Method for measuring the viscosity of a material

A method for measuring the viscosity of a material consists in forming an array of parallel strips of material for constituting a diffraction grating; illuminating the array with a monochromatic light beam which produces a diffraction grating comprised of a main light spot (5) and of a plurality of adjacent diffraction spots, the envelope of which exhibits a major lobe (LO) including the main spot (5) and minor lobes (L1, L2); subjecting the array to a thermal process consisting in rapidly heating it at a predetermined temperature (T) and maintaining it at such temperature; selecting the brightest spot (6) among those of the first lobe (L1) and measuring the evolution of its light intensity (HL1) during the thermal process; determining the time interval elapsing until the first passage by a minimum intensity value (HL1b) of the spot (6); and deducting therefrom the value (.nu.) of the viscosity of the material constituting the array by the formula 1/.nu.=.alpha.d.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for measuring the viscosity of a 
thin-layer material deposited on a substrate. 
The invention especially relates to the field of integrated circuits. 
During manufacturing of integrated circuits, determined materials have to 
be deposited in very thin layers and the shape of these thin layers has to 
be modified by appropriate thermal processes that cause the material to 
flow. The flowing of the material corresponds to a mass transfer resulting 
from superficial forces, this mass transfer being possible only if the 
material has a determined viscosity. 
Generally, the materials that are deposited in thin layer are not intended 
to be heated at high temperatures for a sufficiently long time to provide 
a very substantial flowing. It is simply tried to change, by a determined 
flowing, the shapes of the patterns of the deposited material, 
particularly to round up angles. During manufacturing of integrated 
circuits, a thermal process aiming at achieving a determined flowing of a 
given material can be properly carried out only if the viscosity of the 
material, at the selected temperature, is relatively well known. 
Particularly, the knowledge of this viscosity can serve as a parameter 
introduced in modelling systems which are constituted by softwares 
enabling to foresee the behavior and arrangement of microstructures, 
without involving a high number of actual experiments. Also, it may be 
advantageous to very accurately know the viscosity of a determined 
material, at a given temperature, for any other reason than that mentioned 
above. 
The structure and chemical compositions of materials in thin layers do not 
strictly correspond to that of the corresponding bulk material. Hence, it 
is not possible to deduce, by conventionally measuring a bulk viscosity, 
the viscosity of the same material deposited in thin layer. 
It is reminded that the measurement of viscosity according to conventional 
methods consists in disposing a determined amount of material in a tank 
provided with an aperture and measuring the flowing rate of this material 
through the aperture. 
U.S. Pat. No. 4,813,781 teaches that it is possible to measure the degree 
of flowing of a material deposited in thin layer by forming on a substrate 
an array of parallel strips of the material, so that these parallel strips 
constitute a diffraction grating, by subjecting the grating to the 
conditions wherein the material flows, by lighting the grating with a 
monochromatic light beam and by observing the evolution of the light 
diffracted during flowing. Particularly, the intensity of at least one 
light spot of the diffracted pattern is measured and a determined 
distortion level of the pattern strips is deduced therefrom and, hence, a 
determined flowing value. 
This method enables comparing a flowing level of a material to an equal 
flowing level of the same material previously obtained and which was 
satisfactory. However, this method does not enable to quantitatively 
measure the displacements of materials and hence does not enable to 
determine, for given conditions, the viscosity index of a material. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a simple and reliable method for 
measuring the viscosity of a thin layer of a material. 
To achieve this object, the invention provides a method for measuring the 
viscosity of a material consisting in forming an array of parallel strips 
of the material for constituting a diffraction grating; illuminating the 
array with a monochromatic light beam, the diffracted light of which 
provides a diffraction grating comprised of a main light spot 
corresponding to the specular reflection and a plurality of adjacent, 
aligned, diffraction spots, the envelope of which exhibits a major lobe 
including the main spot and minor lobes among which the first lobe is 
adjacent to the major lobe; subjecting the pattern to a thermal process 
consisting in rapidly heating it at a predetermined temperature and 
maintaining it at this temperature; selecting the brightest spot among 
those of the first lobe and measuring the evolution of the light intensity 
of the spot during the thermal process; determining the time interval d 
elapsing until the first passage through a minimum intensity value of the 
spot; and deducting therefrom the value .nu. of the viscosity of the 
material constituting the pattern by the formula 1/.nu.=.alpha.d, where 
.alpha. is a constant determined by calibration of the apparatus with a 
known material. 
Preferably, the parallel strips of the diffraction grating are regularly 
spaced and each exhibits a cross section in the form of a rectangle having 
a width .zeta. and a thickness e, and are separated by a pitch p, the 
light beam has a wavelength .lambda.. Values .zeta., e and .lambda. meet 
the following criteria: 
EQU 1 .mu.m&lt;.zeta.&lt;60 .mu.m 
EQU 0.1 .mu.m&lt;e&lt;6 .mu.m 
EQU 0.2 .mu.m&lt;.lambda.&lt;10 .mu.m, 
and the ratio p/.zeta. is such that the number of spots of the first lobe 
is higher than 4.

DETAILED DESCRIPTION OF THE INVENTION 
The method for measuring the viscosity according to the invention involves, 
first of all, the formation on a plate 1 of a diffraction grating 
constituted by an assembly of parallel strips 2 regularly spaced apart, 
made in the material, the viscosity of which is to be measured at a 
determined temperature T. The cross section of each strip 2 is 
rectangular, as shown in FIG. 2. 
If plate 1, and consequently the parallel strips 2, is heated at a 
determined temperature T, the viscosity of the material at this 
temperature T becomes sufficiently low so that the forces resulting from 
the superficial stress cause a distortion of parallel strips 2. 
FIG. 3 shows a parallel strip 2 which has been subject to a determined 
flowing. It can be seen that the shapes are rounded up. If, at temperature 
T, the viscosity is extremely low or if, which gives an equivalent result, 
the sample is heated at this temperature for a very long time, the initial 
rectangular shape (FIG. 2) tends to become a flattened spheric shape 
corresponding to the shape of a drop of water on a plane surface. 
According to the invention, the diffraction grating constituted by parallel 
strips 2 is subject to a thermal processing consisting in rapidly heating 
the diffraction grating at a predetermined temperature T. During the 
thermal process, the diffraction grating is illuminated by a monochromatic 
light beam having a wavelength in the range of visible, ultraviolet or 
infrared radiations. 
The diffraction light provides a diffraction grating, an example of which 
is shown in FIG. 4A. This diffraction grating is composed of a main light 
spot 5 corresponding to the specular reflection and a plurality of 
adjacent, aligned, diffraction spots, the envelope of which exhibits a 
major lobe L0 including the main spot 5 and minor lobes L1, L2, etc., 
among which the first lobe L1 is adjacent to the major lobe L0. FIG. 4A 
shows only half of the diffraction grating as it is symmetrical with 
respect to the specular plane. 
The applicant has studied this diffraction grating and its evolution during 
flowing which is continuously achieved when the sample is heated at a 
constant temperature T, in order to provide an accurate and reproducible 
method for measuring the actual value of the material viscosity at 
temperature T. 
According to the invention, the brightest spot 6 among those of the first 
lobe L1 is selected and the evolution of the light intensity HL1 of spot 6 
during the thermal process is measured. When the thermal process of the 
sample is started, at time t.sub.0, a determined diffraction grating is 
obtained, some time is allowed to elapse and, at time ta, another 
diffraction grating corresponding to FIG. 4A is obtained. At a subsequent 
time tb, another diffraction grating corresponding to FIG. 4B is obtained, 
and at a later time tc, a diffraction grating corresponding to FIG. 4C is 
obtained. In fact, the shape of the diffraction grating remains 
substantially equal during the progressive flowing of the sample, but the 
applicant has noticed that the light intensity HL1 of the brightest spot 6 
of the first lobe L1 varies in a relatively substantial way as a function 
of the evolution of the pattern flowing during thermal process. 
FIG. 5 shows the evolution of the light intensity HL1 of spot 6 during 
flowing. At time ta, the light intensity of spot 6 has a relatively high 
value HL1a; at the subsequent time tb, the intensity takes a substantially 
lower value HL1b; and at the later time tc, it can be seen that value HL1c 
is again higher than value HL1b. 
Thus, the patent applicant has noticed that, during flowing, the light 
intensity HL1 of spot 6 starts decreasing, passes through a minimum value 
HL1b, then increases again. Now, it is very easy to view or to calculate 
with automatic apparatuses the time when the value of a light intensity 
goes through a minimum. 
This passage by a minimum value is specific of the brightest spot of the 
first lobe and can be univocally associated with the distortion state of 
the diffraction grating strips. Hence, the time interval d between time 
t.sub.0 corresponding to the beginning of the thermal process and time tb 
is a physical value representative of the value of the viscosity of the 
material constituting the diffraction grating at temperature T at which 
the pattern is heated. In fact, this time interval d is reversely 
proportional to the value of viscosity, the proportionality coefficient 
being liable to be determined by previous calibration carried out with a 
material, having a known viscosity when deposited in thin layer. 
When implementing the method for measuring the viscosity according to the 
invention for the first time on a determined equipment, it is necessary, 
first of all, to calibrate it by forming a first array of a material 
having a known viscosity .nu.O and by measuring, as above described, the 
time interval dO elapsed between time t.sub.0 at the beginning of the 
thermal process and time tb. Then, a constant .alpha. is determined from 
the following formula: 
EQU 1/.nu.O=.alpha.dO. 
The viscosity measurement that is subsequently carried out is made from a 
diffraction grating 2 having a geometrical shape and dimensions strictly 
identical to the first array but formed in the material, the viscosity of 
which is to be measured. Measurement of time interval d is achieved on 
this pattern and the viscosity index .nu. is deducted by the following 
formula: 
EQU 1/.nu.=.alpha.d, 
coefficient .alpha. having been previously determined as indicated above. 
According to another aspect of the invention, the diffraction grating has 
to meet some size requirements so that the above described method for 
measuring the viscosity enables a reliable and reproducible determination 
of the value of the material viscosity. FIGS. 1 and 2 will be referred to 
for explaining these requirements. Each parallel strip 2 has a 
rectangular-shaped cross-section having a width .zeta. and a thickness e. 
Also, p defines the pitch of the pattern, that is, the distance separating 
lateral edges corresponding to two adjacent parallel strips 2. The 
wavelength of the monochromatic light beam is .lambda.. 
For the selected wavelength .lambda., it is necessary to choose .zeta. and 
e values which are such that the obtained diffraction grating has a first 
lobe L1 sufficiently high and that the light intensity HL1 actually 
varies, as a function of flowing, reaching a minimum at a precise time ta. 
It is also necessary to choose a value for e corresponding to the order of 
magnitude of the thickness of the layers it is desired to subsequently 
achieve with the material, the viscosity of which is to be measured. 
Thus, according to an embodiment of the invention, for a wavelength 
.lambda. ranging from 10 to 0.2 .mu.m, values .zeta. and e meet the 
following criteria: 
EQU 1 .mu.m&lt;.zeta.&lt;60 .mu.m 
EQU 0.1 .mu.m&lt;e&lt;6 .mu.m. 
Still for the selected wavelength .lambda., it is also necessary to choose 
a ratio p/.zeta. such that the number of spots of the first lobe L1 of the 
diffraction grating is higher than 4. 
Some materials, the viscosity of which is to be measured cannot be easily 
etched. In that case, it is too difficult or impossible to achieve with 
such materials a satisfactory diffraction grating. To overcome this 
difficulty, a diffraction grating is achieved on a plane substrate, this 
pattern being similar to the one above described but being made of a 
material, the etching of which is well known, and which will not, or very 
little, flow at the measurement temperature T. Then, on this pattern, a 
thin layer of the material to be analyzed is deposited. This layer thus 
takes the shape of the underlying pattern and constitutes in turn the 
diffraction grating used for measuring the viscosity according to the 
invention.