Combination of aqueous baths for electroless gold deposition

Details Combination of aqueous baths for electroless gold deposition Combination of aqueous baths for electroless gold deposition

1993-03-25

1994-06-14

Klemanski, Helene

Compositions: coating or plastic

Coating or plastic compositions

Metal-depositing composition or substrate-sensitizing...

106 126, C23C 1831, C23C 1818

Patent

active

053206673

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a combination of aqueous baths for the electroless deposition of gold and the use of this combination.
The use of chemical-reductive metallization baths for the production of gold films is generally known (Goldie: `Gold as a Surface; Technical and Decorative Gold Plating`, 1983). Gold baths without an external current which contain the disulfitoaurate(I) anion are known from the publications DD-PS 150 762, DD-PS 240 915 or DD-PS 244 768. Furthermore, gold baths without an external current which contain potassium dicyanoaurate(I) are described in the publications DD-PS 273 651 or U.S. Pat. No. 3,506,462. Problems with gold plating occur when depositing on nickel-film surfaces. Either no deposition takes place on nickel or nickel alloy surfaces (U.S. Pat. No. 1,322,203) or it occurs in the form of a cementation deposition (U.S. Pat. No. 3,123,484) Which permits only the deposition of thin gold films (DD-PS 265 915). A further disadvantage of cementation of the gold is that lengthy handling times lead to a worsening of the adhesion of the gold films.
Gold film thicknesses of 0.2-3.0 .mu.m are required for chip or wire bonding. However, the electrolytes with a high deposition rate which are indicated for this are stable only to a limited extent (Rich, D. W.: Proc. Am. Elekctropl. [sic] Soc.; 1971, p. 58 and U.S. Pat. No. 4,169,171). Frequently, alkaline-soluble photoresists or photosensitive resists are used to mask surfaces that are not to be coated during selective coating tasks. Thus, gold electrolytes that work in the alkaline range at >pH 9 cannot be used (Dettke: `Electroless Coating`; Eugen G. Leuze Verlag 1988, pp. 74-78). Similarly, a chemical-reductive gold bath with a high deposition rate is described in DD-PS 263 307. The disadvantage of this bath, however, is that a simultaneous cementative deposition process takes place.
The task which forms the basis of the present invention is to achieve a stable deposit of adherent, bondable gold film of >0.2 .mu.m with particularly short deposition times.
According to the invention, this task is accomplished by a gold-electrolyte combination which consists of a pre-coating bath containing the disulfitoaurate(I) anion [Au(SO.sub.3).sub.2 ].sup.3- as a gold salt, an alkali metal sulfite and/or ammonium sulfite (SO.sub.3).sup.2- as a stabilizer, and a reducing agent, and a primary bath containing the dicyanoaurate(I) anion [Au(CN).sub.2 ].sup.- as the gold salt, thiourea or its derivatives as stabilizers, and cobalt(II) salts, and, optionally, a complexing agent and, optionally, a nickel(II) salt.
The gold electrolyte contained in the pre-coating bath, disulfitoaurate(I), can be used in the form of its ammonium or alkali metal salts. Aldehyde and aldehyde-sulfite adducts are, for example, suitable as the reducing agents in the pre-coating bath. Formaldehyde (methanal) and the formaldehyde/sodium sulfite adduct (Rongalit.RTM.) are especially suitable. The pre-coating bath may contain 0.3-2.0 g/l of gold as Na.sub.3 Au(SO.sub.3).sub.2, 3-15 g/l of Na.sub.2 SO.sub.3, 0.05-1.0 g/l of ethylenediamine, 0.1-0.5 g/l of formaldehyde, 10-25 g/l of NH.sub.4 Cl, and 5-25 g/l of a salt of a hydroxycarboxylic acid.
If required, a complexing agent, such as ethylenediamine, can also be added to the pre-coating bath.
The gold anion used in the primary bath, dicyanoaurate(I), can also be used in the form of its ammonium or alkali metal salts. Thiourea or its derivatives are added to the primary bath as stabilizers, for example thiosemicarbazide. Cobalt(II) salts, for example cobalt(II) halides, cobalt(II) sulfate, cobalt(II) nitrate, cobalt(II) formate or cobalt(II) acetate are added to the electrolyte as an essential component of the primary bath.
If required, a nickel salt, for example a nickel(II) halide, with nickel(II) chloride being especially advantageous, can be added to the primary bath in order to increase the rate of deposition. The primary bath may contain 3-10 g/l of KAu(CN).sub.2, 5-25 g/l of thiourea, 10-30 g/l of CoCl.sub.2.6H.sub.2

REFERENCES:
patent: 4978559 (1990-12-01), Iacovangelo
patent: 5035744 (1991-07-01), Nishiyama et al.
patent: 5130168 (1992-07-01), Mathe et al.

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