INDUSTRY COATINGS
process, select processes can produce a thickness uniformity within a range of +/-1.5 percent on 150 mm wafers. The range of processes capable of delivering this level of uniformity expands with the addition of a second axis of rotation, which is found in the HULA tools.
Figure 10. The calculated values can be compared to experimental values
We have also compared the calculated results for titanium (shown in Figure 8) with measurements on 20 nm-thick titanium films. Comparisons were made by plotting deviations from the average thickness for each substrate, and measuring the titanium film thickness with an optical technique that is sensitive to tiny variations in average thickness over the measurement area. There is good agreement between calculated and measured values, but some discrepancy at levels near +/-0.2 percent (see Figure 10). In this thickness range a 1 percent variation in thickness is comparable to the diameter of an atom, and this level of uniformity indicates a remarkably uniform distribution of atoms across the substrate.
Some of these measurements reveal a small decline in thickness between the inner and outer portions of the wafer. However, there is also other structure apparent, which varies from substrate to substrate. Although these features will partly be caused by measurement uncertainty, it is not possible to rule out substrate mechanical positioning as a contributing factor behind the thickness variation. When films are this uniform, variations in thickness that result from an angular tilt to the substrate or a radial displacement of just 0.04” become apparent.
Wafer-related limitations Another impediment to realising a perfectly uniform film is the flatness of the substrate. For lift-off, the substrate is oriented in such a manner that at its centre, the direction of the impinging incidence flux is normal. Consequently, deposition at the edges of the substrate occurs at a small angle from normal. Making matters worse, the distance to the source is larger at the edges of the substrate than its centre. This difference is magnified as wafer diameter increases, but can be reduced by increasing the
distance between the electron-beam source and the substrate – although this action has downsides, such as reduced deposition rates.
Inserting a shadow mask into the electron-beam evaporator can address variations in film thickness in the radial direction. However, in the direction perpendicular to this – the cross-wafer direction – the mask has a limited influence on reductions in film thickness.
Variation in film thickness due to flat wafers is also influenced by the number of axes of rotation in the system: the HULA motion without a shadow mask gives some slight improvement over the single axis motion without a mask, but the single axis and HULA results are effectively identical after the addition of a shadow mask (see Figure 11).
These uniformities, and those revealed in other plots presented in this feature, indicate that users of electron-beam evaporators have entered a new era for process tolerances and expectations in a production tool. When a single-axis tool is built to tight mechanical tolerances and runs a well-controlled and stable
For the deposition of eutectic films, this level of uniformity can mean the difference between an experimental process and a production wafer. But that’s not all, as this level of uniformity can also drive further manufacturing efficiencies, thanks to reduced product binning − the sorting of finished devices based on variations in performance characteristics, such as voltage output, operational frequency and thermal tolerance. Such variations result from process variables and the inherent limits of each process, with a wider spectrum of product binning increasing parametric yield loss, which is the cost- per-wafer after sorting.
To increase uniformity to inside the +/-1 percent range requires the addressing of additional impairments to perfect film growth. This is hampered by wafer flatness, with success demanding even tighter tolerances on the mechanical systems that support and move the substrates. Programmes directed at uniformities of better than +/-1 percent must also consider whether the metrology is capable of accurately and consistently reporting the correct thickness. This is particularly challenging when the goal is to achieve differences in thickness that are smaller than that of the typical substrate surface roughness, or in the case of thin films, where the difference can be less than the width of an atom.
© 2014 Angel Business Communications. Permission required.
Figure 11. Without a mask, the addition of a second axes of rotation leads to a significant improvement in cross-wafer uniformity. But when the mask is added, this difference is minimal
March 2014
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