Hi everybody,

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 (τ = R ⋅ t = R ⋅ ε_r ⋅ ε_0 ⋅ 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, ε_r ≈ 200 and the distance d between the electrodes is 100 nm. To achieve a reasonable time constant τ, the area of my nearly spherical electrode should be less than 30 μm^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 thick wire. In my current plan, I design the shape of the electrode such that it is circular with a tail of 2x2 μm and I contact only the tail. However, I cannot think of a way to contact this small area without violating the impedance match or increasing the total area of the electrode.

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.

Thanks in advance,

cheers,

Marvin

 

Just out of curiosity, what time constant do you want to achieve, and what current do you expect when charging the capacitor?

 

I want the time constant to be in the area of 20-30 ps. The current I expect at t=0 is 0.3 A.

 

Also I mean circularly shaped electrode, not spherically. See the attached sketch for better visualization. Here I was approaching the problem by using a SPM tip. However, this is very demanding and expensive. That is why I am searching for a better solution.

 

Can´t you make a transmission line on the surface leading to a more suitable connection point?

 

Thanks for the answer kubeek!

Wouldn`t a transmission line increase the size of my capacitor? My heterostructure consists of a conducting bottom layer, an insulating ferroelectric solid and small sputtered circularly shaped top electrodes.

 

If the unit capacitance of the line would be smaller than that of the cpacitor, then you should not have any issues I think.
In any case, your coaxial cable also has some capacitance, but due to the inductance of the transmission line it should be seen a constant impedance by the capacitor.
Maybe you could first try some experiments on a larger physical scale and lower speeds to validate the approach.

 

Thanks again for the answer!

As I said, I am not an electrical engineer, so I do not really have an idea how to make a transmission line on the surface. Could you explain, or recommend some literature about how to do that?

 

Transmssion line can be as simple as a trace above a ground plane with dielectric in between. See this calculator for example https://www.pasternack.com/t-calculator-microstrip.aspx , but that high dielectric constant calls for very thick substrate, air would be better.
Hoewer I am by no means an RF engineer and designing the transistion to your coax to work well at such frequencies with little impedance discontinuities and reflections will be very complicated. Maybe you could try contacting the author from Signal path blog and ask for some advice https://www.youtube.com/user/TheSignalPathBlog/about .