Patent Application
20110226841
The invention relates to a method for forming direct metal-metal bond between metallic surfaces, and in particular, a method for forming direct metal-metal bond between metallic surfaces with the use of a nanostructured organic coating at room temperature. The invention also relates to the nanostructured organic coating for use in the formation of direct metal-metal bond at room temperature. The invention further relates to metallic surfaces coated with the nanostructured organic coating.
The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
Technology feature sizes continue to shrink to meet the ever-increasing performance demands on integrated circuits. To this end, three-dimensional integrated circuits facilitate dramatic reduction in wiring length by stacking multiple device layers on top of one another through direct metal interconnects which are conventionally accomplished by thermocompression bonding.
Metallic micro-bumps of copper and gold are often favoured for three-dimensional integrated circuit applications due to the strong metallurgical bonds formed as well as their excellent electrical and thermal properties. For example, it has been demonstrated that thermocompression copper/copper direct bonding using a thin layer of tin capped onto copper micro-bumps was formed at 240° C. to 450° C. In the case of gold thermocompression widely used in wire and die bonds, recent surface studies indicate that the gold surface would adsorb organic contaminants when exposed to air. Although these organic contaminants can be removed by ultraviolet-ozone cleaning, this requires bonding temperature of about 300° C. At such high temperatures, thermal stresses may be introduced due to the different materials used in the substrate and metallic bumps, leading to dislocation generation, de-bonding or cracking.
Surface cleanliness is one of the key factors that determine the bonding parameters required to form direct metal-metal bonds. In order to facilitate low bonding temperatures such as room temperature and low bonding pressure, aggressive wet and dry cleaning steps are carried out on the metallic surfaces prior to the bonding to remove oxides and organic contaminants which would otherwise damage the active layers with miniaturized features patterned on a semiconductor wafer.
Since device wafers are typically thinned and hence fragile prior to bonding, it is desirable to operate at low bonding temperatures. Therefore, there is a need to provide a room temperature direct metal-metal bonding between metallic surfaces that overcomes, or at least alleviates, the above problems.
Throughout this document, unless otherwise indicated to the contrary, the terms “comprising”, “consisting of”, and the like, are to be construed as non-exhaustive, or in other words, as meaning “including, but not limited to”.
In a first aspect of the present invention, there is provided a method for forming direct metal-metal bond between metallic surfaces. The method comprises:
applying on the first and second metallic surfaces at least a bonding temperature of at least room temperature and/or a bonding pressure for a bonding period to bond the first and second metallic surfaces thereby forming the direct metal-metal bond therebetween.
In a second aspect of the present invention, there is provided a method for forming direct metal-metal bond between metallic surfaces. The method comprises:
applying on the first and second metallic surfaces at least a bonding temperature of at least room temperature and/or a bonding pressure for a bonding period to bond the first and second metallic surfaces thereby forming the direct metal-metal bond therebetween.
In a third aspect of the invention, there is provided a nanostructured organic coating for forming a direct metal-metal bond between metallic surfaces, the nanostructured organic coating comprises an organic phase with nanoparticles dispersed within the organic phase.
In a fourth aspect of the invention, there is provided an article capable of forming a direct metal-metal bond, comprising a metallic surface and a nanostructured organic coating adjacent to the metallic surface comprising an organic phase with nanoparticles dispersed within the organic phase.
In the figures, which illustrate, by way of example only, embodiments of the present invention,
The invention relates to a method for forming direct metal-metal bond between metallic surfaces with the use of a nanostructured organic coating at room temperature. Direct metal-metal bonding is a method of joining two metallic surfaces without an intermediate layer in between the metallic surfaces.
In accordance with a first embodiment of the invention shown in
A first nanostructured organic coating 118 is deposited on the first metallic surface 116 to form a first passivation layer. A second nanostructured organic coating 128 is separately deposited on the second metallic surface 126 to form a second passivation layer. The passivation layers act as oxidation barriers so that the underlying metallic surfaces 116, 126 are protected against oxidation. Passivation of the metallic surfaces 116, 126 is crucial; otherwise the oxidized layer formed on the metallic surfaces 116, 126 will not only affect the resultant metal-metal bond integrity and quality but also the thermal and electrical properties of the wafer substrates thereby leading to device deterioration or failure. Preferably, the passivation layers cover the entire metallic surfaces 116, 126. The thickness of the nanostructured organic coating layer is preferably less than 1 μm. Although it has been described that the two metallic surfaces 116, 126 are each deposited with the nanostructured organic coatings, it is to be appreciated that in certain circumstances only one of the metallic surfaces need to be coated. For example, if the first metallic surface 116 is easily oxidized in ambient environment while the second metallic surface 126 is more resistant to oxidation in ambient environment, only the first metallic surface 116 need to be coated with the nanostructured organic coating. The deposition technique includes, but not limited to, immersion, dipping, spraying, ink-jetting, micro-jetting, spin-coating, dispensing, transferring, Langmuir-Blodgett (LB), evaporation, organic molecular beam deposition (OMBD), and ultrahigh vacuum (UHV). The excess nanostructured organic coatings may be removed from the metallic surfaces by rinsing with solvents such as acetone, isopropyl, alcohol, ethanol, or toluene. Preferably, a further drying process is provided to remove the excess solvent.
The nanostructured organic coatings 118, 128 each comprises essentially an organic phase with nanoparticles dispersed within the organic phase. The organic phase comprises organic molecules having particle sizes between 1 nm and 100 nm. Organic molecules having binding affinity to the underlying metallic surfaces are chosen. Such binding affinity may include both physical adsorption processes as well as covalent and ionic forces. The organic molecules may be auto-organizing or otherwise. Examples of the organic molecules include, but not limited to, nucleic acids, trimesic-acid, carboxyl dimer synthon, porphyrin species, carboxyphenyl species, guanine-based tetramers, amino acids, dimerization of cystein molecules, hexanethiol, dodecanethiol, undecanethiol, octadecanethiol, anionic carboxylate species, methionine, carboxylic acids, perylene species, trimellitic acids, terephthalate, organo-silicon monolayers, sulfides, and disulfides.
The nanoparticles dispersed within the organic phase are present in amounts ranging from 0.01 to 10 weight percent (wt %), based on the total weight of the organic phase including the nanoparticles. Preferably, the nanoparticles are present in amounts ranging from 0.1 wt % to 1 wt %. The size of the nanoparticles is less than the surface roughness of the metallic surfaces. Preferably, the size of the nanoparticles ranges from 2 nm to 100 nm. The nanoparticles comprise inorganic, organic, carbon, or mixture. Inorganic nanoparticles include, but not limited to, metal nanoparticles, metal nanowires, metal nanorods, metal nanofibres, metal nanotubes, or mixture. The metal in the inorganic nanoparticles includes, but not limited to, copper, gold, aluminium, nickel, silver, alloy thereof, and mixture thereof. Organic nanoparticles include, but not limited to, polymer nanoparticles. Carbon nanoparticles include, but not limited to, carbon nanotubes, carbon nanofibres, nanofullerenes, nanodendrimers, graphite nanoparticles, or mixture. The nanoparticles are chosen such that the nanoparticles provide a more uniform surface for bonding, thereby improving the resultant metal-metal bond integrity and quality. This will be discussed later in the paragraphs to follow.
After the deposition of the nanostructured organic coatings 118, 128 on the metallic surfaces 116, 126, the first nanostructured organic coating 126 is brought into contact with the second nanostructured organic coating 128 in a mating engagement.
When the nanostructured organic coatings 118, 128 have been brought into contact, a bonding load is applied across the metallic surfaces 116, 126 for a bonding period sufficient to bond the first metallic surface 116 to the second metallic surface 126. The bonding period is less than 1 min. Preferably, the bonding period is between 20 s and 30 s. The bonding technique includes, but not limited to, compression bonding, thermocompression bonding, diffusion bonding, thermosonic bonding and soldering.
The bonding load comprises at least a bonding temperature of at least room temperature or a bonding pressure. Preferably, the bonding pressure is between 0.1 GPa and 10 GPa. Elevated bonding temperatures are also possible since heating improves bond integrity and quality. However, in applications such as the semiconductor industry, since device wafers are typically thinned and hence fragile prior to bonding, it is desirable to operate at low bonding temperatures. The application of the bonding load depends on the bond integrity and quality desired. For example, if high bond strength is needed, both elevated bonding temperatures above room temperature and elevated bonding pressures are applied. If intermediate bond strength is needed, bonding temperature of room temperature and a bonding pressure are applied. If low bond strength is needed, elevated bonding temperature alone is needed. No bonding pressure is applied in this latter case.
The nanostructured organic coating comprises organic molecules whose adhesion to the metallic surface is strong. Because of the strong adhesion, it is not possible for any further substances, specifically contaminants, to be taken up. Furthermore, the binding affinity of such organic molecules to the metallic surface is high, such that the organic molecules will bind to any exposed metallic surface quickly, thereby depleting available metallic surface regions for oxidation. In the case of some organic molecules such as undecanethiol, these organic molecules provide an additional advantage of reducing the metal oxide inevitably formed on the metallic surface before binding to the metallic surface.
The inclusion of nanoparticles further helps to improve the bonding integrity and quality between two metallic surfaces. Due to the inherent properties of the nanoparticles, the nanoparticles have very high surface energy which enables the nanoparticles to be bonded with metallic surfaces at room temperature or low temperatures. A typical metallic surface profile is uneven or rough on close inspection under the SEM. The surface profile resembles undulating terrain of valleys and peaks. A rougher surface is generally depicted by deeper valleys or taller peaks having higher fluctuations over the average surface height. Due to the ultrathin layer of nanostructured organic coating, the organic molecules and the nanoparticles will be forced to take up the valley space when pressure is applied. When bringing the first and second metallic surfaces to close contact, the taller peaks of the first metallic surface will contact the second metallic surface first and vice versa, while neighbouring shorter peaks may or may not contact the mating metallic surface depending on the amount of bonding pressure applied as well as its work hardening behaviour. The addition of nanoparticles is believed to provide a means to reduce asperity and height fluctuations which translate to lower amount of pressure needed to achieve a more uniform surface for bonding. The added nanoparticles fill up the gaps that can potentially be formed between two neighbouring valleys, thus promoting more exposed regions for bonding between the two metallic surfaces. In some cases where the nanoparticles are metals and more preferably, same metals as the metallic surfaces, even stronger bonding strength will be experienced. This also gives rise to yet another advantage of shortening the electron conduction path due to more availability of electron conductors present in the bond.
Electroplated copper bumps at bump heights of 18 μm, 92.5 μm in average diameter were fabricated on a silicon substrate. The silicon substrate consisted of sputtered layers on 0.65 mm Si wafer in the following sequence: 50 nm Ti, 100 nm TiW, and 200 nm Au. To remove contaminants on the surfaces, the copper bumps and substrates were acid-cleaned using hydrochloric acid and subsequently flushed with acetone. The cleaned copper bumps and substrates were then immersed in a solution containing 1 mM undecanethiol [CH3(CH2)10SH] with 0.5 wt % copper nanoparticles for 2 h. The coated copper bumps and substrates were then re-immersed in another fresh solution containing 1 mM undecanethiol [CH3(CH2)10SH] with 1 wt % copper nanoparticles for 15 min to saturate the surfaces to be bonded. Bonding of the copper bumps between two substrates was performed using flip-chip bonder at constant load at 88.9 N, bonding pressure of 3.28 GPa and bonding period of 30 s at room temperature (approximately 25° C.) under ambient condition of about 70% relative humidity. The experiment was then repeated at 40° C., 60° C., 80° C., 100° C. and 140° C., all other parameters remain unchanged. Several minutes were necessary for the bonder to heat and cool to the set temperature. No additional cleaning steps besides solvent rinsing were employed. After the bonding step, the bonded samples were subjected to a shear strength test using a dynamic mechanical analyzer.
The shear strength results for the above experiments are depicted in
The above experiments were bench-marked against bare copper bumps without the nanostructured organic coating. All the experimental parameters remained the same except no immersion in the nanostructured organic solution was carried out. The shear strength results for this bench-marking exercise are depicted in
The invention is applicable to the manufacture of semiconductor devices, electronic devices, optoelectronic devices, micro-electro-mechanical systems (MEMS), micro-optoelectro-mechanical systems, three-dimensional integrated circuits, systems-in-package, and multi-functional systems.
The invention also finds uses in other similar devices or systems that require direct metal-metal bond integration, such as metal joining of aerospace components for aerospace industry and metal welding of automobile parts.
The afore-described method provides a simple and economical way of forming a direct metal-metal bond having good bonding integrity and quality such as high bond strength of at least 50 MPa with the use of a combination of organic molecules and nanoparticles forming the nanostructured organic coating between metallic surfaces. Advantageously, the bonding is formed at room temperature, although elevated temperatures are also possible. Further, each individual process step, being simple and easy to perform, is compatible with existing bonding techniques and equipments so that the bonding process can be carried out at low cost without much modification to the processing line or equipments. The organic molecules forming the organic phase and the nanoparticles are commercially available reagents and are therefore easily available.
Although the foregoing invention has been described in some detail by way of illustration and example, and with regard to one or more embodiments, for the purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes, variations and modifications may be made thereto without departing from the spirit or scope of the invention as described in the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SG08/00448 | 11/27/2008 | WO | 00 | 5/27/2011 |
