Measuring Plating Uniformity
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Disclosed is a plating uniformity measuring technique which permits 'real time' monitoring of the rate at which metal is deposited on a plane cathode during electrodeposition.
Measuring Plating Uniformity
a plating uniformity measuring technique which
permits 'real time' monitoring of the rate at which metal is
deposited on a plane cathode during electrodeposition.
It is known,
that when plating (electro depositing) a metal
using a plane anode and cathode of equal size and shape, the
thickness of the metal deposited will not be uniform on the surface
of the cathode. It will be thin at the center and thick at the edges
with very thick deposits on corners. The reasons for this are known.
In a simple plating solution like the copper sulphate bath used for
the deposition of copper in printed circuit processing the so-called
primary current distribution depends mainly on the solution
resistance and the diffusion rate (depletion rate) of copper ions
close to the cathode.
It is also
known, and is common practice, to even out this
distribution by adjusting the configuration of the anodes relative to
the workpiece, or by the introduction of shields or robbers into the
plating bath. A shield is an inert plate placed in the bath in order
to increase the effective distance (and hence the resistive path
length) through the solution. A robber is a metal plate connected to
the cathode supply, and strategically placed so that it "robs"
depositing metal from the workpiece, by depleting the concentration
of metal ions in its vicinity. It is usual to use a combination of
skill and empirical evaluation to optimize the deposit uniformity by
plating onto a relevant substrate, measuring the deposit thickness in
several areas, rearranging the electrodes or inserting baffles and
repeating the "deposit and measuring" operation. Because of the time
involved in deposition and measuring, such an exercise may take days
to complete for a specific substrate. It would be far more efficient
if one could monitor the plating rate at various points on the
cathode surface in "real time" and be able to see instantly the
effects of moving the electrodes, shields or robbers.
constant conditions, the amounts of metal deposited on
different parts of the cathode are inversely proportional to the
differences in surface potential on the cathode. Because the whole
of the surface is metal it is not possible to measure this potential
directly. In laboratory conditions, an electrochemist monitors the
surface potential at various points on an electrode using a "Luggen
capillary" versus a calomel electrode but a simpler and more rugged
technique is necessary in a commercial plating shop. The following
describes such a technique.
represents a plating bath, having two commoned anodes at
D and E, and a cathode BAC. The arrangement of the electrodes is
deliberately non symmetrical, and plating thickness is considered in
2 dimensions only, for demonstration purposes. The cathode is a
sheet of FR4, copper clad on the side facing the anodes, 12 inches
from B to C and ab...