Abrading – Precision device or process - or with condition responsive... – By optical sensor
Reexamination Certificate
1998-07-16
2001-07-17
Eley, Timothy V. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
By optical sensor
C356S389000
Reexamination Certificate
active
06261152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an apparatus and method for in-situ process monitoring and more specifically, to an apparatus and method for in-situ monitoring of chemical-mechanical planarization of semiconductor wafers.
2. Background
Planarization of the active or device surface of a substrate has become an important step in the fabrication of modern integrated circuits (ICs). Of the several methods of planarization that have been developed, Chemical Mechanical Polishing (CMP) is perhaps the most commonly used method. This popularity is due, in part, to its broad range of applicability with acceptably uniform results, relative ease of use, and low cost. However, the move to larger diameter wafers and device technologies that require constant improvement in process uniformity requires that an improved planarization system become available.
A typical CMP system uses a flat, rotating disk or platen with a pliable monolithic polishing pad mounted on its upper surface. As the disk is rotated, a slurry is deposited near the center of the polishing pad and spread outward using, at least in part, centrifugal force caused by the rotation. A wafer or substrate is then pressed, typically face down, against the working surface of the polishing pad such that the rotating polishing pad moves the slurry over the wafer's surface. In this manner, surface high spots are removed from the wafer and an essentially planar surface is achieved.
The planarization of an interlayer dielectric is one common use for CMP. As the topography of the underlying surface is not uniform, coating that surface with a dielectric film replicates or even magnifies those non-uniformities. As the surface is planarized, the high spots are removed and then the total thickness of the dielectric film is reduced to a predetermined value. Thus, the planarized dielectric film will be thinner over high points of the underlying surface than over low points of that surface. Typically, it is important to maintain a minimum dielectric thickness over each of the highest points of the underlying layer, both locally (within a die) and globally (across the wafer). Thus, uniform removal of the dielectric layer at all points of the wafer is required.
A problem with most existing CMP systems is their inability to perform in-situ thickness monitoring. As the surface of the wafer is pressed against the polishing pad during removal, typically no measurements as to the progress of the polishing can be made. Thus, wafers are either polished for fixed times, and/or periodically removed for off-line measurement. Recently, Lustig et al., U.S. Pat. No. 5,433,651 (Lustig) proposed placement of at least one viewing window in the working surface through the thickness of the polishing pad to provide access for in-situ measurement. However, a window placed in a polishing pad creates a mechanical discontinuity in the working surface each time the window passes across the surface of the wafer. A more conventional approach is to use a monolithic polishing pad.
Thus there is a need for a CMP apparatus, and method thereof, that provides optical access to the wafer front surface for continuous in-situ process monitoring, without undue process complexity or expense.
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 &mgr;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.
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Eley Timothy V.
Nguyen Dung Van
Nikon Research Corporation of America
Skjerven Morrill & MacPherson LLP
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