Measurement of the forces of unwinding of DNA double helix was conducted. Silicon wafers were chosen as substrates. It is demonstrated that the use of gold for surface modification causes the bimodal probability distribution of rupture forces because of the surface roughness. Surface modification with the help of Langmuir-Blodgett procedure is proposed as an alternative. The use of this method gives unimodal distribution of rupture forces. The forces of double helix unwinding are reported for oligonucleotides containing 20 base pairs depending on salt concentration. Furthermore, forces for the shear mode and unzipping mode are measured.