[SOLVED] Very basic question about CMOS tecnology scaling

[palmeiras]

basic question about CMOS tecnology scaling

Hello everyone,
Please, could you clarify why the gate oxide has been scaling with the development of CMOS technology?

It is clear that scaling the transistor length, on-resistance is decreased and the frequency increased. Moreover, reducing the transistor size, also reduces the parasitic capacitances, and again, improving the frequency. In addition, more transistors can be place in a smaller area.

But decreasing Tox means lower dielectric strength, what results in lower breakdown voltage and higher tunneling current. Moreover, decreasing Tox means higher Cox, what increases the parasitic capacitances for a given transistor aspect ratio. So, what is the advantage of decreasing Tox?

Thank you very much and best regards,

 

[RCinFLA]

You will lose gm if Tox is thicker. As device is scaled down so must the Tox scale in order for the gate field to have same impact on channel resistance.

For deep sub-micro (<65 nm) most processes include at least two Tox options. Thicker or double layer oxide is used for output ports to allow for higher I/O voltage.

Beside reduction in breakdown voltage there is also an increase in leakage current. There are options for two thresholds. Low threshold for speed (with more leakage) or high threshold for low leakage (with slower speed).

 

[palmeiras]

Hi RCinFLA,
Thank you for your reply. Please, could you help me to understand your point?
I think that If we are talking about analog and RF circuits, it is important to increase gm, but I don’t see why this would be helpful in digital circuits. Once the technology scaling is mainly motivated by the digital circuits, I think that higher parasitic capacitances caused by thinner Tox have more impact than higher transcoductance.
What do you mean with “As device is scaled down so must the Tox scale in order for the gate field to have same impact on channel resistance.”?
Thank you very much, and best regards.

 

[shravan87]

Hi palmeiras,

As already mentioned, scaling Tox improves gm. In other words, Id is increased. I can understand that you feel this is an analog circuit issue. But increasing current capacity of a device also means that it can drive more devices. Thus, in digital design, current plays a large role. More current means capacitances can be driven more easily. Of course, nowadays Tox has stopped scaling at the same rate (0.7) as other parameters, due to the problems you mentioned.

-Shravan

 

[palmeiras]

Hi shravan87,

Thanks for your comment. So, do you mean that although more parasitic capacitances (~lower speed) are added because thinner Tox, higher currents (~higher speed) caused by higher gm dominates the first effect?
Thanks and best regards,

 

[polytech1989]

Hello evrybody,

Normally the thickness of the Gate oxide is equal to the minimum featursize/50. This to obtain the correct scaling of our currents. The smaller the oxide, the greater the parasitic capacitance and the greater the leakage current, higher the speed of your FET. When the transistors become smaller you have 2 effects: VDD goes down and due to the thinner Gate oxide the breakdown voltage goes down. The problem is that they go down with a different slope.. there's the limit of our Gate oxide.. We can't go smaller. A solution is to increase the Gate capacity..

Hope it was helpfull..
grts