INDUSTRY COATINGS
These patent-pending tools feature a second axis of rotation, which greatly reduces the range in non-uniformity prior to the addition of a shadow mask.
Figure 8. With the high uniformity lift-off apparatus (HULA), which incorporates a second axis of rotation, unmasked thickness profiles are just +/-1.74 percent to +/-2.36 percent. Note that this plot displays the uniformity across the wafers in the radial direction for the same set of material conditions as shown in Figure 2. The scale of the vertical axis has been reduced to better display the range in thickness. Since the wafers in this design are held in a single tier, the uniformity is representative of the shape for all of the wafers
material, so process engineers only select mask designs for the most critical materials.
Complicating matters, associated with every process is a normal variation in the shape of the evaporant cloud. The shadow mask is a fixed component in the system, designed for a particular process (material and rate), and so its correction cannot always match actively changing conditions. The degree of variation strongly depends on the material, although it can be restrained by employing identical source conditions.
So far, we have only presented plots of film uniformity based on representative deposition conditions for each material. When these deposition conditions vary, thickness profiles change. An example of the extent of this change is illustrated in the variations in growth of titanium films deposited in six identically configured evaporation systems (see Figure 5 for changes in radial thickness profile.) By repeating the same deposition process across six tools we explore the influence that variations in deposition conditions have on the uniformity. These results demonstrate that repeatability of the deposition profile can be kept in the +/-1.5 percent range for this practical range of deposition conditions. The range includes some variation caused by source depletion from within the crucible over the course of multiple runs, as well as other small, unintended differences.
Changes to the height of the evaporant surface also produce variations in the system, but in this case they are independent of material type. To some extent these changes are unavoidable, because as material evaporates from a
source, the height of the evaporant will fall. The impact on uniformity can be significant when uniformity tolerances are tight. For example, changes in uniformity of +/-1 percent and a change in the outer position thickness of nearly 2 percent can occur when the source height shifts from 0.5-inch above to 0.5 inch below the nominal evaporant surface (see Figure 6). This is a realistic scenario for several common sources. Processes that require a tighter range on uniformity cannot allow that large a change in source height, so must break more frequently for source replenishment to maintain an even height.
While repeatability in the +/-1.5 percent range is representative of the exceptional performance capabilities of a well-tuned precision system like the FC-4400, it serves as a noteworthy benchmark for conventional methodologies when compared to the levels of uniformity achievable in the UEFC-5700.
... and two axes To minimize the inclination angle non- uniformity, we have developed the high uniformity lift-off apparatus (HULA).
A tool that we have already mentioned in this article, the UEFC-5700, has a HULA configuration (see Figure 7). This tool features six small domes, which each spin about their own axes while rotating about the tool’s central axis. Rotating the substrates in this manner has several major benefits: it increases collection efficiency by reducing the need to turn to a shadow mask to increase uniformity; and it greatly reduces sensitivity to material type and process conditions, thereby enhancing uniformity.
With the addition of the HULA motion, calculations indicate that the ranges for unmasked thickness profiles of different metals plummet to just +/-1.74 percent to +/-2.36 percent (see Figure 8). The tremendous reduction in overall range for each material and the differences in ranges between material types go hand-in-hand with similar reductions in sensitivity of the uniformity profile to run-to-run variation and source height variation. Those variations that caused shifts in uniformity in a single-axis tool of +/-1.5 percent are reduced in their influence to under 0.2 percent. The upshot of all of this is a system that delivers very robust uniformity characteristics across a wide range of materials.
Further improvements to thickness variation are possible with the addition of a very small shadow mask (see Figure 9). Using one designed for platinum film deposition, thickness variations are of the order of +/-0.5 percent – values so small that the metrology used to measure film thickness becomes very challenging, and the variations in film thickness may exceed the practical measurement accuracy commonly available.
Figure 9. Adding a mask for platinum deposition to a HULA apparatus trims the thickness variations to around +/-0.5 percent
30
www.compoundsemiconductor.net March 2014
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164