Light Damages the Transistors

Hello, Everyone:

I need to do on-wafer measurement on my high frequency
amplifier. But I find the transistors are very sensitive to
microscopic light.
Sometime after I switch off the light and then apply the DC bias,
there will be large leakage current on transistor gate.

Do you have idea on why this happens and any suggestion to avoid this
problem?

thanks,

suyu

 

Su,

Have you tried putting a light shield over the block? For example,
have you placed a piece of top-level metal over the amp? The
reason this occurs is the same reason you can build CCDs and
CMOS Image Sensors.

Best
Regards,


Sheldon

 

Su,

One other question, if you turn the power off and the light
off does the leakage go away?

If the leakage goes away when both power and the light
are off then it maybe that the circuit is latching up. If the
leakage does not go away, then you maybe injecting hot
carriers into the oxide over the B-E junction.

What happens when you don't bias the circuit while the
light is on?

Best Regards,

Sheldon

 

HI, Sheldon:

I did not put top-level metal to block the light. Instead, I've used
"dummy-block" to block any dummy metal layer which could be inserted by
foundry to reduce the parasitic capacitance.
THIs problem has become more severe when we used shorter-channel
device.
But once again, how does the light damage the transistor gate?

Many thanks to your reply,

suyu

 

HI, Sheldon:

Our measurement process is:
1. Landing the probles on the pads.
2. Turn off the microscopic light.
3. BIas the circuit by DC power supply.

At this point, we observed large gate leakage current.

If we did not turn off the light, the circuit was fine when biased.

thanks,

suyu