Vacuum metal deposition

Abstract

Vacuum metal deposition (VMD) is an extremely sensitive technique for the development of latent fingermarks on nonporous and semiporous surfaces. VMD is generally recognized as being more sensitive than cyanoacrylate fuming, which is the most common routine devel¬opment technique for nonporous surfaces [1–3]. The increased sensitivity achieved with VMD is particularly evident in instances where marks are old, have been exposed to adverse environmental conditions, or are present on otherwise difficult surfaces [4,5]. A further advantage of VMD is that, with normal development, the substrate is covered with a layer of metallic zinc that can assist in overcoming background interference such as heavy printing or multicolored patterns. VMD is generally not considered a routine method for fingermark development due to the expense of the equipment (Figure 10.1), the time required for appli¬cation of the technique, and the need for experienced operators to obtain optimum results. The VMD development of latent marks traditionally involves the evaporation and depo¬sition, under vacuum, of gold and then zinc. Zinc will not deposit onto a nonmetallic sur¬face under vacuum unless the surface is very cold or if the surface contains nucleating sites of another metal, such as gold. This property is exploited in VMD to develop latent fingermarks. The clusters (agglomerates) of gold atoms formed during the first evapora¬tion step create nucleation sites to which the zinc can bond, enabling the deposition of a zinc film during the second evaporation step. Fingermark residue, and any other surface contaminants, will generally inhibit zinc deposition. As a result, the zinc film that forms will generally cover the whole surface except where fingermark residue is present, result¬ing in transparent ridges and a metallic (zinc-coated) background (Figure 10.2).