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Copper Electrodeposition and the Effects of Organic Additives on Deposit Growth

Nicholas Sullivan Email: npp8@wildcats.unh.edu Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824

Introduction Copper electrodeposition is commonly used

in the electronics industry for the plating of electrical components such as wires, semiconductors, and printed circuit boards (PCB’s) due to the high thermal conductivity and low electrical resistance of the metal. Copper is also known for its strong resistance to corrosion and its capability to expand and contract with the thermal expansion of plastics [1]. More recently, the use of copper electrodeposition in through-silicon-via technology has become a hot topic due to the many advantages three-dimensional packaging of chips provides. Through-silicon-vias are used in the integration of three-dimensionally packed chips, which enables linear contact between chips as opposed to external connections resulting in much faster signal transmission as well as a significant decrease in the packaging area [2]. The challenges of copper electroplating in through-silicon-via technology include the need for bottom-up superfilling of the vias, which is especially challenging because of their high aspect ratio and depth. A combination of levelers, suppressors, and accelerants is required to effectively fill all vias, free of voids, while maintaining a uniform deposit surface.

Electrochemical deposition with copper plating baths containing organic additives is commonly used when surface complexity makes line-of-sight deposition methods like PVD or CVD processes impractical. Exceptionally smooth plated surfaces can be obtained with the proper mixture of additives, even over very rough surfaces or in deep vias. The deposition process can be controlled through the use of levelers (inhibitors), suppressors, and accelerants (brighteners). The process of leveling occurs when the rate of deposition of metal in recessed areas increases relative to the deposition rate on surface peaks and edges. Leveling works to reduce surface roughness by inhibiting growth on prominent peak features, therefore directing mass transfer to low-set features enabling them to catch up

and promote layer growth as opposed to nucleation coalescence, or column growth [1]. Levelers arrive rapidly at the surface of exposed features and are consumed in the process, thereby rendering their use mass transfer controlled. Suppressors act very similarly to levelers except that they are physically adsorbed to surface features, are not consumed and inhibit deposit growth where they are adsorbed. Accelerants work to selectively remove suppressors from the surface, enabling growth to progress. For reasons that are not very well understood, accelerants tend to accumulate in recessed areas and therefore promote bottom-up filling of trenches and deep vias. Brighteners act in much the same way as levelers, but will smooth surfaces at a smaller scale where visible light interactions are affected [3].

SPS (bis-(3- sulfopropyl)disulfide) is a commonly used accelerant in copper deposition. In a copper plating solution, SPS is reduced to its monomer MPS which acts as an accelerant in the deposition of copper. MPS reacts with copper cations (Cu2+) in solution to reduce them to copper (Cu1+), which forms a thiolate accelerant complex. The catalytic reaction supports the bottom-up filling of trenches [4].

𝟏𝟐𝐒𝐏𝐒 + 𝐇! + 𝐞! → 𝐌𝐏𝐒

𝟐𝐌𝐏𝐒 + 𝐂𝐮𝟐! → 𝐂𝐮 𝐈 𝐭𝐡𝐢𝐨𝐥𝐚𝐭𝐞 +𝟏𝟐𝐒𝐏𝐒 + 𝟐𝐇!

𝐂𝐮 𝐈 𝐭𝐡𝐢𝐨𝐥𝐚𝐭𝐞 + 𝐇! + 𝐞! → 𝐂𝐮 +𝐌𝐏𝐒

Copper electrodeposition from acid solutions is the most common technique used commercially, specifically with solutions of cupric sulfate. Cupric sulfate is relatively low in cost and simple to use compared to other acid copper plating solutions and has been extensively studied since its first use by Bessemer in 1831[1-p63]. Cupric sulfate (copper (II) sulfate) in its pentahydrate form (𝐂𝐮𝐒𝐎𝟒 ∙ 𝟓𝐇𝟐𝐎) is dissolved in sulfuric acid to create the acid copper

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plating solution. Electrodeposition occurs through a plating

process where the desired component to be deposited onto is placed in a solution of dissolved metal salts. The component should ideally have a seed layer of the metal already in place as is common in the Damascene process so as to make initiation of the deposition easier. This seed layer can be deposited in many different ways including physical or chemical vapor deposition (PVD/CVD), both of which are more expensive than electroplating. During the electroplating process, a current is supplied to the solution by an anode composed of the metal being deposited. The component being deposited onto acts as the cathode in the process. The current supplied at the anode oxidizes the metal and dissolves it into solution. The rate at which metal is dissolved from the anode is equal to the rate at which metal is deposited onto the cathode [1].

Copper deposit growth is influenced by several factors including molecular transport of ions in solution to the surface of the deposit/plated device. The transport of ions can either proceed by concentration driven diffusion, or electrical migration [5]. Electrochemical potential plays an important role in electrodeposition; electrical potential differences in a deposition process will serve as driving forces and/or deposition limiting factors. The three major categories of potential difference in electrochemical deposition are concentration overpotential, activation overpotential, and crystallization overpotential. These potential differences occur in the solution at the surface of the electrode, between an ion in solution and an adsorbed ion, and between an adsorbed ion and an ion introduced to the crystal lattice. Concentration overpotential is the phenomena described by a significant reduction in metal ions at the electrode surface as compared to the bulk solution. This potential difference results in deposition limited by the transport of cations to the plating surface. The activation overpotential is a result of the required activation energy for an ion to react with the surface of the metal. Crystallization overpotential is the potential difference due to any hindrance of ions becoming a part of the crystal lattice in a deposit [5].

A paper written by Schilardi, Marchiano, Salvarezza, and Arvia in 1995 shows how through the analysis of aggregate patterns and radial growth velocity, it can be shown that large branch growth is mainly controlled by the electric field between the cathode and anode, while small branch growth is controlled by diffusion [6]. In other words, large

branch growth is driven by the supply of current to the anode, while small branch growth is limited by the diffusion of ions to the metal surface.

Diffusion-limited-aggregation (DLA) is a model describing the physics at the interface of the deposition surface and the plating solution. According to DLA branch growth is dependent on the mass transfer (diffusion) of ions to the plating surface. This model holds true initially in an electroplating process, but as the branch growth proceeds, other factors begin to play major contributing roles. The DLA model assumes that branch growth is controlled by the diffusion of ions through the solution to the plating surface because the motion of ions through the solution is more rapid than the growth velocity of the deposit through the solution. When the branches become more driven by potential, they are essentially pushing through the solution rather than receiving ions by diffusion. When the advance of the growth tips into the solution becomes a larger driving force for deposit growth than diffusion, the DLA model not longer holds. This phenomenon has been shown to occur after the initial growth stage.

Experimental Copper electrodeposition was first tested

without any additives in an acid copper plating solution of sulfuric acid (1M) and cupric sulfate (0.2M) in a glass dish 50mm in diameter and 15mm deep. A copper anode was placed in the solution at the edge of the glass dish while a 0.24mm copper wire was centered in the dish to act as the cathode. The copper wire was insulated by 0.25mm of Teflon shielding which was stripped away at the tip of the wire to allow a deposit to grow. The cell was operated in potentiostatic mode, meaning that the potential difference between the anode and cathode was held constant. First a short potential pulse of (-1) volt was applied for five seconds. Then the potential was stepped down to (-0.5) volts for the remainder of the plating process. The larger initial potential difference helps to initiate growth, but can also result in hydrogen bubbles forming at the cathode. The decreased potential stops hydrogen formation and allows only deposit growth. Copper was deposited for 3,000 seconds.

Results Shielding around the wire inhibited the

growth of copper crystals by forcing branch growth outwards along the shielding, or out into the solution.

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Not until approximately 1,200 to 1,800 seconds of deposition did the deposit reach the edge of the shielding and full three-dimensional growth was able to occur. When viewed macroscopically, the deposit seemed only to grow spherically, but when viewed at a more microscopic level, small bunched dendritic branching was very prominent throughout the entire deposit.

(a)

(b)

(c)

Figure 1. Copper deposit growth in acid copper plating solution at (a) 120 seconds, (b) 1,800 seconds, and (c) 3,000 seconds.

Figure 2. Copper deposit growth as a function of its charge accumulation.

Plotting the natural log of the deposit radius as a function of the natural log of the deposit charge, as is shown in figure 2, linearizes the relationship between deposit growth and charge and expresses the geometry of the deposit. The inverse of the slope of this line should be approximately three for spherical growth. Figure 1 shows how the growth of the deposit is limited by the presence of the Teflon shielding until nearly 1,800 seconds when it surpasses the majority of the shielding. The Teflon shielding limits spherical growth and therefore the inverse of the slope in figure 2 is approximately six for the first 1,200 seconds. Since the deposit growth became more spherical as it grew past the Teflon shielding the inverse of the slope after 1,800 seconds is three as was expected.

Figure 3. Charge accumulation as a function of deposition time. Charge in the deposit increases sharply at first and slowly levels to a linear logarithmic increase

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with time after again approximately 1,200 seconds. The conclusion that can be drawn from the data is that the charge accumulation increases in a logarithmically linear fashion with time for spherical deposit growth. Figures 1 and 2 in combination with figure 3 show the connection between charge, deposit growth and geometry, and time.

Next Steps A new electrochemical cell will be put

together to create a planar growth field. A glass dish will contain plating solution and a much thinner copper wire with less/no insulation, then a glass plate will cover the cell so as to promote two-dimensional growth which will be easier to photograph and measure. Deposition growth will continue to be measured while the effects of organic additives (PEG, SPS, and BV) will be monitored. The setup will look much the same as the setup described in the paper by Schilardi et al entitled ‘The Development of 2D Copper Branched Aggregates’ [6].

References [1] Mordechay Schlesigner, Milan Paunovic, Modern Electroplating (4th Edition), New York, NY: John Wiley & Sons, INC., 2000. [2] P. Dale, Barkey, N. Rohan Akolkar, Kazuo Kondo, Masayuki Yokoi, Copper Electrodeposition for Nanofabrication of Electronics Devices, New York, NY: Springer Science+Business Media, 2014. [3] M.A. Pasquale, D.P. Barkey, A.J. Arvia, “Influence of Additives on the Growth Velocity and Morphology of Branching Copper Electrodeposits,” Dept. Chem. Eng., Univ. NH, Durham, NH, Journal of The Electrochemical Society, 152 (3) C149-C157, Jan. 2005. [4] Philippe M. Vereecken et al, “The Role of SPS in Damascene Copper Electroplating,” IBM, T.J. Watson Res. Cen., Yorktown Heights, NY. [5] C. Thomas Halsey, Michael Lebig, “Electrodeposition and Diffusion-Limited Aggregation,” Univ. Chic., The J. F. Inst. and Dept. of Phys., Chicago, IL, Bost. Univ., Dept. Phys., Boston, MA, Dec. 1989. [6] P.L. Schilardi et al, “The Development of 2D Copper Branched Aggregates,” Univ. La Plata, La Plata, Argentina, Chaos, Solitons & Fractals Vol. 6, pp. 525-529, 1995.