Thickness monitoring

Details Thickness monitoring Thickness monitoring

1999-04-19

2001-05-01

Pham, Hoa Q. (Department: 2877)

Optics: measuring and testing

By configuration comparison

With photosensitive film or plate

C356S389000

Reexamination Certificate

active

06226086

ABSTRACT:

This application is the U.S. national phase application of PCT International Application No. PCT/GB97/02139 filed Aug. 11, 1997.
This invention relates to the field of thin film deposition and/or removal, and more particularly to improved monitoring of thickness during deposition or removal using time domain image recognition applied to optical reflectometry.
Thin films are commonly used to modify surface properties. Typical applications include the coating of optical components to improve their light transmission or reflection properties, the coating of composite materials to improve adhesion behaviour, and the coating of semiconductors to introduce insulation layers or layers with specific electronic properties. Typically these thin films will have thicknesses in the range of 1 nm to 5 &mgr;m and are placed on top of a substrate material which has a very much greater thickness. Frequently the films are structured in stacks, one on top of the other. Such stacks may consist of three or four individual films up to structures containing hundreds of films.
For adequately carrying out the function for which they have been designed these films frequently have to be deposited or, once having been deposited, have to be removed wholly or partially with very great precision. This deposition or removal is frequently carried out under conditions of a vacuum using heated elements and gases or gases excited into the plasma state. Such processes generate considerable quantities of noise in electrical, thermal, optical, vibrational and radio frequency categories.
Equipment that is measuring and/or controlling the thickness and/or rate of deposited or removed film or films therefore has to operate under arduous conditions in the presence of many categories of interfering noised signals. These interfering noise signals frequently upset the measurement technique resulting in processes that are inadequately controlled.
This invention improves the procedure of in-process determination of the thickness of deposited or removed film under these inherently noisy and difficult conditions.
BACKGROUND OF THE INVENTION
Thin film deposition or removal requires either chemical or physical processes or a combination of the two and most frequently takes place under conditions off a partial vacuum. A typical film removal system to which the equipment and method of the current invention could be conveniently applied is depicted in FIG.
1
.
The method of film removal depicted here is commonly referred to as dry etching or reverse sputter etching depending on the pressure level maintained during the process. The substrate
20
is placed on an electrode
21
which may be electrically isolated or part of the electrical ground of the system. A second electrode
22
is connected to the opposite polarity of a power supply unit
25
. Commonly this is the positive polarity. The system is enclosed within a vessel
23
which is evacuated by a pumping means
24
. The application of power from the power supply
25
ionises residual gas in a the vessel or alternatively additional gases may be introduced in order to modify the environment and the process. The ionized gases are attracted to the electrodes with the heavy positively charged ions impinging on the substrate
20
causing film removal by physical means and/or chemical means.
It will be readily observed from the foregoing description and the drawing that the introduction of any probe into the etch region will prevent ions from impinging on the whole substrate and, if the probe is metallic, disturb the electrical profile within the etch region to the detriment of the process. As such it is common and well known to introduce an optical signal which is reflected off the substrate and subsequently detected. A typical optical path is shown at
28
with access to and egress from the system made possible by transparent food through ports or windows
26
,
27
. An alternative system is to provide a small window in the electrode
22
so that light can be directed at the substrate and reflected back along its own path.
An alternative arrangement for deposition rather than removal of thin films is shown in FIG.
2
.
The method of film deposition depicted here is commonly referred to as sputter deposition or plasma enhanced chemical vapour deposition depending on the pressure level maintained during the process. The substrate
30
is placed on an electrode
31
which may be electrically isolated or part of the electrical ground of the system. A second electrode
32
is connected to the opposite polarity of a power supply unit
35
. Commonly this is the negative polarity. The system is enclosed within a vessel
33
which is evacuated by a pumping means
34
. The application of power from the power a supply
35
ionises residual gas in the vessel or alternatively additional gases may be introduced in order to modify the environment and the process. The ionised gases are attracted to the electrodes with the heavy positively charged ions impinging on the chosen material to deposit
39
which is placed on or bonded to the electrode
32
. Material is then deposited by physical or chemical or a combination of methods on the substrate
30
. As a variant on this process there may be no deposition material
39
, with the deposition occurring by a chemical combination of gases enhanced by the plasma.
As with the previous case, it will readily be seen that the introduction of a physical probe, such as may consist of a quartz crystal microbalance, into the deposition region will prevent depositing material from impinging on the whole substrate and, if the probe is metallic, disturb the electrical profile within the etch region to the detriment of the process. As such it is common and well known to introduce an optical signal which is reflected off the substrate and subsequently detected. A typical optical path is shown
38
with access to and egress from the system made possible by transparent feed through ports or windows
36
,
37
. An alternative system is to provide a small window in the electrode
32
so that light can be directed at the substrate and reflected back along its own path. As an alternative if the substrate is transparent then a small hole can be introduced in the electrode
31
with light transmitted through the substrate.
Light that is introduced as described above reflects off the film that is being deposited or removed and the properties of the reflected light are modified (Ref Born and Wolf). Such modification will occur to the intensity of reflection and/or to the polarisation properties and these modifications will depend on the wavelength of the incoming optical radiation. Determination of the film thickness can be by reference to an existing reference standard (Ledger et al, EP 0 545 738 A2) or alternatively oscillations in reflected monochromatic light can be counted (Corlias, GB 2 257 507 A). These methods can be improved by the introduction of additional wavelengths (Canteloup et al, EP 0 735 565 A1) where the additional wavelengths, or indeed white light illurination with spectral analysis of the reflection, is used to remove anomalies in the identification of a particular oscillation extremum.
Prior art assumes an idealized development of the reflection process (
FIG. 3
) with the change in film thickness between extrema in the reflection signal (
50
) occurring in a time &Dgr;T being given by the relationship:
&Dgr;x=&lgr;/(4&mgr;)
where &Dgr;x is the change in film thickness occasioning the change in reflection level;
&lgr; is the wavelength of the light used to probe the film thickness; and
&mgr; is the refractive index of the material at the wavelength of light &lgr;.
In real situations the signal frequently does not meet this ideal and resembles the signal obtained and illustrated in FIG.
4
.
The structure of the film giving this reflected signal during its etch is shown in FIG.
5
. Here a metallic mask
61
is overlying a film of silicon oxide
62
on a silicon substrate
63
and the illumination beam
64
is such that both the mask
61
and th

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