Designing Electrostatic Discharge (ESD) Protection for Monolithic SoCs and Multi-Die Systems

Did you know that over 30% of semiconductor failures are attributed to electrostatic discharge (ESD)? Defined by the ESD Association as "the rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field,” ESD causes equipment malfunction by disrupting the normal operations of electronic systems. ESD-induced damage ranges from leakages and shorts to junction and metallization burnouts, gate oxide ruptures, and resistor-metal interface deterioration. Real-world examples of ESD-induced chip failures could include a smartphone that delivers electric shocks, a fitness tracker with a blinking screen, or a malfunctioning automatic emergency braking system.

To minimize ESD vulnerabilities, semiconductor companies integrate protective devices or circuits into their silicon. Essentially, these components prevent internal circuitry—and the protective elements themselves—from incurring damage during an ESD event by creating low-resistivity discharge current paths. Although protective elements are effective when properly implemented, designing ESD-resistant silicon on the latest process nodes is increasingly challenging. Indeed, engineers pack billions of circuits into dense monolithic systems-on-chip (SoCs), leaving only limited area for protective ESD elements that must be meticulously placed and verified. Moreover, multi-die systems introduce a slew of new ESD vulnerabilities via complex thermal and electrical interactions between processors, memories, and interconnects.

Read on to learn how evolving ESD challenges are spurring semiconductor companies to augment conventional static checkers with a new generation of full-chip design tools that rapidly analyze silicon and simulate millions of transient ESD surges.

From ESD Protection Diodes to Silicon-Controlled Rectifiers

There are three primary devices used to build protection clamps:

• Diodes: Structurally simple, diodes meet key requirements for low-voltage ESD applications. When forward-biased, diodes are considered one of the most effective protective elements– with low turn-on voltage, low on-resistance, and the ability to handle high ESD currents. However, diodes demonstrate high turn-on voltage, high on-resistance, and low current handling capabilities under reverse bias conditions.
• Grounded gate n-channel MOSFET: Grounded-gate n-channel MOSFETs (GGNMOS) are typically leveraged to protect CMOS-based designs. Although GGNMOS are structurally and operationally similar to conventional MOS, layout techniques differ to maximize ESD performance. GGNMOS devices can operate in either active or snapback mode, with the latter considered the most common and effective.
• Silicon-controlled rectifier: The silicon-controlled rectifier (SCR) is frequently rated as the most efficient and robust ESD protective device due to its bipolar conduction mechanism. Although SCRs are prone to “latching up,” or conducting current after an ESD event ends, this shortcoming can be effectively mitigated with optimized silicon designs.

Verifying Resistance to Transient ESD Surges

Semiconductor companies leverage various component-level standards to verify silicon resistance to transient ESD surges, including the human body model (HBM) and Charged Device Model (CDM). As defined by the ESD Association, HBM represents the discharge from the fingertip of a standing individual delivered to the device. Typically modeled by a 100 pF capacitor, HBM is charged by a high-voltage supply through a high-ohmic resistor (typically in the megohm regime) and subsequently discharged via a switching component and 1.5 kW (1,500 ohms) series resistor. A standard HBM waveform includes a rise time of 2–10 ns, a peak current of 0.67 amps/kilovolts, and a double-exponential decay with a width of 200 ns.

A CDM incident occurs when a charged device contacts a grounded object. Specifically, the component is the charge source, which discharges through a grounded body. The CDM test procedure involves placing the device on a field plate with its leads pointing up, then charging it, and discharging the device. All pins are treated equally—and discharged after positive and negative charging. CDM events are the leading cause of ESD failures in modern circuits. Although discharge duration often lasts less than a single nanosecond, peak currents can reach several tens of amperes, causing significant voltage drops and dielectric breakdown.

Comprehensive Full-Chip ESD Analysis

Semiconductor companies are finding it increasingly difficult to routinely perform CDM tests with consistent results. Because CDM is directly affected by the environment, precise chip and package substrate data are needed to accurately define simulation variables. This data is more challenging than ever to obtain and simulate as dense monolithic SoCs typically include billions of circuits—while new multi-die systems introduce complex thermal and electrical interactions between chips placed on a single package.

Indeed, ESD failures can occur in metal interconnects, ESD devices themselves, and the core devices they are meant to protect. Although metal interconnects are crucial elements of the ESD discharge path, they are often evaluated manually or with tools that weren’t designed to independently simulate CDM currents in large, complex chips or multi-die systems. That’s why semiconductor companies now leverage full-chip ESD tools to verify interconnects, ESD devices, and core devices for HBM and CDM events.

Full-chip ESD tools can highlight at-risk designs, pinpoint susceptible devices, and automatically generate reports of current density violations and high-resistance paths. In addition, full-chip ESD tools can perform transient simulation of the full chip and package—analyzing all interconnects, protective elements, and devices including inductors, inductance, and capacitors. Moreover, analysis can be performed on both pre- and post-layout versus schematic (LVS) clean layouts, allowing quick identification and correction of potential issues. Lastly, hierarchical debugging can provide both telescopic and microscopic views of chip designs to deliver granular insights of ESD vulnerabilities.