On a very small surface, a chip, several thousands of oligonucleotides, can be synthesized using a mask technology. Given a set of oligos, the problems tackled here are: What is the minimum number of masks necessary to synthesize one copy of each oligo? How long will be the series of masks if each oligo is synthesized twice, such that the two copies are realized with two completely different series of masks? We establish that, for 20,000 oligos of 20 bases a single copy can be synthesized with around 67 masks and for two copies with less than the double.