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Modeling >> Passive Devices >> capacitance formed by chip and hovering plate??

Message started by trond on Oct 2nd, 2006, 12:23am

Title: capacitance formed by chip and hovering plate??
Post by trond on Oct 2nd, 2006, 12:23am

I did not quite know where to post this, but here I go.

I was wondering whether it is possible to form a capacitance between the routing/devices on a chip and a hovering plate above the chip? If one grounds the hovering membrane then a capacitor should be formed with the charged tracks on chip, should it not?
What, in your opinion, would contribute all the the capacitance, i.e. metal layers on top of the chip, poly....... Is it feasible at all?
I know is a very vague question, but I welcome any feedback.


Title: Re: capacitance formed by chip and hovering plate?
Post by ACWWong on Oct 2nd, 2006, 4:35am

yes you can form capacitors between metal layers. A grounded metal layer will form a capacitance between other metals, poly and substrate/diffusion layers. Of course the coupling capacitance value depends on distance, overlap area, fringe effects. Also for metal-to-nonmetal capacitor formation in paricular the ESR on the non-metal plate can become large.
Most post extraction tools (Assura, diva, calibre) will calculate the C values for you based upon the layout although they are only as good as the PDK rules provided by the foundry.

Title: Re: capacitance formed by chip and hovering plate?
Post by trond on Oct 2nd, 2006, 5:24am

Thanks ACWWong for the response. I was asking since I am interested in simulating a sort of condenser microphone utilizing a ring-oscillator. Essentially the ring oscillator will have a frequency that will change with the capacitance sitting at the inverters nodes. If I use a membrane hovering on top of the ring oscillator I should be able to change this capacitance as the distance will change with the membrane vibrating with sound. Thus, the ring-oscillators frequency of oscillation should be an FM signal.
What would be the most significant capacitance contributor? Assuming I know the distance bewteen the mebrane and chip and air as a dieletric I'd only need the area of the chip components that will form the capacitor with the membrane. How would one approach such a modeling problem?


Title: Re: capacitance formed by chip and hovering plate?
Post by ACWWong on Oct 2nd, 2006, 7:10am

hi trond, i think i misunderstood your initial question (i was talking exclusively on chip).
Anyway now i understand that you what to modulate the frequency of a chip containing a ring oscilator EXTERNALLY by means of a grounded metal plate above the chip.
Now I think the difficulty would be to introduce enough capacitive coupling (through the chip passivation, plastic package, then air) to effect the ring oscilator. So for better chances of success you would need to maximise the top metal area of the ring oscillator nodes on the die, and make the package as thin as possible (bare-die is best) so you can get the plate very close to the ring oscillator. Whilst doing this the want to maximise the osicllator speed (as a smaller delta C has a bigger % change on a smaller fixed C). Hand calculations of coupling C with a knowledge of the fixed C in the ring oscillator should tell you have close the plate would have to be to pull the frequency by x%.
If you are designing this chip, then you would probably go for MEMS techniques to do what you want to do more effectively.
If you are using a chip from the shelf, then be sure the die is the right way up ! some dies are mounted upside down so the backdie metalliation faces up so acting as a ground shield which would cancel any hovering plate effect. Anyway if you have the chip the best course of action is to measure the effects!

Title: Re: capacitance formed by chip and hovering plate?
Post by Geoffrey_Coram on Oct 5th, 2006, 7:16am

My company just put out an article on measuring capacitance to the outside world:

Not exactly what you are trying to do, but gives you a sense of what might be possible.

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