Question about surface area of faraday cage

[DKhosla]

Hi,

I am making a faraday cage to be used in reactive ion etching of silicon. I was wondering if the size of the cage, or the surface area has any impact on it's effectiveness or how it works?

I know the size of the mesh and the material I use, as well as whether or not it is grounded all have an effect on it. I can't seem to find any information on the effect of size though.

 

[Antti]

No size does not matter, although I don't know about ridiculously large cages. There are big cages for sale and for example at CERN they routinely enclose big objects in tin foil or similar material. It's the quality of the seal that matters.

 

[Jeff Rosenbury]

I'm going to disagree with Antti.

Since you seem to be talking about a manufacturing process I'm going to assume the ions are on the inside of the cage. In that case the cage could act as a microwave chamber. That could lead to problems. You will need to consider a way to prevent resonance.

More generally a cage can act as an antenna. High frequencies can slip through the holes. But I think you've considered these effects already.

 

[the_emi_guy]

I would think that in an RIE system you would need to limit parasitic capacitance between electrodes and shield in order to not disturb the geometry of the field. If the shield were too small (too close in proximity to the electrodes), it would disturb the geometry of the field and the plasma flow.

 

[DKhosla]

Thanks for your answers; I will definitely look into all of those things.

I think I should clarify that the cage is going inside an RIE etcher, to shield the silicon wafer. The cage is a triangular prism, and it is being used to do angled etching on the silicon wafer, so it is actually supposed to affect the flow of plasma.

(If the cage works, the potential gradient in the etching process will build up over the face of the cage and accelerate the ions perpendicular to the cage's surface. Since there is no field inside, the ions continue traveling in that direction. The surface of the cage is angled, so the ions hit the wafer at an angle.
Source: http://nano-optics.seas.harvard.edu/publications/Mike_freestanding.pdf [Broken])

 

[Jeff Rosenbury]

Thanks for your answers; I will definitely look into all of those things.

I think I should clarify that the cage is going inside an RIE etcher, to shield the silicon wafer. The cage is a triangular prism, and it is being used to do angled etching on the silicon wafer, so it is actually supposed to affect the flow of plasma.

(If the cage works, the potential gradient in the etching process will build up over the face of the cage and accelerate the ions perpendicular to the cage's surface. Since there is no field inside, the ions continue traveling in that direction. The surface of the cage is angled, so the ions hit the wafer at an angle.
Source: http://nano-optics.seas.harvard.edu/publications/Mike_freestanding.pdf [Broken])

Be aware that accelerating (and more importantly in your case, decelerating) charges emit photons. Basically the ions impacting the silicon will act as an antenna spewing radio waves all over the place -- inside your cage. This effect is called Bremsstrahlung.

One way to reduce this potential problem might be to reinject the missing electrons just before passing the cage. Of course you might want the extra energy of the photons to break substrate molecular bonds.