edfas.org 25 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 1 1.7 microns. This could be very problematic if the target RST is 1.0 microns. All of this is obvious. Just do the same thing, the same way, on every sample. There are variables, though, that must be considered. 1. How well can the sample surface profile be measured and how well does the tool travel reproduce it? 2. How consistent is the material removal process, both in average and across the die surface? 3. How can the irregularities in the material removal process be corrected and to what accuracy? It is not possible to process every sample exactly the same way as there are too many factors that affect the process and the measurement of results. First, the sample must be removed from the processing equipment for cleaning between process steps. Removal and replacement of the sample will change the alignment in all three axes. A single dead human skin cell can be 7 microns thick and about a million of these cells are sloughed off every day by every person. A single cell can interfere with contact profile measurements and, if lodged on the fixture mounting surfaces, will change axial alignments. Add to this, the changes in surface profile and thickness variation resulting from the redistribution of the stresses built into the sample when it was assembled. The diameter of the tool being used and the distance the tool center moves beyond the die surface have effects on the material removal rate and consistency across the die surface. The curvature of the die surface and the average plane deviation from the basic travel plane of the system used for processing will also affect removal consistency. The variation in some supplies can also affect results. Slurries and the adhesive used on lapping films and polishing pads as well as colloidals change with age. This requires using fresh supplies for processing to get as much consistency as possible. Do not buy supplies in bulk. Limit supplies so that they will be consumed in a month or two. PROCESS FLOW The process flow is the same for any sample. The die surface needs to be exposed to allow access for thinning. The die is aligned and profiled. The die is thinned in a way that quickly will remove the bulk of the silicon. Lapping and polishing steps remove the damage to the silicon from the previous step and refine the surface profile to create a sample with a consistent thickness with usable surface quality. EXPOSE THE DIE SURFACE Before a die can be thinned, the backside needs to be exposed. This may require removing a heatsink or spreader, or the removal of encapsulant and a die attach pad. Both processes are easily done by any CNC milling machine. Each process requires the alignment of the sample package to the machine and proper selection of tools used to remove whatever overburden is present. Enough of the die surface needs to be exposed to allow significant travel of the tool center beyond the die edges. DIE ALIGNMENT, LEVELING, AND PROFILING Die alignment is a process that establishes the die size and location in relationship to the processing system’s reference points and defines the area over which the tool must move. This is usually done by moving a die corner to cross hairs or a video monitor. Motion control for this is generally a joystick and either all die corners or two diagonal corners are aligned to the display markings, giving the machine the precise die size and position referenced to the machine coordinates, and, if all corners are aligned, the die edge rotation from the machines X and Y travel axes. The orientation of the mean die surface to the X-Y movement plane of the machine can be critical. Some machines only move the tool in a planar fashion. The Z-axis position of the tool cannot be changed dynamically as the tool is moved over the surface. Other machines are capable of following a programmed or measured profile, continuously adjusting the tool’s vertical position as it travels over the surface to reproduce the reference profile. In any case, either the sample needs to be leveled or the profile measured, or both. GROSS SILICON REMOVAL Depending on the starting die thickness and the desired remaining thickness, it may be necessary to grind off a lot of silicon. For a larger die with a starting thickness of 770 microns and a final thickness of 50 microns, about 640 microns needs to be removed through grinding. This leaves about 130 microns of remaining silicon. The damage done to the silicon by a grinding tool can easily extend as much as 1.5 times the diamond grit diameter. The normal grit diameter for a grinding tool is 66 to 115 microns. Attempting to grind to less than 2 times the grit size invites damage to the active surface and the disintegration of the sample. Controlling in-feed rates and tool linear-travel rate can allow grinding to 125 RST with a 115-micron grit diamond-grinding tool.
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