It is not possible for the Super Bowl to end in a tie. The NFL has long had what is known as a "sudden death" overtime period to determine the winner of a postseason game, which includes the Super Bowl.
If the score is tied at the end of the Super Bowl's regulation playing time, the referee does a coin toss at the center of the field to determine which team is receiving and which team is defending. There is a three-minute intermission. Play continues in 15-minute periods until one of the two teams scores, either by a safety, a field goal or a touchdown.