A Literature Review of Methods Used to Study Electroless Deposition
Introduction
The deposition of selected metal ions is a process which accounts for a myriad of products in
industry. As with all avenues of chemistry, the result is dictated by its means (experimental design/
methodology) rather than the other way around. As such, the two most common methods for this
area of deposition are electroplating and electroless deposition. The primary distinction between the
two systems is that electroplating relies on the use of an external source of electrons to drive this
reaction, while electroless deposition utilises a carefully composed chemical bath which can
generate a free electron population to drive the reaction ‘in house’ or without the need for an
external power supply.
Its discovery and adoption by material scientists didn’t mean the end of electroplating in industry.
There are many differences between the two processes, but each process is suited well to their own
generation of products of specific physical properties. Its discovery was the by-product of
alterations in the experimental design of the electroplating method in an attempt to remove
impurities in the plated metal through the addition of a reducing agent to the chemical bath. The
result was the inadvertent discovery of a method with clear advantages over the standard
electroplating methodology.
The first indication of this was that the deposition rate observed far exceeded that which was
dictated by Faraday’s law, meaning that the reaction was non-Galvanic and so the rate of reaction
was not significantly altered by the presence of this external source of free electrons. This also
indicated a second major benefit: the process was autocatalytic (the products of the reaction act as
catalysts for subsequent reactions). While this experiment was conducted on a small scale (to be
scaled up to industrial size later if successful), the cost-effective nature of the process was obvious
and patents were quickly filed thereafter.
The chemical bath that makes the process of electroless deposition feasible has a few archetypal
components which can be altered to suit the methodology in question; a reducing agent is necessary
to produce the free electron population necessary to drive the process, the use of a complexing
agent allows for the metals to be more readily deposited onto a given material (with the identity of
this complexing agent holding special importance as a slight change in its chemical composition will
produce drastically different physical qualities in the product) and as the reaction requires an even
current distribution, a buffer is also required to maintain a consistent level of free electrons in the
solution. A stabilizing agent is needed to allow for continuous operation of the reaction mixture and
avoid components ‘crashing out’. This is done through ‘masking’ a certain proportion of nuclei to
allow for a steady reaction rate.
Because of this even current distribution, the
‘dog-boning’ effect observed in the products of
electroplating is avoided with the electroless
deposition process. That is to say that the
process is perfect for precision works and small
part manufacture as the metal is deposited
evenly across a given surface. As stated, the
myriad of methodologies which can include the
electroless deposition process provide a
similarly varied set of qualities in its products.
1
As an example, these materials can be highly
Fig. 1: Image illustrating the resulting -dog- bone’ effect present
following electrolytic plating, and its absence in electroless
lustrous while maintaining a high wear and
deposition products.
scratch resistance due to increased hardness
and so make them a favourite in the
jewellery industry and proves their
commercial significance. Conductivity,
ductility and malleability, as well as any
number of other physical characteristics
can be imparted into the products of
this process through careful design of
the chemical bath and selection of
starting materials.
Through the manipulation of the
chemical bath an alloy, composite or
Fig 2: Graphical representation2 of corrosion rates of non-plated
pure metal plating can be deposited,
and various electrolessly plated metals.
speaking to the versatility of the
electroless deposition method and its
ubiquitous inclusion in surface engineering practices. However, while the chemical bath composition
must be carefully designed however, other physical factors play their own role. Temperature, pH (as
stated) and bath volume alongside the operating time must also be carefully controlled so as to
preserve the precise nature of the electroless deposition process.
This literature review however, does not deal with the physical factor requirements for electroless
deposition. Rather, it poses the question: how does one company or another determine the
effectiveness of its electroless deposition experimental design? While there are many methods
available for such analysis, focus on a select few will provide some insight into the mirrored
versatility in the data that can be acquired from a process which itself carries endless experimental
variations and product applications.
Atomic Force Microscopy (AFM)
AFM is a versatile means of analysis in surface
engineering and materials chemistry. Its mode of
operation allows for the direct measurement of physical
thickness while offering an unmatched degree of
precision and allowing for sample thickness to be
obtained to within a fraction of a nanometer.
While other methods allow for the changes in a sample’s
physical parameters to be determined (such as detection
in mass change through gravimetry), electroless
deposition demands a means of measurement which can
support this level of necessary detail.
This is afforded by AFM which, in practice means the
measurement of interatomic potentials generated by
Fig 3: Diagram3 illustrating basic theory and
minute distances. A minute tip is attached to a spring
experimental setup for AFM.
cantilever. The tip then makes ‘physical contact’ with the
sample, when really the interatomic separation of the tip
and sample is so small that interatomic potentials are formed at the surface. These interatomic
potentials then cause the cantilever to bounce up and down as it is ran across the surface of the
sample. As the interatomic distance decreases between these two bodies the potentials rise and
when the interatomic distance increases, they fall. Obviously, a simple tipped spring-cantilever
cannot detect such minute alterations in distance on its own, for this laser light is reflected by the
top of the cantilever and a photosensitive diode receives it. The
sample holder moves the sample up and down via a
piezoelectric scanning tube so as to maintain the preset
interaction force level. The cantilever moves across the material
nanometer by nanometer and a three-dimensional image is
generated by determining the measurements as a z-axis position
as a function of the x and y-axis positions (as can be seen in fig.).
This allows the tipped cantilever to be dragged across the
surface of the sample and so generate a composite topographic
image of the surface of an atomic resolution (This is known as
‘Contact mode’ AFM).
While other modes of AFM exist (non-contact mode and tapping
Fig 4: AFM images4 of a progressing
mode AFM are good examples), they possess considerable
electroless deposition with a: 0s
drawbacks (unstable interference from Van der Waal’s forces as
after reaction start, b: 1s after
an example) when compared to contact AFM measurements for
reaction start, c: 5s after reaction
Electroless deposition sample examination and so will not be
start and d: 30s after reaction start.
described further.
From this topographic image surface roughness can be determined alongside relative changes in
thickness through appropriate
measurements before and after
submersion in the reaction
mixture.
C.M. Edwards et al designed an
experiment which compared the
galvanic displacement of different
deposited metals as a means of
producing semi-conductive
nanostructures as the dimensions
for such structures must be exact.
Fig 5: Diagram5 illustrating Use of AFM to etch nanopatterns into
Contact mode AFM was used to
monolayer and subsequent electroless deposition of metal.
determine the topography of the
surface and ensure that the deposited materials were found only across a specific pattern across the
silicon assembly. This nanopatterning technique is a recent discovery and a strong example of how
broader measurements (such as AFM data being packaged to express rate as a function of time or
temperature) are not always the sole focus of these studies. Similarly, the alteration of experimental
design aspects so as to increase the efficacy of the experiment focuses less around increasing the
rate of the electroless deposition reaction (submersion time in chemical bath, temperature, pH, bath
constituents etc.) but with the means of nanopatterning the silico nanolayer, such as doping the
underlying silicon substrate or modifying pattern line depth as it is etched on said substrate. This
reflects the broader scope of such studies, as electroless deposition occupies an integral part of this
experimental design, but acts as a support for the success of the methodology to be determined
directly through AFM, whilst also allowing conclusions to be drawn about the electroless deposition
methodology specifically.
Topographic information from a given sample allows for expressions of surface roughness to be
obtained. These can be packaged as rate values when compared to the pre-deposition material by
analysing the change per unit time or temperature; or they can be used to gauge the success of the
process through the description of physical parameters to determine their methodological potency
in various areas of commerce.
Similarly the average change in thickness for the material can be measured and compared directly to
the submersion time so as to determine a rate constant that way (conventionally given in units
μm s-1) so as to make the results accessible for outside comparisons.
Electrochemistry
Electrochemistry is a broad term for the properties of a material which reflect differences in the
freedom of electron movement. Capacitance, resistivity or conductance all fall under this one title.
Where AFM (and even simple observation) are effective means of observing the difference between
pre- and post-electroless deposition materials through colour or size, observing the differences in
electrochemistry brought on by the process provides some insight into the changes that are not so
easily observed.
K.G. Mishra and R.K. Paramguru carried out electroless deposition reactions using various metals
and substrates, and used component or total current-potential curves to illustrate the changes of
the materials following the plating reactions involved here. The aim of the study was to determine
the anodic and cathodic regions that form during the electroless deposition of copper and so aims to
prove the mechanism and determine the theory however, this method of determination of
electroless deposition activity within the reaction mixture provides useful information as it can be
inferred that chemical baths for the electroless deposition process tested in this way on a much
smaller scale could be used to optimize chemical bath composition (as can be seen here with the
suggested substitution of the traditional reducing agent formaldehyde with glyoxylic acid, the latter
a more sustainable choice with similar activity to the former.) through electrochemical observation
and so make slight adjustments as required before scaling it up to meet the needs of manufacture.
Additional degrees of control can then be added to these processes through the virtue of
electrochemical study: M.B. Sassin et al designed a self-limiting electroless deposition process
through the use of FeO2-carbon nanofoam substrates which act as capacitors in aqueous solution
and Li2SO4 which act as electrolytes to aid in charge transfer. This controlled the rate of charge
transfer in solution through moderation through its action as a mechanical (rather than chemical)
buffer, in that it maintained the charge transfer ratio over the space of 1000 cycles with a 20%
failure rate in the capacitors at this point. Electrochemical measurements of the solution made it
possible to determine the smallest changes in bath composition and so illustrate another, highly
configurable mode of analysis and optimization for the chemical baths used in these electroless
deposition reactions.
Another facet of electrochemical analysis on electroless deposition reactions is the measurement of
resistivity. Y. Shacham-Diamand et al designed an experimental procedure which aimed to reduce
the resistivity of thin deposited copper films to allow for the production of more efficient PCBs.
Approaching the problem in this way with the product’s resistivity being tested allowed for
manipulation of all aspects of chemical bath design and optimization. This allowed for alterations in
the identities of the reducing agents, catalysts and seeding layers as well as variations in operating
temperature and pressure to obtain easily compressible, low resistance thin copper films.
X-Ray Diffraction (XRD)
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