Electroplating and electropolishing of microelectronic workpieces, such as silicon wafers, typically involves immersing an electrically conductive surface of the wafer into a bath of liquid electrolyte. Electric current is passed through the electrolyte causing metal ions in the electrolyte to plate out onto the surface of the wafer, forming a plated layer or film. The electrical connection to the electrically conductive surface of the wafer may only be made in the so-called edge exclusion zone, as specified by industry standards. The edge exclusion zone is narrow, typically about 3 mm under current industry standards, and 2 mm or less in proposed future industry standards. In the past, contact rings having multiple spring-like contact fingers have been successfully used to provide the electrical connection to the wafer in the edge exclusion zone when processing standard wafers.
Newer wafer level packaging (WLP) applications use a mask design providing the equivalent width of an edge exclusion zone of as little as 0.1 mm (100 microns). To make contact in such a narrow zone requires that the photoresist placement (i.e. centering on the wafer), the wafer size, the wafer centering in the contact ring, the manufacture of the contact fingers, etc. must hold a tolerance within 0.1 mm to make good electrical contact all the way around the circumference of the wafer. If the contact fingers only touch the exposed seed layer on portions of the circumference, then poor electrical contact will cause poor plating uniformity reducing device yield. It is of course very difficult to provide an electroplating apparatus where all of the contact fingers can consistently contact a 0.1 mm edge exclusion zone.
In a similar WLP plating application, a wafer is sawed into individual dies which are tested to identify and discard defective dies. Then, only known good dies are placed into a molding compound layer on a substrate for further processing, essentially as a re-constituted wafer. In this way, only known good die are moved further in the manufacturing process. Use of re-constituted wafers also allows different types of dies to be placed next to each other with electrical connections between them made in subsequent processing steps. This approach is also beneficial for some manufacturing process such as multi-layer RDL (redistribution layer) Fan-Out. Generally, the molding compound layer on the re-constituted wafer results in a very small edge exclusion zone, of the order of about 0.1 mm. In addition, some manufacturing processes, for example multi-layer RDL, may involve wafers where the edge exclusion is different on each layer. These factors present engineering challenges in designing electroplating apparatus.