The best material for copper plating depends on the specific application and desired outcome. Different materials can be used as additives in the copper electroplating bath to achieve specific results. Copper plating is a key process in advanced electronics manufacturing. It allows depositing uniform and high-quality copper films on substrates for various applications like printed circuit boards (PCBs), semiconductors and microelectronics. However, achieving flawless copper plating requires careful optimization of the plating chemistry and materials.

This article will provide a detailed examination of the most effective plating materials, including:

Accelerators – to increase plating rate
Suppressors – to control deposition distribution
Levelers – to obtain planar and smooth deposits
Clays – to improve filling of submicron features
Brighteners – for shiny lustrous deposits
Wetting Agents – to improve surface wettability

It will also outline optimal concentrations and applications for these additives. A discussion on bath parameters like pH and temperature will also be provided.

Critical Role of Accelerators

Accelerators, also known as brighteners, are organic compounds added to the electroplating bath to increase the copper deposition rate. They enhance the kinetics of the cathodic reaction, allowing faster film growth.

Some commonly used accelerators:

Bis(sodiumsulfopropyl)disulfide (SPS)
Janus Green B (JGB)
Coumarin
Sulfonated organics

Accelerators improve the plating rate by adsorbing onto the cathode surface and causing depolarization. This decreases the energy barrier for Cu ion reduction, increasing the kinetics. Faster kinetics leads to quicker film deposition.

The choice and concentration of accelerator depends on the specific application. For example, high accelerator concentrations are used for high-speed plating in advanced packaging to rapidly fill small features. Typical concentrations range from 10-100 ppm.

Too much accelerator can cause non-uniform bumpy deposits. Optimizing the accelerator concentration is key to achieving smooth and shiny copper films at an adequate growth rate. They are an indispensable plating additive for high-quality copper electroplating.

Suppressors – Controlling Deposition Distribution

Suppressors, also known as carriers or levelers, are long-chain polymers that regulate copper deposition distribution. They adsorb strongly on the cathode surface and inhibit copper reduction by blocking growth sites.

By selectively suppressing deposition in certain areas, they can be used to tailor the film morphology and improve thickness uniformity. The main suppressors used are:

Polyethylene glycol (PEG)
Polypropylene glycol (PPG)
Other high molecular weight polyethers

Suppressors achieve preferential growth by differential adsorption strength on the substrate topography. Their inhibitory effect is weakest at corners and protrusions, allowing accelerated growth in such areas. This leads to bottom-up superfilling of small features.

Suppressors are critical for void-free filling of submicron features in PCBs and ICs. Typical concentrations range from 10-100 ppm. Too much can cause plating retardation while too little can result in non-uniform bumpy deposits.

Levelers – Achieving Planar and Smooth Copper Films

Levelers, also known as carriers, are surface active agents that facilitate smooth and planar copper electrodeposition. They contain nitrogen- and oxygen-bearing functional groups that selectively adsorb on overplated bumps and surface protrusions.

This passivates these sites and prevents further metal growth. Levelers thereby minimize growth rate variations and enable uniform copper distribution.

Some commonly used organic levelers:

Janus Green B
Coumarin
SPS
Thiourea

Inorganic levelers like chloride ions also help improve surface planarity. The selective adsorption of levelers enables differential suppression of growth across the substrate, creating smooth and shiny copper films.

Levelers are essential for interconnects and PCB manufacturing where post-plating planarization is required. They facilitate easy CMP after copper plating. Typical concentrations are 10-50 ppm.

Clays – Filling Sub-Micron Features

As feature sizes shrink below 45 nm, suppressors and accelerators are unable to provide void-free copper filling. Clays are emerging as new additives to enable sub-micron plating. Clays are silicates that adsorb strongly within features and enhance bottom-up fill.

Two main clays used are:

Montmorillonite – naturally occurring clay with high adsorption capacity
Laponite – synthetic clay suitable for sub-45nm nodes

The flat plate-like structure of clays facilitates adsorption on feature side-walls. This inhibits growth on the walls and accelerates bottom-up filling. Using just 0.1-1 ppm clay enables void-free copper electroplating down to 10 nm features.

Clays will be indispensable for future IC manufacturing. They can be used along with suppressors and accelerators to optimize filling of ever-shrinking features.

Brighteners – For Lustrous Copper Deposits

Brighteners are organic compounds added to obtain smooth, mirror-like copper finishes. They enhance specular reflection and light scattering to create shiny, lustrous deposits. Common brighteners include:

SPS
Coumarin
Bis(sodiumsulfopropyl)disulfide
Janus Green

Brighteners adsorb preferentially on high growth areas and edges to level out deposition. This evens out roughness and creates reflective mirror-like finishes. Maintaining brightener concentrations between 10-100 ppm is key.

Too much can cause burning while too little may not produce the desired lustre. Brighteners are indispensable for decorative coatings and reflector applications requiring high reflectivity.

Wetting Agents – Improving Surface Wetting

Wetting agents or surfactants decrease the surface tension of the plating bath. This enables deeper solution penetration and improves surface wettability. Better wetting allows uniform mass transport of Cu ions for consistent film deposition.

Common wetting agents for copper plating include:

Sodium lauryl sulfate
Triton X-100
Fluorosurfactants like Zonyl FSN

Wetting agents enhance surface wettability by reducing the electrolyte-substrate interfacial tension. This enables complete wetting of complex geometries and filling of blind microvias.

Using 5-100 ppm wetting agent can significantly improve surface wettability. However, excess concentrations can cause foaming issues. Optimizing the wetting agent addition is vital for uniform plating distribution.

Importance of pH Control and Temperature

Apart from plating additives, bath parameters like pH and temperature also impact copper deposition. Maintaining an optimal pH between 7-9 ensures good quality deposits. Lower pH causes dissolution while higher pH leads to passivation.

Temperature controls reaction kinetics – higher temperatures accelerate plating rate. But too high temperatures can cause non-uniform deposition. Keeping temperatures between 20-30°C provides a good balance.

Careful control of pH and temperature is vital, along with optimal additive concentrations for high-quality copper electroplating.

Conclusion

In summary, copper plating bath additives like accelerators, suppressors, levelers and clays are critical to controlling film uniformity, morphology and filling capability. Brighteners produce shiny lustrous finishes while wetting agents improve surface wettability.

By optimizing the plating chemistry and materials, it is possible to tailor the copper deposition for factors like void elimination, reflectivity, adhesion and smoothness. This enables high-reliability copper films for a range of applications from PCBs, chips to reflector coatings.

As feature sizes continue to shrink, new additives like clays will become indispensable for void-free filling. A comprehensive understanding of the role of different plating materials is key to leveraging them effectively for superior copper electroplating.

References

Djokić, S. S. (Ed.). (2010). Modern aspects of electrochemistry (Vol. 45). Springer Science & Business Media. This book provides a comprehensive overview of electroplating fundamentals and the role of additives. It has chapters dedicated to suppressors, levelers, brighteners, etc.
Paunovic, M., & Schlesinger, M. (2006). Fundamentals of electrochemical deposition. John Wiley & Sons. This is a key textbook on electroplating covering the mechanisms of copper deposition and function of various plating additives.
Kondo, K., Matsumoto, T., & Watanabe, K. (2004). Role of additives for copper damascene electrodeposition. Journal of The Electrochemical Society, 151(4), C250-C255. Research study examining the impact of suppressors, accelerators, levelers and chloride ions on copper filling of sub-micron features.
Akolkar, R., & Landau, U. (2014). Suppression and acceleration in copper electrodeposition from acidic sulfate electrolytes. Journal of The Electrochemical Society, 161(11), D635-D643. Investigates suppressor and accelerator adsorption and its impact on deposition kinetics and morphology.
Hasegawa, M., Negishi, A., Nakamura, T., & Osaka, T. (2015). Effects of polyethylene glycol and chloride ion on superfilling of submicron trenches by copper electrodeposition. Journal of The Electrochemical Society, 162(4), D114-D119. Examines superfilling behavior using PEG suppressor and chloride ions for sub-micron features.