Uniformity and undefined outplating is of great concern to the plating process on silicon wafers. It is important that the over plating is kept to a minimum, as the CMP process following the metal deposition is very cost intensive.

Due to the highly accurate continues flow and pressure capabilities combined with electronic control functions, LEVITRONIX pump systems can be ideally facilitated as consistent pressure source or dynamically as flow controller for uniform plating purposes.

CHALLENGES WITH MAG-DRIVE PLATING PUMPS

Gold sulfite and gold thiosulfate solutions have been developed as an alternative to the highly toxic gold cyanide plating baths. A critical drawback to the use of cyanide-free alternatives is that bath stability is more difficult to control. It is imperative to avoid hotspots, dead-zones, extensive shear, rough surfaces, narrow gaps and fissures.

Magnetically driven plastic centrifugal pumps are widely used in metal plating because of their chemical resistance and leak-free design. In mag-drive pumps, the centrifugal rotor is driven by a magnetic coupling through the pump casing, enabling a seal-less design (Figure 1).

The principle limitation of the mag- drive design is the need of process lubricated slide bearings which are usually fabricated from carbon, silicon carbide or alumina ceramic. When operated with critical plating solutions such as gold sulfite, the metal tends to precipitate on the bearing surface. The precipitate ultimately fills the narrow gap between the stationary bearing spindle and the rotating bearing sleeve. In pump tests performed with gold sulfite solution at elevated temperatures, all mag-drive pumps failed within 3 to 17 weeks. In all cases, the slide bearings were completely clogged with gold which ultimately resulted in increased torque and complete bearing failure. Figure 2 shows the impeller and the bearing of a pump which ceased to function after only 3 weeks of operation.

REVOLUTIONARY MAGNETICALLY LEVITATED CENTRIFUGAL PUMP

Levitronix^® has developed a revolutionary centrifugal pump that has no bearings or seals susceptible to wear or failure. Based on the principles of magnetic levitation, the pump’s impeller is fully suspended inside a sealed casing and is driven by the magnetic field of the motor. The impeller and casing are both fabricated from chemical-resistant, fluorocarbon resins or polypropylene. The impeller with the embedded magnet resides within the outer casing to form the pump head.

By eliminating a mechanical bearing, no mechanical friction occurs that could cause local hotspots. Moreover, no narrow gaps exist where metal could precipitate. The low-shear, dead-zone-free pump design and the smooth, wetted plastic surfaces further aid in avoiding metal deposition in (Figure 4).

In pump tests performed with gold sulfite solution at elevated temperatures, the Levitronix pump showed virtually no metal deposition after six months of operation (Figure 5). In contrast, when mag-drive pumps were tested under the same conditions, they showed heavy gold deposits on all wetted pump parts after significantly shorter operating times. The superior duration of operating time and performance of Levitronix pumps with gold sulfite solution and other critical plating chemistries has made Levitronix the market leader for plating pumps in the semiconductor industry with an installed base of more than 2000 units. Levitronix pumps have also been successfully used for plating of nickel, cobalt, ruthenium, rhodium, silver, palladium, platinum and other metals.

THE SMALLEST PUMP

Mag-drive pumps have an inherently complex design, consisting of an electrical motor, a mechanical coupling between the motor and the magnetic clutch, the magnetic clutch and the pump. Each of the rotating elements including the motor, the magnetic clutch and the impeller require a separate set of mechanical bearings. These elements wear out and fail.

Mag- drive pumps are also large compared to their hydraulic power. Levitronix pumps are 3-5 smaller than mag-drive pumps with comparable hydraulic performance (Figure 9 and 10). The highly integrated design of the pump and motor combined with the absence of mechanical bearings and bulky magnetic couplings, results in a system that greatly reduces the space requirements in your plating equipment.

Since there are no mechanical bearings to clog or wear out, Levitronix pumps can also be placed at locations which are not readily accessible. The high reliability and small size of the Levitronix pumps allows you further freedom in the placement of the pump.