Technique for noise reduction in a torque-based...

Details Technique for noise reduction in a torque-based... Technique for noise reduction in a torque-based...

2001-12-11

2004-05-25

Paladini, Albert W. (Department: 2125)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Mechanical control system

C156S345130, C451S005000

Reexamination Certificate

active

06741913

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to semiconductor processing, and more particularly to noise reduction in the detection of the endpoint for removal of a film by chemical-mechanical polishing.
BACKGROUND OF THE INVENTION
In the manufacture of integrated circuits, the selective formation and removal of films on an underlying substrate are critical steps. Chemical-mechanical polishing (CMP) has become a widely used process for selective film removal and for planarizing a structure where a patterned film overlies another film.
In film removal processes such as CMP, it is extremely important to stop the process when the correct film thickness has been removed (that is, when the endpoint has been reached). In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. Overpolishing (removing too much) of a film renders the wafer unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Underpolishing may sometimes go unnoticed, which also results in yield loss.
FIG. 1
shows a typical CMP apparatus
10
in which a workpiece
100
(such as a silicon wafer) is held face down by a wafer carrier
11
and polished using a polishing pad
12
located on a polishing table or platen
13
; the workpiece is in contact with slurry
14
. The wafer carrier
11
is rotated by a shaft
15
driven by a motor
16
.
An example of an important CMP process is shown in
FIGS. 2A and 2B
. This process involves removal of a polycrystalline silicon (poly-Si) film overlying a patterned film of silicon dioxide (SiO
2
) or silicon nitride (Si
3
N
4
); after removal of a blanket layer of poly-Si, a surface having partly poly-Si and partly SiO
2
or Si
3
N
4
is be exposed. In
FIG. 2A
, a patterned oxide layer
102
is covered by a layer
104
of poly-Si. Generally, it is necessary to remove the target film of poly-Si down to a level
105
so as to completely expose the oxide pattern, while leaving the oxide layer itself essentially intact (FIG.
2
B). A successful endpoint detection scheme must detect exposure of the patterned layer with very high sensitivity, and automatically stop the CMP process within a few seconds after that layer becomes exposed. The endpoint detection scheme should also be effective regardless of the pattern factor of the wafer (that is, even if the area of the exposed underlying pattern is a small portion of the total wafer area).
One widely used approach to monitor and control a CMP process is to monitor a change in the motor current associated with a change in friction between (a) the top surface of the polishing pad
12
and (b) the slurry
14
and the surface being polished (such as the surface of wafer
100
). This method is satisfactory when there is a significant change in friction as the underlying layer is exposed. However, for many applications, including the poly-Si polishing process described just above, the change in friction associated with the interface between layers is too small to result in a motor current change sufficient to be a reliable indicator of CMP process endpoint. This problem is aggravated by a large noise component in the motor current associated with the typical feedback servo current used to drive the wafer carrier at a constant rotational speed. In addition, a small pattern factor (that is, a relatively small area of the underlying patterned layer, compared with the area of the target layer) causes only a small change in friction as the endpoint is reached, limiting the useful signal.
A convenient and highly sensitive method of endpoint detection, applicable to CMP equipment such as shown in
FIG. 1
, is described in U.S. application Ser. No. 09/689,361, “Real-time control of chemical-mechanical polishing processes using a shaft distortion measurement,” the disclosure of which is incorporated herein by reference. According to this method, changes in friction between the surface of the wafer
100
and the polishing pad
12
, in the presence of the slurry
14
, are monitored by directly monitoring the deformation of the carrier shaft
15
. During a polishing process, the shaft
15
driving the wafer carrier
11
can experience changes in torque, bending, thrust and tension. Torque on the shaft (for example, due to rotation by motor
16
in direction
31
being opposed by frictional forces in direction
32
) will induce deformation of the shaft, as shown schematically in FIG.
3
. The degree of deformation depends on the diameter of the shaft, with smaller-diameter shafts being more susceptible to deformation. Such deformations can be measured with extremely high sensitivity at reasonable cost.
When an underlying film of a different material is exposed during the CMP process (for example, when the polishing of layer
104
exposes surface
105
; see FIG.
2
B), the accompanying change in friction results in a change in torque experienced by the shaft
15
. The change in torque induces a change in deformation of the shaft, which is measured by a strain gauge
201
bonded to (or embedded in) the shaft, as shown in FIG.
4
. Strain gauge
201
is connected to a transmitter
202
which broadcasts a signal
203
to a detector
210
. The signal
203
indicates strain caused by deformation of the shaft
15
, which in turn is directly related to torque experienced by the shaft. This arrangement therefore generates a signal indicating changes in friction between the polishing pad
12
and slurry
14
and the wafer
100
. Signals acquired by detector
210
are then processed and analyzed in signal processing unit
220
, which produces an endpoint signal. Processing unit
220
typically includes a computer with a storage medium, the storage medium having software stored therein for performing the endpoint detection algorithm. The endpoint signal may be fed to a control unit
250
to stop the CMP process.
FIG. 5A
shows an example of a detected torque signal
51
acquired and processed during polishing of a poly-Si layer. The sharp change in the signal indicates that the interface between layers has been reached. The actual amount of torque on the shaft may vary from one polishing process to the next, so that a specific value of torque indicating the endpoint cannot be fixed. It therefore is preferable to detect the CMP endpoint in accordance with a change in the torque, as opposed to a predetermined value of torque. This is done by calculating the time derivative
52
of the torque signal (see FIG.
5
B); the peak of the derivative is used to indicate the process endpoint. It is noteworthy that this technique provides real-time, in situ endpoint monitoring and permits closed-loop control of the CMP process.
Since the endpoint signal is based on measurement of the change in torque associated with interaction among the wafer
100
, slurry
14
and pad
12
, the endpoint signal varies with the rotation and oscillation of the wafer carrier
11
. As shown in
FIG. 6A
, shaft
15
causes wafer carrier
11
to rotate with respect to pad
12
(fixed to platen
13
) while carrier
11
oscillates across the pad surface.
FIG. 6B
is a top view of the apparatus of
FIG. 6A
, showing the platen
13
and wafer carrier
11
. The wafer carrier rotates about shaft
15
in direction
61
, and oscillates along a radius of the platen in directions
62
a
and
62
b
. At a given point in time, the amount of torque on the shaft
15
depends on the angular position of the shaft and on the location of the carrier
11
on its radial trajectory. The torque thus varies periodically according to the separate rotation and oscillation periods. These periodic variations in the torque can be great enough to cause false endpoint signals. This noise cannot be eliminated by using a low/high pass filter or a band pass filter, since the noise generally is in the same frequency region as

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