How to ignore background ontriggerenter2d

I have a background sprite that has a boxcollider2d attach to it. When the game starts, it will spawn a circle with circle collider2d attach on the background. If the circle is overlapping the background, what can I do to ignore the background collision if I am touching the circle? I am not using Raycast2d, just simple ontriggerenter2d because I am doing something like slashing.

 

Using layers, you can make certain collisions and triggers ignore each other.

Look in Edit -> ProjectSettings -> Physics2D ... there's a big grid of layers and who interacts with who.

 

But what if I want to detect touch on background if there is no circle exist. Simply speaking its a little bit like which part will be collided first..