Endpoint detection in chemical-mechanical polishing of...

Details Endpoint detection in chemical-mechanical polishing of... Endpoint detection in chemical-mechanical polishing of...

2000-06-28

2001-09-18

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Chemical etching

Combined with the removal of material by nonchemical means

Reexamination Certificate

active

06291351

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the fabrication of read/write heads for magnetic storage devices, and more particularly to improvement of process control in chemical-mechanical polishing (CMP) processes used in fabrication of such heads.
BACKGROUND OF THE INVENTION
In the field of semiconductor processing, chemical-mechanical polishing (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. 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.
FIG. 1
shows a typical CMP apparatus
10
in which a workpiece
100
(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. Accordingly, surface irregularities are removed from the films deposited on the substrate, and a high degree of planarization is obtained. CMP has been used to remove and/or planarize a wide variety of materials from a stack of films on silicon substrates, including polysilicon, silicon oxides and silicon nitride.
More recently, CMP has been used in a cloisonné process in the fabrication of a magnetic read/write head. This process involves polishing of an aluminum oxide (Al
2
O
3
) film. As shown in
FIG. 2A
, a layer
22
of aluminum oxide is deposited over small structures
21
of NiFe; the NiFe structures
21
are disposed on an underlying structure
1
, which may be a substrate or a stack of films. The aluminum oxide layer
22
is then planarized and removed by CMP until the top surface
21
a of each NiFe structure
21
is exposed (see FIG.
2
B). A cloisonné pattern is thus obtained, with the top surface of the Al
2
O
3
layer
22
coplanar with the top surface of the NiFe structures
21
.
In CMP processes generally, it is extremely important to stop the process at a desired predetermined location in the film or stack of films (that is, when the endpoint has been reached). 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. Underpolishing may sometimes go unnoticed, which also results in yield loss. In the above-described cloisonné process, it is particularly important to maintain tight tolerances on the thickness of structures
21
, while assuring that enough of the aluminum oxide layer is removed to expose surface
21
a.
In a 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 simply 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. In particular, the polishing rate of a film generally changes substantially near an interface; this further compounds the problem of predicting the desired polishing time.
Furthermore, as shown in
FIG. 2B
, the desired cloisonné structure has a very small pattern factor; that is, polishing of the aluminum oxide layer
22
must continue until the NiFe surface
21
a
is exposed, but the total exposed NiFe area is only about 2% of the total Al
2
O
3
/NiFe interface area.
Application of CMP processing to head technology therefore requires a CMP endpoint detection technique which is effective with materials such as Al
2
O
3
and NiFe. In addition, it is desirable that the CMP endpoint be detected in situ and in real time; that is, not rely on external measurements such as the current drawn by shaft motor
16
or on extrapolation from previous layer thickness measurements.
SUMMARY OF THE INVENTION
The present invention enables the use of CMP in head fabrication by providing a real-time, sensitive, high-resolution endpoint detection technique for use in the CMP removal of metal oxide films.
In the CMP endpoint detection method of the present invention, an aluminum nitride (AlN) film is embedded as an endpoint-indicating tag in the aluminum oxide where endpoint is desired (e.g. on top of the NiFe structure
21
shown in FIG.
2
A). Reaction of the AlN with the slurry produces a distinct product, namely ammonia (NH
3
), which is then extracted from the slurry and used as a tag to signal the endpoint of the CMP process.
According to a first aspect of the invention, a method of fabricating a cloisonné structure (in which a top surface of a metal oxide layer is made coplanar with a top surface of a metallic structure formed on a substrate) is provided. A nitride layer is deposited on at least the top surface of the metallic structure, and the metal oxide layer is deposited over the metallic structure and the nitride layer. The metal oxide layer is then polished by a chemical-mechanical polishing (CMP) process using a slurry and a suitable polishing pad (optimizing removal rate and polishing uniformity while minimizing scratches). The metal oxide over the nitride layer is removed, and the nitride layer on the top surface of the metallic structure is thus exposed. At this point the metal oxide layer and the nitride layer are polished together by the CMP process; polishing of the nitride layer causes ammonia to be generated in the slurry. The ammonia is then extracted as a gas from the slurry, and a signal is generated in accordance with the concentration of the extracted ammonia. The CMP process is terminated in accordance with a change in the signal. In a preferred embodiment of the invention, the metal oxide is aluminum oxide, the nitride is aluminum nitride, and the nitride layer (the endpoint-indicating tag) is deposited as a conformal layer on the substrate and the metallic structure.
The CMP process may be terminated in accordance with a change pattern in the ammonia concentration signal. An initial increase in the signal indicates that the nitride layer is being polished, and a subsequent decrease indicates the eventual disappearance of the nitride layer as it is removed from the top surface of the metallic structure.
According to another aspect of the invention, a method is provided for detecting the endpoint for removal by CMP of a metal oxide film overlying a metallic structure. An endpoint-indicating film is first provided between the metal oxide film and the metallic structure, on a top surface of the metallic structure. During CMP of the endpoint-indicating film, a chemical reaction product between the slurry and the endpoint-indicating indicating film is generated in the slurry. The chemical reaction product is extracted as a gas from the slurry, and a signal is generated in accordance with the concentration of the extracted reaction product. A change in the signal indicates the metal oxide CMP endpoint.
At the endpoint of the CMP process, a top surface of the metal oxide film is coplanar with the top surface of the metallic structure, so that the CMP process causes formation of a cloisonné structure. The CMP process may be terminated in accordance with the change in the signal. Specifically, the CMP process may be terminated in accordance with a decrease in the signal, since the decrease indicates nearly complete removal of the endpoint-indicating layer from the top surface of the metallic structure.
It should be noted that the endpoint-indicating layer may be located anywhere within the oxide film, so that the CMP process may be terminated at a predetermined location in the film or at a predetermined film thickness.


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