Probe Station

Daniel is currently “master of the probe station”, please see him before using it.

Also see Wiring Up Tubes by Jiwoong, 2000.

Nanotubes burn up like a fuse if currents get too high. This can happen several ways.

• Electrostatic discharge (especially dangerous with quartz substrates). Your body is like a van der Graph generator when insulting shoes walk across an insulating floor. Quartz substrates are also insulting, so the circuit on top of quartz can build up significant charge while it sits in the box - I don't know how (cosmic ray ionization?) it just does.
• Sudden large jumps in the gate voltage (capacitor charging currents).
• Accidental voltage outputs from the computer
• Electromagnetic pulses from high power electronics (e.g. a CRT tv screen being turned on).

1. Ground yourself with a wrist strap then take the chip out of the box.
2. Start up mezurit and set the source drain voltage to 10 mV (remember there may be a voltage divider in the circuit).
3. Cross the source and drain needles like swords and check that current overloads.
4. Touch the two needles to the same pad on a chip and check that current overloads. If it does not, clean the probe needle tips.

1. Ground yourself with a wrist strap then take the chip out of the box.
2. Attach a probe needle to ground via a 10 MOhm resistor.
3. Touch the probe needle to every pad that you plan to use. Built up charge will slowly discharge (I guess the time constant of the RC circuit is < 1 second, but someone should calculate).

Three ways to apply a gate voltage:

1. Place the chip on a glass slide painted on one side with conductive paint (or a covered in a thin film of evaporated metal). The paint will contact the silicon surface on the underside of the chip and apply a gate voltage to entire chip.
2. Scratch a hole in the oxide layer of the silicon and place the probe on the scratch. In this method you also need to place the chip on a glass slide to avoid grounding the chip.
3. Water gating.

It is important to sweep the gate voltage in a continuous fashion, rather than instantaneous jumps. Fast switching puts stress on the insulating dielectric (the silicon oxide). If the insulating dielectric breaks down, large currents will start leaking between the gate and the top electrodes.

If you put a positive bias on the nanotube, the current throught the tube should always be positive. If you see negative currents as you sweep the gate voltages, something is suspicious. Three possibilities

1. Charge is leaking from the gate into the metal electrodes. Check this by lifting the lifting the micromanipulator that applies the source voltage to the nanotube, and see if current changes as a function of gate voltage. These leakage currents will increase with gate voltage.
2. A small capacitive current (~ 1 nA) is seen because you are sweeping the gate very quickly (~ 1 V/s). Capacitive currents are still present when you lift the micromanipulator that applies the source voltage to the nanotube. This current is proportional to dVg/dt and does not change with Vg.
3. If the chip is underwater and you are doing watergating (see below), you might be observing electrochemical reactions occuring at the water-electrode interface. Note that the electrode surface area is much larger than the nanotube surface area. Therefore, elecrochemical currents picked up by the electrodes are typically much larger than electrochemical currents picked up by the nanotube.

We are measuring small currents (nanoAmps) and outside noise sources can interfer with the meausrements.

Labs are sometimes built underground to achieve the lowest possible levels of electro-magnetic noise. Our lab is not underground and we have to be aware of shielding issues. We currently have the probe station inside a cooper mesh Faraday cage (purchased from TMC). This Faraday cage is not sufficient, measurements of an unshielded resistor inside this Faraday cage are much more noisey than measurements of the same resistor when it is inside a solid-metal component box.

A significant source of electrical noise comes from fluorescent lights. When checking for the minimum noise of our probe station, it is important to try with and without the fluorescent lights.

Once Faraday cage shielding is adequate, further improvements can be made. For example, grounding should be done in a star configuration. There is one central ground, with a wire radiating out to each piece of equipment. In a simple approximation of “star grounding”, you plug all measurement equipment into the same powerstrip on the wall. It is important to keep the ground on this power strip as quiet as possible. For example, the computer monitor should be plugged into a different powerstrip (different circuit).

Copper mesh supplier