A strange phenomenon of RC oscillator

[incol]

Dear everyone,

I have met a strange thing when testing the frequency of RC oscillator. The capacitor of RCOSC is the MOM cap, which exploits the effect of lateral capacitive coupling between the plates formed by metal lines. I found when press the chip, the RCOSC frequency changes about 0.25%, when remove this pressure, the frequency is recovered. More interesting is that this RCOSC has been used in different chips with different processes, when put a pressure on these chips, the frequency drift direction is different with different processes, some frequency is increase, some is decrease.
I guess it is the change of MOM cap value that makes the frequency drift, but don't konw how can a vertical pressure influence the lateral cap, and why the frequency can increase, not decrease?
Does anybody meet this problem before?

 

[FvM]

What makes you think that capacitance variation is the dominant effect? All other chip parameters will be also affected.

 

[incol]

Hi FvM.

Thanks for your reply. I think R, C and VDD voltage will influence the frequency. The RCOSC use LDO as the VDD, and tested it has no change. The resistor Low probability to be changed.