How does one electroplate 1mm thick by 0.15mm width by 1.2cm in
length microstructures? I use a watts nickel plating bath at 0.4A/mm^2
d.c. and some experiments with pulse plating. Nucleation/starting a
uniform plate is
the start of problems while observing the process. 0.15mm diameter
feature by 1mm thick appears not to nucleate and plate at the same
rate as the rectangular feature. |
Clarification of Question by
corgi2-ga
on
01 Jul 2003 06:34 PDT
Thanks racercar! The mold is an oxidized 1mm thick silicon wafer
bonded to a gold strike layer.The mold is therefore insulating. The
plating strike layer is conductive. Plating geometry has the
moldfeatures/strike layer facing the anode approximately 1cm distance.
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Request for Question Clarification by
krobert-ga
on
01 Jul 2003 08:52 PDT
corgi2-ga,
I'd like to answer this for you, but would like to make sure that I
provide sufficent information for the price that you are offering.
racecar-ga has the correct idea. You need to create a current that
will deposit plating material inside the cavity. One way of doing this
is to place an electrode at a distance that depends on your particular
bath chemistry and part geometry. In other words, you will need to do
some experiments.
You also need to make sure that you part is clean and that you
actually have a microstructure that the plating can adhere to. Just
because the material is conductive does not mean that it will plate.
The gold strike layer should provide this, so you should be OK.
Another thing to consider is that the rectangular feature -provides-
for nucleation points. The points of the rectangular feature will
concentrate the amperage and nucleate your plating. A circular feature
has no such amperage concentration points. I would bet that your
plating is considerably thicker at the points of your rectangular
part.
Now that I've gone over all of the above... I'm a little confused
concerning the exact geomerty of the feature that you are trying to
plate. Could you please explain the geometry some more for me?
Krobert-ga
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Clarification of Question by
corgi2-ga
on
01 Jul 2003 11:45 PDT
Thank you krobert! I am using a 100mm diameter x1mm thick silicon
substrate. The silicon has microstructures etched through it to form
a mold. The silicon etched wafer is then oxidized to grow a 5um oxide
layer. The oxide is later etched as a mold release. There are 32
features units on 7.5mm center to center spacings. A unit feature
comprise 30ea. "holes" having 0.15mmdiameter on 0.25mm center etched
1mm in depth and 2ea. rectangles 0.15mm by 1.2cm x 1mm deep on 1mm
centers. The 30 holes are split on either end of the short end of the
rectangles and separated from the ends of the rectangles by 1mm. The
silicon mold is bonded to a separate silicon substrate having a
chrome/gold thin film plating strike layer. The strike layer and the
mold interface are either separated by a fixed distance, mechanically
clamped, organic interface bonded or eutectically attached.
The parts are "semiconductor" clean using semiconductor grade
reagents.
The anode material is platinum
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Request for Question Clarification by
krobert-ga
on
07 Jul 2003 08:01 PDT
corgi2-ga,
One thing I should mention is that your use of the term
"microstructure" has been confusing. You may want to use the term
"feature". Microstructure refers (in materials science anyway) to
crystalline grain structure of materials. I think that what you are
referring to is actually an engineered "feature" on the material.
I am thinking that you are going to have quite a bit of difficulty
getting this plating to grow due to two things:
1) Feature size
2) Feature geometry
Have you tried any other plating methods? Must this be made out of
nickel?
It seems like what you are actually trying to do is "grow" a
"positive" feature
out of a "negative" mold. Is electroplating the best method for this?
You may want to try using a vapor deposition method.
These are just some ideas, please respond with where you would like to
go from here with respect to this question.
krobert-ga
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Clarification of Question by
corgi2-ga
on
07 Jul 2003 09:26 PDT
Krobert-ga
Microstructure(s) is a valid descriptor when fabricating features in
the 10-6meter(micron) scale using inorganic and organic material
systems. Microstructure in material sciences does refer to grain
structure amongst others.
Yes, I am attempting to create a "positive" microstructure(feature
element?) from the negative mold.
I am experiencing difficulty in developing this plating process. Why?
would feature size at the 0.15mm(150micron) x 1mmdepth geometry be
problematical? Diffusion, field strength through 1mm depth, what is
the difficulty? Lithography and galvanoforming processes using
PMMA(Poly-Methyl Methacrylate) to depths of 1mm at .060mm(60 microns)
is demonstrated using a u.v. synchrotron source to pattern the PMMA
through its 1mm thickness. Nickel is the preferred metal in plating
these geometries.
Vacuum vapor deposition(thermal, e-beam, sputterring) is normally used
in thin film(<0.010mm or 10 micron) fabrication. These structures are
considered thick film(1mm).
D.C.Pulse plating, A.C. plating methods have been attempted. Gold
plating was attempted. electroless deposition has been attempted.
e-less plating rates topped out at 10um/hr. E-less plating is
considered a thin film method.
Summation: I would like to understand what is the difficulty in
plating 1mm thick x 0.150mm geometries. I am using an oxidized Silicon
wafer as the "negative" mold to plate "positive" features. I have
looked at different methods to interface the silicon mold to a plating
strike layer. My observations lead me to conclude that the plating
strike layer needs to be in intimate contact with the silicon mold. I
observe uneven plating uniformity at the initiation of the plate
cycle.
corgi2
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Request for Question Clarification by
krobert-ga
on
07 Jul 2003 13:24 PDT
Well Corgi2-ga, I don't think I will have a complete answer for you...
but I can say that what you are doing sounds very interesting! I've
included a few more random thoughts below...
Thanks for the note regarding use of the term microstructures. I have
typically been involved in metal alloys, so you can see that you're
talking with a "steelhead". You learn something new every day.
I'm going to have to say that the field strength through 1mm depth is
what is really causing you problems. Your creating a feature that is
much deeper than it is wide. It would definitely make for an
interesting research project in and of itself. The electric field at
the strike layer has to be very weak with respect to just above the
surface of the mold.
If you could somehow create an anode in a geometry that would mate to
your features you would probably have better luck getting the plating
to nucleate. This should at least be able to create a strong electric
field in the vicinity of the feature.
You may also want to look at the nickel nucleation process itself. If
the typical grain size quickly grows to larger than the feature
geometry, it may be preventing the nickel plate from growing very
much.
Also, the whole point of a strike layer is to give the actual plating
something to nucleate on. If you are getting nucleation, then the
strike layer is serving it's purpose.... something else is going on
and I am thinking that the nickel grain growth is the root of the
problem.
Like I wrote... these are random thoughts. I'm sorry that I cannot
provide a complete answer for you, perhaps another researcher can jump
in. If not, then good luck on your quest!
krobert-ga
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Clarification of Question by
corgi2-ga
on
14 Jul 2003 12:00 PDT
Thank you krobert-ga for the time you did spend on my problem. My own
research and labwork indicates that I need to have good isolation of
the plating bath around each 0.15mm feature. Plating nickel from a
Watts Bath one can use pulse reverse plating or D.C. pulse plate and
add coumarin as a leveling additive to achieve good filling of the
mold. One also needs to allow for the metal ion species to diffuse
through the 1mm depth. It was discovered that 1mm silicon depth
reguires pulse off times on the order of 1000 sec. fo a nickel ion.
Kind regards,
Corgi2-ga
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