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Electron-beam liftoff: Collection efficiency & paths to improvement


Liftoff metallization has been classically accomplished in the compound semiconductor industry with masks to achieve a uniform batch process. New process methodologies promise to significantly reduce those mask losses and offer significant improvements in both uniformity and collection efficiency in the future. By Gregg Wallace, Ferrotec USA Corporation, Temescal Division.


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iftoff carriers orient wafers to maintain the perpendicularity rule from a virtual point source near the electron beam impact point to a point on the center of each wafer in a carrier. Considering perfect 90° incidence to an essentially “flat wafer center” it is then obvious that incidence from the same point source at the e-gun to the outer edge of the wafer causes an imperfection to perfect normal incidence. The lack of perpendicularity at the wafer edge represents a process limit for “ideal liftoff conditions”.


These constraints lead to larger diameter wafers requiring greater distance between the E-beam source and the wafers surface to maintain near normal incidence at the wafers edge. In general, +/- 5° from 90° incidence is an accepted rule for good liftoff processing geometry.


Figure 1 maps the radiating zones of gold vapor flux by effective deposition rate at locations above an E-gun. Most simply, this is the flux topography above an E-gun. It is important to note, the map is for a specific set of evaporation conditions as described at the top of the chart.


The flux lines would be different if a material other than gold were evaporated even under the exact same E-Gun conditions.


This image can be mathematically described as cosine radiation from a point source. The product of the equation will be deposition rate which is highest close and directly above the source.


This material flux, under similar conditions, is highly reproducible and this is what offers engineers the benefit of predictable production. However the flux pattern’s shape is ruled by cosine curve mathematics and is highly dependent on control of the conditions listed below:


 Bulk Characteristics of the material being evaporated: density, melting point, thermal coefficients of expansion & conductivity, etc


 Maximum power capacity of the material being evaporated


 Size of crucible used (thermal mass)  Deposition Power  Beam Shape  Use of sweep


Figure 1


Cosine curves are expressed in exponential form as cosine to the “n” power of an angle theta describing a position above the (point) source. As the exponent “n” changes (becoming >1) the cosine


March 2013 www.compoundsemiconductor.net 29


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