The main difference between the Northwood core and the previous P4 core, Willamette, is that now Northwood is built with 0.13µ (micron) production technology and has 512 KB L2 Cache. Moreover the copper material enables the use of thinner wires compared to the aluminum used in Willamette. (To help you get a better idea of the size, say that, hair has 60µ diameter)