Gate density has been increasing with each node and that pushes up power per unit area. This has become an even more significant issue with FinFET processes, where the channels are more thermally isolated than planar processes before them.
Then there is leakage, which in the last few planar nodes was an issue that led to significant power consumption. That has been pegged back somewhat with the latest FinFET nodes but it will continue to be an issue going forward as we look toward the next generation FinFET nodes and beyond.
In addition to these issues, if you are developing for consumer products, smartphones, tablets, that kind of thing then you are always limited in terms of how much heat you can dissipate because you don’t have active cooling systems such as fans, and obviously the upper temperature limit of the product is quite low. In addition, the hotter things get the bigger the issue of reliability and lifetime of device parts which is perhaps the biggest issue going forward, as we are then talking about electro-migration, hot carriers, and BTI effects which we have discussed in the past.
How hot is hot?
That all depends on the application! That said, one thing that is interesting now with the growth in automotive applications, such as ADAS and infotainment is we are starting to see that even 125°C is not high enough as those markets demand higher temperature operation. So for those applications hot is hotter than it may be for say a consumer device where 40°C for the product might be your limit. Then there will be a thermal mass to factor in so you will have devices within that product which are much hotter.
But the key thing for our customers is knowing device temperature accurately. The more accurately they know the temperature the closer to the limit they can operate. That is really what it is all about for modern SoCs; being as close as you can to the limit without stepping over it. And because temperature has an exponential effect in terms of ageing, the accuracy of temperature sensors is correspondingly important.
Trend in use
Certainly a number of years ago when we started developing temperature sensors, they were being used generally just for device characterisation, HTOL, burn in tests and those kind of things. Then they started to be used for high temperature alarms, either to switch off the device or turn on a fan. But we have seen over the last couple of years more applications which rely on these monitors. Applications like Dynamic Voltage and Frequency Scaling (DVFS), Adaptive Voltage Scaling (AVS) and lifetime reliability. These applications make use of the sensor data in a feedback control loop. So certainly the use cases now are much more varied.
The trend for the recent past has been driven by consumer electronics and in those cases you are really trying to get a lot out of a device whilst not making it too hot, because it’s in your pocket, or it’s on your lap or whatever, so this has driven the use cases. I believe that we are moving into a space where just the cost of the advanced node technologies mean you want to get everything out of a device , and all of the different levels of over design that are added to the process, the design flow, take away performance. As a result, having sensors on chip, whether they are temperature sensors, or process or voltage allow you to get that little more performance out of your device and, or improve reliability.
What requirements does that place on temperature sensors?
The most important thing from where we sit is accuracy. The greater the uncertainty in the measured result, the less you can do with it. So for us the key motivation is accuracy. But beyond that the next thing is robustness and testability, because you are now using these sensors in application areas where their failure can cause system failure. This means you need to be able to test them, you need to be able to rely on them. So we are doing a lot in that sense to ensure that there is testability and there is robustness in our products.
How does Moortec address those requirements?
The first thing is that we meet the accuracy requirements and we aim to exceed them. In terms of testability and robustness we have done a lot of work to be able to provide online fault detection and diagnosis of our sensors. This means you can interrogate them and understand if there is a fault. Firstly, it will tell you if there is a fault, and secondly you can then ask it what is wrong and it can give you certain amount of health diagnosis. In addition, we support scan chains to increase overall test coverage.
Then on top of that we believe ease of integration is an important factor. Not because it gives you a more accurate temperature sensor, but to make it easier for the customer to implement and use the product.
About the author:
Oliver King is the Chief Technology Officer of Moortec Semiconductor – www.moortec.com
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