edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 4 34 PROCESSES FOR THINNING AND POLISHING HIGHLY WARPED DIE TO A NEARLY CONSISTENT THICKNESS: PART I Kirk A. Martin RKD Systems, Aptos, California kirk@rkdsystems.org EDFAAO (2022) 3:34-38 1537-0755/$19.00 ©ASM International® INTRODUCTION Previous articles by the author and others partially addressed the need for thinned andpolisheddie that have a remaining silicon thickness (RST) of 50 to 100 microns and variations of +/- 5 microns or less. Tighter thickness tolerance and thinner samples are always desired. Thinner RST is required as doping density increases due to absorption and dispersion in highly doped silicon and better tolerances require less refocusing and less uncertainty in imaging. Finer linewidths, newprobing, and visualization techniques may require RST of 1 to 5 microns with variations of 0.50 microns or less. Thinning the entire die to 1 to 10microns can eliminate the possibility of powering the die after thinning due to heat dissipation considerations, and the thinning can take a rather long time. Thinning to 50 microns RST is fast and most samples can be powered at this thickness. Analysis is then performed, which identifies the area of the die that is of interest. The area of interest is then thinned to the desired thickness, leaving the rest of the die thick enough to allow for powering the sample. Thinning only a small area significantly reduces processing time and results in a much more robust sample. The processes for local area thinning will be discussed in a future article. Thepresent articledescribes processes for finer control of RST through the correction of measured mechanical surface profileswithmultipoint thicknessmeasurements. Also described are the dynamic changes in sample surface profile that make multipoint thickness measurements necessary. These surface profile changes are shown to be a function of the samplemounting process, process time, and the removal of bulk silicon. In addition, the realities of silicon thickness measurements are discussed. REVIEW OF PREVIOUS ARTICLES In previous articles, the authors have described a technique for controlling RST variation by taking low spatial resolution thickness measurements and using thesemeasurements to alter themeasured surface profile used to thin the sample.[1] They have also described the dynamic characteristics of the surfaceprofile of amounted device.[2] The surface profile of a mounted device varies with time as the mounting media cold flows in response to the forces generated during the mounting process. As wax mounting is done at a temperature higher than the waxmelting point, the sample reduces in curvature at the higher temperatures. As the sample cools, without the mounting wax, it would return to the higher curvature it Fig. 1 The thickness variation resulting frommountingwax cold flow during material removal.
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