Plasma Technology  

OPT application lab:
SiN etch by RIE and ICP

Plasmalab 80 Plus (RIE)
with laser interferometer

RIE with
laser interferometer

Laser Interferometry for Failure Analysis

"Interferometric Mode"

The figure above shows the resulting trace from a polyimide over silicon nitride passivation etch. The period of the trace can be used to calculate the process etch rate as 1 period is equal to l /2n for that particular material, l is the wavelength of the laser, n is the refractive index of the material). Therefore, knowing the time taken to complete 1 period and the theoretical thickness calculated from l/2n allows calculation of the real-time etch rate. The change in period at about 500 seconds indicates complete removal of the polyimide. The period then increases due to the much lower etch rate of the silicon nitride in this chemistry. The selectivity of this process was estimated at >10:1.

"Reflectance Mode"

The figure above shows the trace from a silicon nitride over aluminium etch. Once the silicon nitride has cleared (270 seconds), no more light interference takes place resulting in a distinct flat trace. Both conditions demonstrated provide precise and accurate determination of endpoint, allowing excellent process control for layer removal during failure analysis reverse engineering.

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