Electron beam evaporation of aluminium with a porous tantalum rod in melt pool

Abstract

For most metals, evaporation owing to electron beam heating proceeds in an efficient manner at temperatures substantially higher than the melting point. This is particularly true for aluminium, which has a large separation of melting and boiling point (>1000 K). This leads to situations where convective heat transfer plays an increasingly dominant role with increase in incident e-beam power and puts a limit on the surface temperature (and consequently the atomic flux). To mitigate this heat drain, a porous tantalum rod (∼35% porosity) was placed at the point of e-beam impact to act as a convection arrestor and a wick within the aluminium melt pool. The molten aluminium around the porous rod was found to ooze through the capillaries of the porous rod to emerge as a vapour stream. On measuring the atomic flux and surface temperature at e-beam powers (below a value where tantalum softens), it was found that ∼50% higher temperature was reached with the tantalum rod than without it. Also, for generating the same atomic flux, the required e-beam power in the case of the porous rod was about one-sixth of the e-beam power required without the rod. Such an efficient evaporation has been reported earlier but without any qualification on the ion fraction of the vapour stream. In our experiment, the ion content in the vapour stream was measured. It was found that the ionization yield in the case of the porous rod was about five times the yield without the rod. Estimation of ionization yields owing to various processes led to the conclusion that higher ionization in the case of the porous rod can be attributed to a higher emission of secondary electrons from the porous rod causing enhanced electron impact ionization.

Collisional effects on metastable atom population in vapour generated by electron beam heating

“…The uncertainty in the excitation temperature is shown by error bars in the graph and the maximum error in the vapour source temperature is less than ∼20 K. In addition to this, to measure the electron density in the plasma co-expanding with the vapour, a disc-type Langmuir probe (diameter ∼ 7 mm) was kept at a height of 30 cm from the vapour source and the V -I characteristics of the probe were recorded at ∼5 kW e-beam power (figure 3). Based on the method described in [12], the value of ionization yield α was calculated and found to be ∼0.3%.…”

How to solve problems in micro- and nanofabrication caused by the emission of electrons and charged metal atoms during e-beam evaporation

“…So far, the discussion about the amount of ionization has focused on the interaction of the vapor with the primary electrons of the e-beam. In addition, backscattered and secondary electrons, which are created by the interaction of the e-beam with the evaporation material, are important contributors to the overall ionization process, which will be discussed in the next subsection [14,21].…”

“…The escape depth can vary between different materials and can become very high for insulators [22]. The SEY also depends on prior treatments and on the morphology of the surface which gets hit by the e-beam [21,31,32]. Crucially, the SEY can increase in the presence of contaminants, an oxide layer, or adsorbates which cover the evaporation material [32,33].…”

How to solve problems in micro- and nanofabrication caused by the emission of electrons and charged metal atoms during e-beam evaporation

“…In addition, backscattered and secondary electrons, which are created by the interaction of the e-beam with the evaporation material, are important contributors to the overall ionization process, which will be discussed in the next subsection. 14,21…”

“…22 The SEY also depends on prior treatments and particularly on the morphology of the surface which gets hit by the e-beam. 21,31,32 Crucially, the SEY can increase in the presence of contaminants, an oxide layer, or adsorbates which cover the evaporation material. 32,33…”