Form creation sometimes stays behind main form

Person,

I'm creating my child forms as follows.

BuscaUsuario _chld = new BuscaUsuario();
_chld.ShowDialog();
_chld.Dispose();

It turns out that if the user presses alt + tab or for example if I'm debugging and goes to IDE and then I go back to the application the form is always behind the parent form.

Is there a way that in creating the form forces him to always stay on top?