed (see Isotropic)
  • Materials on sample that will be attacked by etchant

Wet etching is a subset of Etching which includes both wet and dry etching; the advantages and disadvantages of wet vs. dry etching are described in that article.

Lift-off

Main article: Lift-off

Lift-off is a method of patterning thin film material. In lift-off, a sacrificial layer is first patterned, and after deposition of the next material, the sacrificial layer is removed, thereby "lifting off" regions of the deposited material that were on top of the sacrificial layer. The most common sacrificial material is photoresist, and the most common deposited materials are evaporated metals.

Lift-off is often suitable/not suitable in the following situations:

Recommended Not Recommended
  • Thin materials (<1µm)
  • Materials that can't be easily or selectively wet etched
  • Materials that can't be dry etched
  • Materials with good adhesion
  • Fine features
  • Evaporated materials
  • Thick materials (>1.5µm)
  • High temperature deposited materials (>100C)
  • Poor adhesion materials
  • Large areas with no pattern
  • More difficult with sputtered materials (See lift-off)

Electroplating

Main article: Electroplating

Electroplating is often used to deposit thick layers onto a metal surface. By applying a bias across a solution with a metal cation in it, the metal ion will reduce at the negative electrode, forming solid metal. When the sample is patterned (Lithography) so that only a small area is exposed, all the current will flow through the exposed area. Since the plating will only occur where the current is flowing, a patterned layer of metal will result.

The following table lists conditions when electroplating may be useful:

Recommended Not Recommended
  • Thick materials (>2µm)
  • Precious metals
  • If non-oxidizing metal seed layer can be used
  • Limited materials available (at LNF Au, Cu, In, Ni, F-Ni)
  • Conductive metals (?)

Figures of Merit

When determining how to process samples and where to process them, a few factors must be considered: compatibility of chemicals with all of the materials in the sample; potential for materials cross-contamination; and if aqueous solutions will cause stiction of devices.

Compatibility

The primary concern when determining how to process a sample using wet chemistry is compatibility with all of the materials on the surface.

Materials segregation

The LNF supports research on many different devices across a wide variety of materials systems. To prevent materials cross-contamination, the LNF restricts certain materials from certain shared labware and wet benches, and maintains redundant capabilities based upon substrate size, substrate material, and materials on the surface.

For safety reasons, use of chemicals is segregated by chemical compatibility, keeping acids in acid benches, solvents in solvent benches, bases in base benches, and other specialized chemical processes in their specific area.

Stiction

Stiction is a term that is used to describe the effect that happens when you have a device structure with two large surface areas separated by a small gap. This is often a concern for structures in MEMS devices. In this situation, if a wet etch/release is done in an aqueous solution, the force that is generated while the water is drying will pull the two surfaces together, and van der Waals forces will keep them stuck together.

Applications

Wet chemical processing is used in a wide range of micro and nanotechnology processing from the deposition and developing of the Lithography materials to the selective etching of underlying materials.


See also

References

Further reading