Direct-Energy Plating: A New Electrodeposition Process for Interconnects

As the life cycle for electronic goods continues to shorten because of the escalating demand for new and leading-edge products, the semiconductor industry is faced with growing calls for innovation. In particular, the pressure is on the packaging industry for smaller form factors, higher performance and lower cost.

Process test array for electrodeposition of copper pillars with various pitch densities.

Although plating offers superior deposition rates, electrochemical deposition (ECD) has limitations in process flexibility and performance. ECD performance is typically measured by deposition rate, uniformity, film morphology, topology and shear strength. The process parameters that determine these metrics include chemistry, temperature, rectifier settings and agitation. The performance has been largely determined by chemistry. Chemistry sets the bath temperature and current output. Although techniques such as periodic pulse-reverse plating¹ can improve performance, the operating window is limited. Agitation affects the plating rate and uniformity. Methods such as fountain streaming or vertical washboard scrubbing² improve the mass transport, but the mechanism takes place away from the surface of the wafer and is limited by flow behavior.

A new technology has been developed to advance ECD called direct-energy plating (DEP). Instead of using mechanical methods, DEP uses vibrational energy that couples directly to the substrate. These energy modes do more than bring about simple agitation. The waveforms can be programmed to create multiple effects: surface cleaning, contact welding, enhanced diffusion through high-aspect-ratio structures, removal of bubbles or reduction of boundary layer for faster plating.

Electroplating fundamentals

Electrodeposition is based on a reduction-oxidation (redux) reaction. An anode and cathode (the plated surface) are connected to a power supply (i.e., rectifier) that supplies current into the system (Fig. 1). At the anode, metal ions are oxidized to create cations in solution with a positive charge (in the case of copper, it would be Cu²⁺). These ions either associate with the anions in solution (SO₄ ^2-) or reduce at the cathode surface, taking on electrons to form a zero valence state and depositing on the surface of the cathode. In solution, it is important to generate mixing to avoid localized reactions and non-uniformities. Plating rates are essentially limited by mass transport. At the surface of the cathode, a boundary layer exists that slows the deposition to the surface.

1. Plating is based on ionic redox kinetics, and deposition rates are greatly affected by the boundary layer near the deposition surface.

Mechanical techniques to generate mixing include fountain spraying and washboard scrubbing (Fig. 2). Fountain spraying creates turbulent flow by directing spray at the surface. Washboard scrubbing creates a shear force along the surface to reduce the boundary layer at the surface from the typical 50 to 10 µm or greater.

2. Fountain spraying and washboard shear scrubbing are the two most common approaches to generating mixing and boundary-layer reduction.

Plating solutions are typically composed of many components, including:

• metal ions as plating materials
• acid that sets the conductivity of the solution
• brighteners for cleaning the surface, with small molecular weight molecules
• levelers to suppress plating at protrusions
• suppressors/carriers to suppress the plating rate, with large molecular weight compounds
• wetting agents to reduce the surface tension

These agents typically dictate the performance of a plating bath. Leveler concentration, suppressors, carriers and wetting agents have an effect on plating into high-aspect-ratio structures. The drawback of this dependence on chemistry is that the organic materials are vulnerable to depletion and will need to be replenished.

Ultrasonic basics

Mechanical agitation is a gross means of mixing or dispersing fluid; the scale that it affects is on the order of microns to millimeters. Plating, however, is a molecular process, and the ability to affect the kinetic energy at the quantum level would have considerable advantages.

One mechanism of affecting the kinetic energy of a fluid is through vibration, and the means of generating this vibration is through ultrasonic energy. Vibrations are basically waves traveling through the media, and the typical frequency range of these transmissions is 0.5 kHz to 10 MHz. Depending on the frequency and modulation of the wave, it can travel in different modes (Fig. 3). These modes include shear, longitudinal, Rayleigh and Lamb waves. Each of these waves can have a different effect on the fluid.

3. Four types of waves can direct fluidic movement in different directions.

The vibrational energy essentially expresses itself in the fluid in the following ways:

• Guided pressure — Swirling, dispersion, mixing, shear and scrubbing are pressure-based phenomena and can be created using the proper waveform.
• Cavitation — When sound waves are produced, they first generate a compression of the molecules outside the source. Like a spring, the fluid then separates and creates a negative pressure. When the negative pressure exceeds the shear limit of the fluid, cavitation takes place and a bubble is formed. The size of the bubble is a function of many factors, such as temperature, viscosity, solubility, vapor pressure, diffusion rate, frequency and power. Microcavitation can cause chemical reactions (sonochemistry) by the energy released caused by the rapid (picosecond) collapse of bubbles.
• Heating — The vibrational energy and imploding bubbles can generate heat in solution.
• Surface reactions — Ultrasonic welding can take place because of energy absorption.

Direct-energy plating

Classic ultrasonic energy operates at one frequency and has a narrow pass band. The drawback to this approach is that cavitation tends to be large, different materials can be driven into resonance, and voids can be generated during deposition. However, we have developed a mixed-signal response to overcome the limitations of standard ultrasonic processes. By sweeping the output frequency and modulating the period of the cycle in the time domain (Fig. 4), mixed modes can be created and wideband responses can be generated. Plating systems are composed of multiple materials with different resonances and responses. A broadband response that can couple to all materials of interest can be pre-programmed by sweeping the frequencies (Fig. 5).

4. Sweeping the base frequency and changing the duty cycle in real time generates a wideband response in the ultrasonic output.

5. A frequency output of wide and narrow band responses.

We developed a system in which the energy can be directed by programming to generate the physical mechanisms for high-performance plating. Figure 6 shows the basic system. There are two ultrasonic sources: side-firing transducers to generate bulk mixing and wafer transducers to couple to the substrate.

6. The impact of the four types of waves on plating performance.

Examples of different kinds of waves are:

• A base longitudinal wave can be created and, by varying the waveform period, this wave can be swept across the chamber. This creates mixing while avoiding issues related to standing waves. By increasing the amplitude, this also serves as a cleaning wave.
• By mixing the frequencies, this same waveform can generate shear, which creates a pressure wave that enhances diffusion toward the surface and reduces the convection time.
• Broadband, moderate energy mix modes can induce Lamb waves that break up the boundary layer and enhance mass transport.
• Rayleigh waves can also be used to generate mixing at the surface (replace depleted regions) and at high energies, actually creating pressure pulses to evacuate bubbles.

7. By modifying the waveform, different physical effects can be generated, such as bubble removal, surface reaction and sweeping.

Vibrational damage — A major concern of a vibrational energy system is the impact on fragile structures or layers. Delicate structures related to plating can be found in either microelectromechanical systems (MEMS) or packaging applications. MEMS actuators and transducers commonly have resonances in the range of 1–10 kHz.³ Vibrational plating does not operate in this range, so DEP is not a major issue for this technology. For the semiconductor industry, a significant concern is the compatibility with low-k dielectrics.

In the drive for higher speeds, lower-k films are being developed to reduce the dielectric constant of traditional oxide-type layers (oxide k=3.9 to 4.2; low-k <3). There are many types of low-k materials, but they can be categorized into three areas: inorganic, organic and hybrid (carbon-doped oxides, SiOC).⁴ The semiconductor processes of cleaning, stripping or conditioning can have a negative effect on these films. Two common characteristics of low-k materials that make them difficult to process are hardness and strength (4-20× less than their oxide counterparts) and porosity (residues change electrical characteristics).

The ultrasonic systems that have been characterized for low-k materials include wire bonders and cleaners. Wire bonders exert physical and ultrasonic force during welding. However, it has been shown that there is a window of operation to avoid damage.^5,6 Thermocompression bonding exerts far greater surface force (5000–10,000 psi) than the worst-case cavitation pressure (1000 psi at 100 kHz) generated by DEP.^7,8 Further, it has been reported that single-wafer ultrasonic configurations have been shown to not damage sensitive structures.⁹ The wideband response and mode modulation of DEP also offer a wider process window for optimization.

The surface chemistry issue is not restricted to only wet systems, but also plasma systems. Post-process cleaning and sealants are potential solutions to this issue. A single-cell ECD system with multiple chemistries would have significant advantages over conventional rack and cups platers.

Boundary layer — In general, electrodeposition processes are primarily controlled by chemistry, bias and temperature. On the wafer surface, however, plating is limited by the boundary layer. For high-aspect-ratio structures, the boundary layer can be quite thick, and with narrow structures (<30 μm) that are deep (>50 μm), plating time in hours is not uncommon. Without mechanical agitation, the boundary layer can be more than 100 μm thick. Ultrasonic agitation within the solution can reduce the diffusion layer to 20 μm, and proper mechanical sweeping can go as low as 10 μm.¹⁰ DEP, however, offers an advantage over ultrasonics because of its multi-frequency/mode operation and because it is coupled directly to the deposition surface rather than the bulk. Figure 8 shows the boundary-layer thickness vs. frequency of a surface couple system. With proper tuning, the layer can be reduced to 1 μm and faster plating rates with opening sizes <10 μm become feasible.

8. As frequency increases, the boundary layer drops off exponentially.

Summary

DEP addresses the limitations of traditional plating and offers a flexible technology. The broadband waveform allows application to different materials and different kinds of substrates, including thinned wafers.

Our new plating process provides features that cannot be attained by mechanical agitation: reducing the dependence on chemistry for performance; handling severe wafer topology; faster deposition rates; and greater number of features, such as surface cleaning. And since it is an energy-based system, potential exists for real-time monitoring and feedback control. Rather than a simple chemistry and tub system, an intelligent tool can be realized.