IR Drop in VLSI

Power distribution within a chip relies on feeding every transistor that is accomplished through the utilization of metal layers. With the new advancement of modern technology, the wires responsible for distributing power have been shrinking in size while the physical dimensions of chips have remained relatively unchanged. Remarkably, despite these changes, the overall power consumed by chips has remained relatively constant.

Moreover, as the current or power flows through a resistor, the voltage drops- this is called an IR Drop.

In this article, we will be discussing IR Drop in VLSI, types of IR Drop and EM.

IR Drop in VLSI is known as an "Intermediate Resistance Drop" at "Very Large Scale Integration". It refers to the variation in electrical potential between the two ends of a conducting wire when current flows through it. This potential difference is determined by the voltage drop across a resistance, which can be calculated by multiplying the current (I) passing through the resistance by its resistance value (R).

V (Voltage) = I (Current) X R (Resistance)

Note: To avoid this, it is important to consider the problems of IR Drop during the design of the VLSI system.

=IstrapAvg*Rs*(W/2)*(1/Wstrap)

Nstrappinspace=Dpadspacing/Lspace.

MIN Ring Width = wring = Ip/Rj Microm

Static IR drop in VLSI refers to the average voltage drop experienced within a VLSI design. This voltage drop is influenced by the RC network of the power grid and plays a crucial role in establishing connections between the power supply and the individual cells.

The magnitude of the average voltage drop is determined by various factors, including the time period and one significant contributor to static IR drop is the gate-channel leakage current.

Vstatic drop= lavy x Rwire (lavy is refer to leakage currents)

Dynamic IR drop in VLSI is dropping in the voltage because of the high switching activity of transistors. The drop happens when the demand for current increases in power supply. It happens due to switching activities within the chip. Moreover, it evaluates the IR drop occurred when a large number of circuitry switches at the same time. Hence causing peak current demand.

Note: IR Drop in VLSI occurs specifically when there is a heightened requirement for current from the power supply, which is triggered by the switching activities of the chip.

Vdynamic_drop = L (di/dt) [L is due to switching current]

The gradual displacement of metal atoms in a semiconductor is called Electromigration. EM occurs when the current density is high enough to cause the drift of metal ions in the direction of the electron flow and is characterised by the ion flux density.

Whereas IR drop intensifies in the presence of Electromigration.

With EM effects, the chances of metal wire bursting into open and shorts are high. As EM increases the wire resistance, causes dropping in voltage. Hence, this will let the device slow down or can cause permanent failure in circuits.

Electromigration (EM) induces a phenomenon where the downstream interconnects in a circuit experience narrowing, while the upstream interconnects and vias undergo metal deposition. This effect of EM results in both the creation and disruption of connections, leading to changes in the resistance of interconnects and vias.

Every Company has their ways of analysing the IR Drop and based on that the precaution has been taken care of.

When the cells switch the drop independently is calculated with the help of wire resistance. The ways to fix the Static IR Drop in VLSI:

The formula for calculating the Dynamic drop happens with the help of switching off the cells. Ways to fix Dynamic IR Drop:

As the technology node is contracting, it leads to decreases in the geometries of the metal layers and the resistance of the wire. Hence, this lead to power supply voltage during CTS, the buffers and inverters which were added along the clock path to balance the skew.

IR drop shows the voltage drop on the current and these issues are highly common. With various techniques and measures, these issues can prevent and also reduce electromigration in lower geometry chip designs.