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High power pulsed magnetron sputtering (HPPMS)

High power pulsed magnetron sputtering (HPPMS/HIPIMS) has attracted considerable attention from industry due its ability to produce thin films and features of excellent adhesion, superior density, decreased roughness, and extreme conformity. The intense pulsed plasma density provides a large concentration of metal ions that produce high-quality, homogeneous coatings. The high ionization fraction allows for fine control of the sputtered species during deposition, a feature well suited to the needs of future semiconductor fabrication techniques.

One of the most constant and consuming challenges of chip fabrication has been the deposition of material into high aspect ratio trenches. As characteristic dimensions have shrunk, interconnect trenches have become narrower, but, because they must often connect features in different layers of the chip, they are often relatively deep. These interconnect trenches must be coated with a diffusion barrier such as titanium nitride and then metalized. Depositing these materials uniformly into the trench is exceedingly difficult in modern chip designs where aspect ratios can be as high as 7:1. In the past thermal evaporation was sufficient. Physical collimators were used when that failed but these too were soon obsolete. A technique known as Ionized Physical Vapor Deposition (IPVD) is the current standard. In IPVD the sputtered material is ionized and directed into the trench by a voltage bias. In this way the trench can be filled quite uniformly.

There are improvements to be made, however. Generally IPVD requires the use of a secondary plasma source to attain a high ionization fraction of the sputtered material. This introduces added complexity that may negatively affect process scalability. HPPMS may provide a highly scalable means of depositing diffusion barrier coatings and metallic features in the high aspect ratio interconnect trenches because a HPPMS power supply may be connected to any existing deposition system to increase performance.

The modulated pulse power (MPP) technique is a new development in the pulsed plasma field pioneered by CPMI partner Zpulser, LLC. MPP can be used to shape an arbitrary voltage waveform that is applied to the cathode. This unprecedented freedom allows control over pulse duration, intensity, duty cycle, and average power. Voltage oscillations during the 1.0 – 3.0 ms pulse on the order of 30-65 kHz induce instabilities in the plasma discharge that may have a marked effect on the level of ionization within the discharge and distribution of these metal ions. Past investigations of MPP have only revealed time-averaged plasma parameters, but knowledge of what happens during the pulse is required to further understanding of the basic mechanisms involved. Because of our extensive experience with intense, short-lived discharges, CPMI is in a unique position to uncover the underlying physics of this technology.

Researchers at CPMI are currently characterizing MPP discharges from a 1000 square centimeter circular planar rotating magnetron (see video) and evaluating the technique’s potential as a chip processing tool. A gridded energy analyzer and quartz crystal microbalance are used to measure the ionization fraction at the substrate under a variety of deposition conditions. The energy spectrum and flux of these ions can also be monitored using this equipment. Time-resolved plasma properties including saturation current, electron temperature, and density will be measured and mapped over a 3D region between the sputter target and substrate level using a triple Langmuir probe built in house. The effects of pulse duration, current density, pulse shape, switching frequency, and target material on the discharge will be explored. Eventually, characterization of film structure, quality, and uniformity over the width of a 200 mm wafer and across surface features will be performed.

A comparative study will be conducted in which MPP discharges are examined in a more current Novellus INOVA hollow cathode magnetron (HCM) system. Vacuum systems of this type are in common use by Intel and other chipmakers. The HCM is intended to develop a greater ionization fraction of the sputtered species; it is thought that the addition of a HPPMS power supply will greatly increase the effectiveness of this design. Once again, films will be deposited and analyzed with a focus of parameter optimization.

Future industry demands will require a trench metallization technique that is effective, efficient, scalable, and reproducible. It is expected that this work will produce a step of the chip manufacturing process that will be suited to the designs of ~2013.