HCOO 2HJ 2e HCHO 3 OH pH 14 1070

Therefore, electroless copper solutions using formaldehyde as reducing agent employ high pH, above pH 12 (typical NaOH concentration is >0.1 N; theoretically 0.1 N = pH 13).

Because simple copper salts are insoluble at pH above about 4, the use of alkaline plating media necessitates use of a complexing, or chelating, component. Historically, complexing agents for electroless copper baths have almost always fallen into one of the following groups of compounds:

• Tartrate salts

• Alkanol amines, such as quadrol (N,N,N',N' tetrakis(2-hydroxypropyl)ethylenediamine) or related compounds

• EDTA (ethylenediamine tetraacetic acid) or related compounds

Glycolic acids and other amines have also been reported (Ref 7).

Tartrates were used in the earliest baths and continue to be used, particularly for low-plating-rate (< 0.5 pm/20 min), low-temperature (near ambient) applications. Tartrates are more easily waste-treatable than the other two classes of chelates, but they have not readily lent themselves to formulation of faster plating systems.

Alkanol amines came into wide use in electroless copper baths in the late 1960s, with the advent of faster plating systems. This type of chelate made it possible to achieve "high-build" (> 2 pm/20 min) electroless copper solutions, and it continues to have wide use even today. Because quadrol and its analogs are liquids, totally miscible with water, they are not easily removed from the waste solution, and hence they are resistant to many conventional waste treatment procedures.

EDTA salts are also widely used for complexing electroless copper solutions. EDTA has certain desirable characteristics versus those of quadrol, based on waste treatability. Specifically, EDTA can be more easily separated (precipitated) from waste solutions by pH adjustment. Starting in the late 1970s, bath additives for EDTA systems (see below) were developed that allowed excellent control of plating rate, grain structure, and other important factors. Because of the very high affinity of EDTA for any metal ions, even small residual amounts of dissolved EDTA can draw potentially toxic metals into the waste stream. This has led to increased legislative efforts (notably in Germany and Japan) against use of this chelate and its derivatives. However, at present, the most commonly used plating baths are based on EDTA.

Besides the copper salt, the reducing agent, the source of alkalinity, and the chelate, other important components are present in commercial electroless copper solutions. These components are generally considered the proprietary portion of the formulation, and they control such parameters as initiation and plating rate, stability (versus dragged-in catalyst; versus excessively high bath activity; versus long shutdown periods; versus Cu(I) oxide), deposit stress, color, ductility, and so on. Prior to development of well-characterized and controlled trace additives, electroless copper baths were prone to "triggering" (spontaneous decomposition of the bath), "plateout" (decomposition over a prolonged standing period), "second day startup" (inability to induce a controlled plating reaction when first stored after makeup), dark deposit color, rough deposit, coarse grain structure, and so on. Literally hundreds of papers and patents have been published relating to these additives. Useful summaries of this data are available (Ref 9, 10).

Additives that stabilize the bath against various manifestations of undesired plateout are referred to as stabilizers. Understanding their composition, mechanism, and optimal replenishment rate is key to successful operation of a bath. They are usually employed at low concentrations, typically 1 to 100 ppm. Principal among the materials reported are compounds such as mercaptobenzothiazole, thiourea, other sulfur compounds, cyanide or ferrocyanide salts, mercury compounds, molybdenum and tungsten, heterocyclic nitrogen compounds, methyl butynol, propionitrile, and so on. Pressure from environmental and regulatory groups over the years has led to near-elimination of cyanide- and mercury-type additives. It is noteworthy that perhaps the most common stabilizer for electroless copper baths is a steady stream of air (i.e., oxygen) bubbled through the solution.

Additives that increase the plating rate of the solution are variously referred to as rate promoters, rate enhancers, exhaltants, or accelerators. This last term is particularly unfortunate and confusing in view of the use of the term accelerator to describe a key process step in electroless copper processes (see the section "Processes" in this article). Materials that have been reported to function as rate promoters include ammonium salts, nitrates, chlorides, chlorates, perchlorates, molybdates, and tungstates. Rate promoters may be present in the electroless formulation at concentrations of 0.1 M or higher.

Other additives may also be incorporated in certain cases. For example, surfactants may be used to improve deposit characteristics (Ref 11), and incorporation of excess halide ion into the formulation permits elimination of the normal accelerator step (Ref 12) (see the section "Processes" in this article).

Typical examples of freshly made-up electroless copper baths are given in Table 1.

Table 1 Examples of electroless copper formulations

Low build (tartrate)

(EDTA)

Copper salt, as Cu(II)

1.8 g/L

2.2 g/L

2.0 g/L

3.0 g/L

0.028 M

0.035 M

0.031 M

0.047 M

Chelate

Rochelle salt

Quadrol

Disodium EDTA dihydrate

Disodium EDTA dihydrate

25 g/L

13 g/L

30 g/L

42 g/L

0.089 M

0.044 M

0.080 M

0.11 M

Formaldehyde, as HCHO

10 g/L

3 g/L

3 g/L

1.5 g/L

Alkalinity, as NaOH

5 g/L

8 g/L

7 g/L

3 g/L

Additives(a)

<2 g/L

<2 g/L

<2 g/L

<2 g/L

Temperature, °C (°F)

20 (70)

43 (110)

45 (115)

70 (160)

Plating rate, ^m/20 min

0.2

2.5

2

1

Note: The terms low build, high build, and full build are defined in the section "Deposit Properties" in this article.

Note: The terms low build, high build, and full build are defined in the section "Deposit Properties" in this article.

(a) Examples of additives: 2-mercaptobenzothiazole, diethyldithiocarbamate, 2,2'-dipyridyl, potassium ferrocyanide, vanadium pentoxide, nickel chloride, polyethylene glycol

The overall electroless copper plating reaction is theoretically given as:

0 0

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