Heterdoyne Thickness Monitoring System

DETAILED DESCRIPTION A Chemical Mechanical Polishing heterodyne in-situ sensor (C-HIS) apparatus and method for enhanced optical access to a wafer surface is provided.
The C-HIS system is based on conventional optical heterodyne interferometry.
In some embodiments, a front surface of the wafer is illuminated through the wafer using an infrared laser source emitting light at a wavelength of 1.
1 .
mu.
m or greater.
In some embodiments, the wafer also comprises a planarization film.
For such embodiments the front wafer surface will be understood to encompass the planarization film.
Light at such wavelengths is transmitted through the wafer and planarization film to the front wafer surface, where it is at least in part reflected back to the C-HIS apparatus.
As the planarization film is polished, the optical path length of the beam propagating through the film is reduced.
This causes the optical frequency of the reflected beam to undergo a Doppler frequency shift.
By measuring this Doppler shift, the instantaneous change in planarization film thickness can be determined.
In some embodiments of the invention, the measured Doppler shift generates an input signal to enable dynamic process control.
Existing optical in-situ sensors are intensity-dependent devices and hence are subject to noise due to source intensity fluctuations and variable transmittance in the optical path.
Unlike those existing in-situ sensors, the embodiments of the present invention provide for measurement based on phase detection independent of intensity, and hence do not suffer from problems related to intensity fluctuations.
Some embodiments are capable of detecting thickness changes of about 2.
5 nm.
In accordance with embodiments of the present invention, C-HIS sensors operate in both polished-to-thickness and polished-to-stop scenarios.
Thus, these embodiments provide a system and method for optically accessing a wafer surface to enable enhanced and versatile in-situ monitoring of a CMP process.