FD-SOI: A Cyber-Resilient Substrate for Secure Automotive Electronics

The paper highlights how Fully Depleted Silicon-On-Insulator (FD-SOI) technology provides a robust defense against Laser Fault Injection (LFI), a precise, laboratory-grade attack method that can compromise cryptographic and safety-critical hardware. As vehicles become increasingly digital and connected, with dozens of microcontrollers and over-the-air updates, hardware-level security has become central to automotive cybersecurity standards such as ISO/SAE 21434.

The Rising Threat of Physical Fault Attacks

Physical fault injection attacks (FIA) can bypass secure boot, unlock protected debug ports, and disrupt program flow. Among these, LFI stands out for its precision, using tightly focused near-infrared laser pulses to flip bits or alter circuit timing. While voltage and electromagnetic glitches can occur in the field, LFI remains the gold standard for systematically probing silicon vulnerabilities in controlled laboratory conditions.

As front-side access becomes harder due to thicker metal layers and shielding, back-side laser access through the substrate is increasingly used. This shift makes substrate engineering—the physical foundation of a chip—a critical security factor.

Why FD-SOI Disrupts Laser Attack Mechanisms

FD-SOI differs from bulk CMOS in that its transistors are built on an ultra-thin silicon layer electrically isolated from the main wafer by a buried oxide (BOX). This structural difference eliminates the main LFI fault mechanisms found in bulk silicon.

Four dominant bulk mechanisms are neutralized by FD-SOI:

1. Drain/body charge collection – FD-SOI’s thin silicon layer and BOX barrier dramatically reduce the photocurrent that lasers generate at PN junctions.

2. Laser-induced IR-drop – In bulk CMOS, current loops between wells and substrate can cause transient voltage drops. FD-SOI, using isolated body-bias networks, removes this conduction path.

3. Substrate diffusion and funneling – Charge carriers cannot spread vertically through the BOX, preventing multi-cell upsets and latch-up.

4. Parasitic bipolar amplification – Only a weak, lateral bipolar effect remains in FD-SOI, which can be further mitigated using reverse body-bias (RBB) to raise the laser energy threshold.

By blocking substrate conduction and confining active regions, FD-SOI significantly reduces the area and energy range vulnerable to laser faults.

Experimental Validation

Experiments comparing 22FDX FD-SOI and 28 nm bulk CMOS devices including D-flip-flops, SRAMs, and AES/ECC crypto cores confirmed the theoretical advantages. In tests, FD-SOI required up to 150× more laser shots to produce the same fault observed in bulk devices. The time-to-first-fault rose from roughly ten minutes to ten hours, while the minimum fault energy threshold increased from 0.3 W to over 0.5 W.

Spatial and depth-sensitivity mapping showed that bulk silicon has wide fault-prone zones, while FD-SOI faults are confined to sub-micron “hotspots” with a narrow focal depth of only about ±1 µm. Attackers must therefore perform ultra-fine spatial scans, drastically increasing effort and cost.

Furthermore, excessive laser power in FD-SOI caused permanent damage or stuck bits effectively creating a natural deterrent since aggressive attempts could destroy the target device.

Implications for Automotive Security Compliance

In the ISO/SAE 21434 framework, reducing attack likelihood directly lowers cybersecurity risk. FD-SOI’s physical resilience therefore simplifies compliance and can help products achieve Common Criteria or SESIP assurance levels (EAL4+ or higher) without extensive additional countermeasures. Because attack duration, equipment complexity, and expertise all increase, FD-SOI provides a quantifiable uplift in assurance for automotive OEMs and tier-one suppliers.

Toward a Next-Generation Secure Substrate

The authors envision extending FD-SOI’s benefits through substrate-level innovation, transforming it from a passive platform into an active cyber-resilient layer. Two emerging techniques are highlighted:

1. Buried optical barriers—highly doped layers under the BOX that absorb or scatter infrared light, reducing LFI energy transmission while enabling anti-counterfeit watermarking.

2. Integrated sensors and PUFs (Physically Unclonable Functions) substrate-embedded monitors that detect tampering or derive unique cryptographic identities from manufacturing variations.

Together, these innovations could allow the substrate to detect attacks, react in real time, and cryptographically bind the silicon identity to the vehicle platform.

Bottom line: FD-SOI represents a material-level breakthrough in hardware security. By eliminating substrate pathways exploited in bulk CMOS, it narrows the laser fault window, increases attack complexity, and provides tunable resilience through body-bias control. These benefits align directly with evolving automotive cybersecurity regulations, offering faster certification and lower system costs.

As substrate engineering continues toward integrated optical barriers and anti-tamper features, FD-SOI is poised to become the reference platform for secure automotive electronics, anchoring trust at the silicon level.

Read the full white paper here.

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