Advanced Packaging - ECPR &#151 Micro-cell Plating for Advanced Packaging

Affordable Accuracy at High Speeds
By Patrik Möller and Mikael Fredenberg, Replisaurus Technologies

The integration of complex functions, such as wireless communication capabilities on chips or packages, puts new demands on manufacturing methods for top metal layers. High density interconnects and integrated passives require a combination of resolution, accuracy, thickness, and uniformity typically offered only by dual-damascene processes. At the same time there is a need for thicker metal, high deposition rates, and low cost per layer that is typically offered only by through-mask plating processes. These combined requirements are difficult to address with existing methods.

However, a different approach called electrochemical pattern replication (ECPR) technology has been developed that combines the precision and resolution of advanced lithography with the efficiency of electrochemical deposition into one single electrochemical metal printing step.¹ ECPR offers a unique combination of resolution, dimensional accuracy, high deposition rates and low cost per layer, bridging the gap between front- and back-end metallization.

This article explains the principles of the ECPR technology, its process characteristics and how it can be used for advanced packaging applications such as integrated passives, redistribution layers, fine-pitch copper pillars and via-filling for 3D integration.

ECPR — Enabling direct printing of metal patterns
In the ECPR process, a template (master electrode), consisting of an electrically conducting electrode layer and a patterned layer of electrically insulating material is used. Cavities of the insulating structures in the master are pre-filled with an anode material, such as copper. During the print step the master is aligned to a substrate that has a seed layer and then pressed against the substrate with an electrolyte applied between the two surfaces Figure 1a.

When put in contact, excessive electrolyte is forced away from the master electrode and substrate interface. Local electrochemical micro-cells filled with electrolyte are formed in the cavities defined by the pattern of the master electrode Figure 1b. When an external potential is applied over the master electrode and substrate surfaces, electrochemical material transfer takes place inside each local micro cell. Metal is dissolved into ions from the pre-filled anode material in the master and transported through the electrolyte in each micro cell and deposited on the cathodic seed layer Figure 1c.

The purpose of the master electrode is both to accommodate the anode surface in each micro cell, and to accurately define each active cathode area on the substrate by a conformal master-to-substrate contact. A conformal contact results in an accurate contact pattern on the substrate, corresponding to the master pattern.

Fab-friendly process — Reducing complexity, cycle times, and environmental impact
The ECPR technology offers an integrated solution involving the tool, processes, and master electrode, reducing complexity in the manufacturing flow. By removing process steps and associated inspection and wafer transfer between multiple tools in the fab, the total cycle time to manufacture a metal layer can be reduced from a few hours to minutes. In comparison to a conventional through-mask plating line, one single ECPR tool replaces six tools used for resist coating, exposure, development, descum, electroplating and stripping. ² The elimination of process steps also reduces the consumption of toxic chemicals, making the ECPR process flow more environmentally friendly than existing methods. The process uses no photopolymers, developers, strippers or descum gases, and only minimum amounts of plating chemistry.

Thickness uniformity independent of pattern density
One of the main technical advantages of ECPR is the ability to achieve a uniform plating height, independent of the pattern density. ECPR has the advantage of maintaining a 1:1 anode-to-cathode area ratio since the electrochemical micro-cells are formed by the cavities between the master and the substrate Figure 1. In this way, the potential field lines do not bend across any isolating material, making it possible to get the same current density and uniform material distribution over the substrate despite varying active-area density, even for complex patterns.

In a conventional electroplating process cell, the anode-to-cathode area ratio can vary over the substrate depending on the density of the pattern. Local spots with low active area density will receive a higher current density than areas with high active area density due to the nature of electrolytic current distribution, known as current crowding. This results in varying deposition rates in different areas and uneven thickness distribution of the plated meta l^{4, 5}. The problem is normally addressed by using advanced agitation equipment, adding organic additives to the electrolyte, and taking the density into account when designing the pattern. Additional dummy patterns may be added to the design and repeated process optimization is typically needed when changing between two different designs. Despite these efforts, non-uniform material distribution is always a problem associated with conventional electroplating processes, with various effects depending on the pattern that is plated.

Microcell concept enables high plating rates
The contact plating concept of ECPR — with each feature in a confined electrochemical micro-cell — makes the electrochemistry and material transfer mechanisms that controls the process quite different from conventional electroplating. Each microcell in ECPR has a typical electrolyte volume of a few pico-liters and an electrode distance less than 10 µm between the anode and cathode surfaces, which can be compared to a typical fountain plating tool often having circulating electrolyte volumes of 100 liters or more, and a distance between the anode and cathode measured in decimeters.

For a conventional macro-cell process, the electrolyte has to be agitated towards the wafer surface to give sufficient supply of metal ions. The efficiency and uniformity of the agitation determines the thickness of the diffusion layer created closest to the wafer surface; the layer where the electrolyte is considered stagnant and the material transfer is exerted mainly by diffusion. A thinner diffusion layer gives higher material transfer rates, and thereby a higher limiting current. In the case of ECPR, the distance between the electrodes is less than 10 µm and each diffusion layer (one on the anode and one on the cathode side) is then less than 5 µm, which is less than what most agitation method can deliver, and significantly thinner than the diffusion layer created in the most advanced fountain plating cells. The thin diffusion layer of ECPR is a unique feature that makes it possible deposit high quality uniform copper at plating rates higher than 5-µm/minute.^{1, 2}

Accurate dimensional control — D and sidewall profile
By accurate thin-resist lithography and pattern transfer by dry etching, superior profile and CD control, high fidelity patterns, and thereby well-controlled cross section of the deposited metal patterns is possible. The pattern definition principle of ECPR is similar to nano-imprinting, and similar resolution capabilities can be expected for ECPR. Currently, replication of 500-nm lines / 250-nm space has been demonstrated and CD variations in the replicated pattern were controlled entirely by the variations in the master electrode. Additionally, variations introduced by the electrochemical pattern transfer itself were so small that they could not be seen in the measurements.³

Integrated passives, such as on-chip inductors for RF and wireless applications as seen in Figure 2 one example where high accuracy pattern definition, and thickness uniformity can enable new advanced designs and functionality.

Summary
ECPR technology introduces a new way to fabricate highly accurate metal patterns. The process combines patterning and metallization into one single process step, reducing complexity, cycle times and material consumption compared to conventional metallization techniques. By simplifying the process sequence a highly reproducible metallization method that offers advanced process control functions and short feedback loops has been developed.

PATRIK MÖLLER, Founder & VP Technology Integration; MIKAEL FREDENBERG, Founder & VP Process Integration, may be contacted at Replisaurus Technologies, Isafjordsgatan 22b, 5fl, 1640 Kista, Sweden; +46 (0)8 752 198; E-mail: patrik.moller@replisaurus.com

References
1. Moller P., Fredenberg M., Wiwen-Nilsson P., in AESF SURFIN / Interfinish 2004, proceedings (2004).
2. Fredenberg M., Möller P., Recent Progress in the Development of ECPR (ElectroChemical Pattern Replication) - Metal Printing for Microelectronics, ECS 208 Fall Meeting, Los Angeles, October 19, 2005.
3. Möller P., Fredenberg M., Dainese M., Aronsson C., Metal Printing of Copper Interconnects Down to 500 nm using ECPR ElectroChemical Pattern Replication, Micro- and Nano Engineering 2005, Microelectronic Engineering 83 (2006) 1410-1413.
4. Mehdizadeh S., Dukovic J. O., Andricacos P.C., Romankiw L. T., Journal of The Electrochemical Society, Vol 139, No. 1, January 1992.
5. Mehdizadeh S., Dukovic J., Andricacos P.C., Romankiw L. T., Journal of The Electrochemical Society, Vol. 140, No. 12, December 1993.

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