Measurement of vaporized atom flux and velocity in a vacuum using a microbalance

The flux and velocity of Cu and Ti vaporized by an electron beam were measured by a microbalance. A small disk was hung horizontally above the crucible and its weight change was measured by the microbalance. The flux was determined from the weight change due to vaporized atom deposition under the disk base. The total momentum of deposited atoms per unit time was determined from the weight change before and after the vapor was turned off by a shutter. The velocity could then be calculated from these two values. The velocity obtained for Cu depended slightly on the vaporization temperature and had reasonable agreement with the theoretical estimation obtained using an ideal gas treatment. The velocity of Ti was slightly higher than the theoretical result. Since Ti was excited to several metastable energy levels by electron beam heating, such an internal energy should be converted to kinetic energy following adiabatic expansion and would account for the velocity increase. The internal energy by excitation to the metastable energy levels must be taken into consideration in the case of high temperature heating, as with an electron beam evaporative source.