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coplanarity issues may occur when fan-out RDL to connect to two TSV- bump joints and overmold of each
separated TSV-bridged dies are used bridge dies. Six HIFOM slices on top HIFOM slice is a little larger than the
to stack three or more dies. Among have a single RDL layer and the base die attached film of a conventional
three possible candidates, the structure HIFOM on the bottom has three RDL package. It is expected that the BLT
in Figure 3b – called HIFOM, as layers along with ball grid array (BGA) can also be reduced by shrinking the
noted previously – was chosen and balls for external interconnections. bump size and overmold thickness
its package character istics were Each slice of the HIFOM is connected later. Table 1 presents the comparison
investigated in this study [5]. through TSV-bridged dies, which of package thicknesses. Given that the
have no real circuitry like transistors die thickness is 60µm and the BLT
HIFOM structure and process or dynamic random access memory of the HIFOM is 40µm, the package
flow (DRAM) cells, and only has a back thickness can be significantly reduced
F i g u r e 4 sho w s t h e d e t a i l e d end of line (BEOL) metallization in HIFOM.
schematic diagram of the HIFOM and TSV for physical interconnects. The overall process flow of HIFOM
package. There are a total of eight Because the HIFOM structure has no is described in Figure 5. Each HIFOM
mobile memory dies with two landings organic substrates and a wire loop on slice has two mobile memory dies and
and four stacks. Two dies are paired the topmost die, the package thickness four TSV-bridged dies. For the RDL
together to form one slice of a HIFOM can be minimized even though the fabrication, the first step is to place the
package and then expanded by using a bond line thickness (BLT) with micro memory dies and the TSV-bridged dies

Figure 5: HIFOM process flow.

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