Determining an endpoint in a polishing process

Details Determining an endpoint in a polishing process Determining an endpoint in a polishing process

2001-10-12

2004-08-24

Rose, Robert A. (Department: 3723)

Abrading

Precision device or process - or with condition responsive...

With indicating

C451S041000

Reexamination Certificate

active

06780086

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to endpoint detection in a polishing process, and more particularly to endpoint detection based on friction between the polishing tool and the structure being polished.
Chemical mechanical polishing (CMP) is widely used in fabrication of integrated circuits.
FIGS. 1 and 2
illustrate fabrication of tungsten plugs
120
that provide electrical contact between a layer
130
and another, overlying layer (not shown). Layer
130
can be a metal layer (e.g. tungsten) formed over a monocrystalline silicon substrate
140
and, possibly, over some other layer or layers
150
. Silicon dioxide
160
is formed over layer
130
. Openings
170
are etched in oxide
160
to expose metal
130
. A thin titanium nitride layer
110
is deposited (e.g. sputtered) over the structure to promote adhesion (tungsten
120
does not adhere well to silicon dioxide). Then tungsten
120
is deposited by chemical vapor deposition to fill the openings
170
and cover the structure. The top surface of the structure is polished by CMP until tungsten
120
and titanium nitride
110
are removed from the top surface. The resulting structure is shown in FIG.
2
. Another conductive layer (not shown) can be formed on this structure. This layer will electrically contact the layer
130
through the metal plugs
120
/
110
in openings
170
. (For brevity, we will refer to plugs
120
/
110
as tungsten plugs
120
.)
The CMP process should remove all of the tungsten
120
and titanium nitride
110
from the top surface of oxide
160
in order to avoid electrical shorts and excessive current leakage between the plugs. The CMP endpoint can be determined by monitoring the friction between the wafer and a polishing pad of the CMP tool.
FIG. 3
illustrates an example CMP tool available from SpeedFam-IPEC of Chandler, Ariz. Wafer
180
, which incorporates the structure of
FIG. 1
, is held upside down on a carrier
210
(the wafer's front side
180
F faces down). A motor (not shown) rotates the carrier
210
, thus causing the wafer to rotate. Another motor
212
rotates a polishing pad
220
which polishes the wafer. (The motor rotates a platen on the pad is positioned). Friction sensor
230
detects the friction between pad
220
and wafer
180
by detecting the current drawn by motor
212
. Controller
240
stops the polishing process based on the friction data from sensor
230
. Suitable controllers
240
and sensors
230
are available from LUXTRON Corporation of Santa Clara, Calif.
FIG. 4
is a chart showing the friction data FR produced by sensor
230
. FR is shown as a function of time. Initially, signal FR decreases as tungsten
120
and titanium nitride
110
are being polished. At some time t1, signal FR levels off, indicating that the polishing pad has reached the oxide
160
. Software programmable controller
240
(
FIG. 3
) is programmed to stop the CMP when the signal FR levels off.
FIG. 5
shows signal FR for another CMP process. In this example, layer
110
is titanium. At some time t2, the friction FR starts to rise. Then FR falls (starting at some time t3), and then levels off at a time t1 when the oxide
160
is reached. Controller
240
is programmed to perform the following steps, in the order shown:
1. Detect rising friction.
2. Detect falling friction.
3. Detect the friction leveling off, and declare an endpoint to stop the CMP.
SUMMARY
The inventors have observed that the endpoint detection method described above (stopping the CMP when FR levels off) results in excessive over-polishing. Too much of oxide
160
gets polished off. In some embodiments of the present invention, the CMP is stopped before FR levels off. In the example of
FIG. 6
, the friction signal FR is as in FIG.
5
. The CMP is stopped a predetermined time “dt” after the time t3 (the time when FR starts falling off). Controller
240
is programmed to:
1. Detect rising friction.
2. Detect when the friction starts to fall.
3. Declare an endpoint the predetermined time dt after the friction starts to fall.
Step 1 (detect rising friction) assumes that at some point of time the signal FR is rising. In
FIG. 4
(the titanium nitride case), the signal FR does not rise. In some embodiments, the titanium nitride deposition parameters are chosen so that the friction signal FR rises at some point (as in FIG.
5
).
Other features of the invention are described below. The invention is defined by the appended claims.


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