Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
1999-04-13
2001-06-12
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S247000
Reexamination Certificate
active
06245208
ABSTRACT:
FIELD OF INVENTION
The present invention relates to an improved gold-tin (Au—Sn) alloy and plating bath composition for codepositing the Au—Sn alloy, a process of making the composition and to a new product produced thereby.
BACKGROUND OF THE INVENTION
Gold-tin (Au—Sn) eutectic solders are commonly used in the optoelectronic and microelectronic industries for chip bonding to dies. Au—Sn solder is classified as a “hard solder” with superior mechanical and thermal properties relative to “soft” solders, such as the Pb—Sn system. Au—Sn solder can be applied in a number of ways, i.e., as Au—Sn preforms, solder paste, by sequential evaporation and sequential electrodeposition. Compared with solder preforms and pastes, evaporated solder is cleaner and provides more precise thickness and positional control. Thin film deposition technology, however, involves expensive vacuum systems.
Electroplating of Au—Sn eutectic solder is an attractive alternative in that it is a low cost process, offering the thickness and positional control of thin film techniques. Au—Sn solder layers have been produced sequentially by depositing Au first on a seed layer, followed by Sn (see for example C. Kallmayer, D. Lin, J. Kloeser, H. Oppermann, E. Zakel and H. Reichl, 1995
IEEE/CPMT International Electronics Manufacturing Technology Symposium,
(1995) 20; C. Kallmayer, D. Lin, H. Oppermann, J. Kloeser, S. Werb, E. Zakel and H. Reichl, 10
th European Microelectronics Conference
, (1995) 440; and E. Zakel and H. Reichl, Chapter 15, in
Flip
-
Chip Technologies,
ed., J. Lau, McGraw-Hill, (1995) 415).
Commercially available Au and Sn baths are utilized from which several microns of solder can be deposited sequentially. Co-electrodeposition of Au and Sn from a single solution offers the same economic advantage of sequential plating relative to vacuum deposition techniques, as well as the prospect of depositing the solder in a single step without oxidation of an outer Sn layer.
The technology for Au and Sn plating is quite well developed and will be briefly reviewed here.
Au Electrodeposition
Electrodeposition of soft Au on electronic devices and componenets is generally performed using a bath containing cyanoaurate (I) ions, because Au cyanide complexes have the highest stability coefficients. Free cyanide ions generated as a result of the Au deposition process attack the interface between the resist film and substrate, lifting the resist and depositing extraneous Au under the resist. Because of this incompatibility, work has focused on developing non-cyanide baths.
Au(I) sulphite complexes have better compatibility towards positive resists and the added benefit of improved throwing power and deposit thickness uniformity compared with cyanide baths. In addition, deposits from Au sulphite solutions are bright, hard and ductile. The Au(I) sulphite complex is subject to a disproportionation reaction, however, forming Au(III) and metallic Au, which causes the bath to decompose spontaneously on standing.
3[Au(SO
3
)
2
]
3−
=2 Au+[Au(SO
3
)
4
]
5−
+2 SO
3
2−
To prevent decomposition, a suitable stabilizing additive is needed.
The first commercial sulphite Au plating solutions were developed in the early to mid 1960s. The sulphite ion is itself in equilibrium with sulphur dioxide according to
SO
3
2−
+H
2
O=SO
2
(g)+2 OH
−
Because the above reaction forms hydroxyl ions, the equilibriun is pH-dependent. Most commercial solutions operate in the alkaline pH range, i.e., at pH values above9.5. When Au is plated out of solution at alkaline pH, the excess sulphite remains and can be oxidized to sulphate at the anode.
There have been several attempts to reduce the operating pH to below neutral for applications involving alkaline-developable photoresists (see for example A. Gemmler, W. Keller, H. Richter and K. Ruess,
Plating and Surface Finishing,
81 (1994) 52; R. J. Morrissey and R. I. Cranston, U.S. Pat. No. 5,277,790, Jan. 11, 1994; R. J. Morrissey,
Plating and Surface Finishing,
80 (1993) 75; and T. Osaka, A. Kodera, T. Misato, T. Homma, Y. Okinada and O. Yoshioka,
J. Electrochem. Soc.,
144 (1997) 3462).
The addition of organic polyamines, such as ethylenediamine, can be used to lower the pH to acidic values, allowing controlled evolution of sulphur dioxide to remove a portion of the excess sulphite (U.S. Pat. No. 5,277,790; R. J. Morrissey,
Plating and Surface Finishing
(above); and A. Meyer, S. Losi and F. Zuntini,
Proc. Fachtagung. Galvanotachnik,
Leipzig (1970), Swiss Patent 506,828 (1969)).
The possibility of electroplating soft Au from a non-cyanide bath containing both thiosulphate and sulphite as complexing agents has been explored (see for example T. Osaka, A. Kodera, T. Misato, T. Homma, Y. Okinada and O. Yoshioka,
J. Electrochem. Soc.,
144 (1997) 3462; T. Inoue, S. Ando, H, Okudaira, J. Ushio, A. Tomizawa, H. Takehara, T. Shimazaki, H. Yamamoto and H. Yokono, Proceedings of IEEE 45th Electronic Components and Technology Conference, May 21-24, 1995; and M. Kato, Y. Yazawa and Y. Okinaka,
International Technical Conference Proceedings,
American Electroplaters and Surface Finishers Society, (1995) 813).
The bath reported by Osaka et al. operates at a pH of 6.0 and a temperature of 60° C. The bath is reported to be stable, although no specific stability data has been given. Three different Au complexes can exist in this system.
Au
+
+2 SO
3
2−
=[Au(SO
3
)
2
]
3−
&bgr;=10
10
Au
+
+2 S
2
O
3
2−
=[Au(S
2
O
3
)
2
]
3−
&bgr;=10
26
Au
+
+SO
3
2−
+S
2
O
3
2−
=[Au(SO
3
)(S
2
O
3
)]
3−
&bgr;=unknown
&bgr; is the stability coefficient for the complex. Thallium(I) ions have been added in the form of Tl
2
SO
4
as a grain refiner to improve the surface morphology of the deposit.
Phosphates, carbonates, acetates and citrates are commonly used as buffering and conducting agents for Au plating baths. In alkaline Au sulphite baths, metals such as Cd, Ti, Mo, W, Pb, Zn, Fe, In, Ni, Co, Sn, Cu, Mn and V in various concentrations are used as brightening additives, while Sb, As, Se and Te semi-metals are also used.
Sn Electrodeposition
There are 2 types of Sn plating solutions: alkaline and acidic (see A.C. Tan, Chapters 8-10, “Tin and Solder Plating in the Semiconductor Industry”, Chapman and Hall (1993)).
Alkaline solutions are based on sodium or potassium stannate. Hydrogen peroxide or sodium perborate is used to oxidize any stannite (bivalent Sn) to the stannate form. Alkaline baths are superior to acid baths in throwing power.
Acidic plating baths contain Sn in the bivalent form, using metal salts that are sulphates, fluoroborates and fluorosilicates. Electrodeposition of Sn from a stannous Sn solution has the obvious advantage of consuming less electricity (half the amount at 100% efficiency) compared with a stannate bath. The problems with acidic baths include poor throwing power and solution instability, with basic tin compounds precipitating on standing. Various additives, including gelatin, glue, cresol sulphonic acid and aromatic hydroxyl compounds, have been used to improve plating quality. When an acidic bath ages, the bath may change colour to darker yellow and may also become turbid. The actual chemistry of this change is relatively poorly understood, but is attributed to the formation of stannic compounds when stannous Sn salt is oxidized to stannic Sn in the presence of dissolved air and elevated temperature. The stannic compounds are colloidal and very difficult to remove. Oxidation of stannous Sn can be minimized by maintaining the solution temperature at 20-25° C., using an airtight plating setup and adding a suitable anti-oxidant such as a phenol compound. It has been reported that oxidation of bivalent Sn can be greatly suppressed or even eliminated by adding at least 1 organic ring compound, which has a radical group such as NH
2
or NO
2
attached in the ortho or
Ivey Douglas Gordon
Sun Wenzhen
Gorgos Kathryn
Governors of the University of Alberta
Keehan Christopher M.
Rowley C. A.
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