INDUSTRY COATINGS
evaporation. Our success includes the development of a process methodology that could reduce losses from uniformity masks and offer significant improvements to collection efficiency (see Compound Semiconductor March 2013, p. 29). We have also explored the more general issue of attainable film uniformity, and here we consider what can be achieved with the historical techniques and the newest methodologies available.
Realising a highly uniform film is not just an end in itself – it is critical to several processes, including the formation of eutectic films used for bonding. In these films, small variations in the ratio of materials can lead to a rise in melting temperature, and ultimately be the source of bonding issues across the substrate. That’s because composition variations have to be addressed by turning to a higher temperature to ensure melting across the entire film. The composition variation also has the unwanted side effect of possible changes in film morphology upon cooling.
Figure 4. In a single-axis rotating dome, adding a shadow mask improves thickness uniformity. Note that the deposition conditions used to generate the plot shown in figure 2 are repeated, but with the addition of an optimised shadow mask. The vertical axis has been changed in order to better display the data
high-volume production: the Temescal FC-4400, which features a wafer carrier with a single axis of rotation that holds up to 30 wafers in three radial tiers; and the Temescal UEFC-5700, a tool with dual axes of rotation that either accommodates: 36 wafers in six domes, each holding six wafers; or 42 wafers in three domes, each holding 14 wafers. Both tools have similar distances between the electron-beam source and the substrates of about 42 inches.
Vapour clouds
Many factors influence the distribution of the evaporant flux, including the inclination angle and azimuthal angle, the power provided to the source material and its evaporation rate, beam cross
section and density (size) and the height and shape of the source.
We have worked hard to minimise the influence of all these parameters by designing the hardware and refining the process so that it provides the most stable, consistent conditions. One of our insights is that it is critical to get high levels of uniformity at the source, because any variation here can carry over into non-uniformities for the vapour cloud. Manufacturers of electron- beam deposition tools often mount the substrates on a rotating dome-shaped carrier to minimize the influence of asymmetries in the azimuthal angle. By taking this approach, substrates experience an average of the deposition
Figure 3. Inserting a shadow mask can improve thickness uniformity in an electron-beam evaporator
Users of electron-beam evaporators face many potential sources of non-uniformity, and it is not possible to cover all of them here. So we will focus on the main factors influencing uniformity, and assume that film deposition is carried out using a well- controlled evaporation source, housed in a tool with tight mechanical tolerances relating to its geometry.
One factor having a big influence on film uniformity is the shape of the vapour cloud. Its impact on the uniformity of films deposited on 150 mm wafers is quite different in two of our tools designed for
Figure 5. Deposition conditions vary from run to run and evaporator to evaporator. These variations, which impact the thickness profile, are considered in this plot that shows deviations in thickness profiles over multiple runs from six evaporators. Note that to remove the slope across the dome – an inherent feature from a mask that was designed to balance the results from two materials – data were normalized to remove that slope from the average profile
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www.compoundsemiconductor.net March 2014
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