The high resolution of some laser irradiation has important applications in micro machining. In drilling micro holes with a diameter close to or less than 1 micrometer, beam diffraction essentially restricts the final drilling depth due to the finite wavelength of all laser irradiation. Besides diffraction, interference can also affects the drilling results. It is of practical interest for process characterization and evaluation to know these substantial restrictions before application. This investigation tries to determine the correlation between the beam properties and the aspect ratio in drilling micro holes. First, the desirable qualities of laser irradiation will be defined in the situation of drilling micro holes. Given the attributes of electromagnetic wave, the diffraction in drilling is then analyzed. The attention is focused on the situation in which the laser wavelength approaches or exceeds the hole diameter. Fraunhofer diffraction is therefore the dominant pattern that determines the wave intensity inside the free space of micro hole. In accordance with a steady-state thermal equilibrium that terminates any further erosion by laser irradiation, the solution of heat conduction equation determines the final erosion front and the maximum depth. Finally, the minimum required power density of a femtosecond laser is estimated by solving a transient heat conduction equation. The effects of ultra-short pulses on diffraction and hole quality are also analyzed.