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deposition  industry


Numbers displayed on a helical arc radiating from the center offer the deposition rate, in Å/s, that would result at the outer edge of the carrier as its’ diameter increases. Note: The process evaporation power is held constant in the model, only the carrier and mask change.


Figure 6-i through 6-iv illustrate the impact of increasing a carrier’s surface area. Carrier size growth is expressed in terms of the half-angle created by the carrier’s outer edge and a vertical center-line dropped directly to the E-Gun source. The first diagram shows a dome with a 25° half angle, and in the succeeding diagrams the half angle increases in increments of 10°.


Figure 7 presents three graphs of modeling data in more detail for a substrate carrier as it grows from 0° half-angle to a 60° half-angle size, they chart: i) dome surface area


ii) the declining rate of deposition at the outer edge of the carrier with each step of growth iii) a calculation of the ideal collection of material beneath the growing dome surface, normalized for liftoff incidence and the use of a mask to make the batch a uniform material thickness. Again, this is for ideal cosine gold deposition from a flux cloud of cosine shape n = 3.3. This represents a typical production deposition condition in a fab.


The graphs in Figure 7 show that a single-axis-of- rotation substrate carrier with a half-angle of approximately 40° yields the best possible collection efficiency. Dome growth beyond ~40° results in more material being lost to the mask than gained on the carrier or wafers. Beyond that point, material collection on the dome declines as a percentage of total material evaporated. The decline is due to the increasing need for uniformity mask growth to compensate for the low deposition rate at a carriers’ outer edge. Ideal* collection is never realized as a small percentage of evaporated materials are


inevitably lost in conditioning and other stages of actual production runs. However, generalized evaporation losses, from an ideal*, are experienced by equally by all sources regardless of system geometry or application. Therefore they can be considered to represent a normalized loss coefficient across all processes being modeled for comparative efficiency purposes. Continued modeling now reveals that maximum collection efficiency in a masked, single axis rotation liftoff process is available from a system with a half-angle of ~40°. “Conceptual construction” of such a tool indicates it would carry 60 wafers in a 53 inch wide carrier, as shown in Figure 8.


While large, it’s conceivable a segmented carrier could be made for loading as in other production tools. “Conceptual modeling” indicates this batch would collect Au material in identical flux conditions at an effective 2.1 Angstrom/second rate (due to larger radius of outer wafer tier).


An ideal* cosine model of the cloud indicates 26.7% of the gold evaporated will be collected on the 60 wafers. To offer a comparison this figure would represent 38% Au collection improvement over Temescal’s FC-4400’s collection efficiency which carries 30 wafers/batch operating under the same flux conditions. Modeling the “ideal geometry” gives rise to another question which is, ‘Is there any way for liftoff coating collection to be improved beyond this?’


Figure 5


In 2002, Temescal’s work in flux mapping software compelled a focus on mask related losses. Experiments were made attempting to reduce the need for uniformity masks in systems. An alternative concept for liftoff wafer motion was developed called HULA an acronym for High Uniformity Liftoff Assembly. The system maintains the wafer’s orthogonal relationship needed for liftoff while also providing planetary motion which moves the wafer


Figure 4.Masked vs. Unmasked Deposition


Figure 6 March 2013 www.compoundsemiconductor.net 31


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