PHOSPHORUS FREE ELECTROLESS PALLADIUM

Abstract

A process for the electroless deposition palladium on a substrate, the process comprising the steps of: (a) preparing the substrate to accept electroless palladium deposited thereon; and (b) bringing the substrate into contact with an electroless palladium composition, the electroless palladium composition comprising: (i) a source of palladium ions; (ii) one or more complexing agents; (iii) a reducing agent; and (iv) a plating rate enhancer, wherein the plating rate enhancer comprises a source of antimony ions.

Claims

1. A process for the electroless deposition of palladium on a substrate, the process comprising the steps of: a. preparing the substrate to accept electroless palladium deposited thereon; and b. bringing the substrate into contact with an electroless palladium composition, the electroless palladium composition comprising: i. a source of palladium ions; ii. one or more complexing agents; iii. a reducing agent; and iv. a plating rate enhancer, wherein the plating rate enhancer comprises a source of antimony ions.

2. The process according to claim 1, wherein the electroless palladium plating composition is at least substantially free of bismuth and/or is at least substantially free of phosphorus.

3. The process according to claim 1, wherein the source of antimony ions is a soluble antimony compound.

4. The process according to claim 3, wherein the source of antimony ions is selected from the group consisting of potassium antimony tartrate, antimony chloride, and combinations thereof.

5. The process according to claim 1, wherein the electroless palladium plating bath is maintained at a temperature of between about 50 C. and about 60 C. while the substrate to be plated is brought into contact with the electroless palladium plating bath.

6. The process according to claim 5, wherein the electroless palladium plating bath is maintained at a temperature of between about 52 C. and about 55 C. while the substrate to be plated is brought into contact with the electroless palladium plating bath.

7. The process according to claim 1, wherein the substrate is brought into contact with the plating bath by immersing the substrate into the plating bath.

8. The process according to claim 1, wherein the concentration of antimony ions in the plating composition is in the range of about 0.5 ppm to about 15 ppm.

9. The process according to claim 8, wherein the concentration of antimony ions in the plating composition is in the range of about 1 ppm to about 10 ppm.

10. The process according to claim 9, wherein the concentration of antimony ions in the plating composition is in the range of about 2 ppm to about 5 ppm.

11. The process according to claim 1, wherein the coefficient of variation of the palladium plating rate is less than about 10% averaged across different sizes of pad features plated on the substrate.

12. The process according to claim 11, wherein the coefficient of variation of the palladium plating rate is less than about 8% averaged across different sizes of pad features plated on the substrate.

13. The process according to claim 12, wherein the coefficient of variation of the palladium plating rate is less than about 7% averaged across different sizes of pad features plated on the substrate.

14. The process according to claim 1, wherein the plating rate is greater than about 0.6 in/minute.

15. The process according to claim 14, wherein the plating rate is greater than about 0.8 in/minute.

16. The process according to claim 1, wherein the plating bath is capable of achieving at least four metal turnovers.

17. The process according to claim 16, wherein the plating bath is capable of achieving at least six metal turnovers.

18. The process according to claim 17, wherein the plating bath is capable of achieving at least eight metal turnovers.

19. The process according to claim 1, wherein the plating time is at least 15 minutes.

20. The process according to claim 19, wherein the plating time is at least 30 minutes.

21. The process according to claim 20, wherein the plating time is at least 45 minutes.

22. The process according to claim 1, wherein the electroless palladium deposit has a thickness of at least about 10 in.

23. The process according to claim 22, wherein the electroless palladium deposit has a thickness of at least about 15 in.

24. The process according to claim 23, wherein the electroless palladium deposit has a thickness of at least about 20 in.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0017] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate various example implementations in accordance with the present invention, in which:

[0018] FIG. 1 depicts a boxplot of the palladium plating rate and shows the effect of different concentrations of antimony ions.

[0019] FIG. 2 depicts a boxplot of the % COV palladium plating rate and shows the effect of different concentrations of antimony ions.

[0020] FIG. 3 depicts a boxplot of palladium plating rate versus temperature.

[0021] FIG. 4 depicts a boxplot of % COV palladium plating rate versus temperature.

[0022] FIG. 5 depicts a boxplot of palladium thickness for plating solutions with and without antimony ions as the stabilizing agent.

[0023] FIG. 6 depicts a chart of % COV palladium thickness for plating solutions with and without antimony ions as the stabilizing agent.

[0024] FIG. 7 depicts a line plot of the mean palladium thickness over time for plating solutions with and without antimony ions as the stabilizing agent.

[0025] FIG. 8 depicts a boxplot of the palladium plating rate and shows the effect of different concentrations of antimony ions and bismuth ions.

[0026] FIG. 9 depicts a boxplot of the % COV palladium plating rate and shows the effect of different concentrations of antimony ions and bismuth ions.

[0027] FIG. 10 depicts a boxplot of palladium plating rate and shows the effect of different concentrations of antimony ions and bismuth ions for different pad types, including key pads, IPC pads, and ball grid array pads.

[0028] FIG. 11 depicts a test coupon and shows various pad types where thickness measurements were taken.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] As described herein, the present invention relates generally to a process for the electroless deposition of palladium on a substrate, the process comprising the steps of: [0030] a. preparing the substrate to accept electroless palladium deposited thereon; and [0031] b. bringing the substrate into contact with an electroless palladium composition, the electroless palladium composition comprising: [0032] i. a source of palladium ions; [0033] ii. one or more complexing agents; [0034] iii. a reducing agent; and [0035] iv. a plating rate enhancer, wherein the plating rate enhancer comprises a source of antimony ions.

[0036] As used herein, a, an, and the refer to both singular and plural referents unless the context clearly dictates otherwise.

[0037] As used herein, the term about refers to a measurable value such as a parameter, an amount, a temporal duration, and the like and is meant to include variations of +/15% or less, preferably variations of +/10% or less, more preferably variations of +/5% or less, even more preferably variations of +/1% or less, and still more preferably variations of +/0.1% or less of and from the particularly recited value, in so far as such variations are appropriate to perform in the invention described herein. Furthermore, it is also to be understood that the value to which the modifier about refers is itself specifically disclosed herein.

[0038] As used herein, spatially relative terms, such as beneath, below, lower, above, upper and the like, are used for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It is further understood that the terms front and back are not intended to be limiting and are intended to be interchangeable where appropriate.

[0039] As used herein, the terms comprises and/or comprising, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0040] As used herein, the term substantially free or essentially free if not otherwise defined herein for a particular element or compound means that a given element or compound is not detectable by ordinary analytical means that are well known to those skilled in the art of metal plating for bath analysis. Such methods typically include atomic absorption spectrometry, titration, UV-Vis analysis, secondary ion mass spectrometry, and other commonly available analytically techniques.

[0041] All amounts are percent by weight unless otherwise noted. All numerical ranges are inclusive and combinable in any order except where it is logical that such numerical ranges are constrained to add up to 100%. The term average is equivalent to the mean value of a sample.

[0042] The terms plating and deposit or deposition are used interchangeably throughout this specification.

[0043] The terms composition and bath and electrolyte and solution are used interchangeably throughout this specification.

[0044] In one embodiment, the palladium compound comprises a palladium ion source suitable for palladium plating. In one embodiment, the palladium compound is a water soluble palladium salt, which may be any water soluble palladium salt compound that aids and allows the dissolution of the metal portion in the bath solution. Such salts may include, for example, sulfates, chlorides, acetates, phosphates, carbonates, sulfamates, among others. Examples of suitable palladium salts include, but are not limited to, water-soluble inorganic palladium salts including palladium chloride, palladium sulfate, and palladium acetate; and water-soluble organic palladium salts including tetraamminepalladium hydrochloride, tetraamminepalladium sulfate, tetraamminepalladium acetate, tetraamminepalladium nitrate, and bis(ethylenediamine) palladium (II) chloride, which may be used alone or in combination. In one embodiment, the palladium compound comprises palladium sulfate.

[0045] The concentration of the palladium ions in the electroless palladium plating bath is preferably within the range of about 0.1 g/L to about 10 g/L, more preferably about 0.2 g/L to about 5 g/L, even more preferably within the range of about 0.2 g/L to about 2 g/L. Too low a concentration may impact the rate of deposition of the plating film and too high a concentration may cause abnormal deposition which could deteriorate the physical properties of the film.

[0046] The reducing agents are electron donors and, when reacted with the free floating metal ions in the bath solution, the electroless reducing agents reduce the metal ions, which are electron acceptors, to metal for deposition on to the article. Thus, the reducing agent has the function of depositing palladium in the electroless palladium plating bath and various kinds of reducing agent can be used. Examples of suitable reducing agents include, but are not limited to formic acid and salts thereof, hydrazines, formalin, and ascorbic acid and salts thereof, and combinations of the foregoing. Examples of the above mentioned salts include: alkali metal salts such as potassium and sodium salts; alkaline earth metal salts such as magnesium and calcium salts; ammonium salts; quaternary ammonium salts; and amine salts containing primary to tertiary amines.

[0047] In one embodiment, the reducing agent comprises formic acid or a salt thereof (e.g., sodium formate). Formic acid and formic acid salts are capable of provided bath stability and desired deposition properties of the plating film and may be used alone or in combination with another reducing agent. The total amount of the reducing agent may be adjusted as appropriate in consideration of the rate of deposition in the plating processing and the stability of the plating bath. In one embodiment, the concentration of the reducing agent is within the range of about 1 to about 100 g/l, preferably about 5 to about 75 g/L, more preferably about 20 to about 50 g/L.

[0048] In addition, while phosphorus compounds and borane compounds are known reducing agents, in one embodiment, the electroless phosphorus plating bath is at least substantially free of any phosphorus-containing compounds, including, for example, hypophosphorous acid and salts thereof, phosphorous acid and salts thereof and is also at least substantially free of any boron-containing compounds such as amine borane compounds (including, for example, dimethylamine borane and trimethylamine borane) and hydroboration compounds (including, for example, alkali metal borohydride salts such as sodium borohydride and potassium borohydride).

[0049] The complexing agent has the main function of stabilizing the solubility of palladium in the electroless palladium plating bath. As the complexing agent, various known kinds of complexing agent can be used. Examples of the complexing agent include ammonia and amine compounds and combinations thereof. Among these, amine compounds are preferred and examples of suitable amine compounds include, but are not limited to methylamine, dimethylamine, trimethylamine, benzylamine, methylenediamine, ethylenediamine, ethylenediamine derivatives, tetramethylenediamine, diethylenetriamine, ethylenediaminetetraacetic acid (EDTA) or alkali metal salts thereof, EDTA derivatives, and glycine, and combinations of the foregoing.

[0050] The total concentration of the complexing agent in the electroless palladium plating bath is generally the concentration necessary to maintain a stable solubility of palladium in solution. In one embodiment, the concentration of the complexing agent is within the range of about 0.1 to about 15 g/L, preferably about 0.5 to about 10 g/L, more preferably about 1 to about 5 g/L, and may be adjusted as appropriate in consideration of the stable solubility of palladium.

[0051] In one embodiment, ammonium hydroxide may be added to the electroless palladium bath to provide stability and OH for oxidation. If added, the concentration of the ammonium hydroxide may be within the range of about 1 to about 50 mL/L, preferably about 2 to about 25 mL/L, most preferably about 5 to about 15 mL/L.

[0052] In addition, in the electroless palladium plating bath according to the present invention, various additives typically used as bath additives in electroless plating compositions, including electroless palladium compositions, can be added in addition to the components described above. Examples of such additives include one or more of a pH adjuster, a buffer, an aminocarboxylic acid compound, a sulfur stabilizer, and a surfactant.

[0053] The pH adjuster is an additive that having the function of adjusting the pH of the plating bath. Examples of the pH adjuster include: acids such as hydrochloric acid, sulfuric acid, nitric acid, citric acid, malonic acid, malic acid, tartaric acid, and phosphoric acid, and alkalis such as sodium hydroxide, potassium hydroxide, and ammonia. These pH adjusters may be used alone or in combination.

[0054] It has been determined that too low pH has a tendency to adversely affect the stability of the electroless palladium plating bath and increase the rate of depositing palladium, whereas a too high pH has a tendency to decrease the rate of depositing palladium. The electroless palladium plating bath according to the present disclosure may have a pH preferably ranging from 4 to 10, and more preferably from 5 to 8.

[0055] A buffer with a buffering function may be added. Examples of the buffer include: carboxylic acids such as citric acids (e.g., trisodium citrate dihydrate), tartaric acid, malic acid, and phthalic acid; phosphoric acids such as orthophosphoric acid, phosphorous acid, hypophosphorous acid, and pyrophosphoric acid; phosphates thereof such as potassium salts, sodium salts (e.g., trisodium phosphate dodecahydrate), and ammonium salts; boric acid; and tetraboric acid. These buffers may be used alone or two or more kinds may be used in a mixture.

[0056] In one embodiment one or more aminocarboxylic acid compounds may be included in the electroless palladium plating composition. Such aminocarboxylic acid compounds may comprise derivatives of aminocarboxylic acids, including derivatives of oxalic acid, adipic acid, succinic acid, malonic acid, maleic acid, propionic acid, and other similar carboxylic acids, and combinations of any of the foregoing.

[0057] In one embodiment a sulfur stabilizer may be included in the palladium plating composition. In one embodiment, the sulfur stabilizer is selected from the group consisting of thioether compounds, thiocyan compounds, thiocarbonyl compounds, thiol compounds, thiosulfuric acids, thiocarboxylic acids, and thiosulfates. Specific examples thereof include thioether compounds such as methionine, dimethylsulfoxide, thiodiglycolic acid and benzothiazole; thiocyan compounds such as thiocyanic acid, potassium thiocyanate, sodium thiocyanate and ammonium thiocyanate; thiocarbonyl compounds such as thiourea and derivatives thereof; thiol compounds such as cysteine, thiolactic acid, thioglycolic acid, mercaptoethanol and butanethiol; and thiosulfates such as sodium thiosulfate. These sulfur-containing compounds can be used alone, or in combinations of two or more. In one embodiment, the concentration of the sulfur stabilizer may be in the range of about 0.1 to about 10 ppm, preferably about 0.5 to about 2 ppm.

[0058] In one embodiment, a surfactant may also be added as necessary to improve the stability, avoid pits, improve the appearance of the plate, and for other purposes. The surfactant is not particularly limited. Various kinds such as nonionic, cationic, anionic, and amphoteric surfactants can be used.

[0059] The electroless palladium plating bath according to the present disclosure is applicable to, for example, a multi-layer plating film including a palladium plating film and a gold plating film. The underlayer on which the palladium plating film is formed is not particularly limited. Examples of the underlayer include: various known base materials such as aluminum and, aluminum-based alloys, copper, and copper-based alloys; and plating films obtained by coating base materials with metal, such as iron, cobalt, nickel, copper, zinc, silver, gold, platinum, and alloys thereof, having catalytic properties for reductive deposition of a palladium plating film.

[0060] In addition, the electroless palladium plating bath according to the present disclosure is applicable to ENEPIG, ENIG, and ENAG process sequences as describe above. In the ENEPIG process, for example, a multi-layer plating film (i.e., the electroless nickel/palladium/gold plating film) is obtained which includes a nickel plating film, the palladium plating film described above, and a gold plating film, in this order on various substrates such as aluminum, an aluminum-based alloy, copper, or a copper-based alloy constituting electrodes.

[0061] As described herein, in one embodiment, the electroless palladium bath composition can be used in a process for depositing a multi-layer plating film on a substrate, to provide a palladium plating film formed by electroless deposition using the electroless palladium plating bath described herein.

[0062] In one embodiment, the process described herein may involve the step of preparing the substrate to accept electroless palladium deposited thereon. This step of preparing the substrate may include a step of cleaning the substrate, a step of catalyzing the substrate, a step of depositing an underlying metal layer, or another such process step that will allow electroless palladium to be deposited thereon.

[0063] In one embodiment, the palladium plating film is deposited on an electroless nickel layer in an ENEPIG multi-layer process. The conditions for forming the palladium plating film are not limited to those described herein but may be changeable as appropriate based on known techniques.

[0064] The plating conditions and plating equipment for electroless nickel plating using an electroless nickel plating bath are generally known and various known methods can be selected as appropriate. For example, a substrate or object to be plated may be brought into contact with an electroless nickel plating bath at a temperature ranging from 50 C. to 95 C. for about 15 to 60 minutes. The thickness of the nickel plating film may be set as appropriate in accordance with required characteristics, and usually ranges from about 3 m to about 7 m. Various known compositions, including, for example, nickel-phosphorus alloys and nickel-boron alloys can be used for the electroless nickel plating bath.

[0065] The plating conditions and plating equipment for electroless palladium plating using an electroless palladium plating bath in accordance with the present invention are set forth herein. The substrate or object to be plated, which may be coated with a nickel plating film or other layer, may be brought into contact with the electroless palladium plating bath described herein at a temperature in the range of about 50 to about 80 C., preferably about 50 to about 60 C., more preferably about 50 to about 55 C. for about 5 to about 60 minutes, preferably about 10 to about 50 minutes, more preferably about 20 to about 45 minutes. The thickness of the palladium plating film may be set as appropriate in accordance with required characteristics, and usually ranges from about 0.001 m to about 1.5 m, preferably about 0.01 m to about 1.5 m, more preferably about a 0.1 m to about 1 m. In one embodiment, the thickness of the palladium film may be, preferably about 0.01 m to about 1.5 m, more preferably about a 0.1 m to about 1 m. In one embodiment, the palladium film has a thickness of at least about 0.25 m or at least about 0.4 m or at least about 0.5 m.

[0066] The plating conditions and plating equipment for electroless gold plating using an electroless gold plating bath are not particularly limited and various known methods can be selected as appropriate. For example, an object to be plated including a palladium plating film may be brought into contact with an electroless gold plating bath at a temperature ranging from 40 C. to 90 C. for about 3 to 20 minutes. The thickness of the gold plating film may be set as appropriate in accordance with required characteristics, and usually ranges from about 0.001 m to about 1 m.

[0067] The electroless palladium plating bath according to the present disclosure is also useful for depositing a phosphorus-free palladium layer on various electronic device components. Examples of these electronic device components include components such as a chip component, a crystal oscillator, a bump, a connector, a lead frame, a hoop material, a semiconductor package, and a printed board, which constitute an electronic device.

[0068] The palladium plating film according to the present disclosure is obtained using the electroless palladium plating bath according to the present disclosure described above. The palladium plating film includes may be a pure palladium film or, alternatively, may be a palladium alloy plating film containing an alloy component. In the instance of a palladium alloy plating film, elements other than palladium may be contained in the palladium plating film depending on the type of the reducing agent to be used. In some cases, components derived from the various additives may also be contained. The rest of the palladium plating film includes palladium and inevitable impurities.

[0069] For example, if formic acid or a salt thereof or hydrazine or a salt thereof is used as the reducing agent, a pure palladium film is obtained. On the other hand, if a phosphoric acid compound such as hypophosphite or phosphite is used as the reducing agent other than the formic acid or the salt thereof, a palladium plating film containing phosphorus is obtained. If a boron compound such as an amine borane compound or a hydroboration compound is used, a palladium plating film containing boron is obtained. If both of a phosphoric acid compound and a boron compound are used, a palladium plating film containing both of phosphorus and boron is obtained. However, as discussed above, it is generally preferred that the electroless palladium composition be at least substantially free of any phosphorus-containing compounds and at least substantially free of any boron-containing compounds.

[0070] In addition, as described herein, it is also desirable that the palladium plating compositions described herein be at least substantially free of bismuth ions or any source of bismuth.

[0071] U.S. Pat. No. 7,981,202 to Kojima et al., the subject matter of which is herein incorporated by reference in its entirety, describes an electroless pure palladium plating solution. However, this patent suggests that the deposition rate of the plating film is dependent upon the palladium concentration as well as the temperature of the plating solution and that the deposition rate of a plating film can be controlled by appropriately setting the palladium concentration in turn facilitates control of the plating film thickness. However, the inventors of the present invention have surprisingly discovered that adding a certain amount of antimony ions to the electroless palladium plating bath has a significant effect on plating rate.

[0072] Furthermore, while antimony compounds are known for use as stabilizers in electroless nickel plating solutions, as described, for example in U.S. Pat. No. 9,401,466 to Walter, the subject matter of which is herein incorporated by reference in its entirety, antimony ions are not disclosed for use in electroless palladium compositions. In addition, other stabilizing agents such as lead ions and bismuth ions which are also suggested for use as stabilizing agents in electroless nickel plating baths do not have the same effect on an electroless palladium bath. In addition, at least bismuth ions, as demonstrated in the examples below, was not found to have any appreciable effect on plating rate and to have an adversely effect on palladium plating. Thus, it was surprisingly discovered that while antimony and bismuth ions may be added to an electroless nickel plating bath to improve bath stability, only antimony was shown to exhibit a positive effect on plating rate in electroless palladium baths.

[0073] In one embodiment, the present invention also relates generally to a phosphorus-free electroless palladium plating composition consisting essentially of: [0074] a. A source of palladium ions; [0075] b. One or more complexing agents; [0076] c. A reducing agent; [0077] d. A plating rate enhancer, wherein the plating rate enhancer comprises a source of antimony ions; [0078] e. A pH adjuster; [0079] f. A buffer; [0080] g. An aminocarboxylic acid or derivative thereof; and [0081] h. A sulfur stabilizer; [0082] wherein the phosphorus-free electroless palladium plating composition is at least substantially free of phosphorus, at least substantially free of boron, and at least substantially free of bismuth ions.

[0083] By at least substantially free what is meant that the electroless palladium plating composition contains less than 1 wt. %, preferably less than 0.1 wt. %, more preferably less than 0.01 wt. %, more preferably less than 0.001 wt. %, even more preferably less than 0.0001 wt. % and most preferably no measurable amount of any of these ingredients.

[0084] The present invention is also directed to the use of antimony ions in a palladium plating bath as described herein to improve palladium plating rate.

[0085] The invention will now be described in reference to the following non-limiting examples. Various electroless palladium compositions were prepared as set forth in Table. 1.

[0086] Each of the plating compositions set forth in Table I were applied to a variety of features of different sizes on a plating coupon at a temperature of 55 C. for 5 minutes and at a pH of 5.5. The properties of the resulting palladium deposits were then evaluated.

[0087] The plating coupon is depicted in FIG. 11 and comprises different pad types and sizes, including key pads (identified as area 1), IPC pads (identified as area 2), and ball grid pads (identified as areas 3 and 4), where the ball grid pads depicted are the same size but for a first set of ball grid pads (area 3), one side is metal defined and for the second set of ball grid pads (area 4), one side is solder mask defined.

[0088] The thickness of the electroless palladium plating layer is determined by an XRF spectrometer using a Fischer XRF film thickness measurement device at various locations on a coupon, such as the one depicted in FIG. 11. As shown in FIG. 11, the coupon may contain features of various sizes (see e.g., regions labeled 1, 2, 3, and 4) and multiple thickness measurements are taken in each of the identified regions. For example, thickness measurements are taken at 5 points of regions 1, 3, and 4 and 10 points of region 2. The average thickness of the palladium plating layer was determined as an arithmetic mean of the thickness measurements taken at the identified points in the palladium plating layer plated on the coupon.

[0089] In the examples below, the coefficient of variation (COV) represents a variation in thickness of the electroless palladium plating layer and is determined by dividing a standard deviation of measure values at the identified points in the thickness of the electroless palladium plating layer by the average thickness of the electroless palladium plating layer. This value is shown as % COV. The thicknesses reported in the examples below are the average for all of the different pad areas (e.g., areas 1-4 as depicted in FIG. 11), not just one size pad feature. In other words, the % COV represents the thickness variation across all of the plated features of the substrate, not simply the thickness variation between features of the same size.

TABLE-US-00001 TABLE 1 plating compositions Comp. Ingredient Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Palladium sulfate, as Pd (g/L) 0.5 0.5 0.5 0.5 0.5 0.5 Ethylenediamine (g/L) 3.0 3.0 3.0 3.0 3.0 3.0 Potassium sodium tartrate (g/L) 15 15 15 15 15 15 Ammonium citrate (g/L) 10 10 10 10 10 10 EDTA, disodium salt (g/L) 2 2 2 2 2 2 pH Buffer (g/L) 2 2 2 2 2 2 Aminocarboxylic acid compound 0.1 0.1 0.1 0.1 0.1 0.1 (g/L) Potassium antimony tartrate (ppm of 0 0.5 1 2 4.5 7 antimony) Sulfur stabilizer (ppm) 0.5 0.5 0.5 0.5 0.5 0.5 28% ammonium hydroxide (mL) 9 9 9 9 9 9 Sodium formate (g/L) 25 25 25 25 25 25 Sulfuric acid (mL) 1.8 1.8 1.8 1.8 1.8 1.8 Bismuth ions (ppm) Comp. Comp. Comp. Ingredient Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Palladium sulfate, as Pd (g/L) 0.5 0.5 0.5 0.5 0.5 Ethylenediamine (g/L) 3.0 3.0 3.0 3.0 3.0 Potassium sodium tartrate (g/L) 15 15 15 15 15 Ammonium citrate (g/L) 10 10 10 10 10 EDTA, disodium salt (g/L) 2 2 2 2 2 pH buffer (g/L) 2 2 2 2 2 Aminocarboxylic acid compound (g/L) 0.1 0.1 0.1 0.1 0.1 Potassium antimony tartrate (ppm of 10 15 antimony) Sulfur stabilizer (ppm) 0.5 0.5 0.5 0.5 0.5 28% ammonium hydroxide (mL) 9 9 9 9 9 Sodium formate (g/L) 25 25 25 25 25 Sulfuric acid (mL) 1.8 1.8 1.8 1.8 1.8 Bismuth ions (ppm) 2 4.5 10

[0090] Comparative Example 1 is used as a baseline composition and does not contain any stabilizer (i.e., Comparative Example 1 is free of both antimony ions and bismuth ions).

[0091] Examples 2-8 provide various concentrations of potassium antimony tartrate as the source of antimony ions, where the concentration of antimony ions ranges from 0.5 ppm to 15 ppm. FIG. 1 provides a boxplot of the plating rate of the various concentration additives in Comparative Example 1 as well as Examples 2-8. As seen in FIG. 1, the best results were obtained with an antimony concentration in the range of 0.5 ppm to 7 ppm in terms of plating rates. For Examples 7 and 8, it can be seen that the higher plating rates of 10 ppm and 15 ppm had a much larger boxplot, demonstrating a much higher degree of plating rate variation at these higher concentrations.

[0092] FIG. 2 provide a chart of % COV palladium plating rate for Comparative Example 1 along with Examples 2-8. As seen in FIG. 2, as with plating rate variation, the best results were obtained for antimony concentrations within the range of 1 ppm and 7 ppm.

[0093] The composition of Example 5 was also used to evaluate the effect of temperature on palladium plating rate for different pad sizes (see FIG. 11). FIG. 3 depicts a boxplot of palladium plating rate versus temperature over a temperature range of 50 to 60 C. As shown in FIG. 3, lower temperatures exhibited lower plating rate. FIG. 4 depicts a chart of % COV palladium plating rate versus temperature over the same temperature range of 50 to 60 C. as shown in FIG. 4, all conditions show excellent thickness distribution as indicated by low % COV.

[0094] FIG. 5 depicts a boxplot of palladium thickness and compares a plating solution without a antimony additive as set forth in Comparative Example 1 to a solution containing an antimony additive as set forth in Example 5. As set forth in FIG. 5, palladium thickness builds up over time with excellent thickness distribution for the palladium solution containing the antimony additive. In contrast, the palladium thickness for the composition that does not contain an antimony stabilizer did not produce a desired plating thickness over the same amount of time and the thickness distribution was also much wider as seen in the boxplot.

[0095] FIG. 6 depicts a chart of % COV plating thickness for the plating solutions with and without an antimony additive over a plating time of 45 minutes. As set forth in FIG. 6, the % COV was significantly less (i.e., less than about 8% COV or less than about 5% COV or less than about 3% COV over time while the bath that did not contain an antimony additive had a much higher % COV of at least 10% COV at the 5 minute mark and increasing to over 30% COV at the 45 minute mark.

[0096] FIG. 7 depicts a line plot of mean palladium thickness over a 45 minute plating time for the plating solutions with and without an antimony additive. For the same plating time, the aged bath with the antimony additive was able to achieve a 50% higher plating thickness than the bath that did not contain an antimony additive. Thus, it can be seen from FIGS. 5 to 7 that the effect of adding an antimony additive is critical in achieving a palladium thickness of at least about 15 in or at least about 20 in for customers who require a thicker deposit.

[0097] Comparative Examples 9 to 11 substitute a bismuth additive for a corresponding amount of antimony ions to demonstrate the effects of bismuth, which is a known electroless nickel stabilizer on the electroless palladium bath. Comparative Examples 9 to 11 are the same as Examples 2 to 8 except that varying amounts of bismuth ions are used instead of the antimony ions.

[0098] FIG. 8 depicts a boxplot of palladium plating rate to demonstrate the effect of bismuth and antimony on the plating rate. As seen in FIG. 8, an increasing trend in plating rate is observed with an increase in the amount of antimony. 4.5 ppm antimony was determined to be an optimum concentration for new bath make up. Higher concentrations of 10 and 15 ppm antimony ions showed a wider plating rate distribution, which is indicated by a higher % COV as shown in FIG. 8 due to a bigger thickness difference between small and large pad areas. Excellent thickness distribution was observed with an antimony concentration in the range of 0.5 to 7 ppm based on lower % COV. In contrast, a decreasing trend in plating rate is noticeable with an increase in bismuth concentration as shown in FIG. 8 and a corresponding wider rate distribution with increasing bismuth amount as indicated by high % COV as shown in FIG. 9.

[0099] FIG. 10 depicts a boxplot of palladium plating rate and shows the effect of the addition of bismuth and antimony for different feature (i.e., pad) types for two different concentrations of the additive 4.5 ppm and 10 ppm antimony and bismuth. As seen in FIG. 10, the plating rate for the composition containing antimony was much higher for each feature type for both concentrations of the additive. In addition, for the concentration of 10 ppm bismuth, the plating rate was significantly lower, especially for smaller features.

[0100] Thus, it can be seen that the % COV of the palladium plating rate is preferably less than about 10%, more preferably less than about 8%, more preferably less than about 7%, averaged across different sizes of pad features plated on the substrate.

[0101] In addition, it is desirable that the palladium plating rate is greater than about 0.6 in/minute, more preferably greater than about 0.8 in/minute.

[0102] In addition, it is desirable that the palladium plating bath be capable of achieving at least four metal turnovers, more preferably at least six metal turnovers, more preferably at least 8 metal turnovers.

Additional Embodiments

[0103] Clause 1. A process for the electroless deposition of palladium on a substrate, the process comprising the steps of: [0104] a. preparing the substrate to accept electroless palladium deposited thereon; and [0105] b. bringing the substrate into contact with an electroless palladium composition, the electroless palladium composition comprising: [0106] i. a source of palladium ions; [0107] ii. one or more complexing agents; [0108] iii. a reducing agent; and [0109] iv. a plating rate enhancer, wherein the plating rate enhancer comprises a source of antimony ions.

[0110] Clause 2. The process according to Clause 1, wherein the electroless palladium plating composition is at least substantially free of bismuth and/or is at least substantially free of phosphorus.

[0111] Clause 3. The process according to Clause 1 or Clause 2, wherein the source of antimony ions is a soluble antimony compound, preferably wherein the source of antimony ions is selected from the group consisting of potassium antimony tartrate, antimony chloride, and combinations thereof.

[0112] Clause 4. The process according to any of Clause 1 to 3, wherein the electroless palladium plating bath is maintained at a temperature of between about 50 C. and about 60 C. while the substrate to be plated is brought into contact with the electroless palladium plating bath, preferably wherein the electroless palladium plating bath is maintained at a temperature of between about 52 C. and about 55 C. while the substrate to be plated is brought into contact with the electroless palladium plating bath.

[0113] Clause 5. The process according to any of Clauses 1 to 4, wherein the substrate is brought into contact with the plating bath by immersing the substrate into the plating bath.

[0114] Clause 6. The process according to any of Clauses 1 to 5, wherein the concentration of antimony ions in the plating composition is in the range of about 0.5 ppm to about 15 ppm, preferably in the range of about 1 ppm to about 10 ppm, more preferably in the plating composition is in the range of about 2 ppm to about 5 ppm.

[0115] Clause 7. The process according to any of Clauses 1 to 6, wherein the coefficient of variation of the palladium plating rate is less than about 10%, preferably less than about 8%, more preferably less than about 7% averaged across different sizes of pad features plated on the substrate.

[0116] Clause 8. The process according to any of Clauses 1 to 7, wherein the plating rate is greater than about 0.6 in/minute, preferably greater than about 0.8 in/minute.

[0117] Clause 9. The process according to any of Clauses 1 to 8, wherein the plating bath is capable of achieving at least four metal turnovers, or at least six metal turnovers or at least eight metal turnovers.

[0118] Clause 10. The process according to any of Clauses 1 to 9, wherein the plating time is at least 15 minutes, or at least 30 minutes or at least 45 minutes.

[0119] Clause 11. The process according to claim 1, wherein the electroless palladium deposit has a thickness of at least about 0.25 m or at least about 0.4 m or at least about 0.5 m.

[0120] Clause 12. a phosphorus-free electroless palladium plating composition consisting essentially of: [0121] a. A source of palladium ions; [0122] b. One or more complexing agents; [0123] c. A reducing agent; [0124] d. A plating rate enhancer, wherein the plating rate enhancer comprises a source of antimony ions; [0125] e. A pH adjuster; [0126] f. A buffer; [0127] g. An aminocarboxylic acid or derivative thereof; and [0128] h. A sulfur stabilizer; [0129] wherein the phosphorus-free electroless palladium plating composition is at least substantially free of phosphorus, at least substantially free of boron, and at least substantially free of bismuth ions.

[0130] Clause 13. Use of antimony ions to improve the plating rate of an electroless palladium plating layer deposited on a substrate by the process of any of Clauses 1 to 11.