Please cite fields including its version and DOI as

  Douglas Nychka, Reinhard Furrer, John Paige, Stephan Sain (2021).
  “fields: Tools for spatial data.” R package version 16.3,
  <https://github.com/dnychka/fieldsRPackage>.

A BibTeX entry for LaTeX users is

  @Misc{,
    title = {fields: Tools for spatial data},
    author = {{Douglas Nychka} and {Reinhard Furrer} and {John Paige} and {Stephan Sain}},
    note = {R package version 16.3},
    organization = {University Corporation for Atmospheric Research},
    address = {Boulder, CO, USA},
    year = {2021},
    url = {https://github.com/dnychka/fieldsRPackage},
  }