micromillimeter vs micromillimetre

micromillimeter and micromillimetre both are nouns.

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

As nouns, micromillimeter and micromillimetre are synonyms defined as:

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

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

micromillimeter (noun)	micromillimetre (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