Christoph Raub

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The Surface Structure of Gold Films
VARIATIONS IN TOPOGRAPHY WITH METHOD OF PRODUCTION
Christoph Raub
Forschungsinstitut für Edelmetalle und Metallchemie, Schwäbisch Gmünd, West Germany
For most industrial applications gold is used in the
form of thin layers. This is especially true for
components in the electronics and telecommunication
industry, where it is applied, for example, in the
production of conductor networks and for many
types of contacts.
The mechanical, chemical and physical properties
of gold make it easy to apply to a variety of substrates,
among which metals and ceramics are, of course, the
most prominent. Depending on the application, and
on the thickness needed, various processes for
deposition are utilised, including vacuum evapora-
tion, sputtering in an inert gas, the production of
inlay contact bi-metal strip by rolling, welding,
electroplating and, particularly on ceramics, deposi-
tion in finely divided form with an admixture of
fluxes, the so-called thick film technology.
In the course of a research programme sponsored
by the Stifterverband der Deutschen Wissenschaft
a comparative investigation has been made on the
properties of fine gold layers produced by thermal
vacuum evaporation, radio frequency sputtering,
electroplating, and rolling. Surface topography, wear
resistance, porosity and annealing behaviour for
layers of various thicknesses were investigated. In
this brief report only the results on surface topo-
graphy and its change with annealing temperature
will be discussed.
Experimental Details
As a basis material 0.5 mm thick by 15 mm wide
nickel strip with a hardness of HV =110 kp /mm2 was
used, produced on a mill with polished rolls. From
this strip samples 20 X 10 mm size were cut to serve
as substrates for the deposition of gold. Nickel-gold
inlays were produced in the usual way by cold
rolling. Samples to be coated were degreased
cathodically in an alkaline electrolyte and cleaned
additionally for coating in vacuum by R.F. sputtering.
The coating in vacuum was done by thermal evapora-
tion of gold in a vacuum of better than 10 -4 torr from
a heated tungsten boat.
Examination
by Electron Microscopy
The surface structure was then investigated by
electron microscopy, using a replica technique.
The micrographs show that each deposition method
produces gold layers with distinct types of surface.
For deposits less than 5µm thick the individual
features are less pronounced but still clearly different
from each other.
The rolled surface (Figure 1) is quite even, only the
polishing structure of the rolls being visible with the
marks running parallel to each other in the direction
of rolling. There is no change in structure apparent
with lower thicknesses.
The electroplated gold surface (Figure 2) shows a
typical grainy structure. It is still fairly smooth, since
the individual grains do not protrude markedly from
their surroundings.
The surfaces of evaporated (Figure 3) and sputtered
gold-films (Figure 4) present a totally different
appearance. On the evaporated samples up to a
thickness of about 8 µm the rolling grooves on the
nickel surface are still visible. The layer itself shows
a homogeneous structure with small rounded grains,
this becoming more pronounced with increasing
thickness. Sometimes small round islets are inter-
spersed, as if during evaporation metal had splattered
out on the surface. This structure agrees nicely with
that recently observed by Rehzig (1) on gold films on
alumina. Whether these islets might have any
connection with pore formation has yet to be
investigated. On the evaporated films no grain
boundaries or crystal faces could be observed.
Figure 4 shows the structure of a gold surface
produced by R.F. sputtering. Even at low thick-
nesses a relatively rough structure appears with many
peaks protruding from a basic structure. At greater
thicknesses of deposit an apparently lamellar structure
builds up, with rather large grains, the orientation of
the lamellae being different within each grain. At
higher magnifications typical growth figures are
found at the faces of the lamellae. The edges them-
selves are rounded. The structure is very similiar to
that previously observed on oxygen-etched noble
metals (2) and is probably caused by an interaction
between the condensation of gold atoms and the
etching of the deposited layers by the bombardment
of ions from the plasma. Sometimes very large
crystals were found embedded in the matrix, caused
by preferential grain growth. Because of the high
condensation temperature the condensed gold shows
70
Fig. 1 Rolled	 X 6000	 Fig. 2 Electroplated	 X 6000
Fig. 3 Vacuum evaporated	 x 6000	 Fig. 4 R.F. sputtered	 x 6000
Figs. 1 to 4 The surface structure of 5µm thick gold films produced by various methods and examined by electron microscopy
the largest grain size found among all the methods
investigated.
Effects of
Heat Treatment
Due to surface diffusion of gold atoms the appear-
ance of the deposits changes during heat treatment
between 100 and 400°C. During this heat treatment,
we tried to avoid the influence of diffusion of nickel
from the basis material by using thicknesses of 5 tm
of gold at not too high temperatures or for long
periods of time.
An attempt to anneal gold layers on quartz sub-
strates proved unsuccessful, since the substrate had to
71
Figs. 5 and 6 The surface
structure of vacuum evap-
orated (above) and sputtered
(below) gold films after heat
treatment in air at 400°C for
15 hours >(6000
be seeded by chromium or titanium, which diffused
into gold during heat treatment.
Annealing for 24 hours at 200°C in air caused the
electroplated and evaporated gold layers to become
smoother. This smoothing effect was less pro-
nounced in sputtered layers and did not occur at all
in the rolled layers. After 15 hours at 400°C the
structure of evaporated and sputtered gold is as
shown in the micrographs of Figures 5 and 6.
The small grains in the structure of the evaporated
sample have grown in size, while the lamellar type
structure of the sputtered surface has almost dis-
appeared. Both surfaces seemed to be much more
even after heat treatment.
The investigations show that, according to its
method of deposition, each type of gold layer exhibits
a typical surface structure, this structure changing
rather slowly during heat treatment. It will be
interesting to see how differences in structure
influence other properties, such as bonding or
contact behaviour, as well as porosity. In this
connection the results obtained by Paatsch (3) and
Rehrig (1) should be considered. Both of these
workers compared the porosity of electrodeposited
and evaporated nickel, chromium and gold layers. In
each case the electrodeposited films showed by far the
lowest porosity at thicknesses below 10 µm.
Rehrig attributed this to differences in the micro-
structure. He observed intergranular grain growth
in the d.c. eecltrodeposited gold layers but not in
the evaporated ones. Even greater intergranular
growth is shown by pulse plated films. Up to about
3 µm thickness the latter showed the least porosity.
References
1 D. L. Rehrig, Plating, 1974, 61, 43
2 E. Raub and W. Plate, Z. Metallkunde, 1957, 48, 529
3 W. Paatsch, Jahrbuch Oberflächentechnik, Metall-Verlag,
1974, p. 301
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The surface structure of gold films

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The surface structure of gold films

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