US 3915717 A
Autocatalytic metal deposition baths stabilized with a vanadate, stannate or silicate.
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United States Patent Feldstein et al.
145 Oct. 28, 1975 STABILIZED AUTOCATALYTIC METAL DEPOSITION BATHS Inventors: Nathan Feldstein, Kendall Park; Joel Alan Weiner, Cranbury, both of NJ.
Assignee: RCA Corporation, New York, NY.
Filed: Nov. 12, 1973 Appl. No.: 415,113
Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. B 415,113. 1
us. 01 106/1; 117/227 Int. Cl. C23C 3/02 Field of Search 106/1 Primary Exah1inerLewis T. Jacobs Attorney, Agent, or FirmGlenn H. Bruestle; William S. Hill  ABSTRACT Autocatalytic Inetal deposition baths stabilized with a vanadate, stannate or silicate.
5 Claims, No Drawings STABILIZED AUTOCATALYTIC METAL DEPOSITION BATHS BACKGROUND OF THE INVENTION Baths for autocatalytic deposition of a number of metals have become widely used throughout the plating industry. The process of deposition is often referred to as electroless plating because no electric current is used.
One of the disadvantages of using these baths is that there is a tendency for spontaneous decomposition, that is,'uncontrolled metal reduction, to occur throughout the bulk of the solution. This unwanted bath decomposition may represent a heavy loss of materials in an industrial installation where thousands of gallons of solution may have to be discarded. Also, the unwanted decomposition may result in much plating time loss while the plating operation is stopped for a thorough clean-out. Also, the quality of the products deposited is generally unacceptable when such baths undergo decomposition.
The first visible indication of bath instability is a loss of solution clarity usually followed by ,the appearance of foam and of a finely dispersed black precipitate which comprises flakes of the metal being plated. One of the substances that can-catalyze the reduction of a metal being deposited electrolessly is the metal, itself. The enormous surface area of the precipitate acts as a very large catalytic surface. Decomposition of the bath therefore becomes autocatalytic.
It has previously been found that spontaneous bath decomposition can be inhibited by using certain bath additives in controlled concentrations. These additives appear to act selectively on the large surface area of the suspensoids rendering them non-catalytic, while only slightly reducing the rate of metal deposition on the substrate being plated rather than stopping deposition completely.
The most commonly used commercial electroless plating solutions for nickel or copper have employed stabilizers which are primarily sulfur-containing compounds. Examples of such compounds are thiourea, thioacetamide and heavy metal sulfides. However, the incorporation of sulfur-containing compounds in plating baths often results in incorporation of sulfides in the metal deposit. Sulfides are known to lower the corrosion resistance of the deposit. Also, compounds like thiourea are known to hydrolyze and hence changes in concentration take place with time, requiring specific analytical procedures for maintenance.
Cyanides have also been used as electroless plating bath stabilizers but they are consumed in some plating bath reactions (e.g., alkaline formaldehyde baths) and they are toxic.
DESCRIPTION OF PREFERRED EMBODIMENTS The present invention comprises use of soluble stannates, vanadates or silicates as stabilizers in electroless plating baths for depositing copper or cobalt, and vanadates for depositing nickel or palladium. In each case, the stabilizer must be added to the plating bath in an amount sufficient to inhibit any tendency for spontaneous decomposition but not so much as to either stop the plating process completely or slow it down to a rate that is uneconomical. The proper amount of stabilizer to add varies from bath to bath and can usually be de- EXAMPLE 1 In this Example the following electroless copper plating bath was used:
CuSO .5H O 15.0 g/liter H CO (37% concentrated) 15.0 ml/liter Tetrasodium salt of propylene diamine tetraacetic acid (40% aqueous solution) 53 ml/liter NaOH 4 g/liter Temperature of bath 25 C To the above bath, Na SiO .9l-I O was added in amounts shown in the Table below. Ceramic wafers 2 inches X 2 inches sensitized with an acidic solution of stannous chloride and activated with an acidic solution of palladium chloride were in the bath for 10 minutes. The plating rate was determined by measuring the increase in weight of each wafer at the end of the plating period.
Table l Moles/liter of Na siO Increase in wt. (mg) None (about) 50 5 x i0- 43 8 X [0 36 l X 10 25 5 X 10' 7.0 8 X 10 0.2
These results show that in this bath, under the conditions employed, apreferred amount of silicate to use as a stabilizer is somewhat less than 5 X 10 moles per liter. It will be obvious to one skilled in this art that this value will change with changes in bath reactivity. For example, increasing the temperature of operation increases bath reactivity and this requires a larger concentration of the stabilizing additive.
EXAMPLE 2 Using the same basic bath (without the silicate) and the same plating conditions as in Example 1, varying amounts of sodium stannate (Na SnO .3l-I O) were added to the plating bath with the results shown in the Table below.
Table 2 Moles/liter of Na SnO Increase in wt. (mg) Inthis bath, about 1 X 10 moles per liter of the stannate should preferably be used as a stabilizer.
EXAMPLE 3 In this example, the same basic copper bath was used as in Example 1 but the substrates were ceramic wafers that had previously been pre-plated with a continuous film of copper. No sensitizing or activating steps were thus needed. Plating time was the same as in the previous examples. Varying amounts of sodium stannate (Na SnO .3H O) were added to the bath as indicated in the Table below.
Table 3 Moles/liter of Na SnO Increase in wt. (mg) In this bath, somewhat less than 1 X 10 moles per liter of stannate should be used to stabilize the bath.
EXAMPLE 4 This Example used the same basic bath as in Example I but the substrates being plated were copper preplated ceramic wafers as in Example 3. Varying amounts of Na SiO .9I-I O were added to the bath with the results shown in the Table below.
Table 4 Moles/liter of Na SiO Increase in wt. (mg) None (about) 50 l X 10" 34 l X 10- 32 5 x to 14 1 X 10' 3 1 X 10- The results shown above indicate that somewhat more than about X moles/liter of the silicate should preferably be used to obtain the desired inhibiting effect.
EXAMPLE 5 In this example, the same basic bath and the same plating conditions were used as in Example 1. Varying amounts of sodium metavanadate were added to the bath with the results shown in the Table below.
Table 5 i Moles/liter of Na VO Increase in wt. (mg) None (about) 50 l X 10 47 3 X 10 32 7 x 10' 21 l X 10" 9 3 X 10'' 0.6
trolessly depositing nickel or palladium. An example of nickel bath stabilization is as follows:
. 6 Vanadates can also be used to stabilize baths for elec- To this bath varying amounts of sodium vanadate were added, producing the results shown in the Table below. Plating time was 10 minutes and the substrates were 2 inches X 2 inches activated ceramic wafers.
Table 6 Moles/liter of Na VO Increase in wt. (mg) 0.25 X 10 83 0.5 X 10 34 1.0 X 10 3.0 X 10 2.3
These results indicate that the amount of sodium vanadate added should preferably be about 1.0 X 10' moles/liter or somewhat more.
The following is an example of palladium deposition bath stabilization.
EXAMPLE 7 PdCl 2 g/liter HCl (concentrated) 4 ml/liter (to dissolve the PdCl NILOH 160 ml/liter NH CI 27 g/liter 3 5 NaH Po l-l o IO g/liter Temp. C.
To this bath, varying amounts of Na;,VO. were added producing the following plating results on 2 inches X 2 These results indicate that the amount of sodium vanadate that should preferably be added for effective stabilization is about 3 X 10 moles/liter or less. Orthovanadates as well as metavanadates can be used in any of the examples that include vanadates.
It has been previously found that brighter copper deposits can be obtained if both an alkali metal cyanide and a non-ionic wetting agent are included in otherwise conventional electroless copper plating baths in which chelating agents of the propylene diamine tetraacetic acid type are used. Stannates, silicates and vanadates can be included in this type of bath for additional stabilization. The cyanide also is an effective stabilizer but is used up as plating proceeds. The present stabilizers are not used up and therefore remain effective as stabilizers throughout the plating operation without replenishment.
EXAMPLE 8 The stabilizers of the present invention can also be used to stabilize electroless cobalt plating baths. In this Example, the objects plated were 2 inches X 2 inches acitvated ceramic wafers. An example of a typical electroless cobalt plating bath is as follows.
( 4)z 4 56 g/liter Na C I-I,O,.2l-I O 90 glliter (sodium citrate) Bath temperature 70 C Varying amounts of Na VO were added to the bath with the plating results indicated in the Table below. Plating time was 10 min.
Table 8 Moles/liter Na VO, Increase in wt. (mg) w er-9 9 9 000wa-m-o In this example, the preferred concentration of the vanadate is about 4-8 X 10 moles/liter. At this concentration, small changes in conditions, such as increases in bath temperature, can be tolerated without risking decomposition of the bath.
EXAMPLE 9 The Table below shows results of adding varying amounts of sodium silicate to the same cobalt bath and In this example a preferred concentration of the silicate is 5 X moles/liter.
EXAMPLE 10 Table 10 below shows results of adding varying amounts of sodium stannate to the same cobalt plating bath and using the same plating conditions as above.
Table 10 Moles/liter Na sno Increase in wt. (mg) 7.5 X 10 6.0 l X 10' 3.5 2 X 10' 0.5 4 x 10" 0 In this example, a preferred concentration of the stannate is about 7.5 X 10 moles/liter.
Although, in all of the above examples, sodium salts of the stabilizing agents have been used because of availability and low cost, the salts of any others of the alkali metals can just as readily be used.
1. In an electroless plating bath comprising a metal salt selected from the group consisting of salts of copper, cobalt, nickel and palladium, a reducing agent for said salt, a chelating agent and a pH adjusting agent, the improvement which comprises adding a stabilizer which consists essentially of an amount of an alkali metal stannate, silicate or vanadate salt in an amount sufficient to inhibit spontaneous decomposition of said bath without unduly slowing down the plating rate, with the provision that when said metal salt is nickel or palladium, the stabilizer is a vanadate salt.
2. In an electroless plating bath comprising a copper salt or a cobalt salt, a reducing agent for said salt, a chelating agent and a pH adjusting agent, the improvement which comprises adding a stabilizer which consists essentially of an amount of an alkali metal stannate, silicate or vanadate salt sufficient to inhibit spontaneous decomposition of said bath without unduly slowing down the plating rate.
3. A bath according to claim 2 in which said alkali metal is sodium.
4. In an electroless plating bath comprising a nickel salt or a palladium salt, a reducing agent for said salt, a chelating agent and a pH adjusting agent, the improvement which comprises adding a stabilizer which consists essentially of an amount of an alkali metal vanadate salt sufficient to inhibit spontaneous decomposition of said bath without unduly slowing down the plating rate.
5. A bath according to claim 4 in which said alkali metal is sodium.