Open a new document. Make the size 3 by 3 and a transparent background. Get out your pencil tool and make the size one pixel large and the color black.

Now zoom in as far as you can go, and draw 3 blocks in a diagonal line.

Go to Edit >> Define Pattern. You're finished! Whenever you want to use it just make a new layer and go to edit >> fill and choose your pattern. Change the blend mode to overlay, soft light, or hard light.