Noise coupling in 3D IC
Noise coupling (or crosstalk) in 3D ICs is one of the biggest reliability issues in 3D ICs. In ICs, two adjacent metal wires form a parallel capacitor, and this capacitive coupling is the source of the noise between the two wires. In 3D ICs, in addition to this wire coupling, two adjacent TSVs have a coupling network between them due to the silicon substrate and silicon dioxide insulator. This TSV-to-TSV coupling could be very problematic in 3D ICs, because TSVs are big and tall so that the coupling between two adjacent TSVs is huge. In addition, these TSVs are connected to metal wires, so the total coupling of two signal paths cause serious crosstalk problems in 3D ICs.
What is TSV (through-silicon via) ?
TSV is a thick, tall metal that penetrates through the silicon substrate. This technology actually stemmed in the early 90’s. However, due to the expensive fabrication cost, this was a technology that was sleeping for a long time.
As seen in the figure, Note that TSV on the left is significantly big comparing to other components such as metal wires (BEOL) and Trench Cap on the device layer. The importance of this is that now we have a way to provide a direct path to the outer I/Os in both directions (chip surface and chip bottom). Without this TSV technology, chip surface was the only connection that we can make to the outer world.
How significant is the coupling?
Let’s think about two things: (1) Coupling capacitance that forms between TSVs and (2) The noise voltage it causes. Unlike the common belief that only the nearest aggressors impact TSV coupling, TSV coupling occurs even between the non-neighbor aggressors. This is because the far aggressor also has a significant amount of capacitance between the victim (close aggressor: 9.46fF, far aggressor: 4.14fF). Though the close aggressor shields the E-field between the victim and the far aggressor, it cannot be perfect. A strong E-field detours the first aggressor and forms capacitance between the far aggressor and the victim.
We can see that the far aggressor affects as much coupling voltage (139.6mV) as the close aggressor (184.6mV) Despite the far aggressor has less than 50\% capacitance of the close aggressor, the noise voltage reduces by only 40mV. This is because of the complicated coupling network that TSVs compose.