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ELECTROLESS PROCESS FOR THE
PREPARATION OF PARTICLE ENHANCED
ELECTRIC CONTACT SURFACES
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to electroless deposition processes and more specifically to methodologies for creating particle enhanced electric joining surfaces through modified electroless deposition processes.
2. Description of Related Art
Electroless nickel bumping (ENB) of contact surfaces is currently a primary technique for providing electrical, thermal, and mechanical connections for integrated circuit chips. The time required to bump a chip by electroless plating is governed by the rate of the metal deposition reaction and the desired height of the bump. Therefore, there is a practical limitation to the throughput of chips in the manufacturing process. Once a chip is bumped, several other steps are necessary to attach the chip to an opposing contact surface. First, oxidation on the opposing contact surface must be removed in order to provide a good electrical connection. Second, in order to attach a bumped chip to an opposing contact surface, the contact joint must be heated and the bumped chip either soldered to the opposing contact surface, or heated enough that the bump re flows to join the contact surface of the chip with the opposing contact surface. In the alternative, a conductive adhesive may be used to attach the bumped chip to the opposing contact surface. These additional steps needed to ensure a good electrical and mechanical connection inject added time and cost into the manufacturing process.
DiFrancesco in U.S. Pat. No. 5,083,697 originally disclosed particle enhancement of contact surfaces to provide improved electrical, thermal, and mechanical connections between contact surfaces. DiFrancesco suggested that the particle enhanced contact surfaces may be formed by employing a variety of techniques, such as electroplating, electroless plating, chemical vapor deposition (CVD), sputter deposition, and evaporation. Many of the methods suggested by DiFrancesco have practical disadvantages. For example, a major disadvantage of electroplating particles to enhance contact surfaces is that the method is not suitable if many electrically isolated contact surfaces are to be coated, for example, the contact surfaces of IC chips.
With particular references to electroless plating, DiFrancesco did not anticipate the following problems. First, metallized conductive particles are incompatible with an electroless plating solution, as metallized particles cause instant solution decomposition because of the large collective surface area of the metallized particles in plating solution. Second, direct deposition of nonconductive particles by conventional electroless plating methods is proven through experimentation to result in poor electrical conductivity because of poor metal coverage on the particles.
The plating of particulate matters onto metal surfaces and other substrates has been commonly practiced since 1960s. The aim of conventional composite electroless plating is to successfully co-deposit particulate matter in a metal film to create a new composite. These electroless composite processes are generally designed to provide a regenerated surface layer for the purpose of corrosion resistance or lubrication. In a conventional composite deposition, significant "mechanical" effort is devoted to ensure either a successful "inclusion" type of co-deposition of lubricating
particles, such as PTFE and graphite fluoride, or a "structural" composite for the purpose of improving corrosion resistance of an electrodeposited structure. Therefore, most particles (95% or more of the deposited particles) in such 5 composite depositions must be densely structurally implanted (without large "black-hole" type defects) in a metallic matrix film. These metal films must be of a certain thickness. Typically the thickness is of at least one order of magnitude greater than the average particle size, in order to achieve the anti-corrosion or lubricating benefits. The con
ventional electroless composite deposition process is not designed to provide an electrical or mechanical connection between two opposing surfaces, nor does it result in such benefits in actuality.
15 SUMMARY OF THE INVENTION
The present invention provides a unique method for co-depositing hard particles and metal on electrical contact surfaces to provide mechanical, thermal, and electrical connections between the contact surfaces, and to enhance the
20 thermal and electrical conductivity between the contact surfaces and their corresponding substrates. The innovative method is able to uniformly deposit metal and particles of any shape, and with a wide range of density and sizes, on contact surfaces, and can be adjusted to provide any desired
25 surface area coverage in desirable deposition patterns. The co-deposited contact surface can, for example, be easily joined to another surface of any type by nonconductive adhesive, resulting in a connection that is mechanically robust, chemically inert, and inherently electrically conduc
30 tive. This eliminates the necessity of using specialized conductive adhesive or extreme heat for soldering or bump reflow for creating electrical surface joints.
In the relevant art, the term "substrate" is used interchangeably to indicate a wafer, an integrated circuit chip, a
35 contact pad on a chip, a circuit board, and various other dielectric and conductive materials and surfaces. In order to avoid potential confusion, in this specification the term "contact surface" is used to indicate that portion of a substrate through which external electrical connections are
40 made. A contact surface would therefore include the contact pads or area array contacts on a wafer, chip, or other substrate, but would not refer to the chip or wafer itself, or other elements thereof. While the embodiments discussed herein focus on the use of the inventive process to co-deposit
45 metal and particles on contact surfaces, this is not meant to indicate any limitation of the process to such surfaces, and indeed the co-deposition process can be applied to other surfaces capable of accepting electroless plating as well. The present invention consists of a modified two-step
50 electroless metal plating process. The first plating step utilizes a modified composite electroless metal plating method to co-deposit metal and particles on a contact surface, wherein the preferred metal is nickel. A particle surface activation step is performed after the co-deposition
55 to ensure adhesion of the metal to exposed particle surfaces during the second metal plating. At this time the first layer of metal may also be activated to achieve a more efficient metal deposition in the second plating process. The second plating step is a conventional electroless metal plating
60 process, again preferably using nickel. The unique process disclosed herein is designed to produce a consistent and uniform dispersion of hard particles on a contact surface through modified electroless plating techniques. The deposits typically contain 5% to 50% by weight of occlusal hard
65 particles, which uniformly cover the contact surface. Higher and lower surface density implantations of particles can be obtained to match any specific applications.
The second electroless metal plating step places a layer of metal film on the hard particles that have already been deposited by the first electroless metal-particle co-deposition process. The second metal layer does more than ensure the bonding strength the implanted particles 5 need. More importantly, it also provides a conductive overcoat on the particles, which allows originally nonconductive particles to function as electrically conductive media. Without the second electroless metal plating, the metal and hard particle deposit of the first deposition often results in inferior 10 electrical conductivity, since the metal coverage on the hard particle outcrops is often poor. Also, directly using conductive particles in an electroless metal deposition process can cause instant decomposition of the plating solution and particle deposition fails. 15
The disclosed two-step electroless metal plating process using the preferred metal, nickel, provides a uniform and less porous metal layer to most configurations of substrate. This provides a stronger bond for the particles and thereby a superior force enhancement for electric contact, i.e., less 20 force is needed to establish conductivity when mounting a particle enhanced component because of the high force per unit area transferred to the particles, which embed in the contact surface.
The two-step electroless metal process proposed in the 25 present invention also offers a much more uniform and desirable particle deposition pattern on contact surfaces. Because the particle deposition results from an electroless plating process, the particles are not subject to the influence of any electric fields, which is inevitable in an electrolytic 30 co-deposition process. No particle treeing and little clustering is observed. As a result, generally uniform, single-layer hard particle deposition is achievable at little extra expense over a normal electroless plating process.
In actual application with common electrical components, the contact surfaces prepared by the present electroless metal-particle co-deposition process offer the following benefits over traditional metal bumping techniques and other particle enhancement techniques. First, the present invention 4Q is ideal for preparing semiconductor wafers for flip chip attachment. All the contact surfaces on the wafer can be treated in a single step without need for electrical bridging.
Second, the methodology provides for the application of nonconductive particles to create a conductive media. The 45 second nickel-plating step deposits an overcoat on the nonconductive particles, which provides an electrical pathway between the joined contact surfaces. This essentially enables any type of hard particles compatible with electroless plating solutions to be used as contact surface joining 50 media, especially a large selection of nonconductive particles, such as most ceramics, glasses, oxides, silicates, nitrides, and diamonds. Many particles in these categories are manufactured in other industries and are commercially available in large quantities and consistent quality. For 55 instance, diamond powders are widely used as abrasive materials.
Third the electroless process results in a desirable contact surface with a designable particle deposition pattern. By a designable particle deposition pattern, it is meant that 60 through the choice of particle material and size, particle concentration in the plating solution, and duration of the plating treatments, the height of the particle bump on the contact surface, the thickness of the metal coating on the particles, the surface density of the particles on the contact 65 surface, and the thermal conductivity of the bond can all be controlled and modified to meet particular specifications for
particular applications. For example, the two-step electroless metal-particle plating method enables the production of flat contact surfaces with single layer, well aligned, nonclustering, non-treeing, uniformly dispersed, particle patterns on the contact surfaces, which is ideal for contact surface joining and is suitable for wide applications, such as smart cards and LED arrays.
Fourth, the particle enhanced contact pads create improved electrical, mechanical, and thermal performances for surface electric connections. Two-step electroless nickelparticle plating, by nature, provides a denser and more mechanically robust co-deposition structure than, for example, electrolytic plating. The second nickel plating and immersion gold steps offer electrical pathways with a very low resistance. The particle-enhanced surfaces also provide exceptionally short circuit pathways aiding the low resistance of the electrical connections. Further, particle enhanced contact surfaces require neither solders nor conductive adhesives, which are often required by chips bumped by electroless nickel. Mechanically, the hard particles have the ability to pierce any nonconductive surface barrier, for example an oxide layer on either or both the co-deposited substrate and the opposing contact substrate, providing good electrical connection without the need for removal of the oxidation or other barrier. In addition, hard particles, such as diamonds, provide excellent thermal conductivity between joined surfaces. The improved thermal conductivity results because of the greater surface area provided by a multiplicity of rugged particles per bond site (typically 5 micron industrial diamond has a surface area of about 1 m2/g) and higher coefficient of thermal conductivity for particles (typically around 10 W/cm/K for the industrial diamonds used) than typical bumped bonds. Altogether, the electroless co-deposition process offers perhaps the best combination of benefits available compared to current technology for joining electrical contact surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. lAis a cross section view of a contact surface before metal-particle deposition, depicting the first step in the series of the co-deposition process.
FIG. IB is a cross section view of the contact surface of FIG. lAwith a metal-particle co-deposition layer, depicting the second step in the series of the co-deposition process.
FIG. 1C is a cross section view of the contact surface of FIG. IB with the addition of a second metal plated layer, depicting the third step in the series of the co-deposition process.
FIG. ID is a cross section view of the contact surface of FIG. 1C with the addition of an immersion gold layer, depicting the fourth step in the series of the co-deposition process.
FIG. 2 is a schematic diagram detailing the steps for a preferred embodiment of the metal and particle co-deposition process of the present invention.
FIG. 3 is a microscopic photograph of a top view of a diamond-nickel co-deposition surface with gold finish prepared according to the process of the present invention.
FIG. 4 is a microscopic photograph of a top view of a diamond-nickel co-deposition surface on an aluminum substrate prepared according to the process of the present invention.
FIG. 5 is a statistical graph for particle cluster size distribution of a diamond-nickel co-deposition surface prepared according to the process of the present invention.
FIG. 6 is a scanning electron microscope photograph showing a microscopic perspective view of a nickeldiamond co-deposition on an aluminum substrate prepared according to the process of the present invention.
FIG. 7 is a scanning electron microscope photograph 5 showing a microscopic perspective view of a nickeldiamond co-deposition prepared according to the process of the present invention without the second electroless nickel plating layer.
DETAILED DESCRIPTION OF THE
The present invention creates a co-deposited metalparticle surface for improved electric joining. Typically, the resulting surface made by the present method includes a co-deposited metal-particle layer, an overcoat of the same metal used in the prior co-deposition, and a thin layer of immersion gold. FIGS. 1A-1D schematically illustrate the evolution series of the inventive process. FIG. 1A shows an
original, single, isolated contact surface 100, for example, of either a copper or aluminum substrate. FIG. IB shows the resulting metal-particle co-deposition on the contact surface 100 plated by a first electroless metal-particle solution. The particles 104 are partially trapped and held on the contact surface 100 by the first metal layer 102 electrolessly deposited on the contact surface 100. FIG. 1C depicts the results of a second electroless metal plating step. A second metal layer 106 is deposited on the previously co-deposited first metal layer 102 and particles 104 on the contact surface 100. ^ Generally, in order to cover the particles 104 with the second metal layer 106, the particles 104 must first be activated to accept the second electroless metal deposition. FIG. ID shows a thin layer of gold overcoat 108 on the top of the second metal layer 106 by immersion gold. FIG. ID repre- 3J sents the typical cross section of a plan view of a contact surface after being processed by the metal-particle co-deposition method of the present invention.
In contrast with prior art composite electroless plating processes, the purpose of the co-deposition process of the 40 present invention is to form a unique conductive, sandpaperlike surface design and provide enhanced electrical contact and thermal transfer. Instead of serving as backbone for the dense inclusion of particulate matters in conventional composition deposition, the electroless metal-particle 45 co-deposition of this invention functions more like glue, bonding the hard particles to the surface contacts. The particles are actually trapped in the metal deposition by happenstance of their location in solution. The process is able to generate a co-deposited metal-particle surface of any 50 thickness, but a surface on the order of average particle size with a single-particle layer is preferred for optimized electrical, mechanical, and thermal connections.
The hard particles contemplated by the present invention may be selected from diamonds (polycrystalline or monoc- 55 rystalline types), ceramics, metals, oxides, silicides, borides, silicates, nitrides, and various other compounds as long as the particle is harder than the applicable substrate contacts, so that it can preferably pierce into the substrate contacts. The particles that can be used in this process encompass a go wide range of particle size and density. Preferably the particle size is from 0.5 to 50 microns, and the density ranges between 1.5 and 8 g/cm3.
The contact surface prepared by the present invention can pierce any electric barriers on the contact surface itself and 65 an opposing surface of any kind when the two surfaces are joined. Superior electrical conductivity can be provided
without any additional steps, for example, soldering or reflow of metallized bumps of other methodologies. In one application of the invention, for example, the attachment of an IC chip to a substrate, a chip mount to a substrate using a chip with co-deposited particles on its contact surfaces can then be completed with a normal nonconductive adhesive to provide a stronger bond between the chip and the substrate, which eliminates the necessity of conductive adhesive underfill.
The schematic drawing shown in FIG. 2 illustrates the principles of a preferred embodiment of the invention. Conceptually, a comprehensive electroless co-deposition procedure 200 consists of surface preparation of substrate pads, modified electroless nickel-particle co-deposition, a second electroless nickel-plating, and immersion gold treatment. While nickel is the preferred metal for use in the metal-particle co-deposition process, other metals, such as cobalt, copper, iron, gold, silver, zinc, palladium, platinum, rhodium, and ruthenium, may be used with the appropriate electroless catalytic processes.
It is well known that good plating starts with proper surface preparation of the contact surfaces. A proper preparation is one whereby surface contamination is removed, which leaves a clean, oxide-free surface. Depending on the type of the contact surface and contaminations, different pretreatments are needed. Typically, surface contaminants that must be removed before plating include one or more of the following: organic contaminants (e.g., oils and lubricants), buffing compounds, oxide films, dirt, and fluxes.
In an embodiment wherein an aluminum contact surface is to be enhanced 210, the contact surface is first surfacecleaned 212 by alkaline cleaning solution followed with a deionized water rinse 214. The alkaline soak cleaning material may contain a mixture of alkaline sodium compounds such as hydroxides, carbonates, phosphates, and/or surfactants. The selection of cleaners can be adjusted according to the nature of the surface contamination. The alkaline cleaning can also include an organic chelating agent and/or sodium cyanide to accomplish surface cleaning and deoxidation simultaneously. However, a separate deoxidization process 216 using acid deoxidizers may also be used in this embodiment. It is believed that acid pickling using combined nitric acid (preferably 50% by volume) and ammonium bifluoride solution helps and benefits the initiation and activation of the contact surfaces for electroless plating. Practically, such a conditioning is desired in this embodiment to remove alloying elements from the substrate surface and to prepare it for uniform zincating. This step may be performed by immersion in strong nitric-acid based solution. Reaction with nitric acid forms a thin, light, uniform oxide film, on the aluminum contact surface that protects it from further attack by the acid. A subsequent zincating process easily removes this film. For wafers that may contain a high percentage of silicon, hydrofluoric acid or fluorides may be added to the nitric acid to dissolve silicon.
Residues from cleaners and deoxidizers can increase the porosity of an electroless nickel plate by creating passive spots on the contact surfaces that will not initiate electroless plating. This in turn creates deposition problems for hard particles. Therefore, in this embodiment a deionized water rinse 218 is a preferred step after deoxidizing, and good rinses are desirable throughout the process of this embodiment. After rinsing, the contact surfaces normally should be dried at an appropriate temperature, i.e., a drying temperature should be chosen according to the type of substrate housing the contact surfaces such that the temperature is not deleterious to the substrate.