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Design >> Analog Design >> threshold voltage for 65 nm technology
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Message started by aaron_do on Apr 30^th, 2010, 1:17am

Title: threshold voltage for 65 nm technology
Post by aaron_do on Apr 30^th, 2010, 1:17am

Hi all,

I've just started looking at a 65 nm PDK, and found that although VTH0 is around 0.3 V, VTH is around 0.5 V for VDS = 0.5 V and L = 60 nm (no typing error - maybe its Leff?), no body bias. I understand that VTH can be length dependent, but my understanding is that the VTH should drop with length.

Does anybody know why VTH would be so high? As a result, a rather high value of VGS is needed to optimize fT.

BTW the transistor model is BSIM4.5. I'm using a so-called low-VT version of the transistor.

thanks,
Aaron

Title: Re: threshold voltage for 65 nm technology
Post by subgold on Apr 30^th, 2010, 2:03am

60nm is the minimum feature size of the lithography of the technology.

but i dont clearly understand your question. do you want to ask why Vth is so high? or do you want to know why there is a difference between vth0 and vth when there is no body bias (i suppose the vth and vth0 you r talking about are referred to the same length)??

Title: Re: threshold voltage for 65 nm technology
Post by aaron_do on Apr 30^th, 2010, 2:10am

Hi,

sorry for the confusion. I'm interested in why the VTH is so high and why does it increase with shorter length rather than decrease.

BTW I didn't think VTH0 varied with length, and it doesn't seem to in the PDK. So i'm only talking about variation of VTH with length.

thanks,
Aaron

Title: Re: threshold voltage for 65 nm technology
Post by subgold on Apr 30^th, 2010, 3:58am

aaron_do wrote on Apr 30^th, 2010, 2:10am:

hi,

threshold voltage depends on the doping process of the technology, so i dont believe "smaller vth at shorter length" is a general rule. in fact, none of the technologies i have used so far follows that.

and when you say vth is too high, you are comparing with which technologies? are those some 0.13um or 0.18um technologies from the same foundry? then it is quite reasonable because the gate oxide thickness has increased.

Title: Re: threshold voltage for 65 nm technology
Post by aaron_do on Apr 30^th, 2010, 8:27am

Hi subgold,

I'm only talking about 65 nm technology. i.e. when comparing two transistors in the same technology, one where I set the gate length at 60 nm and the other where I set the gate length at 200 nm (for example). The longer transistor has a reduced VTH. Is this result normal?

I'm not comparing VTH of different technologies, but it does seem quite strange that the VTH would be so high. It is actually even higher than the VTH of the 0.18 um PDK I am also using (for the 60 nm in 65 nm node versus the 180 nm in 180 nm node anyway).

cheers,
Aaron

Title: Re: threshold voltage for 65 nm technology
Post by subgold on Apr 30^th, 2010, 9:08am

hi aaron,

aaron_do wrote on Apr 30^th, 2010, 8:27am:

i dont see any abnormality with the longer transistor having a reduced vth. as i said, that is actually what i have seen from all the technologies i have used.

i would expect in a short channel transistor, since the gate dimension is smaller, the interface cap is also smaller, so u need a higher voltgate to obatin suffienct charges under the gate to compensate the carriers in the substrate, so as to create the depletion layer.

but i have to say i am no physics guru, do you have some physics proof to show that this shouldn't be the case?

Quote:

I'm not comparing VTH of different technologies, but it does seem quite strange that the VTH would be so high. It is actually even higher than the VTH of the 0.18 um PDK I am also using (for the 60 nm in 65 nm node versus the 180 nm in 180 nm node anyway).

cheers,
Aaron

isnt this what i said in my previous post? they increase the gate oxide thickness when they go from 180nm process to 65nm, maybe for the sake of leakage prevention and so on.

in general i dont think the vth is very high. it is really the case that the vsupply/vth ratio drops drastically when the technology geometry gets slimmer, and the headroom thus becomes one of the biggest obstacles in the design, for example i think u can hardly find a cascode amp in these technologies. (the other critical issue would be the low gm*gds value so you need more gain stages). please notice these technologies are optimized only for digital circuits and they are actually very unfriendly to analog design.

Title: Re: threshold voltage for 65 nm technology
Post by aaron_do on Apr 30^th, 2010, 6:54pm

Hi subgold.

thank for sharing. Yeah i did read your point about the thicker oxide in your original post. But it still seems a little strange. From my understanding VTH still drops with technology, although not as fast as VDD (which you just pointed out). However, I haven't actually heard of VTH increasing with a lower technology node. Since you say it is highly possible, i won't worry about it.

As for the shorter length leading to lower VTH, I was mainly referring to drain-induced-barrier-lowering (DIBL) whereby the drain-bulk depletion region becomes a more significant part of the channel and reduces the VTH. In the technology I am using, this effect seems to dominated by some other effect (of which I don't know) which causes VTH to increase with reduced length. This is my primary concern.

Quote:

i would expect in a short channel transistor, since the gate dimension is smaller, the interface cap is also smaller, so u need a higher voltgate to obatin suffienct charges under the gate to compensate the carriers in the substrate, so as to create the depletion layer.

I think i'll have to do some further reading to confirm this. It seems to me that with a smaller channel length you don't need as much charge anyway to "fill" the channel.

cheers,
Aaron

Title: Re: threshold voltage for 65 nm technology
Post by love_analog on May 1^st, 2010, 8:44am

This is called reverse short channel effect. Google it and you will get some ideas.

Title: Re: threshold voltage for 65 nm technology
Post by ACWWong on May 2^nd, 2010, 9:13am

or search this forum for rsce
http://www.designers-guide.org/Forum/YaBB.pl?num=1168499651/5#5
http://www.designers-guide.org/Forum/YaBB.pl?num=1164621927

Title: Re: threshold voltage for 65 nm technology
Post by loose-electron on May 2^nd, 2010, 4:10pm

Threshold voltage is a complex function of doping density under the channel, gate oxide thickness and a bunch of other things.

Channel doping and oxide thickness are the two primary things.

You can adjust those things and get any threshold you want (within reason of course, but parasitic junctions and second order effects are a different issue)

Threshold is typically set by foundry parameters (see above) to be between 20% and 40% of the power supply voltage that is used on the logic power supply.

Note: Logic Power Supply

The reason for this is to allow digital CMOS to function, with some immunity to ground bounce and power rail variance across a chip.

Now, from there the Logic power supply is limited by things like Vgs limitations (fried gate oxide issues)

What foundry model set is this? There's a lot of bad models out there.

Title: Re: threshold voltage for 65 nm technology
Post by aaron_do on May 2^nd, 2010, 7:16pm

Thanks for all the answers. The PDK is from Global Foundries, and is supposedly based on IBM's 65 nm PDK (although i'm not sure exactly what that means).

thanks,
Aaron

Title: Re: threshold voltage for 65 nm technology
Post by loose-electron on May 3^rd, 2010, 12:28pm

aaron_do wrote on May 2^nd, 2010, 7:16pm:

Thanks for all the answers. The PDK is from Global Foundries, and is supposedly based on IBM's 65 nm PDK (although i'm not sure exactly what that means).

IBM model sets are usually pretty good. Probably one of the community processes. 14SF or something similar would be my guess. (The XXSF series is their generic logic CMOS)

Title: Re: threshold voltage for 65 nm technology
Post by aaron_do on May 3^rd, 2010, 10:00pm

thanks for the info. Yeah actually the manual says: "CMOS 65-nm generation foundry technology developed for static random
access memory (SRAM), logic and mixed-voltage input/output (I/O) applications".

Guess its not really meant for high end Analog RF applications :D

Aaron

Title: Re: threshold voltage for 65 nm technology
Post by casual on May 4^th, 2010, 6:56pm

longer L has smaller since the I-V curve gradient is reduced. It is true for all processes. You could refer to those basic CMOS VLSI books.

Title: Re: threshold voltage for 65 nm technology
Post by ywguo on May 19^th, 2010, 4:55am

http://en.wikipedia.org/wiki/Reverse_short-channel_effect