[lodgezhao]

Hello, I have a question for electrodeposition of gold.

So I tried to electroplate gold on a Si wafer with gold seed layer on it. I used SU-8 as my photoresist to form a pattern. But my electroplated gold was black and granular.

Area is 19mm^2 and desired thickness is 50 um. I set the effective current to be 1.9 mA.

Any ideas?

Thank you so much!!!!!

[eazye1334]

While I don't electroplate gold, I am a Certified Electroplater and Finisher, so I'll take a stab at it.

Two things:

1. You don't say what type of solution you used to do the plating. Cyanide? Sulfide? Chloride? pH? I ask because they all have different current density ranges, and if your current density is too high, you'll burn the plating. You were plating at about 1 A/dm² which is an acceptable current density for some baths, but not all.

2. I have only heard this and can't confirm it's validity, but gold seed layers on Si may not adhere that well. If you have bad adhesion of the seed layer, you will get a poor electrical connection and that will cause burns on the electroplated material as well.

These are my two best guesses as to what could've happened. By your description of the deposited metal, it sounds like a burning problem to me, and these are the two ways I see it burning.

[lodgezhao]

While I don't electroplate gold, I am a Certified Electroplater and Finisher, so I'll take a stab at it.

Two things:

1. You don't say what type of solution you used to do the plating. Cyanide? Sulfide? Chloride? pH? I ask because they all have different current density ranges, and if your current density is too high, you'll burn the plating. You were plating at about 1 A/dm² which is an acceptable current density for some baths, but not all.

2. I have only heard this and can't confirm it's validity, but gold seed layers on Si may not adhere that well. If you have bad adhesion of the seed layer, you will get a poor electrical connection and that will cause burns on the electroplated material as well.

These are my two best guesses as to what could've happened. By your description of the deposited metal, it sounds like a burning problem to me, and these are the two ways I see it burning.

Thank you! I put a layer of Cr underneath the gold so the adherence is warranted.
One more thing I noticed was that seeing under profilometer, I could see clusters of needle-like gold.

I'm not sure about the solution.... but if I understand correctly, your suggestion is to reduce current right?

[eazye1334]

Ok good, that should eliminate that problem.

Yes, that would be my next suggestion. Based on whatever the makeup of your solution, the 1.9 mA you used may be too high of a current for the size of the piece you are plating. If your solution is cyanide based, for example, that would be too high. If you solution is acid based, then that current would actually be quite low.

So yeah, try a lower current. The only thing a lower current is going to do to you in this case is slow down the plating speed, so you'll need to plate longer to reach your target thickness. But if what you have is a burning problem, then this might help.

[Enthalpy]

Please take my message with caution as I have NOT tried by myself. Book knowledge only.

Electrodeposition is said give naturally irregular finishes because the current concentrates at salient locations, and this accentuates the spikes. As opposed, erosion tends to polish layers since it acts more on spikes.

A combination can deposit a smooth layer. It alternates:
- for a longer time a small current density that deposes the metal
- and for a short time a big current density that erodes the layer

Because the effect of current density is more than proportional, a combination of densities and durations can be found that obtains a net deposition AND polish.

A smooth layer is possible under DC conditions; it needs a high bath temperature to combat the current concentration by the spikes, and a small electric field hence current density.

[lodgezhao]

Ok good, that should eliminate that problem.

Yes, that would be my next suggestion. Based on whatever the makeup of your solution, the 1.9 mA you used may be too high of a current for the size of the piece you are plating. If your solution is cyanide based, for example, that would be too high. If you solution is acid based, then that current would actually be quite low.

So yeah, try a lower current. The only thing a lower current is going to do to you in this case is slow down the plating speed, so you'll need to plate longer to reach your target thickness. But if what you have is a burning problem, then this might help.

Thank you eazye1334. My electrolytes is base Sodium gold sulfite. I tuned the current down to 0.2mA but it still gave bad patterns. Should I try lower current? Here is an image of of my pattern, the black dots are electrodeposited gold:[img=http://s23.postimg.org/5jxvjt0nr/image.jpg]

[lodgezhao]

Please take my message with caution as I have NOT tried by myself. Book knowledge only.

Electrodeposition is said give naturally irregular finishes because the current concentrates at salient locations, and this accentuates the spikes. As opposed, erosion tends to polish layers since it acts more on spikes.

A combination can deposit a smooth layer. It alternates:
- for a longer time a small current density that deposes the metal
- and for a short time a big current density that erodes the layer

Because the effect of current density is more than proportional, a combination of densities and durations can be found that obtains a net deposition AND polish.

A smooth layer is possible under DC conditions; it needs a high bath temperature to combat the current concentration by the spikes, and a small electric field hence current density.

Yep smaller and longer fwd current and strong flash of rev current is the parameter I used. The only thing is, I have to make sure the fwd current is low enough...[img=http://s23.postimg.org/5jxvjt0nr/image.jpg]

[eazye1334]

Thank you eazye1334. My electrolytes is base Sodium gold sulfite. I tuned the current down to 0.2mA but it still gave bad patterns. Should I try lower current? Here is an image of of my pattern, the black dots are electrodeposited gold:[img=http://s23.postimg.org/5jxvjt0nr/image.jpg]

No, I wouldn't try any lower. Now knowing that you're using the proprietary sulfite bath, your current density is fine, even at the earlier level. Recommended current density for your setup is 1-10 amps/sq foot, which is exactly the two currents you have tried. It seems like current isn't your problem, and the picture backs that up as that's not really "burning" as far as I can tell.

Some other ideas:

Check your pH, this can have a profound effect on your plating abilities.

Are you filtering the bath while plating? I'm wondering if the problems you're seeing are from some unwanted contamination.

[Enthalpy]

What about increasing the current density during the reverse polarity period?

And I've read nothing about the temperature of the bath. It's said to be important.