Abrading – Precision device or process - or with condition responsive... – By optical sensor
Reexamination Certificate
2002-01-12
2004-09-28
Hail, III, Joseph J. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
By optical sensor
C451S008000, C451S010000, C451S041000, C451S285000, C269S021000, C269S903000
Reexamination Certificate
active
06796879
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor fabrication equipment for the fabrication process of chemical mechanical polishing (CMP), and more particularly to wafer-loss sensors and their holders for such equipment.
BACKGROUND OF THE INVENTION
Chemical mechanical polishing (CMP) is a semiconductor wafer flattening and polishing process that combines chemical removal with mechanical buffing. It is used for polishing and flattening wafers after crystal growing, and for wafer planarization during the wafer fabrication process. CMP is a favored process because it can achieve global planarization across the entire wafer surface, can polish and remove all materials from the wafer, can work on multi-material surfaces, avoids the use of hazardous gasses, and is usually a low-cost process.
FIGS. 1A and 1B
show an example effect of performing CMP. In
FIG. 1A
, a semiconductor wafer
102
has a patterned dielectric layer
104
, over which a metal layer
106
has been deposited. The metal layer
106
has a rough top surface, and there is more metal than necessary. Therefore, CMP is performed, resulting in FIG.
1
B. In
FIG. 1B
, the metal layer
106
has been polished down so that it only fills the gaps within the dielectric layer
104
.
FIG. 2
shows an example CMP system
200
for polishing the wafer
102
of
FIGS. 1A and 1B
. The wafer
102
, with its dielectric layer
104
and metal layer
106
, is placed on a platen
202
connected to a rotatable rod
206
. A polishing pad
204
is lowered over the wafer
102
, specifically over the metal layer
106
thereof. The polishing pad
204
is also connected to a rotatable rod
206
. Slurry
210
is introduced between the polishing pad
204
and the metal layer
106
, and the polishing pad
204
is lowered, pressured against the metal layer
106
, and rotated to polish away the excess, undesired metal from the metal layer
106
. The platen
202
is rotated as in the opposite direction. The combined actions of the two rotations and the abrasive slurry
210
polish the wafer surface.
The polishing pad
204
can be made of cast polyurethane foam with fillers, polyurethane impregnated felts, or other materials with desired properties. Important pad properties include porosity, compressibility, and hardness. Porosity, usually measured as the specific gravity of the material, governs the pad's ability to deliver slurry in its pores and remove material with the pore walls. Compressibility and hardness relate to the pad's ability to conform to the initial surface irregularities. Generally, the harder the pad is, the more global the planarization is. Softer pads tend to contact both the high and low spots, causing non-planar polishing. Another approach is to use flexible polish heads that allow more conformity to the initial wafer surface.
The slurry
210
has a chemistry that is complex, due to its dual role. On the mechanical side, the slurry is carrying abrasives. Small pieces of silica are used for oxide polishing. Alumina is a standard for metals. Abrasive diameters are usually kept to 10-300 nanometers (nm) in size, to achieve polishing, as opposed to grinding, which uses larger diameter abrasives but causes more surface damage. On the chemical side, the etchant may be potassium hydroxide or ammonium hydroxide, for silicon or silicon dioxide, respectively. For metals such as copper, reactions usually start with an oxidation of the metal from the water in the slurry. Various additives may be found in slurries, to balance their ph, to establish wanted flow characteristics, and for other reasons.
One difficulty with CMP semiconductor fabrication equipment is that the semiconductor wafer may slip from the platen during rotation. The platen rotates at a fast speed, such that wafer slippage can be a common occurrence. If the wafer slips out from under the polishing pad and is not detected, the wafer may be flung out by the rotating platen and break. More seriously, if the slipped wafer is not detected, the small pieces into which the wafer breaks may affect semiconductor wafers on neighboring platens, also damaging them. If the polishing pad continues to rotate where the wafer has slipped out from under the pad, the membrane of the polishing pad mechanism can also break. All of these problems are costly.
To avoid this problem, sensors have been developed to detect wafer slippage, or wafer loss. Generally, the wafer is darker in color than the platen and pad, so that if the wafer has slipped from the platen, the change in brightness, or color, can be detected to determine whether wafer loss or slippage has occurred. A single sensor in such cases typically can detect wafer loss. However, some platens and pads are similar in color or brightness to the wafer, rendering the distinction process for determining wafer slippage or loss more difficult to perform. In these cases, conversely, a double sensor has been developed to detect wafer loss. However, it has been determined that the double sensor as currently developed is not properly detecting wafer slippage where the platen and/or pad is similar in color or brightness to the wafer.
FIG. 3
shows a conventional single sensor system
300
, including a single optical wafer loss sensor
304
held at an angle to vertical within a sensor holder
302
. A wafer
308
is on a pad of a platen
306
. Because the platen
306
is lighter in color than the wafer
308
, the sensor
304
can optically detect when the wafer
308
has slipped from the platen
306
. This is accomplished by emitting light
310
that is reflected back as the light
312
. More light will be reflected back from one of the platen
306
and the wafer
308
, depending on their color characteristics, their reflectivity variations, and so on. The sensor
304
can thus be calibrated to properly detect when the wafer
308
has slipped. However, this convention single sensor system
300
does not adequately determine wafer slippage where the pad of the platen
306
has a substantially similar color, brightness, reflectivity, or other attribute to that of the wafer
308
, without generating a significant number of false detections.
FIG. 4
shows a proposed conventional dual sensor system
400
, including dual optical wafer loss sensors
404
and
405
held vertically at no angle within a sensor holder
402
. The wafer
308
is on a pad of a platen
406
that is the same color, or otherwise has a substantially similar attribute, as that of the wafer
308
. In theory, emitting light
410
from the sensor
404
and/or emitting light
411
from the sensor
405
will cause some reflection back as the light
412
and the light
413
, respectively, where different amounts or qualities of the light reflected back can indicate whether the light bounced off the platen
406
, indicating wafer slippage, or from the wafer
308
, indicating no wafer slippage. However, this proposed conventional dual sensor system
400
has been found to not adequately determine wafer slippage where the pad of the platen
406
has a substantially similar color, brightness, reflectivity, or other attribute to that of the wafer
308
, even though this is its designed-for purpose.
Another difficulty with CMP semiconductor fabrication equipment is that the slurry may spray up onto the wafer slippage or loss sensor(s) when the pad or the platen is being rinsed of excess slurry. The slurry then dries on the sensor, and as a white solid, causing false wafer slippage detection by the obfuscated sensor. This situation is shown in
FIG. 5. A
conventional sensor system
500
includes a platen
502
, a sensor holder
504
, and a sensor
506
. To clean the platen
502
, a high-pressure rinse action is performed while the platen
502
rotates. As a result of the high-pressure rinse action, as indicated by the arrow
508
, slurry can be sprayed onto the sensor
506
, and later dry as the dry slurry
510
. This dry slurry
510
obfuscates the sensor
506
, and causes false wafer slippage or loss alarms to be generated.
Therefore, there is a need for CMP t
Cheng Rico
Lai Kevin
Peng Kang-Yung
Hail III Joseph J.
Ojini Anthony
Taiwan Semiconductor Manufacturing Co. Ltd.
Tung & Associates
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