UHV Basics

Modern surface analytical methods over the last three decades have been dominated by those requiring ultrahigh vacuum (UHV) chambers in which to carry out the analyses. This is not a universal requirement for surface analysis and several of the techniques such as Fourier transform infrared (FTIR), scanning tunnelling microscopy (STM) and ellipsometry do not have a mandatory vacuum requirement. Vacuum is of course required by those techniques utilizing beams of particles and higher energy radiation so that the beams may be generated and travel undisturbed until intercepting the surface. The requirement for UHV or vacua of ≦ 10-10mbar (10-8Pa) is fundamental to surface analysis when those beams are employed. This arises due to the flux of residual gas molecules striking the surface i.e. the number of molecules per unit area per unit time, which is responsible for the pressure that those gas molecules exert upon the surface. By knowing the pressure the flux can be evaluated. From the kinetic theory of gases [1] the molecular flux Zis given by the Herz-Knudsen equation:(2.1)% feaaguart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOwaiabg2 % da9maalaaabaGaamOtamaaBaaaleaacaWGJbaabeaaaOqaaiaaisda % caWGwbaaaiaacYcaaaa!3C21! !$$ Z = \frac{{{N_c}}}{{4V}},$$ where N/V is the number of molecules per unit volume and c is the average speed of the molecules.