|
The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Poly resistor self heating https://designers-guide.org/forum/YaBB.pl?num=1325521791 Message started by RobG on Jan 2nd, 2012, 8:29am |
|
Title: Poly resistor self heating Post by RobG on Jan 2nd, 2012, 8:29am On another site a person was wondering how much a poly resistor would self heat if dissipating ~2.5 mW @ 2mA. I would have dismissed it, but I guess it can be a problem. Poly is encased in oxide so the thermal resistance to the outside world is relatively high. Do any of you know about what to expect for temp increase, or the typical recommended current density for poly? |
|
Title: Re: Poly resistor self heating Post by loose-electron on Jan 2nd, 2012, 12:54pm RobG wrote on Jan 2nd, 2012, 8:29am:
Foundry specifications for current density should be your guideline here. It will be foundry unique due to thickness of the poly layer. Heating of substrate can be done using the total wattage on chip and the thermal (degrees C/watt) rating of the package. Localized heating on the die is a little toughter. You can do a thermal model and estimate the thermal conductivity of each layer and all that. Its a time consuming PITA. If you got silicon, you can do an infrared photo to see whats "hot or not" From prior work, a hot element on die, tends to conduct pretty well to whats around it. (based on the "hot or not" pixs) Thermal isolation of oxides, probably not that good because the oxides are so thin. Best I got. |
|
Title: Re: Poly resistor self heating Post by Lex on Jan 3rd, 2012, 12:54am A rough estimate would require a lot of parameters, even to get to the right order of magnitude. I agree with loos-electron on the current density part: they don't want the real resistors to deviate too much from the models they provide. =) I have not encountered any cases with self heating of large poly silicon resistor banks. Most of the heat related issues I had were temperature gradients across the chip, but the standard layout techniques easily covered this. I doubt that the poly silicon height really matters. Usually the height is much smaller than the width and length. In principal it is about the area of the resistor, the distance to the good thermal conducting silicon, and the thermal conductivity of silicon dioxide (probably not a constant vs temp btw). This is of course under the assumption of an ideal substrate. I guess for thinned substrates or even SOI, heat dissipation effects might be more pronounced (no experience here). I wonder how the thermal shocks given by ESD to the devices are handled over there. I tried the heat sensor tool/gun once on a PCB but it was very difficult to use and I couldn't draw any conclusion from the images it gave. Especially the reflection of devices is very confusing. |
|
Title: Re: Poly resistor self heating Post by thechopper on Jan 3rd, 2012, 6:19am One more thing to consider is the driving circuit and TC of your poly resistor: if resistivity decreases with increasing temp, and the driving circuit is capable of provide the necessary power, more current will flow and eventually the resistor could be fused. I guess those limits will depend on the process parameters, and as I suggested whether the TC is positive or negative (depending on the poly implants if any). Best Tosei |
|
Title: Re: Poly resistor self heating Post by RobG on Jan 3rd, 2012, 7:46am I got some numbers for a 0.18u CMOS 250 ohm/sq unsilicided resistor from a friend. They surprised the heck out of me. I won't give you the exact numbers but the change in temperature is proportional to the square of the current density and is approximately: 0.4*Rsheet*(I/W)^2, where I is in mA and W is in um. So, for 2 mA current in a resistor with a width of 1um the temp increase will be 100*(2/1)^2 = 400 C. Wow. At the maximum current density with no similar devices within 6um the temp increase is about 100C. Twice that if you have other resistors close by (I assume these resistors need to have the same current density but the rules didn't say). Don't push the limits for sure. Luckily it is inversely related to the square of the width, so a width of 5um only increase the temp by 16 C. Since the temp change is proportional to current density I would assume that changing the height of the poly would have the same effect as changing the width. |
|
Title: Re: Poly resistor self heating Post by loose-electron on Jan 3rd, 2012, 9:36am RobG wrote on Jan 3rd, 2012, 7:46am:
Thats a lot of current in a very small space. I would get the current density rules and the electromigration rules for the process and start there. |
|
Title: Re: Poly resistor self heating Post by RobG on Jan 3rd, 2012, 9:44am Actually, in my post I also did the calculation with the max current density that were in the rules that my friend gave me. The temp change was 100 C! I'm surprised they'd let it get that hot so the process guys probably put a lot of margin in that number. This wasn't for a design I was doing so I don't have any rules of my own. I was helping a friend in academia. The rules given to students are often incomplete. rg |
|
Title: Re: Poly resistor self heating Post by aaron_do on Jan 3rd, 2012, 7:35pm Hi RobG, Quote:
note that this is equal to 0.4*Power/Area (uW/um2). So it is simply the heat power per unit area generated with some factor added. This result is kind of surprising in that the answer doesn't depend on the surroundings. As others have mentioned, the increase in temperature depends on the thermal resistance between the die and the environment. I'm a bit skeptical of the equation, but maybe because the power dissipation is so localized it loses this dependence? Or maybe it is an instantaneous increase in temperature (i.e. steady state temperature is lower)? regards, Aaron |
|
Title: Re: Poly resistor self heating Post by RobG on Jan 3rd, 2012, 7:59pm It's a steady state temp. They had a second equation where the increase was about 2x if there were similar resistors close by. Maybe the thermal resistance of the oxide is so high that it dominates the paths to the thermal ground (or whatever they call it). They don't talk about packaging either so it is probably just a swag at the answer, but it is good to know when things get really hot. I wonder if you could keep it cooler by covering it with "grounded" metal. |
|
Title: Re: Poly resistor self heating Post by aaron_do on Jan 3rd, 2012, 9:13pm The oxide is extremely thin so normally it would not affect the overall thermal resistance, also Polysilicon itself is a good thermal conductor. I guess you are looking in the right direction though. Perhaps if the resistor is small, then the local thermal resistance dominates the paths to "thermal ground" (i.e. air). So they can guesstimate the temperature. The easiest way to keep it cooler is the one you already pointed out...make it bigger. Quote:
that's also surprising. If you have two similar resistors, then you have double the area and double the power. I can't see why the temperature would increase... Aaron |
|
Title: Re: Poly resistor self heating Post by Lex on Jan 4th, 2012, 12:08am aaron_do wrote on Jan 3rd, 2012, 9:13pm:
The oxide has a much lower thermal conductivity/higher thermal resistance than the substrate. Usually a resistor is made over STI hence it increases the distance to the thermally conducting substrate. So most of the delta T will be between the resistor and the substrate. The thermal resistance between the poly silicon and the substrate will be roughly: Rth = z / ( k * L * W) z : distance between poly silicon and substrate (height of the oxide between the resistor and the substrate) k : thermal conductivity oxide L,W : length and width of poly silicon resistor Of course, this formula is very simple and only valid for large surfaces, for which the temperature increase is probably negligible. |
|
Title: Re: Poly resistor self heating Post by aaron_do on Jan 4th, 2012, 1:04am Hi Lex, Quote:
Are you sure about that? For an unnamed CMOS process which I have access to, the substrate thickness is 625 times thicker than the STI. The thermal conductivity of silicon is 149 W·m−1·K−1 while that of SiO2 seems to be around 1.4 W·m−1·K−1. So the thermal resistance of the bulk is still about 6 times higher. Feel free to correct me here if I'm wrong. regards, Aaron |
|
Title: Re: Poly resistor self heating Post by Lex on Jan 4th, 2012, 1:27am aaron_do wrote on Jan 4th, 2012, 1:04am:
The factor of 6 would hold only if you would have no lateral dispersion of the heat in your substrate. With the dispersion, I think you'll get temperature iso-clines that are related to the square of the distance to the heat source. The reason that I ignore this for the oxide, is that the resistor's sizes are assumed larger than the oxide layer thickness (only true for relatively large resistors). |
|
Title: Re: Poly resistor self heating Post by aaron_do on Jan 4th, 2012, 1:45am Hi Lex, yes I agree. I think RobG's original guess was correct Quote:
because he is dealing with such a small area. regards, Aaron P.S. that leads to a different question. Normally the resistance is temperature dependent and we set the temperature in the simulation options. So is this self-heating taken into account? |
|
Title: Re: Poly resistor self heating Post by vivkr on Jan 4th, 2012, 4:41am RobG wrote on Jan 3rd, 2012, 7:59pm:
As a matter of fact, you can do that. The "grounded" metal must however go to a suitable thermal sink, e.g. substrate and you want to use M1 for this cover as that would be closest to the poly. Given that the exact thermal coefficient will depend on the M1 pattern you lay out on top of your poly resistor, I would expect the foundry to provide a standard resistor with the shield on it, and a switch in the model which accounts for this shield when estimating self-heating. By the way, it might be better to use diffusion resistors rather the poly if current densities are high enough to make self-heating significant. The resistor is then embedded in a well and can dissipate more heat into the substrate directly, and any M1 cover on top of it can be placed much closer to the resistor (vertical clearance). Vivek |
|
Title: Re: Poly resistor self heating Post by loose-electron on Jan 4th, 2012, 6:59am RobG wrote on Jan 3rd, 2012, 9:44am:
However you are probably not accounting for the max temp rules. Got to respect both. If its an academic problem, it does not matter, |
|
Title: Re: Poly resistor self heating Post by RobG on Jan 4th, 2012, 7:53am vivkr wrote on Jan 4th, 2012, 4:41am:
Yes, I believe that is the case.When you include the well and account for the spacings I'm not sure it isn't easier to make a wider poly. I guess it would depend. Aaron, I know some models do account for self heating, but I don't know the details of how it is done. rg |
|
Title: Re: Poly resistor self heating Post by Lex on Jan 5th, 2012, 12:05am aaron_do wrote on Jan 4th, 2012, 1:45am:
I think the VCR2 accounts for that (or at least for some of it), as it is the coefficient that is multiplied by V^2 (a power term). |
|
The Designer's Guide Community Forum » Powered by YaBB 2.2.2! YaBB © 2000-2008. All Rights Reserved. |