Thats a custom patern, although its really easy to make. So what i did was first made the bg, then i made a new layer and did Ctrl + A to select the canvas. Then goto Edit > Fill, then in the drop down menu next to Use: select Pattern, then just select your custom patern and it adds it.
For that patern all i did was make a new image, transperent BG, 3x3 pix and zoomed in all the way. Then i took the pencil tool and put a 1x1 pencil mark in the top left, middle and bottom right and thats it. Then goto Edit > Define Patterm and your done. You can mix it up to make patterns goin | \ / and - or X. You could do anything just remember it repeats so if you put a pixel on the edge it will repeate and the line will be 2 pixles wide.