Process for the measurement of the thickness and refractive index of a thin film on a substrate, and an apparatus for carrying out the process

A process and apparatus for in situ measurement of the thickness of a thin ilm on a substrate using interference effects in the thin film. Thermal radiation of the substrate is utilized as a source of interfering bundles of electromagnetic radiation which intensity thereof is measured with a charge-coupled-device camera, and signal-processing electronics is utilized for determining in accordance with the Airy formula the thickness of the thin film on the substrate in the planar direction of the thin film and the index of refraction thereof. The low time constant for the measurement and evaluation enables the process for the recording of measurements be used for the control of coating or removal procedures.

FIELD OF THE ART 
The invention relates to a process for the spatially resolved measurement 
of the thickness of a thin film on a substrate, as well as to a device for 
performing the process. 
Film thickness measurements are counted among the most significant 
auxiliary means in quality control during semiconductor manufacture, 
especially for checking individual process steps. Since the process 
environment, for example within a semiconductor-producing equipment, can 
vary greatly in spatial respects, it is particularly desirable to measure 
the film thickness over the entire wafer surface. 
With increasing integration of components, the costs per wafer have risen 
considerably; for this reason, a complete check of each individual process 
step is the objective in order to recognize flawed parts as early as 
possible and to be able to sort these out. 
STATE OF THE ART 
A process for measuring the growth rate of an epitaxial layer on a 
substrate has been known from the technical publication by A. J. Spring 
Thorpe et al., "In Situ Growth Rate Measurements During Molecular Beam 
Epitaxie [sic!] Using an Optical Pyrometer", Applied Physics Letter, 55: 
2138-2140 (1989). 
This article describes measurement of the surface temperature of the 
substrate by means of an optical pyrometer during application of the 
layer. During this step, oscillations occur in the temperature which can 
be associated with the growth rate of the layer. However, it is not 
possible to measure, with this process, the absolute value of the layer 
thickness; furthermore, no spatial resolution over the area of the layer 
is possible. 
One possibility of determining the thickness of layers resides in the 
interference of light beams reflected on the two surfaces of the layer. In 
an arrangement as disclosed, for example, in F. Kohlrausch, "Praktische 
Physik" [Practical Physics], vol. 1, 23rd edition, 1984, page 667, the 
phase difference of interfering beams is determined by their angle of 
inclination. Since all bundles of rays with the same angle of inclination 
are imaged independently of their point of origin during interference in 
the same image point, a locally resolved measurement of the layer 
thickness is not possible. 
Moreover, the known interferometric methods for layer thickness 
determination are not directly suitable for measurement during production 
of the layer on account of the time required for the measurement and the 
necessary manipulating mechanisms. 
Furthermore, a process for measuring the film thickness during application 
of the film has been known from DE 19 39 667 A1. In this process, the film 
thickness is determined by detecting the electromagnetic radiation emitted 
by the film. 
Also, a process and apparatus for determining the layer thickness and the 
index of refraction of thin, transparent layers has been known from DE 24 
48 294 A1. 
DISCLOSURE OF THE INVENTION 
The invention is based on the object of indicating a process permitting, 
during application of a film on a substrate, a spatially resolved 
measurement of the thickness of the film over the entire surface. 
Furthermore, a device for carrying out this process is to be made 
available. 
This object has been attained, according to the invention, by measuring and 
evaluating the intensity of two or more interfering electromagnetic 
bundles of rays which exhibit a phase difference after passing through 
differing path lengths in the film, and by providing that the thermal 
radiation of the substrate serves as the source of the electromagnetic 
radiation, and that all frequency proportions are filtered out of the 
continuous spectrum of the thermal radiation except for an approximately 
monochromatic proportion. 
A bundle of rays emanating due to the thermal radiation from any desired 
point in the substrate is refracted on the interface between the substrate 
and the film and, after passing through the film, is separated on the 
vacant surface of the latter by partial reflection into a reflected and a 
transmitted component beam. The reflected beam, after reflection on the 
interface, is again separated by partial reflection into a reflected 
component beam and a second transmitted component beam. Since the two 
transmitted component beams emanate from the same source, they are 
superimposed in coherent fashion and interfere after imaging with an 
imaging optic customary in interferometry in an image point. The intensity 
measured in the image point is a function of the phase difference between 
the two transmitted component beams and thus a function of the thickness 
of a limited area of the film adjacent to the joint source of the 
component beams. With interference of more than two component beams, the 
result does not change in its quality. 
On account of the plurality of sources of electromagnetic radiation in the 
substrate, the spatial distribution of the thickness of the film can be 
measured over the entire area of the film. 
In order to prevent the interferences from being averaged out at varying 
wavelengths of the thermal radiation, all frequency proportions except for 
an approximately monochromatic proportion are filtered out of the 
continuous spectrum. 
According to features of the present invention, the component bundles 
emanating from the film are conducted through a narrowband filter and 
imaged by an imaging optic, in the simplest case by a lens, on a locally 
resolving detector. The output signals of the detector, corresponding to 
the intensities of the interfering component beams, are fed to a 
signal-processing electronic circuit in order to determine, with the aid 
of Airy's formula, the film thicknesses and indices of refraction of the 
film. A multidimensional image of the film thickness and refractive index 
distribution can be built up in the layer by means of the evaluated 
signal. 
In order to avoid the necessity of using appliances of high sensitivity 
(e.g. residual light amplifiers) when selecting a detector, the substrate 
is heated. Thereby, the intensity of the thermal radiation is enhanced. 
According to features of the present invention, line or matrix detectors 
can be utilized as the local-resolution or position-sensitive detectors. 
Thus, the distributions of the film thicknesses and refractive indices can 
be measured either over the entire film or along a line-shaped area. In 
the last-mentioned process, the expenditure in apparatus is minimized 
while foregoing complete information. 
In the process according to this invention, the component bundles 
interfering in an image point yield information averaged over a limited 
region of the film. This region is the smaller, the lower the influence of 
the multiple interferences and the smaller the angle between the surface 
normal of the layer and the exiting component beams. A high spatial 
resolution in the plane of the film surface is achieved by means of the 
process wherein the component beams exit approximately perpendicularly 
from the surface of the film. 
An especially advantages further development of the process provides a 
time-dependent measurement. On account of the time-dependent measurement, 
every change in film thickness is directly monitored. Based on the low 
time constants of the measurement and evaluation of the signals, the 
process of this invention is suitable for measured value recording for 
process control operations. This can involve application procedures 
wherein a layer is applied to a substrate by means of chemical and/or 
physical reactions. Examples of such methods are chemical vapor deposition 
(CVD), plasma enhanced chemical vapor deposition (PECVD), molecular beam 
epitaxy (MBE), or electron beam vapor deposition. 
If an already applied film is to be removed, then it is also possible by 
means of the process of this invention to measure the film thickness 
during a removal process, e.g. dry etching. 
The device for performing the process consists of a substrate holder to 
accommodate the substrate, a narrow-band filter filtering out an 
approximately monochromatic radiation from the continuous spectrum of the 
thermal radiation of the substrate, an imaging optic, and a 
local-resolution or position-sensitive detector. All of these components 
are arranged along an optical axis. The outputs of the local-resolution 
detector are connected to a signal-processing electronic circuit. 
According to a feature of the present invention, the substrate holder is 
arranged in a process chamber in order to be able to measure the thickness 
of a film application to a substrate in the process chamber. In accordance 
with the process, the chamber exhibits inlet valves for the process gas. 
For coupling out of the radiation, a vacuum-tight window is set into a 
wall of the process chamber. All other components required for performing 
the process are mounted externally of the process chamber. 
In accordance with other features of the present invention, cameras with 
semiconductor image converters are advantageously utilized as detectors. 
Suitable CCD cameras (charge coupled device) having a high spatial 
resolution power and adequate sensitivity are also known within the 
wavelength range of thermal radiation. 
When using a camera having semiconductor image converters arranged in line 
pattern, a grating monochromator or a prism monochromator can be employed. 
When utilizing a camera having semiconductor image converters arranged in 
matrix fashion, a narrowband interference filter is utilized for rendering 
the thermal radiation monochromatic. 
The wavelength .lambda..sub.o of the maximum transmission of the filter is 
fixed so that the substrate is opaque to this wavelength, but the film is 
transparent thereto. With a film of silicon dioxide on a silicon 
substrate, the wavelength of the maximal transmission .lambda..sub.o =1 
.mu.m represents a suitable choice. The half transmission width .DELTA. of 
the filter is chosen so that .lambda..sub.o /.DELTA..gtoreq.100. This 
ensures that the interference phenomena pertaining to varying wavelengths 
will not interact in disturbing fashion. 
In order to attain a higher intensity of the electromagnetic radiation, the 
substrate is heated up. Heating is accomplished, for example, by means of 
a heating element mounted in the substrate holder. This arrangement is 
resorted to in case the growth of the film thickness during MBE processes 
is to be observed since such a heater has already been included in the 
associated process chamber. 
For heating the substrate, it is also possible, for relatively large 
regions of the process chamber to serve as a furnace, as is customary in 
silicon oxidation processes. 
The advantages attained by this invention reside particularly in that the 
layer thicknesses of a film can be measured in spatially resolved fashion 
over the entire surface of the film. It is thus safely possible to 
determine local deviations from given desired values. Based on the results 
of the measurement, the device can be optimized for carrying out a 
deposition process or a removal process. 
The process permits rapid recording and evaluation of measured values so 
that the growth procedure of a film on a substrate can be observed as a 
function of time. Due to the small time constant of the measurement, the 
process can also be utilized for process control. 
The process is distinguished by high resolution in the growth direction as 
well as in the plane of the film. The process is insensitive to high 
ambient temperatures and with respect to chemically reactive gases and 
plasmas so that it is suited for the control of many deposition and/or 
removal operations. In contrast to conventional interferometric processes, 
the method of this invention is also insensitive to mechanical 
disturbances, such as, for example, vibrations. 
Since no external light source is required, the process is particularly 
well suited for use in high vacuum (HV) and ultrahigh vacuum (UHV) 
processes. A window is enough for coupling out the thermal radiation. The 
process requires only a small number of components and therefore has a 
favorable cost/utility ratio. 
Areas of usage are preferably the in situ measurement of two-dimensional 
film thickness distributions in layers applied to a substrate in oxidizing 
furnaces, sputtering, deposition, CVD or MBE facilities. It is likewise 
suitable for determining the concomitant variables, such as growth rate, 
reflection and absorption coefficients. 
Since the process according to this invention has not as yet been described 
in the literature it is proposed to introduce therefor the term 
"pyrometric interferometry".

DESCRIPTION OF EMBODIMENTS 
An arbitrary point in a substrate 1 of FIG. 1 acts as the source 2 of 
thermal radiation and transmits the bundle of rays 3 that is refracted 
and, respectively, reflected on the interface between the substrate 1 and 
a film 4 applied to the substrate, as well as on the vacant surface of the 
film 4. Since the thus-produced component beams a, b, c emanate from the 
same source 2, they are superimposed in coherent fashion and interfere 
with one another in a point of the detector 5. The mutual spacing of the 
component beams a, b, c and thus the area of the film 4 covered by the 
component beam 3 is the smaller, the smaller the angle .alpha. between the 
surface normal and the component beams a, b, c. In order to achieve a 
maximally high spatial resolution power, the angle .alpha. is chosen to be 
approximately zero. 
The imaging of the component beams a, b, c on the detector 5 is effected by 
the imaging optic 6, consisting in the illustrated case of a collector 
lens. To eliminate a mutual influencing of the interferences of various 
wavelengths of the thermal radiation, the radiation is passed through a 
narrowband filter 7. 
The component beams emanating from various sources in the substrate are 
superimposed on each other in various points of the detector 5 whereby a 
spatially resolved image of the intensities of the interfering radiation 
emanating from various regions of the film 4 is produced. 
FIG. 2 shows schematically a device in accordance with this invention. The 
substrate 1 with the film 4 is located in a process chamber 10. The 
structure of the process chamber 10 is adapted to the process with the aid 
of which the film 4 is applied to or removed from the substrate 1. The 
process chamber 10 is designed, for example, as a vacuum chamber 
exhibiting inlet valves 11 to afford entrance of process gases. A window 
12 for coupling out the heat radiation 15 of the substrate 1 is installed 
in vacuum-tight fashion in the wall of the process chamber 10. In order to 
raise the intensity of thermal radiation, the substrate 1 can be heated 
with the aid of the heating element 16. In addition, or as an alternative, 
heating elements 17 can be arranged in the process chamber 1 so that the 
chamber acts like a furnace. 
Along the optical axis 13, the filter 7, the imaging optic 6 and the 
detector 5 are disposed outside of the process chamber 10. The angle 
.alpha. between the optical axis 13 and the surface normal 14 of the film 
4 is approximately zero. 
The window 12 must be transparent in the transmission range of the filter 
7. Suitable materials are chemically resistant materials which are of low 
water solubility or water-insoluble and exhibit good transmission 
properties in the wavelength range of near and middle-range infrared 
radiation (0.7 .mu.m to 1.2 .mu.m). Suitable materials are, for example, 
SiO.sub.2, Al.sub.2 O.sub.3, ZnSe, CdTe, ZnS, LaF.sub.3. 
The imaging optic 6 images the surface of the film 4 on the detector 5. The 
same conditions apply for the transmission properties of the imaging optic 
as valid for the window 12. The electrical signal outputs of the detector 
5 are connected to a signal-processing electronic circuit 18.