edfas.org 1 7 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 25 NO . 1 be 1.0 +/- 0.5microns, then a process using 3-micron slurry shouldbe stopped at aminimumthickness of 4.5microns. This requires all but the last two process steps to remove material equal the difference between the last step’s grit size and the previous step’s grit size. This is generallymore removal than isusedprocessing the50-micronsample, but at 50 microns RST, damage that extends a micron or two has no effect on the integrity of the sample. Removing this much material increases the process time but is required to produce a usable result. Increasingpressuremay speedup theprocess but may also increase sample breakage. The strength of the silicon is a function of the cube of the thickness. A 50-micron sample shouldbe able to take 125 times more force than a 10-micron thick sample. The thinner the desired process endpoint, the lower the allowable process force and the finer the permissible abrasive. A typical series of thinning steps, along with starting and ending thicknesses are shown in Tables 1 and 2. It can be seen that the time required to get 1 micron RST is largely the same as 5 micron RST. SELECTING THE SIZE OF THE AREA OF INTEREST The size of the area to be analyzed is determined fromthe evaluation of the 50-micron thick sample. The size of the thinned area must be larger by at least two times the diameter of the tools to be used in the thinning process. If a 2mm square area is needed, and 2 mm diameter tools are to be used, the minimum size of the thinned area should be 6 mm square. Larger is better for several reasons. The outside edge of the total area has the least amount of material removed as it has less time in contact with the tool face. This produces a fillet, or radius, at the intersection of the cavity edge and floor as shown in Fig. 1. This fillet will raise the tool slightlywhen the tool is at the edge causing it to extend further toward the center of the cavity. By then the tool has moved its diameter from the cavity wall; there is no longer any edge effect. Table 1 1 micron final RST Process step Abrasive, microns Starting RST, microns Ending RST, microns Removed, microns Final polish 0.04 1.5 1 0.5 Fine polish 1 4.5 1.5 2.5 Coarse polish 3 10.5 4.5 6 Fine lap 9 21.5 10.5 11 Coarse lap 20 50 21.5 28.5 Table 2 5 micron final RST Process step Abrasive, microns Starting RST, microns Ending RST, microns Removed, microns Final polish 0.04 5.5 5 0.5 Fine polish 1 9 5.5 3.5 Coarse polish 3 15 9 6 Fine lap 9 26 15 11 Coarse lap 20 50 26 24 Fig. 1 The edge fillet is not a radius, but gradually slopes to join the cavity floor. This will lift the tool slightly when it is near the cavity wall, thereby extending the edge distortion toward the center of the work area. Fig. 2 As can be seen, the slope of the area of interest can be faithfully reproduced in the lower end of the cavity. At the upper end, the fillet at the cavitywall will raise the tool creating surface distortion into the area of interest.
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