Abrading – Precision device or process - or with condition responsive... – With indicating
Reexamination Certificate
2003-11-21
2004-12-28
Nguyen, Dung Van (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
With indicating
C451S060000, C451S446000
Reexamination Certificate
active
06835117
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to the fabrication of electronic devices, and more particularly to improvement of process control in chemical-mechanical polishing (CMP) processes used in fabrication of semiconductor structures or ferromagnetic read/write heads.
In the field of semiconductor processing, CMP is a widely used technique for planarization of material and controlled removal of a layer of material from a stack of films on a substrate. A typical application of CMP is shown schematically in
FIGS. 1A-1C
.
FIG. 1A
illustrates a layer of material
1
deposited on an underlying layer of material
2
and overlying structures
3
, prior to planarization of layer
1
. As layer
1
is polished down, the top surfaces
3
a
of some of the structures are exposed, as shown in
FIG. 1B
(that is, a portion of the pattern defined by structures
3
is revealed). The CMP process continues until all the structures
3
have their top surfaces
3
b
exposed (FIG.
1
C); this is the endpoint of the process.
A typical CMP apparatus
20
is shown in
FIG. 2. A
workpiece
10
(such as a silicon wafer with one or more layers deposited thereon) is held face down by a wafer carrier
11
and polished using a polishing pad
12
located on a polishing table
13
; the workpiece is in contact with slurry
14
. The wafer carrier
11
is rotated by a shaft
15
driven by a motor
16
. The entire surface of the workpiece is polished by the polishing pad in the presence of the slurry.
In CMP processes generally, it is extremely important to stop the process at a desired predetermined location in the film or stack of films. Overpolishing (removing too much) of a film renders the workpiece 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. Tight tolerances are often maintained on the heights of structures
3
; this in turn means that the endpoint must be precisely determined, and the process must then be promptly stopped. Moreover, as shown in
FIGS. 1A-1C
, the target surfaces for the process (that is, surfaces
3
b
) are often only a small fraction of the area of the whole wafer (as small as 2% in the case of NiFe structures with an overlying Al
2
O
3
film, as in fabrication of magnetic write heads, for example in a P2 process). An endpoint detection and process control system for such a very low pattern density must detect the exposure of this small area, and do so with high accuracy and in real time.
In one conventional approach to the CMP endpoint detection problem, the thickness of the layer to be removed and the polishing rate are measured for each workpiece, in order to determine a desired polishing time; the CMP process is run for this length of time and then stopped. Since many different factors influence the polishing rate, and the polishing rate itself can change during a process, this approach is far from satisfactory.
More recently, CMP endpoint detection methods have been described which are based on slurry collection, sampling and analysis. A slurry sample
17
, shown schematically in
FIG. 2
, is removed from the CMP apparatus by a slurry collector
21
and conducted through a delivery tube
21
a
into an analyzer unit
22
. The analyzer unit determines whether the endpoint has been reached, based on changes in the composition of the slurry or in the concentration of a reaction product derived therefrom. A device for collecting slurry samples from the apparatus is described in U.S. Pat. No. 6,176,765, “Accumulator for slurry sampling.” As an example of an endpoint detection method based on slurry sample analysis, U.S. Pat. No. 6,440,263, “Indirect endpoint detection by chemical reaction and chemiluminescence,” describes analysis of the concentration of ammonia gas produced in the slurry when a silicon nitride film is exposed by polishing and chemically reacts with the slurry. This particular technique may be employed in polishing a variety of structures. For example, U.S. Pat. No. 6,291,351, “End-point detection in chemical-mechanical polishing of cloisonné structures,” discloses a CMP Al
2
O
3
film removal process in which a nitride film is deposited on top of NiFe structures, so that ammonia production indicates that the desired interface between the NiFe and Al
2
O
3
has been reached. The disclosures of all three of the above-noted patents are incorporated herein by reference.
A variety of analysis techniques have been suggested in addition to the chemiluminescence approach described in the '263 patent. These include atomic emission spectroscopy and mass spectroscopy for elemental analysis of polishing residues dissolved in the slurry, as is understood by those skilled in the art. All of the various detection and analytical techniques face the same difficulty: delivering accurate, reliable performance in a manufacturing environment.
In order to be useful in a manufacturing environment where processing typically is performed 24 hours a day, a CMP slurry-sampling endpoint detection apparatus must satisfy two requirements: (1) The apparatus must operate continuously, with minimal need for operator supervision and minimal downtime for cleaning or maintenance; this is sometimes termed the “robustness” requirement. (2) The apparatus must monitor and control film removal in situ and in real time (with process control in response to endpoint within 1 second), even when the pattern density is as small as 2%; this is sometimes termed the “sensitivity” requirement. Since endpoint detection depends on analysis of slurry samples, the slurry must be sampled throughout the polishing process. However, sampling is generally conducted only while slurry is delivered to the polishing apparatus, and is interrupted when the polishing process stops. This means that the slurry delivery tube
21
a
often becomes clogged with stagnated or partially dried slurry during periods between polishing processes. In addition, the sampled slurry must be flushed out of the endpoint detection apparatus before the next workpiece is polished, so that newly-sampled slurry is not contaminated by slurry from the previous process. This problem is particularly acute when the pattern density is small; in some instances the polishing residues in the slurry from the target surfaces have concentrations below 1 ppm.
There remains a need for a slurry-sampling CMP endpoint detection apparatus which meets the robustness and sensitivity requirements, and in particular is suitable for detecting process endpoint in wafers with a low pattern density in a manufacturing environment.
SUMMARY OF INVENTION
The present invention addresses the above-described need by providing a CMP system and method for transporting slurry through a tube to an endpoint detection apparatus while a CMP apparatus is performing a polishing operation, and for flushing the slurry sample transport tube while the apparatus is not performing a polishing operation.
According to a first aspect of the invention, a method of performing CMP on a workpiece includes providing a slurry sampling tube for conducting slurry from the CMP apparatus to the endpoint detection apparatus; providing a slurry flushing apparatus for flushing slurry from the sampling tube; pumping slurry through the sampling tube in a slurry flow direction from the CMP apparatus into the endpoint detection apparatus while a polishing operation is in progress; and flushing the sampling tube while a polishing operation is not in progress. The flushing of the sampling tube is commenced in accordance with a control signal from the endpoint detection apparatus terminating the polishing operation; the flushing is terminated in accordance with a starting signal for the next polishing operation. The flushing apparatus includes a first flow control valve, and the flushing operation includes opening the valve to commence flushing when the polishing operation is terminated, and closing the valve to terminate flushing when a next polishing operation is commence
Gu Yingru
Guthrie Hung-Chin
Li Leping
Wang Xinhui
Anderson Jay H.
International Business Machines - Corporation
Nguyen Dung Van
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