electroplating recommendations for a new and independent setup
TRANSCRIPT
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
Current SetupCurrent Setup
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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
Current SetupCurrent Setup
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DC POWER SUPPLY
Copper electrode
Hot Plate (w/ stirrer)Temperature = 60°C
Copper used because:
•Good electrical conductor
•Relatively inexpensive(?)
Current SetupCurrent Setup
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DC POWER SUPPLY
Glass beaker
Hot Plate (w/ stirrer)Temperature = 60°C
Glass used because:
•Handles heat well
•Inexpensive
Current SetupCurrent Setup
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DC POWER SUPPLY
Plastic sample plate
Hot Plate (w/ stirrer)Temperature = 60°C
Plastic used because:
•Does not dissolve in bath
•Insulator, so gold does not adhere
Current SetupCurrent Setup
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DC POWER SUPPLY
Gold electroplated copperholds down sample
Hot Plate (w/ stirrer)Temperature = 60°C
•Gold from solution adheres to copper (wasteful), so keep sample as close to surface as possible
•Transfers (-) polarity to sample
Current SetupCurrent Setup
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DC POWER SUPPLY
Sample (e.g. InGaAs)
Hot Plate (w/ stirrer)Temperature = 60°C
•Desired plating thickness: 9 µm
•Time in bath: ~90min
Current SetupCurrent Setup
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DC POWER SUPPLY
Platinum electrode
Hot Plate (w/ stirrer)Temperature = 60°C
•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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DC POWER SUPPLY
Hot Plate (w/ stirrer)Temperature = 60°C
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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DC POWER SUPPLY
Hot Plate (w/ stirrer)Temperature = 60°C
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
Alternative plating materials
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.
Alternative plating materials
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.
Alternative plating materials
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
Alternative plating materials
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.