deposition industry
Figure 2
form of radiation from a point source. That is, a rich inner flux zone deposits thicker films on wafers carried in the inner-tier positions, while a weaker outer flux zone deposits thinner films onto wafers carried in outer-tier positions.
curve exhibits a characteristic elongation changing its shape from a unit circle (n=1) to what may be better described as an inverted teardrop as n exceeds a value of ~3. Temescal has collected hundreds of vapor flux maps from E-Gun deposition tests. As a result, we have developed software which both predicts coating performance from E- Guns and automatically generates uniformity masks used to create batch coatings of uniform thickness. Figure 2 shows how a mask functions in an e-gun system. Figure 3 shows such a prediction for an unmasked film deposited from an e-gun onto a small liftoff carrier with two tiers of wafers.
Figure 3
Across a single-axis-of-rotation carrier, the thickness distribution of an unmasked liftoff film reflects the
In order for a mask to improve uniformity in this environment, it must selectively block material moving to the substrate. This allows a mask to deliver a uniform thickness film to all the wafers on a rotating carrier. System providers regularly re- design equipment, often to accommodate larger wafers by increasing source-to-substrate distance or to provide larger batch sizes. However as the next figure demonstrates, mask designs for liftoff must also change with any change occurring to the wafer carrier. When a carrier’s size is increased (to hold larger, or more wafers) its’ uniformity mask must also grow to balance its’ ability to block the rich inner flux zone to a now larger yet weaker outer flux region of the carrier.
Figure 4 illustrates how the declining trend in the outer tier flux rate would continue as a carrier is enlarged (a continuation of the violet dotted line). This obligates the mask also be enlarged to increase material capture from the central zone to balance the entire batch coating to a common thickness.
In Figure 5, a carrier’s spherical dome radius (the source-to substrate distance) has been superimposed onto the flux map. This indicates the surface where wafers would be carried to make liftoff films. Note the span between different flux or deposition rate zones and their decline as the diameter of a carrier is increased. A reference mask position is also shown indicating the horizontal plane where a mask would be located. The mask’s projected shadow onto the carrier (and wafers) would then trim flux to achieve good uniformity over a specific carrier/wafer batch.
Figure 5 shows flux, the wafer carrier and mask position as viewed through a chamber door looking horizontally into the system. To clearly show mask shape and its shadow effect onto the dome and wafers better, it is more appropriate to change the view to one looking down from the chamber’s top, straight through the wafer and mask onto the centrally mounted E-Gun source.
Figure 6 uses such a top-down projection showing a sequence of growing carrier diameters and their correspondingly growing masks for this specific gold deposition. The carrier is represented by a gray circle in the diagram. The mask starts as a gray outline leaf shape in 6-i) and grows in shape and size as the carrier’s diameter increases.
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www.compoundsemiconductor.net March 2013
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