Vacuum Metal Deposition: Principle, Types & Components.

Vacuum metal deposition (VMD) is probably one of the most sensitive fingermark detection and development technique that is used in fingerprint forensic. 

VMD is also capable of developing bloody fingerprints (you can check our article on “DEVELOPMENT OF BLOODY FINGERPRINTS“) for nonporous surfaces as well as semi-porous surfaces like woods and some fabrics.

This article on “Vacuum metal depositions (VMD)”, focuses on the history of VMD, components of VMD, and detailed explanations about its application in fingerprint development.

What is Vacuum metal deposition (VMD)?

Vacuum Metal Deposition i.e. VMD, is one of the most powerful latent fingerprint development techniques. It develops fingerprints by the process of metal deposition on the surfaces. 

The VMD technique can be used to develop latent fingermarks on a wide range of non-porous and semi-porous surfaces.


History and development of Vacuum Metal Deposition

1964: Tolansky while working on interference filters, stated that the vacuum system can develop the fingermarks using silver metal deposition method.

1968: Theys et al. developed fingerprints on paper using a mixture of zinc, antimony and copper powder, by vacuum metal deposition technique.

1972: Hambley concluded in his thesis that the best combination for developing fingerprint using vacuum metal deposition is gold or silver followed by another layer of cadmium or zinc.

1976: Kent et al. use industrial vacuum metal coaters for fingermarks development on the surface like polythene.

2001: An article in Forensic Science International, solves the challenges of the excess of deposition gold’s film on the surface of low-density Polyethene (LDPE) by vacuum metal deposition technique. (N Jones et al. 2001)

2001: N Jones et al. reported the factors that affect the normal and reverse development of latent fingerprints on the polyethylene substrate. 

2007: Philipson and Bleay developed latent fingermarks on the plastic substrate by using a silver vacuum metal deposition method.

2007: Guraratne et.al. reported that aluminum metal can be used for the development of fingerprints without in combination of any secondary metal coating or deposition.

2010: J Fraser et al. shows that vacuum metal deposition can be used for the development of fingermarks on the fabrics. They use nylon, polyester, polycotton, and cotton to develop prints using gold- zinc metals.

2011: Yu et al. show that oxide of zinc (ZnO) can be used for developing latent fingerprints on polymer surfaces by the deposition of vapors of zinc oxide in the vacuum chamber.

2014: J Fraser et al. reported a comparative analysis of vacuum metal deposition and cyanoacrylate fuming for visualizing of fingermarks on fabrics.  


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Theory and Principle of VMD

The theory of vacuum metal deposition is based on condensation characteristics of metals over the surfaces. 

Metals like Cadmium, Zinc, and Magnesium can vaporize to form very fine particles which are further condensed to form a layer over the surface.

The theory of metal deposition is divided into two parts. These are

  1. A very thin invisible layer of Gold: Gold is first evaporated under vacuum to form a very fine thin layer on the surface which is brought to examined.
  2. Zinc layer making negative Fingermarks:  The second layer of zinc is now evaporated to form a corresponding layer to the surface and zinc can’t able to diffuse into the fingermarks. 

The ridges are therefore left transparent, while the background plated with a layer of zinc in the case of normal vacuum deposition. 

Under normal circumstances, VMD treatment produces negative marks, as zinc deposits on the background substrate and not the print ridges themselves.

Note: Cadmium can be used for thr development of fingermarks, but due to its toxicity it is no longer used.

Types of Vacuum Metal Deposition

There are three types of Vacuum Metal Deposition method, listed below:

  1. Normal development
  2. Over-development
  3. Reverse development

1. Normal development

In normal development, the primary layer of gold covers the entire substrate. Gold also get diffuse into the ridges of latent prints.

And the secondary layer of zinc covers the whole surface leaving the friction ridges. And the developed prints have the entire surface covered with zinc except for the ridges. 

And this creates a different contrast of friction ridges to the surrounding and hence prints are visible.

2. Overdevelopment: 

This may occur in certain types of surfaces like HDPE (high-density polyethylene) when an excess amount of primary coating of gold gets deposited on the surface. 

In overdevelopment, the gold clusters overgrow on the ridges and when the corresponding layer of zinc is introduced it covers the entire area even over the finger ridges.

Then there is no clearly ridge details are seen, and it becomes nearly impossible to develop clear fingerprints using this method.

3. Reverse development: 

Surface-like polyethylene LDPE (low-density polyethylene) shows the reverse development of the fingermarks.

In this development, the secondary layer of zinc deposited more rapidly on the latent finger ridges than the surface. 

Theories to describe the reverse development

There are two theories to describe why the reverse development occurs in Vacuum Metal Deposition;

1. Based on the surface porosity:

Porosity is the quality of being porous. If the surface has semi-porous or porous then, in this case, the gold nuclei diffused more through the porous surfaces.

And the resultant corresponding zinc development leads to reverse the development of fingerprints.

2. Prints are dried enough: 

In another explanation, it may be possible the fingerprints or contaminants like grease, are dried enough so that the gold particles unable to diffuse into the fingermarks. 

This result in the deposition of larger gold nuclei on the surface. Hence, after the application of the second layer, the zinc deposition is concentrated on the latent ridges and formed a thick layer than the surface.

So, the resultant prints are reserve to the normal development having thick zinc coating on the latent prints than the entire surface.


Process of using Vacuum Metal deposition:

The deposition process includes the formation of metal nuclei of different sizes that can be deposited on the different regions of the surfaces. 

In VMD, a very fine primarily mist vapor, mainly gold is used to create a very fine and thin layer on the surface. 

Then the next layer is brought to be thicker which is developed by applying the corresponding layer of zinc metal.

Now discuss the development process in details;

# First stage: Layer of Gold

Gold is evaporated and resultant nuclei are very small but discrete in size that can’t be seen through the eyes. The gold nuclei form a very fine layer on the surface but these layers are discontinuous. 

Example: One can co-relate deposition of gold muclie with the talcum powder is on a table. Talcum powder form a fine layer to the surface but are discrete and discontinuous to the surface.

To maintain even coating, there must be less collision between the nuclei of golds so the low pressure is used in the VMD chamber for coating.

When the gold nuclei bombarded to the surfaces then they not only disperse over the surface, but they also get diffused into the fatty acids present in the ridges or fatty acid from contaminants like grease. 

As the gold film layer is very thin which are not visible, further development of the fingermarks needed.

Properties of gold used in vacuum metal deposition 

In VMD the gold is used in cylindrical wire form usually 0.5mm of diameter. The gold wire is about 99% pure. And for a single deposition procedure, 5mm of length wire is used. The gold particle is typically boiled at 1600° C at 10-4Mbar.

# 2nd Stage: Layer of Zinc

The zinc layer allows the user to control the deposition factors because of higher chamber pressure and the ability of zinc to re-evaporate. 

The more air pressure into the chamber serves the major purpose of maintaining the uniformity of the zinc layer across the surface.

Properties of Zinc used in VMD

Zinc has a melting point of around 420°C. When the pressure reduces the zinc starts to sublime i.e. evaporate directly to gas. Once the boiling point of zinc is reached the zinc particles agitates very fast.

As zinc starts to sublime, the zinc particles have a tendency to travel both via straight and curved trajectories depending upon the obstacles in the path. 

So, even the obstacle like shutting the shutter does not stop the zinc particles to get deposit on the surface.

Components of Vacuum Metal Deposition Unit

  1. A vacuum coating chamber
  2. Thickness Monitor
  3. Shutter
  4. Sample holder
  5. Baffle Valve
  6. Vacuum Direction Valves
  7. Rotatory Pump
  8. Diffusion Pump

#1 Coating Chamber

A typically Vacuum metal deposition chamber is a spacious compartment fitted with a thickness monitor, a sample holder, a shutter and a tungsten boat.

The evaporation and deposition of metals occur in this chamber.  

#2 Thickness Monitor

It a thin quartz film coated with gold from both sides. Its diameter is about 8-10 mm. 

It serves two major purposes in vacuum metal deposition chamber:

  1. It monitors the evaporation rate in terms of counts per second.
  2. Total number of counts

Thickness monitors value are based on the principle of resonance frequencies. According to this, any change in mass results to corresponding resonance frequencies. 

So, when the deposition of metal on quartz film takes place, then there is a corresponding resonance frequency. And deposited mass is get converted into arbitrary unit know as “counts”. 

The thickness monitor is used to check only gold evaporation and deposition rate because zinc can re-evaporate after deposition.

#3 Shutter

It is used to control the amount of gold deposition. The shutter operates with a programmed time period in accordance with the thickness monitor. 

Initially, the shutter is kept closed until the gold is evaporated. Then opened to bombarded gold nuclei to the surface. Typically shutter is programmed to get opened after the 30s of passing electric current to gold for heating.

#4 Sample Holder

It is a removable, semi-cylinder that covers the half vacuum metal deposition chamber. It can be aligned horizontally or vertically to the VMD chamber. The samples to develop are fixed with small magnets.

#5 Baffle Valve

A large heavy valves that are opened when the VMD chamber pressure below the threshold pressure i.e. ~8 x 10-2 Mbar. It also prevents diffusion pump oil from oxidation.

#6 Vacuum Direction Valves

There are two operational direction valves of the shutting and opening function to achieve the desired conditions required in vacuum metal deposition.

#7 Rotatory Pump

It is a two-stage rotatory pump that is used to create vacuum pressure until the threshold pressure. 

Note 2: A two-stage rotatory pump is a pump with two rotators and valves. The first stage creates the vacuums and the second stage cleans the system, which creates a deeper ultimate vacuum level.

#8 Diffusion Pump

A diffusion pump contains oil chambers at the bottom which is used to pump oils to achieve and maintain the VMD chamber pressure. 

When the oil is heated to its boiling point oil steam fills the cylinder and the fast oil molecules capture air molecules and concentrate them at the bottom of the pump. 

Then, the coolant around the diffusion pump condenses the oil that then travels down to the oil sump. 

Frequently asked questions on Vacuum Metal Deposition

Q1 What difference in Gold Layer and Zinc Layer film?

The gold vapors not only cover the whole surface of the sample but also penetrates the ridges marks of the fingerprint.
Whereas in the case of zinc particles, they are not able to diffuse into the friction ridges.

Q2 What are the factors affecting the development phases in the VMD chamber?

1. Presence of dirt and oily substances on the surface like grease.
2. Local Difference in chemical composition.
3. The local difference in oxidation state.
4. Abrasion of the surface

Q3 Why Gold is used as the primary layer in VMD?

This is because of the following reason;
1. The gold layer acts as the primer for the deposition of the secondary zinc layer.
2. As gold is not selective in nature because it gets disperse on entire surfaces even on the ridges of fingermarks.
3. Because of gold’s Inert nature, it does not react with fingermarks and hence other evidentiary chemicals remain unaffected.
4. Without the initial layer of gold, zinc deposition may be inhibited by the presence of many greasy contaminants.

Q4 Why Zinc is used as the Secondary layer in VMD?

Reason 1: Zinc is used to trace down the fringer ridges details by the unseen deposition of gold nuclei on the surface of ridges and surfaces.
Reason 2: The zinc particles get easily re-evaporated so that the deposition factors can be controlled. The controlled deposition factor defined by the type of surface and contracted needed.

Q5 What happens to fatty acid in high vacuum vapor pressure?

The latent prints contain many fatty acids which are mostly insoluble in water and have a long-chain structure of fatty acids.
As the chain structure increases the vapor pressure of compounds decreases. This means that these fatty acids remain stable even in high chamber pressure.

Q6 Why shutter is not required in Zinc deposition?

The zinc particles have a tendency to travel both via straight and curved trajectories depending upon the obstacles in the path. So, even the obstacle like shutting the shutter does not stop the zinc particles to get deposit on the surface.

Conclusion

Vacuum metal deposition is one of the best fingerprint development methods. However, for the optimum development of fingerprints by Vacuum metal deposition method, requires an appropriately  configured VMD chamber (with a thickness monitor and shutter highly recommended),  and an operator experienced in applying the technique on a range of different surfaces

References:

  • Fraser J, et.al. (2014). A comparison of the use of vacuum metal deposition versus cyanoacrylate fuming for visualization of fingermarks and grab impressions on fabrics. [URL]
  • Jones, N., et.al. (2001). Vacuum metal deposition: factors affecting normal and reverse development of latent fingerprints on polyethylene substrates. [URL]
  • Jones, N., et.al. (2001). Vacuum metal deposition: developing latent fingerprints on polyethylene substrates after the deposition of excess gold. [URL]
  • Vacuum deposition of high-quality metal films on porous substrates. (1985).  [URL]
  • Affinito, J., & Gross, M. (1995). Vacuum deposited polymer/metal multilayer films for optical application. [URL]
  • Bleay, S. M., Croxton, R. S., & Puit, M. D. (2018). Fingerprint development techniques: theory and application. [URL]
  • I.HengYu, et.al. (2011). Development of latent fingerprint by ZnO deposition. [URL]