Patent No. 6261152 Heterodyne thickness monitoring system

 

Patent No. 6261152

Heterodyne thickness monitoring system (Aiyer, Jul 17, 2001)

Abstract

A CMP heterodyne in-situ sensor (C-HIS) system utilizes optical heterodyne interferometry. A wafer undergoing CMP is illuminated through the wafer thickness using an infrared laser beam at a wavelength of 1.1 .mu.m or greater. The beam is transmitted through the wafer and is reflected from the front wafer surface. As the wafer is polished, the optical beam path through the wafer is shortened, causing the reflected optical frequency to undergo a Doppler shift. By measuring this shift, the change in wafer thickness is determined. The frequency shift generates a signal, which enables dynamic process control. In embodiments where the wafer includes a planarization film, the frequency shift provides a measurement of changing film thickness. Embodiments of the invention utilize phase detection independent of intensity, and hence do not suffer from intensity fluctuations. Some embodiments detect thickness changes less than 2.5 nm. C-HIS sensors operate in both polished-to-thickness and polished-to-stop scenarios.

Notes:

SUMMARY OF THE INVENTION

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.

-----------------------------------

Although the invention has been described in terms of a certain preferred embodiment, other embodiments apparent to those skilled in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims which follow.