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What's the maximum current density for poly resistors?

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DoctorX

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current density ma/um

I am trying to make a 50-ohm poly resistor for on-chip termination, driven by +/- 100 mV. I already lowered the amplitude to reduce power dissipation.

What's the general rule-of-thumb for maximum current density for precision resistor poly without blowing it up? I found no such info from vendor, but I assume that I can find the thickness of poly somewhere, if someone can tell me a max current per unit width together with an estimated poly thickness.

Thanks all.
 

For poly resistors current density(J) depend on sheet resistance (ρ).
For example, if ρ = 20 Ω/sq than J = 1 mA/um,
if ρ = 600 Ω/sq than J = 0.1 mA/um,
if ρ = 3.5 KΩ/sq than J = 0.02 mA/um.
 
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    sohaee

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Thanks, great. It seems to me that there is roughly a maximum power per unit area, is that right? For the three cases, I got:

if ρ = 20 Ω/sq than J = 1 mA/um, P = 2e-5 W/sq um
if ρ = 600 Ω/sq than J = 0.1 mA/um, P = 6e-6 W/sq um
if ρ = 3.5 KΩ/sq than J = 0.02 mA/um, P = 1.4e-6 W/sq um

Due to other factors, this maximum power varies a little depending on poly doping and process. Did I understand this correctly?
 
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    sohaee

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Well, I agree sort of. It is dependent upon sheet rho, but is ALSO very dependent upon film thickness. This is why power density is different. The three films are different besides the sheet resistivity. Electromigration is a definite issue. Remember that the resistor sheet "ohms per square" is a two D rule. The variability comes from resistor etch variations in widths but also film thickness. Thinner films will not be able to withstand higher current densities.
 

jonrhan said:
Well, I agree sort of. It is dependent upon sheet rho, but is ALSO very dependent upon film thickness.
Click to expand...

Great, make sense to me. There is also a saying that poly resistors are less subject to failure under high currents compared with metal (AL) resistors. Is that because poly retains its shape better under high temp, while metal doesn't? Thanks.
 

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