Automotive IC Design Drives Simulation Innovation

By Roberto Stella, STMicroelectronics and Ahmed Eisawy, Mentor Graphics

STMicroelectronics invented the Bipolar, CMOS, DMOS (BCD) technology for the intelligent power applications demanded by automotive ICs. This technology is widely-adopted by the automotive IC industry. But, designing automotive ICs is very challenging. It requires innovative techniques to ensure that the ICs can stand up to the harsh environments and high reliability requirements that are key to technologies such as airbags, automatic braking systems (ABS), fuel injectors, power steering, and 800 volt hybrid power systems. No driver wants to find out what happens when any of these systems fail. These design challenges led STMicroelectronics to partner with Mentor Graphics to pioneer three key features for the Eldo circuit simulator that benefit the IC design community:

• Electro-thermal modeling and simulation
• Advanced aging simulation
• Safe operating area (SOA) simulation and analysis

Electro-thermal modeling and simulation

Advanced BCD technologies use deep trench isolation (DTI) to optimize area and to suppress parasitic effects in bi-polar junction transistors (BJTs). However, DTI increases thermal resistance and self-heating effects up to a factor of 2 versus junction isolation technologies. In order to properly simulate self-heating resistance effects over time, a model based on RC networks for the DTI technology was developed:

This self-heating network is added to an internal thermal node sub-circuit. The circuit simulator calculates the power dissipated by any element in the sub-circuit which is transformed into an equivalent current generator and automatically applied to the thermal node, taking into account only transport currents. The thermal network is extracted and added to the thermal node. The simulator updates the model parameters according to the instantaneous local temperature.

STMicroelectronics teams use the electro-thermal simulation feature of Eldo to evaluate components that dissipate large quantities of power and for checking current mirror mismatches due to self-heating effects.

Advanced aging simulation

Aging simulations are critical for automotive applications because:

• Bias and thermal conditions are very stressful.
• Automotive devices must be stable and fully working over a long period of time.
• The aging simulation can predict reliability issues so that they can be rectified at the design level.

PMOS technology degrades over time due primarily to negative bias temperature instability (NBTI). This degradation is activated by negative gate to source voltage, which is a normal condition for PMOS. The degradation is due to the creation of positively-charged, localized states in the oxide or at the Si/SiO₂ interface. The net effect of this degradation is an increase in |Vth| drift. In order to account for the effect of NBTI on V_ds values, STMicroelectronics developed an advanced model:

This model illustrates a simple current mirror, extracted from a real circuit where non-homogeneous NBTI led to an important drift. If the switch MN1 is on during the stress event, both MP1 and MP2 operate in saturation and experience the same degradation level. No shift in mirrored current results from NBTI. However, if the switch MN1 is off during the stress event, MP1 operates in saturation and it degrades less than MP2 operating in a linear region. This results in reduction of mirrored current.

STMicroelectronics simulated a comparator within an automotive product for ABS and found an offset problem due to asymmetric NBTI stress. At high temperature, one transistor in the comparator showed a reduction in the comparator threshold, while the other did not. After the stress point, the two transistors were mismatched. Using the aging simulation feature, the design team discovered a V_gs drift over a period of 3,500 hours at 175 C. High-temperature operating life (HTOL) readouts measured a drift in line with the simulation results.

Safe operating area (SOA) simulation and analysis

Automotive ICs are simulated to check if the component works inside its SOA. These checks determine if there are any dangerous situations that occur over the life of the component. However, these checks can be challenging because:

• The components on the IC can belong to very different voltage classes.
• Short voltage spikes are generated by switching applications due to parasitic inductances.
• Maximum operating voltage (MOV) can be exceeded up to the absolute maximum rating (AMR) if the cumulative duration of the violation is lower than a designer-specified critical value.

To address these challenges, Mentor Graphics developed a SOA browser to analyze and interact with violations:

Using the browser, it is possible to merge all the same violations reported over the total duration and a total, relative duration. It is also possible to filter the violation report for a specific lifetime, such as 1000 hours for reliability trials or 10 years for circuit life. In this way, the designers can detect any dangerous MOV violations.

For a fuel injector driver circuit, STMicroelectronics used the SOA browser to analyze MOV violations. It was proven that MOV violations did occur for short transients. But, with the lifetime filters activated, these violations were proven to not be dangerous. Without this feature, the designers could have decided to over-design the circuit or to increase the IC size in order to adopt higher voltage class components.

For a detailed look at how STMicroelectronics and Mentor Graphics worked together to drive simulation innovation for automotive IC design, view the whitepaper.