In my current project, I want to apply a voltage to a capacitor (top electrode on a thin film heterostructure) very rapidly. Since capacitors have their own charging rate \$(\tau = R \cdot t = R \cdot \epsilon_r \cdot \epsilon_0 \cdot\frac{A}{d} )\$ and the only variable I can access is \$A\$, I need to keep the area as small as possible. In my case, I have 50 ohm impedance, \$ \epsilon_r \approx 200 \$ and the distance \$ d \$ between the electrodes is 100 nm. To achieve a reasonable time constant \$ \tau \$, the area of my nearly circularly shaped electrode should be less than \$30 \mu m^2\$.

In my special case, I want to probe the area of the electrode with a laser shortly after the application of the voltage pulse. Thus, I cannot contact the electrode in the middle or with a bigger wire. In my current plan, I design the shape of the electrode such that it is circular with a tail of 2x2 \$ \mu m \$ and I contact only the tail with a SPM tip as sketched in the picture. However, this is very demanding and I cannot think of an alternative way to contact this small area without violating the impedance match or increasing the total area of the electrode.

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Is there anybody with experience in this who can help me? Since I am not an electrical engineer, I would be very grateful, if the answers are as detailed as possible.

  • 2
    \$\begingroup\$ I gather you are talking about a sphere with \$r\approx 8\:\mu\text{m}\$ and \$\tau\approx 4\:\text{ps}\$. Besides a point that this seems a question for people with experience in experimental design for physics, I think you need to provide a clear diagram. There are a number of questions I don't have any answer for, reading over your description several times already. A picture would help, I think. Along with what's under more precise control and what is not well controlled. \$\endgroup\$ – jonk Oct 16 '18 at 7:09
  • \$\begingroup\$ With areas in that range, bonding a gold contact comes to mind, but I don't really have experience in this. \$\endgroup\$ – Arsenal Oct 16 '18 at 7:44
  • \$\begingroup\$ Jonk: I am talking about circularly shaped electrodes. No spheres. And with the dimensions mentioned in the sketch I get \$\tau ≈ 30 \$ ps. I hope that the sketch made it a bit clearer. If not, please tell me which parts I should explain in more detail. Arsenal: Thanks for the idea, but bonding usually increases the size of my capacitor (the bottom electrode is a full conductive layer). Do you know if there are bonding techniques creating only a very small contact area? \$\endgroup\$ – Marv_92 Oct 16 '18 at 9:31
  • \$\begingroup\$ I`m sorry, very stupid mistake :/ \$\endgroup\$ – Marv_92 Oct 16 '18 at 9:57
  • \$\begingroup\$ What is the gap between the electrodes filled with and how is the current structure made? Lithography? Could you add more layers on top and etch them away again? \$\endgroup\$ – Jack B Oct 16 '18 at 10:15

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