Once you complete the ACD, assuming you have not been previously convicted of a crime, the record will be sealed except for the purpose of determining if you qualify for a subsequent ACD, and the arrest and prosecution will be deemed a nullity. See CPL Sec. 170.56. While nobody can promise that a background check won't turn up a sealed case, it's unlikely. Prior to the order to seal, however, the arrest and charge may show up in a background check.