The "single beam" laser interferometer is recommended for in situ
rate/ depth measurements.
It is often used in the "reflectance mode", where we just monitor changes in the surface reflection.
It can also be used in the "interferometric mode" , where interference signals from two interfaces
(e g the top of a transparent layer and its bottom or the bottom of an etch and the coated substrate backside).
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.
With laser interferometry it is often useful to run a simulation
|Laser Interferometry for Failure Analysis|