Dual Beam Laser Interferometry
|
Dual Beam Laser Interferometry |
|
|
The "dual beam" or "twin spot" laser interferometer is
recommended for in situ rate/ depth measurements.
It is used in the "interferometric mode" only and uses the interference
signal from the reflection of beam A
from the etch mask and beam B from the bottom of the etch.
In-situ etch rate monitoring
Endpoint does not require etch stop layer
Endpoint can be chosen anywhere within the layer once etch rate has been established.
675 nm is the standard wavelength for laser interferometry.
However, for certain III-V applications, e.g. InP-related materials, 905 nm
is often more suitable,
since the index contrast between InP-related materials is greater (and absorption
is lower) at 905nm.
A 905 nm laser endpoint system with high gain amplification of endpoint signal
is available from OPT
for these demanding endpoint applications.
For GaAs VCSEL DBR stack etching, it is common to use a 675 nm laser interferometer,
since this gives
a clearer endpoint trace, typically with each ‚ripple’ relating
to each layer within the structure.
For thicker GaAs layers (> about 0.5 microns of GaAs film thickness) 905 nm
is typically recommended,
since absorption of the laser light is much lower at this wavelength.
Laser endpoint traces can be modelled by OPT for any stack of materials, allowing
optimum choice of laser wavelength for any given endpoint requirement.
For the processes with switching between etching and deposition special software
and training
is available to extract the etch depth/ rate:
| single beam laser interferometry | |
| Laser Interferometry for Failure Analysis | |