Written by Adrian O’Shaughnessy, Senior Layout Engineer

ESD: Do you need some shock therapy?!

In the electronics industry, losses associated with ESD are estimated at between a half billion and five billion dollars annually.

An ESD damaged chip can be worse than a complete failure of a chip as a damaged chip still may test good but cause intermittent problems or erratic behavior further down the line. So ESD can not only destroy a component but ESD can shorten a component's life.

Having ESD awareness and following through with good ESD layout practices are essential in reducing quality failures due to ESD. It is very important when designing and implementing an ESD layout program to know the ESD susceptibility of the ESD sensitive (ESDS) devices you are trying to protect. Classification of these devices should include all simulation models such as the Human Body Model (HBM), Machine Mode (MM), Charged-device Model (CDM) and Transmission Line Pulsing (TLP).

Figure 1 tracks ESD-related failures, as determined in Failure Mode Analysis, with deviations from good ESD layout practices:

The principles behind different ESD protection schemes must be fully understood as well as various ESD schemes that aim to create low impedance discharge paths to shunt ESD transients, clamp the pad voltages to sufficiently low levels and techniques that limit ESD currents from reaching the core.

% of FailuresDue to ESD

Figure 1: Correlation of ESD failure occurrence and deviations from good ESD layout practices

Deviations fromgood layout

practice

IC Mask Design’s new ESD course aims to fully explain these protection schemes as well as various other ESD topics such as ESD device compositions, parasitic effects of ESD structures, noise isolation, ground/power strategies, ESD electromigration issues, etc.Let’s take a look some specific ESD layout guidelines (taken from new ICMD ESD training course):

Vias must be placed correctly

Electromigration is the process of conductor heating and fusing due to excessive current and is growing as a design problem due to increased interconnect current densities related to IC down-scaling. Care is always given to supply rails, but electromigration is very prevalent in signal interconnect, especially ESD signal interconnect.

Contact holes and vias are particularly susceptible to electromigration. The current here commonly encounters a narrowing of the conductive pathway and so multiple vias must be organized such that the resulting current flow is distributed as evenly as possible through all the vias.

Avoid 90-degree corner bends

Pay attention to bends in interconnect. 90-degree corner bends must

Figure 2: The lower left via is overloaded while the upper right via hardly carries any current at all

Better Current Distribution

Current Crowding

Avoid 90-degree corner bends

Pay attention to bends in interconnect. 90-degree corner bends must be avoided in ESD layout structures as corners are highly stressed during ESD event. The use of 45-degree layout is useful in certain areas and is often used for optimal routing of wide signals. It’s also highly recommended in I/O cells in order to reduce power surge in corners and possible spikes that can result from an ESD event.

Metal routing is critical

Avoid metal routing that results in current crowding through a device. Figure 3 shows good and bad layout practices for metal routing. In the bottom diagram of figure 3, the right-most finger has least metal resistance and will experience the current crowding effect on the right side of the device.The upper diagram shows a good layout practice for metal routing as all fingers have equal metal resistance which ensures a more even current distribution.

ESD current in

ESD current in

ESD current out

ESD current out

Correct

Incorrect

Figure 3: Good and bad layout practice for metal routing

Figure 4 shows possible methods for good metal routing:

Current

Current

Current

Current

Current Current

Figure 4: The two figures at the right both illustrate uniform current distribution through the MOS device. The left figure illustrates the better option for contact-to-via placement

Adrian O’ShaughnessyEmail: adrianos@icmaskdesign.com

Tel: + 353 1 505 4540Website: www.icmaskdesign.com