Fringe Field of an asymmetrical parallel capacitor

[smartmac]

I have a problem bothering me for a long time. Here it is: suppose I have an asymmetrical parallel plate capacitor with one electrode much larger than the other. For example, the capacitor could be made by depositing small-area metal electrode on heavily doped wafer with 300 nm thermal oxide. Is there a way to calculate the fringe field component parallel to the SiO2 surface at the edge of the small metal electrode? A figure is attached to make the question clear. Thanks.

 

[Meir Achuz]

The bottom electrode seems large enough to consider as an infinite plane.
Then you could use an image electrode, so the the field would be that of a capacitor with a
6 nm gap.

 

[smartmac]

Thanks. Could you please give some details on how you got the 6 nm thickness?

 

[Meir Achuz]

Sorry. I meant 600 nm, twice the space on the picture.

 

[smartmac]

Sorry I am still a little bit confused. Is the field calculated with 600 nm thickness vertical or horizontal? I am interested in the horizontal fringe field.

 

[Meir Achuz]

It's not an easy calculation, but you calculate the fringe field of a symmetric capacitor with 600 nm between the plates.

 

[smartmac]

Thanks, I will try that.