The STM8L (8-bit ultra-low-power) of STMicroelectronics is really lower power with 570uA at 4MHz down to 400nA in halt mode.
Then if ypou need more performance and need to go lower in power consumption you should have a look at the STM32L (Cortex-M3 ultra-low-power).
What are the engineering and design challenges in creating successful IoT devices? These devices are usually small, resource-constrained electronics designed to sense, collect, send, and/or interpret data. Some of the devices need to be smart enough to act upon data in real time, 24/7. Specifically the guests will discuss sensors, security, and lessons from IoT deployments.