In the US, completion of a minor can generally be certified on the transcript and the diploma, and in that case it is "certification". However, when a separate document is issued certifying completion of a minor, which seems to be what we're dealing with here, I'm not sure that document can be called a "certification" (though I'm not a native speaker of American English); I think it is a certificate.
This is from Penn State:
"7. The associate dean or designee will certify completion of minor requirements on each student record.
8. The Office of the University Registrar will prepare a minor certificate for each approved student. The certificate will include the signatures of the President of the Board of Trustees [etc.]. The minor certificate will be issued to the graduate along with the diploma [...].
9. The Registrar is responsible for recording on the student's academic record the successful completion of the minor program"
http://undergrad.psu.edu/aappm/L-6-minors-entrance-and-certi...