Duplicate movie and depth

1

I’m not too sure how to add depth to this code. I want to place the duplicate MC’s in the background of the movie. Anyone able to help with this?

Here is the code:

onClipEvent (enterFrame) {
duplicateMovieClip ("/box", “bn” add n, n);
_root[“bn”+n]._alpha = (2*(n));
_root[“bn”+n]._x = 350;
y = 1;
y = y+1;
_root[“bn”+n]._y = ((300)+(7*(n)/(50/(n))));
n = n+1;
if (Number(n)>28) {
n = n-1;
_root["/bn"+n]. _visible = 0;
}
}