Spectre Tech Tips: Introducing Electrothermal Simulation

Understanding the thermal performance of integrated circuits has been essential to avoid the overheating that can cause circuit malfunctions. With greater integration, the power dissipation in integrated circuits is becoming increasingly important in order to limit the on-chip temperature. Spectre provides thermal analysis of IC designs to help IC designers to manage thermally-aware designs and improve the thermal property of their products.

For example, for the chip design shown in the figure below, a thermal sensor is placed on the chip to monitor the temperature. However, the actual hot spot is located some distance away from the sensor. Because of the separation between the hot spot and the sensor, there is a temperature difference and thermal propagation delay between their thermal profiles. Therefore, the shutdown circuit needs to take into account the temperature difference and the delay in order to properly protect the circuit from overheating. Thermal analysis can be used to characterize the temperature difference and thermal propagation delay.

As another example, the following figure shows a floor planning case where the heat source on the chip produces a temperature gradient around it, and the devices sitting on different isothermal contour lines will behave differently due to the temperature difference. Therefore, for devices that are designed to operate under matching conditions, such as differential pairs, it’s important to make sure they are placed on the same temperature contour to allow the circuit to function correctly.

Spectre provides thermal analysis of IC designs by performing electrothermal co-simulation or electrothermal analysis. Because the electrical behavior of semiconductor devices is strongly temperature dependent, an accurate analysis of an electrical circuit must be carried out with the thermal and electrical analyses being coupled as shown in the following figure:

Electrical simulation is performed to produce the average power consumption of the devices in the circuit. These power consumption values are then used as the heat source for the thermal simulation which calculates the temperature on the devices. The resulting device temperatures are then used to update the device models used by the next electrical simulation, and the iteration continues until the power and temperature values settle.

As shown in the flowchart below, Spectre thermal simulation requires the following inputs:

• An input netlist which serves as the testbench
• The design to be simulated for thermal analysis must be in the DSPF format because it provides device location and geometry information in addition to the electrical models and connections
• Thermal configuration files for thermal set up and control

The thermal model used for thermal calculation is provided by the Cadence Celsius Thermal Solver through an API which is transparent to the users. Both the electrical and thermal simulations are carried out in Spectre. Therefore, the user needs to interact with Spectre user interface only.

In addition to the usual electrical simulation data, such as voltage and current waveforms, Spectre electrothermal simulation also generates thermal simulation output data, such as device or thermal grid temperatures.  Both of them can be viewed under Cadence Virtuoso/ADE tool such as shown in the following figures:

Device temperature waveform

Heatmap display of thermal grid temperatures

Related Resources

• Spectre® Classic Simulator, Spectre APS, Spectre X, and Spectre XPS User Guide

You may also contact your Cadence support AE for guidance.

For more information on Cadence products and services, visit www.cadence.com.

About Spectre Tech Tips

Spectre Tech Tips is a blog series aimed at exploring the capabilities and potential of Spectre®. In addition to providing insight into the useful features and enhancements in Spectre, this series broadcasts the voice of different bloggers and experts, who share their knowledge and experience on all things related to Spectre. Enter your email address in the Subscriptions box and click SUBSCRIBE NOW to receive notifications about our latest Spectre Tech Tips posts.