Fabrication: Electrostatic discharge in micro devices
After fabrication, it is joyful measurement time!
To start measurement successfully, handling of electrostatic discharge is important.
Find out broken devices early
Before losing your precious time by measuring broken devices.
1. Check resistance of all gates and Ohmic contacts.
Measure the current through a gate (or Ohmic) with connecting all other terminal to ground is an easy way.
Keep applied voltages between +-1 mV.
(You cannot use a conventional DMM which applies several hundreds mV.)
2. If you find small gate resistances, check the leakage paths by floating other Ohmics or gates.
If resistances between gates are under 1 MOhm, the device might be damaged by electrostatic discharge.
It might be better to switch to the next device.
Examine what is wrong in the device
This process might be a little bit annoying, but is important for the success in the next measurement.
1. From the values of the resistances measured above, imagine what happened in the device.
2. Probe the device with a scanning electron microscope (SEM), etc.
The right pictures show a SEM image of a blown up quantum dot device by electrostatic discharge.
The left (right) is before (after) the damage.
Heating in tiny area by discharge current melts and removes gate electrodes.
If you find these features, you need additional countermeasures against static electricity to protect your precious devices.
Countermeasures against static electricity
The followings are effective countermeasures in bonding and mounting processes of devices.
1. Prevent to wear specific clothes which easily create electrostatic charge.
Prevent sweater, fleece, etc.
Using fabric softener also decreases charging by clothes.
2. Utilize grounding wrist strap.
Check the resistance through human body to ground is several MOhm before use.
3. Neutralize electrostatic charge by using ionizers.
In many cases, we can prevent troubles caused by electrostatic discharge with these three countermeasures.
If they are not enough,
4. Utilize sprays to prevent charging of clothes.
5. Use only tools designed to prevent electrostatic discharge.
Those tools are usually labeled with ESD mark.
6. Use humidifiers.
7. Use shoes, floor, chairs, etc. designed to prevent electrostatic discharge.
8. Check electrostatic charging in every process with a surface electrometer.
Standing up from a chair, Wiping desks, trays, Pulling out a toothpick from the container, Peeling off tapes, etc.
These are all possible to cause electrostatic charge.
With appropriate handling of electrostatic discharge, enjoy your wonderful measurements!