Laser enhanced plating
0
IBM Corp.,
ThomasJ. Watson Research Center
, Yorktown Heights,
N.Y., U.S.A
Presentprocessesforgenerating metallizedpatternsfor microcircuitry involve many steps including mask fabrication. Maskless metallixation of patterns at acceptable resolution would reduce the number of processing steps, add flexibility to the production scheme and facilitate repair of circuitry. Laser enhanced deposition ofgold and other metals is believed to represent a significant advance in this field. Recently, a new technique for depositing from and etching in aqueous solutions was described in which a focussed laser beam is used to define arbitrary patterns without the use of masks (1 to 5). The rate of plating or etching is much greater in the region of laser absorption than in the non- irradiated areas so that a pattern can be traced by movement of the beam relative to the cathode or, in the case of etching, the anode. Laser enhanced plating bas been used with a variety of substrates to achieve gold, nickel and copper deposition rates as high as 10 Itm/s for laser power densities in the range 1 to 103 kW / cm2. Thin film lines as narrow as 2m have been generated by simple motion of the beam relative to the cathode. More complicated plated patterns have been generated with a computerized table to provide the necessary relative beam-cathode movement.
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Plating Mechanisms
Using the apparatus shown in Figure 1, an extensive study of the
copper/cupric ion plating system has yielded an understanding of
the enhancement mechanisms, which are also believed to be
fundamental to gold deposition. Special cathodes were designed to
limit the irradiated plated area to that of a small spot several
hundred micrometres in diameter, or approximately equal to that
of the laser beam diameter(4). With the beam directed through an
opening in the platinum anode onto the cathode, the plating
current was measured as a function of applied overpotential, using
a potentiostat in connection with a three-electrode system. It was
found that the plating current increased by between 2 and 3 orders
of magnitude with the laser beam incident on the cathode,
compared to values obtained without laser irradiation. Enhancement
was observed over the entire polarization curve extending from 0 to
about 800 mV of applied overpotential relative to SCE (saturated
calomel electrode). These results, in combination with earlier
experiments using widely differing thermally conducting
substrates, led to the following thermal model for laser
enhancement of electrodeposition or etching:
(1) At low overpotentials, enhancement occurs due to an increase
in the thermal kinetics of the plating process, resulting from
laser energy absorption which causes a higher laser energy rate
of charge transfer at the cathode
(2) At higher (more negative) potentials, in the absence of laser
irradiation, the plating current becomes limited by the depletion
of available ions in the plating region with nearby ions unable
to diffuse towards the cathode sufficiently rapidly to replenish
the supply required at the surface for faster plating (mass
transport limited region). The laser beam causes strong thermal
gradients in the surface region, which in turn produce intense
local stirring of the electrolyte. The rapid replenishment of the
metal ion concentration at the solution/cathode interface,
which is due to local thermal convection, allows for a very high
local deposition rate.
Types of Laser Plating
Three types of laser enhanced plating potentially applicable to
improved gold deposition have been experimentally identified.
The first, laser enhanced electrodeposition has been described
above and requires an anode-cathode arrangement with an
externally applied potential.
The two other types require no external potential and are
thermally driven by the absorbed laser power. The first of these
is laser enhanced electroless plating for which the deposition of
nickel has been demonstrated (2, 5). In this process, charge
conservation during plating is maintained via a catalyst in the
plating solution. Sinc the performance of electroless solutions is
generally quite temperature-sensitive, these solutions invariably
operate well only at temperatures significantly above room
temperature. Hence, in electroless plating, the effective
deposition rate enhancement, that is the ratio of local plating produced
by the heating effect of a laser beam compared to background
plating, is very large and can approach infinity. The increase in
temperature with laser irradiation locally also produces relatively
high rates of plating though not up to the same levels as those
obtainable with electroplating.
A third improved form of plating is a type of laser enhanced
exchange plating (5). Here a local thermobattery is created
between hot (laser irradiated) and cool regions of the cathode
immersed in a conventional electroplating solution. The heated
portion performs as the active cathode and becomes plated, with
the co (...truncated)