Real-time control of chemical-mechanical polishing...

Details Real-time control of chemical-mechanical polishing... Real-time control of chemical-mechanical polishing...

1998-12-08

2001-06-26

Mills, Gregory (Department: 1763)

Semiconductor device manufacturing: process

Chemical etching

Combined with the removal of material by nonchemical means

C438S007000, C438S010000, C438S706000, C216S088000, C216S085000, C216S086000, C216S060000, C216S061000

Reexamination Certificate

active

06251784

ABSTRACT:

This application is related to application Ser. Nos. 09/073,601, 09/073,602, 09/073,603, 09/073,604, 09/073,605, 09/073,606 and 09/073,607, all filed May 6, 1998; and to application Ser. Nos. 09/129,003, 09/129,102, 09/129,103, 09/129,104 and 09/129,107, all filed Aug. 4, 1998. All of these related applications are assigned to the same assignee as the present application. The disclosures of all these related applications are incorporated herein by reference.
FIELD OF THE INVENTION
This invention relates to semiconductor processing, and more particularly to detection of the endpoint for removal of one film overlying another film.
BACKGROUND OF THE INVENTION
In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. Typical processing steps involve (1) depositing a film; (2) patterning areas of the film using lithography and etching; (3) depositing a film which fills the etched areas; and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP).
In film removal processes, 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 moving the wafer, in a rotating or linear motion, 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.
A number of methods have been suggested for obtaining reliable endpoint detection in CMP processing. These involve the following types of measurement: (1) timing the process; (2) friction or motor current; (3) capacitive; (4) optical; (5) acoustical; and (6) inductive.
In addition, U.S. Pat. No. 5,399,234 to Yu et al. describes monitoring the CMP endpoint by sending acoustic waves through a slurry containing potassium hydroxide. A chemical reaction between the potassium hydroxide and the layer being polished yields a reaction product whose concentration decreases as the endpoint is reached; this decrease is associated with a change in the acoustic velocity.
These endpoint detection methods each have inherent disadvantages, such as a lack of sensitivity, an inability to provide real-time monitoring, or requiring removal of the wafer from the process apparatus to test for endpoint.
U.S. Pat. No. 5,559,428 to Li et al. describes an in-situ endpoint detection scheme for conductive films, using an induction method. There remains a need for an in-situ, real-time endpoint detection scheme suitable for use with non-conductive films. Such a scheme should also have high detection sensitivity and fast response time (preferably less than 1 second). In addition, it is desirable that the detection apparatus be robust, inexpensive and require little maintenance.
A particularly crucial application of endpoint detection in CMP processing involves removal of a silicon dioxide (SiO
2
) film overlying a silicon nitride (Si
3
N
4
) film.
FIG. 1A
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
13
.
FIG. 1B
is a detail view showing a thin layer
102
of nitride with an overlying layer
104
of oxide. Generally, it is necessary to remove the target film of oxide so as to completely expose the stopping film of nitride, while leaving the stopping film essentially intact (see FIG.
1
C). An additional requirement is that, when the process endpoint is reached, the distance between the silicon
itride interface
107
and the surface
105
of the oxide be controlled within ±200 Å of the target distance. This is referred to in the art as a target window of ±200 Å. Since the oxide removal rate is typically 50 Å/sec, the process time must be controlled to within ±4 sec. Accordingly, a successful endpoint detection scheme must detect exposure of the nitride layer with very high sensitivity, and automatically stop the CMP process within a few seconds after the nitride becomes exposed (that is, no operator intervention should be required when endpoint is reached). Furthermore, the endpoint detection scheme should be effective regardless of the pattern factor of the wafer (that is, even if the area of the exposed underlying layer is a small portion of the total wafer area).
SUMMARY OF THE INVENTION
The present invention addresses the above-described need for endpoint detection and control of a film removal process by providing a reliable, real-time, in-situ method of endpoint detection.
The present invention will be described with reference to chemical-mechanical polishing merely as a specific example, and is not meant to limit applicability of the invention to semiconductor processing technology. Those skilled in the art will appreciate that the invention is broadly applicable to any process in which it is desirable to detect the endpoint for removal of a target film overlying a stopping film. In accordance with the present invention, this is done by removing the target film with a process that generates a chemical reaction product from the target film or the stopping film (or from both films, but at different levels of concentration); converting the chemical reaction product into a separate product; exposing the separate product to ionizing radiation (for example, alpha particles); and monitoring the ionization current generated by the radiation as the target film is being removed. A change in the current indicates a change in the concentration of the separate product, and therefore in the concentration of the chemical reaction product. This change can be correlated to the process endpoint, thereby providing real-time, in-situ monitoring capability and process control.
More specifically, the endpoint detection method of the present invention may be used with a dry etching process or a CMP process. When the method is used with a CMP process, the chemical reaction product may be extracted as a gas from the polishing slurry. In particular, the slurry may be brought into contact with one side of a membrane which is hydrophobic but permeable to the gas molecules. A carrier gas stream below atmospheric pressure contacts the other side of the hydrophobic membrane, so that the chemical reaction product is pulled through the membrane, is entrained in the gas stream and subsequently is introduced into the detection unit.
The endpoint detection method of the present invention may include a step of stopping the film removal process when the endpoint has been reached, thereby providing automatic control of the film removal process.
According to another aspect of the invention, an apparatus for detecting the endpoint of a film removal process is provided, including: a converter for converting the chemical reaction product to a separate, easily detectable product; means for exposing the separate product to ionizing radiation; and a monitor for monitoring an ionization current generated by the radiation as the target film is removed. The apparatus may also include means for stopping the film removal process when the endpoint has been reached. The apparatus may also include an extractor for extracting the chemical reaction product as a gas from a polishing slurry, when the film removal process is chemical-mechanical polishing.


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