Roll-to-Roll processed foil inlay as an electrostatic chuck for flexible thin semiconductor wafer handling
As sub-millimetre package heights are becoming prevalent, it is imperative to have a solution to reliably and cost effectively handle and process wafers which are 20 -100 mu m thin in order to enable such miniaturized IC packages. The challenge with such thin silicon or GaAs wafers is that at such low wafer thickness the wafer itself is flexible and tends to warp under its own weight, which causes processing errors and stoppages in the conventional production line. In this paper, we present a foil based electrostatic chuck system (E-Foil) which allows the thin wafers to be processed in parallel with existing thick wafers on a conventional line, without requiring any modification of the conventional processing line or without the use of any adhesive or bonding / de-bonding process. The advantage of this system is that it makes no difference to the conventional processing line, which by itself is unaware, whether a standard or thinned wafer is being processed on the line. The E-Foil is setup as a bi-polar parallel plate capacitor on a polyimide (PI) foil substrate produced on a continuous Roll-to-Roll (R2R) line. The E-Foil is designed as an inlay to be placed between the thinned wafer and the chuck. The foil inlay is 85 mu m thick and is made up of an insulator (PI) - conductor (Copper / ITO) - insulator (PI) stack. The current design enables for the processing of up to 200 mm diameter wafers. Two different variants of the chuck have been developed, one using 100 nm sputtered copper as the metal layer which has been photo-lithographically structured onto the PI foil substrate and the other one using 200 nm sputtered Indium Tin Oxide (ITO) which has been structured using a screen printing etch step on the PI foil substrate in a R2R manner. The electrical and mechanical characterization of the setup has been performed in air, on a heated chuck and in a plasma atmosphere.