Due to the simple planar topography of the synthesis surface, the coupling reaction is fast, about 15 s for DNA, and the low scale of photolithographic synthesis, circa 20 pmol (10 pmol/cm 2 ), versus 25 nmol to 15 μmol for standard column-based synthesis, means that the phosphoramidite is not significantly depleted during the coupling reaction and the DNA phosphoramidites can be used at a low concentration of 30 mM . Following the coupling reaction, there is a drying step in which the synthesis surface is dried for 10 to 20 seconds using a stream of helium. This step has been found to be necessary, but so far, we have not been able to determine a reason for its utility . Oxidation is the same as in column-based synthesis; the phosphite triester (P(III)) formed in the coupling step is unstable to strongly acidic or basic conditions and must be converted to the stable natural phosphate triester (P(V)) form. In conventional synthesis, this step must always be performed in each cycle prior to the acidic deblocking step (typically di-or trichloroacetic acid in dichloromethane).