Reversal of the spontaneous polarization direction under an applied electric field is a basic property of ferroelectrics. However the traditional techniques used for fabrication of domain gratings have been able to produce domains not smaller then 2 micrometers. Sub-micron and nanometer scale domains may be fabricated using atomic force microscopy based techniques; however, to date there was no success in fabricating stable domains that elongate without widening throughout thick ferroelectrics. A breakthrough in the field emerged with the recent development of the high voltage atomic force microscope that enabled to obtain sub-micrometer stable domain configurations in bulk ferroelectrics. Diverse stable domain configurations were fabricated in several ferroelectric crystals like LiNbO<sub>3</sub> and RbTiOPO<sub>4</sub>. Studying the influence of the applied high voltage, and the tip velocity on the domain strips has allowed fabricating domain gratings (with a domain width of 590 micron) useful for backward propagating quasi-phase-matched frequency conversion. It is found that string-like domains are formed due to the super-high electric field of the high voltage atomic force microscope tip. The domains, which resemble channels of an electrical breakdown, nucleate under an electric field of around 10 in a power of seven Volts per centimeter at the ferroelectric surface, and grow throughout the crystal bulk where the external electric field is practically zero. A theory explaining the shape of the formed domains shows that the driving force for the domain breakdown is the decrease of the total free energy of the system with increasing domain length.