# Coupling capacitance between interconnects in VLSI

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1. May 4, 2017

### omar alaa

Hello all,

I'm trying to measure the effect of adding charges in the silicon dioxide separating two interconnects in VLSI circuits and how it will effect the coupling capacitance between these two wires. How can I measure this effect ? The VLSI circuit I'm working on has frequency of 500 Mhz.

What I did so far is playing a little with FastFieldSolver tool and see the effect of changing the dielectric constant on the coupling capacitance between two wires....but still I don't know how the dielectric constant is related to charge (impurities) concentration in the silicon dioxide.

My goal is to be able to tune the dielectric constant of the silicon dioxide by adding charges (impurities) in the silicon dioxide ...and how to count the effect of operating frequency in my calculations.

Thanks

2. May 4, 2017

### omar alaa

Is there a way to increase artificially the dielectric constant of a material (i.e. silicon dioxide) ?

3. May 4, 2017

### Staff: Mentor

Are you familiar with Electrically Controlled Capacitance?

https://en.wikipedia.org/wiki/Variable_capacitor

I don't know if it might be related to what you are asking... By varying the reverse bias voltage across a semiconductor junction, you change the width of the depletion layer, which changes the capacitance across the junction. The technique is commonly used in radio circuits and other circuits that need a variable capacitance for tuning purposes.

4. May 4, 2017

### omar alaa

No I'm not talking about dielectric in transistors (sorry for not clarifying), I'm talking about parallel plate capacitor. I want to increase its capacitance, but I don't have control over goemetries..I can just modify the dielectric. I was wondering if I can increase the dielectric constant with doping of ions or free charges, if yes is there an equation governing this ?

5. May 4, 2017

### Fred Wright

The Wikipedia articles on high-k and low-k dielectric may help.

6. May 4, 2017

### omar alaa

Is there a way to modify the silicon dioxide itself such as doping the dielectric ?

7. May 4, 2017

### Staff: Mentor

I would think that would mostly alter the conductance, not the dielectric constant. How leaky can this capacitor be? Leaky capacitance is usually a bad thing, no?

8. May 4, 2017

### Fred Wright

You can lower the dielectric constant of your SiO2 substrate by doping it with fluorine. This is done with plasma enhanced CVD. Lowering the dielectric constant will reduce the capacitive coupling between your traces. If you want to measure the result, hook your circuit to a network analyzer.

9. May 4, 2017

### omar alaa

Actually I want to do the opposite. I want to increase the capacitive coupling

10. May 4, 2017

### Fred Wright

I suggest you do a literature search on hafnium based high-k dielectrics. I am not familiar with the processing techniques involved but I suspect that it isn't trivial. Also, industry has used oxynitride, where a small amount of nitrogen is infused into the SiO2 substrate.