Electroplating Recommendations for a new and independent setup

ElectroplatingElectroplating

Recommendations for a new Recommendations for a new and independent setupand independent setup

Current SetupCurrent Setup

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DC POWER SUPPLY

Pura Gold 401 Solution

Copper electrode

Glass beaker

Plastic sample plate

Gold electroplated copperholds down sample

Sample (e.g. InGaAs)

Platinum electrodeHot Plate (w/ stirrer)Temperature = 60°C


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DC POWER SUPPLY

Pura Gold 401 Solution

Hot Plate (w/ stirrer)Temperature = 60°C

Contents: •Distilled water (85%)•Non-hazardous salts (20%)•Potassium Gold Cyanide (2%)•Thallium (0.1%)•Other soluble compounds

Manufacturer: Ethone-OMI Inc. New Haven, CT


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DC POWER SUPPLY

Copper electrode


Copper used because:

•Good electrical conductor

•Relatively inexpensive(?)


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Glass beaker


Glass used because:

•Handles heat well

•Inexpensive


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Plastic sample plate


Plastic used because:

•Does not dissolve in bath

•Insulator, so gold does not adhere


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Gold electroplated copperholds down sample


•Gold from solution adheres to copper (wasteful), so keep sample as close to surface as possible

•Transfers (-) polarity to sample


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Sample (e.g. InGaAs)


•Desired plating thickness: 9 µm

•Time in bath: ~90min


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Platinum electrode


•Electrically conductive

•Won’t dissolve in bath

Current Setup Current Setup ParametersParameters

Temperature: 60±10°C

•Direct current: ~3mA

•Negative photoresist

Photoresist bake ~1hrHeat solution ~ 1hrPlating ~1.5hrLiftoff ~8 hours

Total Time: ~3 hours attended plus overnight unattended

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Current Setup Current Setup PROBLEMSPROBLEMS

Cyanide highly toxic and environmentally unfriendly

Cyanide eats under photoresist if not baked properly, causing plating in undesired areas

Negative photoresist required, meaning long liftoff times

Evaporation makes replenishing solution frequently necessary

Copper holder too large for tiny sample

Gold shrinks during cooling at different rate from semiconductors, putting strain on sample

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Possible New Strategies

Replace Platinum anode with Titanium or Cadmium coated with Platinum (if this is not currently the case), reducing cost

Alternative plating materials Cyanide vs. non-cyanide baths Electroless plating Back-side plating Computerized process

Alternative plating materials

Important factors: Thermal Conductivity

Plating designed for cooling Adhesion

Must adhere well to III-V compounds in samples and copper in heat sink

Density Denser plating means better properties

Thermal Expansion Coefficient When cooling occurs, a TEC different from that of

the plated material causes stress on the system


Thermal Adhesion Density Thermal Applications

Material conductivity to III-V at 20degC expansion coeff. list includes

(cal/cm3/°C/s @20°C) Semiconductors (g/cm3) (x10-6/°C) electronics?

Gold (Au) 0.71 ? 19.32 14.2 Y

Copper (Cu) 0.94 ? 8.96 16.5 N

Silver (Ag) 1 ? 10.49 19.7 N

Nickel (Ni) 0.22 ? 8.9 13.3 Y

Zinc (Zn) 0.27 ? 7.133 39.7 N

Aluminum (Al) 0.53 ? 2.699 23.9 ?

Pallladium (Pd) 0.17 ? 12 11.8 ?

From Graham, A. Kenneth, ed. Electroplating Engineering Handbook. New York: Van Nostrand Reinhold Co. 1971. p12-15.


Thermal Adhesion Density Thermal Applications

Material conductivity to III-V at 20degC expansion coeff. list includes

(cal/cm3/°C/s @20°C) Semiconductors (g/cm3) (x10-6/°C) electronics?

Gold (Au) 0.71 ? 19.32 14.2 Y

Copper (Cu) 0.94 ? 8.96 16.5 N

Silver (Ag) 1 ? 10.49 19.7 N

Nickel (Ni) 0.22 ? 8.9 13.3 Y

Zinc (Zn) 0.27 ? 7.133 39.7 N

Aluminum (Al) 0.53 ? 2.699 23.9 ?

Pallladium (Pd) 0.17 ? 12 11.8 ?

From Graham, A. Kenneth, ed. Electroplating Engineering Handbook. New York: Van Nostrand Reinhold Co. 1971. p12-15.


Thermal Applications

Material expansion coeff. list includes

(x10-6/°C) electronics?

Gold (Au) 14.2 Y

Copper (Cu) 16.5 N

Silver (Ag) 19.7 N

Nickel (Ni) 13.3 Y

Zinc (Zn) 39.7 N

Aluminum (Al) 23.9 ?

Pallladium (Pd) 11.8 ?

From http://www.ioffe.rssi.ru/SVA/NSM/Semicond/

Thermal expansion coefficients of commonly-used semiconductors (x10-6/°C):

AlAs: 5.20

GaAs: 5.73

InAs: 4.52

InP: 4.60

In0.53Ga0.47As: 5.09

Al0.48In0.52As: 4.85


Thermal Applications

Material expansion coeff. list includes

(x10-6/°C) electronics?

Gold (Au) 14.2 Y

Copper (Cu) 16.5 N

Silver (Ag) 19.7 N

Nickel (Ni) 13.3 Y

Zinc (Zn) 39.7 N

Aluminum (Al) 23.9 ?

Pallladium (Pd) 11.8 ?

Thermal expansion coefficients of commonly-used semiconductors (x10-6/°C):

AlAs: 5.20

GaAs: 5.73

InAs: 4.52

InP: 4.60

In0.53Ga0.47As: 5.09

Al0.48In0.52As: 4.85

Gold – The Strategies

From “Some Recent Developments in Non-Cyanide Gold Plating for Electronics Applications,” p2

Cyanide Electroplating Baths

Alkaline pH ~11, excess of free cyanide Sacrificial gold anode Incompatible with many elements of microelectronics, degrades

photoresist Acid

pH ~4 with aid of citrate buffer Anode is platinized titanium or gold Low current densities required, slowing process Hydrogen gas = unwanted byproduct

Neutral pH ~7 Anode is platinized titanium or gold Relatively low current density (2-5mA/cm2), 60-70°C Au(III) = unwanted byproduct (creates process control problem)

From Modern Electroplating p205-213.

Cyanide vs. Non-Cyanide

Gold Cyanide complex = more stable More widely used and available Health/safety concerns Lower plating efficiency Incompatible with positive photoresists Residual stress can be controlled in non-

cyanide baths But this uses Thallium, a health hazard

From Modern Electroplating p213-14.

Non-Cyanide Baths

Thiosulfate [Au(S2O3)2]3-

Has never been used for a practical bath Stability constant = 1026

Thiosulfate ion itself is unstable

Sulfite: [Au(SO3)2]3-

Unstable (stability constant = 1010, as compared with 1039 for Cyanide-based [Au(CN)2]-)

Stabilizing additives required Additives fortunately allow lower pH levels,

increasing compatibility with photoresists

From “Some Recent Topics in Gold Plating for Electronics Applications,” p4

Non-Cyanide Baths

Thiosulfate-Sulfite “Mixed Ligand” Bath Highly stable even without stabilizers added Gold deposit contains sulfur as an impurity

element, increasing hardness Sulfur content should be minimized Advantage: pH = 6.0, making it better for

standard, positive photoresists

From “Some Recent Topics in Gold Plating for Electronics Applications,” p5

Non-Cyanide Baths

Both Sulfite and Thiosulfate-Sulfite baths now available commercially Sulfite: Aurofab BP Thiosulfate-Sulfite: ECF60mod

Advantages: Reduction in toxicity Compatibility with positive photoresists possible More freedom with material of anode

Sulfite bath appears to be more widely used currently

Electroless Plating

Specifically “Autocatalytic processes,” or “Chemical reduction plating”

Eliminates need for DC source, electrodes Reduces cost

Significant pretreatment required Not as much control over thickness Best Plating rate: 1.5µm/h

Resulting time in bath = 6 hrs

From “Some Recent Developments in Non-Cyanide Gold Plating for Electronics Applications,” p6-10

Reducing Strain – Back-side Processing Theoretically, equal and opposite strain from

each side cancels out I was unable to find record of back-side

plating being used for strain reduction Likely reduces adhesion, gold may detach

Perhaps the selective nature of the frontal plating makes equality of strain difficult

x

Computerized process

Robotic arm performed functions as early as 1982

Advantages: Assured uniformity in plating thickness Researcher need not be present(?)

Disadvantages: Cost increase Space issues

Available primarily for large-scale, non-research operations

Recommendations from Igor

Reference Electrode Plastic lid (e.g. polystyrene) should be

used to prevent evaporation Better, smaller sample holder Greater thickness control (how?)

Deposit gold in troughs just below ridges Better heat conduction

Sources

Journals: Green, T.A., Liew, M.J., and Roy S. ,”Electrodeposition of Gold from a Thiosulfate-Sulfite Bath

for Microelectronic Applications” Journal of The Electrochemical Society. v150 n3 C104-C110. 2003.

Holliday, R. and Goodman, P. “Going for Gold.” IEEE Review. May 2002. p15-19. Kato, Masaru and Okinaka, Yutaka. “Some Recent Developments in Gold Plating for

Electronics Applications.” ??? p1-15. Okinaka, Y. and Hoshino, M. “Some Recent Topics in Gold Plating for Electronics Applications.”

Electrochemical Technology Applications in Electronics. Proceedings of the Third International Symposium (Electrochemical Society Proceedings Vol.99-34), 2000, p 132-44.

Wang, K., Beica, R., and Brown, N. “Soft gold electroplating from a non-cyanide bath for electronic applications.” IEEE/CPMT/SEMI 29th International Electronics Manufacturing Technology Symposium. 2004. p 242-6.

Young, E., et. al. “Characterization of Electroplated Gold for Back-Side Processing of GaAs Wafers.” 2002 GaAs MANTECH Conf. Digest of Papers. 2002. p180-3.

Osaka, T., Okinaka, Y., and Kato, M. “Non-cyanide electrolytes for electroytic and electroless gold deposition processes.” ???.

Books: Graham, A. Kenneth, ed. Electroplating Engineering Handbook. New York: Van Nostrand

Reinhold Co. 1971. Kanani, Nasser. Electroplating: Basic Principles, Processes, and Practice. Oxford: Elsevier

Ltd. 2004. Schlesinger, M. and Paunovic, M. Modern Electroplating: Fourth Edition. New York: John Wiley

& Sons, Inc. 2000.

Note: the Okinaka sources each present some unique information, but they do have a great deal of overlap.

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Electroplating Recommendations for a new and independent setup