Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-12-21
2004-09-28
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S646000, C428S672000, C428S935000
Reexamination Certificate
active
06797409
ABSTRACT:
TECHNICAL FIELD
A layered composite material comprised of layers of an alloy and a process for producing the layered composite material.
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 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 or codeposition 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.
One of the challenges with Au—Sn alloy plating baths is preventing the oxidation of Sn(II) to Sn(IV), as discussed in D. R. Mason, A. Blair and P. Wilkinson,
Trans. Inst. Met. Finish
., 52 (1974) 143. Oxidation of Sn can be minimized by using soluble Sn anodes. However, Au is deposited on the anodes unless they are isolated by semi-permeable diaphragms.
It has been reported that Au—Sn alloys containing up to 30 at (i.e. atomic) % Sn could be deposited from baths containing no free cyanide, and containing the Sn as its stannate complex formed with KOH (see E. Rau and K. Bihlimaier,
Galvanische Weissgolniederschlage, Mitt. Forschungsinst. Probierants. Edelmetalle Staatl. Hoheren Fachschule Schwab. Gmund
, 11 (1937) 59. Later claims concerning Au—Sn alloy plating, however, have been based on the use of alkaline and acid cyanide electrolytes, where Sn in many cases has been incorporated with the goal of obtaining brightening effects rather than producing deposits with significant amounts of Sn.
Several cyanide based systems have been reported (see T. Frey and W. Hempel, DE 4406434, (1995); W. Kuhn, W. Zilske and A.-G. Degussa, Ger. DE 4,406,434, Aug. 10, 1995: N Kubota, T. Horikoshi and E. Sato,
J. Met. Fin. Soc. Japan
, 34 (1983) 37; and Y. Tanabe, N. Hasegawa and M. Odaka,
J. Met. Fin. Soc. Japan,
34 (1983) 8.
Frey and Hempel developed a bright Au—Sn plating bath with a pH of 3-14, comprised of potatassium dicyanoaurate, soluble Sn(IV), potassium hydroxide, potassium salt of gluconic, glucaric and/or glucaronic acid, conductivity salt, piperazine and a small amount of As. The bath was used to plate small parts with an alloy containing 5-25 wt % Sn. Bright deposits were obtained for thicknesses greater than 0.1 &mgr;m and the solution exhibited long term stability without the use of soluble Sn anodes.
A.-G. Degussa, Ger. DE 4,406,434 teaches using potassium dicyanoaurate and tin chloride and claims a deposit composition of 8 wt % Sn and thickness of 5 &mgr;m.
Au—Sn codeposition from a cyanide system using pyrophosphate as a buffering agent was studied by Kubota et al (N. Kubota, T. Horikoshi and E. Sato,
J. Met. Fin. Soc. Japan
, 34 (1983) 37; and N. Kubota, T. Horikoshi and E. Sato,
Plating and Surface Finishing
, 71 (1984) 46. The basic formula consisted of K
4
P
2
O
7
, Kau(CN)
2
and SnCl
2
—2H
2
O. The mass transfer was investigated to clarify reaction mechanisms between monovalent Au or bivalent Sn and pyrophosphate ions, by measuring conductivity, kinematic viscosity and limiting current density of the bath components. Two pyrophosphate ions were complexed with one stannous ion, with excess pyrophosphate acting as a supporting constituent.
Tanabe et al, referred to above, obtained various Au—Sn alloy compositions by electrodeposition from cyanide baths containing HauCl
4
—4H
2
O, K
2
SnO
3
—3H
2
O, KCN and KOH. Although a linear relationship was not found between the Sn content in the bath and the Sn content in the alloy formed, a relationship was found between the two alloys which permitted formation of alloys of desired compositions. The composition of electrodeposited Au—Sn was shifted by about 10% to the Sn side in comparison with alloys at thermal equilibrium; thus exhibiting the &zgr; phase in the 25-29 at % range. AuSn, AuSn
2
and AuSn
4
were also electrodeposited.
Gold chloride electrolytes were used in the early days of Au plating, but today are employed almost exclusively in the electrochemical refining of Au. An extensive investigation of the cathodic behaviour of Au in chloride solutions has shown that the quality of the cathode deposit is strongly influenced by the relative amounts of Au(I) and Au(III) in the solution. The reduction of Au(III) chloride to the metal can be expected to involve the formation of Au(I) as an intermediate species. Under plating conditions, Au will be deposited from both the Au(III) and Au(I) species. Since Au(I) has a more positive plating potential (1.154 V) than Au(III) (1.002 V), a limiting current density for Au(I) will be reached first and it can be expected that the deposits will be of relatively poor quality, i.e., they tend to be bulky and porous. Gold fines will be present in the solution as a result of the following disproportionation reaction:
3AuCl
2
−
=2Au+AuCl
4
−
+2Cl
−
Detailed studies of the anodic and cathodic reactions have shown that the use of low temperatures and periodic interruption of the current are major factors that can contribute to reduced Au(I) concentration.
Japanese Patent JP 56 136994 to Masayoshi Mashiko describes a process carried out under alkaline conditions and employing a bath composition containing gold, tin and copper and sodium sulphite or potassium sulphite was used as a stabilizer for the gold.
Japanese Patent to S. Matsumoto and Y. Inomata, JP 61 15,992 [86 15.992], (Jan. 24, 1986) discloses a Au—Sn plating bath (pH=3-7) containing KauCl
4
, SnCl
2
, triammonium citrate, L-ascorbic acid, NiCl
2
and peptone. A 7 &mgr;m Au—Sn alloy (20±2 wt % Sn) layer was plated out on a 50 mm diameter Si wafer at 208° C. and a current density of 0.6 A/dm
2
in 30 minutes using a Pt coated non-consumable Ti anode. The stability of the bath seemed to be the weak link in this process as stability decreased dramatically when the Sn salt was added.
U.S. Pat. No. 6,245,208 (Ivey et al), issued on Jun. 12, 2001 describes a relatively stable, weakly acidic, non-cyanide electroplating solution for codeposition of Au—Sn alloys over a range of compositions, including the technologically important eutectic and near eutectic compositions. In the preferred embodiment, the solution consists of Au and Sn chloride salts, as well as ammonium citrate as a buffering agent and sodium sulphite and L-ascorbic acid as stabilizers.
Ivey et al discusses the use of both direct current and pulsed current power sources and describes relationships between Sn content and average current density, Sn content and pulsed current “ON time”, and Sn content and pulsed current “OFF time”. These relationships indicate that within certain ranges, the Sn content of the resulting Au—Sn alloy will increase with an increas
Djurfors Barbara M.
Doesburg Jacobus Cornelius
Ivey Douglas G.
Kuharch Terrence N.
Rodman & Rodman
The Governors of the University of Alberta
Zimmerman John J.
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