micromillimetre vs millimicron

micromillimetre and millimicron both are nouns.

Word	Noun	Adjective	Verb	Adverb
micromillimetre	Yes	No	No	No
millimicron	Yes	No	No	No

As nouns, micromillimetre and millimicron are synonyms defined as:

micromillimetre and millimicron: a metric unit of length equal to one billionth of a meter

Other synonyms of micromillimetre include micromillimeter, nanometer, nanometre, nm.

micromillimetre (noun)	millimicron (noun)
a metric unit of length equal to one billionth of a meter	a metric unit of length equal to one billionth of a meter