INDUSTRY COATINGS
rate across all azimuthal angles, and thus have a smooth film thickness profile, with high values in the centre and lower ones towards the outer edges of the dome.
Engineers will often curve-fit the shape of the thickness profile observed across the wafers and carrier to better predict and understand the influence of measurement errors and small random fluctuations in conditions. One curve function offering a close fit to the observed thickness variation is the cosine power law – using this, one can predict thickness resulting from changes to the hardware, not including measurement error. Here we report the results of the application of that methodology to predict the best uniformity that can be achieved under realistic conditions.
The shape of an azimuthally symmetric cloud of gold vapour emanating from a 6 kW source is shown in Figure 1. This plot reveals the effective deposition rate across a substrate carrier placed normal to the flux. Regardless of source- substrate distance, the deposition rate varies with angle and is highest directly above the source. Given this, one could say that evaporation from the source is ‘directed upwards’, with a degree that changes significantly with different evaporants − and may even change significantly via modifications to the evaporation rate of a material.
Rotation about one axis… Over the years, we have expended considerable effort in documenting vapour cloud profiles for numerous materials over a range of process conditions. In this article, we draw on a small fraction of this database to highlight the sensitivity of the system. For example, the choice of material has a big impact on film thickness variation across all substrates: it can range from +/-12.8 percent to +/-24.0 percent in a tool employing a standard single rotation axis, and featuring a lift-off dome configuration (see Figure 2).
Adding one or more ‘shadow masks’ to block some of the evaporant flux is a conventional approach for increasing the uniformity along the radius of the dome (see Figure 3 for an illustration of this approach). The evaporant flux is greatest towards the centre of the rotating dome, so shadow masks are carefully shaped to block more of the flux heading towards the inner portion of the dome, compared to that travelling to the outer regions.
Figure 6. Even variations in the height of the source produce differences in film thickness. This plot show the variations in a single axis rotating dome with a mask, with source heights of + 0.5 inch (blue line) and -0.5 inch (red line)
With a fixed-position shadow mask, each and every material, regardless of deposition conditions, gets the same correction for relative thickness. The improvement can be far from modest – see Figure 4 to gauge the benefit wrought by the addition of a shadow mask designed for the average thickness profiles from the data in Figure 2. The improved thickness uniformity with the addition of one shadow mask varies from material to material, from a low of +/-0.9 percent to a high of +/-6.9 percent. These results highlight
that a well designed mask can tune the uniformity for a given process condition to exceedingly tight standards. However, although these gains are significant, there is a limit to what a single mask can produce, in terms of tuning the uniformity for multiple processes. The obvious solution – and one that has been applied when there is a select set of processes with critical uniformity tolerances – is to work with multiple shadow masks, which are moved into and out of the evaporant cloud at appropriate times. However, it is often impractical to do this for every
Figure 7. Higher levels of film thickness uniformity are possible by switching from a single-axis tool to one that provides dual axes of rotation. An example of the latter is the Temescal UEFC-5700, which accommodates 36 wafers in six domes, each holding six wafers
March 2014
www.compoundsemiconductor.net 29
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