Interferometric method for endpointing plasma etch processes

Details Interferometric method for endpointing plasma etch processes Interferometric method for endpointing plasma etch processes

1999-09-30

2002-06-04

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By light interference

For dimensional measurement

C356S498000, C356S511000

Reexamination Certificate

active

06400458

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to plasma etch processes, and more particularly to an interferometric method and apparatus for monitoring a plasma etch process.
In the fabrication of integrated circuits, the removal of various layers or thin films of materials formed on a silicon wafer to define device patterns is commonly accomplished by means of an etching process. Etching techniques in use include wet, or chemical etching, and dry, or plasma etching. The latter technique is typically dependent upon the generation of reactive species from process gases that are impinged on the surface of the material to be etched. A chemical reaction takes place between the material and these species and the gaseous reaction product is then removed from the surface.
An important consideration in all etch processes is control of the extent to which the wafer is etched and determining a time, referred to as the endpoint, at which to end the process. Common methods for monitoring the etch process and determining the endpoint include spectroscopy and interferometry. Interferometric methods known in the prior art include laser interferometry and plasma emission interferometry as disclosed in U.S. Pat. No. 5,450,205 to Sawin et al. In the laser interferometric method, a laser beam I generated by laser
10
is directed through an optical window
12
and onto an area of a wafer
14
undergoing etching within a plasma chamber
16
as shown in FIG.
1
. The intensity of the reflected beam R is detected by a detector
18
and recorded as a function of time. The detector may be a bandpass filter coupled with a silicon photodiode, a spectrometer, or a CCD camera.
When the material being etched is relatively transparent to the incident light, such as layer A as shown in
FIG. 2
, and overlies a reflective surface, such as layer B, the detected light intensity goes through a series of maxima and minima. As layer A is transparent to the incident light, the incident light is both reflected from the upper surface of the layer A and is refracted through the material. At the reflective surface of layer B, the refracted light is also reflected upwardly through layer A, exiting layer A to interfere with the light reflected from the upper surface of layer A. As layer A is etched, the optical path through layer A decreases in length resulting in varying interference patterns.
Plasma emission interferometry also analyzes the interference of light reflected from the surface of a wafer but uses etch reactor plasma optical emission as the light source. As shown in
FIG. 3
, incident light I′ generated from plasma emission
20
formed within the plasma chamber
22
is reflected from the surface of a wafer
24
disposed within the chamber
22
. The reflected light R′ from the wafer
24
passes through an optical window
26
and is detected by a detector
28
.
A plasma chamber
30
having a top portion
32
formed of a dielectric material transmissive to radiation is shown in FIG.
4
. In the case where the dielectric material is transparent, such as fused silica, the top portion
32
can serve as an optical window
33
. As shown, a light source
34
provides an incident beam I″ which illuminates the surface of a wafer
36
through the optical window
33
. The reflected light R″ exits a plasma chamber
38
through the optical window
33
and is detected by detector
39
. Although not shown, optical emission generated by the plasma may also be detected by the detector
39
in the case where no light source
34
is used.
A common problem with the prior art systems shown in FIGS.
1
and
3
-
4
relates to the difficulty in maintaining the optical quality of a window exposed to the plasma. The plasma either etches the window, in which case the window loses its clarity, or the plasma deposits material onto the window, which also leads to a loss of clarity. In the case of optical window
33
shown in
FIG. 4
, these problems are further exacerbated by the fact that a bottom surface
31
facing the plasma
35
of the top portion
32
is typically roughened. Deposited materials adhere better to roughened surfaces than to smooth surfaces and are less likely to flake off onto the wafer being etched. As a consequence of roughening the bottom surface
31
, the optical window
33
becomes translucent rather than transparent and is not very useful for as an optical window for prior art interferometric monitoring methods.
A solution to maintaining the optical quality of a window is disclosed in pending application Ser. No. 09/282,519 to Ni et al. assigned to LAM Research Corporation. With reference to
FIG. 5
, a plasma chamber
40
includes a radiation transmissive top portion
42
having a recessed optical window
44
formed therethrough. Process gas flows into the plasma chamber
40
through an inlet
45
in communication with a prechamber
46
, the prechamber being in communication with the interior of the plasma chamber
40
. The flow of process gases prevents the plasma
47
from etching or depositing material on the optical window
44
. Interferometry is then performed conventionally using a light source
48
and detector
49
.
While the optical window
44
works optically well, it suffers from the disadvantage of increasing the cost of the fused silica dielectric window. In addition to the cost of machining a hole in the dielectric window to accommodate the prechamber
46
, the window is structurally weakened by the machining of the hole. As the dielectric window serves as portion of a vacuum chamber, it must be made thicker to restore the loss in structural strength. This further increases the cost of the dielectric window and reduces the effectiveness of the dielectric window in coupling the radiation to the plasma. An additional drawback of the recessed window solution disclosed is that the top center of the plasma chamber is not the optimum location for the process gas feed for all purposes.
It would therefore be desirable to be able to detect interferometric signals from a wafer being etched without incurring the additional costs, shifting the process, or constraining the gas injection as is required by prior art methods of keeping the optical window clean.
SUMMARY OF THE INVENTION
The present invention provides an interferometric method and apparatus for monitoring a plasma etch process that interposes a diffusing or scattering element between the wafer and the detector. The diffusing or scattering element eliminates the need for a hard-to-maintain transparent optical window located in the top wall of the chamber. It either replaces the transparent window, or allows the transparent window to be moved from a position in the top wall of the chamber to a position in the side wall of the chamber, wherein the window is less susceptible to being degraded by high-density plasma.
More particularly, the present invention is embodied in a plasma chamber having a top wall formed of fused silica, the top wall having a top surface and a bottom surface facing the interior of the plasma chamber. In a first embodiment of the invention, light generated by plasma emission is reflected from the wafer, is scattered at the bottom surface of the top wall, and is transmitted through the top surface of the top wall. Detecting apparatus comprising a lens, optical fiber and a detector detect the light opposite the top wall from the wafer.
In another embodiment of the invention, a light source is provided for illuminating the wafer. Light from the light source passes through the top surface of the top wall and is diffused at the bottom surface of the top wall. The diffused light from the light source illuminates the wafer and is reflected from the wafer. The light reflected from the wafer illuminates the bottom surface of the top wall, is scattered by that surface and is transmitted through the top surface of the top wall. Detecting apparatus comprising a lens, optical fiber and a detector detect the light opposite the top wall from the wafer.
In another embodiment of the invention, a screen is

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