Probe Station

Matt is currently “master of the probe station”, please see him before using it for the first time.

We use labview programs to control voltage sources and acquire data. For documentation on the MeaSureit program see Vera's program development site. It is straightforward to use programs that are already written. If you need to write your own labview program, it will take some time investment to learn this graphical programming language. The time is well spent because labview the industry standard for software control of processes. There are tutorials, videos and exercises on the labview website and further material at labview for students section. On the T: drive there is a folder Manuals/Labview where we keep some example programs.

More information:

• Installing labview
• Learning labview
• DAQ systems from National Instruments

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 (capacitive currents as the charge on the electrodes changes).
• 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 to a silicon/silicon oxide chip:

1. Place the chip on a conducting surface (a piece of copper on top of a piece of glass, a glass slide painted on one side with conductive paint, a covered in a thin film of evaporated metal). The conducting surface must be isolated from all other conductors. The surface will contact the silicon underside of the chip and apply a gate voltage to the 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: see Using the fluid cell below.

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.

The most common reason for unusually high noise can be fixed by connecting the shield on the BNC wires which carry source, drain and gate signals (see Figure below).

Other noise considerations

• The reference electrode we use is RE-6 from Bioanalytical systems. A manual is available. It is critical that the electrode is always wet, and always stored in 3M NaCl solution.
• Liquid gating is more effective than back gating because electrolyte surrounds the nanotube. Smaller gate voltages are required. In general use a factor of 10 less (+/-500 mV is safe).
• Before using a syringe flick the top like a nurse to get rid of any bubbles
• Before loading a syringe pump some liquid into the hole first. When pushing the syringe in place some liquid should leak out the sides. This prevents bubbles from getting into the system.
• Use clean room towels to dry the fluid cell
• To clean the fluid cell sonicate all fluid carrying parts in ethanol for 20 minutes.

• Pour buffer solution (typically PBS) into fluid cell from above
• Lower the cell onto a clean glass slide
• Push the gate electrode in. It should fit snugly against the bottom of the hole.
  • Pump some buffer through the cell with a syringe. Make sure the electrode doesn't move and no liquid leaks up the sides.
• Replace the glass slide with a chip and lower the fluid cell. Use the alignment marks on the cell and the chip to center the o-ring over the nanotube devices. Be careful apply too much pressure and break the chip in half.
• Pump some ethanol through the cell to wet the chip.
  • Note: ethanol dissolves photoresist. Skip this step if using photoresist windows.
  • Make sure there are no bubbles
• Pump in the electrolyte buffer
• With source electrode disconnected sweep the gate voltage
  • Any current observed is due to capacitive effects and electrochemistry
  • The most desirable scenario is when capacitive currents dominate. The IV curve should be flat and the current value depends on the direction and speed of the gate voltage sweep. Starting around +/-150 mV ionic currents between the gate and drain electrodes should be visible. Ionic currents larger than ~1 nA indicate something is wrong.
• If the gate voltage sweeps look good connect the source electrode and begin the experiment

Dilution Calculator - Be careful to double check though!