There ISN'T anything called a "backhand serve" in tennis.
It is called a service ace!
If the player is unable to hit the serve then it is an ace. But if the player hits the ball but does not return it, then it is a forced error.
Because you hit the ball with some kind of racket and that is why you call it a serve.
The legal serve where the opponent fails to touch the tennis ball is called 'ACE'.