Riding High on SOI

Toyota has a long history of SOI innovation.

Quietly but steadily, Toyota has lead the industry in the use of SOI-based chips in key automotive applications.

A few years ago, Toyota published a paper entitled “Multi-Voltage SOI BiCDMOS for 14V & 42V Automotive Applications” (Kawai et al, ISPSD Proceedings, 2004), in which researchers presented the company’s new technology for high-power, high-voltage automotive applications. Toyota had developed this SOI-based technology to answer the growing list of requirements for electronic control units (ECUs), which help create more environmentally friendly, safer and more comfortable automobiles.

ECUs requirements included:

  • BiCMOS for processing analog and digital signals from sensors and switches;
  • Power MOSFETS for output control actuators for motors, solenoids, lamps, and so forth;
  • Compatability with both conventional 14V batteries and a new generation of 42V batteries;
  • More densely-packed signal processing circuits;
  • LDMOS (lateral DMOS) with lower onresistance as well as stability under harsh automotive conditions (high temperature and voltage surges, for example).

All in one

For their starting substrate, the Toyota research engineers chose a 200mm bonded-SOI wafer, with a very thick (12 μm) top silicon layer, specially tailored to their design. Their specifications, they said, resulted in a reduced chip cost.

The single-chip design integrated:

  • Six LDMOSs (which required only two additional mask steps) with 35V, 60V and 80V breakdown voltages;
  • A range of highly-packed power MOSFETs and BJTs;
  • Deep trench isolation;
  • A cost-effective (rather than leading edge) CMOS process.

In addition to operating stability under voltage surges and in high-temperature conditions, the choice of the less expensive CMOS process on the bonded SOI wafer proved to be a costeffective solution, they said.

Taking advantage of the new technology, Toyota developed a range of ICs for automotive applications, including throttle control, suspension control, body control, hybrid control and brake control (see Figure 1). They concluded that, “This technology can be expected to demonstrate high cost-performance in future automotive applications.”

Figure 1. Using its SOI-BiCDMOS technology, Toyota developed a generation of SOI-based automotive ICs. (Courtesy: Toyota)

Braking ahead

A few years later, another team of Toyota research engineers presented a paper at the IEEE SOI Conference, entitled “An SOI-BiCDMOS Chipset for Automotive Electronically Controlled Brake System” (Wasekura et al, 2006).

Prompted by the specific braking requirements of hybrid vehicles, but needing a cost-effective solution that could be deployed across all vehicles, this team leveraged the SOI Bi-CDMOS technology in an electronically controlled brake (ECB) system.

As pointed out in the paper, brake-by-wire (non-mechanical) braking systems for hybrid cars get a lot of attention because of the need to coordinate a regenerative brake and a friction brake. A role of the ECB in hybrid cars is to distribute the brake force to each of the four wheels. In all vehicles, it is also part of the system for throttle activation, power steering and improved dynamic performance.

Hot spots

But to make it sufficiently cost-effective to be used in conventional vehicles, an electronic control unit (ECU) and the ECB actuators had to be installed closely together in the engine compartment. This made it necessary to also reduce the size of the units and the number of wire harnesses that are typically deployed to ensure fail-safe operation.

To ensure stability in the harsh, high-temperature conditions, two critical LSIs in the ECU (see Figure 2) were fabricated using the SOI BiCDMOS process:

  • An input-signal-control LSI (shown as LSI1 in the figure), which served as a switching voltage regulator, a “watchdog” for CPU behavior, and handled processing for wheel-speed, pedal-stroke and hydraulic pressure sensors.
  • A linear-drive-current driver (shown as LSI2 in the figure), capable of accurate current measurement under a very wide temperature range (from –40° to 125°C).

Figure 2. A block diagram of the ECB ECU. LSI1 and LSI2 were developed using the SOI-BiCDMOS process technology. (Courtesy: Toyota)

Figure 3. The new SOI-based ECB (a) is 13600mm2, 41% small than the previous non-SOI generation (b) (Courtesy: Toyota)


As a result of the SOI-based process used in the ECB, the size of the ECU was reduced by 41% over the previous model, while matching its performance (see figure 3).

Cost-effectiveness, high-performance and reliability under the extreme conditions found under the hood – this explains why Toyota has already started deploying SOI-based chips throughout the top end of its lines.

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