Chapter 16 Electroless Plating Of Platinum Group Metals Yutaka Okinaka and Catherine Wolowodiuk

Interest in the deposition of platinum group metals and alloys arises often in connection with applications to the fabrication of electronic devices and components, and also as a means of providing corrosion protection to basis metals in various other applications. For example, electroplated palladium and its alloys have received considerable attention i n recent years as substitute materials for the gold plated on connectors and printed circuit board contacts. Physically deposited palladium and platinum are used as a barrier layer t o prevent interaction between the first layer (e.g., Ti) and the top layer of gold i n multilayer conductors used on semiconductor devices and circuit boards. There is also an interest in palladium as a material for forming metallic contacts on compound semiconductors such as GaAs and InP. Ruthenium is another metal known to be suitable as a contact finish for certain applications. The use of plated rhodium as a wear-resistant decorative finish is well known. Platinum group metals are also known for their catalytic activities for numerous chemical reactions, and the catalysts containing those metals have been deposited from solutions. The principle of electroless plating would be of interest i n all of the above applications. However, as far as the authors are aware, there is no established industrial application of electroless plating of platinum group metals. A survey of the literature shows that development activities i n this area are inconspicuous at present. On the other hand, some commercial baths have recently become available for plating palladium and rhodium. This chapter reviews those electroless (autocatalytic) processes described in the literature for plating platinum group metals and alloys. Displacement processes and specialized deposition techniques such as those developed for catalyst manufacture will not be covered in spite of the fact that these processes often are also called "electroless". An extensive coverage will be given to each known autocatalytic process, including listings of essential bath compositions and operating conditions in the hope that the reader can experiment with the various processes based on the information found i n this chapter alone. The following metals and alloys will be covered: Pd, Pd-P, Pd-B, Pd-Ni-P, Pd-Co-P, Pd-Zn-P, Pt, Pt-Rh, Pt-lr, Pt-Pd, Ru, and Rh. 421

ELECTROLESS PLATING

422

PURE PALLADIUM, PALLADIUM-PHOSPHORUS, AND PALLADIUM-BORON Reducing agents used for electroless plating of palladium include hydrazine, hypophosphite, amine borane, and formaldehyde. Deposits produced with hypophosphite or amine borane contain 1-3 percent by wt of phosphorus or boron, but they will be discussed in this section. Alloys with other elements will be treated in the next section.

Hydrazine Baths Rhoda (1) The first electroless palladium plating bath was developed by Rhoda in 1958 using hydrazine as the reducing agent. Two bath formulations are given in Table 16.1. Palladium is used in the form of tetraammine chloride, Pd(NH,)&I:, in these examples, but Rhoda states that amine complexes can be used as well. The solution of the tetraammine complex can be prepared either by dissolving palladium diammine chloride, Pd(NH3)GI?,in dilute ammonia, or by adding a solution of PdCL (e.g., 50 g/L Pd in 2M HCI) to dilute ammonia and heating until the precipitate that forms initially dissolves. The overall plating reaction is believed to be represented by 2Pd(NH3);' + NzH4 + 40H-

-.2Pd + 8NH3 + NZ + 4H20

[I1

The plating rate increases linearly with temperature between 40 and 80°C from 3.8 to 14.7 pm/hr. The EDTA salt is added as a stabilizer. Without EDTA, the bath decomposes spontaneously when the temperature exceeds 70°C. If the

Table 16.1 Hydrazine Bath for Pd (1) (Rhoda)

Pd(NHl),CI?, g/L as Pd Na?EDTA, g/L NH,OH, g/L Hydrazine, g/L Hydrazine ( l M ) , mL/hr Temperature, " C Plating rate, pm/hr Plating area, cm'/L

Bath A"

Bath B

5.4 33.6 350 0.3

7.5 8.0 280

-

-

8

80 25.4

3525 0.89 1000

100

'Bath A for rack plating, Bath B for barrel plating

Electroless Plating of Plafinurn Group Metals

423

bath is allowed to stand idle at the operating temperature, the plating rate decreases rapidly with time. For example, the initial rate of 15 pm/hr decreases to 3.8 pm/hr in two hours, and practically to zero in four hours. This phenomenon is attributed to catalytic decomposition of hydrazine by palladium. The deposit is reported to be at least 99.4 percent pure. It has a density of 11.96 g/cm’ and an average Knoop hardness of 257, with a range of 150to 350 at a 25 g load. The hardness depends on the plating rate, with the higher rates producing the softer and more ductile deposits. Rhoda states that good deposits were obtained on AI, Cr, Co, Au, Fe, Mo, Ni, Pd, Pt, Rh, Ru, Ag, steel, Sn, W, graphite, carbon, and properly activated glass and ceramics. Copper and its alloys were plated with palladium after first plating with a catalytic metal such as a displacement-type gold.

Kawagoshi (2) Bath instability is aserious drawback of theoriginal Rhodaformulation. In order to obtain an improved stability, Kawagoshi added a small amount of thiourea. The bath composition given in Table 16.2 yields 3 g of palladium deposit on 1 m’ of a steel substrate in one minute at 80°C. This result corresponds to a plating rate of approximately 16 prn/hr.

Table 16.2 Hydrazine Bath for Pd (2) (Kawagoshi) PdCI?,g/L NaZDTA. g/L NaXO,, g/L

NHIOH (28% NHI). mL/L Thiourea, g/L Hydrazine, g/L Temperature, “ C Plating rate

5 20 30 100

0.0006 0.3 80 0.26pm/min

Laub and Januschkowetr (3) The baths developed by these workers contain both a stabilizer and an accelerator. As the stabilizer, a mercaptoformazan with the general formula /NH-NHR

s=c

\N=NR is used. Examples are N,N‘-di-p-naphthyl-C-mercaptoformazan, N,N’-di-mcarboxyphenyl-C-mercaptoformazan, N,N‘-diphenyl-C-mercaptoformazan,

424

ELECTROLESS PLATING

and N,N'-p-phenyl-sulfonic acid-C-mercaptoformazan. As the accelerator, a water-soluble benzene derivative with two or three functional groups is added. The following compounds are listed as examples: 3,4-dimethoxybenzoic acid, 2-hydroxy-3-methylbenzoic acid, 2-hydroxy-4-methylbenzoic acid, 2-hydroxy4-methoxybenzoic acid, 3-hydroxy-4-methylsulfonic acid, and 2-amino-5-oxosulfonic acid. Palladium is added to the baths as Pd acetate, PdCI, or Pd(NHI)J(NO?):with a carboxylic acid and ammonia as complexing agents. Either hydrazine or sodium hypophosphite can be used as the reducing agent. Two examples of bath compositions and operating conditionsare listed in Table 16.3. Both baths apparently behave similarly under identical operating conditions. The patent states that pure, bright deposits are obtained on Fe, Ni, Co, Cu, Cu-alloys, AI, Ag, Au, Pt, and activated plastic materials. It is indicated that the baths must be replenished periodically during operation to maintain the given compositions. In spite of the improvement brought about by the addition of stabilizing agents, the hydrazine baths suffer from a serious drawback: the plating rate decreases rapidly during bath operation to a much greater degree than expected from the depletion of palladium in the bath, because of the catalytic decomposition of hydrazine itself. Hypophosphite baths do not have this problem and have been investigated more extensively, as described in the next section.

Table 16.3 Hydrazine or HypophosphiteBath for Pd (3) (Laub and Januschkowetz)

Pd acetate, g/L PdCI:, g/L NH, acetate, g/L Na succinate, g/L NH,OH (25% NH,), mL/L N,N'-di-o-tolyl-Crnercaptoforrnazan. g/L N,N'-di-p-phenylsulfonic acidC-mercaptoformazan, g/L 3,4-dirnethoxybenzoic acid, g/L 2-amino-5-oxo-sulfonic acid, g/L Hydrazine hydrate, 8%, mL/L Na hypophosphite. g/L PH Temperature, " C Loading, drn'/L Plating rate, pm/hr

Hydrazine Bath

Hypophosphite Bath

8.5

-

-

7

70

-

-

100

75 100

0.02 -

10

0.015

-

-

10

10

-

-

15 8.5-9.0 70 5 15-20

8.5-9.0 65-70 5 15-20

Electroless Plating of Platinum Group Metals

425

Hypophosphite Baths Sergienko (4) Sergienko was the first to disclose the composition of a hypophosphite bath for electroless palladium plating. An example of specific bath formulation and operating conditions is given in Table 16.4. EDTA and ethylenediamineare said to serve as stabilizers (in addition to forming complexes with palladium) because metallic impurities are also complexed with these agents, minimizing deposition of such impurity metals. The following specific instruction is given in the patent for proper preparation of the bath-"Add PdCI:, ethylenediamine, and disodium EDTA to water and dissolve the solids. Permit the solution to stand until chelation is complete. Chelation can beaccomplished in approximately24 hours if the temperature of the solution is maintained at approximately 160" F (71°C). After chelation is complete, cool the solution to 20°C, add sodium hypophosphite, and adjust the pH to 8.5. Heat the bath through a water jacket to 71°C for plating." The bath can be used, stored, and reused. It can be replenished with PdCI:, which may be added directly to the bath, and with acid. No information is given on plating rate or deposit properties.

Table 16.4 Hypophosphite Bath for Pd (4) (Sergienko) PdCIz, g/L NazEDTA, g/L Ethylenediamine, g/L NaHzP02.g/L pH at 20°C (with HCI) Temperature, " C

10.0 19.0 25.6 4.1

8.5 71

Pearlstein and Weightman (5,6) Independently of Sergienko (4), these investigators developed a hypophosphite bath using ammonia as the complexing agent and made a detailed study of the process. Palladium is present in this bath as palladous ammine complex, which is prepared by adding slowly, with agitation, a stock solution of PdCI: (20 g dissolved in 40 mL of 38% HCI and the solution diluted to one liter) into an appropriate volume of NH,OH. This solution is allowed to stand at room temperature for at least 20 hours and filtered befor? use. Ammonium chloride is added to stabilize the bath for extended use, although it lowers the plating rate. Hypophosphite is added last, and the bath is brought to final volume. A recommended bath composition is given in Table 16.5. Behavior of this bath is well documented in the original publications (5,6).The effect of Pd concentration on deposition rate was determined by depleting the

426

ELECTROLESS PLATING

Table 16.5 HypophosphiteBath for Pd (5) (Pearlstein and Weightman) PdCI?, g/L HCI (38%), mL/L NHIOH (28% NHI), mL/L NHaCI. g/L NaH:PO:.H:O, g/L PH Temperature, " C Plating rate

2 4 160 27 10

9.8+2.0 50-60 2.5 pm/hr

bath up to 90 percent. The rate decreased almost linearly with the extent of depletion. When the bath was replenished by addition of Pd-ammine complex, the rate was restored almost to the original value, even without addition of hypophosphite. The determination of hypophosphite utilization efficiency showed that only about 31 percent is consumed for the reduction of palladium and the remainder simply decomposes to evolve gaseous hydrogen from the bulk of the solution. The plating rate increases with temperature and hypophosphite concentration up to 60°C and 20 g/L, respectively, above which the bath becomes unstable. Electroless palladium plates spontaneously on copper, brass, gold, steel, or electroless nickel, but there is an initial incubation period ranging from 20 sec to 1.5 min, depending on the substrate. Pretreatment with a mixture of 0.1 g/L PdClz and 0.5 mL/L HCI (38%) for 30 sec at room temperature followed by DI water rinse reduces the incubation time. The plating is almost instantaneous on freshly plated nickel or non-metallic surfaces activated by using a SnCI2-PdCl2process. The palladium deposit produced by using this bath has a hardness of approximately 165 kg/rnm' on the Vickers scale. The existence of stresses in the deposit was revealed in a microscopic observation of crosssectionsof thedeposit showing acrack pattern. It should be noted that the deposit produced by this process contains about'1.5 percent phosphorus. The Pearlstein-Weightman bath was used by Hsu and Buxbaum (7) to coat zirconium with an adherent, 5-lm-thick palladium deposit. Zirconium, which is an important metal used in the nuclear and chemical industries, often requires a coating of thin metal films for corrosion protection or for modification of surface properties. A series of special pretreatment steps, which are required to produce deposits without flaking, peeling, or blistering, are described in the original article (7). Pearlsteinand Weightman alsoobtained various palladium alloys by operating the same basic bath with the addition of suitable alloying elements, as will be discussed in a subsequent section.

Electroless Plating ol Platinum Group Metals

427

Zayats et ai. (8) These investigators found that the addition of a small amount of thiosulfate is a very effective way of stabilizing the bath. Within a limited range of thiosulfate concentration, the bath yields, at otherwise identical conditions, a plating rate much greater than that obtained with the additives used in the two baths described above. A recommended bath composition is given in Table 16.6. The thiosulfate concentration must be controlled carefully, because when it is