We have developed a new method for writing of biomolecular patterns on silicon in sub-micron scales using the interference of light. In the first instance DNA patterned surfaces were fabricated. Covalent attachment of DNA oligonucleotides accessible towards hybridisation with a complementary sequence was achieved with very high density (one oligonucleotide per 16 nmò). Further development of the surface attachment chemistry has provided DNA modified patterned substrates for hybridisation with long DNA fragments.