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Stratus Systems
1. Why is Stratus the optimum electroplating configuration for flip chip?
Solder, gold and copper layers used in flip chip and WLCSP processes are thick; even at high deposition rates, the deposit times are well over 10 minutes per wafer. To achieve at least a 40 wph machine throughput, many wafers must be plating simultaneously. The high throughput process cell allows for a line configuration that is readily matched to the process requirements. For example, in a copper mini-bump solder bump process, four copper and 16 solder wafers are depositing simultaneously. Shear Plate™ paddle agitation allows for higher deposition rates.
2. Isn’t paddle-agitated deposition an unusual technique?
No. Vertical paddle agitation deposition was developed by IBM in the 1970’s for critical electroplating processes used in the thin film heads industry where fountain process cells could not deliver the required results. A paddle moving rapidly back and forth in front of the wafer surface creates much better fluid mixing than occurs in horizontal fountain cells, providing a variety of process advantages. Horizontal fountain cell plating has been used
because, prior to Stratus, equipment vendors had not developed a cost-effective and reliable perimeter fluid seal and electrical contact suitable for vertical orientation. The Stratus Shear Plate electro-deposition process cell is an economical and reliable means of achieving a thin boundary layer.
3. How has NEXX made thin boundary layer deposition cost effective?
The critical success factor for reliable thin boundary layer deposition equipment is a completely leak-free sealed contact to the wafer edge. Our patented integrated contact/seal and fail-safe wafer holder provide a compliant seal and continuous electrical contact in the outer 1.75 mm of the wafer edge. Also, the contact/seal extends no more than 1.5 mm in front of the wafer allowing very close agitator-to-paddle spacing, facilitating superior fluid mixing at the wafer surface. These innovations are the foundation of a process cell with the process advantages of thin boundary layer deposition in a configuration that is reliable and cost-effective.
4. What are the advantages of thin boundary layer deposition from a process perspective?
Higher deposition rates result from a thinner boundary layer at the wafer surface. Better uniformity results because the Shear Plate agitation decouples the fluid flow and electric field at the wafer surface, providing more range for field uniformity optimization. In fountain cells, the fluid boundary layer thickness varies with radial position on the wafer surface; with Shear Plate agitation, the boundary layer is uniform in space and time.1 Complete filling of deep photoresist patterns results from vigorous mixing and avoidance of trapping air bubbles as occurs in face-down horizontal fountain cell systems.
5. Why is NEXX’s Stratus the best electro-deposition system?
Stratus systems offer several patent-pending advantages. Highest deposition rates result from extremely close wafer-to-agitator spacing (< 1.5 mm) than other commercial paddle systems; for example, copper can be deposited at over 100 ma/cm2 1. Limiting current measurements demonstrate a five-fold reduction in boundary layer thickness. Better reliability results from a paddle agitation mechanism with no bearings to wear and degrade. And better cost of ownership results from a compact wafer holder cathode that transports two wafers back-to-back simultaneously, doubling the effective throughput per process cell.
6. Is automation of a vertical system more expensive than for a fountain system?
No. It is less costly, and it is simpler to maintain. Wafers are handled only once, by the standard front-end wafer handling robot, which loads and unloads them from the wafer holders. There is no need for the expensive robotic precision necessary to reliably move wafers between process chambers because, in Stratus, the movement occurs while wafers are securely held in the fail-safe wafer-holder. This feature also makes Stratus an ideal platform for electro-deposition on thin wafers; 4 mil thick wafers have been tested. Wafer breakage is reduced and automation costs are less.
7. How is Stratus better than a "rack plating" system?
The Stratus wafer holder is not your typical cathode paddle with a bolt-on contact ring. An innovative flexure mechanism provides a fail-safe sealing force of over 40 Newtons to the wafer perimeter. Compliance in both the contact/seal and the flexure mechanism ensure uniform perimeter sealing and contact. Finally, to avoid scrap that would occur with misloads, the Stratus load station uses a proprietary method of checking the seal integrity
after each wafer is loaded.
1Diffusion Boundary Layer Studies in Stratus (PDF)
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