Endpoint detector and method for measuring a change in wafer...

Details Endpoint detector and method for measuring a change in wafer... Endpoint detector and method for measuring a change in wafer...

1999-05-19

2001-03-27

Kim, Robert (Department: 2877)

Optics: measuring and testing

By light interference

For dimensional measurement

C356S507000

Reexamination Certificate

active

06208425

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an endpoint detector and a method for quickly and accurately measuring a change in thickness of a semiconductor wafer during chemical-mechanical polishing of the wafer.
BACKGROUND OF THE INVENTION
Chemical-mechanical polishing (“CMP”) processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a porous material such as blown polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
FIG. 1
schematically illustrates a conventional CMP machine
10
with a platen
20
, a wafer carrier
30
, a polishing pad
40
, and a slurry
44
on the polishing pad. The platen
20
has a surface
22
upon which the polishing pad
40
is positioned. A drive assembly
26
rotates the platen
20
as indicated by arrow “A”. In another type of existing CMP machine, the drive assembly
26
reciprocates the platen back and forth as indicated by arrow “B”. The motion of the platen
20
is imparted to the pad
40
because the polishing pad
40
frictionally engages the surface
22
of the platen
20
. The wafer carrier
30
has a lower surface
32
to which a wafer
60
may be attached, or the wafer
60
may be attached to a resilient pad
34
positioned between the wafer
60
and the lower surface
32
. The wafer carrier
30
may be a weighted, free-floating wafer carrier, or an actuator assembly
36
may be attached to the wafer carrier
30
to impart axial and rotational motion, as indicated by arrows “C” and “D”, respectively.
In the operation of the conventional polisher
10
, the wafer
60
is positioned face-downward against the polishing pad
40
, and then the platen
20
and the wafer carrier
30
move relative to one another. As the face of the wafer
60
moves across the planarizing surface
42
of the polishing pad
40
, the polishing pad
40
and the slurry
44
remove material from the wafer
60
.
In the competitive semiconductor industry, it is highly desirable to maximize the throughput of CMP processes to produce accurate, planar surfaces as quickly as possible. The throughput of CMP processes is a function of several factors, one of which is the ability to accurately stop the CMP process at a desired endpoint. Accurately stopping the CMP process at a desired endpoint is important to maintaining a high throughput because the thickness of the dielectric layer must be within an acceptable range; if the thickness of the dielectric layer is not within an acceptable range, the wafer must be re-polished until it reaches the desired endpoint. Re-polishing a wafer, however, significantly reduces the throughput of CMP processes. Thus, it is highly desirable to stop the CMP process at the desired endpoint.
In one conventional method for determining the endpoint of the CMP process, the polishing period of one wafer in a run is estimated using the polishing rate of previous wafers in the run. The estimated polishing period for the wafer, however, may not be accurate because the polishing rate may change from one wafer to another. Thus, this method may not accurately polish the wafer to the desired endpoint.
In another method for determining the endpoint of the CMP process, the wafer is removed from the pad and wafer carrier, and then the thickness of the wafer is measured. Removing the wafer from the pad and wafer carrier, however, is time-consuming and may damage the wafer. Moreover, if the wafer is not at the desired endpoint, then even more time is required to remount the wafer to the wafer carrier for repolishing. Thus, this method generally reduces the throughput of the CMP process.
In still another method for determining the endpoint of the CMP process, a portion of the wafer is moved beyond the edge of the pad, and an interferometer directs a beam of light directly onto the exposed portion of the wafer. The wafer, however, may not be in the same reference position each time it overhangs the pad because the edge of the pad is compressible, the wafer may pivot when it overhangs the pad, and the exposed portion of the wafer may vary from one measurement to the next. Thus, this method may inaccurately measure the change in thickness of the wafer.
In light of the problems with conventional endpoint detection techniques, it would be desirable to develop an apparatus and a method for quickly and accurately measuring the change in thickness of a wafer during the CMP process.
SUMMARY OF THE INVENTION
The inventive endpoint detector measures the change in thickness of a semiconductor wafer while the wafer is attached to a wafer carrier and during chemical-mechanical polishing of the wafer. The endpoint detector has a reference platform, a measuring face, and a distance measuring device. The reference platform is positioned proximate to the wafer carrier, and the reference platform and measuring device are positioned apart from one another by a known, constant distance for all of the measurements of a single wafer. The measuring face is fixedly positioned with respect to the wafer carrier at a location that allows the measuring device to engage the measuring face when the wafer is positioned on the reference platform. Each time the measuring device engages the measuring surface, it measures the displacement of the measuring face with respect to the measuring device. The displacement of the measuring face is proportional to the change in thickness of the wafer between measurements.
In an inventive method of the invention, the wafer is placed on the reference platform before it is polished, and then the measuring device engages the measuring surface to determine a baseline measurement of the position of the measuring face with respect to the measuring device. After the wafer is at least partially polished, the wafer is re-placed on the reference platform and the measuring device is re-engaged with the measuring face to determine a subsequent measurement of the position of the measuring face with respect to the measuring device. The displacement of the measuring face from the baseline measurement to the subsequent measurement is proportionate to the change in thickness of the wafer.


REFERENCES:
patent: 4717255 (1988-01-01), Ulbers
patent: 5369488 (1994-11-01), Morokuma

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