Combination of Aqueous Baths for Electroless Gold Deposition

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 O, 5-15 g/l of NiCl.sub.2.6H.sub.2 O, and 10-30 g/l of an ammonium salt of a hydroxycarboxylic acid.

In order to coat nickel or nickel alloy surfaces, it can be necessary, in addition, to use a pickling solution. This pickling solution consists of a hydroxycarboxylic acid or its salts and ammonium chloride. Tartaric acid or citric acid are, for example, suitable as hydroxy acids. A pickling solution containing 1-10 g/l of a hydroxycarboxylic acid or its salts and 5-25 g/l of NH.sub.4 Cl may be used.

The pickling solution is to be used if the nickel or nickel alloy surface which is present shows high cementation during coating with gold; additional dissolution of the nickel film occurs because of the acidic electrolyte solution. Solutions of the alkali metal salts of carboxylic acids or the ammonium salts of carboxylic acids, such as sodium citrate with ammonium chloride at pH 6-8 passivate nickel surfaces or nickel alloy surfaces and nickel film surfaces. The cementation rate is reduced as a result of this.

A `gold-strike electrolyte` working in the neutral range up to a maximum pH of 8, which also contains an alkali metal salt of a carboxylic acid or an ammonium salt of a carboxylic acid and ammonium chloride, ensures adherent depositions of gold films. There is no additional dissolution of the passive nickel films (for example, selectively underneath the gold films). The leak-proof preliminary gold film protects the nickel surface from selective dissolution in the acidic primary gold electrolyte. The acidic primary gold electrolyte is additionally stabilized by the use of thiourea or its derivatives whereby, in the case of permanent stability, an acceleration of the catalytic reduction reaction--and thus a deposition rate of 2 .mu.m/h--is achieved by means of an adjusted concentration of nickel ions (nickel chloride) in the electrolyte.

The initial reaction in depositing the primary gold layer can be accelerated by a pretreatment solution in the case of additional gold plating onto gold and/or gold-plated surfaces. The pretreatment solution consists of heavy metal ions, e.g. Ni.sup.2+ ions in an acidic medium, which are adsorbed at the gold surface and change the surface potential. A 10-30 g/l NiCl.sub.2.6H.sub.2 O and 5-15 g/l CoCl.sub.2. 6H.sub.2 O pretreatment solution may be used that has been heated to 60.degree.-90.degree. C.

The invention further relates to a process for gold deposition using the electrolyte combination according to the invention and, optionally, the pickling solution.

Examples of embodiments:

The subject matter of the invention is especially well-suited to the deposition of gold films on ceramic chip carrier housings that have been made conductive. In this process, they are metallized with chemical-reductive nickel-phosphorus films and gold (2-3 .mu.m) as the external film contact system.

The nickel-phosphorus film is first treated with the following pickling solution for 30 hours at room temperature:

10 g/l sodium citrate

15 g/l ammonium chloride

Gold plating is then performed with the combination according to the invention.

During incipient alloying of the chip on the chip-bond surface, a eutectic coverage of >75% was achieved. The tear resistance of the chip was greater than 20N. With ultrasound wire bonds, high resistance levels were achieved in regard to the connecting wires (pull test 9.2.+-.1.1 cN).

A miniature printed circuit board partially covered with "Riston 3615.RTM." alkaline-soluble fixed resist is to be selectively gold-plated on existing, chemically reduced nickel-plated copper contact-film structures. The pickling solution used was the same as in Example 1. Gold plating then follows with the following combination in accordance with the invention:

The adhesive strength of the Ni.sub.x P.sub.y -Au film system amounted to 1300N/cm.sup.2 in the frontal pulling test. The au wires (.phi.20 .mu.m) which had been bonded using the Thermosonic process showed tear resistances of 9.1.+-.1.3 cN (pull test). The Au films are capable of soldering by reflow techniques (checked with the printed circuit board bending test).

A line circuit on Al.sub.2 O.sub.3 ceramic is to be built with the film system Ni.sub.x P.sub.y -galv. Cu - Ni.sub.x P.sub.y -Au. The conductor structure incl. galv. copper is first produced with the lift-off technique. After removing the photosensitive resist, the film system Ni.sub.x P.sub.y -galv. copper is coated with Ni.sub.x P.sub.y from an alkaline electrolyte and gold-plated with the combination as follows which is in accordance with the invention:

The adhesive strength of the film system amounted to 1800.+-.100N/cm.sup.2 (frontal pull test).

A Ni.sub.x P.sub.y structure is produced from an alkaline electrolyte on Al.sub.2 O.sub.3 ceramic by means of laser metallization. This structure is to be gold-plated in order to create wire bonding islands.

A special pickling solution is not used.

The gold plating is then performed with the combination in accordance with the invention.

The IC chip is attached by gluing.

The ultrasound bonding of the Ni.sub.x P.sub.y -Au film system with 25 .mu.m AlSil wires yielded adhesive strengths of 7.5.+-.0.6 cN (pull test).